Catalytic activity of highly stable oligo-ethlyene glycol (OEG)–modified gold nanoparticles for the detection of mercury ions (Hg2+) in seawater

Mercury is extremely toxic, and continues to cause major threats to aquatic life, human health and the environment. Mercury mainly arises from seawater as a product of atmospheric deposition, therefore it is an important global issue to monitor and regulate oceanic levels, to prevent mercury reaching the human population through seafood consumption. Methods commonly used to detect Hg2+ include ICP-MS, XRD, and HPLC, however these methods require lengthy procedures and expensive, bulky equipment. Few studies have recently reported on detecting Hg2+ which exploit peroxidase-mimicking activities of gold nanoparticles (AuNPs) for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to generate a blue oxidised product. However, detection of Hg2+ in seawater is extremely challenging, as the catalytic activities of AuNPs are severely diminished under high electrolyte of the environment. In fact, we found that AuNPs reported in previous studies reduced their catalytic activity for TMB by more than 80% when NaCl concentration higher than 1.168%, which is much lower than that of in seawater and thus cannot be used for its detection in the harsh matrix. Herein, we report that once the AuNPs are successfully functionalized with oligo-ethlyene glycol (OEG), the OEG-AuNPs are highly stable not only in high NaCl concentration (up to 20%) but also in a wide range of pH (3-14). In addition, the catalytic activity of the OEG-AuNPs for TMB oxidation is strongly suppressed, and only recovered upon Au-Hg amalgamation. These novel findings underlies a straightforward yet highly selective detection for Hg2+ in seawater with sensitivity down to ppb level. The entire detection is less than 30 min and costs only a few pennies, it will help improve the overall health of both humans and aquaculture in rapid, low-cost and ultrasensitive manners.