We attempted to detect ROS generated upon blue light irradiation of PpIX using an ESR technique used specifically for detecting ROS. Blue light irradiation of PpIX generates 1O2, a critical ROS (Fig. 2A,B). The inhibition of this ROS by typical scavenger L-histidine confirms it as 1O2 39, 40 (Fig. 2C). 1O2 is generated by photochemical reactions through the transfer of excitation energy from a suitable triplet state sensitizer or by radical interactions41. Furthermore, 1O2 is a powerful oxidizing molecule, and starts further oxidation reactions in closed environments such as bacterial cell walls, lipid membranes, enzymes, or nucleic acids42–44. 1O2 generated upon the blue light irradiation of PpIX increased in proportion to irradiation intensity (Fig. 2A,B).