Effects of water content and compositions in deep eutectic solvents on the green biosynthesis of silver nanoparticles and their activities

Deep eutectic solvents (DESs) were evaluated for their role in enhancing green tea extract (GTE)-based biosynthesis of silver nanoparticles (Ag NPs). Four types of DESs with varying water contents (10–50 %) and hydrogen bond acceptor (HBA) mole fractions (0.17–0.83) were evaluated. While HBA composition had little effect on catechin and phenolic compound extraction, higher water content usually improved yields by reducing DES viscosity. Compared with water-based GTE, all DES-based GTEs produced Ag NPs with higher surface plasmon resonance intensities, smaller particle sizes, and narrower distributions. In GTEs derived from non-urea-based DESs, these enhancements were consistent regardless of the water content and HBA mole fraction, suggesting inherent beneficial effects of the DESs. However, urea-based DESs that contained ammonia as a byproduct during DES preparation, exhibited different characteristics: although the geometric sizes of the Ag NPs remained unchanged, increasing water content led to larger hydrodynamic sizes, indicating intensified surface modification of the Ag NPs. The surface modification correlated positively with antioxidant activity, negatively with catalytic activity, and exhibited no significant association with antibacterial activity. Overall, DESs generally enhance the GTE-based biosynthesis of Ag NPs without the need for precise condition optimizations. However, ammonia-generating DESs may require tailored conditions to balance synthesis and activities.