Leu-M

This work addresses the determination of arbitrarily shaped particle size distributions (PSDs) from PbS and PbSe quantum dot (QD) optical absorbance spectra in order to arrive at a relationship between band gap energy and particle size over a large size range. Using a modified algorithm which was previously developed for ZnO, we take only bulk absorption data from the literature and match the PSDs derived from QD absorbance spectra with those from transmission electron microscopical (TEM) image analysis in order to arrive at the functional dependence of the band gap on particle size. Additional samples sized solely from their absorbance spectra with our algorithm show excellent agreement with TEM results. We investigate the influence of parameters of the TEM image analysis such as threshold value on the final result. The band gap versus size relationship developed from analysis of just two samples lies well within the bounds of a number of published data sets. We believe that our methodology provides an a...

Determination of the quantum dot band gap dependence on particle size from optical absorbance and transmission electron microscopy measurements.

Quantum-sized ZnO nanoparticles have been synthesized at room temperature by a mild sol–gel process using tetraethylorthosilicate (TEOS) as the capping agent to control the particle growth of ZnO. The crystal structure, particle size and optical properties have been investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), photoluminescence (PL) spectra and Raman spectra, respectively. The results show that the ZnO nanoparticles exhibit hexagonal wurtzite structure and the average crystallite size is 5.7 nm which is a little less than TEM results. It has been testified by room-temperature PL spectra that the TEOS capped the surface of ZnO nanoparticles and obviously reduced grain size, as an emission at 520 nm almost disappeared and a new peak with an anomalous blue shift as great as 9 nm, appeared for the TEOS capped ZnO. The sensing tests indicate that the ZnO based sensors not only show a high response to NO2 but also exhibit high selectivity over CO and CH4 at a low operating temperature of 290 °C. The response increases with NO2 concentration and decreases with calcination temperature, and is in agreement with Raman and XRD results.

Quantum-sized ZnO nanoparticles: Synthesis, characterization and sensing properties for NO2

The microenvironment of cancer plays a very critical role in the survival, proliferation and drug resistance of solid tumors. Here, we report an interesting, acidic cancer microenvironment-mediated dissolution-induced preferential toxicity of ZnO nanocrystals (NCs) against cancer cells while leaving primary cells unaffected. Irrespective of the size-scale (5 and 200 nm) and surface chemistry differences (silica, starch or polyethylene glycol coating), ZnO NCs exhibited multiple stress mechanisms against cancer cell lines (IC(50)∼150 μM) while normal human primary cells (human dermal fibroblast, lymphocytes, human umbilical vein endothelial cells) remain less affected. Flow cytometry and confocal microscopy studies revealed that ZnO NCs undergo rapid preferential dissolution in acidic (pH ∼5-6) cancer microenvironment causing elevated ROS stress, mitochondrial superoxide formation, depolarization of mitochondrial membrane, and cell cycle arrest at S/G2 phase leading to apoptosis. In effect, by elucidating the unique toxicity mechanism of ZnO NCs, we show that ZnO NCs can destabilize cancer cells by utilizing its own hostile acidic microenvironment, which is otherwise critical for its survival.

Rapid dissolution of ZnO nanocrystals in acidic cancer microenvironment leading to preferential apoptosis

Visible light driven photocatalytic degradation of Rhodamine B and Direct Red using cobalt oxide nanoparticles

Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

Highly stable and optically transparent colloidal ZnO QDs have been synthesized at room temperature without using any surface capping agent. The as synthesized QDs show phase singularity of ZnO particles having wurzite (hexagonal) structure. TEM as well as AFM studies indicate that the average crystalline size of QDs is ∼ 7nm. Further, high resolution TEM image shows formation of hexagonal shape particles having lattice fringes along (001) plane of wurzite phase. Photoluminescence (PL) studies show bright luminescence with peak maximum at 530nm due to oxygen vacancy centers (V o ) present in QDs. The optical transmission spectrum of colloidal QDs of ZnO shows sharp absorption at 3.48eV which is blue shifted as compared to bulk ZnO (3.36eV) due to the quantum confinement effect. The band gap energy observed in QDs of ZnO is consistent with the energy calculated by using effective mass approximation model of Brus.

V.S.a Choudhry

Papers

Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

Optically transparent colloidal suspensions of single crystalline ZnO quantum dots prepared by simple wet- chemistry