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.

Detection of enzyme biomarkers originating from either bio-fluids or contaminating microorganisms is of utmost importance in clinical diagnostics and food safety. Herein, we present a simple, low-cost and easy-to-use sensing approach based on the switchable peroxidase-mimicking activity of plasmonic gold nanoparticles (AuNPs) that can catalyse for the oxidation of 3, 3o, 5o5-tetramethylbenzidine (TMB) for the determination of protease enzyme. The AuNP surface is modified with casein, showing dual functionalities. The first function of the coating molecule is to suppress the intrinsic peroxidase-mimicking activity of AuNPs by up to 77.1%, due to surface shielding effects. Secondly, casein also functions as recognition sites for the enzyme biomarker. In the presence of protease, the enzyme binds to and catalyses the degradation of the coating layer on the AuNP surface, resulting in the recovery of peroxidase-mimicking activity. This is shown visually in the development of a blue colored product (oxidised TMB) or spectroscopically as an increase in absorbance at 370 and 650 nm. This mechanism allows for the detection of protease at 44 ngdmL^-1 in 90 min. The nanosensor circumvents issues associated with current methods of detection in terms of ease of use, compatibility with point-of-care testing, low-cost production and short analysis time. The sensing approach has also been applied for the detection of protease spiked in ultra-heat treated (UHT) milk and synthetic human urine samples at a limit of detection of 490 and 176 ngdmL^-1, respectively, showing great potential in clinical diagnostics, food safety and quality control.