Patterned and switchable surfaces for biomaterial applications

Chapter 3 – Comparison of the binding mode of plasmid DNA to allylamine plasma polymer and poly(ethyleneglycol) surfaces3.3. Results and discussion3.3.1. Surface characterisationSurfaces used for DNA adsorption and desorption studies were initiallycharacterised by contact angle, XPS and -potential measurements. Results fromcontact angle measurements are shown as Table 3.1. ALAPP deposition on silicongives rise to an increase in contact angle from 36° to 70°. This increase inhydrophobicity is a result of the deposition of a crosslinked organic polymer filmcontaining moderately polar functional groups (e.g. amines and imines). Notably alarge decrease in contact angle from 70° to 13° is observed when the high densityPEG coating is grafted onto the ALAPP layer, corresponding to a significant increasein hydrophilicity.XPS atomic percentages for bare silicon and ALAPP and PEG films are shown inTable 3.2. The absence of a silicon signal on the Si-ALAPP surfaces stronglysuggests that pinhole-free ALAPP films with a thickness of more than 10 nm (theapproximate penetration depth of XPS for polymers) were formed. The oxygencontent of this film is due to the reactions of atmospheric oxygen with reactivespecies in the polymer film such as free radicals resulting in the incorporation ofoxygen, which occurs once the film is exposed to air [183, 184, 189]. The inclusionof oxygen within the ALAPP film also suggests the formation of anionic groups onthe polymer surface. Upon subsequent grafting of PEG to the amine groups of theALAPP coating, a significant decrease in the nitrogen content (12.0% to 6.4%) andan accompanying rise in the oxygen content (11.5% to 23.6%) was observed,confirming the successful grafting of a high density PEG layer. High-resolution XPSspectra of the C 1s peak are shown in Figure 2.4 and discussed in section 2.3.1. The3-108