Detection of mercury ions based on mercury-induced switching of enzyme-like activity of platinum/gold nanoparticles

Abstract

In this study, bimetallic platinum/gold nanoparticles (Pt/Au NPs) were found to exhibit peroxidase-like activity, and the deposition of mercury was found to switch the enzymatic activity to a catalase-like activity. Based on this phenomenon, we developed a new method for detecting mercury ions through their deposition on bimetallic Pt/Au NPs to switch the catalytic activity of Pt/Au NPs. Pt/Au NPs could be easily prepared through reduction of Au³⁺ and Pt⁴⁺ by sodium citrate in a one-pot synthesis. The peroxidase catalytic activity of the Pt/Au NPs was controlled by varying the ratios of Pt to Au. The Pt_0.1/Au NPs (prepared with a [Au³⁺]/[Pt⁴⁺] molar ratio of 9.0/1.0) showed excellent oxidation catalysis for H₂O₂-mediated oxidation of Amplex® Red (AR) to resorufin. The oxidized product of AR, resorufin, fluoresces more strongly (excitation/emission wavelength maxima ca. 570/585 nm) than AR alone. The peroxidase catalytic activity of Pt_0.1/Au NPs was switched to catalase-like activity in the presence of mercury ions in a 5.0 mM tris(hydroxymethyl)aminomethane (Tris)-borate solution (pH 7.0) through the deposition of Hg on the particle surfaces owing to the strong Hg–Au metallic bond. The catalytic activity of Hg–Pt_0.1/Au NPs is superior (by at least 5-fold) to that of natural catalase (from bovine liver). Under optimal solution conditions [5.0 mM Tris-borate (pH 7.0), H₂O₂ (50 mM), and AR (10 μM)] and in the presence of the masking agents polyacrylic acid and tellurium nanowires, the Pt_0.1/Au NPs allowed the selective detection of inorganic mercury (Hg²⁺) and methylmercury ions (MeHg⁺) at concentrations as low as several nanomolar. This simple, fast, and cost-effective system enabled selective determination of the spiked concentrations of Hg²⁺ and MeHg⁺ in tap, pond, and stream waters.