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.

In this paper, we propose a one-step procedure for fabricating a solution-gated ultrathin channel indium tin oxide (ITO)-based field-effect transistor (FET) biosensor, thus providing an ″all-by-ITO″ technology. A thin-film sheet was placed on both ends of a metal shadow mask, which were contacted with a glass substrate. That is, the bottom of the metal shadow mask corresponding to the channel was slightly raised from the substrate, resulting in the creeping of some particles into the gap during sputtering. Owing to this modified metal shadow mask, a thin ITO channel (<30−40 nm) and thick ITO source/drain electrodes (ca. 100 nm) were simultaneously fabricated during the one-step sputtering. The thickness of ITO films was critical for them to be semiconductive, depending on the maximum depletion width (∼30− 40 nm for the ITO channel), similarly to 2D materials. The ultrathin ITO channel worked as an ion-sensitive membrane as well owing to the intrinsic oxidated surface directly contacting with an electrolyte solution. The solution-gated 20-nm-thick channel ITO-based FET, with a steep subthreshold slope (SS) of 55 mV/dec (pH 7.41) attributable to the electric double-layer capacitance at the electrolyte solution/channel interface and the absence of interfacial traps among electrodes formed in one step, demonstrated an ideal pH responsivity (∼56 mV/pH), resulting in the realtime monitoring of cellular respiration and the long-term stability of electrical properties for 1 month. Moreover, the chemical modification of the ITO channel surface is expected to contribute to biomolecular recognition with ultrahigh sensitivity owing to the remarkably steep SS, which provided the exponential pH sensitivity in the subthreshold regime. Our new device produced in this one-step manner has a great future potential in bioelectronics. KEYWORDS: solution-gated field-effect transistor (FET), one-step procedure, ultrathin channel, indium tin oxide (ITO), 2D material

In this paper, we proposed to enhance a signal-tonoise (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-Augate 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 report a direct and quantitative analytical method of small-biomolecule recognition with a molecularly imprinted polymer (MIP) interface, taking advantage of the potentiometric principle of a field-effect transistor (FET) sensor, which enables the direct detection of ionic charges without using labeling materials such as fluorescent dyes. The interaction of low-molecular-weight oligosaccharides such as paromomycin and kanamycin with the MIP interface including phenylboronic acid (PBA) was directly and quantitatively analyzed from the electrical signals of an MIP-coated FET sensor. In particular, the change in the potential response of the FET sensor was derived on the basis of the multi-Langmuir adsorption isotherm equations, considering the change in the molecular charges of PBA caused by the adsorption equilibrium of the analytes with the vinyl PBA-copolymerized MIP membrane. Thus, the potentiometric adsorption isotherm analysis can elucidate the formation of selective binding sites at the MIP interface. The electrochemical analysis of the functional biointerface used in this study supports the design and construction of sensors for small biomarkers.

A molecularly imprinted polymer (MIP)-based membrane with phenylboronic acid (PBA) molecules, which induces the change in the density of molecular charges, is suitable for the bioelectrical interface of field-effect transistor (FET) sensors.