It has been proposed that melanin may play a role in the antimicrobial defense system of a human body. Furthermore, it has been found that melanin has efficacy against oxidative stress, tumor, venin, virus, and heavy metal ions. In view of the great potential of melanin in medical applications revealed by its photodynamic actions, we develop techniques based on mechanical stir and photo-fragmentation with femtosecond laser pulses respectively for nanonization of melanin to produce

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... d and water-dispersible melanin, in order to more reliably study melanin as a photodynamic-inactivation photosensitizer. In vitro experiments on the antimicrobial efficacy of nanonized melanin were conducted. It was found that both Sepia melanin and synthetic melanin do not have a significant effect on lowering the survival rate of Gram-positive bacteria Streptococcus mutans with or without light irradiation. Therefore, it is concluded that melanin, though capable of generating melanin radicals and reactive oxygen species, is not a practical agent for photodynamic inactivation.