Oxidase-like mimic of Ag@Ag3PO4 microcubes as a smart probe for ultrasensitive and selective Hg2+ detection

An oxidase-like mimic system based on facilely synthesized Ag@Ag3PO4 microcubes (Ag@Ag3PO4MCs) was designed and utilized to detect mercury ions with high selectivity and ultrasensitivity. Ag@Ag3PO4MCs with an average size of ca. 1.6 μm were synthesized by the reaction of [Ag(NH3)2]+ complex and Na2HPO4 and subsequent photoreduction under ultraviolet light. The as-prepared Ag@Ag3PO4MCs can effectively catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) and o-phenylenediamine (OPD) in the presence of dissolved oxygen in slightly acidic solution, exhibiting oxidase-like activities rather than peroxidase-like activity. Interestingly, the introduction of Ag nanoparticles (AgNPs) on the surfaces of Ag3PO4MCs can dramatically enhance the oxidase-like activities due to a synergistic effect between AgNPs and Ag3PO4MCs, as evidenced by the faster oxidation speed of TMB and OPD than that of native Ag3PO4MCs in the presence of dissolved oxygen. The enzyme kinetics can be well-explained by the Michaelis–Menten equation. As “poisoning” inhibitor, Hg2+ ions can inhibit the enzyme reaction catalyzed by Ag3PO4MCs or Ag@Ag3PO4MCs. On the basis of this effect, a colorimetric Hg2+ sensor was developed by the enzyme inhibition reaction of Ag3PO4MCs or Ag@Ag3PO4MCs. The excellent specific interaction of Hg–Ag or Hg2+–Ag+ provides high selectivity for Hg2+ over interfering metal ions. Meanwhile, the sensitivity of this sensor to Hg2+ is extremely excellent with a limit of detection as low as 0.253 nM for Ag@Ag3PO4MCs. Considering the advantages of low detection limit, low cost, facile preparation, and visualization, the colorimetric Ag@Ag3PO4MCs sensor shows high promise for the testing of Hg2+ in water samples.