Size-controlled synthesis of Cu2O nanoparticles: Size effect on antibacterial activity and application as a photocatalyst for highly efficient H2O2 evolution

Abstract

We propose a green approach for the size-controlled synthesis of Cu2O nanoparticles (NPs) with simplified chemical deposition technology by adjusting the concentration of the precursor copper source. The morphologies, phase compositions and structures were recorded for the obtained Cu2O NPs with sizes of 10, 50, 100 and 200 nm. In the dark, all of the Cu2O NPs showed good antibacterial activity towards Escherichia coli K-12 (E. coli). A size-dependent antibacterial effect was also observed as a result where decreasing the size of the Cu2O NPs led to increasing the antibacterial activity. The smallest Cu2O NPs (Cu2O-10) with the minimum inhibitory concentration (≤1 μg mL-1) achieved the highest antibacterial efficiency, with the inactivation of 7-log E. coli in 80 min with a concentration of 5 μg mL-1 in the dark. Under visible light (VL), medium concentrations (10, 20 and 50 μg mL-1) of Cu2O-10 showed enhanced antibacterial activity compared to that in the dark. For low (5 μg mL-1) or high concentrations (200 μg mL-1) of Cu2O-10, the antibacterial activity under VL was almost the same as that in the dark. The mechanisms of antibacterial activity are proposed and discussed in detail. "Contact killing" and photogenerated h+ together with reactive oxygen species (such as OH and H2O2) were found to be responsible for bacterial inactivation in the dark and under irradiation, respectively. Cu2O-10 proved to be a good photocatalyst, capable of the highly efficient evolution of H2O2, whose stable equilibrium concentration could be as high as 150 μM (using 20 μg mL-1 of Cu2O-10). Study on the membrane-separated system shows that the concentration of diffusing H2O2 can be as high as 50 μM, contributing to a complete inactivation of 7-log E. coli in 7 h. For comparison, the inactivation kinetics of the Cu2O NPs and H2O2 were calculated, and could be fitted into the Weibull and shoulder-linear-tail models, respectively.