Photocatalytic bactericidal activity of silver-sensitized titanium dioxide on Micrococcus lylae

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Under UV irradiation, titanium dioxide (TiO2) exhibits a strong bactericidal activity through the generation of hydroxyl radicals (•OH). Silver (Ag) sensitization is an effective way to enhance photocatalytic activity of TiO2. In the present study, Micrococcus lylae was used as a model bacterium to compare the bactericidal activity of Ag-sensitized TiO2 (in two different Ag/TiO2 molar ratios) and pure TiO2 (P25). When the concentration of photocatalysts was fixed on 0.2 mg/ml with 300 rpm stirring, no obvious difference observed among the three photocatalysts. However, the Ag-sensitized photocatalysts with higher Ag/TiO2 ratio showed better bactericidal efficiency when their concentration decreased (0.1 mg/ml) or the stirring speed increased (380 rpm). The results indicated that optimizing the phosico-chemical conditions of reaction promoted the efficiency of photocatalyst. Moreover, transmission electron microscopy (TEM) was used to observe the sub-cellular structural changes of M. lylae during photocatalytic oxidation (PCO). According to the TEM images, the disruption of cell wall occurred at a relatively long time after the cell death. The cause of cell death was the destruction of plasma membrane induced by membrane permeable •OH. These results supported that both modification on photocatalyst properties and optimization on reaction conditions enhanced the bactericidal efficiency of PCO.