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Surface and structural properties of gold nanoparticles and their biofunctionalized derivatives in aqueous electrolytes solution

Gold nanoparticles (AuNP) prepared by reduction with sodium citrate and commercial gold colloid particles (AuCP) were examined and compared. The properties (size, shape, inner structure, electrokinetic potential, colloid stability) of both gold particles and their biofunctionalized derivatives with L-cysteine and L-glutathione were studied in the presence of inert electrolyte (sodium nitrate). The structural investigations by XRD and SAXS measurements indicated an aggregated inner structure for AuNP, d ~ 10 nm and compact structure for AuCP d ~ 500 nm. The isoelectric point of AuCP was found to be at pHiep around 2.5 which was explained by considering the distribution and accumulation of H+ and OH– ions within the interfacial water layer, being more pronounced for OH– ions. In the presence of citrate ions isoelectric point of AuCP was slightly shifted to lower pH value pHiep around 2. In contrast to AuCP, citrate-reduced AuNP were negatively charged in the whole pH region, i.e. no isoelectric point was observed in the examined pH region. This observation cannot be explained by the simple adsorption mechanism but rather by considering the presence of citrate species in the course of reduction and precipitation of gold nuclei. The functionalization of AuCP particles resulted in a slightly higher isoelectric point, pHiep 3. However, the functionalization of citrate-reduced AuNP changed their properties drastically. The stability of gold dispersion of both AuCP and AuNP was found to be related to electrokinetic charge. Dispersions were stable in negative but unstable in the isoelectric region.

gold nanoparticles; inner structure; surface properties; isoelectric point; XRD; SAXS

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