Polyvinylpyrrolidone-mediated synthesis of ultra-stable gold nanoparticles in a nonaqueous choline chloride–urea deep eutectic solvent

Abstract

This study investigates the role of polyvinylpyrrolidone (PVP) as a capping and stabilizing agent for the synthesis of gold nanoparticles (AuNPs) in a nonaqueous choline chloride–urea deep eutectic solvent (DES). AuNPs are obtained via chemical reduction, using L-ascorbic acid at 60 °C. Specifically, the effect of PVP's molecular weight (M_n of 10, 40, and 360 kDa) on the AuNP morphology, size, and colloidal stability is studied. The effect of the PVP's M_n on the resulting AuNPs is evaluated through UV-vis spectroscopy, via localized surface plasmon resonance (LSPR). Meanwhile, the morphology and size distribution of the nanoparticles are analyzed using electron microscopy. Our findings indicate the presence of spherical AuNPs with sizes ranging between 15 and 23 nm for PVP of 40 and 360 kDa (LSPR = 540 nm), irrespective of the media's viscosity. Notably, PVP of 40 kDa also produces luminescent gold clusters of ca. 2 nm size, the formation of which depends on the PVP's concentration. PVP of 10 kDa produces heterogeneous-sized particles due to a limited stabilization effect, while PVP of 360 kDa imposes difficulties in transferring to water. The most promising outcomes are observed with PVP of 40 kDa, as the nonaqueous colloids exhibit exceptional colloidal stability, even with drastic thermal changes. Finally, we introduce a proof of concept for dual strain-responsive and plasmonic eutectogels due to the DES' ionic conductivity and overall colloidal stability of PVP-capped AuNPs. This is achieved by immobilizing the gold colloids within gelatin-based eutectogels, showcasing potential for innovative applications in responsive transient and iontronic soft materials.