S. Ponnusamy

Morphology-directed synthesis of ZnO nanostructures and their antibacterial activity

Zinc oxide (ZnO) nanostructures of various morphologies were produced in an aqueous system, with pyridine as a shape-directing agent. X-ray diffraction (XRD) and selected-area electron diffraction (SAED) revealed hexagonal wurtzite crystal structure. Variation in surface morphology was analyzed using transmission electron microscopy (TEM). Changes in surface morphology were attributed to the absence of steric stabilization in pyridine during synthesis process. Pyridine concentration affected morphology and optical properties. Fourier transform infrared spectroscopy (FTIR) confirmed the presence/absence of pyridine on the surface of ZnO nanostructures (ZnO-NSs). Optical measurements carried out using UV-visible spectrophotometer (UV-vis) and photoluminescence (PL) indicated the presence of defects. All the samples exhibited two PL peaks, at 350-370 nm and 560-624 nm. Variation in the intensities of PL peaks corresponded to the changes in the surface morphology from nanoparticles to rods and origin of deep-level defect luminescence is attributed to surface recombination. The toxicity of the nanostructures was tested on model Gram-negative and Gram-positive pathogens. Smaller nanorods were most toxic among the nanostructures tested.

Amino acid-mediated synthesis of zinc oxide nanostructures and evaluation of their facet-dependent antimicrobial activity

ZnO nanostructures (ZnO-NSs) of different morphologies are synthesized with the amino acids L-alanine, L-threonine, and L-glutamine as capping agents. X-ray diffraction (XRD) shows the formation of a crystalline wurtzite phase of ZnO-NSs. The surface modification of ZnO-NSs due to the capping agents is confirmed using Fourier transform infrared (FTIR) spectroscopy. Photoluminescence spectroscopy reveals that the concentration of surface defects correlates positively with the number of polar facets in ZnO-NSs. The antimicrobial activity of the ZnO-NSs has been tested against Escherichia coli and the common pathogens Staphylococcus aureus, Klebsiella pneumoniae, and Bacillus subtilis. Culture-based methods in rich medium show up to 90% growth inhibition, depending on the ZnO-NSs. Flow cytometry analyses indicate that the reactive oxygen species (ROS) generated by ZnO-NSs contribute mostly to the antibacterial activity. Control experiments in minimal medium show that amino acids and other reducing agents in Luria-Bertani (LB) medium quench ROS, thereby decreasing the antimicrobial activity of the ZnO-NSs.

Enhanced visible light induced photocatalytic activity on the degradation of organic pollutants by SnO nanoparticle decorated hierarchical ZnO nanostructures

One (1D) and two-dimensional (2D) nanostructures of zinc oxide and tin oxide (ZnO/SnO) nanocomposites were synthesized by a hydrothermal method using ethylenediamine (EDA) as a capping ligand. The effect of Sn concentration on the morphology of the nanocomposites has been investigated. X-ray diffraction analysis indicated good crystallinity of samples with the presence of both ZnO and SnO phases. The morphological analysis revealed the morphological transformation from ZnO nanorods to ZnO/SnO nanosheets by adding Sn. X-ray photoelectron spectra analyses showed significant peak shift in the electronic state of Zn at the higher concentration of Sn. Elemental mapping results clearly evidenced that both ZnO and SnO moieties were uniformly distributed in the nanosheets. Photocatalytic degradation of methylene blue using as-prepared ZnO/SnO nanocomposites was nine times faster than that of pure ZnO under visible light irradiation. It could be attributed to the formation of a hetero-junction between ZnO and SnO. Our experimental results revealed that photogenerated superoxide (O2−˙) radicals were the main reactive species for the degradation of MB. The maximum degradation efficiency was observed for the sample with 1 wt% of tin chloride, the MB related absorption peak completely disappeared after 6 min of irradiation. ZnO/SnO composites extended the light absorption spectra of ZnO to a visible light region and enhanced the visible light photocatalytic activity.

Effect of organic-ligands on the toxicity profiles of CdS nanoparticles and functional properties.

CdS nanoparticles are one among the most promising agents for fluorescent imaging. Hence, it is essential to develop new strategies to overcome the cytotoxicity of these nanoparticles. Surface modification is one of the simplest and effective techniques. This paper assesses the effect of surface modification on toxicity of the CdS nanoparticles. Unmodified CdS and surface-modified CdS nanoparticles were synthesized in an aqueous medium using a wet chemical route at room temperature. The surface modification of the CdS nanoparticles with polyvinylpyrrolidone (PVP) and cysteine was confirmed using infrared absorption studies. The diameters of unmodified CdS, PVP-modified CdS, and cysteine-modified CdS nanoparticles were determined using HRTEM. They exhibited luminescence in the range from 500 to 800 nm. The cytotoxic effects of these CdS nanoparticles were investigated in cultures of Vero cells. The results indicated that Vero cell viability was higher for the surface-modified CdS nanoparticles than for the unmodified CdS nanoparticles. The reduction in the toxicity was related to the nature of the capping agents used for the surface modification, and the particle size.

Preliminary investigations on the antibacterial activity of zinc oxide nanostructures

In this study, we present a systematic investigation on the evolution of nanorods of diameter 35–40xa0nm and 1–2xa0μm length from nanoparticles of diameter 30–35xa0nm by varying the concentration of 2,6-lutidine which acts as a shape-directing agent in the synthesis process. This variation in morphology was studied using transmission electron microscopy. The surface capping agent was subsequently removed by heating during the synthesis process and confirmed using Fourier Transform Infra-red spectroscopy. Sufficient quantity of surface defects in the form of oxygen vacancies was observed from the photoluminescence analysis of the synthesized nanostructures. The concentration of defects decreased as the shape transits from nanoparticles to nanorods. The synthesized samples were preliminarily studied for their antibacterial activity against four model (gram-positive and gram-negative) pathogens by disk diffusion method and growth curve analysis. The calculated generation time indicates higher activity for nanoparticles than nanorods. However, the difference in the activity against different pathogens and their dependence on the concentration of defects indicate oxidative stress in addition to mechanical membrane damage as the major toxicity mechanism. Overall, the experimental findings are preliminary evidence supporting the possibility of developing zinc oxide nanostructures as antibacterial agents against a wide range of microorganisms to control and prevent the spreading of bacterial infections.

Enhanced biocidal activity and optical properties of zinc oxide nanoneedles

Zinc oxide nanoneedles were successfully synthesized by simple wet chemical method. X-ray diffraction studies reveal the formation of wurtzite-type of ZnO. Optical studies indicate the presence of defects in the form of oxygen vacancies and zinc interstitials. As an application study, this sample was tested for its antibacterial activity. These nanoneedles were found to exhibit excellent biocidal activity against both gram positive and gram negative bacteria.