A. Erdem, D. Metzler, H.W. Chou, H.Y. Lin and C.P. Huang
The effects of photocatalytic nano-TiO2 on the survival or die-off of E. coli (e.g. TK2) were investigated under ambient conditions. Experimentally, a 28-h E. coli culture was exposed to photocatalytic nano-TiO2 at various concentrations, e.g., 0 to 1000 mg/L and particle size, e.g., 5 to 500 nm both in darkness and the presence of several light sources including a simulated solar light. Preliminary results indicated that E. coli underwent a two-stage response to nano-TiO2 particles independent of UV light irradiation; a rapid decrease in population density within the first 10 min then slowly decreased upon extended treatment time, e.g., 30-60 min. Results also show that there is bacteria die-off in the presence of nanoscale TiO2 in dark. Generally it appears that the growth rate decreases as the particle size decreases. The presence of light irradiation significantly enhanced the killing of E. coli due to additional photocatalytic activity. Photocatalytic reactions generated hydroxyl radical, which is strong oxidation agent that can cause great stress and damage to the cell. Cell damage was observed in terms of lipid peroxidation (e.g., production of malondialdehyde, MDA), cellular respiration (e.g., reduction of 2,3,5-triphenyltetrazolium chloride, TTC) and antioxidant enzyme (e.g., glutathione-S-transferase). Upon exposure of E. coli to nano-TiO2 the photocatalytic activity that was generated has markedly increased the production of MDA, TTC and GST. SEM observations vividly indicate cell wall damages.