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 µM. 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.