Nonfouling behavior of polycarboxybetaine-grafted surfaces: structural and environmental effects

Zwitterionic carboxybetaine (CB) has unique dual functionality for ligand immobilization on a nonfouling background. The properties of CB groups depend on their spacer groups between the positive quaternary amine groups and the negative carboxyl groups and environmental factors (e.g., ionic strengths and pH values). In this work, five polycarboxybetaines were prepared, including one polycarboxybetaine methacrylate (polyCBMA) and four polycarboxybetaine acrylamides (polyCBAAs) with different spacer groups. The polymers were grafted from a gold surface covered with initiators using surface-initiated atom transfer radical polymerization. Fibrinogen adsorption was measured as a function of ionic strengths and pH values using surface plasmon resonance sensors. The responsive protein adsorption on four polyCBAAs was mapped out. Results show that most of these surfaces exhibit high protein resistance in a wide range of ionic strengths and are more effective than zwitterionic self-assembled monolayers. Although protein adsorption tends to increase at low ionic strength and low pH value, it is still very low for polycarboxybetaines with a methylene, an ethylene, or a propylene spacer group but is more evident for polyCBAA with a longer spacer group (i.e., a pentene group). The response to ionic strengths and pH values can be attributed to the antipolyelectrolyte and protonation/deprotonation properties of polycarboxybetaines, respectively. Both of these properties are related to the spacer groups of CBs.