N-lauryltyramine capped copper nanoparticles exhibit a selective colorimetric response towards hazardous mercury(ii) ions and display true anti-biofilm and efflux pump inhibitory effects in E. coli

Abstract

The surface plasmon resonance properties of copper nanoparticles (CuNPs) are rarely explored for colorimetric sensor applications. Herein, we demonstrated the CuNPs prepared with a stabilizing agent, N-lauryltyramine (NLTA) selectively induce a colorimetric response toward mercury(II) ions from wine red to gray color at micromolar concentrations. The reported sensor system can detect Hg²⁺ in complex water bodies, which was proven through sensing Hg²⁺ in sewage water and also in the presence of interfering ions. The mechanism of sensing Hg²⁺ was through a galvanic displacement reaction. The as prepared CuNPs exhibited antibacterial activity by releasing copper ions from CuNPs, and we were interested in exploring the ability of NLTA capped CuNPs to prevent biofilm formation. Strikingly, the anti-biofilm activity of the CuNPs was not mediated by copper ion release and the minimum biofilm inhibitory concentration (MBIC) was observed to be much lower than the minimum inhibitory concentration (MIC), which implies that NLTA–CuNPs display a true anti-biofilm effect. At 0.5×MIC, CuNPs also displayed a remarkable 128 fold reversal in EtBr MIC and a drastic 256 fold reversal in ciprofloxacin MIC, implying an efflux pump inhibitory effect in E. coli. When tested against multidrug resistant (MDR) K. pneumoniae and A. baumanii, NLTA–CuNP displayed a remarkable 16 fold reversal in ciprofloxacin MIC validating its EPI effect against MDR Gram-negative bacteria. Mechanistic studies revealed that CuNPs alter the outer membrane permeability to retain an efflux inhibitory effect while displaying a true anti-biofilm effect.