Mechanistic insights into the phase-directed green synthesis of antibacterial copper nanoparticles in ethaline deep eutectic solvent

Abstract

Developing green and controllable routes for copper nanoparticle synthesis remains important for expanding their functional applications. Herein, a non-electrochemical one-pot chemical reduction strategy was developed for synthesizing copper nanoparticles (Cu NPs) in ethaline deep eutectic solvent (DES). The phase evolution of copper species was systematically investigated by regulating reaction temperature and time. The results revealed a stepwise transformation pathway from soluble [CuCl₄]²⁻ complexes to an amorphous Cu^II–O/OH–H₂O precursor, a crystalline Cu₂O intermediate, and finally metallic Cu⁰ nanoparticles. This transformation was driven by the combined effects of KOH-induced coordination perturbation, NaH₂PO₂-mediated stepwise reduction, and ethaline-regulated interfacial confinement. Under the optimized condition of 383 K for 6 h, high-purity (>99 wt%), highly crystalline, quasi-spherical Cu NPs with a median particle size of 94.6 nm were obtained without external capping agents. The optimized Cu NPs exhibited stronger antibacterial activity than commercial ultrafine Cu powder against Staphylococcus aureus and Escherichia coli under agar-dilution conditions. At 0.5 g L⁻¹ after 12 h incubation, no visible colony formation was observed for the Cu NP-treated groups. Concentration-dependent antibacterial tests, FESEM observations, and ICP-MS analysis further suggest that the antibacterial activity is associated with nanoscale particle–bacteria interactions, bacterial envelope damage, and enhanced availability of soluble copper species. This work provides mechanistic insight into DES-mediated phase-directed Cu nanoparticle formation and offers a practical route for preparing functional antibacterial copper nanomaterials.