Synthesis of gold nanoparticles using deep eutectic solvents and their incorporation into polyvinyl alcohol–based eutectogels: structural, chemical, and functional analysis

The research in the article aims to address the problem of limited functionality and chemical stability of composite hydrophilic materials when using traditional solvents and reaction media, fillers and synthesis methods. For the first time, deep eutectic solvents (DES) are simultaneously used as a green medium for AuNP (DES–AuNPs) synthesis and integrated into polyvinyl alcohol (PVA)-based eutectogels and also DES, creating a stable hybrid material with improved stability and functional properties. A series of deep eutectic solvents (DESs) based on choline chloride, proline, xylitol and betaine were synthesized. The success of the DES synthesis was confirmed using the nuclear magnetic resonance method (¹H NMR). The main physical and chemical properties of solvents (viscosity, thermal stability and surface tension) were studied. The impact of the chemical’s properties of the DES (viscosity, surface tension, pH) on the effectiveness of synthesis and parameters of the obtained gold nanoparticles was investigated. These DES-synthesized AuNPs were subsequently incorporated into a PVA-DES-based eutectogel, forming a hybrid nanocomposite hydrogel system. The DES-assisted synthesis yielded stable, uniformly dispersed gold nanoparticles with controlled size. Incorporation of AuNPs into the PVA eutectogel resulted in a hybrid material with enhanced mechanical strength, thermal stability and potential conductive and optical functionalities. The eutectogel matrix facilitated uniform nanoparticle distribution without requiring additional stabilizers. The size of nanoparticles, depending on the type of DES used, varies in the range from 10 to 50 nm. The results show that the formation of DES-AuNPs significantly pronounced with the moderate acidic DES and with moderate viscosity. It has been shown that DES ensures the stability of the obtained DES-AuNPs. The second part of the work was devoted to the synthesis of eutectogel based on PVA and DES-3 (betaine–xylitol) as well as the production of a composite based on it by incorporation of DES-AuNPs. The amphiphilic nature, thermal stability and high viscosity of DES provide a unique opportunity for their introduction into polymeric materials. Thus, the current study was aimed to prepare a novel PVA-based eutectogel from PVA, DES (betaine–xylitol) and with DES-AuNPs obtained with DES (betaine–xylitol). Casting method was used to prepare eutectogel composite material PVA/DES (betaine–xylitol)/DES (betaine–xylitol)-AuNPs. The eutectogel and composite eutectogel also exhibited good functional properties such as stability in various pH and absorptivity capacity. Every sample showed maximal swelling at acidic pH and reduced swelling at neutral pH; eutectogel and composite began to swell once again at pH 9.18.