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

Abstract

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.