Gold nanoparticle-based sensing of "spectroscopically silent" heavy metal ions

A simple colorimetric technique for the detection of small concentrations of aqueous heavy metal ions, including toxic metals such as lead, cadmium, and mercury, is described. Functionalized gold nanoparticles are aggregated in solution in the presence of divalent metal ions by an ion-templated chelation process; this causes an easily measurable change in the absorption spectrum of the particles. The aggregation also enhances the hyper-Rayleigh scattering (HRS) response from the nanoparticle solutions, providing an inherently more sensitive method of detection. The chelation/aggregation process is reversible via addition of a strong metal ion chelator such as EDTA. Suggestions for improving the sensitivity and selectivity of the technique are given.