Amalgamated gold-nanoalloys with enhanced catalytic activity for the detection of mercury ions (Hg²⁺) in seawater samples

Mercury (Hg) is extremely toxic, and continues to cause major threats to aquatic life, human health and the environment. Hg²⁺ mainly derives from seawater as a product of atmospheric deposition, therefore there is great demand for sensing approaches that can detect Hg²⁺ in seawater samples. Herein, we demonstrate that the peroxidase-mimicking activity of gold nanoparticles (AuNPs) or so-called nanozymes, can be exploited for the detection of Hg²⁺ ions in various water samples. In a high electrolyte environment, the catalytic activity for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) was significantly diminished due to poor stability of the bare-AuNPs. This activity was reduced by ~ 73.7% when the NaCl concentration was higher than 1.168%, which is much lower than that of seawater (~ 3.5%), thus presenting its unsuitability for detecting Hg²⁺ in harsh water matrices. To overcome this limitation, AuNPs were first functionalized with oligo-ethylene glycol (OEG), of which their colloidal form presented high stability in NaCl concentrations up to 20% and across a wide range of pHs from 1-14. Interestingly, the catalytic activity of OEG-AuNPs for the oxidation of TMB was strongly suppressed by the coating, but enhanced upon formation of Au-Hg amalgamation. This novel finding underlies a straightforward, sensitive, and highly selective detection platform for Hg²⁺ in water samples. The approach could detect the exposure limit level for Hg²⁺ in drinking water (i.e., 2 ppb for tap and bottled water) as set by the United States Environmental Protection Agency (EPA) and the World Health Organization (WHO). When Hg²⁺ was spiked into a 3.5% saline solution and a coastal seawater certified reference material (CRM), the detection limits were found to be 10 and 13 ppb, respectively, which exceed the Hg²⁺ concentrations commonly found within seawater (~ 60-80 ppb). The whole procedure takes less than 45 min to conduct, providing a highly innovative, rapid and low-cost approach for detecting Hg²⁺ in complex water matrices.