Catalytic gold nanostars for SERS-based detection of mercury ions (Hg2+) with inverse sensitivity

Gold nanostars (AuNSt) exhibit outstanding catalytic and plasmonic properties derived from their nano-size and shape. Their ability to mimic natural enzymes is exploited herein to oxidise 3,3 ',5,5 '-tetramethybenzidine (TMB) into an oxidised product (oxTMB), a highly Raman-active compound. AuNSt with different surface coatings including sodium citrate (NaCit), polyethylene glycol (PEG), oligoethylene glycol (OEG) and 11-mercaptoundecanoic acid (11-MUA) were prepared and characterised both as catalysts and Raman enhancers. AuNSt-PEG showed the best results as SERS substrates for oxTMB, therefore being chosen for the detection of mercury ions (Hg2+). Hg is one of the most toxic elements known, and is a major environmental concern due to its adverse health effects. Hg is also known for forming amalgams with most metals, a property that is used in this study to alter the shape and size of AuNSt, thus limiting their Raman enhancement nature by amalgamating their sharp tips which are responsible for most SERS 'hot spots'. As a result, the Raman peak of oxTMB at 1190 cm(-1) shows an inverse linear dependence with increasing Hg2+ ion concentration. This approach achieved a limit of detection of 0.2 ppb in seawater (certified reference materials, CRM), and a linear response between 0.1 and 1000 ppb. Thus, this ground-breaking technique could contribute to the protection of water systems, aquaculture and human health, whilst providing a biosensing platform for numerous applications in the future.