Probing charge transfer through antifouling polymer brushes by electrochemical methods: The impact of supporting self-assembled monolayer chain length

Abstract

Ultrathin surface-tethered polymer brushes represent attractive platforms for a wide range of sensing applications in strategically vital areas such as medicine, forensics, or security. The recent trends in such developments towards "real world conditions" highlighted the role of zwitterionic poly(carboxybetaine) (pCB) brushes which provide excellent antifouling properties combined with bio-functionalization capacity. Highly dense pCB brushes are usually prepared by the "grafting from" polymerization triggered by initiators on self-assembled monolayers (SAMs). Here, multi-methodological experimental studies are pursued to elucidate the impact of the alkanethiolate SAM chain length (C₆, C₈ and C₁₁) on structural and functional properties of antifouling poly(carboxybetaine methacrylamide) (pCBMAA) brush. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a custom-made 3D printed cell employing [Ru(NH₃)₆]^3+/2+ redox probe were used to investigate penetrability of SAM/pCBMAA bilayers for small molecules and interfacial charge transfer characteristics. The biofouling resistance of pCBMAA brushes was characterized by surface plasmon resonance; ellipsometry and FT-IRRAS spectroscopy were used to determine swelling and relative density of the brushes synthesized from initiator-bearing SAMs with varied carbon chain length. The SAM length was found to have a substantial impact on all studied characteristics; the highest value of charge transfer resistance (R_ct) was observed for denser pCBMAA on longer-chain (C₁₁) SAM when compared to shorter (C₈/C₆) SAMs. The observed high value of R_ct for C₁₁ implies a limitation for the analytical performance of electrochemical sensing methods. At the same time, the pCBMAA brushes on C₁₁ SAM exhibited the best bio-fouling resistance among inspected systems. This demonstrates that proper selection of supporting structures for brushes is critical in the design of these assemblies for biosensing applications.

Keywords: 3D printed electrochemical cell; Antifouling; Cyclic voltammetry; Electrochemical impedance spectroscopy; Polymer brushes; Self-assembled monolayers.