Functionalization of Polymeric Nanofilter Biointerface for Small Biomarker Sensing

Small biomolecules contained in body fluids are often recognized as indicators of diseases and health conditions. Therefore, the development of biosensors capable of recognizing significant biomarkers from body fluids such as blood, sweat, tears, and saliva, leads to early diagnosis and contribute to the realization of next-generation medical care. In this regard, we have recently developed the potentiometric biosensor based on the extended-gate field-effect transistor (EG-FET) for small-biomolecule recognition. The catalytically active extended-Au electrodes have the capability to detect various small biomolecules with high sensitivity on the basis of the surface electrochemical reaction. However, since the Au electrode may react with various electrochemically active biomolecules, it is necessary to limit the interfering biomolecules from approaching the Au surface in order to enhance target selectivity. Therefore, a suitable polymeric nanofilter was developed on the Au electrode where interfering biomolecules are prevented from penetrating through the nanofilter, and only the target biomolecules can access the Au sensing surface to generate an electrical signal.1,2 The polymeric nanofilter biointerface is composed of two layers. The first layer is an anchor layer directly grafted on the Au surface, which controls the density of the polymeric nanofilter to make enough space for only small biomolecules to penetrate through and prevent large-biomolecule interferences from approaching the Au sensing surface. Then, a polymeric filter layer as the second layer is precisely grafted from the surface of the anchor layer by atom transfer radical polymerization (ATRP). The filter layer selectively captures the small-biomolecule interferences outside the Debye’s length; therefore, the interfering small-biomolecules will not cause the change in Au surface potential. In our previous study, we designed and developed the polymeric nanofilter by using aryldiazonium reduction chemistry and boronate affinity. Aryldiazonium reduction chemistry enabled the grafting of chemically stable multilayer film on the Au surface, having an adequate density in order to suppress the penetration of large biomolecules. Phenylboronic acid was included in the polymeric filter layer to capture the model interfering small biomolecule, L-DOPA, via a reversible PBA/diol boronate affinity binding. Resultantly, the polymeric nanofilter-coated EG-FET biosensor specifically detected the model target biomolecule, L-cysteine. In this study, we designed and developed the second polymeric layer on the polymeric nanofilter to give an additional function. Although the space-controlled nanofilter suppresses the penetration of the large biomolecules, the non-specific adsorption can cause severe fouling of the nanofilter surface. Therefore, in order to improve the anti-fouling function of the nanofilter interface, poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC) was chosen as the second polymeric layer. PMPC is a bio-inspired polymer that has been widely used as a biocompatible, anti-fouling coating material. In order to investigate the anti-fouling function of pMPC coating, we firstly grafted the pMPC layer directly from the anchor layer surface via surface-initiated ATRP. Then, the response of pMPC-nanofilter coated EG-FET biosensor to human-serum albumin (HSA) was compared with the response of the control (unmodified) sensor. Resultantly, the pMPC-coated sensor suppressed the non-specific response up to 60% compared with the control sensor. Therefore, we verified the successful enhancement of the anti-fouling property on the nanofilter surface. Reference (1) Nishitani, S.; Sakata, T. ACS Appl. Mater. Interfaces 2019, 11 (5), 5561–5569. (2) Himori, S.; Nishitani, S.; Sakata, T. Langmuir 2019, 35 (10), 3701–3709.