The influence of silver and copper ions on the antibacterial activity and local electrical properties of single sepiolite fiber: A conductive atomic force microscopy (C-AFM) study

This is a study to analyze antibacterial and electrically-conductive sepiolite, and to evaluate its potential usage as biomaterials, medical products, sensors and electronics. Silver and copper ions were used to modify the properties of sepiolite through a multi-step procedure. During the modification of the sepiolite fibers, the most significant adsorption rates were obtained at metal uptake concentrations of 50 mg/g for Ag⁺ and 80 mg/g for Cu^{2 +}. This was found to fit the Langmuir isotherm model, thereby indicating the formation of a monolayer on the sepiolite surfaces by silver and copper ions. The effects of adsorbed silver and copper ions at the optimum metal uptake concentrations were then compared in terms of their antibacterial activity and electrical conductivity. Serial broth dilution and Kirby-Bauer disk diffusion susceptibility tests on Escherichia coli (ATCC 25922) and Staphylococcus aureus (ATCC 25923) were both utilized in this study. Metal ions that neutralized bacteria, and their minimum inhibitory concentrations (MIC), were measured at 50 mg/L for Ag⁺ and 100 mg/L for Cu^{2 +}. Additionally, the current distributions of metal ions over a single sepiolite fiber could also be characterized through the use of C-AFM. Interestingly, for the first time, local electric current variations and the potential gradients of Ag⁺ and Cu^{2 +} sorped sepiolite fiber surfaces were detected. Their C-AFM generated current image and line scans showed that silver electrons transfer much more rapidly than those of copper; however, raw sepiolite demonstrates no antibacterial or current properties. Therefore, the fibrous clay mineral used in this study might be a good candidate for highly effective and innovative clay-based applications.