Natural zwitterionic organosulfurs as surface ligands for antifouling and responsive properties

Natural sulfur-containing zwitterionic compounds, l-cysteine (Cys), l-methionine, and glutathionine (GSH), have been employed as surface ligands to prevent protein nonspecific adsorption on planar substrates. These organosulfur compounds form self-assembled monolayers (SAMs) on gold substrates by gold-sulfur interaction. The chemical elements of SAMs were confirmed using x-ray photoelectron spectroscopy. The surface wetting tests for SAMs show that films prepared from Cys and GSH exhibited super-hydrophilicity (contact angles of θ = ~5°) due to their high coverage and strong hydration via ionic solvation and formation of hydrogen bonding. Quartz crystal microbalance with dissipation sensor was used to quantitatively and qualitatively monitor the adsorption of bovine serum albumin (BSA) from buffer onto these SAMs. It was found that the GSH film enables the resistance of BSA adsorption to the best extent at a physiological pH. Moreover, the surface charges of modified substrates were modulated by varying the pH value to control BSA adsorption. The effect of electrostatic repulsion on the antifouling behavior becomes prominent at a pH where the protein and the surface carry same charges. Consolidating the BSA adsorption measurements at different pH values, the antifouling properties of GSH-modified Au should be attributed to prevention of entropy gain and enthalpy loss, making BSA adsorption energetically unfavorable. It is believed that the surface modification with natural organosulfur ligands holds great potential in improving the biocompatibility of medical devices and in offering intelligent biointerfaces in response to environmental stimuli.