Specific turn-on near infrared fluorescence from non-fluorescent gold nanoclusters bearing sulfhydryl oligopeptides

Abstract

We describe the development of near-infrared (NIR) fluorescence from non-fluorescent gold nanoclusters as a result of the reaction with sulfhydryl oligopeptides. Non-fluorescent gold nanoclusters with a diameter of about 2nm were synthesized by reducing HAuCl4 with NaBH4 in the presence of tetraethyleneglycol. The fluorescence of the gold nanoclusters bearing sulfhydryl compounds arose from the ligand-to-metal charge transfer from the sulfhydryl compounds to the gold atoms at the gold nanocluster surface. The sulfhydryl oligopeptides cysteinylglycine, glutamylcysteine, and glutathione produced a fluorescence band around 830nm in the NIR region, whereas the sulfhydryl monopeptides cysteine and homocysteine produced fluorescence in the visible region. Among the sulfhydryl oligopeptides, only cysteinylglycine exhibited stable fluorescence without heating the solution. Because the fluorescent gold nanoclusters bearing cysteinylglycine were so stable, monothiols, amine compounds except polyethyleneimines, transition metal ions, alkali metal ions, and alkaline earth metal ions did not affect the fluorescence properties of the nanoparticles.