Hydrothermal Synthesis of Near-Infra-Red Emitting Quantum Dots for Fluorescent and Magnetic Bimodal Imaging

MISSING IMAGE

Material Information

Title:
Hydrothermal Synthesis of Near-Infra-Red Emitting Quantum Dots for Fluorescent and Magnetic Bimodal Imaging
Physical Description:
1 online resource (148 p.)
Language:
english
Creator:
Saha,Ajoy Kumar
Publisher:
University of Florida
Place of Publication:
Gainesville, Fla.
Publication Date:

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Materials Science and Engineering
Committee Chair:
Moudgil, Brij M
Committee Members:
Powers, Kevin W
Holloway, Paul H
Batich, Christopher D
Walter, Glenn A

Subjects

Subjects / Keywords:
bimodal -- bio -- contrast -- fluorescent -- magnetic -- near -- quantum -- relaxivity -- water
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre:
Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract:
Synthesis and characterization of water dispersible, near-infra-red (NIR) emitting and magnetic QDs of sizes between 3-6 nm for magnetic and fluorescent bimodal imaging are reported in this dissertation. QDs are semiconducting materials that exhibit quantum confinement with sizes below the excitonic Bohr radius of the material. NIR emitting QDs have potential to act as excellent probes for non-invasive monitoring of biological processes because NIR photons permit deep tissue penetration due to low absorption by water and other tissue components and also due to minimum tissue autofluorescence in the NIR wavelength regime of 700 ? 900 nm. QDs synthesized by the conventional organometallic route require surface modifications with hydrophilic ligands to enable dispersion in aqueous biological conditions. However, these procedures result in significant reduction in their optical properties such as quantum yields (QYs). In this research 3-6 nm alloyed QDs were synthesized by heating the precursor solutions at 180 ?C for various time intervals (30 ? 100 min) under hydrothermal conditions. No separate ligand exchange steps for the QDs were necessary for water dispersibility. NIR emission tunability was achieved by modifying the sizes of the QDs and also by developing a CdS rich shell on core CdTeS QDs. The alloy core and core/shell structure of the QDs were characterized using TEM, XRD, Energy dispersive X-ray spectroscopy (EDS) and XPS. The functionalization of the QDs with a non-toxic N-Acetyl-Cysteine (NAC) creates surface carboxylic acid groups which also allow subsequent bio-conjugation for targeted delivery. The QDs exhibited fluorescence in the visible-NIR 530-820 nm range and yielded high photoluminescence QYs with the maximum being about 60%. The functionality of the QDs was evaluated using in vitro mouse phantom experiments. The 800 nm emitting core/shell QDs exhibited bright photoluminescence inside the mouse phantom when excited with NIR light (710-745 nm) in the Xenogen IVIS? Spectrum Biophotonic Imager indicating their viability as NIR contrast agents. In order to produce QDs that could also be traced by magnetic resonance imaging (MRI) the synthesis route was modified to develop novel magnetic QDs. This was achieved by controlled doping of the CdTeS QDs with Fe. Bimodal contrast agents with optical and magnetic properties integrated into one single nanoparticle (NP) resulted in a group of new materials that can be utilized in two highly complementary imaging techniques viz. fluorescence imaging and MRI. The fluorescent and magnetic Fe doped CdTeS QDs were characterized by superconducting Quantum interference device (SQUID) and MRI measurements. Fe doped QDs emitting between 530-740 nm exhibited QY within 40-60%. The saturation magnetization (Ms) values for QDs emitting at 740 nm and 730 nm were measured to be 2.8 emu/gm and 1.7 emu/gm, respectively, at room temperature. The relaxivity coefficient of the Fe doped QD emitting at 740 nm (732.4 mM-1s-1) was determined to be 88% higher than that for Feridex? I.V. (389.2 mM-1s-1), a commercial magnetic contrast agent (now withdrawn). The performance of the magnetic QDs was determined by in vitro labeling with J774 macrophages. In vivo experiments were also performed by injecting QD labeled macrophages into the leg muscle of mouse followed by whole animal fluorescence imaging and MRI. Significant fluorescence and magnetic contrasts were generated by the QDs with respect to the neighboring tissues inside the animal. These 3-6 nm QDs because of their small size can be cleared from the blood circulation in a short time span. The magnetic QDs have significant potential as biological contrast agents because of their dual fluorescent and magnetic properties in addition to their small size.
General Note:
In the series University of Florida Digital Collections.
General Note:
Includes vita.
Bibliography:
Includes bibliographical references.
Source of Description:
Description based on online resource; title from PDF title page.
Source of Description:
This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility:
by Ajoy Kumar Saha.
Thesis:
Thesis (Ph.D.)--University of Florida, 2011.
Local:
Adviser: Moudgil, Brij M.

Record Information

Source Institution:
UFRGP
Rights Management:
Applicable rights reserved.
Classification:
lcc - LD1780 2011
System ID:
UFE0043250:00001


This item is only available as the following downloads:


Full Text

PAGE 1

1 HYDROTHERMAL SYNTHESIS OF NEAR INFRA RED EMITTING QUANTUM DOTS FOR FLUORESCENT AND MAGNETIC BIMODAL IMAGING By AJOY K. SAHA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE R EQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2011

PAGE 2

2 2011 Ajoy K. Saha

PAGE 3

3 Dedicated to my parents, who taught me the importance of perseverance and patience, and encouraged me to pursue my dreams

PAGE 4

4 ACKNOWLEDGEMENTS Five years have gone by doing my PhD research. As I look back I am surprised by the amount of learnin g that I have had during these years. I also feel grateful for all the help that I have received from numerous people during my study. First, I would like to acknowledge the help received from my advisor, Dr. Moudgil for his guidance and support. My since re thanks go to Dr. Batich, Dr. Holloway, Dr. Walter, and Dr. Powers for serving on my PhD supervisory committee. I am grateful to my research group members at the Particle Engineering Research Center (PERC), Dr. Sharma, Vijay, Amit, Scott, Maria, Paul, De bamitra, Kalyan, Gary, Gill, Angelina and Megan for their invaluable guidance, constructive critique and helpful suggestions with my research. I would like to express my sincere thanks to Dr. Hebard and his students Ritesh and Siddhartha for their assistan ce with particle c haracterization and helpful discussions. To Kerry Siebien, Dr. Craciun, and Eric Lambers from Major Analytical Instrumentation Center (MAIC) for characterization help with t ransmission electron m icroscopy ( TEM ) x ray d iffraction ( XRD ) an d x ray photoelectron spectroscopy ( XPS ) After joining the Materials Science and Engineering (MSE) department at the University of Florida in 2006 I have worked for a year with Hess group doing research on Kinesin motor proteins. Thanks are due to Dr. He ss, Thorsten, Asutosh, Parag, Rob, Yoli, Elizabeth and Issac for their help with my research. Many thanks to Dr. Santra and his students from the University of Central F lorida (UCF) for all the research discussions that we had during the biweekly teleconf erences Thanks to Dr. Shah for his guidance with my research presentation and helpful discussions, to Jiaqing and Dr. Svoronos for their work with flow synthesis of quantum dots ( QDs ) My sincere thanks and gratitude

PAGE 5

5 to Jo Anne, Denise, Greg, Mary Alice, Jamie, Ashley, Marc, Sophie and Alberta for their help related to office work ever since I joined PERC Th anks to Niclas, Maria and Celine from the Walter group for their help with all in vitro in vivo c haracterizations related to magnetic resonance ( MR ) and near infra red ( NIR ) fluorescence imaging of the QDs. To Dr. Schanze and his student Dongping from the Department of Chemistry for their help with photoluminescence ( PL ) decay measurements of the QDs. I had a great time mentoring my undergraduate stude nts Mark, Marston, Oseas, Tracey, Mariam and Luis. It was Mark who started the hydrothermal synthesis of QDs at PERC which developed into the research that I am presenting in this dissertation. Special thanks go to my friends from the student organization Asha for E ducation and to my running coaches Rick and Dana for making my stay in Gainesville a memorable one. Last but not the least, I would like to acknowledge the support I have received from my sister, parents and family throughout my academic career w ithout which I could not have come this far.

PAGE 6

6 TABLE OF CONTENTS page ACKNOWLEDGEMENTS ................................ ................................ ............................... 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 14 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 17 1.1 Quantum Dots ................................ ................................ ................................ ... 17 1.1.1 What are Quantum Dots? ................................ ................................ ........ 17 1.1.2 Optical Properties and Quantum Confinement in QDs ............................ 17 1.1.3 Visible NIR QDs ................................ ................................ ...................... 17 1.1.4 Why is the Study of QDs Important? ................................ ....................... 18 1.1.5 Advantages of QDs over Dyes as Fluorescent Labels ............................ 19 1.1.6 Magnetic and Fluorescent Bimodal QDs ................................ ................. 20 1.1.7 QD Toxicity ................................ ................................ .............................. 24 1.2 Literature Review ................................ ................................ .............................. 25 1.2.1 Quantum Dot Applications, Synthesis and Characterization ................... 25 1.2.1.1 Applications and types of NIR QDs ................................ ................ 25 1.2.1.2 Types of NIR QDs ................................ ................................ .......... 27 1.2.1.3 Synthesis and characterization of NIR QDs ................................ ... 31 1.2 .2 Magnetic NPs ................................ ................................ .......................... 35 1.2.3 Bimodal Fluorescent and Magnetic NPs ................................ ............... 36 1.2.4 QD Toxicity ................................ ................................ .............................. 41 2 MATERIALS AND METHODS ................................ ................................ ................ 54 2.1 Core/Shell Design Considerations ................................ ................................ .... 54 2.1.1 CdTe as QD Core Material ................................ ................................ ...... 54 2.1.2 CdS as QD Shell Material ................................ ................................ ........ 55 2.1.3 N Acetyl Cysteine (NAC) as the Dispersing Ligand ................................ 55 2.2 Synthesis of the QDs ................................ ................................ ........................ 56 2.2.1 Synthesis of the Core C dTeS QDs ................................ .......................... 56 2.2.2 Coating of the Core CdTeS QDs with CdS Shells ................................ ... 57 2.2.3 Synthesis of Magnetic Fe Doped CdTeS QDs ................................ ........ 58 2.3 Characterization of the QDs ................................ ................................ .............. 58 3 RESULTS AND DISCUSSIO NS ................................ ................................ ............. 66

PAGE 7

7 3.1 Synthesis of Core CdTeS QDs Emitting up to 750 nm ................................ ...... 66 3.1.1 Effect of Reactant Concentration ................................ ............................. 66 3.1.2 Effect of P recursor pH ................................ ................................ ............. 67 3.1.3 Effect of Heating Temperature ................................ ................................ 67 3.1.4 Effect of Heating Time ................................ ................................ ............. 67 3.2 Characterization of the Core CdTeS QDs ................................ ......................... 68 3.2.1 Particle Size of the Core CdTeS QDs ................................ ...................... 68 3.2.2 Chemical Composition Variation in the Core CdTeS QDs ....................... 69 3.2.3 Fluorescence Quantum Yield of the Core CdTeS QDs ........................... 70 3.2.4 Extinction Co Efficient Calculations for the Core QDs. ............................ 71 3.3 Synthesis of Core/Shell CdTeS QDs Emitting up to 820 nm ............................. 72 3.4 Characterization of the Core/Shell CdTeS/CdS QDs ................................ ........ 74 3 .4.1 Determining Core/Shell Structure of the QDs Using Chemical Composition Studies ................................ ................................ ..................... 76 3.4.1.1 Characterization of the QDs using E DS ................................ ......... 76 3.4.1.2 Characterization of the QDs using XPS ................................ ......... 76 3.4.2 Time Resolved Luminescence Properties of Core and Core/Shell QDs .. 78 3.5 Performance Assessment of the NIR Emitting Core/Shell QDs ........................ 80 3.6 Bimodal Imaging Using Magnetic QDs ................................ ............................. 81 3.6.1 Hydrothermal Synthesis of Fe Doped Core CdTeS QDs ......................... 82 3.6.2 Characterization of the Fe Doped Core CdTeS QDs ............................... 84 3.6.3 Performance Assessment of the Magnetic Fe Doped QDs ..................... 91 4 CONCLUSIONS AND FUTURE WORK ................................ ............................... 130 4.1 Conclusions ................................ ................................ ................................ .... 130 4.2 Suggestions for Future Work ................................ ................................ .......... 131 APPENDIX: DETERMINATION OF QUANTUM DOT MOLECULAR WEIGHTS ........ 134 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 148

PAGE 8

8 LIST OF TABLES Table page 1 1 Co mparison of characters, modification strategy and application of QDs ........... 44 1 2 Properties of T 2 contrast agents. ................................ ................................ ........ 47 1 3 Properties of T 1 contrast agents. ................................ ................................ ......... 48 3 1 Particle size comparison (Measured versus calculated) ................................ ..... 94 3 2 Chemical composition of the core QD67 0, core QD750 versus core/shell QD750 determined by EDS. ................................ ................................ ............... 94 3 3 Surface chemical composition of the core QD670, core QD750 versus core/shell QD750 determined by XPS. ................................ ............................... 94 3 4 Surface chemical composition (atomic %) of the core QD670 determined using XPS after etching with Argon for 0, 5 and 10 minutes. .............................. 95 3 5 Surface che mical composition (atomic %) of the core QD750 determined using XPS after etching with Argon for 0, 5 and 10 minutes. .............................. 95 3 6 Surface chemical composition (atomic %) of the core/shell QD750 de termined using XPS after etching with Argon for 0, 5 and 10 minutes. ........... 95 3 7 Characteristics of clinically used imaging modalities ................................ .......... 96

PAGE 9

9 LIST OF FIGURES Figure page 1 1 Density of states in one band of a semiconductor as a function of dimension.. .. 49 1 2 Energy diagram of large a nd small NPs with different magnetic spin alignment.. ................................ ................................ ................................ .......... 50 1 3 Size dependent m agnetization properties of ferromagnetic and superparamagnetic NPs. ................................ ................................ .................... 51 1 4 Absorption coefficient versus wavelength plot. ................................ ................... 52 1 5 Schematic representation of Type I and Type II QDs.. ................................ ....... 53 2 1 Energy gap (eV) versus lattice parameter () for bulk semiconducting materials. ................................ ................................ ................................ ............ 61 2 2 Band gap (eV) versus lattice spacing () for compound semiconductors.. ......... 62 2 3 Structure of the N Acetyl Cysteine (NAC) ligand. ................................ ............... 63 2 4 Schematic representation of the core and core/shell QD synthesis by hydrothermal techniqu e. ................................ ................................ ..................... 64 2 5 Flowchart for the synthesis of magnetic Fe doped CdTeS QDs. ........................ 65 3 1 Fluorescence from Vis NIR QDs when excited with UV light .............................. 97 3 2 Emission wavelength versus heating time for core QDs. Heating temperature ................................ 98 3 3 TEM image of core CdTeS QDs emitting at 700 nm. Insets show the SAD pattern and the size of a single QD. ................................ ................................ ... 99 3 4 EDS spectra for the core 560 nm and core 700 nm emit ting QDs. .................. 100 3 5 Chemical composition variation in QDs determined using EDS spectra analysis. ................................ ................................ ................................ ........... 101 3 6 Determination of extin ction co efficient of core QDs QD630 and QD710. ........ 102 3 7 Energy diagram of CdTeS/CdS core/shell QDs represented schematically ..... 103 3 8 Emission t unability in core/shell quantum dots ................................ ................. 104 3 9 Zeta potential variation for core CdTeS 690 nm emitting QDs as a function of pH. ................................ ................................ ................................ .................... 105

PAGE 10

10 3 10 TEM image of core/shell QDs emitting at 720 nm. Insets show the SAD pattern and the size of a single QD. ................................ ................................ 106 3 11 XRD of core and core/shell QDs. ................................ ................................ ...... 107 3 12 EDS spectra for core CdTeS QDs emitting at 750 nm and core/shell CdTeS/CdS QDs emitting at 750 nm ................................ ................................ 108 3 13 XPS of the core QD670, core QD750 and core/shell QD750 XPS showing the Cd peaks for all the samples. ................................ ................................ ..... 109 3 14 XPS showing the Te and S peaks in core QD670, core QD750 and core/shell QD750 samples. ................................ ................................ ............................... 110 3 15 Normalized PL decay curves for core 620 nm and core/shell 800 nm emitting QDs. ................................ ................................ ................................ ................. 111 3 16 Normalized PL decay curves for core 670 nm, core 750 nm a nd core/shell 750 nm emitting QDs. ................................ ................................ ....................... 112 3 17 J774 mouse cells labeled with core/shell C dTeS/CdS QDs emitting at 800 nm ................................ ................................ ................................ ................... 113 3 1 8 Xenogen IVIS Spectrum Biophotonic imaging of mouse with core/shell CdTeS/CdS QDs emitting at 800 nm ................................ ................................ 114 3 19 TEM image of Fe doped CdTeS QDs havin g emission wavelength of 730 nm 115 3 20 XRD of the magnetic Fe doped and undoped CdTeS QDs. ............................. 116 3 21 EDS spectrum for the Fe doped QD740 ................................ ........................... 117 3 22 XPS of the Fe doped QD710 sample etched with Argon for 10 minutes .......... 118 3 22 Continued. XPS of the Fe doped QD710 sample etched with Argon for 10 minut es showing the Fe and Te peaks ................................ ............................. 119 3 22 Continued. XPS of the Fe doped QD710 sample etched with Argon for 10 minutes showing the S peak. ................................ ................................ ............ 120 3 23 Magnetometry measurements using SQUID at 10K and 300K for magnetic QDs and Feridex I.V. NPs normalized with respect to particle mass. ............ 121 3 24 Magnetometry measurements usi ng SQUID at 10K and 300K for magnetic QDs and Feridex I.V. NPs normalized with respect to Fe content. ................. 122 3 25 Magnetization versus temperature plot at a constant magnetic field of 80 Oe for Fe doped QD730 and Fe doped QD740. ................................ .................... 123

PAGE 11

11 3 25 Continued. Magnetization versus temperature plot at a constant magnetic field of 80 Oe for commercial Feridex I.V. particles. ................................ ....... 124 3 26 MRI in vitro T 2 weighted images of serially diluted Fe doped QDs (in DI water) and Feridex I.V. partic les (in 0.25% agarose solution) ......................... 125 3 27 R 2 and R 2 versus Fe concentration plots for Fe doped QD740 (in water) and Feridex I.V. NPs (in 0.25% agarose solution). ................................ ................ 126 3 28 Fe doped QDs and and Feridex I.V. labeled J774 macrop hages were loaded into glass capillaries and imaged using 14T MRI at 600 MHz. .............. 127 3 29 Fluorescence imaging of J774 macrophages labeled with Fe doped QD740 and Feridex I.V. NPs injected insid e mouse ................................ ................... 128 3 30 MRI of Fe doped QDs and Feridex I.V. labeled J774 macrophage s injected inside mouse ................................ ................................ ................................ .... 129

PAGE 12

12 LIST OF ABBREVIATION S CLIO Cross linked iron oxide DAPI diamidino 2 phenylindole DI Distilled DMSA DMSA Dimercaptosuccinic acid EDC N (3 dimethylaminopropyl) N ethylcarbodiimde hydrochloride EDS Energy dispersive x ray spectroscopy EPR Electron paramagnetic resonance EXAFS Extended x ray ab sorption fine structure FC Field cooled HRTEM High resolution transmission electron microscopy ICP Inductively coupled plasma mass spectrometry IR Infra R ed LED Light emitting device MAA Mercaptoacetic acid MEIO Metal doped iron oxide MNP Magneti c nanoparticle MPA Mercapto propionic acid MR Magnetic Resonance MRI Magnetic Resonance Imaging MUA Mercaptoundecanoic acid MWIR Mid wavelength infrared NAC N Acetyl Cysteine NHS N hydroxysuccinimide

PAGE 13

13 NIR Near Infra Red NPs Nanoparticles PET Posi tion emission tomography PL Photo luminescence PTFE Polytetrafluoroethylene QD Quantum dot QD750 Quantum dot emitting at 750 nm QY Quantum Yield ROI Reactive oxygen intermediates SAD Selected area diffraction SEM Scanning Electron Microscopy SPECT Single photon emission computed tomography SQUID Superconducting quantum interference device SWIR Short wavelength infrared TAT Trans activator of transcription TEM Transmission Electron Microscopy TGA Thioglycolic acid TSPETE Triacetic acid trisodi um salt Vis Visible WSIO Water soluble iron oxide XPS X ray photoelectron spectroscopy XRD X ray diffraction ZFC Zero field cooled

PAGE 14

14 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy HYDROTHERMAL SYNTHESIS O F NEAR INFRA RED EMITTING QUANTUM DOTS FOR FLUORESCENT AND MAGNETIC BIMODAL IMAGING By Ajoy K. Saha August 2011 Chair: Brij M. Moudgil Major: Materials Science and Engineering Synthesis and characterization of water dispersible, near infra red (NIR) emitting and magnetic QDs of sizes between 3 6 nm for magnetic and fluorescen t bimodal imaging are reported in this dissertation. QDs are semiconducting materials that exhibit quantum confinement with sizes below the excitonic Bohr radius of the material. NIR emitting QDs have potential to act as excellent probes for non invasive monitoring of biological processes because NIR photons permit deep tissue penetration due to low absorption by water and other tissue components and also due to minimum tissue auto fluorescence in the NIR wavelength regime of 700 900 nm. QDs synthesized by the conventional organometallic route require surface modifications with hydrophilic ligands to enable dispersion in aqueous biological conditions. However, these procedures resu lt in significant reduction in their optical properties such as quantum yields (QYs). In this research 3 6 nm alloyed QDs were synthesized by heating the 100 min) under hydrothermal conditions. No separate ligand exchange steps for the QDs were necessary for water dispersibility. NIR emission tunability was achieved by modifying

PAGE 15

15 the sizes of the QDs and also by developing a CdS rich shell on core CdTeS QDs. The alloy core and core/shell structur e of the QDs were characterized using TEM, XRD, Energy dispersive X ray spectroscopy ( EDS ) and XPS. The functionalization of the QDs with a non toxic N Acetyl Cysteine (NAC) creates surface carboxylic acid groups which also allow subsequent bio conjugation for targeted delivery. The QDs exhibited fluorescence in the visible NIR 530 820 nm range and yielded high photoluminescence QYs with the maximum being about 60%. The functionality of the QDs was evaluated using in vitro mouse phantom experiments. The 800 nm emitting core/shell QDs exhibited bright photoluminescence inside the mouse phantom when excited with NIR light (710 745 nm) in the Xenogen IVIS Spectrum Biophotonic Imager indicating their viability as NIR contrast agents. In order to produce QDs that could also be traced by magnetic resonance imaging ( MRI ) the synthesis route was modified to develop novel magnetic QDs. This was achieved by controlled doping of the CdTeS QDs with Fe. Bimodal contrast agents with optical and magnetic properties inte grated in to one single nanoparticle (NP) resulted in a group of new materials that can be utilized in two highly complementary imaging techniques viz. fluorescence imaging and MRI. The fluorescent and magnetic Fe doped C dTeS QDs were characterized by super conducting Quantum i nterference d evice (SQUID) and MRI measurements. Fe doped QDs emitting between 530 740 nm exhibited QY within 40 60%. The saturation magnetization (M s ) values for QDs emitting at 740 nm and 730 nm were measured to be 2.8 emu/gm and 1. 7 emu/gm respectively at room temperature. The relaxivity coefficient of the Fe doped QD emitting at 740 nm (732.4 mM 1 s 1 ) was determined to be 88 % higher than that for Feridex I.V. (389.2 mM

PAGE 16

16 1 s 1 ), a commercial magnetic contrast agent (now withdrawn) The performance of the magnetic QDs was determined by in vitro labeling with J774 macrophages. In vivo experiments were also performed by injecting QD labeled macrophages into the leg muscle of mouse followed by whole animal fluorescence imaging and MRI. S ignificant fluorescence and magnetic contrast s w ere generated by the QDs with respect to the neighboring tissues inside the animal. These 3 6 nm QDs because of their small size can be cleared from the blood circulation in a short time span. The magnetic Q Ds have significant potential as biological contrast agents because of their dual fluorescent and magnetic properties in addition to their small size.

PAGE 17

17 CHAPTER 1 INTRODUCTION 1.1 Quantum Dots 1.1 .1 What are Quantum Dots? Quantum dots (QDs) are semicondu cting materials in which the excitons (electron hole pairs) are confined in all three dimensions 1 These nanoparticles (NPs) are smaller in size relative to the spatial extension of the holes and electrons in the bulk semiconductors and show quantum size effects The size regime in which these effects are typically observed is 1 10 nm. The material properties of QDs lie in between molecules and bulk semiconducting materials. 1. 1. 2 Optical Properties and Quantum Confinement in QDs QDs can vary in size from less t han 1 nm to greater than 20 nm in diameter. Those with sizes less than 1 nm are nearly molecular and contain less than 100 atoms while those with diameter greater than 20 nm can contain more than 100,000 atoms 1 The emission wavelen gths of QDs depend on th eir size due to quantum confinement, which is t he change of the electronic states from semiconducting bulk materials to QDs ( Figure 1 1 ) 2 In a bulk semiconducting material an exciton is typically bound within a certain length called the exciton Bohr radiu s. The properties of the semiconducting material change when the excitons are constrained further by changing the size of the semiconductor. 1. 1. 3 Visible NIR QDs Among the different properties that QDs exhibit the most striking is the optical property va riation with size. With reduction of the size of a nanocrystal the electronic excitations transitions into higher energy. Thus tunability of emission wavelength is

PAGE 18

18 attained with nanocrystal size variation. For example, the band gap of CdSe can be tuned fro m 1.7 eV to 2.4 eV with reduction of the cluster size from 200 to 20 and changes in light emissions in the visible wavelength regime of the electromagnetic spectrum from deep red to green 2 The band gap of CdTe is tunable from 1.51 eV to 2.34 eV with change in emission wavelengths from NIR emission to visible green emission with change in particle diameter from 200 to 20 . 1.1.4 Why is the Study of QDs Important? Interest in QDs is growing over the years because of their unique properties that incl ude size tunable emission, good stability of the QDs against photobleaching relative to organic dyes and continuous size dependent absorption profiles from the ultraviolet wavelengths to the visible. Due to these properties QDs have numerous potential appl ications in biology and optoelectronics 3 5 .They have attracted considerable research interest recently than conventional dyes due to their excellent photostability along with narrow and symmetric emission peaks and broad absorption spectra 3,5,6 Size tunab le QDs can emit in different wavelength ranges. Visible QDs emit in the 400 700 nm while NIR QDs emit in the 700 900 nm wavelength regimes. The near infrared (NIR) QDs emitting within the wavelength region 700 and 900 nm are of particular interest beca use in this range the tissue absorbance and autofluorescence are minimum leading to low background noise 7 In the field of life sciences investigation and understanding of various fundamental processes is dependent on reliable, fast and sensitive detectio n of the interactions of biomolecules among themselves and also with other ionic/molecular species. Fluorescence spectroscopy is one among the techniques available that is well suited to realize these goals. The different properties that are required in s uitable fluorescent

PAGE 19

19 label are 8 (a) convenient excitability of the fluorophore alone without the biological matrix being excited simultaneously and it should be detectable using conventional techniques and instrumentation (b) brightness i.e. high molar ext inction co efficient at the wavelength of excitation with high fluorescence QY (c) solubility in cell culture media, body fluids and relevant buffers (d) stability in relevant experimental conditions (e) presence of functional groups for site specific l abeling (f) availability of reported data on its photophysics and (g) availability in reproducible quality. There are other requirements depending on the application of the fluorescent label. They are (h) size, steric effects (i) toxicity (j) possibilit y of the label to be delivered to cells (k) suitability for multiplexing and (l) compatibility for signal amplification techniques. These properties were compared for two different types of fluorescent labels viz. QDs of type II VI (CdTe, CdSe CdS etc.) III V (GaAs, InP etc.) and organic dyes, which are widely used for diagnosis in medicine and biological analysis. 1.1.5 Advantages of QDs over Dyes as Fluorescent Labels QDs in comparison to organic dyes have the desirable property of increased absorption at shorter wavelengths below the first excitonic peak. The emission bands of QDs are also narrower and symmetrical compared to the organic dyes. The quantum size effect phenomenon allows emission wavelength tunability in QDs by variation of size. The emis sion peak width is mainly a function of the particle size distribution. The broad absorption in QDs makes free selection of excitation wavelength possible, which allows for straightforward separation of emission from that of the excitation. The molar absor ption co efficient of QDs at the first absorption band are usually larger compared to the organic dyes. For QDs the typical values of the molar absorption coefficients are within the range of 100,000 1,000,000 M 1 cm 1 whereas for the dyes the typical valu es

PAGE 20

20 lies within 25,000 250,000 M 1 cm 1 at the main absorption band 8 In the visible wavelength regime (400 700 nm) the QY of QDs with proper surface passivation are high in most cases. The QY re ported for CdSe was within 65 85% for CdS the value was less than 60% and for CdTe, CdHgTe in the N IR range of >700 nm was within 30 75% For PbS and PbSe emitting above 80 0 nm the QY value ranges were 30 70% and 10 80% respectively 8 In comparison to QDs organic dyes have high fluorescence QYs in the vis ible range but moderate in the NIR wavelength regime. In NIR imaging applications QDs are favored over organic dyes because of the combined limitations of low QY along with low photostability of the NIR wavelength dyes. The fluorescence lifetimes for QDs are typically large in the range of five to hundreds of nanoseconds compared to that of organic dyes which are about 5 ns and 1 ns (few exceptions such as acridone 9 dyes) in visible and NIR wavelengths respectively. The extended lifetime of QD fluorescence also offers some advantages for using QDs over organic dyes in imaging applications that involve lifetime measurements. 1. 1. 6 Magnetic and Fluorescent Bimodal QDs Bimodal QDs designed with two functionalities that of fluorescence and magnetic property, in tegrated in one single nanoparticle are an exciting class of new bioimaging materials. Size controllable multimodal nanoparticles/QDs ranging from a few nanometers to tens of nanometers are of particular interest because they are smaller than cells but com parable to viruses, genes and proteins. Because of the small size these particles can interact with biomolecules by crossing the biological membranes. These magnetic and fluorescent QDs can be used to perform two important but complementary diagnosti c imag ing of tissues and cells viz. fluorescent and magnetic

PAGE 21

21 resonance imaging (MRI). Like their fluorescent counterparts the magnetic properties of NPs are different from bulk magnetic materials 10 Unique properties of magnetic NPs Magnetic nanoparticles (MNP s) exhibit variety of phenomena which are unique and drastically different from their bulk counterparts. These properties could be utilized for various applications which include their use as storage media in magnetic memory devices, as probes and vectors in biomedical research etc. 10 The magnetic properties of the nanoparticles vary from bulk magnetic materials mainly in two aspects. The magnetic coupling/interaction of the surface atoms in MNPs with neighboring atoms will be different from those in bulk m aterials resulting in mixed surface and volume magnetic properties 10 This is due to the different local environment experienced by the surface atoms in MNPs which have large surface t o volume ratio relative to the bulk magnetic materials. Superparamagnet ism is one of the size dependent phenomena exhibited by magnetic NPs. T he magnetic anisotropic energy (U) of bulk materials is much larger than the thermal energy (kT). However, for the small NPs the magnetic anisotropy energy is lower than the thermal ene rgy (Figure 1 2). The thermal energy of the small NPs is high enough to readily invert the magnetic spin direction but it is not sufficient to overcome the spin spin exchange coupling energy. This kind of magnetic behavior in NPs leads to a net magnetizati on of zero and is called superparamagnetism. The temperature at which the transition occurs from ferromagnetism to superparamagnetism is called blocking temperature (T B ). The property of superparamagnetism occurs for a single particle which is also a singl e domain. The magnetization behavior of ferromagnetic and superparamagnetic NPs in the presence and absence of an external

PAGE 22

22 applied field is illustrated in Figure 1 3 (a). In the presence of external magnetic field domains in the ferroelectric NPs aligned i n the direction of the field. The single domain of superparamagnetic NP also aligned with the external field. In the absence of the external magnetic field the ferromagnetic NPs maintained a net magnetization whereas there was no net magnetization for supe rparamagnetic NPs. In Figure 1 3 (b) the relationship between the size of magnetic NPs and their magnetic domain structures is indicated. MNPs with sizes below D s exhibit superparamagnetism and each particle is a single domain. With NP sizes above D c the M NPs acquire multidomain structure 11 For f erromagnetic materials the intensity of the applied magnetic field required to bring the magnetization of the material to zero after it was subjected to saturation magnetization is called coercivity (H c ) or coerci ve field/force of the ferromagnetic material. The unit of coercivity is oersted or amp/meter. Magnetic coercivity of ferromagnetic NPs is different from those of bulk ferromagnetic materials. NPs exist as single magnetic domains below a critical size, D c w here there is unidirectional alignment of all magnetic spins. Coercivity of MNPs increase with size according to the relationship 10 ( 1 1 ) where, m s is the saturation magnetization, K u is the magnetic aniso tropic constant and V is the volume of NP Mult idomain magnetism exists for NP sizes greater than D c and a small amount of magnetic field (H c ) is required to bring down the net magnetization to zero. MRI using MNPs. By using MRI 3 dimensional images of soft and opaque tissue can be generated with rela tively high tissue contrast and spatial resolution. Hence it is

PAGE 23

23 the most versatile diagnostic imaging tool available in the clinic. The magnetic property of the MNPs enhances the sensitivity of MRI. Very high magnetic moment of the MNPs/ QDs along with high transverse relaxivity value s makes high detection sensitivity possible for MRI 1 2 Contrast agents generate contrast in MRI by shortening the 1 H relaxation times of the surrounding water 1 2 By the process of relaxation the protons in a magnetic field that were initially excited by a radio frequency magnetic pulse in the MRI scanner return to thermal equilibrium. The process of relaxation is divided into two principle relaxation types: longitudinal relaxation or spin lattice (characteristic time T 1 [s] or rel axation rate R 1 [s 1 ]) and transverse relaxation (T 2 R 2 ). Magnetic field inhomogeneities can accelerate the rate of relaxation in the transverse process. In such a case the relaxation process is referred as T 2 For the T 1 weighted MR sequences, bright ima ges are produced in areas with short T 1 (positive contrast). While for T 2 weighted MR sequences, dark images are produced in areas of short T 2 (negative contrast). The strength of a contrast agent to produce contrast by accelerating the relaxation rate is defined by the change in the relaxation rate per unit concentration of the contrast agent. For longitudinal relaxation the proportionality constant is denoted by r 1 and for transverse relaxation the proportionality constant is denoted as r 2 Their unit is mM 1 s 1 where r 2 1 A contrast agent can never exclusively be a positive or a negative contrast agent because the T 1 and T 2 processes are not completely independent of each other. The ratio r 2 /r 1 can be used to determine the suitability of a contrast agent either as T 1 weighted positive contrast agent or as T 2 weighted negative contrast agent. Contrast

PAGE 24

24 agents with r 2 /r 1 between 1 and 2 are usually suitable as T 1 contrast agent while those with ratio larger than r 2 /r 1 are suitable as T 2 contrast agent 12 1. 1. 7 QD Toxici ty Toxicity of QDs has become an important area of research in recent years because of the emergence of studies involving biological imaging using QDs. As discussed earlier QDs are attractive compared to dyes because of their high photoluminescence QY and broad absorption spectrum which permits excitation with multiple wavelengths simultaneously. However, most of the QDs that are optically suitable for biological imaging are c admium based materials. The most popular among QDs that possess high fluorescence QY for light emissions across the visible and near infrared wavelength regime of the electromagnetic spectrum are the CdSe and CdTe QDs. The elements of Cd, Se and Te using which the QDs were synthesized are considered to be highly toxic to cells and orga nisms 13 These elements were thought to be released from the QDs in the form of ions when the surfaces of the QDs were oxidized. QD toxicity was attributed to leaching of Cd, Te, Se ions from the QDs and also due to the formation of reactive oxygen interme diates (ROI) 1 4 Synthesis of DNA, RNA and proteins gets inhibited by Cadmium which is also responsible for damaging DNA strands and mutating chromosomes 15 16 One way to minimize toxicity by QDs is to enclose the core QD with a shell and thus prevent the o xidation and leaching of ions from the QDs. But, core encapsulation is not full proof and there were reports 13 on ions leaching from the cores and subsequent toxicity associated from the leached ions for even well protected cores. Reactive oxygen intermedi ates (ROI) were also reported to be responsible for some amount of toxicity. Encapsulation of QD cores can minimize leaching of toxic ions and mitigate toxic effects to some degree H owever, toxicity due to

PAGE 25

25 the produced ROIs is less controllable as the ROI s are produced due to the transfer of energy from QDs to molecular oxygen and can happen without any barrier 17 1 9 1.2 Literature R eview 1.2.1 Q uantum Dot Applications, Synthesis and Characterization QDs can be tailor made to fluoresce over a wide range o f wavelengths which includes visible (400 700 nm) and NIR (700 1000 nm) wavelength regimes of light. Visible light emitting QDs have applications in light emitting devices (LEDs) 2 0 2 2 photonic 2 3 2 4 structures, solar cells 2 5 etc. whereas NIR QDs have a pplications in biomedical and solar cells. Visible light emitting QDs have been widely researched during the last few decades and a large volume of literature on such QDs are available. The interest in NIR emitting QDs is newer in comparison to visible wav elength emitting QDs and the amount of literature available on their application, synthesis and characterization are relatively fewer than those available for the visible QDs. Here in this dissertation we will focus on the application, synthesis and charac terization of NIR emitting QDs. 1. 2.1.1 Applications and types of NIR QDs While visible applications of QDs are popular, the need for QDs emitting in the NIR is also apparent. For example, solar cells can benefit from QDs that absorb across 2 5 the visible a nd into the NIR. The wavelengths of light energy emitted by the sun span the visible (400 700 nm) along with NIR (700 1000 nm) short wavelength infrared (SWIR) (1000 2000 nm) and mid wavelength infrared (MWIR) (2000 8000 nm) While visible light a beyond 700 nm into the IR region 2 5 Solar cells made of silicon can transform only 25%

PAGE 26

26 converted if solar cells are used containing layers of visible and infrared photovoltaic devices 2 5 This construct requires the integration of various semiconductor crystals of various compositions, band gaps, and lattice structures onto a single substrate. Lattice mismatch causes crystal strain and defects, resulting in less efficient solar energy conversion; thus, it becomes necessary to find materials which can minimize the lattice mismatch between the semiconductors. Varying the QD composition can allow for continuous lig ht absorption through the visible and NIR because of broad QD absorption spectra. With a single semiconductor system composed of QDs, only their size needs to be changed to alter their optical properties; thus, the ability to precisely control size in a ro bust system using process control is vital to the scaled up manufacturing of QDs. In analytical research fluorescent probes are very popular however applying these labels for bioimaging of multicellular organisms poses a lot of challenges 7 These probes em it light mostly in the visible wavelength region which has poor transmission through animal tissues. Thus most of the emitted light from the probes will be attenuated. Also the scattering properties of tissues can decrease the signal intensity. In addition endogenous fluorophores like collagen emit in t he visible wavelength spectrum ( 400 700 nm) which is also the wavelength spectrum for visible fluorophores. This can produce an overlap of the signals making the detection of visible fluophores difficult. Fo rtunately, there ex ists a clear wavelength region between 650 900 nm in most biological tissue where tissue absorption is the lowest with low Rayleigh scattering which make this region suitable for fluorescence imaging (Figure 1 4 ) 26 This allows NIR emi ssion wavelengths to penetrate tissues deeply leading to excellent non invasive imaging applications. QDs emitting in the near infra red (NIR) region of 700 900 nm is

PAGE 27

27 useful for biomedical applications because of minimal autofluorescence and maximum tissue penetration in this wavelength range 2 7 NIR QDs can also be used in light emitting devices (LEDs), which have potential applications in the medical field. Photobiomodulation is a therapy that uses low intensity light in the far red to NIR (630 1000 nm) t o modulate various cellular functions 28 These NIR LEDs can accelerate wound healing and attenuate degeneration of damaged optic nerves. They can also improve healing of ischemic injury of the heart. All of these effects happen due to the improvement in th e mitochondrial energy metabolism and production when subjected to NIR light 28 QDs can even potentially be used as photosensitizers 29 3 0 1. 2.1.2 Types of NIR QDs NIR QDs can be of different types depending on their composition. Typically NIR QDs are mad e from II VI, III V and IV VI binary alloys. However, research on core/shell and ternary alloy (AB x C 1 x ) QDs are increasingly being reported 3 1 Binary alloy NIR QDs The QDs of type II VI forms the largest group among QDs. The Cd based QDs belong to this g roup. The sulfide, telluride and selenide QDs of Cd, Zn and Hg are uniform in shape and size compared to other QDs. At room temperature they also display sharp absorption profiles with good emission features 7 relative to other QDs. But the number of NIR wa velength emitting II VI type QDs are less compared to visible wavelength emitting QDs. The QDs containing Hg are highly toxic for the environment which limits their application. CdTe QDs can emit NIR light around the 800 nm wavelength region. From Table 1 1 it is evident that NIR emitting CdTe QDs synthesized by the organometallic technique has a QY less than 20% whereas some of the QDs synthesized using aqueous techniques has higher QY > 50%. Thus by

PAGE 28

28 aqueous synthesis it is possible to obtain good quality QDs with good emission properties. The III V type QDs can have NIR emissions. InAs and GaAs QDs fall under this category. These QDs are mainly used in optoelectronic devices. Al i visatos 3 2 group reported the fabrication of InAs QDs having sizes within 2.5 nm and 6 nm by the organometallic synthesis technique. The synthesis of III V QDs which emit in the NIR wavelength region is complicated and difficult 7 compared to II VI type QDs. The QDs synthesized have wide size distribution, poor stability and low fluo rescence efficiency with QY of 2.5%. Hence the literature available on the synthesis of III V type QDs is much less than that available for II VI type QDs. The IV VI type NIR emitting QDs are Pb based materials viz. PbTe, PbSe and PbS. These Pb based QDs are environmentally hazardous and are thus less attractive for applications. However, these QDs can be easily size tuned in the NIR or infra red ( IR ) wavelength regions. Core/shell NIR QDs Core/shell QDs are especially desirable for bio imaging because thick shell QDs exhibit less blinking and greater photostability 3 3 Recently some researchers 3 4 3 6 reported the synthesis of thick shelled NCs with suppressed blinking in the single particle level. The reported core/shell QDs were all prepared using multis tep organometallic techniques. Deng et al. 3 3 reported the fabrication of core/shell QDs in the aqueous phase. The core/shell QDs have a structure that resembles an onion with layers. More than two semiconducting materials are used in core/shell QDs. In t hese QDs the basic fluorescence properties are controlled by the core. Core/shell QDs containing the

PAGE 29

29 different semiconducting compound materials viz. II VI, III V and IV VI can be classified as type I, reverse type I and type II depending on the band align ment in core/shell QDs. The bandgap of the shell material along with the relative electronic energy level positions of the core and the shell are responsible for the different optical properties of the core / shell material. In case of type I core/shell QDs the band gap of the shell is larger than that of the core and in this case all electrons and holes remain confined in the core. In reverse type I alignment the shell has smaller band gap than that of the core. For the reverse type I core/shell QDs the thic kness of the shell determines whether the confinement of the electrons and holes inside the shell will be complete or partial. In case of type II core/shell QDs the band alignment is staggered with the result that the effective band gap of the material is smaller than either core or shell 3 7 The staggered alignment of the band gaps results in the spatial separation of the holes and electrons in different regions inside the core/shell QDs (Fig ure 1 5 ) 38 The shell in type I QDs is used to minimize the surfa ce defects in order to improve their fluorescence efficiency. The dangling bonds at the surface are reduced by the growth of the shell. These dangling bonds when present can minimize the QY of the QDs by acting as surface trap states for charge carriers. A nother function of the shell in type I core/shell QDs is that it separates the optically active core physically from the surrounding environment. Thus the effect of surrounding water or oxygen molecules on the surface of the QDs is minimized 3 7 One example of this system is CdSe/ZnS. In this case the ZnS shell improves the fluorescence QY significantly and also contributes toward stability against photobleaching. The shell growth gives rise to small red shift in emission waelength by 5 10 nm which is due to partial leakage of electrons from the

PAGE 30

30 core into the shell 3 7 In reverse type I core/shell QDs, the shell material has a narrower band gap than the core. The charge carriers in this system are delocalized at least partially into the shell material and the wavelength of emission can be fine tuned by varying the thickness of the shell. The change is shell thickness is usually accompanied by a red shift in the emission wavelength. ZnSe/CdSe 39 CdS/HgS 4 0 and CdS/CdSe 4 1 are the most extensively studied systems o f this type. Photobleaching properties and QY for these QDs can be improved by coating these particles with another shell over their core/shell structure. In the core/shell type II system, the growth of the shell leads to significant redshift of the wavele ngth of emission. The effective band gap of the material due to the staggered alignment is reduced to a value that is smaller than either of the core or the shell material. Interest in having this kind of system lies in the tunability of the emission wavel ength to values which will be difficult to achieve in other systems. These type of QDs can have NIR emissions. Some common examples are CdTe/CdSe and CdSe/ZnTe. Relative to type I QDs, type II systems have long PL decay times due to smaller overlap of exci tonic wavefunctions. In this case either the hole or electrons will be located in the shell and a second shell will be required to improve their fluorescence efficiency and photostability. Ternary alloy NIR QDs Alloyed quantum dot semiconductors (AB x C 1 x ) 3 1 have potential in research areas related to nanoscale engineering due to their physical and optical properties which can be continuously tuned by varying their chemical composition variable x. This allows for an added degree of freedom by which the ex citon energy of the QDs can be fine tuned keeping size unchanged to obtain emission

PAGE 31

31 wavelength ranges that may be difficult to achieve using binary QDs. Different other properties can be achieved in these QDs by fine tuning the composition either homogeneo usly or by producing a gradient throughout the nanocrystal. The fluorescence properties of these QDs are good and on par with binary QDs 3 1 The different areas where these QDs can find applications include LEDs 4 2 bio imaging 4 3 4 6 and solar cells 4 7 48 In case of LEDs the QDs should emit at a specific wavelength, for bio imaging the QDs are required to be of small size with emission wavelength in the NIR region while for the solar cells wide absorption wavelength range with small size are preferred. All of these properties are achievable with alloy core QDs. Synthesis of various II VI alloyed nanocrystals have been reported viz. CdS x Te 1 x 3 1 Cd x Zn 1 x Se 49 5 0 CdS x Se 1 x 5 1 Cd x Zn 1 x S 5 2 CdSe x Te 1 x 43 and HgSe x S 1 x 5 3 These materials can be categorized into two groups according to the elements they contain. a. The compounds in this group contain one transition metal element along with two chalcogen atoms CdS x Te 1 x CdS x Se 1 x CdSe x Te 1 x and HgSe x S 1 x b. This group will contain compounds having two transition elemen ts and one chalcogen atom Cd x Zn 1 x Se and Cd x Zn 1 x S It has been reported in recent articles that the ratio of the chalcogen atoms in the first group of alloys has a significant non linear influence on the properties of these materials. The properties of t he second group of alloys change quasi linearly with their compositions. The non previously for bulk semiconductors and explained by Bernard and Zunger 5 4 1.2. 1.3 Synthesis and c haracterization of NIR QDs The conventional organometallic route developed by Murray 5 5 is currently the most widely used technique for synthesizing QDs. However, despite numerous efforts by various researchers preparation of high quality NIR QDs by the organometallic proce ss

PAGE 32

32 that will be useful for bioimaging applications remained elusive 3 3 There are reports on the synthesis of various NIR QDs viz. CdTeS/ZnS, InAs etc. by the organometal l ic synthesis techniques. However for using them in biological applications the surface of these QDs need to be modified using hydrophilic ligands in aqueous solution. This is a cumbersome process which gives rise to several drawbacks like low QY and limited stability. Typical NIR QDs that have been synthesized by various researchers are lis ted in Table 1 1. This table is adapted from the review paper by Ma et al. 7 (with some modifications) and contains information on emission wavelength, size, capping agent, applications, synthesis technique and QY of the QDs. Thiol coated II VI quantum dots synthesized by aqueous synthesis processes provide an alternative route to the conventional synthesis processes that use high boiling organic solvents. Thiol usage in QDs is useful not only for surface functionality leading to high dispersibility in water but also for controlling the kinetics of particle formation, stability in solution and passivation of surface dangling bonds 8 1 Various applications of thiol coated QDs have been reported viz. in optoelectronics like light emitting diodes (LEDs), photosen sitive films. They are useful for bioimaging applications. In the following section some of the published research on NIR emitting QDs synthesized using different thiol ligands viz. mercaptopropionic acid (MPA) 78 N Acetyl cysteine (NAC) 79 and thioglycoli c acid (TGA) 80 etc. are discussed. CdTeS alloyed QDs emitting in the NIR wavelength range and capped by MPA was reported by Mao et al. 78 Hydrothermal synthesis technique was used for the preparation of the QDs. The synthesis process was carried out at a te mperature of 180

PAGE 33

33 800 nm. The maximum QY of 68% was achieved for QDs having emission wavelength of 726 nm. The authors discussed the effects of precursor concentrations on the emission wavelengt h of the synthesized QDs. The 800 nm emitting QDs were obtained for CdCl 2 concentration of 30 mM. With lower CdCl 2 precursor concentration of 2, 10 and 20 mM only smaller QDs with emission wavelengths much less than 800 nm were obtained. Zhao et al. 79 fir st repor ted the synthesis of water dispersible, NIR emitting CdTe/CdS QDs using NAC as the capping agent by the hydrothermal process at 200 The QY of the QDs varied within 45 62%. H K 1 cells were labeled using QDs emitting at 685 nm and subjected to fluorescence imaging to demonstrate the potential of these QDs for bio imaging applications. Zhang et al. 80 synthesized CdTe nanocrystals using hydrothermal technique from CdCl 2 and NaHTe solution with TGA as the capping agent. The as prepared nanocrystals emitted wavelengths varying from green to red. The highest QY achieved for the nanoparticles was 30%. The nanoparticles were then utilized for labeling L929 mouse cells. Rogach et al. 8 1 reported the synthesis of thiol capped CdTe nanocrystals by aqueous method. In this process they used Cd(ClO 4 ) 2 as the source of Cd and H 2 Te gas as the source of Te with MPA as the stabilizer. The emission wavelengths of the nanoparticles synthesized varie d within 500 800 nm with 40 60% QY (QY). The authors compared the effects of using two different stabilizers, TGA and MPA on the optical properties of the synthesized nanoparticles. They observed that by using MPA

PAGE 34

34 nanoparticles with sizes more than 4.5 nm emitting upto 800 nm can be prepared which is not possible for TGA capped nanoparticles. By using TGA maximum emission wavelength of 750 nm was achievable. There are only a few reports on the aqueous synthesis of core/shell NIR emitting QDs. Deng et a l. 3 3 published a paper on the multistep synthesis of MPA capped core/shell CdTe/CdS QDs using aqueous techniques. The core/shell QDs were tetrahedral in shape and emitted NIR wavelengths upto 820 nm. The QDs were synthesized using various temperatures with in 20 maximum QY achieved was 70% for the 715 nm emitting QDs. Qian and co workers 7 4 reported the synthesis and characterization of core/shell CdHgTe/CdS QDs by a multistep process. The precursors used for the syn thesis were CdCl 2 Hg(ClO 4 ) 2 and NaHTe in the presence of MPA as stabilizer. The p hotoluminescence (PL) exhibited by the QDs ranged from 600 830 nm while the QY varied within 20 50%. Type II NIR emitting core/shell CdTe/CdSe QDs were synthesized in the aqueous system by Zhang et al. 80 The QDs can emit within the wavelength range of 613 813 nm. The maximum QY achieved for these particles was 12%. He et al. 8 2 developed aqueous NIR emitting CdTe QDs using a microwave synthesis process. The QDs exhibited P L in the 700 800 nm wavelength regimes with QY variation within 15 20%. These QDs were conjugated to antibodies and then used subjected to in vivo imaging inside mouse. The authors demonstrated that these QDs have potential as NIR emitting optical contrast agents for biological applications.

PAGE 35

35 1.2.2 Magnetic NPs During the last two decades scientists and researchers investigated numerous NPs as contrast agents for MRI. Magnetic iron oxide NPs have been extensively studied by the researchers as MRI contrast a gents as they have the ability to reduce the T 2 relaxation time in animal tissues 8 3 Iron oxide NPs can be classified into micrometer sized paramagnetic iron oxide ( MPIO ; several micrometers) superparamagnetic iron oxide ( SPIO ; hundreds of nanometers) an d ultrasmall superparamagnetic iron oxide ( USPIO ; less than 50 nm) depending on their particle sizes. SPIOs coated with dextran have been clinically used for the diagnosis of liver diseases because of the selective uptake of SPIOs in the Kupffer cells of l iver, bone marrow and spleen 8 3 USPIOs were used for imaging lymph nodes. However, the T 2 con trast agents have several disadvantages for clinical applications. The T 2 contrast agents being negative imaging agents produce a signal decreasing effect. There are possibilities of confusing the dark signal produced by the contrast agents with other pathogenic conditions. Thus T 2 contrast agents produce images of lower contrast than those produced by T 1 contrast agents. T 2 contrast agents also possess susceptibil ities high enough to distort the magnetic field on the surrounding normal tissues. This produces obscure images and demolishes the background of the region of interest. To overcome the limitations of the iron oxide based T 2 contrast agents researchers re cently conducted extensive research to develop T 1 contrast agents based on NP technology. The T 1 contrast agents are mostly Gadolinium complexes conjugated to nanostructured materials viz. dendrimers, silicas, perfluorocarbon NPs and nanotubes. Researchers also have investigated Gadolinium compounds 8 3 such as Gd 2 O 3 GdF 3 and GdPO 4 as T 1 MRI contrast agents. Beside s Gadolinium compounds

PAGE 36

36 other T 1 contrast agents like MnO NPs have also been investigated. The various properties of the NP based T 2 and T 1 contra s t agents viz. magnetization and relaxivity coefficients (r 1 r 2 ) are listed in Table s 1 2 and 1 3 respectively. 1.2. 3 B imodal Fluorescent and M agnetic NPs Bimodal nanoparticles, with magnetic and fluorescent functionalities b roaden 99 the applicability of nanocrystals in different applications like bioseparation, pathogenic detections, immunoassay along with multimodal imaging. These particles can be classified into four different categories depending on their structure and composition. Type A Core/sh ell and heterostructures In this type the QD and the MNPs form a fused heterostructure or core/shell. Magnetic and semiconductor nanocrytals usually have a large lattice mismatch. However, inspite of this mismatch, researchers demonstrated that combinatio n of the two materials into a single nanocrystal is possible. Gao et al 100 demonstrated the synthesis of ~3 nm core FePt nanoparticles with a ~3 5 nm thick shell of CdSe or CdS. The NPs were synthesized by the organometallic technique. These nanoparticle s emitted at wavelengths around 465 nm and had a QY of 7 10% which was much lower relative to the CdSe QDs alone. The synthesized particles were paramagnetic and had a blocking temperature of 14K. The quenching of fluorescence was due to the interaction of the magnetic core with that of the fluorescent shell. Gu et al 101 reported FePt CdS heterodimers with magnetofluorescent properties and synthesized using the organometallic process. According to the authors the FePt/CdS core/shell initial structure tran sformed on heating into a heterodimer structure. The final product emitted at 438 nm and had a QY of 3%. They were superparamagnetic with a coercivity of 0.85 kOe at 5K and had a blocking temperature of 11K.

PAGE 37

37 Type B Doped QDs Fluorescent and magnetic NPs can be synthesized by doping fluorescent QDs with paramagnetic ions. This is a direct synthetic procedure by which dual functionalities like fluorescence and magnetic properties can be achieved in a single nanoparticle. Doping paramagnetic ions into se miconductors have been tried by various researchers over several decades 102 Although some successes have been reported however for unknown reasons most efforts in doping crystals have failed 103 Bhargava et al. 104 published one of the first reports on dop ed semiconductor nanocrytals after which research in this field increased rapidly. In this publication the authors reported the optical properties of ZnS nanocrytals of size 3.5 to 7.5 nm doped with Mn 2+ The nanocrystals emitted in the 590 nm region and e xhibited QY of 18%. Doped nanocrytals can be of two types: Type B (i ) Paramagnetic ion doped in QDs D uring the last few years synthesis and characterization of various doped magnetic QDs of this type have been reported 105 109 viz. ZnS:Mn 2+ ZnO:Co 2+ Z nSe:Mn 2+ CdSe:Mn 2+ CdS:Mn/ZnS. These nanoparticles emit light in the visible wavelength range. One important aspect in this research is the doping of the paramagnetic ion into the core of the semiconducting nanoparticle. Proving the location of the dopan t ion inside the core or on the outside surface of the particle is difficult. Various characterizing techniques viz. extended X ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR) have been used to verify the location of the dop ant 110 The potential of doped nanocrytals for bioimaging has been realized. Santra et al. 109 demonstrated the use of CdS:Mn/ZnS QDs for multimodal imaging. These NPs were synthesized using the microemulsion technique and were 3.1 nm in size. The QD s emitt ed at wavelength ~ 575 nm and

PAGE 38

38 exhibited a hysteresis loop when characterized using SQUID. These QDs were imaged in vivo using MRI after they were conjugated with HIV 1 trans activator of transcription (TAT) peptide and injected into the brain of a rat. Det ails of the MRI measurements were not reported. Yong reported the preparation of fluorescent MNPs that emit in the NIR wavelength region 111 To the best of our knowledge this is the only publi shed report on magnetic NIR emitting QDs. The authors reported the synthesis and characterization of Mn doped CdS capped CdTe 0 25 Se 0 75 QDs of size 4 5 nm by the organometallic synthesis route. The amount of Mn doping in the QDs was 3 atom%. The QDs emitted at 822 nm with a QY of 15%. The Mn doped QDs exhibited hyster esis and had coercivity of 125 G at room temperature. Type B ( ii ) Paramagnetic ion doped in the shell of core/shell QDs In this type the paramagnetic ions were doped inside the shell of the core/shell QDs. Wang et al. 112 reported the synthesis of CdSe /Zn 1 x Mn x S QDs which have both magnetic and fluorescent properties. The ~ 5 nm QDs were synthesized using the organometallic technique and emitted within 570 650 nm wavelength range. The QDs had a QY of more than 20%. The Mn 2+ ions were located inside th e CdS shell and exhibited r 1 relaxivity values of 10 18 mM 1 s 1 The suitability of these QDs for bimodal imaging was assessed by attaching these QDs with macrophages imaging these macrophages using fluorescence spectroscopy and MRI. Type C Integrated composite particles containing semiconducting and magnetic nanoparticles In this type a carrier material contains two different nanoparticles, one magnetic and the other fluorescent integrated into a single entity.

PAGE 39

39 This type of nanoparticles can be furth er classified into two types depending on their structure. Type C (i ) Incorporation of the fluorescent and magnetic nanoparticles inside the carrier material In the type C (i ) case the carrier material either contains all the nanoparticles inside it or attached to its outside. The carrier material can be silica or polymer based particle. The final particles containing the fluorescent and MNPs entities are usually larger than the type A or type B particles. Yi et al. 113 demonstrated the synthesis of sil ica particles of size ~ 50 nm that are doped with CdSe/ZnS (size 3.5 nm) QDs and iron oxide NPs (size 12 nm) by a microemulsion synthesis technique. The composite particle exhibited PL at 554 nm with a QY of 5%. The QY of CdSe/ZnS QDs decreased from 15% to 5% on being incorporated into the silica matrix. The reason for the decrease of QY was probably the interaction of the QDs with iron oxide or due to the incorporation of the QDs in the silica shell by the microemulsion process. Magnetization values report ed were 60 80 Fe 2 O 3 at 5K and and 40 60 Fe 2 O 3 at 300K. Kim et al. 114 described a similar process of incorporating CdSe/ZnS QDs and iron oxide NPs inside 150 nm silica matrix. The composite particle showed both fluorescent and magnetic prope rties. Type C (ii ) Incorporation of QDs inside the carrier material with MNPs attached to the outside of the carrier material or vice versa The interaction of the QDs with MNPs results in the decrease of the fluorescence QY of the QDs which makes the composite particles less bright. One way to overcome this limitation is to increase the distance between these particles inside the composite material. Maceira et al. 115 synthesized a composite particle where the MNPs were incorporated into silica particl es

PAGE 40

40 70 nm in size while the QDs were attached to the surface of the silica particles using charged polymers. The resulting silica particles carrying the MNPs and QDs were further coated with a 20 nm silica shell. The final particle was 220 nm in diameter. T he authors maintained that quenching of the QDs were prevented using this technique. Two similar particles were synthesized which emitted at 567 nm and 623 nm. Magnetic measurements were carried out on the particles. Saturation magnetization of 1.34 emu/gm and a coercivity of 175 Oe (at 5K) along with a blocking temperature of 150 K were recorded. Type D Paramagnetic Gd coated on QDs using chelates Gd 3+ is used as a paramagnetic T 1 contrast agent for MRI. It has a high magnetic moment and its ground st ate is symmetrical. There is toxicity issues associated with Gd 3+ Hence, to decrease the effect of toxicity it is often used along with organic chelates in the form of a Gd complex. Chelated Gd DTPA (diethylenetriamine pentaacidic acid) is a widely used contrast agent. However, there are many other compounds which are being studied for better relaxivity properties and varied biomedical applications 116 The paramagnetic chelated complexes are attached to dyes and QDs to impart fluorescent property to the c omplex. The attachments of the Gd complexes to the fluorescent materials are done by covalent/non covalent bonding. Gd complexes can be directly attached to the QDs or they can be attached on the shell material surrounding the QDs. Depending on the how the Gd chelates are attached to the QDs the Gd chelate QD composite particles can be divided into two types. Type D ( i ) Gd chelates attached to QDs Mulder et al. 1 17 first reported Gd chelate attachments to QDs. The QDs were surrounded by a lipidic micelle to facilitate

PAGE 41

41 non covalent attachment of the Gd chelates to the QDs. The composite particle synthesized emitted at around 560 nm and exhibited r 1 relaxivity of 12.4 mM 1 s 1 and r 2 relaxaivity of 18 mM 1 s 1 The r 2 /r 1 ratio was 1.5 which is good enough for these particles to be suitable for imaging in the T 1 mode. Type D ( ii ) Gd chelates attached to the shell surrounding the QDs The number of paramagnetic ions present on the surface of the QDs will determine the overall relaxivity of the composite parti cles. Hence it will be useful to increase the surface area of the particles by coating them with a shell and then attaching the paramagnetic ions on the shell. Yang et al. 118 reported the synthesis of 3 nm CdS:Mn/ZnS QDs coated with 7 nm thick silica shell s. Gd ions were chelated with Triacetic acid trisodium salt (TSPETE) and the Gd TSPETE complex was attached to the silica shell coating on the surface of the QDs. The QY of the composite particles emitting yellow light was 28% and the r 1 r 2 relaxivity val ues reported were 20.5 and 151 mM 1 s 1 respectively. The r 2 /r 1 ratio is 7.4 indicating that the composite particles are most suitable as T 2 contrast agents. 1.2. 4 QD Toxicity The nature of cadmium, selenium and tellurium whether bounded covalently as CdTe/ CdSe in the QDs or released as free ions will determine the toxicity of the QDs 1 19 Cadmium toxicity studies on animals were carried out by Manca et al. 1 20 Rats were administered 25 1250 g cadmium/kg in the form of CdCl 2. The Cd dosage was found to be to xic to the animals which were sacrificed 24 hours after treatment. Lin et al. 119 carried out experiments to investigate the chemical fate of Cd/Se/Te based QDs emitting at 705 nm on mice. The authors reported that 100% of the QDs were retained in the body after 16 weeks of exposure and free Cd was released from the QDs. Su et

PAGE 42

42 al. 1 21 investigated short and long term toxicity of CdTe QDs synthesized by the aqueous method. CdTe QDs of concentration 0.2 nmol and volume 0.1 ml was intravenously injected into the tail vain of mice. Post injection the QDs accumulated in the liver in a short time span of (0.5 4h) and then into the kidneys in increasing amounts after (15 80 days). However there was no overt toxicity from the QDs even after 80 days of exposure. T he first in depth research on the toxicity effects of CdSe QDs coated with various coatings on hepatocytes was reported in 2004 by Derfus and coworkers 13 CdSe QDs coated with mercaptoacetic acid (MAA) and prepared by organometallic synthesis technique and kept in an inert atmosphere showed no toxicity. However, when these QDs were exposed to air prior to MAA coating, the viability of the cells decreased with increasing dosage of QDs. The reason for cell viability decrease was attributed to leaching of Cd i ons due to the surface degradation following surface oxidation. Photoluminescence data indicating blue shifting of emission wavelength due to the decrease of particle size and lower peak amplitude reinforced the finding by the authors that the surface was degraded by oxidation. Release of Cd and associated cytotoxicity was also increased with increasing exposure of QDs to UV light. MAA coated CdSe/ZnS core/shell QDs were found to be less cytotoxic than bare QDs when they were subjected to oxidation solely b y air. Some degradation of the ZnS coating was observed while the core CdSe remained unaffected. The stability of the same QD decreased when exposed to UV light and was expected to be more cytotoxic. The authors concluded that the QDs can be safely used in the cells without much damage if used in low concentrations and subjected to limited UV exposure.

PAGE 43

43 The effect of QD capping agent on cytotoxicity was also investigated by the authors 13 The capping ligands used for the study were Mercaptoundecanoic acid ( MUA), thioglycerol and cysteamine. Cytotoxicity of QDs coated with a mixture of these cappling ligands was also evaluated. Cell deaths were reported for QDs coated with MUA. QDs coated with other ligands did not show significant cell death and DNA damage. The authors concluded that cytoxicity of QDs were due to coatings and not due to the material composition.

PAGE 44

44 Table 1 1. Comparison of characters, modification strategy and application of QDs (Ref. 33, 56 79) QDs Emission Size/ Modification strategy Area o f Synthesis Quantum (core/shell) peak/nm nm or coupling reagent application Approach (Reference) yield CdTe/CdS 715 6 7 3 Mercaptopropionic acid capped a AS c (33) 70% InAs/ZnS 750 4 16 Coated with different lengths of short chain PEGs Orga n and tissue selective biodistribution OR b (56) 20% InAs 820 1240 2 6 TOP capped a Colloidal chemical synthesis (57) 2.50% InAs/CdSe 700 1400 3 5 TOP capped a OR b (58) Up to 90% InAs/ZnCdS 700 900 10 (1) Coupled with poly(amino P EG11)25% PIL (2) Covalent conjugation of streptavidin Imaging of HeLa cells in vivo OR b (59) 35 50% InAs/InP/ZnSe 800 15.9 (1) Encapsulated with poly(ethylene glycol) (2) Coupled with arginine glycine aspartic acid or arginine alanine asparti c acid peptides Imaging of subcutaneous U87MG tumor OR b (60) 19% PbS 900 1600 5.3 Mercaptoundecanoic acid capped Imaging of HT29 cells AS c (61) 10% PbS 700 1600 10 0.5 Straightforward mercaptan PEGylation ligand exchange a OR b (62) 26% PbSe 1200 1500 7 12 Coated with a silica shell Imaging of NIH 3T3 cells and HepG2 cells OR b (63) a CdTe/CdSe 750 7 Loaded in PLGA nanosphere a OR b (64) a

PAGE 45

45 Table 1 1. Continued. QDs Emission Size/ Modification strategy A rea of Synthesis Quantum (core/shell) peak/nm nm or coupling reagent application Approach (Reference) yield CdTe/CdSe 743 9.5 L Cysteine capped Fixed HeLa cell staining Layer by layer (65) epitaxy approach 8% CdTe/CdSe 752.2 4.5 Thiol capped Sensin g of copper ASc (66) 11.40% CdTe/CdSe 840 860 10 Oligomeric phosphine organic coatings Sentinel lymph node mapping OR b (67) a CdTe/CdSe 750 5 Cysteine capped Detecting cysteine, homocysteine and glutathione Layer by layer (68) epitaxy approach a CdSe/CdTe/ZnSe 850 1100 6 3 Mercaptopropionic acid capped a OR b (69) 60% CdTeSe 750 5 6 Aminoethanethiol capped a Chemical aerosol flow synthesis (70) CdSe 0.25 Te 0.75 /CdS 859 7 Coated with lysine Long term targeted imaging in vivo Hot c olloidal synthesis approach (71) 10 15% CdTe x Se 1 x /CdS 650 850 a Exchanged with mercaptoundecanoic acid Multiplexed imaging OR b (72) a CdSeTe/CdS 785 10 0.2 d Binding of Gd 3+ DOTA complexes Fluorescence/magnetic resonance imaging OR b (73) a CdHgTe 800 5.68 Capped with a CdS shell In vivo imaging of a mouse AS c (74) <20% CdHgTe 775 40d Loaded in gelatin nanospheres Imaging of cells and mouse AS c (75) a Mn:CdTe 720 a Conjugated with EDC and NHS FRET AS c (76) 15 20% Cu:InP 630 1100 4 TOP capped a OR b (77) 20% CdTe/CdS 820 11 3 Mercaptopropionic acid capped a AS c (78) 70% CdTe/CdS 735 4.3 N Acetyl L cysteine capped Imaging of HK 1 cells Hydrothermal route(79) 60.02%

PAGE 46

46 Table 1 1. Continued. QDs Emission Size/ Modification strategy Area of Synthesis Quantum (core/shell) peak/nm nm or coupling reagent application Approach yield QD705 705 15 20 d Coated with arginine glycine aspartic acid peptide Detecting U87MG glioblastoma tumors Purchased a QD800 800 a Coated with 4 (maleimidomethyl) 1 cyclohexanecarboxylic acid N hydroxysuccinimide ester Imaging of Human prostate cancer C4 2B Purchased a QD800 800 17.6 d Formed PEG PE QDs based micelle Quantification of tumors Purchased a QD705/800 705/ 800 a Conjugated with long chain (2000 Da) amino PEG, (5000 Da) methoxy PEG Analysis of chick CAM vasculature Purchased a where, a indicates no record. b c synthesis route. d Indicates the diameter after modification.

PAGE 47

47 Table 1 2 Properties of T 2 contrast agents. (Adapted from Ref. 83 ) Name Core Material Surface Diameter of Core (nm) HDD c (nm) Magnetization (emu/gm) a r 2 (mM 1 s 1 ) B 0 (T) Ref Ferumoxid es (Feridex) Fe 3 O 4 Fe 2 O 3 Dextran 4.96 160 45 120 1.5 84 Ferucarbotran (Combidex) Fe 3 O 4 Carboxydextran 4 60 186 1.5 85 Ferumoxtran (Combidex) Fe 3 O 4 Dextran 5.85 35 61 65 1.5 85 CLIO Tat Fe 3 O 4 Dextran 5 30 60 62 1.5 86 WSIO (MEIO) Fe 3 O 4 DMSA b 4 25 78 1.5 10 87 6 43 106 10 87 9 80 130 10 87 12 101 218 10 87 FeNP Fe PEG 10 70 129 1.5 88 MnMEIO MnFe 2 O 4 DMSA b 6 68 208 1.5 10 89 9 98 265 10 89 12 110 358 10 89 CoMEIO CoFe 2 O 4 DMSA b 12 99 172 1.5 10 89 NiMEIO NiFe 2 O 4 DMSA b 12 85 152 1.5 10 89 Au Fe 3 O 4 Fe 3 O 4 PEG 20 114 3.0 90 Au FePt FePt{fcc} PEG 6 59 3.0 91 a Magnetic properties were measured at 1.5 T external field. b 2, 3 Dimercaptosuccinic acid. c Hydrodynamic diameter *B 0 magnetic field CLIO Cross linked iron oxide MEIO Metal doped iron oxide WSIO Water soluble iron oxide

PAGE 48

48 Table 1 3. Properties of T 1 contrast agents. (Adapted from Ref. 83) Name Core Material Diameter of Core (nm) Relaxivity based on concentration of whole atoms Relaxivity base d on number of particles Relaxivity based on surface B 0 (T) Ref. r 1 (mM 1 s 1 ) r 2 (mM 1 s 1 ) r 1 (mM 1 s 1 ) r 2 (mM 1 s 1 ) r 1 r 2 Gd DTPA Gd ion 4.1 4.9 4.1 4.9 7 92 Dextran SPGO Gd 2 O 3 4.8 16.9 7 93 PEG Gd 2 O 3 Gd 2 O 3 3 9.4 13.4 1.5 94 GadoSiPEG Gd 2 O 3 2.2 8.8 11.4 3700 4800 7 92 3.8 8.8 28.8 18600 60700 4.6 4.4 28.9 38800 65000 PGP/dextran K01 GdPO 4 13.9 15 0.47 95 GdF 3 :cit GdF 3 3.17 2.0 10 7 227.3 a 14.2 96 GdF 3 /LaF 3 :AEP GdF 3 /LaF 3 2.71 8.8 10 5 77.2 a 14.2 96 PGP/d extran K01 GdPO 4 13.9 15 0.47 95 MnO MnO 7 0.37 1.74 3000 14000 33 b 154 b 3 97 15 0.18 0.57 15000 46000 34 b 121 b 20 0.13 0.52 25000 99000 33 b 102 b 25 0.12 0.44 46000 165000 39 b 139 b FeCo/GC FeCo 4 31 185 1.5 98 7 70 644 a Relaxivities based on the Gd 3+ on the shell (unit: mM 1 s 1 ) b Relaxivities based on the surface area of the nanoparticles (unit: m s 1 ) B 0 magnetic field

PAGE 49

49 Figure 1 1. Density of states in one band of a semiconductor as a function of dimension. Adapted f rom Ref. 2

PAGE 50

50 Figure 1 2. Energy diagram of large and small NPs with different magnetic spin alignment. Ferromagnetism and superparamagnetism exhibited by the large and small NPs respectively. Adapted from Ref. 10.

PAGE 51

51 Figure 1 3 (a) Magnetization properties of ferromagnetic and superparamagnetic NPs with and without the influence of external magnetic field (b) Dependence of magnetic domain structures on NP sizes, Ds and Dc are the thresholds for size. Adapted from Ref. 11

PAGE 52

52 Figure 1 4 Absorption coefficient versus wavelength plot. Adapted from Ref. 2 6 ( Hb is h emoglobin HbO 2 is oxy hemoglobin)

PAGE 53

53 Figure 1 5 Schematic representation of Type I and Type II QDs. The positions of condu ction and valence bands of the core (center) and the shell materials are represented using rectangles. Adapted from Ref. 3 8

PAGE 54

54 CHAPTER 2 MATERIALS AND METHOD S 2.1 Core/S hell Design Considerations Efficient NIR QDs have attracted a lot of attention for various applications. As discussed earlier bio imaging using these QDs holds a lot of potential. The properties that are desirable in a good NIR emitting material used for bio imaging are: good fluorescence property with QY > 10%, small in size with hydrod ynamic diameters < 10 nm to facilitate transportation 56 and circulation inside living organisms, good stability when exposed to physiological environment and against photobleaching 65 QDs synthesized by aqueous techniques like hydrothermal can have the abo ve mentioned properties. The criteria used for choosing the different materials used in this research are provided below. 2. 1.1 CdTe as QD Core M aterial As explained in section 1.2.1.2, II VI systems are the largest group among QDs. The synthesis of CdX, Z nX and HgX (X=Te, Se, S) are widely reported because these QDs when synthesized have uniform shape and size compared to other QDs. At room temperature they also display sharp absorption profiles with good emission features 7 relative to other QDs. Among the se particles Zn based QDs emit in the visible range (Figure 2 1). While the Hg based QDs emit in the mid NIR range, they are highly toxic to the environment. Thus the Cd based QDs appears to be the material of choice. However from Figure 2 1 it is evident that CdSe and CdS emit in the visible region while CdTe in the NIR region. CdTe has a bulk bandgap 1.5 eV which corresponds to 828 nm emission 7 8 Thus CdTe was chosen as the core QD material because it can emit around 800 nm which is desirable for NIR ap plications.

PAGE 55

55 2.1.2 CdS as QD S hell M aterial In addition to band alignment the other requirement for core/ shell nanocrystals with good optical properties is epitaxial shell growth. Epitaxial growth of shells on core nanoparticles signifies that the shell ma terial will crystallize on the core with very little lattice mismatch. 1 2 3 Large lattice mismatch between core and shell leads to strain which generates defect states at the interface of core and shell or inside the shell. These defects can in turn act as t rap states for charge carriers resulting in the reduction of fluorescence QY 1 2 3 The difference in lattice parameter of CdTe (a=6.482) and CdS (a=5.818) is small. Thus CdTe/CdS form good core/shell materials because they have a small lattice mismatch (11 .4 %) as illustrated in Figure 2 2. CdTe core with CdS shell forms a pseudo type II QD 1 2 4 By varying the thickness of the CdS shell the emission wavelength of the QD can be fine tuned In the type II QD the staggered band alignment produces smaller band ga p which gives rise to significant red shift of the emission wavelength. 2.1.3 N Acetyl Cysteine (NAC) as the Dispersing L igand The common thiols used for nanoparticle synthesis are thioglycolic acid (TGA) 12 6 L cysteine/mercaptoethylamine 1 2 7 1 thyoglicer ol 12 7 1 2 8 mercaptopropionic acid (MPA) 1 2 9 However, MPA and NAC are the only thiol reported that have been utilized for synthesizing NIR emitting QDs 79 CdCl 2 forms a white precipitate with MPA during the initial mixing stages with pH lower than 7.3. Abov e pH > 7.3 the precipitate dissolves to form a clear solution. MPA also has toxic properties with awful odor 79 With NAC CdCl 2 do not form any precipitate in the wide pH range of 2.4 12 and NaOH solution can be used to vary the pH. This provides flexibi lity of QD synthesis and allows production of QDs at any pH lying in the above mentioned range. Also, NAC is used as an antioxidant

PAGE 56

56 and there are reports which indicate that NAC protects cells from oxidative stress and cyto toxicity arising from toxic QDs 7 9 All of these factors led to the choice of NAC as the desirable ligand for synthesizing NIR QDs reported in this work. The chemical structure of the ligand is shown in Figure 2 3. 2.2 Synthesis of the QDs 2.2.1 Synthesis of the Core CdTeS QDs Hydrotherma l synthesis of core CdTeS QDs was carried out in Teflon lined autoclaves. The flowchart for the synthesis of core and core/shell QDs is outlined in Figure 2 4. The synthesis process is similar to the method used by Zhao et al. 79 In a typical process 0.23 g m CdCl 2 (1.26 mmol) was dissolved in 100 ml Argon saturated DI water in a 250 ml round bottom flask to produce a 12.5 mM solution. N Acetyl Cysteine (NAC) was added to the solution so that Cd:NAC molar ratio was 1:2.5. In a separate 20 ml air tight glass b ottle 0.05 gm (1.32 mmol) NaBH 4 and 0.08 gm (0.63 mmol) Tellurium powder were mixed in 2 ml Argon saturated DI water to produce sodium hydrogen telluride (NaHTe). The molar ratio of NaBH 4 to Te used was 2:1. The glass vial containing the solution was seale d with paraffin film to make it air tight. The as prepared NaBH 4 This reaction produces a deep red solution of NaHTe. The time period allowed for reaction was more than 8 hr s. The equation for the reaction is as follows: 2NaBH 4 + Te + 2H 2 O = NaHTe + NaBO 2 + 11/2H 2 ( 2 1) It is very i mportant to use DI water that is oxygen free or with negligible oxygen content. Te is susceptible to oxidation and its oxidation number can vary from 2(H 2 Te) to +6(TeO 4 2 ) 130 In CdTe the oxidation number of Te is 2. In case of oxidation of Te atoms, the CdTeS QD properties prepared from partially oxidized tellurium will be poor.

PAGE 57

57 Hence it is important to use Argon saturated DI water for maki ng solutions. The c olor of the NaHTe solution turned wine red. The pH of the precursor solution was adjusted to (8.0 8.4) by using 2M NaOH solution. Several polytetrafluoroethylene (PTFE) lined autoclaves (Parr acid digestion bombs) were then used for he ating the solution. 10 ml of solution was filled in each of these containers with maximum capacity of 23 ml. Subsequent heating of the solutions were carried out for different time intervals (30 100 min) coolin g process after the completion of heating The QDs thus prepared were taken out from the autoclaves and characterized after washing. Washing of the QDs were done using Amicon Ultra 15 Centrifugal Filt er Unit with membranes suitable for 30 kDa protein filtration and centrifuging them at 2600 rpm for 4 5 minutes. The QDs were washed 8 9 times with DI water to get rid of unreacted reactants. 2.2.2 Coating of the Core CdTeS QDs with CdS Shells 10 ml of t he crude CdTeS QDs were taken in a 100 ml two necked round bottom flask and 0.5 gm (2.73 mmol) of CdCl 2 was added to the solution. In another two necked 100 ml round bottom flask 0.03 gm (0.39 mmol) of anhydrous Na 2 S and 0.125 gm (0.77 mmol) of NAC were ad ded to 20 ml of DI water (Figure 2 4). This solution was bubbled with Argon gas for 30 minutes to get rid of any dissolved oxygen. The Argon saturated solution was then added dropwise to the crude CdTeS QD solution very slowly using a syringe pump at the rate of 300 l per minute The solution thus obtained was heated in batches of 10 ml (60 150 min) The QDs thus obtained were centrifuged at 2600 rpm and washed with DI water using 30 kDa Amicon Ultra 15 Centrifugal Filter Units The washing steps were

PAGE 58

58 repeated 8 to 9 times to get rid of all the unreacted chemicals. The washed QDs were then characterized by different techniques. 2. 2.3 Synthesis of Magnetic Fe Doped CdTeS QDs The synthesis of Fe doped CdTeS QDs is schematically represented in Figure 2 5. The basic synth esis technique was the same as used for core CdTeS QDs. The Cd:Te molar ratio was maintained as 2:1. In a typical process a 30 mM solution of CdCl 2 in DIW was prepared by adding 0.55 gm (3 mmol) of CdCl 2 to 100 ml DI water. Sodium hydrogen telluride (NaHTe ) was prepared from NaBH 4 and Tellurium powder in Argon saturated DI water. The molar ratio of NaBH 4 to Te used was 2:1. The as prepared NaBH 4 The time period allowed for re action was more than 8 hrs. CdCl 2 (30 mM) solution in 100 ml DI water was prepared in a 250 ml round bottom flask. NAC was added to the solution so that Cd:NAC molar ratio was 1:2.5. FeCl 2 .4H 2 O was added to the resulting solution so that Cd:Fe molar ratio was 8.5:1. The solution was Argon bubbled for more than 30 min and then the NaHTe solution was added to this solution. The color of the solution turned wine red. The pH of the precursor solution was adjusted to (8.0 8.4) by using 2M NaOH solution with su bsequent heating in PTFE lined autoclaves for different time intervals (30 100 min) cooling process after the heating was completed The QDs thus prepared were taken out and characterized after washing. 2 .3 Characterization of the QDs The QDs thus prepared were characterized using fluorescence spectroscopy, TEM, EDS, XRD and XPS to determine their particle size, chemical and phase composition. SpectraMax M5 was used for ultraviolet visible absorption emiss ion

PAGE 59

59 spectra at room temperature. The PL QYs of the core and core/ shell particles were determined at room temperature by comparing with Rhodamine 6g dissolved in ethanol which was taken as 95% 1 3 1 The absorbance or optical density (OD) of Rhodamine 6g, the core and the core / shell QDs were adjusted to less than 0.1 for QY determination. JEOL 2010F was used for high resolution transmission electron microscopy (HRTEM) of the QDs. The transmission electron microscopy ( TEM ) sample preparations were done by droppi ng the nanoparticles suspended in water on carbon coated Cu grids and dried overnight at room temperature. The powder diffraction patterns of the radiation. XPS studies were done using Perking Elmer 5100 XPS system. The x ray photoelectron spectroscopy ( XPS ) sample pre parations were done b y drying drops of core and core/ shell suspensions on silicon wafers at room temperature. Performance assessment of the QDs was carried out by incubating the QDs with macrophages and subsequent imaging of the QD doped cells with NIR det ecting camera. J774 mouse macrophage/monocytic cells were grown using standard serum, glutamax and penicillin/streptomycin. Prior to labeling, cells were seeded into 4 well cham ber slides and allowed to attach before subsequent incubation with 200 g/ml core/ shell CdTeS/CdS QDs, emitting at 800 nm, for 6 hours. The cells were imaged at diamidino 2 phenylindole (DAPI) for visualization of cell nuclei. Near infrared images was acquired using a custom made infrared camera setup with a 15 second exposure time and subsequently pseudo colored red using ImageJ software (NIH website). The

PAGE 60

60 time resolved Phot o luminescence (PL) spectra measurement were done using time correlated single photon counting (PicoQuant PicoHarp 300). The samples were excited by 375 nm light. The hysteresis curves for the commercial Feridex I.V. sample and the magnetic Fe doped Cd TeS QDs were obtained using a superconducting quantum interference device (SQUID) magnetometer at different temperatures (10K and 300K) The applied field was varied from 1500 Oe to 1500 Oe. Magnetic measurements were performed on a series of QDs emitting between 660 740 nm. T he diamagnetic contribution from the substrate was subtracted from the crude data to obtain the QD magnetization values. Non magnetic filter paper was soaked with 15 L QD sample and placed inside non magne tic empty gelatin capsule. The gelatin capsule w as placed within the measuring straw inside the SQUID. Magnetization loops were obtained by varying the magnetic field over the range (5 T) and during the measurement the temperature was kept constant

PAGE 61

61 Figure 2 1. Energy gap ( eV) versus lattice parameter () for bulk se miconducting materials. Adapted from Ref. 12 2. Energy gap (eV) Lattice parameter ()

PAGE 62

62 Figure 2 2. Band gap (eV) versus lattice spacing () for compound semiconductors Adapted from Ref. 125.

PAGE 63

63 Figure 2 3. Structure of the N Acetyl Cysteine (NA C) ligand

PAGE 64

64 Figure 2 4. Schematic representation of the core and core/shell QD synthesis by hydrothermal technique

PAGE 65

65 Figure 2 5. Flowchart for the synthesis of magnetic Fe doped CdTeS QDs

PAGE 66

66 C HAPTER 3 RESULTS AND DISCUSSI ONS 3.1 Synthesis of Co re CdTeS QDs Emitting up to 750 nm Aqueous synthesis technique using short chain thiols as stabilizing agents is a useful alternative 8 1 to the oil based organometallic synthesis of CdSe 1 3 2 CdS 13 3 CdTe 12 7 CdHgTe 13 4 HgTe 13 5 and ZnSe 13 6 quantum dots. Thi ols not only passivate the dangling bonds at the surface of the QDs but also help in controlling the kinetics of the nanoparticle synthesis. Thus chemical stability of the QDs along with surface functionality and good water dispersibility of the particles were obtained. The different aqueous methods of nanoparticle synthesis reported using thiols include hydrothermal 78 80 ultrasonic 13 7 microwave irradiation 1 3 8 1 3 9 and illumination 1 40 In this research aqueous synthesis of QDs using NAC thiol as the dis persing ligand is reported. Highly luminescent CdTeS QDs were prepared by reacting CdCl 2 with NaHTe and NAC in teflon lined autoclaves by a process similar to that reported by Zhao et al 79 The quality of the QDs synthesized by this process is highly influ enced by molar ratio of reactants, reaction temperature, pH, reaction time, and Cd 2+ concentration. Therefore to obtain QDs that are of high quality all these parameters were strictly controlled. 3.1 .1 Effect of R eactant C oncentration The Cd:Te:NAC molar ratio used for QD synthesis plays an important role in the quality of the resultant QDs 79 The optimal reactant ratios were determined after experimentation. In the core CdTeS QD synthesis the optimum molar ratio was found to be Cd:Te:NAC 1:0.5:2.4.

PAGE 67

67 3. 1. 2 Effect of P recursor pH The pH of the precursor solution is an important parameter that influences the QY as well as the nanocrystal growth rate 79 The pH of the precursor solution for the core CdTeS QDs was kept within 8 9 because for CdTe/CdS QDs pH high er or lower than this range causes particle aggregation resulting in the decrease of their fluorescence property 79 3. 1. 3 Effect of H eating T emperature The structure of the QD surface is dependent on the reaction temperature. Faster growth rate due to hig her temperatures minimizes the formation of surface defects and thus improve the fluorescence properties of the QDs 79 One important advantage of hydrothermal synthesis is that it permits high temperature synthesis of QDs. The heating temperature was kept autoclave 79 3. 1.4 Effect of H eating T ime Heating time is directly proportional to the size of the QDs. Longer heating t ime produced larger QDs that emitted at higher wavelengths. The solutions were heated in different time int ervals within 3 0 100 minutes. During heating the emitted wavelengths evolved from green to NIR as larger particles were produced when heated for lo nger time periods (Figure 3 1) The evolution of emitted wavelength with heating time for the core QDs is presented in Figure 3 2 This experiment was repeated several times to ascertain the variation of emission wavelength with heating time The emission wavelength variation obtained for QDs heated for a particular time interval that lies within the 30 100 min period at a constant temperature of

PAGE 68

68 Above 96 minutes heating time which produced QDs that emitted at 750 nm particles st ar t ed to agglomerate in the autoclave The agglomerated QDs with emission wavelengths longer than 750 nm exhibited significant ly lower QY s than the non agglomerated QDs emitting below 750 nm. The probable reason for the ag glomeration of the QDs emitting ab ove 750 nm is the decomposition of high amount of NAC ligand leading to the decrease in the amount of ligand available for dispersion of the QDs 3.2 Characterization of the Core CdTeS QDs 3. 2.1 Particle Size of the Core CdTeS QDs The core CdTeS QDs were characterized using TEM to determine their particle sizes. Figure 3 3 shows the TEM image of QD700. Size of the CdTeS QDs were measured from TEM and compared with values calculated using the equation reported by Yu et al. 1 4 1 The core QDs emitting within 5 30 750 nm have particle size within 3 6 nm. The calculated and measured sizes of the QDs are tabulated in Table 3 1. (3 1) where, D (nm) is the diameter of the quantum dot corresponding sample There is some variation observed in the measured and calculated particle sizes. The particle sizes reported by Yu et al. 1 4 1 are for CdTe QDs However the QDs reported in this dissertation have CdTe 1 x S x alloy composition ( discussed later in section 3.2.2 ) The change in lattice parameter and density due to the change in composition might be the reason behind the difference in values observed between the measure d

PAGE 69

69 and the calculated particle size s The particle sizes of the QDs were also measured using Dynamic Light Scattering (DLS). However, because of the small size of the QDs there were large variations in the obtained results. The data were not reproducible an d hence not reported in this dissertation. 3. 2.2 Chemical Composition Variation in the Core CdTeS QDs The selected area diffraction ( SAD ) pattern of the core CdTeS QDs emitting at 700 nm is shown in the inset of Figure 3 3 The 3 rings correspond to the p eaks observed in the XRD pattern of the core CdTeS as depicted in Figure 3 1 1 The d value for [111] in core CdTeS 700 nm is 3.59 . This value is intermediate between CdS, cubic [111] (3.36 ) and CdTe, cubic [111] (3.74 ) which indicate that the QDs hav e an alloy composition of Cd, Te and S. The S in the QDs comes from the decomposition of the NAC ligand 79 at high temperature. The chemical compositions of the QDs were also determined by Energy dispersive X ray spectroscopy (EDS) analysis and X ray phot oelectron spectroscopy (XPS). The EDS spectra for core QDs emitting at 560 nm and 700 nm with Cd, Te and S peaks are shown in Figure 3 4 The variation of chemical composition with increasing emission wavelength and corresponding increase in particle size is illustrated in Figure 3 5 From the figure it is evident that the amount of Te decreases while that of S increases with increasing emission wavelength. The amount of Cd remains almost constant around 50 atomic%. The bigger QDs with higher emission wave length s have more S and less Te than the QDs having smaller size and lower emission wavelength. No significant change in their optical properties that are desirable for bioimaging applications seem s to be affected by the size dependent variation in QD chem ical composition Ohata et al. 1 4 2

PAGE 70

70 reported the change in phase, density and lattice parameter with S addition to bulk powders of CdTe. With increasing value of x in CdTe 1 x S x the density of the bulk powder d with cubic CdTe changing to wurtzite CdS. The QDs might follow similar change in phase, lattice parameter and density with change in chemical composition as happens with bulk CdTe 1 x S x powders. More investigations are required to figure out the change of p hase and lattice parameter of the CdTeS QDs with increasing size and sul fur content. 3. 2.3 Fluorescence Quantum Yield of the Core CdTeS QDs The QY of the QDs w as determined relative to the standard Rhodamine 6g dye which has a QY of 95% in ethanol 13 1 Th e optical densities for both QDs and Rhodamine 6g were determined using SpectraMax M5 Value of absorbance in either case was kept below 0.08 at the wavelength of excitation. Same excitation wavelength was used for both the QD samples and the standard dye. T he integrated emission intensity from the QDs was compared to that of the dye using the following equation 1 3 1 (3 2) where, QY and QY S t are quantum yields for sample and standard A and A St are absorbance values of sample and standard at the excitati on wavelength and St are refractive indices of th e sample and standard solvents, and I and I St are the integrated emission areas for the QD samples and the standard, respectively.

PAGE 71

71 The QY thus measured for the core CdTeS emitting in the 530 700 nm region varied with in 40 60%. The QDs emitting in the wavelength range 700 740 nm exhibited QY between 20 40%. 3.2.4 Extinction Co Efficient Calculation s for the Core QDs. The extinction co efficients of two core QDs viz. QD630 and QD710 per mole of particles ( ) were calcul ated. The calculations were based on Beer law 1 4 1 (3 3) w here, A is the absorbance for a sample at the first exciton absorbance peak C is the molar concentration (mol/L) of the QDs of the same sample L is the path length (cm) of the exciting light us ed for obtaining the absorbanc e spectrum The value of L was fixed at 1 cm. The extinction co efficient was calculated per mole of the QDs and its unit is L (mol) 1 (cm) 1 or M 1 cm 1 The sizes of QD630 and QD710 calculated from Equation 3 1 were 3.7 nm and 4.4 nm respectively. Molecular weights (MWs) of the QDs were determined (calculations available in Appendix) and their absorbance versus concentration plots obtained from the UV/Vis spectra (Figure 3 6). From the slopes of the linear plo ts extinction co efficients were estimat ed. The extinction co efficient values for QD630 and QD710 were found to be 5.8 10 5 M 1 cm 1 and 3.7 10 5 M 1 cm 1 respectively. The extinction co efficient values for CdTe reported by Yu et al. 1 4 1 are also in the 10 5 M 1 cm 1 range. However, extinction co efficients for more QDs need to be determined and a trend line plotted with increasing particle size to figure out the variation of extinction co efficient with changing QD sizes.

PAGE 72

72 3.3 Synthesis of Core/ S hell CdTe S QDs Emitting up to 820 nm As discussed earlier the objective of this research was to obtain QDs emitting close to 800 nm. However, using the one pot synthesis technique core QDs with good fluorescence property above 750 nm could not be prepared. This nec essitated the search for alternative techniques that can overcome the above mentioned limitation. Core/shell QDs provided a good alternative and by fabricating a CdS shell over the CdTeS cores emission wavelength tunability near the 800 nm region was achie ved. By changing the thickness of the shell emission wavelength tunability over a wide range is possible 3 3 In principle CdS can be considered as a good shell material for CdTe core QDs because of three factors viz. CdS has a wider band gap (2 .5 eV ) than C dTe ( 1.5 eV ) relatively small lattice parameter mismatch of CdTe with CdS (11. 4 %) when compared to other candidates like ZnS (16.5%) and ZnSe (12.5%) 1 24 Core/shell CdTeS/CdS will probably be Type II QDs (Figure 3 7 ) as their electrons are considered to be confined in the CdS shells while the holes are mostly in the CdTeS cores as depicted by their band offsets 1 24 According to the hypothesis by Zeng et al. 1 24 the CdTe/CdS core/shell system slowly evolves from Type I to Type II system with increasing shel l thickness as represented in Figure 3 7 (b). CdS Coatings on the Core CdTeS QDs : The CdS coatings of the core CdTeS QDs were done using the experimental p rotocol presented in Figure 2 4 The core QDs used for CdS coatings have emission wavelengths of 575 nm, 720 nm and 735 nm respectively. The emission wavelengths for these QDs when coated with the CdS shell increased with increasing thickness of the shell produced by prolonged heating of the core QDs in the precursor solution containing Cd and S

PAGE 73

73 With an increase in CdS shell thickness first exciton peak in absorption along with the fluorescence peak undergo a significant redshift. This redshift caused emission wavelength to increase for all three core CdTeS QDs (Figure 3 8 ). With the CdS shell formation on the CdTeS core QDs maximum tunability upto 8 20 nm was achieved. For overcoating QDs with an inorganic shell there are a few criteria that need to be followed 1 23 (a) the conditions for shell deposition should be such that the core QDs will be able to wi thstand those conditions (b) both the core and shell materials should have similar surface energies so that the barrier for heterogeneous nucleation of the shell will be lower t han homogeneous nucleation. (c) under the conditions of shell deposition the co re and shell material must not readily interdiffuse. In a typical core/shell synthesis the core QDs synthesized by one of the standard techniques are redispersed into a solution of stabilizers and solvent. The inorganic shell precursors are then gradually added to the solution which is heated and held at a particular temperature. During this process the shell materials are heterogeneously nucleated on the core QDs. The rate of precursor addition is controlled so that it never exceeds the rate of deposition of shell material on the core QDs. In such as case the precursor concentration does not cross the threshold for homogeneous nucleation of the shell material. In the case of core/shell CdTeS/CdS QD synthesis the above criteria were followed. Temperature for shell formation was different than that was used for the synthesis of core QDs. The CdS shell ium in a two necked 100 ml flask. In another two necked 100 ml flask Na 2 S was dissolved in DI water containing NAC. The Na 2 S solution was then slowly added to the solution

PAGE 74

74 containing the CdTeS core QDs in a dropwise manner using a syringe pump so that the precursor concentration never reaches the homogeneous nucleation threshold. The pH of the final solution was adjusted to 12.0 12.2. The resultant solution was then heated of the CdS shell. After the heating was completed the autoclaves were taken out of the oven and cooled by running tap water. The QDs were then washed with DI water several times and characterized using TEM, XRD, fluorescence spectrophotometer etc. Rogach et al. 8 1 reported that high pH of the precursor solution in the range of 11 12 facilitate high growth rate of CdTe QDs coated with thioglycolic acid (TGA). In the research reported here the optimum pH range for coating the NAC stabilized core QDs with the CdS shell was also found to be 12.0 12.2, which is similar to the pH used by Rogach. The emission wavelength tunability achieved at pH lower than 12 was smaller than 180 nm. The zeta potential (by the Huckel method) of the precursor solution containing cor e CdTeS QDs emitting at 690 nm decreased from 70 mV to 20 mV with pH change from 7 to 12 (Figure 3 9 ). Thus with high pH the core QDs probably had smaller distance of separation among them leading to high growth rate. At pH 12 the maximum wavelength tuna bility achieved for a core QD emitting at 620 nm was 180 nm when heated at 180 these results we can conclude that high pH of the precursor solution favors high growth rate of Q Ds. 3.4 Characterization of the Core/ S hell CdTeS/CdS QDs The core/shell QDs synthesized were characterized using TEM, XRD, EDS and XPS techniques. TEM was used to determine the size and shape of the QDs whereas XRD, EDS and XPS gave the phase and chemical composition of these particles. The

PAGE 75

75 TEM image of core/shell QDs emitting at 720 nm is shown in Figure 3 10 That the QDs are crystalline is evident from the presence of lattice fringes in the TEM image. From the TEM images it was determined that all core/ shell QDs emitting between 530 n m 820 nm have sizes within 3 6 nm. The core/shell CdTeS/CdS QDs have the same size range of 3 6 nm as that of the core CdTeS QDs which implies that the CdS shell on the core CdTeS QDs is less than 1 nm. The core/shell stru cture of the QDs was not distinct in the TEM image. Chemical composition studies using EDS and XPS were performed to determine the core/shell characteristics of these QDs. The XRD of the core CdTeS QDs and the core/shell CdTeS/CdS QDs are presented in Fig ure 3 1 1 From the figure 3 distinct peaks can be identified that correspond to planes [111], [220] and [311] respectively. The 3 diffraction peaks for both cubic CdS peaks. blende CdS the peaks of the co re and core/shell QDs lie within the corresponding peaks for CdTe and CdS illustrate the fact that these QDs are alloys of CdTe and CdS. According to Ohata et al. 1 42 CdTe 1 x S x type bulk mixed crystals possess cubic zincblende structure for x < 0.2 and hex agonal wurtzite structure for x > 0.2. More investigations are required to confirm the phase composition of the core and core/shell QDs reported in this dissertation.

PAGE 76

76 3. 4.1 Determining Core /S hell Structure of the QDs Using Chemical Composition Studies The core/ shell structure of the QDs was not distinct in the (Figure 3 10 ) TEM picture. EDS and XPS studies of the QDs were carried out to investigate the presence of CdS shell. The chemical composition of 3 particles were analyzed viz. core 670 nm, core 750 nm and core/shell 750 nm emitting QDs. The core/shell 750 nm emitting QDs were obtained by coating the core 670 nm emitting QDs with CdS shells. 3. 4 1.1 Characterization of the QDs using EDS The EDS spectra for the core QD750 and core/shell QD750 are shown in Figures 3 1 2 (a) and (b) respectively. The Cd, Te and S peaks depicted in these figur es confirm the presence of the elements in these samples. The O peak in the samples comes from the NAC ligand attached to the QDs while the Cu peak is from the Cu TEM gr ids used as sample holders. The chemical composition values are for all of the three QDs are shown in Table 3 2. The amount of sulfur in the core/shell QD750 (47.3 atomic %) is significantly higher than the sulfur present in the core QD670 (34.7 atomic %) from which they were developed using a CdS coating. These results indicate the presence of CdS rich shell on the surface of CdTeS core QDs. Also the sulfur amount is higher in the core/shell 750 nm emitting particle than just the core 750 nm (35.5 atomic%) emitting QDs. Hence the core/shell 750 nm particle has a different chemical composition than that of a core 750 nm emitting particle without a shell which is expected. 3. 4.1.2 Characterization of the QDs using XPS X ray photoelectron spectroscopy (XPS) is regarded as a sensitive tool for determination of the surface chemical composition of materials. Hence, this technique

PAGE 77

77 was utilized for the surface analysis of the QDs. The surface composition of 3 different QDs viz. core QD670, core QD750 and core/shell QD750 were analyzed using XPS. Peaks due to Cd, Te, O, C and S appear in the XPS spectra of the QDs shown in Figure 3 1 3 (a). C and O came either from the NAC ligand or from the environment. The appearance of Cd, Te and S peaks were due to the fact that the se elements were present in the core and shell of the QDs. The high resolution spectra showing the plot of binding energy vs. intensity for the elements Cd, Te and S species are depicted in Figures 3 1 3 (b), 3 1 4 (a) and (b) respectively. Peaks of Cd 3d 5/2 at 405 eV, Cd 3d 3/2 at 411 eV, Te 3d 5/2 at 571 eV, Te 3d 3/2 at 582 eV, S 2p at 161 eV and S 2p at 225 eV appeared in all the XPS spectra. Thus the presence of these elements in the QDs was confirmed. The elemental composition of the QD surfaces is present ed in Table 3 3. The amount of S for the core/shell QD750 (37.9 atomic %) is higher than that in the core QD750 (34.0 atomic %). The amount of S in core QD670 (32.5 atomic %) is the lowest among the 3 QDs. The reverse trend in observed for Te. Among the th ree QDs the amount of Te in the core QD670 (12.5 atomic%) is the highest followed by the amount of Te in core QD750 (8.8 atomic%) and core/shell QD750 (3.0 atom ic%) The trend observed with the S and Te amounts present in the QDs by XPS is the same as was observed from the chemical analysis of the QDs using EDS. The results indicate that the amount of S on the surface increases with increasing QD size i.e. as the emission wavelength increases for the core QDs. Also, the core/shell QDs have even higher S on the surface relative to the core QDs due to the presence of CdS rich shell on the core CdTeS QDs The QDs were further etched using Argon and the chemical composition of the surfaces determined using XPS. The chemical compositions of the etched

PAGE 78

78 surfaces of the same three particles viz. core 670 nm, core 750 nm and core/shell 750 nm emitting QDs were determined after subjecting them to Ar etching for 0, 5 and 10 minutes. Tables 3 4, 3 5 and 3 6 show all the chemical composition values obtained after etching the samples with Argon for 0, 5 and 10 minutes. For all the QD samples the amount of Te increases with increasing etching time which is expected because as discussed earlier the QDs are Te rich inside and are S rich on the outside. These values agree with the previous finding that the QDs are of gradient alloy composition with a CdS rich shell. The presence of Te in all the XPS results confirms that the shell is an alloy of Cd, Te and S. 3.4.2 Time Resolved Luminescence Properties of Core and Core/ S hell Q Ds The fluorescence decay curves for the 620 nm emitting core QDs and that of the core / shell 800 nm are shown in Figure 3 1 5 The core / shell 800 nm emitting QDs were obtained by coating the 620 nm emitting QDs with CdS rich shells. The PL decay measurement s were taken after exciting the QDs with 375 nm laser. The 620 nm and 800 nm emitting QDs have lifetimes of 33 ns and 121 ns respectively. Figure 3 1 6 shows the comparisons of lifetimes among CdTeS cores 670 nm and 7 50 emitting QDs along with core/ shell 75 0 nm emitting QDs. The core/shell 750 nm QDs were obtained by coating the 670 nm emitting QDs with CdS shells. The lifetimes for t he core 670 nm, 750 nm and core/ shell 750 nm emitting QDs are 51 ns, 75 ns and 82 ns respectively. For the core QDs an increas e in the average PL decay times with increasing size was observed. This phenomenon is typical of II VI QDs and reported by other researchers 14 3 The PL decay times can be higher for the larger QDs with longer wavelength of emission for two reasons:

PAGE 79

79 a. The num ber of discrete energy levels increases in larger QDs which can trap electron hole pairs and thus delay the recombination process resulting in larger PL decay times. b. The number of excitons increases with larger QDs. Thus it takes longer for the decay to b e complete. PL decay times for core/shell QDs are larger than the core QDs. There are two probable reasons behind this phenomenon: a. Type I nanocrystals transform into type II nanocrystals during the growth of CdS shells on the core CdTe QDs 3 3 This causes the electrons to be accomodated the shell and holes in the cores. As a result the overlap in the electron hole integral decreases producing significant increase in the PL lifetimes 1 4 4 b. The shell reduces the surface states on the surface of the core QDs. Th us the high amount of non radiative relaxations originating from the surface states gets minimized. When the percentage of radiative relaxations rises due to the passivation of the surface states the average PL decay time for the core/shell QDs increases Deng et al. 3 3 reported the PL decay times for CdTe/CdS core/shell QDs made by classic aqueous reflux method. The lifetimes measured for QDs emitting at 623, 660, 740, 760, 800, 820 nm were 44, 58, 110, 138, 183, 245 ns respectively. The PL decay values fo r core 620, 670 and 750 nm emitting QDs obtained from our experiments were 33 ns, 51 ns and 75 ns which are close to the values reported by the above researchers. However, our value for 800 nm emitting QDs was 121 ns which is much lower than 183 ns reporte d by Deng et al 3 3 The possible reason for this difference in

PAGE 80

80 lifetime might be the higher shell thickness for the QDs prepared by the authors They prepared the core/shell 800 nm emitting QDs from 465 nm cores whereas in our system the 800 nm emitting co re/shell QDs were prepared from 620 nm cores. 3. 5 Performance Assessment of the NIR Emitting C ore/S hell QDs The usefulness of the QDs for applications in bioimaging was investigated by performing a staining test of J774 macrophages/monocytes cells using co re/shell CdTeS/CdS 800 nm wavelength emitting QDs. These cells were i ncubated with 200 For nuclei visualization the cells were stained with DAPI before ima ging. As depicted in Figure 3 1 7 (a) the CdTeS/CdS core/ shell QDs emitting at 800 nm were up taken by the J774 macrophages/monocytes. That the QDs have been absorbed by the cells was confirmed by comparing the images given in Figure 3 1 7 for cells incubated (a) with the QDs and (b) without the QDs. However, from the images it was difficult to ascertain whether the QDs were on the surface of the cells or inside the cells. The QDs were also imaged with mouse phantoms to determine their usefulness for animal imaging. Figure 3 1 8 shows the mouse phantom imaged with core/shell CdTeS/CdS QD e mit ting at 800 nm embedded in it and using different excitation wavelengths and emission filters. The picture represented in Figure 3 1 8 (a) is that of the mouse phantom containing the 800 nm QDs when it is excited with 640 nm light and imaged using 720 nm em ission filters. The auto fluorescence from the phantom when viewed with the excitation wavelength/emission filter combination of 640 nm/720 nm overshadows the emission from the QDs a t 720 nm. The QDs are distinctly visible only when the emission wavelength s of the QDs as well as the emission filters are in the NIR range. By just changing the excitation wavelength/filter combination to the NIR region

PAGE 81

81 and keeping everything else exactly the same as before the QDs can be easily detected in the phantom as shown in the re maining figures. In Figures 3 1 8 (b) and (c) the excitation wavelength/emission filter combinations used are 710 nm/780 nm and 710 nm/840 nm respectively. In both these images the fluorescence from the QDs from inside the mouse phantom could be d etected without any difficulty. The imaging results from experiments with both the J774 cells and the mouse phantom d emonstrate that the QDs emitting at 800 nm can be useful for bioimaging of animal cells and tissues. 3.6 Bimodal Imaging Using Magnetic QD s Visualization and understanding of tissues and specific molecules in vivo can be achieved using molecular imaging. The molecular imaging techniques that are currently used for various disease diagnoses are position emission tomography (PET), single photo n emission computed tomography (SPECT), MRI, optical and ultrasound 1 4 5 Table 3 7 illustrates the different characteristics of these imaging techniques. None of these techniques are perfect and each one has its own advantages and disadvantages. The infor mation required for comprehensive imaging can be obtained only by synergistic combination of the different imaging modalities. Nanoparticles, QDs which are 100 10,000 times smaller in size than the cells have the ability to pass through the cell membrane easily and reach the target site. Thus nanoparticles, QDs can potentially enhance not only the imaging sensitivity and resolution but also specificity. Multimodal imaging probes based on nanoparticles, QDs which are detectable by the above mentioned imagin g modalities can be used for multitargeting and monitoring, as well as in improving diagnostic/ therapeutic effects simultaneously 1 4 6 Hence, their importance is growing.

PAGE 82

82 A nanoprobe with bifunctional magnetic and fluorescence properties can take advantag e of the high sensitivity and high resolution characteristics of the fluorescence phenomena along with high spatial resolution and noninvasiveness characteristics of MRI. With these properties such a multifunctional nanoprobe will be useful for bioimaging applications. Magnetic Fe Doped CdTeS NIR QDs for Bio I maging Application s. The optical, electrical and magnetic properties of bulk semiconductors are strongly influenced by doping. However, despite a few successes many of the efforts to dope semiconductin g nanocrystals have failed for unknown reasons 10 3 To the best of our knowledge there are no reports available currently on Fe doped NIR emitting QDs synthesized in the aqueous system that have been used for MR imaging of mouse Here in this dissertation s ynthesis and charact erization of novel water dispersible Fe doped CdTeS QDs that emit in the NIR region and can be detected by MRI inside a mouse are reported. 3.6.1 Hydrothermal S ynthesis of Fe Doped C ore CdTeS QDs The Fe doped CdTeS QDs were synthesized in a way which is very similar to the technique used for CdTeS core QDs. In a typical process sodium hydrogen telluride (NaHTe) was prepared from NaBH 4 and Tellurium powder in Argon saturated DI water. The molar ratio of NaBH 4 to Te used was 2:1. The as p repared NaBH 4 /Te sol ution was from expanding. The vial containing the solution might explode if the reaction is allowed to proceed at room temperature. The time period allowed for reaction to be completed was more than 8 hrs. 100 ml solution of a 30 mM solution of CdCl 2 in Argon saturated DIW was prepared separately in a 250 ml round bottle flask. NAC was added to the

PAGE 83

83 solution so that Cd:NAC molar ratio was 1:2.5. FeCl 2 .4H 2 O was added so tha t Cd:Fe is 8.5:1. The solution was Argon bubbled for more than 30 min and then the NaHTe solution was added to this solution. The color of the solution turned wine red. The pH of the precursor solution was adjusted to (8.0 8.4) by using 2M NaOH solution with subsequent heating in PTFE lined autoclaves for different time intervals (30 100 min) co oling process after heating. The QDs thus prepared were taken out and characterized after washing them several times with DI water Effect of r eactant c oncentration The CdCl 2 concentration was important for obtaining NIR emitting Fe doped QDs. For the undoped core CdTeS QDs, 12.5 mM CdCl 2 concentration produced QDs which emitted upto 750 nm. However, for the fluor escent magnetic Fe doped core CdTeS QDs emitting in the NIR wavelength regime, 30 mM CdCl 2 concentration was found optimum. Solution with lower concentration of CdCl 2 i.e. 12.5 mM produced QDs that can emit only upto 650 nm. With this concentration, effort s to obtain Fe doped QDs that emitted above 650 nm were unsuccessful. When 30 mM CdCl 2 solution was used Fe doped QDs emitting upto 750 nm were obtained. For optimum fluorescence efficiency the Cd:Fe molar ratio was kept constant at 8.5:1. When the amount of Fe added was lower than this value the quantum yield of the Fe doped CdTeS QDs were the same as that of the undoped CdTeS QDs. Addition of more Fe resulted in significant quenching of the QDs. Effect of precursor pH, heating t emperature and t ime The p H of the precursor solution used for synthesizing Fe doped CdTeS QDs was kept unchanged as was for undoped CdTeS QDs in the range of 8 9. The heating temperature was also kept

PAGE 84

84 ncy (40 60%) wit hin the wavelength range of 530 nm 74 0 nm when h eated for various times within 3 0 to 100 minutes. 3.6.2 Characterization of the Fe Doped C ore CdTeS QDs The Fe doped core QDs synthesized by the hydrothermal technique were characterized usi ng TEM, SAD, XRD, ICP, EDS, XPS, SQUID and MRI for determining the size, structure, chemical composition, magnetization and relaxivity of these particles. TEM, SAD and XRD of the Fe d oped QDs The TEM image of Fe doped QD740 is presented in Figure 3 1 9 L attice fringes in the TEM image reveal the crystalline nature of the QDs. The first inset shows the SAD pattern obtained from the QDs. The second inset shows the magnified image of one single QD. The particle sizes of all Fe doped QDs emitting within the w avelength range (530 nm 740 nm) lie within 3 6 nm. The particles als o appear to be elongated and non spherical. The SAD pattern has three rings that correspond to the [111], [220] and [311] planes. The same three planes were represented by 3 peaks in th e XRD of the QDs shown in Figure 3 20 The XRD patterns of the undoped QD and substrate are also provided for comparison. From the positions of the undoped QDs. Als o the peaks of the doped material looks less sharp compared to the undoped samples. Both these effects are probably due to the change in lattice parameter as a result of Fe doping. The calculated d value for the [111] planes of Fe doped QDs is 4.71 which is larger than 3.59 for undoped CdTeS QDs. For

PAGE 85

85 Ohata et al. 1 42 reported that bulk Cd Te 1 x S x type mixed crystals have cubic zinc blende (x < 0.2) and hexagonal wurtzite (x > 0.2) structure. The chem ical composition of the Fe doped QDs QD740 (S = 39.0 atom%) and QD730 (S = 39.4 atom%) as described in the section below have more than 20 at om % S and might possess the hexagonal wurtzite structure assuming that the phase behavior of bulk and QDs is si milar More investigation is required in this area to confirm the phase composition of these magnetic QDs. ICP, EDS and XPS of the Fe d oped QDs The chemical compositions of these particles were determined using ICP. The amounts of S in these QDs were determined by subtraction. For the Fe doped QD740 the compositions in atomic % were Cd (44.40.3), Te (11.10.2), Fe (5.60.1) and S (39.0). For the Fe doped QD730 these numbers are Cd (46.91.5), Te (10.60.3), Fe (3.10.1) and S (39.4). The composition for the Fe doped QD740 was confirmed using EDS. The EDS spectrum in (Figure 3 2 1 ) indicates the presence of Fe (4.50.7) in the material in addition to Cd (49.41.1), Te (10.80.7) and S (35.40.8). The chemical composition values obtained from EDS for Fe doped QD730 are Cd (51.6 0.8), Te (9.7 1.9) and S (38.8 1.9). Due to small amount of Fe doping (< atomic 5%) in the QD730 which is below the detect ion limi t of EDS Fe peak s did not show up in the EDS spectra of the QD The atomic% data for all the other elements Cd, Te, and S from both of these measurements match closely. XPS studies were done to cross check the presence of the elements in the QDs. Fe doped QD710 was analyze d using XPS after etching it with Argon for 10 minutes. The XPS results from the experiment are illustrated in Figures 3 2 2 (a) (e). The typical peaks of the elements Cd and Te with the following binding energies: Cd 3d 5/2 (404.9 eV), Cd

PAGE 86

86 3d 3/2 (411.8 eV) Te 3d 5/2 (572.2 eV) and Te 3d 3/2 (582.6 eV) were obtained. The S 2p peak at 161.4 eV indicates the presence of monosulfide (S 2 ). The appearance of monosulfide peak indicates broken S S bond which may be present at the Ar etched QD surface due to the bre aking of CdS or FeS bonds. The Fe 2p 3/2 peak at 708.6 eV signifies the presence of Fe(II)S in the material 14 7 Magnetic p roperty measurements of the Fe d oped QDs Magnetic measurements using SQUID showed that the Fe doped QDs were superparamagnetic at roo m temperature (Figure 3 2 3 ). The temperatures at which the measurements were carried out were 10K and 300K. The saturation magnetization (M s ) values of Fe doped QD7 3 0 (Figure 3 2 3 (a)) was determined to be 1.7 emu/gm at 10K and at room temperature, 300K. For the Fe doped QD7 4 0 (Figure 3 2 3 (b)) the M s values were 3.3 emu/gm at 10K and 2 8 emu/gm at 300K. The M s values of the commercial contrast agent, Feridex I.V. w ere also measured using SQUID. At 10K and 3 00K the M s values for Feridex I.V were 12.3 e mu/gm and 7.5 emu/gm respectively as indicated in Figure 3 2 3 (c). In all the above cases the magnetization values were calculated taking into account the total weight of the particles. However these calculations can also be done with respect to the amount of Fe in these materials. The saturation magnetization values of the Fe doped QDs and Feridex I.V. NPs with respect to the Fe content are depicted in Figure 3 24. T he Fe content in the QDs was determined using ICP and EDS. The results indicate d that the amount of Fe in QD730 was 3.1 atomic% (2.2 wt%) and in QD740 was 5.6 at% (3.9 wt%). The manufacturer reported the Fe content of Feridex I.V. as 11.2 mg/ml. The M s values therefore for QD730 is 76 emu/gm[Fe] at 10K and 300K for QD740 the values are 85 em u/gm[Fe] (10K), 71 emu/gm[Fe] (300K) Fig. 18 (a)

PAGE 87

87 and for Feridex I.V. they are 119 emu/gm[Fe] (10K) and 72 emu/gm[Fe] (300K). The reported Ms value for Feridex I.V at room temperature is 64.4 emu/gm[Fe] 14 8 Thus we see that the M s values obtained for the Fe doped Q Ds are comparable to that of Feridex I.V. None of the particles tested exhibited coercivity at room temperature indicating their superparamagnetic behavior at room temperature However, in all of these particles coercivity was found to be present when th eir magnetic properties were measured at 10K. The 10K magnetization loops show hysteresis curve with a H c ~ 240 Oe for QD740 particle, H c ~ 290 Oe for QD730 and H c ~ 90 Oe for Feridex I.V This phenomenon was observed for MNPs. Sun et al. 1 4 9 reported the ma gnetization of 16 nm MNPs of CoFe 2 O 4 with large hysteresis loop at 10K with coercivity of 20 kOe compared to a much smaller loop at 3 00 K with coercivity of only 40 0 Oe. However, for Fe 3 O 4 NPs having the same size of 16 nm but without Co doping the coerciv ity exhibited by the NP assembly was 450 Oe at 10K and zero at 300K. This indicated that the incorporation of the Co cation in the matrix of Fe O highly enhanced the magnetic anisotropy of the MNPs. For the Fe doped CdTeS QDs reported here the presence of large coercivity at 10K with almost negligible coercivity at 300K indicate s similarity of increased magnetic anisotropy as that of the Co doped Fe O NPs at low temperature The M vs H curves discussed above were obtained by measuring the magnetization o f the NPs with varying magnetic field at a constant temperature. Now, the variation of magnetization of the NPs with varying temperature at a fixed magnetic field will be discussed While the MH curves provide d information on the saturation magnetization a nd coercivity of NPs, the field cooled (FC) and zero field cooled measurements provide d information on the b locking temperature (T B ) of the NPs

PAGE 88

88 (Figures 3 25 (a) (c)) In the ZFC measurements the QD and Feridex I. V. sample s were cooled without any appl ied magnetic field to a temperature much below the anticipated blocking temperature, T B Then the temperature of the system was raised and the magnetization measured as a function of temperature at a relatively low fixed magnetic field of 80 Oe The FC mag netization measurements were carried out by cooling the sample in an applied magnetic field (80 Oe) and magnetization measured as a function of increasing temperature. The temperature dependent magnetizations, M versus T curves for both FC and zero field c ooled ZFC cases were plotted for all the samples viz QD730, QD740 and Feridex I.V. NPs The blocking temperatures (T B ) for the NPs were determined from the ZFC curves. As the samples were heated the transition from ferromagnetism to superparamagnetism occ urred at the blocking temperature. The free movements of the magnetic moments in the samples were B by the anisotropy leading to their ferromagnetic behavior. Above T B the NPs exhibit super paramagnetic behavior. From th e curves, T B for QD730 was determined to be 190K whereas, for QD740 it was 150K. By fitting the 3/2 equation in the region T << T C : (3 3 ) where, M s Saturation magnetization (emu/gm) T c T C ~ 570 K for QD740, T C ~ 720 K for QD730 nm particle and Tc ~283 for Feridex I.V. were also o btained. F or both of these QDs c urie temperatures, T c is above the room temperature. From the magnetization data we conclude that Fe d oped QDs show

PAGE 89

89 ferromagnetism at low temperatures which is evident from their magnetization loops with non zero H c values and low field saturation. MR imaging of magnetic Fe doped QDs and Feridex I.V. NPs The T 2 weighed images of Fe doped CdTeS QDs (QD7 40) and those of Feridex I.V. NPs are depicted in Figures 3 2 6 (a) and (b). In both cases serial dilutions of the particles were loaded into glass capillaries and imaged using a 750 MHz 17.6 T 89 mm bore MRI. The Fe doped QDs were well dispersed and hence they were imaged after diluting them with DI water However, the Feridex I.V. NPs were found to be poorly dispersed and agglomerated Hence they were imaged after diluting them with 0.25% agarose solution. The Fe doped QD740 sample concentrations used fo r capillaries numbered 1, 2, 3, 4, 5 and 6 (Figure 3 26 (a)) were 8.6, 4.3, 2 .15, 1.08, 0.54 and 0.27 mg/ml r espectively. DIW was used for glass capillary numbered 7 and was used as a control sample. For the commercial Feridex I.V. samples ( in 0.25% agar ose solution ) imaged in Figure 3 2 6 (b) the sample concentration used were 8.6, 4.3, 2.15, 1.08, 0.54, 0.27 and 0.14 mg /ml for capillaries numbered 1 7 respectively. The 8 th capillary was filled with distilled water and was used as the control sample From the figures it is evident that various concentrations of the contrast agent, Fe doped particles generated different contrast with respect to distilled water. The efficiency of the synthesized Fe doped QD740 and the Feridex I.V. NPs both T 2 agents in g enerating MRI contrast can be evaluated by comparing their relaxivity coefficient s (r 2 ), which is related to T 2 through the following equations 1 50 (3 4 ) (a)

PAGE 90

90 (3 5 ) where, C is the concentration of the contras t agent T 2 is the observed relaxation time in the presence of the contrast agent and is the relaxation time for pure water R 2 or (1/T 2 ) is the relaxation rate in the presence of contrast agent or (1/ ) is the relaxation rate of pure water From E quation 3 4 it is evident that and r 2 being constants the relaxation time is inversely proportional to the concentration of the contrast agent. Thus, as the concentration of the contrast agent incre ases the relaxation time decreases and the MRI image appears darker. Also, the higher the value of r 2 for a contrast agent, smaller the concentration required to generate the same contrast. Plots represented in Figures 3 2 7 depict the (a) T 2 weighted rela xation rate, R 2 and (b) T 2 weighted relaxation rate, R 2 versus metal ion concentration (Fe) for Fe doped QD740 and Feridex I.V. NPs The relaxivity co efficient, r 2 for both contrast agents was determined from the slope of the plots. The r 2 values obtai ned for the Fe doped QD740 and Feridex I.V. sample were 732.36 (mM 1s 1 ) and 389.18 (mM 1s 1 ) respectively whereas the r 2 values for the samples were 730.75 (mM 1s 1 ) and 438.11 (mM 1s 1 ) respectively. Thus the relaxivity coefficient (r 2 ) f or Fe doped QD7 40 is 88% higher than that of Feridex I.V. NPs. This phenomenon is unusual and the underlying reason behind the higher relaxivity exhibited by the Fe doped QDs when compared to Feridex I.V. NPs was not understood and needs further investigation. From the data it can be concluded that these particles are magnetic and can have useful applications as MRI contrast agents. As discussed before these particles are also

PAGE 91

91 fluorescent unlike the Feridex I.V. particles. Thus these particles can have novel bimodal fl uorescent and magnetic applications and can be tracked more efficiently compared to Feridex I.V. which can only generate magnetic contrast. 3.6.3 Performance Assessment of the Magnetic Fe D oped QDs Experiments were performed to evaluate the performance of the magnetic QDs as in vitro and in vivo MRI contrast agents. J774 macrophages were incubated with Feridex I.V. NPs and Fe doped QDs separately at 37 C for 6 hours Prior to incubation, Fe doped QD740 and Feridex I.V. NPs were added to J774 macrophages of concentration 110 6 cells per ml. The concentration achieved for both particles w as 0.2 mg/ml. Macrophages containing the particles were then loaded into glass capillaries and imaged with 14 T MRI at 600 MHz. Macrophages containing Feridex I.V. NPs we re used as control for the experiment. Some of these macrophages were then injected into a live mouse to ascertain the efficacy of the Fe doped QDs as in vivo optical and MRI contrast agent. The macrophages containing the QDs were injected into the left le g of the mouse while the Feridex I.V. particles were injected into the right leg of the mouse. The mouse was imaged using Xenogen IVIS imaging system for assessing the in vivo fluorescent contrast generating efficiency of the QDs. MR imaging was also per formed on the mouse to find out the suitability of the magnetic QDs as in vivo MRI contrast agents. MR Imaging of magnetic Fe doped QDs and Feridex I.V. NPs after injecting them into J774 m acrophages MR imaging of the QD labeled macrophages was performe d using 14T MRI at 600 MHz. Figure 3 2 8 shows the MRI images of capillaries filled with cells attached to both the QDs and the Feridex I.V. NPs There is also a capillary containing just the cells without any particles as control sample.

PAGE 92

92 From the images depicted in Figure 3 2 8 the contrast generated by the two capillaries containing both the QDs and the Feridex I.V. particles relative to the capillary containing cells without any of the particles is significant and can be easily detected by the unaided e ye. The Feridex I.V. NPs generated more contrast than the QDs. This was due to the higher iron content (10.4 wt%) of the Feridex I.V. NPs relative to the Fe doped QDs (3.9 wt%) as discussed earlier. Optical and MR i maging of mouse after i njecting it with cells loaded with Fe d oped QDs and Feridex I.V. p articles containing Fe doped QDs (emi tting at 740 nm) was injected into the left leg of a mouse while the right leg was injected with cells containing Feridex I.V. NPs The mouse was then imaged using Xenogen IVIS imaging instr ument. As depicted in Figure 3 2 9 the fluorescence from the QDs inside the left leg of the mouse could be seen whereas there was no detectable fluorescence from the Feridex I.V. NPs (non fluorescent) in the right leg. The MRI of the mouse was carried o ut at 600 MHz. As shown in Figure 3 30 both the Fe doped QDs as well as the Feridex I.V. particles generated contrast with respect to the surrounding tissue. The contrast generated by the Feridex I.V. NPs w as more than that generated by the Fe doped QDs. This phenomenon is due to the stronger magnetic property of the Feridex I.V. NPs because of its higher Fe content (10.4 wt%) compared to the Fe doped QD 740 (3.9 wt%) However the important point to note here is that the QD particles have enough magnetic property that can generate MRI contrast when injected into animals. Thus these QDs show potential for dual fluorescent and magnetic bimodal contrast agents.

PAGE 93

93 The amount of Fe content of the QDs can be increased by coating the Fe doped core QDs (emitting at 730, 740 nm) with Fe doped CdS shells. The formation of Fe doped shells on magnetic core QDs will not only increase the ir iron content thus improv ing their magnetic property but als o help in tuning the emission wavelength toward 800 nm. Better magnetic p roperties and NIR emission around 800 nm are both desirable in these QDs for bio imaging applications.

PAGE 94

94 Table 3 1. Particle size comparison (Measured versus calculated) Serial Quantum Dot Measured (D m ) (nm) Calculated (D c ) (nm) 1 Q D580 3.8 0.4 3. 3 2 QD630 4.00.5 3. 8 3 QD700 4.20.4 4. 6 QD580 QD630 and QD700 CdTeS quantum dot s emitting at 580 nm 630 nm and 700 nm respectively. Table 3 2 Chemical composition of the core QD670, core QD750 versus core/shell QD750 determined by ED S. Element Core 670 nm Core 750 nm Core/shell 750 nm Cd 49.8 53.3 49.4 Te 15.5 11.2 3.3 S 34.7 35.5 47.3 Table 3 3 Surface chemical composition of the core QD670, core QD750 versus core/shell QD750 determined by XPS. Element Core QD670 Core QD750 Core/shell QD750 Cd 55.1 0.1 57.2 0.7 59.52.0 Te 12.5 0.3 8.8 0.1 3.01.1 S 32.5 0.2 34.0 1.1 37.91.6

PAGE 95

95 Table 3 4 Surface chemical composition (atomic %) of the core QD670 determined using XPS after etching with Argon for 0, 5 and 10 min utes. Element Cd Te S 0 min Ar etching 55.10.1 12.50.3 32.50.2 5 min Ar etching 53.40.7 16.80.4 29.81.1 10 min Ar etching 51.80.2 18.00.1 30.20.3 Table 3 5 Surface chemical composition (atomic %) of the core QD750 determined using XPS afte r etching with Argon for 0, 5 and 10 minutes. Element Cd Te S 0 min Ar etching 56.3 8.9 34.8 5 min Ar etching 55.1 12.4 32.5 10 min Ar etching 54.1 13.0 33.0 Table 3 6 Surface chemical composition (atomic %) of the core/shell QD750 determined using XPS after etching with Argon for 0, 5 and 10 minutes. Element Cd Te S 0 min Ar etching 59.9 3.1 37.9 5 min Ar etching 58.4 3.5 38.1 10 min Ar etching 56.9 4.0 39.2

PAGE 96

96 Table 3 7. Characteristics of clinically used imaging modalities (Basilion et al. 1 4 5 ) Modality Resolution Depth Cost Sensitivity Imaging agents PET ++ ++++ ++++ ++++ Radioisotope SPECT + ++++ ++ +++ Radioisotope MRI ++++ ++++ +++ + Paramagnetic ion (Gd 3+ Mn 2+ ) Paramagnetic nanoparticles Superparamagnetic nanoparticles( iron oxide) Optical ++ + ++ ++++ Organic dye Fluorescent protein QDs RE materials Carbon nanotube Ultrasound ++ +++ + ++ Microbubble Perfluorocarbon, nanoparticles

PAGE 97

97 (a) (b) Figure 3 1. (a) Fluorescen ce from Vis NIR QDs when excited with UV light (b) PL intensity versus wavelength plot for the QDs (some wavelengths are omitted for clarity)

PAGE 98

98 Figure 3 2. Emission wavelength versus heating time for core QDs. Heating temperature was kept constant at 1

PAGE 99

99 Figure 3 3. TEM image of core CdTeS QDs emitting at 700 nm. Insets show the SAD pattern and the size of a single QD.

PAGE 100

100 (a) (b) Figure 3 4 EDS spectra for the (a) core 560 nm and (b) core 7 0 0 nm emitt ing QDs.

PAGE 101

101 Figure 3 5 Chemical composition variation in QDs determined using EDS spectra analysis.

PAGE 102

102 Figure 3 6. Determination of extinction co efficient of core QDs (a) QD630 and (b) QD710 (a) (b)

PAGE 103

103 Figure 3 7 (a) Energy diagram of Cd TeS/CdS core / shell QDs represented schematically adapted from Ref. 1 24 (b) Red shift in emission wavelength with increasing CdS shell thickness. ( a ) (a) (b)

PAGE 104

104 Figure 3 8 Emission t unability in core/ shell quantum dots (a) core quantum dots (b) and (c) change in wave length of the core dots due to CdS shell coating on the CdTeS core QDs

PAGE 105

105 Figure 3 9 Zeta potential variation for core CdTeS 690 nm emitting QDs as a function of pH.

PAGE 106

106 Figure 3 10 TEM image of core/shell QDs emitting at 720 nm. Insets show the SAD patt ern and the size of a single QD.

PAGE 107

107 Figure 3 1 1 XRD of core and core/shell QDs. The standard diffraction lines for cubic CdTe are shown at the bottom axis while for cubic CdS are shown at the top axis of the plot.

PAGE 108

108 (a) (b) Figure 3 1 2 EDS spe ctra for (a) core CdTeS QDs emitting at 750 nm and (b) core / shell CdTeS/CdS QDs emitting at 750 nm

PAGE 109

109 F igure 3 1 3 (a) XPS of the core QD670, core QD750 and core/shell QD750 (b) XPS showing the Cd 3d 5/2 and Cd 3d 3/2 peaks for all the samples. ( a ) (b)

PAGE 110

110 Fi gure 3 1 4 XPS showing the (a) Te 3d 5/2 & Te 3d 3/2 peaks and (b) S 2p & S 2s peaks in core QD670, core QD750 and core/shell QD750 samples. ( a ) (b)

PAGE 111

111 Figure 3 1 5 Normalized PL decay curves for core 620 nm and core/shell 800 nm emitting QDs.

PAGE 112

112 Figure 3 1 6 Normal ized PL decay curves for co re 670 nm, core 750 nm and core/ shell 750 nm emitting QDs.

PAGE 113

113 Figure 3 1 7 (a) J77 4 mouse cells labeled with core/ shell CdTeS/CdS QDs emitting at 800 nm (b) unlabeled DAPI stained cells as control. ( a ) (b)

PAGE 114

114 Figure 3 1 8 Xen ogen IVIS Spectrum Biophotonic imaging of mouse with core / shell CdTeS/CdS QDs emitting at 800 nm with different excitation wavelengths and emission filters (a) excited at 640 nm and imaged with 720 nm filter (b) excited with 710 nm and imaged with 780 nm filter (c) excited with 710 nm and imaged with 840 nm filter. ( a ) (b) (c)

PAGE 115

115 Figure 3 1 9 TEM image of Fe doped CdTeS QDs having emission wavelength of 730 nm. Insets show the SAD pattern and the size of a single QD.

PAGE 116

116 Figure 3 20 XRD of the magnetic Fe doped and undoped CdTeS QDs.

PAGE 117

117 Figure 3 2 1 EDS spectrum for the Fe doped QD740

PAGE 118

118 Fi gure 3 22 XPS of the Fe doped QD710 sample etched with Argon for 10 minutes showing the (a) Cd, Te, Fe, S, O and C peaks and (b) Cd 3d 5/2 and Cd 3d 3/2 duplet peaks. (b) (a)

PAGE 119

119 Figure 3 2 2 Continued. XPS of the Fe doped QD710 sample etched with Argon for 10 minutes showing the (c) Fe 2p 3/2 and Fe 3p 1/2 duplet peaks (d) Te 3d 5/2 and Te 3d 3/2 duplet peaks (d) (c)

PAGE 120

120 Figure 3 2 2 Continued. (e) XPS of the Fe doped QD710 sample etched with Argon for 10 minutes showing the S 2p peak. (e)

PAGE 121

121 Figure 3 2 3 Magnetometry measurements using SQUID at 10K and 300K for (a) 730 nm and (b) 740 nm emitting Fe doped CdTeS QDs and (c) c ommercial Feridex I.V. NPs normalized with respect to pa rticle mass (b) (c) (a) (a)

PAGE 122

122 Figure 3 24. Magnetometry measurements using SQUID at 10K and 300K for (a) 730 nm and (b) 740 nm emitting Fe doped CdTeS QDs and (c) c ommercial Feridex I.V. NPs normalized with respect to Fe content (b) (c) (a)

PAGE 123

123 Figure 3 2 5 Magnetiza tion versus temperature plot at a constant magnetic field of 80 Oe for (a) Fe doped QD730 and (b) Fe doped QD740. (b) (a)

PAGE 124

124 Figure 3 2 5 Continued. (c) Magnetization versus temperature plot at a constant magnetic field of 80 Oe for commercial Feridex I.V. part icles. (c)

PAGE 125

125 (a) (b) Figure 3 2 6 MRI in vitro T 2 weighted images of serially diluted (a) Fe doped QDs (in DI water) and (b) Feridex I.V particles (in 0.25% agarose solution) loaded into glass capillaries.

PAGE 126

126 (a) (b) Figure 3 2 7 (a) R 2 and (b) R 2 versus Fe concentration plot s for Fe doped QD 740 (in water) and Feridex I.V. NPs (in 0.25% agarose solution)

PAGE 127

127 Figure 3 2 8 Fe doped QD s and and Feridex I.V. labeled J774 macrophages were l oaded into glass capillaries and imaged using 14T MRI at 600 MHz. J774 macrophages labeled with commercial Feridex I.V. NPs J774 macrophages without any particles as control sample J774 macrophages labeled with magnetic Fe doped QDs emitting at 740 nm.

PAGE 128

128 Figure 3 2 9 J774 macrophages labeled with Fe doped QD740 and Feridex I.V. were injected into the left and right leg of the mouse respectively and excited with 710 nm light. Fluorescence from J774 macrophages labeled with magnetic Fe doped QDs emitting at 740 nm. No fluorescence from J774 macrophages labeled with magnetic Feridex I.V NPs

PAGE 129

129 Figure 3 30 Fe doped QD s and Feridex I.V. labeled J774 macrophages were injected into the left and right leg of the mouse respectively and imaged with MRI. MRI contrast generated using Fe doped QD labeled J774 macrophages injected into the left leg of the mouse MRI contrast generated using Feridex I.V. labeled J774 macrophages injected int o the right leg of the mouse

PAGE 130

130 CHAPTER 4 CONCLUSIONS AND FUTU RE WORK 4.1 Conclusions In this research we demonstrat e the synthesis and characterization of two kinds of QDs by the hydrothermal synthesis technique (a) core and core/shell CdTeS/CdS QDs emitting in the visib le NIR wavelength regime of 53 0 820 nm and (b) novel bimodal Fe doped CdTeS QDs that exhibit dua l fluorescent and magnetic properties. Either types of QDs were capped with NAC and were highly dispersible in water. Through the hydrothermal synthesis process the emission wavelength of the QDs could be easily tuned by changing the size of the QDs while keeping the heating temperatur e constant. Longer heating times (30 100 min) produced larger particles with higher emission wavelengths in the 530 750 nm regimes. Particle agglomeration with QDs emitting above 750 nm was observed. Decomposition and deg radation of the dispersing ligand (NAC) in high amounts is the probable reason behind QD agglomeration. Wavelength tunability above 750 nm was achieved by coating the core CdTeS QDs with a CdS shell. Maximum emission tunability of 200 nm was achieved by co ating CdS layer on core CdTeS QDs (emitting at 620 nm) which produced 820 nm emitting CdTeS/CdS core/shell QDs. The sizes of core and core/shell QDs varied within 3 6 nm and the ir fluorescence QY ranged from 2 0 to 60%. The QY for the vis NIR core 530 70 0 nm emitting QDs varied between 40 60% while that of core/shell 700 820 nm emitting QDs varied within 20 40%. Fe doped QDs exhibited similar values for size and QY. The core CdTeS QDs possessed a gradient alloy composition where the amount of S increased while that of Te decreased with increasing QD size. The amount of Cd remained constant at around 50% for the entire QD size range of 3 6 nm. The suitability

PAGE 131

131 of these core/shell QDs as optical contrast agents for NIR fluorescence imaging was assessed by at taching the QD800 with J774 macrophages and imaging them using NIR detecting camera. The QDs were bright enough to be imaged by the camera. The Fe doped magnetic QDs were highly fluorescent and emitted within the wavelength range of 530 750 nm. The amou nts of Fe in the 730 740 nm emitting QDs were determined to be within 4 5 atomic% and they exhibited ~ 40% QY The magnetic properties of these QDs were evaluated by measuring the saturation magnetization ( M s ) using SQUID and the values were found to be i n the range of 1.5 4.0 emu/gm. J774 macrophages incubated with Q D740 were further injected into the left leg of a mouse and imaged using Xenogen IVIS to find out the suitability of the se QDs for in vivo imaging. The QDs inside the mouse were visible wh en imaged with Xenogen IVIS and they were also successfully imaged by MRI. The relaxivity coefficient, r 2 obtained for QD740 ( 732.4 mM 1 s 1 ) was 88% higher than that of Feridex I.V. NPs (389.2 mM 1 s 1 ). The underlying reason for the higher relaxivity ex hibited by the Fe doped QDs per unit Fe compared to Feridex I.V. is not understood. This is an unusual phenomenon and needs further investigation. In summary, these particles are highly fluorescen t and magnetic in nature and they have potential as bimodal contrast agents for biological applications. 4.2 Suggestions for Future Work In this research it was demonstrated that fabrication of magnetic QDs, as small as 3 6 nm in diameter with significant fluorescence and magnetic properties for bimodal biological imaging applications is possible by aqueous hydrothermal synthesis techniques. This research can be further extended in various ways. Few of them are reported here:

PAGE 132

132 The QDs have elements Cd and Te which are considered to be toxic to animals. QDs can be sy nthesized with elements that are minimally toxic/non toxic to animals using processes similar to the hydrothermal techniques described in this research One of the elements contained in the QDs need to have thiol affinity for the hydrothermal synthesis wit h thiol stabilizers to be successful. For e.g. CuInS 2 is a good potential candidate for NIR imaging of biological materials. These QDs have already been synthesized by the organometallic route and they exhibit good fluorescence property in the NIR waveleng th regime of the electromagnetic spectrum around 800 nm. Li et al. 15 1 reported the synthesis of CuInS/CdS QDs (emitting 709 nm) having QY > 80%. C u is known to be relatively less toxic to human beings than Cd and also has affinity for thiol Indium has so me toxicity however the amount of Indium used in CuInS 2 QDs was relatively smaller than Cd used for Cd based QDs and th us these Cu/ In based QDs shou l d be much less toxic compared to Cd based QDs. Also, these Cu based QDs can be made bimodal with optical an d magnetic properties by doping them with paramagnetic ions similar to the Fe doped CdTeS QDs reported here. The QDs in this research were doped with Fe, which is a T 2 contrast agent for MRI. However, paramagnetic ions such as Mn can be doped in the CdTeS QDs in a similar way. Mn is a T 1 contrast agent and has complementary MR imaging characteristics to Fe. Thus the hydrothermal process described here can also be utilized to develop T 1 contrast agents for bio imaging applications. The amount of Fe /Mn conte nt in the QDs can be increased by coating these core QDs with Fe /Mn doped CdS shells. The formation of paramagnetic ion doped shells on core QDs will not only increase the amount of paramagnetic ion content of the QDs,

PAGE 133

133 thus improving their magnetic propert y, but also help in tuning the emission toward the 800 nm regime, both of which are desirable for bio imaging applications. The QDs synthesized can be coated with silica or a polymer to reduce toxicity and improve their biocompatibility. He et al. 138 demon strated that CdTe QDs prepared by aqueous synthesis techniques can be made bio compatible by coating them with protein amino groups. The authors used crosslinkers, N (3 dimethylaminopropyl) N ethylcarbodiimde hydrochloride (EDC) and N hydroxysuccinimide (N HS) as conjugates for coating the QDs.

PAGE 134

134 APPENDIX DETERMINATION OF QUA NTUM DOT MOLECULAR W EIGHTS The MWs of QDs were calculated by a method that is similar to the one reported by Quall et al. 1 5 2 Discussions on the extinction coefficient are available in section 3.2.4. Molecular w eight of QD630 max = 600 nm Diameter d = 3.66 nm CdTe bond length = 0.28 nm The number of Formula units across the QD diameter is given by: FU = Diameter/CdTe bond length = 3.66 nm/0.28 nm = 13.07 units The number of CdT e units present in a CdTe spherical particle of size d= 3.66 nm is: (FU/2) 3 07 /2) 3 = 1168.44 Therefore, Molar Mass of QD630 with diameter 3.7 nm = ( 1168.44 units of CdTe) (molar mass of Cd + Molar mass of Te) = 1168.44 (11 2.41 + 127.60) = 280436 g/Mol Molecular w eight of QD710 max = 6 5 0 nm Diameter, d = 4. 34 nm CdTe bond length = 0.28 nm The number of Formula units across the QD diameter is given by: FU = Diameter/CdTe bond length = 4.34 nm/0.28 nm = 15.5 units The number of CdTe units present in a CdTe spherical particle of size d=3.7 nm is: 3 15.5 /2) 3 = 1948.83 units Therefore, Molar Mass of QD630 with diameter 4.6 nm = ( 1948.83 units of CdTe) (molar mass of Cd + Molar mass of Te) = 1948.83 (112.41 + 127.60) = 467738 g/Mol

PAGE 135

135 LIST OF REFERENCES 1 Guan J 2008 Synthesis and structural characterization of ZnTe/ZnSe Core/shell Tunable Quantum Dots MS thesis Chemistry MIT 2 Alivisatos AP 1996 Semiconductor Clusters, Nanocrystals, and Quantum Dots Science 271 933 937 3 Bruchez M, Moronne M, Gin P, Weiss S and Alivisatos AP 1998 Semiconductor Nanocrystals as Fluorescent Biological Labels Science 281 2013 2016 4 Fleischhaker F, Arsenault AC, Kitaev V, Peiris FC, Freymann GV, Manners I, Zent el R and Ozin GA 2005 Photochemically and Thermally Tunable Planar Defects in Colloidal Photonic Crystals J. Am. Chem. Soc. 127 9318 9319 5 Zrenner A, Ester P, Vasconcellos SMD, Hubner MC, Lackmann L, Stufler S and Bichler M 2008 Coherent optoelectronics wi th single quantum dots J. Phys.: Condens. Matter 20 454210 454215 6 Chan WCW and Nie S 1998 Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection Science 281 2016 2018 7 Ma Q and Su X 2010 Near infrared quantum dots: synthesis, functionalizatio n and analytical applications Analyst 1 35 1867 1877 8 Genger UR, Grabolle M, Jaricot SC, Nitschke R and Nann T 2008 Quantum dots versus organic dyes as fluorescent labels Nature methods 5 (9) 763 775 9 Mihindukulasuriya SH, Morcone TK and McGown L 2003 Chara cterization of acridone dyes for use in four decay detection in DNA sequencing Electrophoresis 24 20 25 10 Jun YW, Seo JW and Cheon J 2008 Nanoscaling Laws of Magnetic Nanoparticles and Their Applicabilities in Biomedical Sciences Accou. of Chem. Res. 41 (2) 179 189 11 Dave SR and Xiaohu G 2009 Monodisperse magnetic nanoparticles for biodetection, imaging, and drug delivery: a versatile and evolving technology Wiley Interdisciplinary Reviews: Nanomedicine and Nanobiotechnology 1 (6) 583 609 12 Strijkers GJ Mulder WJM Tilborg GAF and Nicolay K 2007 MRI Contrast Agents: Current Status and Future Perspectives Anticancer Agents Med Chem 2007 7 ( 3 ) 291 305 13 Derfus AM, Chan WCW, and Bhatia S N 2004 Probing the Cytotoxicity of Semiconductor Quantum Dots Nano Lett. 4 (1) 11 18

PAGE 136

136 14 Stohs SJ and Bagchi D 1995 Oxidative mechanisms in the toxicity of metal ions Free Rad. Biol. and Med. 18 (2) 321 336 15 Prakash AS, Rao S and Dameron C 1998 Cadmium Inhibit s BPDE Alkylation of DNA in the Major Groove but Not in the Minor Groove Biochem. Biophys. Res. Commun. 244 198 203 16 Hossain Z and Huq F 2002 Studies on the interaction between Cd ions and DNA Journal of Inorganic Biochemistry 90 85 96 67:303 323 1991. 17 Ana s AA, Akita H, Harashima H, Itoh T, Ishikawa M, and Biju V 2008 Photosensitized Breakage and Damage of DNA by CdSe ZnS Quantum Dots J. Phys. Chem. B 112 10005 10011 18 Samia ACS, Chen X, and Burda C 2003 Semiconductor Quantum Dots for Photodynamic Therapy J Amer. Chem. Soc 125 15736 15737 19 Samia ACS, Dayal S and Burda C 2006 Quantum Dot based Energy Transfer: Perspectives and Potential for Applications in Photodynamic Therapy Photochemistry and Photobiology 82 617 625 20 Caruge JM, Halpert JE, Wood V, Bulovi c V and Bawendi MG 2008 Colloidal quantum dot light emitting diodes with metal oxide charge transport layers Nature Photonics 2 247 250 21 Gaponik NP, Talapin DV, Rogach AL 1999 A light emitting device based on a CdTe nanocrystal / polyaniline composite Phys. Chem. Chem. Phys. 1 1787 1789 22 Gaponik NP, Talapin DV, Rogach AL, Eychmuller A 2000 Electrochemical synthesis of CdTe nanocrystal/polypyrrole composites for optoelectronic applications J. Mater. Chem. 10 2163 2166 23 Subramania G, Lee YJ, Fischer AJ, Luk TS, Brinker CJ and Dunphy D 2009 Emission modification of CdSe quantum dots by titanium dioxide visible logpile photonic crystal Appl. Phys. Lett. 95 151101 151101 3 24 Gaponenko SV, Kapitonov AM, Bogomolov VN, Prokofiev AV, Eychmuller A, Rogach AL 1998 Electr ons and photons in mesoscopic structures: quantum dots in a photonic crystal JETP Lett. 68 142 147 25 Sargent EH 2008 Solar Cells, Photodetectors, and Optical Sources from Infrared Colloidal Quantum Dots Adv. Mater. 2008 20 (20) 3958 3964 26 Weissleder R 2001 A clearer vision for in vivo imaging Nat. Biotech. 19 316 317

PAGE 137

137 27 Kim S, Lim YT, Soltesz EG, Grand AMD, Lee J, Nakayama A, Parker JA, Mihaljevic T, Laurence RG, Dor DM, Cohn LH, Bawendi MG and Frangioni JV 2003 Near infrared fluorescent type II quantum dots for sentinel lymph node mapping Nat. Biotech. 22 (1) 93 97 28 Desmet KD, Paz DA, Corry JJ, Eells JT, Riley MTTW, Henry MM, Buchmann EV, Connelly MP, Dovi J, Liang HL, Henshel DS, Yeager RL, Millsap DS, Lim J, Gould LJ, Das R, Jett M, Hodgson BD, Margolis D and Whelan HT 2006 Clinical and Experimental Applications of NIR LED Photobiomodulation Photomedicine And Laser Surgery 24 (2) 121 128 29 Yaghini E, Seifalian AM and MacRobert AJ 2009 Quantum dots and their potential biomedical applications in photosensitization for photodynamic therapy Nanomedicine 4 (3) 353 363. 30 Bakalova R, Ohba H, Zhelev Z, Ishikawa M and Baba Y 2004 Quantum dots as photosensitizers? Nat. Biotec. 22 (11) 1360 1361 31 Gurusinghe NP, Hewa Kasakarage NN and Zamkov M 2008 Composition Tunable Proper ties of CdS x Te x Alloy Nanocrystals J. Phys. Chem. C 112 (33) 12795 12800 32 Guzelian AA, Banin U, Kadavanich AV, Peng X and Alivisatos AP 1996 Colloidal chemical synthesis and characterization of InAs nanocrystal quantum dots Appl. Phys. Lett 69 1432 1434 33 Deng Z, Schulz O, Lin S, Ding B, Liu X, Wei XX, Ros R,Yan H and Liu Y 2010 Aqueous Synthesis of Zinc Blende CdTe/CdS Magic Core/Thick Shell Tetrahedral Shaped Nanocrystals with Emission Tunable to Near Infrared J. Am. Chem. Soc. 2010 132 5592 5593 34 Chen Y, Vela J, Htoon H, Casson JL, Werder DJ, Bussian DA, Klimov VI, and Hollingsworth JA 2008 Suppressed Blinking J. Am. Chem. Soc. 130 5026 5027 35 Mahler B, Spinicelli P, Buil S, Quelin X, Hermier JP and D ubertret B 2008 Towards non blinking colloidal quantum dots Nat. Mat. 7 659 664 36 Mahler B, Lequeux N and Dubertret B 2010 Ligand Controlled Polytypism of Thick Shell CdSe/CdS Nanocrystals J. Am. Chem. Soc. 132 953 959 37 Reiss P, Protiere M and Li L 2009 Co re/Shell Semiconductor Nanocrystals s mall 5 154 168 38 http://www.nanotrio.com/board/list.php?board_num=11

PAGE 138

138 39 Zhong XH Xie RG Zhang Y Basch T and Knoll W 2005 High Quality Violet to Red E mitting ZnSe/CdSe Core/Shell Nanocrystals Chem. Mater. 17 4038 4042 40 Mews A, Eychmuller A, Giersig M, Schooss D and Weller H 1994 Preparation, Characterization, and Photophysics of the Quantum Dot Quantum Well System CdS/HgS/CdS J. Phys. Chem. 98 934 941 41 Battaglia D, Li JJ, Wang YJ and Peng XG 2003 Colloidal Two Dimensional Systems: CdSe Quantum Shells and Wells Angew. Chem. Int. Ed. 42 5035 5039 42 Steckel JS, Snee P, Sullivan SC, Zimmer JR, Halpert JE, Anikeeva P, Kim LA, Bulovic V and Bawendi MG 2006 C olor Saturated Green Emitting QD LEDs A ngew Chem. Int. Ed. 45 5796 5799 43 Bailey RE, Strausburg JB and Nie SM 2004 A New Class of Far Red and Near Infrared Biological Labels Based on Alloyed Semiconductor Quantum Dots J. Nanosci. Technol 4 (6) 569 574 44 Bec ker A, Hessenius C, Licha K, Ebert B, Sukowski U, Semmler W, Wiedenmann B and Grotzinger C 2001 Receptor targeted optical imaging of tumors with near infrared fluorescent ligands Nat. Biotechnol 19 (4) 327 331 45 Zaheer A, Lenkinski RE, Mahmood A, Jones AG, C antley LC and Frangioni JV 2001 In vivo near infrared fluorescence imaging of osteoblastic activity Nat. Biotechnol. 19 (12) 1148 1154 46 Muraca EMS, Houston JP and Gurfinkel M 2002 Fluorescence enhanced, near infrared diagnostic imaging with contrast agents Curr. Opin. Chem. Biol. 6 642 650 47 Li YC, Zhong HZ, Li R, Zhou Y, Yang CH and Li YF 2006 High Yield Fabrication and Electrochemical Characterization of Tetrapodal CdSe, CdTe, and CdSe x Te 1 x Nanocrystals Adv. Funct. Mater. 16 1705 1716 48 Zhou Y, Li YC, Zhon g HZ, Hou JH, Ding YQ, Yang CH and Li YF 2006 Hybrid nanocrystal/polymer solar cells based on tetrapod shaped CdSe x Te x nanocrystals Nanotechnology 17 4041 4047 49 Zhong X, Zhang Z, Liu S, Han M and Knoll W 2004 Embryonic Nuclei Induced Alloying Process fo r the Reproducible Synthesis of Blue Emitting Zn x Cd 1 x Se Nanocrystals with Long Time Thermal Stability in Size Distribution and Emission Wavelength J. Phys. Chem. B 108 15552 15559 50 Zhong X, Han M, Dong Z, White TJ and Knoll W 2003 Composition Tunable Zn x Cd 1 x Se N anocrystals with High Luminescence and Stability J. Am. Chem. Soc. 125 8589 8594

PAGE 139

139 51 Swafford LA, Weigand LA, Bowers MJ, McBride JR, Rapaport JL, Watt TL, Dixit SK, Feldman LC and Rosenthal SJ 2006 Homogeneously Alloyed CdS x Se 1 x Nanocrystals: Synth esis, Characterization, and Composition/Size Dependent J. Am. Chem. Soc. 128 12299 12306 52 Zhong, X, Feng, Y, Knoll, W and Han M 2003 Alloyed Zn x Cd 1 x S Nanocrystals with Highly Narrow Luminescence Spectral Width J. Am. Chem. Soc. 125 13559 13563 53 Kuno M, Higginson KA, Qadri SB, Yousuf M, Lee SH, Davis BL and Mattoussi H 2003 Molecular Clusters of Binary and Ternary Mercury Chalcogenides: Colloidal Synthesis, Characterization, and Optical Spectra J. Phys. Chem. B 107 (24) 5758 5767 54 Bernard JE and Zunger A 1987 Electronic structure of ZnS, ZnSe, ZnTe, and their pseudobinary alloys Phys. Rev. B 36 (6) 3199 3228 55 Murray CB, Noms DJ, and Bawendi MG 1993 Synthesis and Characterization of Nearly Monodisperse CdE (E = S, Se, Te) Semiconductor Nanocrystalli tes J. Am. Chem. Soc. 115 8706 8715 56 Choi HS, Ipe BI, Misra P, Lee JH, Bawendi MG and Frangioni JV 2009 Tissue and Organ Selective Biodistribution of NIR Fluorescent Quantum Dots Nano Lett. 9 (6) 2354 2359 57 Guzelian AA, Banin U, Kadavanich AV, Peng X and A livisatos AP 1996 Colloidal chemical synthesis and characterization of InAs nanocrystal quantum dots Appl. Phys. Lett. 69 (10) 1432 1434 58 Xie RG and Peng XG 2008 Synthetic Scheme for High Quality InAs Nanocrystals Based on Self Focusing and One Pot Synthesi s of InAs Based Core Shell Nanocrystals Angew. Chem. Int. Ed. 47 7677 7680 59 Allen PM, Li u WH, Chauhan VP, Lee J, Ting AY, Fukumura D, Jain RK and Bawendi MG 2010 InAs(ZnCdS) Quantum Dots Optimized for Biological Imaging in the Near Infrared J. Am. Chem. S oc. 132 470 471 60 Gao JH, Chen K, Xie RG, Xie J, Yan YJ, Cheng Z, Peng XG and Chen XY 2010 In Vivo Tumor Targeted Fluorescence Imaging Using Near Infrared Non Cadmium Quantum Dots Bioconjugate Chem. 21 604 609 61 Deng DW, Zhang WH, Chen XY, Liu F, Zhang J, Gu YQ and Hong JM 2009 Facile Synthesis of High Quality, Water Soluble, Near Infrared Emitting PbS Quantum Dots Eur. J. Inorg. Chem. 3440 3446

PAGE 140

140 62 Hinds S, Myrskog S, Levina L, Koleilat G, Yang J, Kelley SO and Sargent EH, 2007 NIR Emitting Colloidal Quantum Do ts Having 26% Luminescence Quantum Yield in Buffer Solution J. Am. Chem. Soc. 129 7218 7219 63 Tan TT, Selvan ST, Zhao L, Gao SJ and Ying JY 2007 Size Control, Shape Evolution, and Silica Coating of Near Infrared Emitting PbSe Quantum Dots Chem. Mater 19 311 2 3117 64 Lim YT, Kim S, Nakayama A, Stott NE, Bawendi MG and Frangioni JV 2003 S election of quantum dot wavelengths for biomedical assays and imaging Mol. Imaging 2 50 64 65 Zhang Y, Li Y and Yan XP 2009 Aqueous Layer by Layer Epitaxy of Type II CdTe/CdSe Qu antum Dots with Near Infrared Fluorescence for Bioimaging Applications small 5 185 189 66 Xia YS and Zhu CQ 2008 Aqueous synthesis of type II core/shell CdTe/CdSe quantum dots for near infrared fluorescent sensing of copper(II) Analyst 133 928 932 67 Kim S, Li m YT, Soltesz EG, Grand AMD, Lee J, Nakayama A, Parker JA, Mihaljevic T, Laurence RG, Dor DM, Cohn LH, Bawendi MG and Frangioni JV 2004 Near infrared fluorescent type II quantum dots for sentinel lymph node mapping Nat. Biotechnol. 22 93 97 68 Zhang Y, Li Y and Yan XP 2009 Photoactivated CdTe/CdSe Quantum Dots as a Near Infrared Fluorescent Probe for Detecting Biothiols in Biological Fluids Anal. Chem. 81 5001 5007 69 Blackman B, Battaglia D and Peng X 2008 Bright and Water Soluble Near IR Emitting CdSe/CdTe/Zn Se Type II/Type I Nanocrystals, Tuning the Efficiency and Stability by Growth Chem. Mater. 20 4847 4853 70 Bang JH, Suh WH and Suslick KS 2008 Quantum Dots from Chemical Aerosol Flow Synthesis: Preparation, Characterization, and Cellular Imaging Chem. Mater. 20 4033 4038 71 Yong KT, Roy I, Ding H, Bergey EJ and Prasad PN 2009 Biocompatible Near Infrared Quantum Dots as Ultrasensitive Probes for Long Term in vivo Imaging Applications Small 5 1997 2004 72 Jiang W, Singhal A, Kim BYS, Zheng JN, Rutka JT, Wang C and Chan WCW 2008 Assessing Near Infrared Quantum Dots for Deep Tissue, Organ, and Animal Imaging Applications J. Assoc. Lab. Autom. 13 6 12

PAGE 141

141 73 Jin T, Yoshioka Y, Fujii F, Komai Y, Seki J and Seiyama A 2008 Gd 3+ functionalized near infrared quantum dots for in v ivo dual modal (fluorescence/magnetic resonance) imaging Chem. Commun. 4 5764 5766 74 Qian HF, Dong CQ, Peng JL, Qiu X, Xu YH and Ren JC 2007 High Quality and Water Soluble Near Infrared Photoluminescent CdHgTe/CdS Quantum Dots Prepared by Adjusting Size an d Composition J. Phys. Chem. C 111 16852 16857 75 Wang YQ, Ye C, Wu LH and Hu YZ 2010 Synthesis and characterization of self assembled CdHgTe /gelatin nanospheres as stable near infrared fluorescent probes in vivo J. Pharma.& Biomed. Anal. 53 235 242 76 Liang G X, Pan HC, Li Y, Jiang LP, Zhang JR and Zhu JJ 2009 Near infrared sensing based on fluorescence resonance energy transfer between Mn:CdTe quantum dots and Au nanorods Biosens. Bioelectron. 24 3693 3697 77 Xie RG and Peng XG 2009 Synthesis of Cu Doped InP Nan ocrystals (d dots) with ZnSe Diffusion Barrier as Efficient and Color Tunable NIR Emitters J. Am. Chem. Soc. 131 10645 10651 78 Mao W, Guo J, Yang W, Wang C, He J and Chen J 2007 Synthesis of high quality near infrared emitting CdTeS alloyed quantum dots via the hydrothermal method Nanotechnology 18 485611 485617 79 Zhao D, He Z, Chan WH and Choi MMF 2009 Synthesis and Characterization of High Quality Water Soluble Near Infrared Emitting CdTe/CdS Quantum Dots Capped by N Acetyl L cysteine Via Hydrothermal Metho d J. Phys. Chem. C 113 1293 1300 80 Zhang H, Wang L, Xiong H, Hu L, Yang B and Li W 2003 Hydrothermal synthesis for High Quality CdTe nanocrystals Adv. Mater. 15 (20) 1712 1715 81 Rogach AL, Franzl T, Klar TA, Feldmann J, Gaponik N, Lesnyak V, Shavel A, Eychmul ler A, Rakovich YP and Donegan JF 2007 Aqueous Synthesis of Thiol Capped CdTe Nanocrystals: State of the Art J. Phys. Chem. C 111 (40) 14628 14637 82 He Y, Zhong Y, Su Y, Lu Y, Jiang Z, Peng F, Xu T, Su S, Huang Q, Fan C, and Lee ST 2011 Water Dispersed Near Infrared Emitting Quantum Dots of Ultrasmall Sizes for In Vitro and In Vivo Imaging Angew. Chem. Int. Ed. 50 1 5 83 Na HB, Song IC, and Hyeon T 2009 Inorganic Nanoparticles for MRI Contrast Agents Adv. Mater. 21 2133 2148

PAGE 142

142 84 Jung CW and Jacobs P1995 Physical and chemical properties of superparamagnetic iron oxide MR contrast agents: Ferumoxides, ferumoxtran, ferumoxsil Magnetic Resonance Imaging 13 (5) 661 674 85 Wang YXJ, Hussain SM and Krestin GP 2001 Superparamagnetic iron oxide contrast agents: physicochemica l characteristics and applications in MR imaging Eur. Radiol. 11 2319 2331 86 Josephson L, Tung CH, Moore A and Weissleder R 1999 High Efficiency Intracellular Magnetic Labeling with Novel Superparamagnetic Tat Peptide Conjugates Bioconjugate Chem. 10 (2) 186 191 87 Jun YW Huh YM Choi JS Lee JH Song HT Kim S Yoon S Kim KS Shin JS Suh JS Cheon J 2005 Nanoscale Size Effect of Magnetic Nanocrystals and Their Utilization for Cancer Diagnosis via Magnetic Resonance Imaging J. Am. Chem. Soc. 127 5732 573 3 88 Hadjipanayis CG, Bonder MJ, Balakrishnan S, Wang X, Mao H, and Hadjipanayis GC 2008 Metallic Iron Nanoparticles for MRI Contrast Enhancement and Local Hyperthermia small 4 (11) 1925 1929 89 Lee JH, Huh YM, Jun YW, Seo JW, Jang JT, Song HT, Kim S, Ch o EJ, Yoon HG, Suh JS and Cheon J 2007 Artificially engineered magnetic nanoparticles for ultra sensitive molecular imaging Nature Medicine 13 (1) 95 99 90 Xu C, Xie J, Ho D, Wang C, Kohler N,Walsh EG, Morgan JR, Chin YE, and Sun S 2008 Au Fe 3 O 4 Dumbbell N anoparticles as Dual Functional Probes Angew. Chem. Int. Ed. 47 173 176 91 Choi JS, Jun YW, Yeon SI, Kim HC, Shin JS, and Cheon J 2006 Biocompatible Heterostructured Nanoparticles for Multimodal Biological Detection J. Am. Chem. Soc. 128 15982 15983 92 Faure JLBAC, Laurent S, Rivire C, Billotey C, Hiba B, Janier M, Josserand V, Coll JL, Elst LV, Muller R, Roux S, Perriat P, and Tillement O 2007 Hybrid Gadolinium Oxide Nanoparticles: Multimodal Contrast Agents for in Vivo Imaging J. Am. Chem. Soc 129 (16 ) 5076 5084 93 McDonald MA and Watkin KL 2006 Investigations into the Physicochemical Properties of Dextran Small Particulate Gadolinium Oxide Nanoparticles Academic Radiology 13 (4) 421 427 94 Fortin MA, Petoral RM, Soderlind F, Klasson A, Engstrom M, Ve res T, Kall PO and Uvdal K 2007 Polyethylene glycol covered ultra small Gd 2 O 3 nanoparticles for positive contrast at 1.5 T magnetic resonance clinical scanning Nanotechnology 18 39550 39559

PAGE 143

143 95 Hifumi H Yamaoka S Tanimoto A Citterio D and Suzuki K 2006 Gadolinium Based Hybrid Nanoparticles as a Positive MR Contrast Agent J. Am. Chem. Soc. 128 15090 15091 96 Evanics F Diamente PR Veggel FCJMV Stanisz GJ Prosser RS 2006 Water Soluble GdF 3 and GdF 3 /LaF 3 Nanoparticles s Physical Characterization and NMR Relaxation Properties Chem. Mater. 18 2499 2505 97 Na HB Lee JH An K Park YI Park M Lee IS Nam DH Kim ST Kim SH Kim SW Lim KH Kim KS Kim SO Hyeon T 2007 Development of a T 1 Contrast Agent for Magnetic Resonance Imaging Using MnO Nanoparticles Angew. Chem. Int. Ed. 46 5397 5401 98 Seo WS Lee JH Sun X Suzuki Y Mann D Liu Z Terashima M Yang PC Mcconnell MV Nishimura DG and Dai H 2006 FeCo/graphitic shell nanocrystals as advanced magnetic resonance imaging and near infrared agents Nat. Ma ter. 5 971 976 99 He R, You X, Shao J, Gao F, Pan B and Cui D 2007 Core/shell fluorescent magnetic silica coated composite nanoparticles for bioconjugation Nanotechnology 18 315601 315607 100 Gao JH, Zhang B, Gao Y, Pan Y and Zhang XX 2007 Fluorescent magnetic nanocrystals by sequential addition of reagents in a one pot reaction: a simple preparation for multifunctional nanostructures J. Am. Chem. Soc. 129 (39) 11928 11935 101 Gu H Zheng R Zhang X Xu B 2004 Facile one pot synthesis of bifunctional heterodimers of nanoparticles: a conjugate of quantum dot and magnetic nanoparticles J. Am. Chem. Soc. 126 (8) 5664 5665 102 Norris DJ, Efros AL and Erwin SC 2008 Doped Nanocrystals Science 319 1776 1779 103 Erwin SC, Zu L, Haftel M, Efros A, Kennedy AK and Norris DJ 2005 Dopin g semiconductor nanocrystals Nature 436 (7) 91 94 104 Bhargava RN and Gallagher D 1994 Optical Properties of Manganese Doped Nanocrystals of ZnS Phy. Rev. Lett. 72 (3) 416 419 105 Bol AA and Meijerink A 1998 Long lived Mn 2+ emission in nanocrystalline ZnS:Mn 2+ Phy s Rev B 58 (24) R15997 R16000 106 Sagar RV, Buddhudu S 2010 Structural and Magnetic Properties of Co 2+ :ZnO Nanoparticles Adv. Sci. Lett. 3 (4) 461 464

PAGE 144

144 107 Hofmann A, Graf C, Boeglin C, and Ruhl E 2007 Magnetic and Structural Investigation of Mn 2+ Doped ZnSe Semico nductor Nanoparticles ChemPhysChem 8 2008 2012 108 Magana D, Perera SC, Harter AG, Dalal NS, and Strouse GF 2006 Switching on Superparamagnetism in Mn/CdSe Quantum Dots J Am Chem Soc 128 2931 2939 109 Santra S, Yang H, Holloway PH, Stanley JT, and Mericle RA 200 5 Synthesis of Water Dispersible Fluorescent, Radio Opaque, and Paramagnetic CdS:Mn/ZnS Quantum Dots: A Multifunctional Probe for Bioimaging J Am Chem Soc 127( 6) 1656 1657 110 Koole R, Mulder WJ, Schooneveld MMV, Strijkers GJ, Meijerink A, Nicolay K 2009 Magn etic quantum dots for multimodal imaging Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 1 (5) 475 491 111 Yong KT 2009 Mn doped near infrared quantum dots as multimodal targeted probes for pancreatic cancer imaging Nanotechnology 20 015102 015112 112 Wang S, Jarrett BR, Kauzlarich SM, and Louie AY 2007 Core/shell quantum dots with high relaxivity and photoluminescence for multimodality imaging J Am Chem Soc 129 (13) 3848 3856 113 Yi DK, Selvan ST, Lee SS, Papaefthymiou GC, Kundaliya D and Ying JY 2005 Silica coat ed nanocomposites of magnetic nanoparticles and quantum dots J Am Chem Soc 127 (14) 4990 4991 114 Kim J, Lee JE, Lee J, Yu JH, Kim BC 2006 Magnetic fluorescent delivery vehicle using uniform mesoporous silica spheres embedded with monodisperse magnetic and se miconductor nanocrystals J. Am. Chem. Soc. 128 (3) 688 689 115 Maceira SV Duarte MAC Spasova M Marzan LML and Farle M 2006 Composite silica spheres with magnetic and luminescent functionalities Adv. Funct. Mater. 16 ( 4 ) 509 514 116 Frullano L and Meade TJ 2007 Multimodal MRI contrast agents J. Biol. Inorg. Chem. 12 ( 7 ) 939 949 117 Mulder WJM, Koole R, Brandwijk RJ, Storm G, Chin PTK, Strijkars GJ, Donega CDM, Nicolay K and Griffioen AW 2006 Quantum dots with a paramagnetic coating as a bimodal molecular imaging pro be Nano Lett 6 (1) 1 6 118 Yang HS Santra S Walter GA and Holloway PH 2006 Gd III functionalized fluorescent quantum dots as multimodal imaging probes Adv. Mater 18 ( 21 ) 2890 2894

PAGE 145

145 119 Lin CH, Chang LW, Chang H, Yang MH, Yang CS, Lai WH, Chang WH and Lin P 2009 The chemical fate of the Cd/Se/Te based quantum dot 705 in the biological system: toxicity implications Nanotechnology 20 215101 215110 120 Manca D, Ricard AC, Tro ttier B and Chevalier G 1991 Studies on lipid peroxidation in rat tissues following administration of low and moderate doses of cadmium chloride Toxicology 67 (3) 303 323 121 Su Y, Peng F, Jiang Z, Zhong Y, Lu Y, Jiang X, Huang Q, Fan C, Lee ST and He Y 2 011 In vivo distribution, pharmacokinetics, and toxicity of aqueous synthesized cadmium containing quantum dots Biomaterials 32 5855 5862 122 http://www.tf.uni kiel.de/matwis/amat/semitech_en/kap_2/backbone/r2_3_1.html 123 Murray CB, Kagan CR and Bawendi MG 2000 Synthesis and characterization of monodisperse nanocrystals and close packed nanocrytal assemblies Annu. Rev. Mater. Sci. 30 545 610 124 Zeng Q, Kong X, Sun Y, Zhang Y, Tu L, Zhao J, and Zhang H 2008 Synthesis and Optical Properties of Type II CdTe/C dS Core/Shell Quantum Dots in Aqueous Solution via Successive Ion Layer Adsorption and Reaction J. Phys. Chem. C 112 8587 8593 125 Law WC, Yong KT, Roy I, Ding H, Hu R, Zhao W, and Prasad PN 2009 Aqueous Phase Synthesis of Highly Luminescent CdTe/ZnTe Core /Shell Quantum Dots Optimized for Targeted Bioimaging small 2009 5 (11) 1302 1310 126 Gao M, Kirstein S, and Mohwald H 1998 Strongly Photoluminescent CdTe Nanocrystals by Proper Surface Modification J. Phys. Chem. B 102 8360 8363 127 Gaponik N, Talapin DV, Rogach AL, Hoppe K, Shevchenko EV, Kornowski A, Eychmuller A, and Weller H 2002 Thiol Capping of CdTe Nanocrystals: An Alternative to Organometallic Synthetic Routes J. Phys. Chem. B 106 7177 7185 128 Rogach AL, Katsikas L, Kornowski A, Su D, Eychmuller A and Well er A 1996 Synthesis and characterization of thiol stabilized CdTe nanocrystals Bunsen Ges. Phys. Chem. 100 (11) 1772 1778 129 Zhang H, Zhou Z, and Yang B 2003 The Influence of Carboxyl Groups on the Photoluminescence of Mercaptocarboxylic Acid Stabilized CdTe Nanoparticles J. Phys. Chem. B 107 8 13 130 Ba LA, Doring M, Jamier V and Jacob C 2010 Tellurium: an element with great biological potency and potential Org. Biomol. Chem. 8 4203 4216

PAGE 146

146 131 Bera D, Qian L, Tseng TK and Holloway PH 2010 Quantum Dots and Their Multi modal Applications: A Review Materials 3 2260 2345 132 Rogach AL, Kornowski A,Gao M, Eychmuller A and Weller H 1999 Synthesis and Characterization of a Size Series of Extremely Small Thiol Stabilized CdSe Nanocrystals J. Phys. Chem. B 103 (16) 3065 3069 133 Vossm eyer T, Katsikas L, Giersig M, Popovic IG, Diesner K, Chemseddine A, Eychmuller A, and Weller H 1994 CdS Nanoclusters: Synthesis, Characterization, Size Dependent Oscillator Strength, Temperature Shift of the Excitonic Transition Energy, and Reversible Abs orbance Shift J. Phys. Chem 98 7665 7673 134 Harrison MT, Kershaw SV, Burt MG, Eychmuller A, Weller H and Rogach AL 2000 Wet chemical synthesis and spectroscopic study of CdHgTe nanocrystals with strong near infrared luminescence Materials Science and Engine ering B69 70 355 360 135 Rogach A, Kershaw S, Burt M, Harrison M, Kornowski A, Eychmuller A and Weller H 1999 Colloidally Prepared HgTe Nanocrystals with Strong Room Temperature Infrared Luminescence Adv. Mater. 11 (7) 552 555 136 Shavel A, Gaponik N and Eychm uller A 2004 Efficient UV Blue Photoluminescing Thiol Stabilized Water Soluble Alloyed ZnSe(S) Nanocrystals J. Phys. Chem. B 108 5905 5908 137 Wang CL, Zhang H, Zhang JH, Li MJ, Sun HZ, and Yang B 2007 Application of Ultrasonic Irradiation in Aqueous Synth esis of Highly Fluorescent CdTe/CdS Core Shell Nanocrystals J. Phys. Chem. C 111 2465 2469 138 Li L, Qian H and Ren J 2005 Rapid synthesis of highly luminescent CdTe nanocrystals in the aqueous phase by microwave irradiation with controllable temperature Chem Commun. 4 528 530 139 He Y, Sai LM, Lu HT, Hu M, Lai WY, Fan QL, Wang LH and Huang W 2007 Microwave Assisted Synthesis of Water Dispersed CdTe Nanocrystals with High Luminescent Efficiency and Narrow Size Distribution Chem. Mater. 19 359 365 140 Bao H, Gon g Y, Li Z, and Gao M 2004 Enhancement Effect of Illumination on the Photoluminescence of Water Soluble CdTe Nanocrystals: Toward Highly Fluorescent CdTe/CdS Core Shell Structure Chem. Mater. 16 3853 3859 141 Yu WW, Qu L, Guo W, and Peng X, 2003 Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals Chem. Mater. 15 2854 2860

PAGE 147

147 142 Ohata K, Saraie J and Tanaka T 1973 Phase Diagram of the CdS CdTe Pseudobinary System JJAP Vol. 12 (8) 1198 1204 143 Crooker SA, Hollingsworth JA, Tret iak S, and Klimov VI 2002 Spectrally Resolved Dynamics of Energy Transfer in Quantum Dot Assemblies: Towards Engineered Energy Flows in Artificial Materials Phys. Rev Lett. 89 (18) 186802 1 186802 4 144 Smith AM, Mohs AM and Nie S 2009 Tuning the optical and electronic properties of colloidal nanocrystals by lattice strain Nat. Nanotech. 4 56 63 145 Basilion JP, Yeon S and Botnar R 2005 Magnetic resonance imaging: Utility as a molecular imaging modality Curr. Top. Dev. Biol. 70 1 33 146 Janczewski D, Zhang Y, Das GK, Yi DK, Padmanavan P, Bhakoo KK, Tan TTY and Selvan ST 2011 Bimodal Magnetic Fluorescent Probes for Bioimaging Microscopy Research and Technique 74 563 567 147 Kim EJ and Batchelor B 2009 Synthesis and characterization of pyrite (FeS 2 ) using microwave irra diation Mater. Res. Bull. 44 1553 1558 148 Lee N, Kim H, Choi SH, Parka M, Kima D, Kim HC, Choib Y, Lin S, Kim BH, Jung HS, Kim H, Park KS, Moon WK, and Hyeon T 2011 Magnetosome like ferrimagnetic iron oxide nanocubes for highly sensitive MRI of single cells and transplanted pancreatic islets PNAS 108 2662 2667 149 Sun S, Zeng H, Robinson DB, Raoux S, Rice PM, Wang SH and Li G 2004 Monodisperse MFe 2 O 4 (M = Fe, Co, Mn) Nanoparticles J. Am. Chem. Soc. 126 273 279 150 Joshi HM, Lin YP, Aslam M, Prasad PV, Sikma EAS, Ede lman R, Meade T and Dravid VP 2009 Effects of Shape and Size of Cobalt Ferrite Nanostructures on Their MRI Contrast and Thermal Activation J. Phys. Chem. C 113 17761 17767 151 Li L, Pandey A, Werder DJ, Khanal BP, Pietryga JM, and Klimov VI 2011 Efficient Syn thesis of Highly Luminescent Copper Indium Sulfide Based Core/Shell Nanocrystals with Surprisingly Long Lived Emission J. Am. Chem. Soc. 133 1176 1179 152 www.sos.siena.edu /Chemistry/files/doc/Exp_6_Number_of_ Shells .doc

PAGE 148

148 B IOGRAPHICAL SKETCH Ajoy K Saha was born and brought up in K olkata, India and obtained his b c eramic t echnology in 1999 from the College of Ceramic Technology, University of Calcutta He then worked for a short while as an engineer in N itco Tiles, Mumbai. Later he joined Banaras Hindu University (BHU) at Varanasi and obtained his Master of Te chnology degree in ceramic e ngineering in 2002. Following this he arrived in the United State s of America to contin ue his education and completed lemson University in 2005. He joined UF in Fall 2006 to pursue a PhD deg ree in materials s cience and e ngineering and earned his degree in Summer 2011