Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity

Abstract

The photocatalytic properties of titanium dioxide are well known and have many applications including the removal of organic contaminants and production of self-cleaning glass. There is an increasing interest in the application of the photocatalytic properties of TiO(2) for disinfection of surfaces, air and water. Reviews of the applications of photocatalysis in disinfection (Gamage and Zhang 2010; Chong et al., Wat Res 44(10):2997-3027, 2010) and of modelling of TiO(2) action have recently been published (Dalrymple et al. , Appl Catal B 98(1-2):27-38, 2010). In this review, we give an overview of the effects of photoactivated TiO(2) on microorganisms. The activity has been shown to be capable of killing a wide range of Gram-negative and Gram-positive bacteria, filamentous and unicellular fungi, algae, protozoa, mammalian viruses and bacteriophage. Resting stages, particularly bacterial endospores, fungal spores and protozoan cysts, are generally more resistant than the vegetative forms, possibly due to the increased cell wall thickness. The killing mechanism involves degradation of the cell wall and cytoplasmic membrane due to the production of reactive oxygen species such as hydroxyl radicals and hydrogen peroxide. This initially leads to leakage of cellular contents then cell lysis and may be followed by complete mineralisation of the organism. Killing is most efficient when there is close contact between the organisms and the TiO(2) catalyst. The killing activity is enhanced by the presence of other antimicrobial agents such as Cu and Ag.

Fig. 1

Number of publications on photocatalytic disinfection

Fig. 2

Scanning electron micrographs of photocatalytically treated E. coli. a Untreated cells. b, c Cells after 240 min. d Cells after 30 min. Catalyst TiON thin film. From Wu et al. (2010a, b)

Fig. 3

Transmission electron micrographs of photocatalytically treated P. aeruginosa. Untreated cells transverse section showing normal thickness and shape cell wall (arrows). b–d Cells after 240 min treatment showing abnormal wavy cell wall (arrows) (b), cytoplasmic material escaping from the cell with damaged cell wall (arrows) (c) and cell showing two “bubbles” of cellular material with cell wall (arrows) (d). Catalyst TiO₂ thin film. Bar marker = 200 nm. From Amezaga-Madrid et al. (b)

Fig. 4

Comet assay of DNA from cells of E. coli on photoirradiated TiO₂ and CuO–TiO₂ catalysts. Upper photographs show fragmented DNA entering the gel like the tail of a comet. The graph shows viability (control, open circle; TiO₂ catalyst, closed circle; TiO₂–CuO dual catalyst, downturned triangle) and tail moment (TM = Tail length × %DNA in tail/100; Olive et al. 1990) as the measure of the extent of DNA damage (TiO₂ catalyst, black square; TiO₂–CuO dual catalyst, gray square) against time

Fig. 5

Role of ROS in photocatalytic killing of bacteria. Direct oxidation of cell components can occur when cells are in direct contact with the catalyst. Hydroxyl radicals and H₂O₂ are involved close to and distant from the catalyst, respectively. Furthermore, ⋅OH can be generated from reduction of metal ions, e.g. Cu²⁺ by H₂O₂ (Sato and Taya 2006c)

Fig. 6

Transmission electron micrograph of E. coli showing adhesion betwen cells and TiO₂ in suspension. Catalyst Degussa P25 pH 6.0. From Gumy et al. (2006b)

Fig. 7

Scheme for photocatalytic killing and destruction of bacteria on TiO₂. Contact between the cells and TiO₂ may affects membrane permeability, but is reversible. This is followed by increased damage to all cell wall layers, allowing leakage of small molecules such as ions. Damage at this stage may be irreversible, and this accompanies cell death. Furthermore, membrane damage allows leakage of higher molecular weight components such as proteins, which may be followed by protrusion of the cytoplasmic membrane into the surrounding medium through degraded areas of the peptidoglycan and lysis of the cell. Degradation of the internal components of the cell then occurs, followed by complete mineralisation. The degradation process may occur progressively from the side of the cell in contact with the catalyst