Label-Free Cysteamine-Capped Silver Nanoparticle-Based Colorimetric Assay for Hg(II) Detection in Water with Subnanomolar Exactitude

Precisely probing heavy metal pollutants in water warrants novel methods and materials. To this end, functionalization of nanoparticles using biologically important substances through a green route is a novel aspect in the design of an optical sensor. In this article we report a green preparative strategy for the synthesis of cysteamine stabilized silver nanoparticles (Ag-Nps) in aqueous medium. The water-soluble Ag-Nps are found to be highly sensitive and selective for rapid colorimetric detection of Hg(II) ion with a limit of detection (LOD) of 0.273 nM (55 ppt). This system also enables us to detect Hg(II) through the naked eye with an LOD of 2.73 nM (0.55 ppb) which is below the World Health Organization (WHO) permissible limit (10 nM or 2 ppb). Cysteamine undergoes cooperative coordination with the mercury ion leading to spontaneous formation of a mercury–cysteamine complex and consequently forms Ag–Hg nanoalloy, which in turn changes the surface plasmon property of Ag-Nps to allow detection of Hg(II) ion with subnanomolar precision. Furthermore, Ag-Nps were tested for detection of Hg(II) in different real water samples with satisfying recoveries over 96–102%.