The effect of amino acids on the photocatalytic toxicity of titanium dioxide (TiO2) was investigated. Various amino acids were treated with ultraviolet irradiation along with rutile and anatase types of TiO2 particles. The photooxidized amino acids, especially cysteine, generated hydrogen peroxide in the presence of copper ions during the auto-oxidation process. In the case of aromatic amino acids, hydrogen peroxide formation from photocatalyzed phenylalanine was significantly larger than that from tyrosine. Hydrogen peroxide is a long-lived reactive oxygen species. Furthermore, photocatalyzed amino acids gradually generate hydrogen peroxide. This secondary generation of hydrogen peroxide may play an important role in the cytotoxicity of the TiO2 photocatalyst after photoirradiation.
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