Stability of phosphonic self assembled monolayers (SAMs) on cobalt chromium (Co–Cr) alloy under oxidative conditions

Cobalt chromium (Co–Cr) alloys have been widely used in the biomedical arena for cardiovascular, orthopedic and dental applications. Surface modification of the alloy allows us to tailor the interfacial properties to address critical challenges of Co–Cr alloy in medical applications. Self assembled monolayers (SAMs) of Octadecylphosphonic acid (ODPA) have been used to form thin films on the oxide layer of the Co–Cr alloy surface by solution deposition technique. The SAMs formed were investigated for their stability to oxidative conditions of ambient laboratory environment over periods of 1, 3, 7 and 14 days. The samples were then characterized for their stability using X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and contact angle measurements. Detailed high energy XPS elemental scans confirmed the presence of the phosphonic monolayer after oxidative exposure which suggested that the SAMs were firmly attached to the oxide layer of Co–Cr alloy. AFM images gave topographical data of the surface and showed islands of SAMs on Co–Cr alloy surface, before and after SAM formation and also over the duration of the oxidative exposure. Contact angle measurements confirmed the hydrophobicity of the surface over 14 days. Thus the SAMs were found to be stable for the duration of the study. These SAMs could be subsequently tailored by modifying the terminal functional groups and could be used for various potential biomedical applications such as drug delivery, biocompatibility and tissue integration.