Picomolar selective detection of mercuric ion (Hg(2+)) using a functionalized single plasmonic gold nanoparticle

A highly sensitive method for the selective detection and quantification of mercuric ions (Hg(2+)) using single plasmonic gold nanoparticle (GNP)-based dark-field microspectroscopy (DFMS) is demonstrated. The method is based on the scattering property of a single GNP that is functionalized with thiolated molecules, which is altered when analytes bind to the functionalized GNP. The spectral resolution of the system is 0.26 nm and a linear response to Hg(2+) was found in the dynamic range of 100 pM-10 microM. The method permits Hg(2+) to be detected at the picomolar level, which is a remarkable reduction in the detection limit, considering the currently proscribed Environmental Protection Agency regulation level (10 nM, or 2 ppb) and the detection limits of other optical methods for detecting Hg(2+) (recently approx. 1-10 nM). In addition, Hg(2+) can be sensitively detected in the presence of Cd(2+), Pb(2+), Cu(2+), Zn(2+) and Ni(2+), which do not interfere with the analysis. Based on the findings reported herein, it is likely that single-nanoparticle-based metal ion sensing can be extended to the development of other chemo- and biosensors for the direct detection of specific targets in an intracellular environment as well as in environmental monitoring.