Patterned and switchable surfaces for biomaterial applications

Chapter 2 – Spatially controlled electro-stimulated DNA adsorption and desorption for biodeviceapplications2.3.2. DNA adsorption and desorption studiesDNA adsorption and desorption from the Si-ALAPP-PEG ablated surfaces wasinitially studied using a custom-built flow cell. Initially, short (16mer)oligonucleotides labelled with 6-FAM at the 5’ end were used.Oligonucleotide solution was injected over the Si-ALAPP-PEG patterned surfaceand analysed microscopically. The preferential adsorption of the DNA to the ALAPPregions (green regions) as opposed to the PEG regions (dark regions) was observed(Figure 2.6A), demonstrating the spatially controlled adsorption of DNA [187]. It issuggested that the mechanism for this adsorption is due mainly to electrostaticinteractions of surface tethered, protonated amino groups with the negatively chargedbackbone of DNA. Electrostatic interactions between DNA and cationic polymershas been demonstrated (Appendix 2). The presence of amino groups in the ALAPPfilm was demonstrated by N 1s high resolution XPS spectra (results not shown), andindirectly by the successful grafting of PEG to the ALAPP film as confirmed by XPS(Table 2.1; Figure 2.4B). As experiments were conducted at a pH 7.4, and given thetypical isoelectric point of aliphatic amines is 10.6 [43] the amino groups present onthe ALAPP film were expected to be protonated. However, it cannot be saidconclusively from the fluorescent images as to whether the PEG coatingquantitatively prevents DNA adsorption. Figure 2.6A and Figure 2.6B both show agreen background on the PEG regions that could be due to oligonucleotide adsorbingto the PEG surface, to free oligonucleotide in solution or to autofluorescence effects.The flow cell design allowed the microscopic observation of the surface whilstapplying an electrical bias. Spatially controlled DNA adsorption was observed on theALAPP-PEG surface by visualisation of the green pattern after injection of the2-82