Herein, we proposed a simple method to measure the inner resistance of a photoelectrochemical (PEC) sensor. Three photoanodes, including TiO2 nanotubes array (TiO2 NTs), g-C3N4/TiO2 NTs and Mn-3(BTC)(2)/g-C3N4/TiO2 NTs (BTC = 1,3,5-benzene-tricarboxylic acid), were chosen as the models. It was shown the change in inner resistance of the photoanode is the main factor for the photocurrent response in a PEC sensor. Under light excitation, the inner resistance of the photoanode is reduced remarkably, resulting in the increment in the photocurrent. The influence of the light intensity on the inner resistance is less than that on the photocurrent, due to the synchronous change in photovoltage. With increasing concentration of hole scavengers (e.g., H2O2, ascorbic acid), the steady inner resistances of the three photoanodes are decreased. In the PEC sensor with Mn-3(BTC)(2)/g-C3N4/TiO2 NTs, the Mn-3(BTC)(2) layer is served as a size-exclusion filter to prevent the diffusion of molecules with size larger than the apertures in crystals, which improves the selectivity of the PEC sensor to smaller molecules (e.g., H2O2). The catalytic effect of Mn-3(BTC)(2) to H2O2 enhances the PEC response sensitivity to H2O2. Under optimized conditions, the reciprocal of the averaged steady effective inner resistance of Mn-3(BTC)(2)/g-C3N4/TiO2 NTs is in a linear correlation with the logarithmic concentration of H2O2 in the range of 0.003 to 10 mM with a limit of detection of 1 nM. This PEC sensor was applied for H2O2 determination in exhaled breath condensate samples. (C) 2017 Elsevier Ltd. All rights reserved.