Temperature‐Resolved Fluorescence Quenching on Ultrasmall Gold Nanoparticles Conjugated with Dye‐Labeled Peptides

Fluorescent peptides were covalently attached to the surface of ultrasmall gold nanoparticles (2 nm), that is, FITC‐AlaCys, FITC‐Ala‐Ala‐Ala‐Cys, and Trp‐Ala‐Cys (WAC). For comparison, the fluorescent dye AlexaFluor‐647 was attached to glutathione‐coated gold nanoparticles via click chemistry. Each particle carried between 10 and 20 fluorescent ligands, except for WAC where about 60 peptides were attached to each nanoparticle. The photophysical properties of dissolved and nanoparticle‐conjugated dyes were assessed by UV‐Vis and fluorescence spectroscopy, including measurements of the absolute photoluminescence quantum yields at the same dye concentration. After conjugation to the nanoparticles, the quantum yield decreased by a factor of 10 to 20 in the FITC systems but only by a factor of 1.5 in the AlexaFluor‐647 system. The intrinsic fluorescence was almost completely lost for WAC after conjugation to the nanoparticles. The fluorescence intensity at temperatures up to 85°C showed a decrease with temperature in all cases which was stronger for dissolved dyes than for nanoparticle‐conjugated dyes. The fluorescence was fully recovered after cooling back to ambient temperature, that is, there was no permanent change of the particles caused by heating.