1O2 generated upon blue light irradiation of PpIX was analyzed quantitatively using ESR spectroscopy48, 49. It was detected by adding 1O2 trapping agent (100 mM 4-OH-TEMP) to PpIX solutions irradiated with blue light (460 nm, 400 mW/cm2). The concentration of the PpIX solution was 100 µM and blue light irradiation doses were 0, 1, 10, and 100J. We assessed the inhibitory effect of ROS in blue light irradiated PpIX solutions with 5 mM L-histidine34. We compared the double integrals of 4-OH-TEMP experimental spectra with those of a 10 μM TEMPOL standard measured under identical settings to estimate 1O2 adduct concentration50. ESR was performed using a JES-RE1X (JEOL, Tokyo, Japan) connected to a WIN-RAD ESR Data Analyzer (Radical Research, Tokyo, Japan) at the following instrument settings: microwave power, 8.00 mW; magnetic field, 335.8 ± 5.0 mT; field modulation width, 0.1 mT; sweep time, 1 min; and time constant, 0.03 s. All experiments were repeated three times.