Surface composition XPS analysis of a plasma treated polystyrene: Evolution over long storage periods

A polystyrene surface (PS) was initially treated by cold nitrogen and oxygen plasma in order to incorporate in particular amine and hydroxyl functions, respectively. The evolution of the chemical nature of the surface was further monitored over a long time period (580 days) by chemical assay, XPS and contact angle measurements. Surface density quantification of primary amine groups was performed using three chemical amine assays: 4-nitrobenzaldehyde (4-NBZ), Sulfo succinimidyl 6-[3'(2 pyridyldithio)-pionamido] hexanoate (Sulfo-LC-SPDP) and iminothiolane (ITL). The results showed amine densities were in the range of 2 per square nanometer (comparable to the results described in the literature) after 5min of nitrogen plasma treatment. Over the time period investigated, chemical assays, XPS and contact angles suggest a drastic significant evolution of the chemical nature of the surface within the first two weeks. Beyond that time period and up to almost two years, nitrogen plasma modified substrates exhibits a slow and continuous oxidation whereas oxygen plasma modifed polystyrene surface is chemically stable after two weeks of storage. The latter appeared to "ease of" showing relatively mild changes within the one year period. Our results suggest that it may be preferable to wait for a chemical "stabilization" period of two weeks before subsequent covalent immobilization of proteins onto the surface. The originality of this work resides in the study of the plasma treated surface chemistry evolution over long periods of storage time (580 days) considerably exceeding those described in the literature.