Patterned and switchable surfaces for biomaterial applications

Chapter 3 – Comparison of the binding mode of plasmid DNA to allylamine plasma polymer and poly(ethyleneglycol) surfacesinterface. Being a polyelectrolyte, any charge at the surface would play a significantrole in DNA adsorption due to long range electrostatic interactions. -potential(Figure 3.1) and XPS (Table 3.2) measurements of freshly formed ALAPP suggestthe presence of both anionic and cationic functionalities. The presence of cations andanions is thought to influence DNA adsorption in three ways. Firstly, the cations insolution should shield the anionic charge of the DNA, minimising any electrostaticrepulsion from the surface [25, 26]. This is important as ALAPP has a net -potentialof ≈-20 mV at pH of 7.4. It is thought that there would be localised positive charge atthe surface that could stabilise DNA on the surface. Secondly, the presence of cationsshould shield anionic functionalities on the DNA from adjacent charges on nearbyDNA strands or on the same strand. This should allow the DNA to form a denserlayer on the surface [25, 26]. Thirdly, and more apparent for higher valency cations,the presence of cations in solution can act as bridging ions between anionic chargeson the surface and on the DNA strands. This has been observed particularly for DNAadsorption onto silica and mica [20, 23, 43]. To investigate whether the chargedsurface functionalities contribute to DNA adsorption via attractive electrostaticinteractions, a plasmid solution was incubated with ALAPP samples and theresulting DNA was measured at varied [NaCl]. As expected, when the [NaCl] wasincreased, an increase in DNA was observed (Figure 3.5). The DNA increased up to 3M salt solutions. Higher salt concentrations interfered with Picogreen ® fluorescence.This result suggests that electrostatic interactions play a significant role in DNAadsorption to ALAPP. However, the pathway by which this enhanced DNAadsorption occurs is not clear, and all three above mentioned effects, the shielding ofrepulsive electrostatic DNA-surface interactions, compaction of DNA allowinghigher density DNA layers and formation of bridging ions between the surface and3-118