We first revealed that the bare fluorine doped tin oxide (FTO) under the cathodic polarization over -0.7 V (vs.SCE) shows very sensitive to the irradiating light in a wide wavelength region 850-400 nm in the aqueous solution free of a redox couple, and its band gap of energy E-g is determined to be 1.38 eV via the photoelectrochemical method. The bare FTO can effectively catalyze electrochemically L-cysteine (CySH) oxidation and especially shows the photocatalytic ability toward CySH oxidation. Thus the bare FTO electrode can be directly used for determination of CySH concentration using cyclic voltammetry in both the dark and light illumination and it can be used to recognize CySH among 20 alpha-amino acids found in proteins, based on the low oxidation peak potential and unique photoelectric response. The rate-determining step for the photocatalytic oxidation of CySH on the bare FTO electrode is controlled by supply of charge inside the FTO film to the electrode surface, which exhibits the typical characteristics of semiconductors. (C) 2016 Elsevier Ltd. All rights reserved.