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In this paper, we proposed to enhance a signal-to-noise (S/N) ratio for detecting a primary stress marker, serotonin, using a potentiometric biosensor modified by a well-designed nanofilter film. An extended-Au-gate field-effect transistor (EG-Au-gate FET) biosensor exhibits highly sensitive electrochemical detection toward various small biomolecules, including serotonin. Therefore, to enhance the S/N ratio for the serotonin detection, we designed an appropriate nanofilter film on the Au electrode by combining the aryldiazonium salt reduction strategy and boronate affinity. That is, only serotonin can approach the Au sensing surface to generate an electrical signal; interfering biomolecules are prevented from penetrating through the nanofilter, either because large interfering biomolecules cannot permeate through the highly dense, nanoporous multilayer film, or because phenylboronic acids included in the nanofilter captures small interfering biomolecules (e.g., catecholamines). The potentiometric biosensor modified by such a nanofilter film detected serotonin in a model sample solution containing catecholamines, cortisol, and human serum albumin with a high S/N ratio for the serotonin levels in the blood. Furthermore, we found that the effect of the nanofilter directly reflects the binding affinity of the receptors such as phenylboronic acids included in the nanofilter; thus, the selectivity and dynamic range of small target biomolecules can be tuned freely by designing the appropriate receptors for the nanofilter. The results show that a well-designed nanofilter biointerface can be a versatile biosensing platform for point-of-care testing, particularly for a simple stress check.

In this paper, we propose a novel concept of a biointerface, a polymeric nanofilter, for the potentiometric detection of small biomolecules using an extended-Au-gate field-effect transistor (EG-Au-FET). A Au electrode has the potential capability to detect various small biomolecules with ultrasensitivity at nM levels on the basis of a surface redox reaction, but it exhibits no selective response to such biomolecules. Therefore, a suitable polymeric nanofilter is designed and modified on the Au electrode, so that a small target biomolecule reaches the Au surface, resulting in an electrical signal, whereas low-molecular-weight interferences not approaching the Au surface are captured in the polymeric nanofilter. The polymeric nanofilter is composed of two layers. The first layer is electrografted as an anchor layer by a cyclic voltammetry method. Then, a filtering layer is precisely polymerized as the second layer by a photo-mediated surface-initiated atom transfer radical polymerization method. The thickness and density of the polymeric nanofilter are controlled to specifically detect a small target biomolecule with high sensitivity. As a model case, l-cysteine as the small target biomolecule at nM levels is specifically detected by filtering l-DOPA as a low-molecular-weight interference using the polymeric nanofilter-grafted EG-Au-FET on the basis of the following mechanism. The phenylboronic acid (PBA) that copolymerizes with the polymeric nanofilter captures l-DOPA through diol binding, whereas l-cysteine reaches the Au surface through the filter layer. The polymeric nanofilter can also effectively prevent the interaction between biomacromolecules such as albumin and the Au electrode. A platform based on a polymeric nanofilter-grafted EG-Au-FET biosensor is suitable for the ultrasensitive and specific detection of a small biomolecule in biological samples such as tears and sweat, which include small amounts of low-molecular-weight interferences, which generate nonspecific electrical signals.