Anhydrous metal nanoparticle suspensions using deep eutectic solvents (DES) – Green approach to metal nanoparticles production

Following the principles of green chemistry and the concept of sustainable development, a method was developed to obtain suspensions of metal nanoparticles, i.e. Ag, Au, Cu, Ni and Sn in deep eutectic solvents (DESs). The choice of DES (type III quaternary ammonium salt + hydrogen bond donor) as the base for the preparation of metal nanoparticles allowed the elimination of unnecessary solvents and the removal of the introduction of additional reactants, as DES acted as both a metal ion reducer and a stabiliser of the formed nanoparticles in the system. Running the process to obtain metal nanoparticle suspensions in an anhydrous environment (possible trace water content) ensured high stability of the nanoparticles. Suspensions with a nanoparticle concentration of 250 mg/kg were characterised by a reduced pH of 5–6, which in the future increases their application potential in, among others, coatings exposed to skin or in acidic concentrates used for cleaning. Spherical nanoparticles with an average size of approx. 14.40, 14.00, 60.38, 26.50 and 16.36 nm were obtained for Ag, Au, Cu, Ni and Sn NPs, respectively. The developed method will enable the future use of metal nanoparticle suspensions in DESs as additives both in materials sensitive to the aqueous environment and in products with neutral or acidic pH. The developed method is also a versatile approach to the synthesis of nanoparticles, following the principles of green chemistry by maximising the potential of the reactants used.