Probing antibacterial activity of ZnO and ZnO:S thin films against E. coli and S. typhi with structural, optical and electrical properties

Abstract

This study analyzed the antibacterial properties of ZnO and ZnO:S thin films under dark conditions against two different gram-negative bacteria (E. coli and S. typhi) along with their structural, optical and electrical properties. A simple spray pyrolysis method was employed to deposit the thin films using Air and N2 as carrier gases. All samples showed antibacterial activity against E. coli. Further, all samples except ZnO:S (Air) showed antibacterial activity against S. typhi. A simple mechanism has been proposed based on electrostatic interaction between bacteria cell and zinc ions to explain the observed antibacterial activity under dark conditions. The noneffectiveness of ZnO:S (Air) thin film against S. typhi is attributed to the presence of virulence polysaccharide capsule which prevents neutralization of surface charge of bacteria cell by low ionic zinc ions. X-ray photoelectron spectroscopic analysis revealed a decrease of binding energies of Zn 2p and O 1s by 0.8 eV in ZnO:S (Air) thin film, confirming the reduction of ionicity of Zn2+ ions. This may be as a result of incorporation of S+1/S+2 charge states into the host ZnO in presence of oxygen owing to its lower electronegativity than oxygen. The carrier gas affected the growth of nanoparticles leading to variable morphology in all cases. However, all films had (0 0 2) preferred orientation and optical band gap in the range 3.20-3.00 eV. The variation of bandgap and bulk carrier concentration had no significant impact on the antibacterial activity of ZnO and ZnO:S thin films in the dark conditions. ZnO:S (N2) thin film was found to possess maximum antibacterial efficacy against both E. coli and S. typhi; and can be a useful antibacterial material for processed food industries to enhance the shelf life of the packaged items.