Probing the interactions between disulfide-based ligands and gold nanoparticles using a functionalised fluorescent perylene-monoimide dyeThis article is published as part of a themed issue in appreciation of the many important contributions made to the field of molecular photophysics by Jan Verhoeven.Electronic supplementary information (ESI) available: Additional quantum chemical predictions of fluorescence quenching and details of the fluorescence correlation analaysis. See DOI: 10.1039/c0pp00

The binding of disulfides to gold nanoparticles was investigated using fluorescence spectroscopy and a perylene-monoimide dye coupled to a dissymmetric disulfide via a tetraethyleneglycolalkyl chain (PMImSS). Quantum chemical calculations using the polarizable continuum model (PCM) predict a strong quenching of perylene-monoimide fluorescence by gold nanoparticles as a result of efficient excitation energy transfer from the dye to the particle. Such quenching is indeed observed when unfunctionalised gold nanoparticles are added to a solution of PMImSS. The fluorimetric titration curves show behaviour indicative of the existence of an equilibrium between free and bound ligands (association constant 5 × 10 5 M −1 ), whereas the affinity of thiols and disulfide for gold surfaces is in general assumed to be much higher. Gold nanoparticles fully functionalised with PMImSS were synthesised and purified. Fluorescence correlation spectroscopy shows the appearance of free PMImSS ligands in dilute (approx. pM) suspensions of these PMImSS-functionalised nanoparticles over a period of several days. The fluorescence of a perylene-monoimide-based disulfide ligand reveals an unexpected lability of the thiolate-gold particle linkage in solution.