When titanium dioxide (TiO2) is irradiated with near-UV light, this semiconductor exhibits strong bactericidal activity. In this paper, we present the first evidence that the lipid peroxidation reaction is the underlying mechanism of death of Escherichia coli K-12 cells that are irradiated in the presence of the TiO2 photocatalyst. Using production of malondialdehyde (MDA) as an index to assess cell membrane damage by lipid peroxidation, we observed that there was an exponential increase in the production of MDA, whose concentration reached 1.1 to 2.4 nmol · mg (dry weight) of cells−1 after 30 min of illumination, and that the kinetics of this process paralleled cell death. Under these conditions, concomitant losses of 77 to 93% of the cell respiratory activity were also detected, as measured by both oxygen uptake and reduction of 2,3,5-triphenyltetrazolium chloride from succinate as the electron donor. The occurrence of lipid peroxidation and the simultaneous losses of both membrane-dependent respiratory activity and cell viability depended strictly on the presence of both light and TiO2. We concluded that TiO2 photocatalysis promoted peroxidation of the polyunsaturated phospholipid component of the lipid membrane initially and induced major disorder in the E. coli cell membrane. Subsequently, essential functions that rely on intact cell membrane architecture, such as respiratory activity, were lost, and cell death was inevitable.

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The gas-phase photocatalytic oxidation of trichloroethylene (TCE) over titanium dioxide was investigated as a potential method for destroying this common pollutant. The results from this study agree with earlier studies in that high levels of destruction of TCE were achieved. Accompanying these high rates of destruction were high quantum yields (approaching unity). However, directsampling mass spectrometry and gas-phase Fourier transform infrared (FTIR) spectroscopy revealed that there are significant quantities of byproducts produced [phosgene, dichloroacetyl chloride (DCAC), carbon monoxide, molecular chlorine]. The DCAC has been rationalized on the basis of a chemical reaction mechanism in which the TCE molecules are oxidized in a chain reaction involving C1 atoms. This mechanism appears to be validated by tests with other chlorinated ethylenes (perchloroethylene, dichloroethylenes). Phosgene may arise at least partially from the photocatalytic oxidation of DCAC, and molecular chlorine may result from the recombination of chlorine atoms. The results of this study are discussed relative to aqueous-phase photocatalytic oxidation of TCE where chlorinated intermediates have been observed.

We report carbon mass balance and kinetic data for the total oxidation of cells, spores, and biomolecules deposited on illuminated titanium dioxide surfaces in contact with air. Carbon dioxide formation by photocatalytic oxidation of methanol, glucose, Escherichia coli, Micrococcus luteus, Bacillus subtilis (cells and spores), Aspergillus niger spores, phosphatidylethanolamine, bovine serum albumin, and gum xanthan was determined as a function of time. The quantitative data provide mass balance and rate information for removal of these materials from a photocatalytic surface. This kind of information is importantfor applications of photocatalytic chemistry in air and water purification and disinfection, self-cleaning surfaces, and the development of self-cleaning air filters.