Surface characterization and platelet adhesion studies for the mixed self-assembled monolayers with amine and carboxylic acid terminated functionalities

The mixed self-assembled monolayers (SAM) prepared from long chain alkanethiols, HS(CH₂)₁₁NH₂ and HS(CH ₂)₁₀COOH, on gold are employed as the model surface for investigating the interactions between the biological environment and synthetic surface. A distinctive SAM preparation scheme was utilized in this investigation. The triethylamine was added to the alkanethiol solution during SAM formation and then followed by additional rinsing of SAM with 10% CH ₃COOH or 1% HCl ethanolic solution. The contact angle values of NH₂ + COOH mixed SAMs were between those of the pure SAMs, except that it was prepared with solution mole fraction of amine-terminated alkanethiol at 0.2. X-ray photoelectron spectroscopy (XPS) analysis has indicated that these two distinctive SAM preparation procedures had both resulted in a reduction in oxidized sulfur species on pure - NH₂ terminated SAM. However, the procedure utilizing 1% HCl ethanolic washing solution was more effective in reducing the unbound thiol fraction and to form a pure - NH ₂ SAM with better quality. XPS analysis has also revealed that the surface of NH₂ + COOH mixed SAMs was "amine-rich". In vitro platelet adhesion assay has shown that the amount of adherent platelets on pure positive charged - NH₂ terminated SAM is less than that on anionic - COOH terminated counterpart in both acidic ethanolic washing schemes. Moreover, the lowest platelet adhesion density was noted on the mixed SAM surfaces with surface amine mole fraction at 0.51 and 0.57. This finding suggests that the surface charge with near neutrality might be of importance in reducing platelet adhesion and activation on artificial biomaterial.