Elevation in surface enhanced fluorescence performance mediated by glutathione protonation in acidic environments

Surface enhanced fluorescence (SEF) plays a key role in sensing and cell imaging. However, in order to prevent cytotoxicity, the concentration of noble metal nanoparticles (Ag) needs to be lowered, which leads to poor SEF performance. Herein, different Ag@C (core-Ag, shell-carbon) nanoparticles were synthesized, and Ag@C nanoparticles prepared under 4 h heating (AC4.0) had the largest SEF enhancement factor (EFsef). Dilution of AC4.0 markedly reduces EFsef. Subsequently, Ag@C-GSH nanoparticles (ACG) were prepared by modifying glutathione (GSH) on the surface of diluted AC4.0. EFsef of ACG was improved at pH 4, and similar result for diluted AC4.0 at pH 8. This recovery of EFsef at low nanoparticle concentrations was attributed to a mechanism of electric field strength enhancement: acid-induced protonation of glutathione mediates aggregation of ACG via hydrogen bonding, which amplifies electric field strength at “hot spots”.