For example, for an oxidized red phosphorus/C3N4 photocatalyst , the absorbance of the dark sample was virtually identical to that of the DPD-POD assay without H2O2 (A = 0.03), thus indicating neither presence of H2O2 nor cross-sensitivity. The irradiated sample showed an approximate H2O2 concentration of 5 -10 mg(H2O2) L -1 . Therefore, it was diluted by a factor of 30 before performing the DPD-POD assay. The resulting absorbance was A = 0.15. Using the calibration function from Supplementary Figure , this gave a H2O2 concentration of 6.7 mg(H2O2) L -1 . Since the dark sample did not show any positive result, this value does not have to be corrected by the dark measurement. If the DPD-POD assay showed any response in the dark sample, the corresponding H2O2 concentration would be used to correct the value of the irradiated sample.