Ag-β-Cyclodextrin-Graphene Oxide Ternary Nanostructures with Peroxidase-Mimicking Activity for Hg2+ Detection

To strengthen the properties of nanozymes, a Ag-β-cyclodextrin-graphene oxide ternary nanocomposite (Ag-β-CD-GO) was developed first by a homogeneous redox-active self-assembly method. Taking advantage of the excellent environmental compatibility, high surface areas, and strong hydrogen-bonding ability of β-cyclodextrin and graphene oxide, the proposed Ag-β-CD-GO exhibited superior peroxidase-mimicking activity, high stability, and nontoxicity as well. It could accelerate the oxidation–reduction reaction of the common colorimetric substrate 3,3′,5,5′-tetramethylbenzidine (TMB) and the oxidant H2O2 with Michaelis constants (Km)/maximal reaction rates (Vmax) of 3.3 mmol·L–1/2.45 × 10–8 mol·L–1·s–1 and 0.13 mmol·L–1/2.52 × 10–8 mol·L–1·s–1, respectively. Interestingly, toxic Hg2+ could decrease the characteristic UV–vis absorbance at 653 nm of the Ag-β-CD-GO-TMB-H2O2 system exclusively with an obvious color change from blue to colorless, expressing a visual hypochromic effect. Under the optimal testing conditions (pH 4.0, 180 μL of 1.5 mmol·L–1 TMB, 180 μL of 1.0 mol·L–1 H2O2, incubation for 20 min at 25 °C), the target Ag-β-CD-GO-TMB-H2O2 was efficiently utilized for visual monitoring of toxic Hg2+ in natural water, drink, and fruit juice samples with quite a low detection limit, i.e., 8.3 × 10–10 mol·L–1 (S/N = 3), far below the 3.0 × 10–8 mol·L–1 permitted in drinking water by the World Health Organization (WHO). The synergetic enhancement peroxidase-mimicking activity of Ag-β-CD-GO and the exclusive recognition mechanism to Hg2+ were further investigated in detail.