Fluorescence Probing of Thiol-Functionalized Gold Nanoparticles: Is Alkylthiol Coating of a Nanoparticle as Hydrophobic as Expected?

Understanding the interaction of fluorescent dyes with monolayer-protected gold nanoparticles (AuNPs) is of fundamental importance in designing new fluorescent nanomaterials. Among a variety of molecular sensors and reporters, fluorescent probes based on a 3-hydroxychromone (3HC) skeleton appear to be very promising. They exhibit the phenomenon of dual band emission, resulting from excited-state intramolecular proton transfer (ESIPT), known to be highly sensitive to a nature of microenvironment surrounding a fluorophore. In this study, dodecanethiol-protected gold nanoparticles were synthesized, and, owing to the transmission electron micrograph imaging, their average diameter was found to be ∼1.4 nm. Fluorescence titrations of the 3HC ESIPT probes with AuNPs in toluene solutions demonstrate significant changes in the intensity ratio of their normal and tautomeric emission bands, suggesting that the probe molecules become noncovalently bound to a dodecanethiol layer of AuNPs. Despite expected fluorescence quenching induced by close proximity to the metal surface, no fluorescence lifetime decrease was observed, indicating that a bound-fluorophore is shielded from a nanoparticle core. Further spectral analysis revealed that the ratiometric fluorescence changes could be interpreted as a consequence of intermolecular hydrogen bonding between a probe and residual ethanol molecules, trapped into the dodecanethiol shell of AuNPs during the synthesis. Evidences for residual traces of ethanol in the ligand shell of the nanoparticles were also observed in NMR spectra, suggesting that alkylthiol-coated gold nanoparticles may not be as hydrophobic as one could expect. To elucidate structural features of dodecanethiol-stabilized gold nanoparticles at the supramolecular level, a molecular dynamics (MD) model of AuNP was developed. The model was based on the all-atom CHARMM27 force field parameters and parametrized according to available experimental data of the synthesized AuNPs. Our MD simulations show that in bulk toluene the 3HC probe molecule becomes weakly bound to a dodecanethiol monolayer, so that a fluorophore favors residence in an outer shell of AuNP. In addition, MD simulations of transfer of AuNP from bulk ethanol to toluene demonstrate that a small population of ethanol molecules are able to penetrate deeply into the dodecanethiol layer and may indeed be trapped into the ligand shell of alkylthiol-functionalized gold nanoparticles. The results of our fluorescence experiments and molecular dynamics simulation suggest that 3-hydroxychromones can be used as a noncovalent fluorescent labeling agent for alkylthiol-stabilized noble metal nanoparticles.