Protein adsorption can be reversibly switched on and off on mixed PEO/PAA brushes.

Adsorption of proteins on surfaces placed in biological fluids is a ubiquitous and mostly irreversible phenomenon, desirable or not, but often uncontrolled. Adsorption of most proteins on poly(ethylene oxide) (PEO) brushes is very limited, while the amount of proteins adsorbed on poly(acrylic acid) (PAA) brushes varies with the pH and ionic strength (I) of the protein solution. Mixed brushes of PEO and PAA are designed here to reversibly adsorb and desorb albumin, lysozyme, collagen and immunoglobulin G, four very different proteins in terms of size, solubility and isoelectric point. Protein adsorption and desorption are monitored using X-ray photoelectron spectroscopy, as well as with quartz crystal microbalance for in situ and real-time measurements. Large amounts of protein are adsorbed and then nearly completely desorbed on mixed PEO/PAA brushes by a simple pH and I trigger. The mixed brushes thus nicely combine the properties of pure PAA and pure PEO brushes. These adsorption/desorption cycles are shown to be repeated with high efficiency. The high-performance smart substrates created here could find applications in domains as diverse as biosensors, drug delivery and nanotransport.

Quartz crystal microbalance study of ionic strength and pH-dependent polymer conformation and protein adsorption/desorption on PAA, PEO, and mixed PEO/PAA brushes.

The conformation of polymer chains grafted on a substrate influences protein adsorption. In a previous study, adsorption/desorption of albumin was demonstrated on mixed poly(ethylene oxide) (PEO)/poly(acrylic acid) (PAA) brushes, triggered by solutions of adequate pH and ionic strength (I). In the present work, homolayers of PEO or PAA are submitted to saline solutions with pH from 3 to 9 and I from 10(-5) to 10(-1) M, and their conformation is evaluated in real time using quartz crystal microbalance with dissipation monitoring (QCM-D). Shrinkage/swelling of PAA chains and hydration and salt condensation in the brush are evidenced. The adsorption of human serum albumin (HSA) onto such brushes is also monitored in these different saline solutions, leading to a deep understanding of the influence of polymer chain conformation, modulated by pH and I, on protein adsorption. A detailed model of the conformation of PEO/PAA mixed brushes depending on pH and I is then proposed, providing a rationale for the identification of conditions for the successive adsorption and desorption of proteins on such mixed brushes. The adsorption/desorption of albumin on PEO/PAA is demonstrated using QCM-D.

Design of mixed PEO/PAA brushes with switchable properties toward protein adsorption.

Adsorption of proteins at interfaces is an ubiquitous phenomenon of prime importance. Layers of poly(ethylene oxide) (PEO) are widely used to repel proteins. Conversely, proteins were shown to adsorb deeply into brushes of poly(acrylic acid) (PAA), and their subsequent partial release could be triggered by a change of pH and/or ionic strength (I). Mixed brushes of these polymers are thus promising candidates to tune protein adsorption onto new smart surfaces. In this work, the synthesis of such mixed brushes was performed based on a "grafting to" approach, the two polymers being either grafted sequentially or simultaneously. Detailed characterization of the obtained brushes using static water contact angle measurements, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and polarization-modulation reflection-absorption infrared spectroscopy is presented. While sequential grafting of the two polymers for different reactions times did not give rise to a broad range of composition of mixed brushes, simultaneous grafting of the polymers from solutions with different compositions allows for the synthesis of a range of mixed brushes (mass fraction of PEO in the mixed brushes from 0.35 to 0.65). A key example is then chosen to illustrate the switchable behavior of a selected mixed PEO/PAA brush toward albumin adsorption. The adsorption behavior was monitored with a quartz crystal microbalance. The mixed brush could adsorb high amounts of albumin, but 86% of the adsorbed protein could then be desorbed upon pH and I change. The obtained properties are thus a combination of the ones of PEO and PAA, and a highly switchable behavior is observed toward protein adsorption.