Photoelectrochemical Sensor with a Z‑Scheme Fe₂O₃/CdS Heterostructure for Sensitive Detection of Mercury Ions

Mercury (Hg²⁺) is a highly toxic element and can seriously affect human health. This work proposed a photoelectrochemical (PEC) sensor with a Z-scheme Fe₂O₃/CdS heterostructure and two thymine-rich DNA strands (DNA-1 and Au@DNA-2) for sensitive detection of Hg²⁺. The light excitation of the Fe₂O₃/CdS composite accelerated the electron transfer among Fe₂O₃, CdS, and the electrode to produce a stable photocurrent response. Upon the recognition of Hg²⁺ to thymine bases (T) in two DNA strands to form a stable T-Hg²⁺-T biomimetic structure, the photocurrent response increased with the increasing concentration of Hg²⁺ due to the opening of electronic transmission channels from Au nanoparticles to Fe₂O₃/CdS nanocomposite. Under the optimal conditions screened by the Box–Behnken experiments, the proposed PEC sensor showed excellent analytical performance for Hg²⁺ detection with high sensitivity, a detection limit of 0.20 pM at a signal-to-noise ratio of 3, high selectivity, a detectable concentration range of 1 pM–100 nM, and acceptable stability. The good recovery and low relative standard deviation for the analysis of Hg²⁺ in lake and tap water samples demonstrated the potential application of the designed Z-scheme Fe₂O₃/CdS heterostructure in the PEC detection of heavy metal ions.