Graphene oxide nanohybrids for electron transfer-mediated antimicrobial activity†
The rapid increase in the prevalence of antibiotic-resistant bacterial strains poses a global health risk. In this scenario, alternative strategies are needed to combat the alarming rise in multidrug-resistant bacterial populations. For example, metal-incorporated graphene derivatives have emerged as model nanomaterials owing to their intrinsic antibacterial activity together with their biocompatibility. Interestingly, photon-activated phthalocyanine sensitizers have also shown promising physiochemical biocidal effects against pathogenic bacteria populations when conjugated with diverse nanomaterials. Herein, we report the facile synthesis of graphene oxide incorporated zinc phthalocyanine (ZnPc–GO) nanohybrids showing bactericidal activity against Gram-negative Escherichia coli (E. coli) cells, in the absence of any photo-excitation. The ZnPc–GO hybrid nanomaterials were synthesized by the in situ deposition of GO flakes on ZnPc-coated indium tin oxide (ITO) substrates. Two types of morphologically different ZnPc molecules, potato-chip-like α-phase ZnPc, namely ZnPc(A), and nanorod-like β-phase ZnPc(B), were used for the synthesis of the ZnPc(A/B)–GO nanocomposites. The interactions of GO with the underlying ZnPc(A/B) entities in the ZnPc–GO systems were investigated using multiple characterization techniques. It was observed that the GO flakes in the ZnPc(B)–GO nanocomposite possess stronger π–π interactions and thus show a more efficient electron transfer mechanism when compared with the ZnPc(A) counterpart. Furthermore, the E. coli bacterial cells with an electronegative surface demonstrated a profound adherence to the electron-withdrawing ZnPc(B)–GO surface. The death kinetics of bacteria with ZnPc(B)–GO were further investigated using surface potential mapping and Kelvin probe force microscopy (KPFM) analysis. Upon direct contact with ZnPc(B)–GO, the adhered bacterial cells showed outer cell deformation and membrane protein leakage, induced by a proposed charge-transfer mechanism between negatively charged cells and the electron-withdrawing ZnPc(B)–GO surface. These new findings may provide insights into the design of potential ZnPc–GO-based novel antimicrobial nanomaterials or surface coatings.