Three different PDMAEMA brushes were investigated: a dense and thick brush (s of 0.5 chains/nm 2 ; h of 30 nm), a dense and thin brush (s of 0.5 chains/nm 2 ; h of 10 nm) and a sparse brush (s of 0.1 chains/nm 2 ; h of 20 nm) polymerised for the same amount of time as the dense thick brush. The aim of such comparison was to study a range of brushes displaying distinct RNA absorption properties, as previously studied by surface plasmon resonance . Whereas brushes generated with the same density of ATRP initiators display differences in chain length, as their thickness is controlled via the polymerisation time, brushes generated at reduced densities but polymerised for the same amount of time are expected to display comparable polymer length (although recent studies indicated a slightly reduced rate of polymerisation with increasing grafting densities and compared to free polymers formed in solution ). The densities studied in this work are based on those of brushes previously investigated and for which densities were determined upon grafting from the surface of silica nanoparticles (prior to dissolution and analysis by size exclusion chromatography, transmission electron microscopy and thermogravimetric analysis) . The comparatively low surface area of silicon wafers and silicon blocks used in the present study did not allow direct measurement of grafting densities from their surface, but the thickness of resulting brushes was comparable to that of brushes previously studied (from silica nanoparticles and silicon wafers) . In addition, PMETAI brushes were generated by quaternization of dense and thick PDMAEMA brushes, to afford brushes with dry ellipsometric thicknesses of 35 nm. This was selected to ensure comparable grafting densities to dense PDMAEMA brushes, and as it was previously found that such brush quaternisation was essentially quantitative.