**6. Electron transfer mechanism and antimicrobial photodynamic therapy**

PDT can be applied for disinfection and sterilization [4–7]. Microbial, including bacterium and viruses can be removed by photosensitized reaction. The physical treatment, such as PDT, is advantageous against antibiotic-resistant bacteria [91, 92]. PDT for microbial treatment is called as aPDT and/or PACT. Red light (relatively long wavelength visible light) is used for aPDT. Because 1 O2 can be easily produced by relatively small energy photons, it is considered as the important reactive species for aPDT process. Phenothiazine dyes, such as Methylene Blue is used as the photosensitizer for aPDT [93], because Methylene Blue can absorb relatively long-wavelength visible light and its ΦΔ value is relatively large [94]. However, the aPDT mechanism has not been well-understand. Biological environments are under a hypoxic condition [95], the mechanism mediated by <sup>1</sup> O2 generation mechanism may be restricted. Therefore, the electron transfer mechanism may play an important role in the aPDT mechanism.
