Retarding oxidation of core@shell Ag/Cu nanoalloys: the role of Ag shell, PVP and thiolate capping agent

Metallic nanoparticles, such as silver (AgNP) and copper (CuNP), have been extensively used as alternatives to microbial resistance. Nevertheless, AgNP’s high cost and CuNP’s easy oxidation limit their use. Ag/Cu bimetallic NPs emerged as an alternative, but recent studies do not explore the variables that affect their long-term stability in the aqueous phase. In this scenario, we propose optimizing the synthesis of Ag/Cu nanostructures using factorial design statistics evaluating how different variables affect Ag/CuNP synthesis and stabilization. Increasing the reaction temperature hampered nanoparticle formation, while N2 purging during the reaction did not affect long-term stability. On the other hand, the simultaneous use of polyvinylpyrrolidone (PVP), 3-Mercaptopropionic acid (3-MPA), and creating an external Ag shell (@Ag) on the NPs surface significantly retarded or inhibited samples oxidation. Major driving forces were the chelating power of the polymer, NP’s surface passivation, and inhibition of the silver electromigration mechanism. Dynamic light scattering and UV-Vis spectroscopy analyzes confirmed the stability of the optimized PVP-capped Ag/Cu@Ag core@shell NPs. In addition, transmission electron microscopy confirmed their core@shell morphology and an Ag/Cu nanoalloy phase in the particle core. Antimicrobial activity studies showed selectivity toward bacteria, including the biofilm-forming Pseudomonas Aeruginosa. PVP-capped Ag/Cu@AgNPs resistance to oxidation allowed a slow metal ions release, which induced a synergic antimicrobial effect between Ag and Cu ions at low copper contents. Therefore, a rational synthesis design and the application of statistical tools allowed for obtaining highly stable antibacterial nanostructures. The results show that PVP-capped Ag/Cu@AgNPs are promising new-generation antibacterial agents.