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X-ray photoelectron spectroscopy and impedance spectroscopy studies of Nitinol modified by thin organic films

The self-assembly of organic molecules is a cost effective way of functionalization of metallic and inorganic surfaces for various applications in medicine and biochemical engineering. In the present work the Nitinol (Ni-Ti alloy) surface was modified with octadecylphosphonic acid by two different methods: immersion and aerosol spraying. Electrochemical properties of modified Nitinol samples were investigated in simulated body fluid (Hank's solution) using polarization and electrochemical impedance spectroscopy (EIS) measurements. It was found that the self-assembled films of phosphonic acid improve the corrosion resistance and biocompatibility of the Nitinol surface. The composition and the thickness of self-assembled films on the Nitinol surface were investigated by X-ray photoelectron spectroscopy (XPS). Angle resolved XPS analysis was explored to perform a non-destructive depth profiling of the surface layer. This analysis enables: (i) the determination of molecular orientation within the self-assembled layer, especially the orientation of carbon, phosphorus and oxygen, which are the main components of these surface layers ; (ii) the determination of the oxidation state and the chemical environment of individual elements at the surface. The results indicate that the formation of layers occurred through chemical bonding with the alloy surface. Self-assembled films prepared by immersion of Nitinol in phosphonic acid are several nm thick, whereas films prepared by spraying are much thicker, up to 20 nm.

Nitinol; Phosphonic acid; Self-assembled films; X-ray photoelectron spectroscopy; Electrochemical impedance spectroscopy.

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