Bactericidal mode of titanium dioxide photocatalysis

When exposed to near-UV light, titanium dioxide (TiO2) exhibits a strong bactericidal activity. However, the killing mechanism(s) underlying the TiO2 photocatalytic reaction is not yet well understood. The aim of the present study is to investigate the cellular damage sites and their contribution to cell death. A sensitive approach using o-nitrophenol β-D-galactopyranoside (ONPG) as the probe and Escherichia coli as model cells has been developed. This approach is used to illustrate damage to both the cell envelope and intracellular components caused by TiO2 photocatalytic reaction. Treatment of E. coli with TiO2 and near-UV light resulted in an immediate increase in permeability to small molecules such as ONPG, and the leakage of large molecules such as β-D-galactosidase after 20 min. Kinetic data showed that cell wall damage took place in less than 20 min, followed by a progressive damage of cytoplasmic membrane and intracellular components. The results from the ONPG assay correlated well with the loss of cell viability. Cell wall damage followed by cytoplasmic membrane damage leading to a direct intracellular attack has therefore been proposed as the sequence of events when microorganisms undergo TiO2 photocatalytic attack. It has been found that smaller TiO2 particles cause quicker intracellular damage. Evidence has been obtained that indicated that the TiO2 photocatalytic reaction results in continued bactericidal activity after the UV illumination terminates.