SYNTHESIS OF CdTe QUANTUM DOTS AND STUDY OF THEIR PHOTO-

STABILITY AND THERMAL-STABILITY

A Thesis

Presented to

The Faculty of the Department of Chemistry

Sam Houston State University

In Partial Fulfillment

of the Requirements for the Degree of

Master of Science

by

Poorna Tharaka Wansapura

August, 2014

SYNTHESIS OF CdTe QUANTUM DOTS AND STUDY OF THEIR PHOTO-

STABILITY AND THERMAL-STABILITY

by

Poorna Tharaka Wansapura

APPROVED:

Dr. Thomas G. Chasteen Thesis Director

Dr. Richard E. Norman

Dr. Donovan C. Haines Approved: Dr. John B. Pascarella, Dean College of Sciences

ABSTRACT

Wansapura, Poorna Tharaka., Synthesis of CdTe quantum dots and study of their photo-stability and thermal-stability. Master of Science (Chemistry), August, 2014, Sam Houston State University, Huntsville, Texas.

Purpose

The purpose of this research was (1) to synthesize glutathione-capped cadmium

telluride quantum dots (QDs), (2) to determine the Cd and Te molar ratios in those QDs,

(3) to determine the effect of sonication on QDs in solutions, and (4) to determine the

photo-stability and thermal-stability of the GSH-capped CdTe QDs.

Methods

Glutathione-capped CdTe QDs were made synthetically using cadmium and

tellurium-containing salts in a buffer solution with glutathione as a reducing agent. Freshly

prepared QDs, QDs in citrate-borate buffer, and QDs in deionized water solutions were

stored in disposable, plastic-capped cuvettes and analyzed using fluorescence

spectrometry. QDs exposed to different times of sonication were examined in separate

experiments. Cadmium and tellurium elemental compositions were determined using

inductively coupled plasma atomic emission spectrometry (ICP-AES). Dried QDs, QDs in

buffer, and QDs in deionized water were stored at 4oC (under dark and light conditions),

at -80oC (under dark conditions), under sunlight, at room temperature (under dark and

light), and under 1200 lumens fluorescent light in replicate samples. Fluorescence and

absorbance spectrometric data were collected and analyzed periodically for 76 days. Before

and after degradation, sulfur contents were determined in QDs in deionized water,

supernatant and precipitate using ICP-AES.

iii

Findings

Glutathione-capped CdTe QDs were successfully synthesized in powder form and

in solutions. QDs degraded under the effects of sonication, and that degradation increased

with the sonication time. The Cd:Te molar ratio in QDs is approximately 2:1, but grows

slightly with incubation time. Glutathione-capped CdTe QDs undergo thermal and

photodegradation. QDs are stable when they are stored in dried powder form, showing no

decrease in fluorescence peak emission over 76 days. If QDs are resuspended in deionized

water or citrate-borate buffer and stored in low temperatures (-80oC) and in low light

intensities (dark conditions), they are more stable than at higher temperatures (25oC) and

in higher light intensities (~70% drop in peak fluorescence over 76 days at -80oC). QD

photodegradation is associated with the loss of sulfur from GSH-capped QDs.

KEY WORDS: Nanoparticles, Fluorescence Spectrophotometry, Photodegradation, Nanocrystals.

iv

ACKNOWLEDGMENTS

First of all, I would like to thank my research supervisor, Dr. Thomas G. Chasteen

for the continuous support for my research, for his patience, motivation, enthusiasm, and

immense knowledge. He has truly been an amazing mentor throughout my studies at Sam

Houston State University. I would also like to acknowledge Dr. Richard E. Norman and

Dr. Donovan C. Haines for their support and guidance in my research and thesis writing.

I would like to thank Chasteen’s research group - Desiré Lopez, Gayan Adikari,

Sajini Hettiarachchi and Harshani Rathnaweera. Their help and assistance in the laboratory

was really beneficial for my research. I would also like to acknowledge the financial,

academic and technical support of the Department of Chemistry and its staff. In addition,

I would like to thank Dr. José Manuel Pérez Donoso and Dr. Waldo A. Dıáz-Vásquez,

Microbiology and BioNanotechnology Research Group, Universidad de Santiago de Chile;

and Dr. Rachelle Smith, and the staff of the Texas Research Institute for Environmental

Studies (TRIES).

Most of all, I would like to thank specially for my parents and my wife Iresha

Premarathna. Without their endless love, support, and encouragement, I would not be

where I am today.

v

TABLE OF CONTENTS

Page

ABSTRACT ....................................................................................................................... iii

ACKNOWLEDGMENTS .................................................................................................. v

TABLE OF CONTENTS ................................................................................................... vi

LIST OF FIGURES ......................................................................................................... viii

CHAPTER

I INTRODUCTION .................................................................................................. 1

Quantum dots .......................................................................................................... 1

Cadmium (Cd) ........................................................................................................ 1

Tellurium (Te) ......................................................................................................... 2

CdTe quantum dots (CdTe QDs) ............................................................................ 2

Synthesis of CdTe QDs ........................................................................................... 3

Stability study of CdTe QDs ................................................................................... 4

II MATERIALS AND METHODS ............................................................................ 5

Synthesis of CdTe QDs ........................................................................................... 5

Synthesizing QDs.................................................................................................... 5

Collecting powdered QDs ....................................................................................... 6

Determination of the effect of sonication on QDs in solution ................................ 7

Determination of the Cd and Te molar ratios ......................................................... 7

Stability study of the QDs ....................................................................................... 7

Determination of the sulfur content ........................................................................ 9

III RESULTS ............................................................................................................. 10

vi

Synthesis of CdTe QDs ......................................................................................... 10

Fluorescence determination .................................................................................. 11

Cd and Te molar ratio ........................................................................................... 14

Stability study of the QDs ..................................................................................... 14

Light intensity of storage locations ....................................................................... 25

Determination of the effect of sonication on QDs in solution .............................. 27

Determination of the sulfur content ...................................................................... 27

IV DISCUSSION ....................................................................................................... 29

Synthesis of CdTe QDs ......................................................................................... 29

Cd and Te molar ratio ........................................................................................... 30

Stability study of the QDs ..................................................................................... 30

Determination of the effect of sonication on QDs in solutions ............................ 32

Determination of the sulfur content ...................................................................... 32

V CONCLUSIONS................................................................................................... 35

Synthesis of CdTe QDs ......................................................................................... 35

The effect of sonication on QDs in solutions........................................................ 35

Cd:Te molar ratio and stability study of the QDs ................................................. 35

BIBLIOGRAPHY ............................................................................................................. 37

APPENDIX A ................................................................................................................... 41

VITA ................................................................................................................................. 42

vii

LIST OF FIGURES

FIGURE Page

1 Freshly prepared QDs and resuspended QDs in buffer solutions (1-6 hours

incubated) under room light. ..................................................................................10

2 Fluorescence under (302 nm) transilluminator. A-Freshly prepared QDs (0-6

hours incubated). B-After the precipitation process. C-After the drying process.

D-After resuspension in buffer solution. ...............................................................11

3 The fluorescence emission spectra of freshly prepared QDs and resuspended

QDs. .......................................................................................................................12

4 The emission and excitation spectra of freshly prepared QDs and resuspended

QDs. .......................................................................................................................13

5 Dried QDs incubated at 90oC for 2 hours photographed under room light. ..........15

6 Dried QDs incubated at 90oC for 2 hours photographed on the transilluminator

(302 nm). ................................................................................................................15

7 Photographs of QDs under visible and UV light conditions in citrate-borate

buffer, imaged periodically for 76 days. ................................................................17

8 Fluorescence intensity (ex: 375 nm / em: 510 nm) vs time for QDs in citrate-

borate buffer measured periodically for 76 days. ..................................................18

9 Absorbance (475 nm) vs time for QDs in citrate-borate buffer measured

periodically for 76 days. ........................................................................................19

10 Photographs of QDs under visible and UV light conditions in deionized water,

imaged periodically for 76 days. ............................................................................20

viii

11 Fluorescence intensity (ex: 375 nm / em: 510 nm) vs time for QDs in deionized

water measured periodically for 76 days. ..............................................................21

12 Absorbance (475 nm) vs time for QDs in deionized water measured periodically

for 76 days. ............................................................................................................22

13 Photographs of dried powdered QDs under visible and UV light conditions in

deionized water, imaged periodically for 76 days. ................................................23

14 Fluorescence intensity (ex: 375 nm / em: 510 nm) vs time for dried QDs in

deionized water measured periodically for 76 days. ..............................................24

15 Absorbance (475 nm) vs time for dried, powdered QDs resuspended in deionized

water measured periodically for 76 days. ..............................................................25

16 Light intensity at all storage locations vs time. (Lab: under lab/room light;

Window: against a glass, outside laboratory window; Bulb: under 1200 lumens

fluorescent bulb; 4C light: in 4oC glass refrigerator.) ............................................26

17 Light intensity vs time at three storage locations. (Lab: under lab/room light;

Bulb: under 1200 lumens fluorescent bulb; 4C light: in 4oC glass refrigerator.) ..26

18 The effect of sonication on QDs in solution with time. .........................................27

19 Structure of Glutathione (GSH) .............................................................................33

ix

1

CHAPTER I

INTRODUCTION

Quantum dots

Quantum dots (QDs) are nanoparticles (NPs) which are made by chalcogenides

such as selenides, tellurides, and by sulfides of metals such as cadmium, zinc and copper.

The diameter of a quantum dot is in the range of 2-10 nanometers. These NPs were

discovered by Alexei Ekimov and by Louis E. Brus in the early 1980s (Ekimov and

Onushchenko, 1981). Mark Reed was the one who named these nanoparticles as quantum

dots (Reed et al., 1988; Murray et al., 2000). These NPs absorb excitation light at a specific

wavelength and emit light at a longer wavelength (Pérez-Donoso et al., 2012). Since the

nanoparticles used in this study are CdTe, I’ll first give a little background on these

elements.

Cadmium (Cd)

Cadmium was discovered in 1818 by German scientists Friedrich Stromeyer and

Karl Samuel Leberecht Hermann (Eedle, 2011). The most common oxidation state is +2

and a very few compounds have been reported in which cadmium exists in the +1 oxidation

state. Cadmium has a low melting point compared to other d-block metals and the

concentration in the earth’s crust is 0.1 ppm. Greenockite (CdS) is the most common

cadmium containing mineral found in the earth’s crust. There are eight naturally occurring

cadmium isotopes: 113Cd, 12.22% and 116Cd, 7.49% are the two naturally radioactive

isotopes; the others are 106Cd, 1.25%; 108Cd, 0.89%; 110Cd, 12.49%; 111Cd, 12.80%; 112Cd,

24.13%; and 114Cd, 28.73%. Elemental cadmium is insoluble in water. Cadmium dissolved

in hydrochloric acid, sulfuric acid and nitric acid can form cadmium chloride (CdCl2),

2

cadmium sulfate (CdSO4), and cadmium nitrate (Cd(NO3)2), respectively. Cadmium is

resistant to corrosion and has no known biological function in higher organisms (Weast,

1975; Lane and Morel, 2000; Wedepohl, 1995). Some activated Cd compounds, such as

CdS are stable under photochemical conditions. Extremely intense laser beams can

decompose Cd compounds into elemental Cd (Spanhel et al., 1987). CdSe/CdS core/shell

nanocrystals show great stability under chemical, thermal and photochemical conditions

(Guo et al., 2003).

Tellurium (Te)

Tellurium was discovered in 1782 by Franz-Joseph Müller von Reichenstein

(Eedle, 2011). The tellurium isotopes are 130Te, 34.48%; 128Te, 31.79%; 126Te, 18.71%;

125Te, 6.99%; 124Te, 4.61%; 122Te, 2.55%; and 123Te, 0.87%. Common oxyanions of Te are

tellurite (TeO32-) and tellurate ([TeO2(OH)4]2-). Tellurate is slightly soluble in water.

Calaverite (AuTe2) is the most common mineral which contains Te. Crystalline tellurium

is silvery-white in color. Tellurium is a p-type semiconductor and can dope with other

elements. Tellurium is used in ceramics, fuel cells and solid state devices. (Weast, 1975).

Some selenium and tellurium compounds are unstable under photochemical and thermal

conditions (Piers et al., 1994).

CdTe quantum dots (CdTe QDs)

During the past two decades CdTe quantum dots have been used for making solar

cells (Bang and Kamat, 2009; Kongkanand et al., 2008), photonic crystal devices (such as

light-emitting diodes) (Faraon et al., 2007), and as fluorescent probes in cell-imaging

(Hoshino et al., 2004; Chang et al., 2008). QDs have important advantages over organic

fluorophores. They have unique characteristics which are useful for industrial and

3

analytical chemistry-related research: narrow emission spectra, reasonable chemical

stability, controllable spectroscopic properties, photochemical stability, high quantum

yields, and the ability to provide specific color fluorescence with a specific single

excitation wavelength (Pérez-Donoso et al., 2012).

Synthesis of CdTe QDs

QDs are synthesised by different methods: electron beam irradiation (Li et al.,

2011), polyol-hydrolysis (Xin et al., 2011), microwave-assisted aqueous synthesis (Yang

et al., 2013), photochemical synthesis (Liu et al., 2012), UV-irradiation (Chang et al., 2008)

or chemical precipitation (Lang et al., 2011). The two main synthetic methods for QDs are

organometallic synthesis (Ehlert et al., 2008) and hydrothermal synthesis (Yu et al., 2012).

CdTe QDs have mostly been produced by organic synthetic procedures, using organic

solvents (Rogach et al., 2007; Chang et al., 2008; Talapin et al., 2001). Recently, several

aqueous syntheses were discovered (Bao et al., 2006; He et al., 2006). CdTe QDs have

poor biocompatibility in biological systems (Chang et al., 2008; Lovric et al., 2005;

Schneider et al., 2009). Therefore, aqueous synthesis of CdTe QDs capped with different

thiols has been used to create QDs which have high stability and biocompatibility with

biological systems (Schneider et al., 2009; Gaponik et al., 2002; Yu et al., 2012; Monrás

et al., 2012; Rogach et al., 2007; Pérez-Donoso et al., 2012).

CdTe QDs can be synthesized by using thioglycolic acid (Tian et al., 2009),

glutathione (Zheng et al., 2007), cysteine (Bao et al., 2006) and mercaptosuccinic acid

(Ying et al., 2008). Synthesis of QDs via aqueous methods produces less toxic pollutants

compared to other synthetic methods and also supplies many different types of QDs with

high luminescence: CdTe, HgTe, ZnSe, CdHgTe (Gaponik et al., 2002; Rogach et al.,

4

2007). Aqueous inorganic synthesis of glutathione (GSH) capped CdTe was discovered

and developed recently by Chilean researchers (Pérez-Donoso et al., 2012). This inorganic

synthetic method can be used to synthesize CdTe-GSH quantum dots in solution and via

precipitation to a powder form without using high temperatures or high pressure conditions.

Stability study of CdTe QDs

Understanding the stability of CdTe QDs is important because of their wide range

of applications. The fluorescence properties of QDs can be varied under different

conditions such as temperature and absorbed light intensity (Yu et al., 2012). This current

research is designed to synthesize CdTe QDs using inorganic synthetic procedures (Pérez-

Donoso et al., 2012) and to study their stability under a variety of storage conditions

involving different light intensities and temperatures. The primary goal is to determine

whether CdTe QDs can be synthesized, precipitated from solution, stored in a powder form,

and reused for scientific research. The ultimate goal is to determine the optimal conditions

for storing CdTe QDs for further experiments, which is important for various industries

and for research. As these QDs emit light during excitation, fluorescence spectroscopic

methods were used to analyze freshly-prepared QDs and powdered QDs. Inductively

coupled plasma atomic emission spectroscopic methods were used for analyzing the Cd to

Te ratio in QDs (Pérez-Donoso et al., 2012; Montes, 2012). QDs can undergo degradation

under sonication conditions, variation of temperature, time (76 days) and under sunlight.

These conditions were tested in this research and optimal conditions for storing CdTe-GSH

QDs for future research were found. Fluorescence spectroscopic methods and UV

absorbance spectroscopic methods were used for degradation studies by analyzing the

absorption and fluorescence spectra of QDs stored under different conditions.

5

CHAPTER II

MATERIALS AND METHODS

Synthesis of CdTe QDs

Reagents

Reagents used in these experiments included: potassium tellurite (Sigma-Aldrich,

St. Louis, MO, USA), cadmium chloride (Sigma-Aldrich), sodium citrate

(Na3C6H5O7.2H2O - Fisher Scientific, Fair Lawn, NJ, USA), sodium borate

(Na2[B4O5(OH)4].8H2O - Sigma-Aldrich), L-glutathione reduced (GSH) (Sigma-Aldrich)

and ethyl alcohol (Pharmco, Brookfield. CT, USA).

Equipment and instrumentation

Equipment used in these experiments included: RiOs 3 water purification system

from Millipore for water deionization (Billerica, MA, USA), sonicator (Branson 2510,

Danbury, CT, USA), and UV transilluminator (UVP high performance 2UVTM

transilluminator, Upland, CA, USA).

Instruments used include: UV/vis spectrophotometer (Jasco, V-550, Easton, MD,

USA), fluorescence spectrophotometer (Hitachi, F-4500, Schaumburg, IL, USA)

inductively coupled plasma atomic emission spectrometer (ICP-AES) (Spectro CIROS

Vision ICP-OES, Mahwah, NJ, USA) and light meter (Control Company, Friendswood,

TX, USA).

Synthesizing QDs

QDs were synthesized using the procedure of Pérez-Donioso et al., (2012). This

involved reducing tellurium oxyanions with GSH in the presence of cadmium cations in a

6

buffer solution. Stock solutions of GSH (30 mM in deionized water), citrate-borate buffer

at pH ~ 9 (30 mM in deionized water), cadmium chloride (25 mM CdCl2 in deionized

water) and potassium tellurite (7 mM K2TeO3 in deionized water) were prepared. Citrate-

borate buffer was prepared by using sodium citrate (Na3C6H5O7.2H2O) and sodium borate

(Na2[B4O5(OH)4].8H2O). To prepare the buffer 0.441 g of sodium citrate and 0.572 g of

sodium borate were dissolved in 50 ml of deionized water and pH was checked by pH

meter. The concentrations of the sodium citrate (30 mM) and sodium borate (30 mM) were

the same. To a test tube 1.60 mL of 25 mM CdCl2, 3.64 mL of 30 mM citrate-borate

solution (buffer) and 3.33 mL of 30 mM GSH were added and then vortexed vigorously

and subsequently allowed to sit at room temperature for 5 minutes. Below the surface of

the vortexed solution, 1.43 mL of 7 mM K2TeO3 was added slowly via micropipette. The

final ratio of CdCl2: GSH: K2TeO3 in this synthetic mixture was 4:10:1. The reaction flask

was then placed in a 90oC hot water bath to initiate nucleation. Samples were taken out for

analysis at one hour time intervals from the start of the reaction (incubation time at 90oC,

1 hour to 6 hours), analyzed using fluorescence spectrometry, and checked by UV

transilluminator illuminated at 302 nm (Pérez-Donoso et al., 2012).

Collecting powdered QDs

Synthesized QDs in aqueous solution were precipitated with two volumes of

ethanol (200 proof) and centrifuged for 20 minutes at 12000 rpm (Pérez-Donoso et al.,

2012). Powdered QDs were dried for 24 hours in a desiccator over calcium chloride.

Dried, powder CdTe QDs (incubation time 1 hour to 6 hours) were dissolved in

citrate-borate buffer solution (QDs in buffer), analyzed using fluorescence spectrometry,

and checked by UV transilluminator.

7

Determination of the effect of sonication on QDs in solution

Freshly prepared QDs (fresh QDs), QDs in citrate-borate buffer, and QDs in water

solutions were stored in disposable, plastic-capped cuvettes and sonicated using 40 kHz

frequency at different time intervals (1 to 80 minutes, at 25oC) and analyzed using

fluorescence spectrometry.

Determination of the Cd and Te molar ratios

Dried QDs were dissolved in 10% nitric acid, and cadmium and tellurium elemental

composition were determined using ICP-AES (Montes, 2012). Standard Cd solutions (SCP

Science, Champlain, NY, USA) and standard Te solutions (PEAK PERFORMANCE CPI

International, Santa Rosa, CA USA) were used for calibration. Analytical wavelengths for

Cd and Te were 228.802 nm and 214.281 nm respectively. The nitric acid content of all

samples was ten percent.

Stability study of the QDs

For the stability studies, QDs incubated at 90oC for 2 hours were used. Dried QDs,

QDs in citrate-borate buffer, and QDs in water were stored at 4oC (under dark and light

conditions), at -80oC (under dark conditions), under sunlight (against a glass window;

AFGD H ASTM E774 84 A Class A glass), at room temperature conditions (under dark

and light), and under a 1200 lumens fluorescent bulb in replicate samples. To make

solutions of QDs for storage, 2.5 mg of dried QDs were re-suspended in 2.5 mL of

deionized water/citrate-borate buffer and stored in disposable, plastic-capped cuvettes

(BRAND, Wertheim, Germany). Empty, disposable, plastic-capped cuvettes were also

stored under the above mentioned conditions as controls. Dried, powdered QDs not in

solution were also stored in the same storage locations (2.5 mg in plastic-capped cuvettes).

8

These solids were daily shaken for 5 seconds to redistribute the NPs in the containers. In

order to collect spectrometric data, 2.5 mg of stored, dried QDs were resuspended in

deionized water and analyzed. One cuvette (from each location) with dried QDs was

analyzed for each day. After analysis, the QDs were discarded. A total of 126 cuvettes with

dried QDs were analyzed. Fluorescence and absorbance spectrometric data were collected

and analyzed periodically for 76 days. Light intensities of each storage location were

measured using a light meter. List of the samples are mentioned below.

QDs in buffer LB = Room temp: in the lab SB = Room temp: under sunlight BB = Room temp: under 1200 lumens bulb 4dB = 4oC Under dark 4LB = 4oC Under room light / dark -80dB = -80oC Under dark LdB = Room temp: under dark QDs in water LW = Room temp: in the lab SW = Room temp: under sunlight BW = Room temp: under 1200 lumens bulb 4dW = 4oC Under dark 4LW = 4oC Under room light / dark -80dW = -80oC Under dark LdW = Room temp: under dark Dried powdered QDs in water LN = Room temp: in the lab SN = Room temp: under sunlight BN = Room temp: under 1200 lumens bulb 4dN = 4oC Under dark 4LN = 4oC Under room light / dark -80dN = -80oC Under dark LdN = Room temp: under dark

9

Determination of the sulfur content

QDs incubated for 2 hours at 90oC were synthesized and sulfur content measured

by using ICP-AES as described below for fresh QDs, QDs degraded under sunlight, and

the supernatant from samples of sunlight-degraded QDs: dried QDs were resuspended in

deionized water and concentrated nitric acid was added to give a final concentration of

10% nitric acid. Dried QDs from the same sample were resuspended in deionized water

and kept under sunlight (against a glass window) for 96 hours. These degraded samples

were centrifuged for 20 minutes at 12000 rpm. Sulfur content in the supernatants—with

10% HNO3—were measured. Precipitates were collected and dissolved in 10% nitric acid

and sulfur contents were measured. The analytical wavelength for sulfur analysis was

180.731 nm. ICP-AES calibration standards were prepared by using single elemental sulfur

standards (PEAK PERFORMANCE CPI International) in 10% nitric acid solutions.

10

CHAPTER III

RESULTS

Synthesis of CdTe QDs

Freshly prepared QDs and resuspended QDs in buffer solutions showed the same

color intensities under room light. There was no significant visual difference between the

two solutions (Figure 1).

Figure 1. Freshly prepared QDs and resuspended QDs in buffer solutions (1-6 hours incubated) under room light.

Freshly prepared QDs, resuspended QDs in buffer solutions, precipitated QDs and

dried QDs were visually checked for fluorescence. QD solutions and powders were excited

at 302 nm under the transilluminator. Eight replicates were checked.

11

Fluorescence determination

Figure 2. Fluorescence under (302 nm) transilluminator. A-Freshly prepared QDs (0-6 hours incubated). B-After the precipitation process. C-After the drying process. D-After resuspension in buffer solution.

The fluorescence spectra of QDs incubated for 1 to 6 hours were recorded. Figures

3-4 show the fluorescence spectra using 25 nm increments in excitation wavelengths and

10 nm increments in emission wavelengths (excitation 200-600 nm, emission 450-600 nm).

0h 1h 2h 3h 4h 5h 6h

1h 2h 3h 4h 5h 6h

1h 2h 3h 4h 5h 6h

12

Figure 3. The fluorescence emission spectra of freshly prepared QDs and resuspended QDs.

13

Figure 4. The emission and excitation spectra of freshly prepared QDs and resuspended QDs.

Emission and Excitation Scan-Fresh- QDs

Emission and Excitation Scan-Resuspended QDs

14

Cd and Te molar ratio

Table 1 shows the Cd/NP weight ratios, Te/NP weight ratios and Cd/Te molar ratios

which were determined using ICP-AES. The dried samples were weighed on a four place

balance.

Table 1

Summary of Cd/NP and Te/NP weight ratio and molar ratio. Standard deviations are for four replicates.

Incubation time (hours) Cd/NP (w/w) Te/NP (w/w) Cd/Te (molar ratio)

1 0.309 ± 0.016 0.169 ± 0.005 2.079 ± 0.091

2 0.328 ± 0.028 0.176 ± 0.006 2.115 ± 0.143

3 0.349 ± 0.050 0.186 ± 0.032 2.141 ± 0.138

4 0.357 ± 0.044 0.188 ± 0.018 2.146 ± 0.099

5 0.377 ± 0.028 0.198 ± 0.014 2.163 ± 0.072

6 0.380 ± 0.048 0.198 ± 0.025 2.180 ± 0.066

Stability study of the QDs

Photographs of the same sample of QDs (incubated 2 hours (at 90oC)) are shown

in Figures 5 and 6, the first under room light and the second on the transilluminator.

15

Figure 5. Dried QDs incubated at 90oC for 2 hours photographed under room light.

Figure 6. Dried QDs incubated at 90oC for 2 hours photographed on the transilluminator (302 nm).

Fluorescence and absorbance spectrometric data were collected periodically over

76 days for dried QDs (resuspended in DI water), QDs in citrate-borate buffer, and QDs in

water stored at different light and temperature conditions. Photographs of samples of QDs

16

under visible and UV light in citrate-borate buffer in plastic cuvettes are shown in Figure

7.

17

Figure 7. Photographs of QDs under visible and UV light conditions in citrate-borate buffer, imaged periodically for 76 days.

18

Changes in the intensity of peak fluorescence for solutions of QDs resuspended in

buffer under multiple light and temperature conditions were followed in time course

experiments. Fluorescence intensity (excitation 375 nm and emission 510 nm) vs time

(days) data for QDs in citrate-borate buffer are shown in Figure 8.

Figure 8. Fluorescence intensity (ex: 375 nm / em: 510 nm) vs time for QDs in citrate-borate buffer measured periodically for 76 days.

Absorbance (at 475 nm) vs time graph for QDs in citrate-borate buffer is shown in

Figure 9.

0

100

200

300

400

500

600

700

0 20 40 60 80

Fluo

resc

ence

Inte

nsity

( ex:

375

nm /

em: 5

10nm

)

Time (days)

LBSBBB4dB4LB80dBLdB

19

Figure 9. Absorbance (475 nm) vs time for QDs in citrate-borate buffer measured periodically for 76 days.

Photographs of samples under visible and UV light of QDs in deionized water are

shown in Figure 10.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80

Abs

orba

nce

(475

nm)

Time (days)

LBSBBB4dB4LB80dBLdB

20

Figure 10. Photographs of QDs under visible and UV light conditions in deionized water, imaged periodically for 76 days.

21

Fluorescence intensity (at excitation 375 nm and emission 510 nm) vs time (days)

data for QDs in deionized water are shown in Figure 11.

Figure 11. Fluorescence intensity (ex: 375 nm / em: 510 nm) vs time for QDs in deionized water measured periodically for 76 days.

Absorbance (at 475 nm) vs time data for QDs in deionized water are shown in

Figure 12.

0

100

200

300

400

500

600

700

800

0 20 40 60 80

Fluo

resc

ence

Inte

nsity

( ex

: 375

nm /

em: 5

10nm

)

Time (days)

LWSWBW4dW4LW80dWLdW

22

Figure 12. Absorbance (475 nm) vs time for QDs in deionized water measured periodically for 76 days.

Figure 13 shows photographs of samples, under visible and UV light, of powdered

QDs resuspended in deionized water.

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80

Abs

orba

nce

(475

nm)

Time (days)

LW

SW

BW

4dW

4LW

80dW

LdW

Due to black color,

degraded particles

23

Figure 13. Photographs of dried powdered QDs under visible and UV light conditions in deionized water, imaged periodically for 76 days.

24

Fluorescence intensity (at excitation 375 nm and emission 510 nm) vs time (days)

data for dried, powdered QDs in deionized water are shown in Figure 14.

Figure 14. Fluorescence intensity (ex: 375 nm / em: 510 nm) vs time for dried QDs in deionized water measured periodically for 76 days.

Absorbance (at 475 nm) vs time data for dried, powdered QDs resuspended in

deionized water are shown in Figure 15.

0

100

200

300

400

500

600

700

800

0 20 40 60 80

Fluo

resc

ence

Inte

nsity

( e

x: 3

75nm

/ em

: 510

nm )

Time (days)

LNSNBN4dN4LN80dNLdN

25

Figure 15. Absorbance (475 nm) vs time for dried, powdered QDs resuspended in deionized water measured periodically for 76 days.

Light intensity of storage locations

Light intensities of each location where QDs were stored were measured by a light

meter (in daylight mode—wavelength range not available). These data were recorded (on

August to October in 2013) in the late summer through early fall. Light intensity vs time

(24 hours) data are shown in Figure 16. Error bars represent the standard deviation of four

replicates in Figures 16 and 17.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80

Abs

orba

nce

(475

nm)

Time (days)

LNSNBN4dN4LN80dNLdN

26

Figure 16. Light intensity at all storage locations vs time. (Lab: under lab/room light; Window: against a glass, outside laboratory window; Bulb: under 1200 lumens fluorescent bulb; 4C light: in 4oC glass refrigerator.)

Under “window” showed low light intensities after 16:30 h. Under “Lab”, “Bulb”

and “4C light” also showed low intensities throughout the experiment. Light intensities vs

time for “Lab”, “Bulb” and “4C light” are shown in Figure 17 with a zoomed y axis.

Figure 17. Light intensity vs time at three storage locations. (Lab: under lab/room light; Bulb: under 1200 lumens fluorescent bulb; 4C light: in 4oC glass refrigerator.)

0

20000

40000

60000

80000

100000

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Ligh

t int

ensi

ty (L

ux)

Time (24 hours)

LabWindowBulb4C light

0200400600800

100012001400

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Ligh

t int

ensi

ty (L

ux)

Time (24 hours)

LabBulb4C light

27

Determination of the effect of sonication on QDs in solution

Degradation of resuspended QDs by sonication experiments were carried out using

40 kHz frequency at 25oC. The changes of the fluorescence intensities with the sonication

time of freshly prepared QDs, QDs in citrate-borate buffer, and QDs in water solutions are

shown in Figure 18.

Figure 18. The effect of sonication on QDs in solution with time.

Determination of the sulfur content

Sulfur content determinations of QDs (incubated 2 hours at 90oC) are summarized

in Table 2 before and after the degradation caused by exposure to sunlight for 96 hours

while in solution. Nanoparticle sulfur content represents the weight ratio of sulfur to NP

(w/w). Sulfur percentage represents the percentage of sulfur in NPs by mass.

0

100

200

300

400

500

600

700

800

0 10 20 30 40 50 60 70 80 90

Fluo

resc

ence

Inte

nsity

(e

x: 3

75 n

m /

em: 5

10 n

m)

Time (min)

Freshly Prepared QDsQDs in BufferQDs in Deionized water

28

Table 2

Summary of ICP-AES analysis for sulfur before and after the degradation process. Standard deviations are for five replicates.

Step Sulfur content S/NP (w/w) Sulfur percentage by mass of NP

Before degradation 0.152 ± 0.049 —

After degradation, in supernatant 0.110 ± 0.037 72

After degradation, in precipitate 0.026 ± 0.012 17

The percent recovery (ratio of sulfur content in supernatant and precipitate after

the degradation to initial sulfur content in NPs) was approximately 89%.

29

CHAPTER IV

DISCUSSION

Synthesis of CdTe QDs

In these experiments, a synthetic procedure, which was developed by our Chilean

collaborators, Pérez-Donoso et al., was used for synthesizing CdTe QDs. GSH was used

as the reducing agent. GSH, even in its reagent bottle, reacts with atmospheric oxygen very

quickly. Therefore, all solutions which were used for this method were freshly prepared

every day before the experiments, and newly-opened GSH bottles were used for the

preparation of GSH solutions.

Results (Figures 1 and 2) show freshly prepared QDs and resuspended QDs in

buffer after the precipitation process, which were visually the same under both room light

and UV light (302 nm). Also fluorescence spectra showed (Figures 3 and 4) the same

characteristics (excitation and emission wavelengths) in both solutions. But fluorescence

intensities were lower ( ̴ 8% on average) after the precipitation process. That could be due

to sample loss, which can occur during the precipitation and collection process, because

the number/mass of NPs is proportional to the fluorescence intensity.

Data in Figures 1 to 4 showed the typical characteristics of the GSH-capped CdTe

QDs (Pérez-Donoso et al., 2012). As the incubation time increases, the particle size

increases. Therefore, excitation wavelengths and emission wavelengths also shifted to

longer wavelengths. Colors of the QDs under room light visually changed from yellow to

light orange and under the UV light, fluorescence emission visually changed from yellow

to light green as incubation times grew from 1 to 6 hours. Fluorescence data also showed

maximum excitation wavelengths changed from 360 nm to 450 nm and maximum emission

30

wavelengths changed from 490 nm to 560 nm with longer incubation times. When the size

of the QD increases, the energy gap between the ground state energy level and excited state

energy level decreases. Therefore larger QDs showed emission at longer wavelengths (Bao

et al., 2006; Chang et al., 2008; Gaponik et al., 2002).

Cd and Te molar ratio

As QD incubation time increased, Cd and Te content (element mass per NP mass)

increased in the nanoparticles forming in the hot synthetic mixture (Table 1). The Cd/Te

ratio of QDs also increased with the incubation time. ICP-AES data showed that the Cd:Te

molar ratio is approximately 2:1, but grows slightly with incubation time (Pérez-Donoso

et al., 2012; Wang and Liu, 2012.; Montes, 2012).

Stability study of the QDs

Two hour incubated QDs were used for the stability study (Figures 5 and 6) because

they showed a fair amount of fluorescence intensity—useful for the stability study—and

could be easily synthesized within 2 hours. Intensities of fluorescence emission at 510 nm

(excitation at 375 nm) were collected from the fluorescence spectra because that emission

wavelength had the highest intensity for the green QDs, with the least contribution from

particle scattering. Maximum absorbance was observed at 475 nm for these nanoparticles.

Fluorescence and absorbance spectrometric spectra of empty plastic-capped

cuvettes (controls) did not show any significant differences during the experiments (0 to

76 days). Therefore the changes of the NP spectra over time were due to degradation of

QDs not changes in their storage cuvettes.

Among the three sets of samples, the dried, powdered QDs (Figures 13 to 15)

showed the most stability under different light intensities (~ 0-60000 Lux) and different

31

temperatures (-80oC to 25oC). Dried, powdered QDs have less surface area exposed to light

when compared to the other two sample sets (both QDs in solution). Due to its dried, solid

form, these QDs apparently also had less possibility to react with other chemical substances

too even given 5 second, periodic shaking (daily) during storage. CdTe QDs can react with

small amounts of dissolved oxygen in solutions. Te2- ions can oxidize and form Te-

containing oxyanions (Ribeiro et al., 2013) and surface-bound thiols can oxidize to the

disulfide (Gaponik et al., 2002).

Both QDs in buffer and deionized water showed a similar decreasing pattern in

fluorescence intensities and absorbance at 475 nm. Data confirmed that QDs in storage

locations which had low temperatures and low light intensities had higher stability (Figures

7 to 12). Among the seven storage locations, QDs in the -80oC refrigerator in the dark were

the most stable and QDs in the sunlight (against the glass window) were the least stable.

Fluorescence intensities of the QDs decreased 90% within 3 days in QDs which were stored

in sunlight. QDs in solutions (buffer or water) have higher surface areas than dried,

powdered QDs, and therefore more contact with sunlight. Liquid solutions, therefore,

provide a good reaction medium for the degradation. Both QD liquid storage experiments

showed that fluorescence intensity dropped by 50% within 10 days. All of the above data

confirmed that QDs degraded faster under higher light and warmer conditions.

Figures 16 and 17 confirmed that the different locations have different light

intensities during the day. Between direct sunlight and light intensity against the glass

window, there was a ̴ 30000 Lux difference during the day. Therefore, if QDs are stored

in sunlight, we can confirm substantial degradation if the QDs are suspended in water or

buffer.

32

Determination of the effect of sonication on QDs in solutions

Analysis of the data confirmed that QDs in solutions showed degradation during

the sonication process and the degradation increased with sonication time (Figure 18). QDs

in water and the buffer showed a lower degradation rate compared to freshly prepared QDs

taken before the precipitation process still in the synthetic mixture, a solution that contained

unreacted CdCl2, GSH, K2TeO3, and buffer. Therefore freshly prepared QDs solutions had

more ions (a higher ionic content) compared to QDs resuspended in water or buffer, and

therefore collisions and interactions of ions with NPs would be higher in freshly prepared

QDs and that might reasonably cause faster degradation of QDs under sonication, and

therefore rapid decrease of the fluorescence intensity. Freshly-prepared QDs show 17%

lower fluorescence after only 1 min of sonication and a continuous fluorescence drop

throughout the entire sonication experiment.

Determination of the sulfur content

Pérez-Donoso and coworkers showed that CdTe QDs synthesized using this recipe

were capped with glutathione (Pérez-Donoso et al., 2012). GSH contains thiol groups,

which they proposed reacted with Cd on the CdTe nanoparticle surface. Using IR

spectroscopy they reported the disappearance of the S–H vibrational modes seen in GSH

solutions when GSH-capped CdTe NPs were examined. This, they proposed, indicated the

formation of Cd-S bonds between GS- and Cd (Pérez-Donoso et al., 2012). Our

experiments tracing sulfur composition in freshly-prepared NPs and in those particles after

degradation (and in their supernatant) were an effort to preliminarily determine what

happened during the photodegradation of GSH-capped CdTe nanoparticles.

33

Figure 19. Structure of Glutathione (GSH)

The sulfur content experiments (Table 2) confirmed that sulfur was removed from

NPs during the photodegradation process: ~72% of the original sulfur detected in freshly-

prepared NPs was found in the supernatant and ~17% was found in the centrifuged

precipitate after NPs in suspension were exposed to sunlight for 96 hours. Sulfur released

from GSH-capped NPs by photodegradation can be dissolved in the supernatant in

principle as ions or oxidized glutathione fragments (glutathione disulfide, GSSG), and in

the spun down precipitate as Cd-SG, CdS, CdSO3, CdSO4, etc. While the results showed

that most of the original NP sulfur ended up dissolved in the supernatant, the percent

recovery of all sulfur for that experiment was ~89%. That relatively low rate of recovery

can reasonably be due to small sample losses, which could occur during the precipitation

and collection process given the low masses involved (~10 mg for each run in Table 2,

n=5). Therefore the photodegradation of GSH-capped CdTe QDs examined in this study

could reasonably be viewed as the loss of S-containing compounds initially from the

surface of sunlight-exposed QDs and subsequent decomposition of the exposed QDs no

longer protected by surface capping. GSH capping is important for the thermal, chemical,

and photostability of the GSH-capped CdTe QDs (Hardman, 2006; Gaponik et al., 2002;

Fang et al., 2010) and others have chosen GSH-capping to improve biocompatibility and

decrease toxicity in NP bio-applications (Yu et al., 2012; Liu and Yu, 2010). As the

34

photographs in Figures 7 and 10 show for sunlight-exposed QDs in solution (samples

labeled SB and SW), degraded QDs increasingly settled at the bottom of the cuvette soon

after shaking, and their supernatants did not exhibit green fluorescence. Compare this to

the resuspended samples of dried, powered QDs in Figure 13. For the powdered QDs, only

after 76 days of storage is the settling process caused by degradation apparent, a process in

the solutions-stored samples that is already clear at 10 days.

35

CHAPTER V

CONCLUSIONS

Synthesis of CdTe QDs

GSH-capped CdTe QDs were successfully synthesized in powder form and

solutions. The final ratio of CdCl2:GSH:K2TeO3 in this synthetic mixture was 4:10:1. Dried

QDs can be resuspended in water or citrate-borate buffer (pH ~9). Initially, they showed

the same fluorescent characteristics as freshly prepared QDs in the synthetic mixture. After

synthesis, the dried QDs could be stored or used for other experiments.

The effect of sonication on QDs in solutions

QDs degraded during the sonication process. The degradation increased with the

sonication time. If QDs are used for experiments which involve sonication, a decrease of

QD fluorescence intensity due to degradation is expected.

Cd:Te molar ratio and stability study of the QDs

The Cd:Te molar ratio in QDs is approximately 2:1, but grows slightly with

incubation time. GSH-capped CdTe QDs undergo thermal and photodegradation over time.

QDs are very stable when they are stored in dried, powdered form (over 76 days, no change

in peak fluorescence for dried, powdered QDs). If QDs were resuspended in deionized

water or citrate-borate buffer, in low temperatures (-80oC) and in low light intensities (dark

conditions), they are more stable than at higher temperatures (25oC) and at higher light

intensities. At -80oC in the dark, peak fluorescence decreased by 70% over 76 days. The

best storing condition for GSH capped CdTe QDs is the dried, powdered form at low

temperatures under dark conditions.

36

The degradation process probably occurs due to the release of sulfur or GSH from

the GSH-capped QDs, although the form of that sulfur is unknown beyond a form that is

water soluble. In QD solutions, photodegraded in sunlight, ~72% of sulfur found on

freshly-prepared QDs was found in the solution supernatant.

37

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41

APPENDIX A

Chemical Abstract Service Registry Numbers

Compound CAS Number

cadmium chloride hydrate 654054-66-4

ethyl alcohol 64-17-5

L-glutathione reduced 70-18-8

potassium tellurite 123333-66-4

sodium borate octahydate 1303-96-4

sodium citrate dihydrate 6132-04-3

nitric acid 7697-37-2

elemental sulfur standards 7704-34-9

elemental tellurium standards 10605-80-9

elemental cadmium standards 74404-43-9

42

VITA

Poorna Tharaka Wansapura was born on June 1, 1984 in Kandy, Sri Lanka, to

Munidasa Wansapura and Anula Bulumulla. He is the third of three children and second in

his family to pursue a degree beyond that of a Bachelor’s degree. In September of 2005 he

began working towards his undergraduate degree at Institute of Chemistry, Ceylon. He also

worked two years as a research and development chemist in Sri Lanka. He was offered a

scholarship as a graduate teaching assistant at Sam Houston State University, USA in June

2012. There, he began working on his Master’s degree in Chemistry in September 2012.

He carried out research with Dr. Thomas G. Chasteen in analytical chemistry. During his

graduate studies he was able to win the graduate school scholarship in 2012 and 2013. He

also won the best research award at SHSU’s University Graduate Research Exchange

Program 2014 for his graduate research. He has been the co-author of articles that have

been published in Electrochimica Acta (2007) and BioMetals (2014).