Prior to investigating the complexation of a range of nucleic acid molecules by polymer brushes, the impact of the complexation buffer on siRNA capture and knock down efficiency was investigated. The adsorption profile of a 20 base pair (bp) siRNA sequence, dissolved into three different buffers (deionised water, 150 mM NaCl and PBS), onto dense 30 nm PDMAEMA brushes was quantified by SPR (Supplementary Figure ). In agreement with previous observations made in PBS , Gmax in the range of 3.20-3.45 µg/cm 2 were observed for the three buffers tested. The kinetics of adsorption recorded were comparable, with saturation at 20 µg/mL occurring near 150 s of injection. These adsorption levels correspond to densely packed RNA complexes, with near neutralisation of the brush coating . This suggests that the ionic strength and presence of phosphate electrolytes does not significantly impact siRNA complexation within PDMAEMA brushes. To investigate the impact of buffer type on transfection efficiency, we quantified the knock down of CD31 in HUVECs (Supplementary Figure ). We observed comparable levels of knock down when the complexes between CD31 siRNA and 30 nm PDMAEMA brush-coated nanoparticles (330 nm silica core ) were formed in deionised water, PBS or OPTIMEM, a buffer routinely used to form complexes and inject them on cultures in vitro.