Effect of Interfacial Charge Distribution in Mixed Charge-Equilibrated SAMs on the Attachment of Pathogens

Zwitterions consisting of positively and negatively charged groups confer hydrophilicity while retaining overall charge neutrality. Both properties were identified as decisive prerequisites for protein-resistant coatings. In this work, we studied the electrostatic contributions to the bacterial attachment process by altering the interfacial charge distribution of the two charges and correlated the results with bacterial adhesion data. Therefore, we generated a set of well-defined, quasi-zwitterionic, charge-equilibrated self-assembled monolayers on gold-coated substrates. As cationic component (11-mercaptoundecyl)-N,N,N-trimethylammonium was combined in a 1:1 ratio with anionic thiols of varying alkyl spacer lengths. By embedding 8-mercaptooctanoic acid, 12-mercaptododecanoic acid, or 16-mercaptohexadecanoic acid, the distance of the anionic moiety to the surface could be varied while maintaining the distance of the cationic moiety to the substrate. Thereby, the interfacial charge distribution and thus the average orientation of the zwitterionic dipoles of the charge-equilibrated mixed self-assembled monolayers have been systematically varied. The resistance against the nonspecific adsorption of the blood-related proteins human serum albumin and fibronectin as well as the attachment-inhibiting effect against the pathogenic bacteria Escherichia coli, Pseudomonas fluorescens, and Bacillus subtilis was tested. It turned out that the change in dipole orientation affected the proteins and the bacteria in different ways with an equilibrated charge distribution within the surface plane being in total the superior one. The results are further discussed based on streaming current data revealing net surface charge of the self-assembled monolayers and the apparent zeta potential of the bacteria to understand to what degree electrostatic interactions contribute to the attachment process.