New challenges for photocatalysts

A photocatalyst is a material that can capture and absorb light, and use this energy to produce reactive chemical species to increase the rate of a reaction. In the case of self-cleaning windows the reaction is the oxidation of organic molecules. The undesirable greasy, organic deposits on a surface of a window are thermodynamically unstable in the presence of oxygen, producing carbon dioxide and water: 

Under ordinary conditions reaction (i) is so slow that most organic chemicals can be considered stable even in the presence of oxygen. If the window surface is coated with a photocatalyst - typically titania, TiO₂, at 30 nm thick, ie  around 1/50 000 the thickness of a human hair - and exposed to sunlight, the greasy coating will react with oxygen at room temperature. This is sometimes referred to as 'cold combustion', owing to the similarity to a combustion reaction, but without the necessary high temperatures.  

From a chemist's viewpoint bacteria are simply collections of organic molecules. The cell linings are made up of mostly carbon and hydrogen, much like the greasy layer on the windows. The destruction of even part of the bacteria's cell walls would kill the bacteria.