Colorimetric and visual detection of silver(I) using gold nanoparticles modified with furfuryl alcohol

The authors describe a colorimetric assay for Ag(I) ions that is highly selective over other metal ions. It is based on the measurement of changes in the surface plasmon resonance absorbance (at 520 nm) of gold nanoparticles (AuNPs) modified with furfuryl alcohol (Fu-AuNPs). The AuNPs were modified with mixed monolayers of 6-nitrohexane-1-thiol and octanethiol as capping ligands. Next, these AuNPs were modified with furfuraldehyde via an interfacial Henry reaction. The unique structure and presence of heteroatoms in the resulting product of the Henry reaction enable the Fu-AuNPs to recognize very low concentrations of Ag(I) ions, and this results in a visually and instrumentally detectable color change from pale-brown to dark blue. TEM images and optical absorption data show that this color change is the result of an aggregation of the Fu-AuNPs upon addition of Ag(I). In contrast, divalent ions such as Cu(II), Zn(II), Co(II), and Pb(II) do not aggregate, or they cause the formation of a black precipitate. The recognition mechanism is attributed to the formation of a sandwich between Ag(I) ion and two furfuryl alcohol moieties that are attached to separate nanoparticles. This simple and fast method can be used to determine Ag(I) ions with a limit of detection as low as 12 nM.