Single-drop gold nanoparticles for headspace microextraction and colorimetric assay of mercury (II) in environmental waters

A novel headspace colorimetric nanosensor strategy for specific detection of Hg(II) was developed based upon analyte induced etching and amalgamation of gold nanoparticles (AuNPs). The Hg(II) was first selectively reduced to its volatile form, Hg(0), by stannous chloride (SnCl2) through chemical cold vapor generation (CVG) reaction. Then, the Hg(0) was headspace extracted into 37 mu L thioglycolic acid functionalized AuNP aqueous suspension containing 10% methanol as extractant and simultaneously reacted with AuNPs through the strong metallophilic Hg-Au interaction, resulting in a red-to-blue color change. Parameters influencing the chromogenic and chemical vapor generation reactions were optimized. The limit of detections were determined as 5 nM through inspection by naked-eye and 1 nM based on measurements by UV-Vis spectrometer, which are below the safe limit of Hg(II) in drinking water set by the US Environmental Protection Agency, showing excellent potential for monitoring ultralow levels of Hg(II) in environmental water samples. The assay was not interfered by the presence of other common metal ions even at 1000-fold excess over Hg(II) concentration. The outstanding selectivity results from the combined effect of selective reduction of Hg(II) by SnCl2, efficient separation of sample matrix through headspace extraction, and amalgamation process that occurs specifically between Hg and AuNPs. The method was successfully applied to the visual detection of Hg(II) in environmental water samples at a 10 nM spiking level, with recoveries in the range of 86.8-99.8%. More importantly, compared to classical calorimetric assays for detection of Hg(II), this method is featured with simplicity, quite high sensitivity and excellent selectivity. The method is also superior to most colorimetric methods for detection of Hg(II) in terms of its applicability to matrix-rich real samples including wastewater.