Rapid detection of mercury contamination in water by surface enhanced Raman spectroscopy

Abstract

Mercury (Hg) is a potent neurotoxin in fish, wildlife, and humans. The detection of Hg(II) ions in water therefore requires accurate, ultra-sensitive, rapid and cost effective analytical methods. We present a novel nanosensor for the field detection of Hg(II) in water by surface enhanced Raman spectroscopy (SERS). In the new SERS nanosensor, aminodibenzo-18-crown-6 (ADB18C6) was coupled with mercaptopropionic acid and the resultant crown ether derivative (TCE) was self-assembled as a recognition surface layer for Hg(II) onto the surface of a nanostructured gold substrate. The coordination of Hg(II) to the oxygen atoms of TCE led to the spontaneous binding of the metal ion into the cavity of the crown ether layer. This caused the intensity of the Raman band at 1501 cm⁻¹ for the crown ether to increase with the concentration of Hg(II) in the range of 1 × 10⁻¹¹ M to 1 × 10⁻⁶ M. Complexation between TCE and Hg(II) was further confirmed by UV-visible spectrometry, spectrofluorimetry and electrochemistry. The method was successfully applied to the determination of Hg(II) in tap water using a handheld Raman spectrometer and it demonstrated high selectivity towards Hg(II) in the presence of Pb(II) and Cd(II). This eliminated the need for extensive sample preparation and extraction procedures prior to the analysis. The limit of Hg(II) quantification by the new SERS nanosensor and the limit of detection were 1000 fold below the EPA and WHO defined levels for Hg(II) ions in water.