Ion-Specific Conformational Behavior of Polyzwitterionic Brushes: Exploiting It for Protein Adsorption/Desorption Control

The conformation of polyzwitterionic brushes plays a crucial role in the adsorption/desorption of proteins on solid surfaces. By use of quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance (SPR), we have systematically investigated the conformational behavior of poly(sulfobetaine methacrylate) (PSBMA) brushes as a function of ionic strength in the presence of different ions. The frequency change demonstrates that the effectiveness of anions to weaken the inter/intrachain association and to enhance the hydration of the grafted chains increases from kosmotrope to chaotrope in the low ionic strength regime, but the ordering of anions is almost reversed at the high ionic strengths. The dissipation change indicates that some heterogeneous structures are formed inside the brushes in the presence of chaotropic anions with the increase of ionic strength. In SPR studies, the change of resonance unit (ΔRU) with ionic strength is determined by the balance between the increase of thickness and the decrease of refractive index of the brushes. No anion specificity is observed in the SPR measurements because ΔRU is insensitive to the coupled water molecules inside the brushes. For the control of protein adsorption/desorption, our studies show that the brushes can more effectively resist the protein adsorption in the presence of a more chaotropic anion and a more chaotropic anion can also more effectively induce the protein desorption from the surface of the brushes. In addition, no obvious cation specificity can be observed in the conformational change of the brushes in either QCM-D or SPR measurements.