Dopamine assisted PMOXA/PAA brushes for their switchable protein adsorption/desorption

Protein adsorption on interfaces immersed in biological fluids is a prevalent and mostly irreversible phenomenon of great importance and is often uncontrolled. Poly(2-methyl-2-oxazoline) (PMOXA) is shown to possess excellent protein-resistant properties, whereas proteins can be adsorbed deeply inside the swollen brushes of poly(acrylic acid) (PAA). In this work, mixed brushes of PMOXA/PAA were prepared by sequentially grafting amine-terminated PMOXA (PMOXA-NH2) and thiol-terminated PAA (PAA-SH) onto poly(dopamine) (PDA)-coated substrates. The constituents of the mixed brushes can be adjusted by changing the grafting time of PMOXA-NH2 or PAA-SH to the PDA-coated surfaces. PAA chains with different molecular weights in the mixed brushes of PMOXA/PAA were used to investigate the effect of the length of the PAA chains on the surface wetting and protein adsorption on the prepared interfaces. The prepared brushes were rigorously characterized in terms of the surface chemical composition, thickness and hydrophilicity using X-ray photoelectron spectroscopy (XPS), ellipsometry and the static water contact angle (WCA) tests. Protein adsorption/desorption on the brushes were monitored using fluorescence microscopy as well as surface plasmon resonance (SPR) for in situ and real-time measurements. The results demonstrated that the WCA on mixed brushes could be reversibly tuned with the variation of pH and I, the amount of bovine serum albumin (BSA) adsorbed on mixed brushes could be well regulated by the grafting time and the molecular weight of PAA-SH, and the maximal amount (87%) of adsorbed BSA could then be desorbed upon changing pH and I. The highly switchable protein adsorption/desorption properties are therefore a nice combination of the protein-resistance of PMOXA and the salt and pH-sensitivity of PAA.