A Versatile Method For Surface Functionalization And Hydrophobization Of Gold Nanoparticles

A convenient, one-step, approach for surface modification with hydrophobic thiol or its dye-derivative of hydrophilic citrate-capped Au nanoparticles (Cit-AuNPs) is demonstrated. This method depended on the solubility of both Cit-AuNP and thiol or its derivative in the reaction solvent. We investigated a variety of solvents for Cit-AuNPs with 15-75 nm sizes and a capping molecule, octadecanethiol (ODT) or a disulfide derivative of hydrophobic porphyrin (PC10S)(2). As to ODT, surface immobilization of Cit-AuNPs with ODT was successful in 2-propanol, while N,N-dimethylformamide was best in the case of (PC10S)(2). In both cases, surface modified AuNPs were redispersible even after storing under dried powder conditions. Two-dimensional monoparticle films of surface-modified AuNPs by the Langmuir-Blodgett method with ethylene glycol as subphase could be formed and exhibit surface-enhanced Raman scattering. Spectra from the porphyrin skeleton were certainly observed, with the maximum photoluminescence enhancement of 15-fold, around 50-nm size. As to AuNP with 17-nm diameter, mixed monolayer assemblies of (PC10S)(2) and octanethiol with different concentration ratios were fabricated and spectroscopic enhancement effects due to localized surface plasmon resonance were investigated.