Enhanced antimicrobial activity of ZnO nanofluids in sonophotocatalysis and its mechanism

This study investigated the inactivation efficiency of ZnO nanofluids against E. coli in sonophotocatalysis with the aeration of nitrogen, oxygen, argon and their mixtures. The results showed that inactivation efficiency was increased when aeration was combined with sonophotocatalysis. Addition of different types of gases could lead to the different inactivation efficiency. The inactivation efficiencies were shown in the following order: no aeration < nitrogen < argon < oxygen < Ar/O₂(3:7) < Ar/O₂(7:3) < Ar/O₂(5:5). The production of hydroxyl radicals was explored to understand the inactivation mechanism. Compared with sonophotocatalysis without aeration, more hydroxyl radicals were produced in sonophotocatalysis with aeration, which could lead to changes of cellular substances. Furthermore, characterization of E. coli cells using Raman spectroscopy and FTIR illustrated that sonophotocalysis could affect the cellular substances containing carbohydrates, proteins and P containing molecules. Results suggested that the enhanced antimicrobial activity with aeration was originated from stronger cavitational activity, together with the formation of hydroxyl radicals. Compared to sonophotocatalysis without aeration, more dissolved oxygen was existed in sonophotocatalysis with aeration, which could enhance the formation of hydroxyl radicals.