Time-of-flight-secondary ion mass spectrometry study of the temperature dependence of protein adsorption onto poly(N-isopropylacrylamide) graft coatings.

Stimuli-responsive materials show considerable promise for applications that require control over biomolecule interactions at solid material interfaces. Graft coatings of poly(N-isopropylacrylamide) (pNIPAM) are of interest for biomedical and biotechnological applications due to their temperature-dependent switching of surface properties between adhesive and nonadhesive states for cells and proteins. The characterization of protein adsorption to these switchable coatings is a formidable task since switching not only influences the affinity for proteins but at the same time induces a significant change in the coating. Here, the highly sensitive analytical technique of time-of-flight-secondary ion mass spectrometry (TOF-SIMS) combined with principal component analysis (PCA) was used for the characterization of protein adsorption onto pNIPAM coatings prepared by free radical polymerization onto surface-bound polymerizable groups. Adsorption of bovine serum albumin and lysozyme onto pNIPAM coatings from phosphate buffered solutions was investigated at temperatures above and below the polymer's lower critical solution temperature (LCST). Below the LCST, no adsorbed proteins could be detected even with this ultrasensitive method. Whereas above the LCST, adsorbed protein was detected in amounts corresponding at less than the monolayer. PCA loadings plots showed that adventitious contaminants, which might lead to confounding or misleading spectral changes upon protein exposure, were not observed.