Role of surface adsorbed anionic species in antibacterial activity of ZnO quantum dots against Escherichia coli.

We have studied the role of surface bound anionic species on zinc oxide (ZnO) quantum dots (QDs) for the antibacterial activity against Escherichia coli (E. coli) bacteria. The ZnO QDs with surface adsorbed anionic species of acetate ions and nitrate ions have been synthesized using wet chemical route. X-Ray diffraction studies reveal single-phase hexagonal wurtzite structure of as synthesized ZnO QDs. The particle size was found to be 3-5 nm for acetate adsorbed ZnO QDs and 4-7 nm for nitrate adsorbed ZnO QDs. Minimum inhibitory concentration (MIC) measurements and growth kinetics studies for E-coli show a marked difference in the antibacterial activity of ZnO QDs with both anionic species. The MIC for acetate adsorbed ZnO QDs was found to be 2.5 mM in light and 3 mM in dark. However, nitrate adsorbed ZnO QDs exhibits MIC about 6 mM in light and no significant bacterial growth inhibition was observed upto 30 mM under dark. The enhanced bacterial growth inhibition observed for acetate adsorbed ZnO QDs is attributed to the inherent ability of acetate ions to generate reactive oxygen species. The acetate adsorbed QDs having excellent antibacterial activities suggests its potential application for practical bactericidal realization.