Chain-End Carboxy Functionalization of Zwitterionic Polymethacrylate Brushes Grafted from Polymer Particles for Bioconjugation without Compromising Protein Repellency

Combining both biomolecule immobilization and suppression of nonspecific biomolecule adsorption is a key requirement for advanced biointerfaces. In particular, polymer brush surfaces prepared via surface-initiated atom transfer radical polymerization (SI-ATRP) represent a promising approach for suppressing nonspecific adsorption while allowing the introduction of reactive groups into polymer side chains or chain ends. However, direct one-step chain-end functionalization remains challenging for polymers bearing tertiary alkyl bromide termini. Here, the terminal tertiary alkyl bromides of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes prepared by SI-ATRP were directly converted into reactive carboxy groups via thiolate-mediated substitution using 3-mercaptopropionic acid (MPA). The reaction conditions were first optimized using a low-molecular-weight model compound and free PMPC and subsequently applied to PMPC brushes on polymer particles. The density of the surface-accessible carboxy groups was determined to be 0.040 groups/nm2 by fluorescence labeling, corresponding to 32% of the total number of polymer brush chains, which was comparable to that obtained for free PMPC. Importantly, the adsorption of bovine serum albumin and lysozyme remained unchanged after carboxy functionalization, demonstrating that the protein repellency of the PMPC brushes were preserved. Furthermore, superfolder green fluorescent protein (sfGFP) was successfully immobilized through the introduced carboxy groups, as confirmed by fluorescence spectroscopy and microscopy. These results demonstrate that chain-end functionalization of PMPC brushes enables the introduction of reactive sites for biomolecule conjugation without compromising protein repellency. The strategy developed in this study provides a versatile route to end-functionalized polymethacrylate brushes and offers a useful platform for the design of biofunctional and low-fouling polymer interfaces.