Characterization of sequentially grafted polysaccharide coatings using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and principal component analysis (PCA)

Sequentially grafted polymer layers are playing an increasingly important role in a variety of biomedical applications. Characterization of the chemistry and structure of these coatings has typically followed a layer-by-layer approach, ignoring potential interactions between the adjacent layers and the effects that these interactions may have on the resulting physicochemical characteristics of the surface. Using a sequential grafting protocol to couple the polysaccharide carboxymethyldextran (CMD), previous surface force and x-ray photoelectron spectroscopy studies have suggested that there was some intercalation of the CMD and intermediate poly(ethyleneimine) (PEI) layers. We have utilized the surface and chemical specificity of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate the intercalation of the polymer layers. Owing to the complexity of the data, principal component analysis (PCA) was used to interpret the spectra of the grafted polymer films. The results of the PCA indicated that variations between spectra could be related directly to the carboxyl density (1:2 or 1:30 carboxyls per glucose unit) of the CMD. Variations between the spectra were shown to be due to the presence of nitrogen-rich species from the PEI at the outermost surface in the CMD 1:30 samples, whereas the CMD 1:2 spectra contained oxygen-rich species related to the presence of the polysaccharide. These results enabled a model to be developed combining the new data with the previous studies to show the carboxyl-density-dependent intercalation of the CMD and PEI layers in this polymer graft. The application of ToF-SIMS and PCA allowed conclusive determination of the surface structure of this complex grafted polymer layer