Naked-eye colorimetric detection of Cu2+ and Ag+ ions based on close-packed aggregation of pyridines-functionalized gold nanoparticles

A series of nano gold-based colorimetric sensors has been developed for the selective detection of Cu2+ and Ag+ metal ions in aqueous solution. Monodisperse gold nanoparticles protected with octanethiol have been prepared facilely and then, modified with pyridine moieties, chelidamate or 2-aminopyridine to give chelidamate-modified and 2-aminopyridine-modified gold nanoparticles, respectively. Aggregation of these pyridine-modified AuNPs was induced immediately upon addition of Cu2+ and Ag+ ions, yielding a color change. The aggregation was first monitored using the naked eye and then UV–vis spectroscopy and TEM.The recognition mechanism is attributed to the unique structures of these modified nanoparticles and sandwich coordination (2 + 1) between metal ion and two pyridine moieties that are attached to separate nanoparticles. In the case of Cu2+ ion sensing, the inter-particle cross-linking of modified nanoparticles results in an aggregation and formation of the black precipitates. On the other hand, sensing of Ag+ ion can be monitored with an apparent color change from brown to purple. TEM experiments and optical imaging support the optical absorption data proving aggregation of the nanoparticles upon addition of metal ions. These gold nanoparticle based colorimetric assay is a facile and robust method and allows fast detection of metal ions at ambient temperatures. More importantly, the developed technique does not utilize enzymatic reactions, light-sensitive dye molecules, lengthy protocols or sophisticated instrumentation.