Synthesis and Stabilization of Gold Nanoparticles Using Water-Soluble Synthetic and Natural Polymers

Gold nanoparticles (AuNPs) were synthesized and stabilized using the one-pot method and growth seeding, through utilization of synthetic polymers, including poly(N-vinylpyrrolidone) (PVP), poly(ethylene glycol) (PEG), and poly(vinylcaprolactame) (PVCL), as well as natural polysaccharides, including gellan, welan, pectin, and kappa-carrageenan. The absorption spectra, average hydrodynamic size, zeta-potential, and morphology of the gold nanoparticles were evaluated based on various factors, such as polymer concentration, molecular mass of polymers, temperature, and storage time. The optimal polymer concentration for stabilization of AuNPs was found to be 4.0 wt % for PVP, 0.5 wt % for gellan, and 0.2 wt % for pectin, welan, and kappa-carrageenan. The values of the zeta-potential of polymer-stabilized AuNPs show that their surfaces are negatively charged. Most of the AuNPs are polydisperse particles, though very monodisperse AuNPs were detected in the presence of a 0.5 wt % gellan solution. At a constant polymer concentration of PVP (4 wt %), the average size of the PVP-AuNPs decreased with the decrease of molecular weight, and in the following order: PVP 350 kDa (similar to 25 nm) > PVP 40 kDa (similar to 8 nm) > PVP 10 kDa (similar to 4 nm). The combination of Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy revealed that the functional groups of polymers that are responsible for stabilization of AuNPs are lactam ring in PVP, carboxylic groups in gellan and welan, esterified carboxylic groups in pectin, and SO2 groups in kappa-carrageenan. Viscometric and proton nuclear magnetic resonance (H-1 NMR) spectroscopic measurements showed that the temperature-dependent change in the size of AuNPs, and the gradual increase of the intensity of AuNPs at 550 nm in the presence of gellan, is due to the rigid and disordered conformation of gellan that affects the stabilization of AuNPs. The AuNPs synthesized in the presence of water-soluble polymers were stable over a period of 36 days. Preliminary results on the synthesis and characterization of gold nanorods stabilized by polymers are also presented.