Green chemistry synthesis of biocompatible ZnS quantum dots (QDs): their application as potential thin films and antibacterial agent

We are presenting here the synthesis of quantum dots (QDs) of direct band gap semiconductor, cubic ZnS through modified green chemistry-mediated chemical precipitation reaction. Green chemistry-synthesized (GCS) ZnS QDs were characterized using powder X-ray diffraction and high-resolution transmission electron microscope techniques. Analysis of results, revealed by both the techniques for the synthesized QDs, is complementary as far as the size range (2-6 nm) of ZnS QDs is concerned. UV-Vis spectrophotometric spectrum (lambda(max) = 314 nm) showed a conspicuous blue shift than the bulk. The Fourier-transformed infrared spectra convincingly reported a Zn-S bond stretching frequency at 649 cm(-1). The characterized QDs were subjected to the preparation of thin films over SiO2 template (57 nm thickness) using photoresist spin coating technique at the ambient condition. The surface topology of nanoscale-thick films was studied by atomic force microscope (roughness parameter -33.28 nm, rms; for a scan area of 3.48 x 3.48 m(2)). The symmetrical (skewness = 1.68) and random distribution (kurtosis = 2.93) of the peaks and valleys revealed the nanoscale-thick films of ZnS QDs. Zeta potential (-9.2mV) fairly proved stable existence of ZnS QDs. The GCS QDs were found to be non-toxic toward L929 mouse fibroblastic cells and human erythrocytes. However, they demonstrated significant inhibitory effects against seven bacterial pathogens with an average zone of inhibition of 1.5 cm at 100 g/ml concentration. The minimum inhibitory concentrations determined were in the range of 75 to 125 g/ml for gram-positive and 100 to 150 g/ml for gram-negative bacterial pathogens.