Correlation between defects in capped ZnO nanoparticles and their antibacterial activity

• ZnO nanoparticles capped with PEG and AsA exhibited green emission due to oxygen vacancy defects.
• Green emission peak and hence oxygen vacancy not observed in T-80 and MAA capped ZnO nanoparticles.
• T-80 and MAA capped ZnO showed higher antibacterial activity than PEG and AsA capped ZnO nanoparticles.
• Low ROS measured for PEG and AsA capped batches, due to trapping of photoexcited charge carriers.
• Structure of capping agent was hence vital for ROS generation and antibacterial effect of ZnO.

Antibacterial activity of ZnO nanoparticles (NPs) triggered by generation of reactive oxygen species (ROS) depends on the fate of photoexcited charge carriers. Batches of wide band gap ZnO NPs of 7-9 nm sizes, capped with polyethylene glycol (PEG), ascorbic acid (AsA), mercaptoacetic acid (MAA) and polysorbate 80 (T-80) were synthesized by precipitation method. These capped ZnO NPs exhibited ROS induced antibacterial activity, where the ROS was measured by TBARS assay. The PEG capped and AsA capped ZnO NPs exhibited weaker antibacterial activity and were correlated with strong and broad green emission peak owing to oxygen vacancies. The oxygen vacancies were trap sites of photoexcited electrons which inhibited interaction between the photoexcited electrons and oxygen on the surface of the ZnO NPs and accounted for lesser ROS generation and subsequently weaker antibacterial activity. Contrastingly MAA capped and T-80 capped ZnO NPs did not exhibit significant green emission peak, but exhibited 13% and 43% inhibition of growth of E. coli, respectively. The lack of oxygen vacancy defects in MAA capped and T-80 capped ZnO NPs perhaps led to lesser trapping of charge carriers, which is favorable for higher ROS generation and consequently higher antibacterial activity.