Solvent and concentration effects on the surface characteristics and platelet compatibility of zwitterionic sulfobetaine-terminated self-assembled monolayers.

The structural organization of a monolayer influences biological responses as the material makes contact with the bodily fluid. Zwitterionic materials containing the sulfobetaine functionalities have been shown to exhibit protein-repelling characteristics. In this study, the effect of solvent and thiol concentrations on the sulfobetaine-terminated SAM (self-assembled monolayer) is discussed. Four different types of solvents were selected: deionized water, PBS, methanol and ethanol. The total thiol concentration was set at either 2mM or 0.1 mM. X-ray photoelectron analyses indicated that all SAMs demonstrated similar chemical configurations. Reflection adsorption infrared spectroscopy showed that conformation of the SAMs was more organized when prepared from a 0.1 mM solution compared to a 2 mM solution. The contact angle of the SAMs prepared from 2 mM concentration was dependent upon the solvent utilized and was more hydrophobic than the SAMs prepared from 0.1 mM concentration. Moreover, all of these sulfobetaine-terminated SAMs showed a fairly negative zeta potential in PBS at pH 7.4. After contact with blood, these sulfobetaine-terminated SAMs demonstrated distinct platelet reactivity among each other. The highest platelet compatibility was shown on the SAMs prepared in 0.1 mM solution and the one formed in 2 mM ethanolic solution, where they exhibited a more organized conformation and enhanced hydrophilic properties. These properties might be caused by the different hydration layers, which are affected by the assembly conditions on the topmost monolayer. This study demonstrated that optimizing solvent and concentration conditions could control the structural organization of zwitterionic sulfobetaine-terminated SAMs and, consequently, modify biomedical properties.