Development of an anion probe: Detection of sulfate ion by two-photon fluorescence of gold nanoparticles

Anion-selective detection is demonstrated for sulfate ion in aqueous solutions by using two-photon excited fluorescence of gold nanoparticles (AuNPs) modified with a thiourea-based anion receptor, bis[2-(3-(4-nitrophenyl)thioureido)ethyl]-disulfide. The fluorescent intensity increased with the change of the sulfate concentration in the solution from 10^-4 to 10^-3 M. In comparison with an unadsorbed receptor molecule in bulk acetonitrile solution, the molecule on AuNPs in water showed improved affinity for sulfate ion. The controllability of the hydrophobicity around receptor molecules on AuNPs is considered a dominant contributing factor for improved sulfate affinity. This unique feature of the surface enables us to detect anionic species in an aqueous phase where a dye-type indicator has poor sensitivity.