Keywords: fluorescence, gold nanoparticles, self-assembled monolayers, PEGylation, disulfides, fluorescent chromophores, nanotechnology, bioanalytical detection, biological imaging, phototherapy, self-assembly, quenching, monolayer chemistry
Hydrophilic monolayer-protected gold nanoparticles and their functionalisation with fluorescent chromophores
The efficient implementation of metal nanoparticles in bioanalytical detection schemes, biological imaging and phototherapy requires these particles to be stable against aggregation and chemical desintegration. The functionalisation of these particles with specific (bio)molecular units is a further necessity. These requirements may be met by covering the particle surface with self-assembled monolayers of functional thiols and disulfides. We have synthesised organic disulfides carrying alkyl-oligoethylene glycol chains that self-assemble into a molecular monolayer on the surface of gold nanoparticles to give rise to robust water-soluble nano-objects. These monolayer-protected gold nanoparticles can be purified using centrifugation, dialysis and gel permeation chromatography. The number of available disulfide binding sites on the gold nanoparticles was determined experimentally, and found to be in reasonable agreement with theoretical estimates, indicative of the formation of a dense protecting monolayer. The introduction of molecular functions through amino-terminated oligoethylene glycol disulfides is exemplified using a fluorescent chromophore, fluorescein. The quenching of the fluorescence of fluorescein by the metal nanoparticle core is investigated quantitatively, and employed as a fluorimetric tool for the investigation of the monolayer chemistry of these water-soluble functionalised gold nanoparticles.