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 mu M. This meets the detection requirement of the World Health Organization for the arsenic concentration in drinking water [less than 10 mu g L-1 (130 nM)]. In addition, whether it was cationic or anionic interferents except phosphate (PO43-), 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.