Colloidal stability and rheological properties of gold nanoparticle–loaded polymeric hydrogels: impact of nanoparticle's shape, surface modification, and concentration

Gold nanorods (GNR) and gold nanospheres (GNS) of different surface charges; neutral -poly ethylene glycol (PEG), negatively charged poly acrylic acid (PAA), and positively charged poly allyl amine hydrochloride (PAH), were synthesized and characterized, and their colloidal stability was investigated using UV-Vis absorption spectra and zeta potential measurements. Gold nanoparticles (GNP) were loaded at different concentrations (from 1.0 to 6.0 nM) into two hydrogels prepared in water; 18% w/v Poloxamer 407 and 1.5% w/v hydroxypropyl methylcellulose (HPMC). The GNP hydrogels were characterized in terms of GNP colloidal stability, and the rheological properties of the GNP hydrogels. GNP loaded into HPMC hydrogel demonstrated poor stability and exhibited separation into two layers. On the contrary, colloidal stability of the GNP was preserved upon loading into Poloxamer 407 hydrogel. The rheology study demonstrates that Poloxamer 407 hydrogel was able to accommodate GNR or GNS at concentrations lower than 6.0 nM with no apparent change in the measured rheology parameters. PAH-coated GNP caused the largest decrease in the low shear rate viscosity values. The elastic modulus was higher than the viscous modulus at the entire concentration range and the values of the phase shift angle did not exceed 10 degrees indicating prevail elastic property. As a conclusion, Poloxamer 407 hydrogel is a good candidate as a topical vehicle for both GNR and GNS at concentrations lower than 6.0 nM.