pH-Triggered conversion of soft nanocomposites: in situ synthesized AuNP-hydrogel to AuNP-organogel

Amino acid based amphiphilic gelators (carboxylate salts) were employed for the in situ synthesis of gold nanoparticles (GNPs) in hydrogel networks at room temperature without using any external reducing or capping agents for the development of AuNP-hydrogel soft composite. Synthesized AuNP-hydrogel composites were then successfully converted to AuNP-organogel composites simply by lowering the pH of the aqueous medium, as the hydrogelating amphiphilic carboxylates were transformed to corresponding carboxylic acids that are efficient organogelators. These water insoluble carboxylic acids spontaneously moved from the aqueous phase to the nonpolar organic media (toluene) along with the synthesized GNPs to form the AuNP-organogel composite. The phase transfer of the GNPs from a hydrogel network to an organogel network was investigated by UV-Vis spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies. Supramolecular networks of both the gels played a crucial role in stabilization of the GNPs. Fluorescence spectroscopy was used to investigate the mechanistic detail of the in situ GNP synthesis. The characterizations indicated the formation of spherical and uniform sized GNPs and even phase transfer of the nanoparticles did not result in change of shape or size of the nanoparticles. Rational designing of the gelator/nongelator molecules helped us to recognize the key structural components required for the efficient synthesis and stabilization of the GNPs in both the phases. Rheological study suggested that the hydrogel-GNP composites possess improved viscoelastic property than the native hydrogel.