Antibacterial Surfaces Prepared through Electropolymerization of N-Heterocyclic Carbene Complexes: A Pivotal Role of the Metal.

Abstract

N-Heterocyclic carbene (NHC) complexes are known to have antibacterial properties in solutions. However, these complexes have never been immobilized on solid supports to prepare antibacterial surfaces. Here, we tackled this lack and succeeded in immobilizing these NHC complexes on gold surfaces by electropolymerization. For this, we synthesized a series of various NHC complexes of different low-valent transition metals (M = Ag(I), Au(I), Rh(I), Ru(II), Cu(I)) bearing a pyrrole function at the five-membered carbenic cycle. We measured the antibacterial properties of these complexes against two Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and two Gram-positive bacteria (Staphylococcus aureus and Listeria innocua) by determining their minimum inhibitory concentration (MIC) values. All NHC complexes presented interesting antibacterial properties that are metal-dependent. The silver-NHC complex showed higher antibacterial activity against Gram-negative bacteria (MIC = 16 μg·mL^-1) than against Gram-positive bacteria (MIC = 32 μg·mL^-1) and was poorly efficient against L. innocua. All other metal-NHC complexes were more efficient against Gram-positive bacteria, with MIC values in the range 4-16 μg·mL^-1. These NHC complexes were then electropolymerized on gold substrates using their pyrrole function. Efficient incorporation of these NHC species into polypyrrole (PPy) films was confirmed by X-ray photoelectron spectroscopy (XPS) measurements with metal contents ranging from 0.8% (Cu) to 12.3% (Ag). Scanning electron microscopy (SEM) and profilometry measurements ascertain that the homogeneity, structure, and thickness of the films depend on the metal. The antibacterial activities of the polypyrrole films were then determined by the halo inhibition method. A very good match between the antibacterial properties of the films and those of the monomers with Ag(I), Au(I), and Rh(I) complexes was found. For the other complexes, the metallic content was too low to obtain interesting antibacterial properties. The cytotoxicity of the films was finally evaluated on normal human dermal fibroblasts (NHDF). Our study reveals a strong impact of the doping anions of polypyrrole on cell viability.