Argon plasma treatment of polystyrene microtiter wells. Chemical and physical characterisation by contact angle, ToF-SIMS, XPS and STM

The use of plasma treatment for the modification of polystyrene microtiter wells has been evaluated by contact angle measurements, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The contact angle data suggests that the effect of plasma treatment is first to clean the surface of volatile contamination, increasing the hydrophobicity, and then to introduce oxygen functionality into the surface, decreasing the hydrophobicity. The cleaning effect appears to occur in the first few seconds of treatment while the oxygenation effect increases with increased exposure to the plasma. The XPS and ToF-SIMS measurements show increasing surface oxygen concentration with plasma treatment time, with a concomitant reduction in aromaticity. Scanning tunnelling microscopy (STM) imaging reveals that plasma treatment significantly affects the adsorption of bovine serum albumin (BSA). Untreated surfaces exhibited areas where no BSA adsorption occurred. These regions ranged in size between 20 and 60 nm in diameter. Plasma treated surfaces, however, exhibited no such areas, with BSA adsorption appearing to be more uniform across the surface. The regions on the untreated surfaces where no BSA adsorption occurred are thought to be hydrocarbon (volatile) in nature possibly from the moulding process, which is removed in the first few seconds of plasma treatment.