FeS and MnS NPs Synthesized in Deep Eutectic Solvents: Antioxidant and Pro-Oxidant Behavior of Green Suspensions

This study reports the synthesis of FeS and MnS nanoparticles (NPs) using deep eutectic solvents (DESs) as a green, highly stabilizing reaction medium. FeS NPs exhibited reduced particle sizes compared to aqueous synthesis (190 nm and 167 nm in DES based on betaine (DES(B)) and proline (DES(P)), respectively). MnS synthesised in water formed large agglomerates (from 22 to 168 nm), whereas DES(B) yielded stable, nonaggregated particles (39 nm). FeS NPs showed significantly enhanced oxidative activity in the Fenton reaction, generating ∼60% more ∙OH radicals and increasing the reaction rate constant from 0.2215 min^{- 1} (Fe^{2 +}) to ∼1.30 min^{- 1} (FeS-DES). MnS demonstrated superior H₂O₂ decomposition, exceeding enzymatic catalase (2,000-5,000 units/mg protein, 59.1%) with efficiencies above 99.9%. Antioxidant assays (DPPH, FRAP, TMB, and ABTS) confirmed up to a sixfold increase in activity for DES-derived NPs. Under∙OH radical exposure, Fe^{2 +} in water lost all activity, while FeS-H₂O retained 60% activity and FeS-DES(P) maintained ∼90%. The results emphasize the potential application of FeS and MnS NPs as highly stable and efficient biocatalysts in redox reactions, which, due to their resistance to oxidative stress and high activity, can provide a competitive, sustainable, and economically beneficial alternative to conventional enzymes.