Colorimetric Detection of Heavy Metal Ions Using Label-Free Gold Nanoparticles and Alkanethiols

We have developed a simple method for the selective colorimetric detection of aqueous mercuric (Hg2+), silver (Ag+), and lead (Pb2+) ions by using label-free gold nanoparticles (Au NPs) and alkanethiols. The degree of alkanethiol-induced aggregation of the Au NPs decreases in the order of 6-mercaptohexanol (6-MH) similar to 4-mercaptobutanol (4-MB) > 11-mercaptoundecanol (11-MU) > 2-mercaptoethanol (2-ME). The specific and strong interactions of these alkanethiols with Au NPs and heavy metal ions enabled us to develop label-free assays for the sensitive and selective detection of Hg2+ ions using the 4-MB/Au NPs probe, as well as Ag+ and Pb2+ ions using the 2-ME/Au NPs probe. The presence of strong Hg2+-S bonds alleviated the extent of 4-MB-induced aggregation of the Au NPs, resulting in a declining ratio of the extinction coefficients at 650 to 520 nm (Ex(650/520), a measure of the molar ratio of the aggregated to the dispersed Au NPs) of the Au NP solution. In contrast, the presence of Ag+, Cu2+, and Pb2+ ions led to a severe aggregation of the Au NPs, mediated by the deposition of these ions on the surfaces of the Au NPs in the 2-ME/Au NPs system. In the presence of masking agents [ethylenediaminetetraacetic acid (EDTA), Na2S], the 2-ME/Au NP-EDTA and 2-ME/Au NP-Na2S sensors permitted the selective detection of Ag+ and Pb2+ ions, respectively, at concentrations down to the nanomolar range. This cot-effective process also allowed the rapid and simple determination of the concentrations of heavy metal ions in real environmental samples (river water and Montana soil). These alkanethiol/Au NP-based sensor probes enabled us to detect three different heavy metal ions, and we feel confident that, because of the simplicity, rapidity, and cost-effectiveness of these analyses, such systems demonstrate great potential for the practical detection of heavy metal ions in real samples.