A luminol chemiluminescence method was used to evaluate the porphyrin-induced photogeneration of hydrogen peroxide (H
2O
2). This method enabled us to detect H
2O
2 in the presence of a high concentration of porphyrin, which was not possible using conventional colorimetry. The limit of detection was about 1 µM. We compared the ability to generate H
2O
2, using uroporphyrin (UP), hexacarboxylporphyrin (HCP), coproporphyrin (CP), hematoporphyrin (HP), mesoporphyrin (MP), and protoporphyrin (PP). The amount of H
2O
2 photoproduced was strongly related to the state of the porphyrin in the aqueous solution. UP and HCP, which existed predominantly in a monomeric form, had a good ability to produce H
2O
2. HP and MP, existing as dimers, showed weak activity. CP, forming a mixture of monomer and dimer, had a moderate ability to produce H
2O
2. PP, which was highly aggregated, had a good ability. These results demonstrated that the efficiency of porphyrins to produce H
2O
2 was strongly dependent on their aggregated form, and the dimer suppressed the production of H
2O
2.
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