Highly Selective Photoelectrochemical Assay of Arsenate Based on Magnetic Co₃O₄–Fe₃O₄ Cubes and the Negative-Background Signal Strategy

Water pollution presents a significant environmental concern on earth. Herein, due to the serious environmental harmfulness of arsenate [As­(V)], an iron phthalocyanine (FePc)-induced switchable photocurrent-polarity platform was developed for highly selective assay of As­(V). First, magnetic Co₃O₄–Fe₃O₄ cubes were obtained by calcination of the CoFe Prussian blue analogue and then functionalized with oligonucleotide (S1). In the presence of As­(V), S1 could be released based on the stronger affinity between As­(V) and Co₃O₄–Fe₃O₄ cubes. After magnetic separation by Co₃O₄–Fe₃O₄ cubes, the released S1 was used to trigger the catalytic hairpin assembly (CHA) and hybridization chain reaction, resulting in the formation of lots of G-quadruplex structures on the AgInS₂/ITO electrode. Then, the capture of FePc by the G-quadruplex led to the switch of the photocurrent polarity of the AgInS₂/ITO electrode from the anode to the cathode. Thus, As­(V) was sensitively assayed with a low detection limit of 1.0 nM and a wide linear response range from 10 nM to 200 μM. This meets the detection requirement of the World Health Organization for the arsenic concentration in drinking water [less than 10 μg L^–1 (130 nM)]. In addition, whether it was cationic or anionic interferents except phosphate (PO₄^3–), only As­(V) could generate the cathodic photocurrent, effectively avoiding the false-positive or false-negative results during As­(V) assay. Interestingly, As­(V) was also simultaneously separated from the detection system by Co₃O₄–Fe₃O₄ magnetic cubes. The proposed photoelectrochemical platform may have a great potential application for the selective detection of As­(V) in environmental fields.