LAUSR

Gold nanoparticles (AuNPs) exhibit interesting physico-chemical properties, which is a reason why they have become a highly useful and intensely studied material. The optical properties of AuNPs arise from their localised surface plasmon resonance (LSPR) property where the colour generated depends foremost on size and shape of the AuNPs. The surface chemical properties of AuNPs are generally a function of the ligand molecules that adsorb or bind onto the NP surface. This occurs either during synthesis where the reducing agent and or its product adsorb onto the AuNP surface, or by post-synthesis modifications of the AuNPs via, e.g., ligand-exchange reactions. Engineering or functionalising the surface chemistry of NPs allows for customising the surface chemical properties of such NPs for particular applications, e.g., biosensing. To facilitate this surface engineering or functionalisation, it is desirable that the surface is essentially free of adsorbed entities, i.e., it is 'naked'. However, naked NPs tend to agglomerate/aggregate easily and need arises for stable naked NPs. Here, we demonstrate the synthesis and functionalisation of stable 'naked' AuNPs in aqueous medium using Na2S2O5 to reduce an Au3+ precursor salt at room temperature and within a few minutes. Such AuNPs have optical properties similar to those of commonly used AuNPs produced with trisodium citrate (TSC) and are stable for several months at ambient conditions. The zeta potential of such AuNPs is close to zero mV suggesting that only a surface layer adsorption of SO42- ions occurs to provide this near electrically neutral NP surface. The 'naked' AuNPs were subsequently functionalised with poly(methacrylic acid), polyvinylpyrrolidone, Tween 20 and following this, with 16-mercaptohexadecanoic acid to facilitate binding of the resulting functionalised AuNPs to a particular reactive target. We provide evidence for the successful and reliable functionalisation of AuNPs produced with Na2S2O5, which can open up a range of applications.