Non-enzymatic detection of glucose levels in human blood plasma by a graphene oxide-modified organic transistor sensor

We herein report an organic transistor functionalized with a phenylboronic acid derivative and graphene oxide for the quantification of plasma glucose levels, which has been achieved by the minimization of interferent effects derived from physical protein adsorption on the detection electrode.

Molecular Level Sucrose Quantification: A Critical Review

Sucrose is a primary metabolite in plants, a source of energy, a source of carbon atoms for growth and development, and a regulator of biochemical processes. Most of the traditional analytical chemistry methods for sucrose quantification in plants require sample treatment (with consequent tissue destruction) and complex facilities, that do not allow real-time sucrose quantification at ultra-low concentrations (nM to pM range) under in vivo conditions, limiting our understanding of sucrose roles in plant physiology across different plant tissues and cellular compartments. Some of the above-mentioned problems may be circumvented with the use of bio-compatible ligands for molecular recognition of sucrose. Nevertheless, problems such as the signal-noise ratio, stability, and selectivity are some of the main challenges limiting the use of molecular recognition methods for the in vivo quantification of sucrose. In this review, we provide a critical analysis of the existing analytical chemistry tools, biosensors, and synthetic ligands, for sucrose quantification and discuss the most promising paths to improve upon its limits of detection. Our goal is to highlight the criteria design need for real-time, in vivo, highly sensitive and selective sucrose sensing capabilities to enable further our understanding of living organisms, the development of new plant breeding strategies for increased crop productivity and sustainability, and ultimately to contribute to the overarching need for food security.

Hydrogel-Coated Gate Field-Effect Transistor for Real-Time and Label-Free Monitoring of β-Amyloid Aggregation and Its Inhibition

In this paper, we propose a hydrogel-coated gate field-effect transistor (FET) for the real-time and label-free monitoring of β-amyloid (Aβ) aggregation and its inhibition. The hydrogel used in this study is composed of poly tetramethoxysilane (TMOS), in which Aβ monomers are entrapped and then aggregate, and coated on the gate insulator; that is, Aβ aggregation is induced in the vicinity of the sensing surface. With the Aβ hydrogel-coated gate FET, the steplike decrease in the surface potential of the Aβ hydrogel-coated gate electrode is electrically monitored in real time, according to the stepwise aggregation of Aβ monomers to form into fibrils through oligomers and so forth in stages. This is because the capacitance of the Aβ-hydrogel membrane decreases depending on the stage of aggregation; that is, the hydrophobicity of the Aβ-hydrogel membrane increases stepwise depending on the amount of Aβ aggregates. The formation of Aβ fibrils is also confirmed in the measurement solution using a fluorescent dye, thioflavin T, which selectively binds to the Aβ fibrils. Moreover, the addition of daunomycin, an inhibitor of Aβ aggregation, to the measurement solution suppresses the stepwise electrical response of the Aβ hydrogel-coated gate FET. Thus, a platform based on the Aβ hydrogel-coated gate FET is suitable for a simple screening system for inhibitors of Aβ aggregation, which may lead the identification of potential therapeutic agents for Alzheimer's disease.

Mesoporous Silica-Based Metal Oxide Electrode for a Nonenzymatic Glucose Sensor at a Physiological pH

To construct an electrochemical biosensing platform, we propose a glucose sensor whose electrode interface was modified by mesoporous silica (MPSi) as an electronic signal transmission interface between a biomarker and an electrochemical device. We develop an enzyme-free glucose sensor using an MPSi-coated Ta₂O₅ electrode in an actual biological fluid such as blood serum. MPSi includes a phenylboronic acid (PBA) molecule, in which glucose binds to a synthesized PBA-silane compound in an ca. 150 nm thick MPSi nanolayer, which changes the density of molecular charges of the PBA/glucose complex on the surface of MPSi. The charge changes derived from the equilibrium reaction of PBA with glucose lead to changes in surface potential of the Ta₂O₅ electrode, and the surface potential changes depending on glucose concentration were measured by a potentiometric detector. As a result, a remarkable surface potential response was observed in the vicinity of neutral pH. K_d = 6.0 mM and V_max = 194 mV were obtained from the fitting curve of the Langmuir adsorption isotherm. Finally, we confirmed the glucose response of the PBA-MPSi-coated Ta₂O₅ substrate in human serum by considering the influence of various contaminants. Although the surface potential change was suppressed by approximately one-third of that in the buffer system, it was suggested that it could be applied to measurements in the blood glucose concentration range. From the results of this study, it was clarified that blood-level glucose response could be monitored using a PBA-MPSi-coated Ta₂O₅ substrate, which suggests the possibility of using a nonenzymatic glucose sensor as an alternative to the existing enzyme sensor.

A Hippocampus-Inspired Dual-Gated Organic Artificial Synapse for Simultaneous Sensing of a Neurotransmitter and Light

Organic neuromorphic devices and sensors that mimic the functions of chemical synapses and sensory perception in humans have received much attention for next-generation computing and integrated logic circuits. Despite recent advances, organic artificial synapses capable of detecting both neurotransmitters in liquid environments and light are not reported. Herein, inspired by hippocampal synapses, a dual-gate organic synaptic transistor platform with a photoconductive polymer semiconductor, a ferroelectric insulator of P(VDF-TrFE), and an extended-gate electrode functionalized with boronic acid is developed to simultaneously detect the neurotransmitter dopamine and light. The developed synaptic transistor enables memory consolidation upon repetitive exposure to dopamine and polychromatic light, exhibiting effectively modulated postsynaptic currents. This proof-of-concept hippocampal-synapse-mimetic organic neuromorphic system combining a chemical sensor and a photosensor opens new possibilities for developing low-power organic artificial synaptic multisensors and light-induced memory consolidative artificial synapses, and can also contribute to the development of human-machine interfaces.

Molecularly imprinted polymer-based bioelectrical interfaces with intrinsic molecular charges

For enzyme-/antibody-free and label-free biosensing, a molecularly imprinted polymer (MIP)-based membrane with phenylboronic acid (PBA) molecules, which induces the change in the density of molecular charges based on the small biomolecule–PBA diol binding, has been demonstrated to be suitable for the bioelectrical interface of biologically coupled gate field-effect transistor (bio-FET) sensors. MIP-coated gate FET sensors selectively detect various small biomolecules such as glucose, dopamine, sialic acid, and oligosaccharides without using labeled materials. In particular, the well-controlled MIP film by surface-initiated atom transfer radical polymerization (SI-ATRP) contributes to the quantitative analysis of small biomolecule sensing, resulting in potentiometric Langmuir isotherm adsorption analysis by which the parameters such as the binding affinity between small biomolecules and MIP cavities are evaluated. Also, the output electrical signal of even a random MIP-coated gate FET sensor is quantitatively analyzed using the bi-Langmuir adsorption isotherm equation, showing the adsorption mechanism of small biomolecules onto the template-specific MIP membrane. Thus, a platform based on the MIP bioelectrical interface for the bio-FET sensor is suitable for an enzyme-/antibody-free and label-free biosensing system in the fields of clinical diagnostics, drug discovery, the food industry, and environmental research.

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.

Molecularly Imprinted Artificial Biointerface for an Enzyme-Free Glucose Transistor

A platform based on a highly selective and sensitive detection device functionalized with a well-designed artificial biointerface is required for versatile biosensors. We develop a molecularly imprinted polymer (MIP)-coated gate field-effect transistor (FET) biosensor for low-concentration glucose detection in biological fluid samples such as tears in an enzyme-free manner. The MIP includes glucose templates (GluMIP), in which glucose binds to vinylphenylboronic acid in the copolymerized membrane, resulting in the change in the density of molecular charges of the phenylboronic acid (PBA)/glucose complex. The FET biosensor can detect small biomolecules as long as biomolecular recognition events cause intrinsic changes in the density of molecular charges. As a result, the changes in the output voltage detected using the GluMIP-based FET sensor are fitted to the Langmuir adsorption isotherm equation at various concentrations of sugars, showing the low detection limit of 3 μM and the high sensitivity of 115 mV/decade from 100 μM to 4 mM glucose. On the basis of the equation, the stability constant ( K_a) of PBA with glucose is calculated and found to markedly increase to K_a = 1192 M^-1, which is higher by a factor of a few hundreds than K_a = 4.6 M^-1 obtained by nonelectrical detection methods. Moreover, the GluMIP-coated gate FET sensor shows an approximately 200-fold higher selectivity for glucose than for fructose. This is because glucose binds to PBA more selectively than fructose in the templates, resulting in the generation of negative charges. The electrical properties of the MIP-coated electrode are also evaluated by measuring capacitance. Our work suggests a new strategy of designing a platform based on the MIP-coated gate FET biosensor, which is suitable for a highly selective, sensitive, enzyme-free biosensing system.

Catecholamine Detection Using a Functionalized Poly(l-dopa)-Coated Gate Field-Effect Transistor

A highly sensitive catecholamine (CA) sensor was created using a biointerface layer composed of a biopolymer and a potentiometric detection device. For the detection of CAs, 3-aminophenylboronic acid (3-NH₂-PBA) was reacted with the carboxyl side chain of l-3,4-dihydroxyphenylalanine (l-dopa, LD) and the PBA-modified l-dopa was directly copolymerized with LD on an Au electrode, resulting in a 3.5 nm thick PBA-modified poly(PBA-LD/LD) layer-coated Au electrode. By connecting the PBA-LD-coated Au electrode to a field-effect transistor (FET), the molecular charge changes at the biointerface of the Au electrode, which was caused by di-ester binding of the PBA-CA complex, were transduced into gate surface potential changes. Effective CAs included LD, dopamine (DA), norepinephrine (NE), and epinephrine (EP). The surface potential of the PBA-LD-coated Au changed after the addition of 40 nM of each CA solution; notably, the PBA-LD-coated Au showed a higher sensitivity to LD because the surface potential change could already be observed after 1 nM of LD was added. The fundamental parameter analyses of the PBA-LD to CA affinity from the surface potential shift against each CA concentration indicated the highest affinity to LD (binding constant (K_s): 1.68 × 10⁶ M^-1, maximum surface potential shift (V_max): 182 mV). Moreover, the limit of detection for each CA was 3.5 nM in LD, 12.0 nM in DA, 7.5 nM in NE, and 12.6 nM in EP. From these results, it is concluded that the poly(PBA-LD/LD)-coated gate FET could become a useful biosensor for neurotransmitters, hormones, and early detection of Parkinson's disease.

Potentiometric Adsorption Isotherm Analysis of a Molecularly Imprinted Polymer Interface for Small-Biomolecule Recognition

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.

Biocompatible Poly(catecholamine)-Film Electrode for Potentiometric Cell Sensing

Surface-coated poly(catecholamine) (pCA) films have attracted attention as biomaterial interfaces owing to their biocompatible and physicochemical characteristics. In this paper, we report that pCA-film-coated electrodes are useful for potentiometric biosensing devices. Four different types of pCA film, l-dopa, dopamine, norepinephrine, and epinephrine, with thicknesses in the range of 7-27 nm were electropolymerized by oxidation on Au electrodes by using cyclic voltammetry. By using the pCA-film electrodes, the pH responsivities were found to be 39.3-47.7 mV/pH within the pH range of 1.68 to 10.01 on the basis of the equilibrium reaction with hydrogen ions and the functional groups of the pCAs. The pCA films suppressed nonspecific signals generated by other ions (Na⁺, K⁺, Ca²⁺) and proteins such as albumin. Thus, the pCA-film electrodes can be used in pH-sensitive and pH-selective biosensors. HeLa cells were cultivated on the surface of the pCA-film electrodes to monitor cellular activities. The surface potential of the pCA-film electrodes changed markedly because of cellular activity; therefore, the change in the hydrogen ion concentration around the cell/pCA-film interface could be monitored in real time. This was caused by carbon dioxide or lactic acid that is generated by cellular respiration and dissolves in the culture medium, resulting in the change of hydrogen concentration. pCA-film electrodes are suitable for use in biocompatible and pH-responsive biosensors, enabling the more selective detection of biological phenomena.

Dynamic electrical behaviour of a thermoresponsive polymer in well-defined poly(N-isopropylacrylamide)-grafted semiconductor devices

Herein, we found that the phase transition behaviour from swelling state to deswelling state in response to temperature change was electrically detected in real time by using the poly(N-isopropylacrylamide)-grafted gate field effect transistor.