Cleaner synthesis of betaine-based natural deep eutectic solvents and the effect of their composition on silver nanoparticle suspensions

A key challenge in nanotechnology remains the development of methods to produce nanoparticles in high concentrations that are stable over time, without relying on toxic or costly reagents. The aim of the study was to analyse the effect of the composition of natural deep eutectic solvents on the stability, concentration and size of formed silver nanoparticles (AgNPs). By selecting the composition of the natural deep eutectic solvent (NDES), it was feasible to obtain a product that simultaneously performed the functions of reductant, stabiliser and solvent for AgNPs while maintaining the low price of the NDES and its non-toxicity. AgNPs obtained in an NDES based on betaine-glucose-ethylene glycol were characterised by a particle size of about 15 nm, and 3 −8 nm nanoparticles represented more than 80 %vol. Depending on the composition, AgNPs suspensions were characterised by a stability of more than 180 days, and the possibility of increasing the concentration of AgNPs from 250 to 5000 ppm (mg/kg) was confirmed. The study proved the relevance of NDESs in nanotechnology as a versatile and multifunctional material, especially in the synthesis of stable suspensions of high-concentration AgNPs. • NDES enables stable, high-concentration silver nanoparticle suspensions. • Betaine-glucose-glycol NDES yields Ag NPs of 3–10 nm with > 180 days of stability. • NDES enables the synthesis of 5000 ppm silver nanoparticle suspensions. • NDES acts as a solvent, reducer, and stabilizer for multifunctional Ag NPs. • Careful selection of NDES composition ensures nanoparticle quality and safety.