Udit Soni

Chitosan encapsulated quantum dots platform for leukemia detection.

We report results of the studies relating to electrophoretic deposition of nanostructured composite of chitosan (CS)-cadmium-telluride quantum dots (CdTe-QDs) onto indium-tin-oxide coated glass substrate. The high resolution transmission electron microscopic studies of the nanocomposite reveal molecular level coating of the CdTe-QDs with CS molecules in the colloidal dispersion medium. This novel composite platform has been explored to fabricate an electrochemical DNA biosensor for detection of chronic myelogenous leukemia (CML) by immobilizing amine terminated oligonucleotide probe sequence containing 22 base pairs, identified from BCR-ABL fusion gene. The results of differential pulse voltammetry reveal that this nucleic acid sensor can detect as low as 2.56 pM concentration of complementary target DNA with a response time of 60s. Further, the response characteristics show that this fabricated bioelectrode has a shelf life of about 6 weeks and can be used for about 5-6 times. The results of experiments conducted using clinical patient samples reveal that this sensor can be used to distinguish CML positive and the negative control samples.

Simultaneous Type-I/Type-II Emission from CdSe/CdS/ZnSe Nano-Heterostructures

Core/intermediate/shell (C/I/S) structures with Type-I emission are well-known and are gaining immense importance due to their superior luminescence properties. Here, we report a unique C/I/S structure composed of CdSe/CdS/ZnSe that exhibits both Type-I and Type-II phenomena. The structures have been well characterized using a combination of optical and structural techniques. The photoluminescence (PL) and photoluminescence excitation (PLE) data indicate the formation of a combined Type-I and Type-II structure in one material, results supported by simple theoretical calculations. Single particle fluorescence reveals colocalization of both the emissions. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results confirm the structure of these particles. The time-resolved fluorescence studies show the possibility of tuning the lifetime of these materials by changing the Type-I/Type-II thickness ratios. It is possible to form these two separate excitons in the same system separated by a CdS intermediate layer that acts both as a barrier and an active member of the Type-II system allowing the generation and recombination of two excitons, in violation of Kashas rule.

Photocatalysis from Fluorescence-Quenched CdSe/Au Nanoheterostructures: A Size-Dependent Study

We report a systematic study of the electron-transfer process from CdSe quantum dots (QDs) to the Au tips as a function of the QD diameter and also the size of the Au-tip. For Au-tips smaller than ∼3 nm, that is, when they are still not metallic, a reduction in PL behavior is observed as the excited electrons are transferred from the QD-conduction band to Au, with quenching being higher for larger tips and smaller QDs. A combination of steady state and time-resolved studies establish the mechanism of charge transfer that is further confirmed by dye-degradation studies, showing the possibility of ambient day light photocatalysis.

Nanopatterned Cadmium Selenide Langmuir–Blodgett Platform for Leukemia Detection

We present results of the studies relating to preparation of Langmuir-Blodgett (LB) monolayers of tri-n-octylphosphine oxide-capped cadmium selenide quantum dots (QCdSe) onto indium-tin oxide (ITO) coated glass substrate. The monolayer behavior has been studied at the air-water interface under various subphase conditions. This nanopatterned platform has been explored to fabricate an electrochemical DNA biosensor for detection of chronic myelogenous leukemia (CML) by covalently immobilizing the thiol-terminated oligonucleotide probe sequence via a displacement reaction. The results of electrochemical response studies reveal that this biosensor can detect target DNA in the range of 10(-6) to 10(-14) M within 120 s, has a shelf life of 2 months, and can be used about 8 times. Further, this nucleic acid sensor has been found to distinguish the CML-positive and the control negative clinical patient samples.

Wurtzite or zinc blende? Surface decides the crystal structure of nanocrystals

We show for the first time that the stability of the wurtzite or the zinc blende phase for CdE (E = S, Se, Te) nanocrystals strongly depends on the surface of these nanocrystals. Anion rich surfaces prefer to crystallize in the zinc blende structure whereas cation rich surfaces tend to favor the wurtzite phase. Transformations from one phase to the other can be effected by merely changing the surface of the particles and annealing at a specified temperature. Our results show that it is not only important to choose the synthesis temperature and ligands in order to achieve crystallization in a preferred phase but also the surface of the particles is a deciding factor.

Development of functionalized quantum dot modified poly(vinyl alcohol) membranes for fuel cell applications

This paper describes the preparation and characterization of membranes based on poly(vinyl alcohol) (PVA) cross-linked using functionalized CdSe quantum dots [FQDs] (size 4 ± 0.2 nm). FQDs having –COOH and –SO3H functionalities on the surface were synthesized by the ligand exchange method using a mixture of 3-mercaptopropionic acid and 2-mercaptoethanesulfonic acid. Both sulfonic (–SO3H) and carboxylic (–COOH) groups are expected to enhance the dimensional/thermal stability of PVA [by cross-linking] and proton conductivity [as proton donors]. Several membranes were prepared by mixing an aqueous solution of PVA with varying amounts of FQDs and then cross-linked by thermal treatment. Cross-linked membranes were characterized for structural (FTIR-ATR) and thermal behaviour (TGA, DSC, DMA), morphology (SEM, TEM, XRD), water uptake and proton conductivity. Hydrolytic stability and Tg increased with increasing degree of cross-linking. All of the cross-linked membranes had proton conductivity of 10−3 S cm−1 in the temperature range of 25 °C to 70 °C at 100% relative humidity.

Synthesis of trap emission free cadmium sulfide quantum dots: Role of phosphonic acids and halide ions

Changing the capping ligands of CdS nanocrystals during their synthesis controls the optical properties of these nanocrystals. Addition of a little amount of CdCl2:Phosphonic Acid (PA) completely eliminates the surface state emission with an increase in quantum yield of band edge emission from these nanocrystals. The role of halide ions in surface passivation along with the structural dependance of these nanocrystals on surface states has also been studied. These as prepared nanocrystals are uniform as well as monodispersed and their size can be tuned by varying the reaction time. These nanocrystals have been well characterized using a combination of optical and structural techniques.

Synthesis of Core–Shell Quantum Dots and Their Potential Application

We report the synthesis of core–shell CdSe/ZnS, CdSe/CdS, CdSe/CdS/ZnS nanoparticles. Fluorescent properties of these core/shell nanoparticles depends on the type of ions present on the surface of nanoparticles; a cation-rich surface enhances the emission intensity, while an anion-rich surface leads to deterioration of the fluorescence signal. We used this phenomenon for the synthesis of fluorescent and non-fluorescent quantum dots. We synthesized CdSe/S in which the fluorescence emission is completely quenched, and CdSe/S/Cd and CdSe/S/Zn type of core–shell nanoparticles, which exhibit high fluorescence emission. Although various fluorescence sensors for metal cations based on nanoparticles were previously developed, to the best of our knowledge we first explored the potential application of non-fluorescent quantum dots for the detection of cadmium and zinc in organic medium.

Tumor cell targetting using folate conjugated core/shell CdSe/CdS/ZnS nano rods

Core-shell-shell nano rods have been widely investigated as fluorescent biomarkers, due to their photochemical stability and high brightness, which makes them a good alternative to organic fluorophores. There are several unique optical properties that can make nano rods (NRs) potentially more appealing bioimaging probes. Hydrophobic core-shell-shell CdSe/CdS/ZnS nano rods (NRs) were synthesized by successive ion layer adsorption and reaction (SILAR) technique. Synthesized NRs (CdSe/CdS/ZnS) were made water dispersible by ligand exchange with thioglycolic acid and were conjugated with folic acid (FA) using EDC/NHS techniques for targeting human cancer cells expressing folate receptor (FR). The Folate receptor (FR) is ideally suited for this study because it is preferentially expressed with high binding affinity for folic acid in several cancers. In vitro cytotoxicity of the folate-conjugated CdSe/CdS-TGA and CdSe/CdS/ZnS-TGA NRs were investigated also by employing MCF-7 cells (Breast cancer cell line) through the 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. This assay shows that core-shell-shell (CdSe/CdS/ZnS) NRs are biocompatible than core- shell (CdSe/CdS) for targeting the cancer cells. The result of MTT viability assay shows that the percentage of living cells gradually decreases for CdSe/CdS rods where as in case of CdSe/CdS/ZnS rods ≥ 80% percentage of cell viability has been observed. These experiments confirms that FA conjugated NRs are preferentially internalized by MCF-7 tumor cells suggesting their potential utility as targeted fluorescent imaging agent for early stage cancer detection. The MTT cell viability assay indicated that covering the CdSe/CdS rods with ZnS shell reduces their cell toxicity.

BIOCONJUGATED QUANTUM DOTS BASED RAPID DETECTION OF PATHOGENIC BACTERIA FROM WATER SAMPLES

A sensitive and rapid method for the detection of pathogenic bacteria (Salmonella typhi) in water sample was developed using core-shell CdSe/ZnS quantum dots (QDs) as fluorescence label. Surface-functionalized core-shell quantum dots were synthesized by successive ion layer adsorption and reaction (SILAR) technique and were made hydrophilic by ligand exchange method. Developed hydrophobic and hydrophilic QDs were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and spectrofluorimetry. Carboxy- terminated QDs were conjugated with bacteria-specific antibodies (S. typhi-specific IgG) for the preparation of photostable fluorescent label and were characterized by various techniques like spectrofluorimetry and enzyme-linked immunosorbent assay (ELISA) for their photoluminescence and successful bioconjugation. Antibody (Ab)-conjugated QDs were incubated with bacteria-contaminated water for S. typhi detection. Microscopic images and spectral profile of bacteria–Ab conjugated QDs complex were recorded by confocal laser scanning microscopy (CLSM). A sensitivity of 103 organisms/mL of targeted bacteria (S. typhi) could be attained in a period of about 2 h.