Tuning the interparticle distance between assembled copper nanocrystals to improve the optical plasmon resonance, photothermal, and antibacterial properties under visible light

Here, we report the assembly of plasmonic copper nanoparticles capped by poly (vinyl alcohol) via esterification with oxalic acid (OA) as a crosslinking agent. As-synthesized copper nanoparticles measure 8.6 nm and are single crystals, displaying a plasmonic resonance at 392 nm. An increase in the reaction time from 15 to 240 min leads to the assembly of nanoparticles into clusters with decreasing interparticle distances. This is reflected in the appearance of the second plasmon resonance at a longer wavelength, whose intensity and position depend on the reaction time. The influence of the interparticle distance between the assembled copper nanoparticles on the wave propagation of visible light and the optical parameters was studied. The results revealed that decreased interparticle distance between the copper nanocrystals increases the plasmon pulsation frequency and the penetration depth of visible light. This new finding remarkably enhanced the surface plasmon polariton and the surface plasmon resonance along the interface between the dielectric medium and copper nanocrystals. Therefore, the developed nano‑copper assembly with various interparticle distances exhibited outstanding photothermal performance under visible light irradiation for the first time. The study of the antibacterial activity of assembled copper nanoparticles against hospital-associated methicillin-resistant Staphylococcus aureus (HA-MRSA) under visible light irradiation revealed a remarkable efficiency of the nanoparticle sample obtained after 30 min of the reaction with OA and containing medium-size clusters with the interparticle distance of 0.9 nm. This was attributed to an optimal cluster size, allowing easy penetration of the bacterial cell membrane and the comparatively small interparticle distance between copper nanoparticles, leading to strong interparticle interactions and influencing antibacterial activity. This unique feature may open a new avenue for using the proposed synthetic recipe as an alternative for photothermal therapy under visible irradiation instead of laser or UV irradiation with serious harmful effects on living cells.