Biphasic Approach for the Synthesis of Au Nanoparticles via Diffusional Release of Gallic Acid from Supramolecular Hydrogels to Aqueous Solutions: Implications for Biomedical Applications

Owing to the unique optical and chemical properties of plasmonic nanoparticles, new synthetic approaches for their fabrication are constantly being developed to control their size, shape, colloidal stability, and surface functionalization. This work presents a biphasic method based on the controlled release of a reducing agent from a supramolecular polymer hydrogel to produce gold nanoparticles (Au NPs). Specifically, Au NPs are obtained by gallic acid (GA) diffusion from a poly(vinyl alcohol) (PVA)/GA hydrogel to a Au3+ solution using the reducing properties of polyphenols. The formation process of Au NPs is investigated under different conditions, such as changing the PVA/ GA stochiometric ratio and Au3+ concentration. It is demonstrated that a subtle change in the conditions leads to different Au NPs sizes and morphologies. Au NPs of small size can be obtained (<20 nm) using this method, in contrast with Au NPs produced by simply mixing GA with Au3+ at any GA/Au3+ concentration ratio. For instance, when 5 wt % GA in the PVA-GA hydrogels in combination with 0.25 mM Au3+ solution is used, spherical and highly monodispersed (7 +/- 2 nm) GA-functionalized Au NP colloidal dispersions can be obtained, giving a suitable substrate for biomedical applications. The results are rationalized in terms of different standard diffusional models, morphological characterization of the PVA-GA hydrogels, and UV-vis spectroscopy in combination with electrodynamic simulations.