Direct transduction of allele-specific primer extension into electrical signal using genetic field effect transistor

Signal transduction interfaces for field-effect transistor-based biosensors

Biosensors based on field-effect transistors (FETs) are suitable for use in miniaturized and cost-effective healthcare devices. Various semiconductive materials can be applied as FET channels for biosensing, including one- and two-dimensional materials. The signal transduction interface between the biosample and the channel of FETs plays a key role in translating electrochemical reactions into output signals, thereby capturing target ions or biomolecules. In this Review, distinctive signal transduction interfaces for FET biosensors are introduced, categorized as chemically synthesized, physically structured, and biologically induced interfaces. The Review highlights that these signal transduction interfaces are key in controlling biosensing parameters, such as specificity, selectivity, binding constant, limit of detection, signal-to-noise ratio, and biocompatibility.

Control of interface functions in solid-state biosensors for stable detection of molecular recognition

Significant progress has been achieved in the field of solid-state biosensors over the past 50 years. Various sensing devices with high-density integration and flexible configuration, as well as new applications for clinical diagnosis and healthcare, have been developed using blood, serum, and other body fluids such as sweat, tears, and saliva. A high-density array of ion-sensitive field effect transistors was developed by exploiting the advantages of advanced semiconductor technologies and commercialized in combination with an enzymatic primer extension reaction as a DNA sequencer in 2011. Different types of materials such as inorganic materials, metals, polymers, and biomolecules are mixed together on the surface of the gate while maintaining their own functions; therefore, compatibility among different materials has to be optimized so that the best detection performance of solid-state biosensors, including stability and reliability, is achieved as designed. Solid-state biosensors are suitable for the rapid, cost-effective, and noninvasive identification of biomarkers at various timepoints over the course of a disease.

Technical Perspectives on Applications of Biologically Coupled Gate Field-Effect Transistors

Biosensing technologies are required for point-of-care testing (POCT). We determine some physical parameters such as molecular charge and mass, redox potential, and reflective index for measuring biological phenomena. Among such technologies, biologically coupled gate field-effect transistor (Bio-FET) sensors are a promising candidate as a type of potentiometric biosensor for the POCT because they enable the direct detection of ionic and biomolecular charges in a miniaturized device. However, we need to reconsider some technical issues of Bio-FET sensors to expand their possible use for biosensing in the future. In this perspective, the technical issues of Bio-FET sensors are pointed out, focusing on the shielding effect, pH signals, and unique parameters of FETs for biosensing. Moreover, other attractive features of Bio-FET sensors are described in this perspective, such as the integration and the semiconductive materials used for the Bio-FET sensors.

Detection of cell membrane proteins using ion-sensitive field effect transistors combined with chemical signal amplification

The capture and detection of cells expressing a breast-cancer related membrane protein, namely a BT474 cell line expressing HER2, is demonstrated using ion-sensitive field effect transistors (ISFETs). BT474 cells were exposed to anti-HER2 antibodies and urease-conjugated secondary antibodies to induce chemical signal amplification by adding urea.

In Situ Electrical Monitoring of Methylated DNA Based on Its Conformational Change to G-Quadruplex Using a Solution-Gated Field-Effect Transistor

Methylated DNA is not only a diagnostic but also a prognostic biomarker for early-stage cancer. However, sodium bisulfite sequencing as a "gold standard" method for detection of methylation markers has some drawbacks such as its time-consuming and labor-intensive procedures. Therefore, simple and reliable methods are required to analyze DNA sequences with or without methylated residues. Herein, we propose a simple and direct method for detecting DNA methylation through its conformation transition to G-quadruplex using a solution-gated field-effect transistor (SG-FET) without using labeled materials. The BCL-2 gene, which is involved in the development of various human tumors, contains G-rich segments and undergoes a conformational change to G-quadruplex depending on the K⁺ concentration. Stacked G-quadruplex strands move close to the SG-FET sensor surface, resulting in large electrical signals based on intrinsic molecular charges. In addition, a dense hydrophilic polymer brush is grafted using surface-initiated atom transfer radical polymerization onto the SG-FET sensor surface to reduce electrical noise based on nonspecific adsorption of interfering species. In particular, control of the polymer brush thickness induces electrical signals based on DNA molecular charges in the diffusion layer, according to the Debye length limit. A platform based on the SG-FET sensor with a well-defined polymer brush is suitable for in situ monitoring of methylated DNA and realizes a point-of-care device with a high signal-to-noise ratio and without the requirement for additional processes such as bisulfite conversion and polymerase chain reaction.

A suspended graphene-based optical interferometric surface stress sensor for selective biomolecular detection

Graphene-based sensors are of great interest in research due to their high specific surface area and high electron mobility that make them suitable for numerous advanced applications. In this paper, selective molecular detection using an antigen-antibody reaction on suspended graphene with a cavity-sealing structure was demonstrated. The suspended graphene sealed nanocavities in a pre-patterned Si substrate, which increased robustness and allowed the use of wet chemical processes for surface functionalization of the suspended graphene to achieve selective molecular binding. The selectivity was evaluated by nanomechanical deflection induced by molecular adsorption on the suspended graphene, resulting in spectral shifts in the optical interference between the suspended graphene and Si substrate. The chemically functionalized suspended graphene enables the analysis of intermolecular interactions and molecular kinetics by colorimetry using optical interference.

Molecular-Charge-Contact-Based Ion-Sensitive Field-Effect Transistor Sensor in Microfluidic System for Protein Sensing

In this paper, we demonstrate the possibility of direct protein sensing beyond the Debye length limit using a molecular-charge-contact (MCC)-based ion-sensitive field-effect transistor (ISFET) sensor combined with a microfluidic device. Different from the MCC method previously reported, biotin-coated magnetic beads are set on the gate insulator of an ISFET using a button magnet before the injection of target molecules such as streptavidin. Then, the streptavidin—a biotin interaction, used as a model of antigen—antibody reaction is expected at the magnetic beads/gate insulator nanogap interface, changing the pH at the solution/dielectric interface owing to the weak acidity of streptavidin. In addition, the effect of the pH or ionic strength of the measurement solutions on the electrical signals of the MCC-based ISFET sensor is investigated. Furthermore, bound/free (B/F) molecule separation with a microfluidic device is very important to obtain an actual electrical signal based on the streptavidin–biotin interaction. Platforms based on the MCC method are suitable for exploiting the advantages of ISFETs as pH sensors, that is, direct monitoring systems for antigen–antibody reactions in the field of in vitro diagnostics.

Biologically Coupled Gate Field-Effect Transistors Meet in Vitro Diagnostics

In this paper, recent works on biologically coupled gate field-effect transistor (bio-FET) sensors are introduced and compared to provide a perspective. Most biological phenomena are closely related to behaviors of ions and biomolecules. This is why biosensing devices for detecting ionic and biomolecular charges contribute to the direct analysis of biological phenomena in a label-free and enzyme-free manner. Potentiometric biosensors such as bio-FET sensors, which allow the direct detection of these charges on the basis of the field effect, meet this requirement and have been developed as simple devices for in vitro diagnostics (IVD). A variety of biological ionic behaviors generated by biomolecular recognition events and cellular activities are being targeted for clinical diagnostics as well as the study of neuroscience using the bio-FET sensors. To realize these applications, bioelectrical interfaces should be formed between the electrolyte solution and the gate electrode by modifying artificially synthesized and biomimetic membranes, resulting in the selective detection of targets based on intrinsic molecular charges. Various types of semiconducting materials, not only inorganic semiconductors but also organic semiconductors, can be selected for use in bio-FET sensors, depending on the application field. In addition, a semiconductor integrated circuit device is ideal for the massively parallel detection of multiple samples. Thus, platforms based on bio-FET sensors are suitable for use in simple and miniaturized electrical circuit systems for IVD to enable the prevention and early detection of diseases.

Control of Potential Response to Small Biomolecules with Electrochemically Grafted Aryl-Based Monolayer in Field-Effect Transistor-Based Sensors

In this paper, we demonstrate the use of a monolayer film electrografted via diazonium chemistry for controlling the potential response of a field-effect transistor (FET)-based sensor. 4-Nitrobenzenediazonium salt is electrografted on an extended-Au-gate FET (EG-Au-FET) with or without using a radical scavenger by cyclic voltammetry (CV), resulting in the formation of a monolayer or multilayer. In particular, the surface coverage of the aryl-derivative monolayer on the Au gate electrode gradually increases with increasing number of potential cycles in CV. Here, Au exhibits a strong catalytic action, resulting in the oxidation of organic compounds. Uric acid is used as a low-molecular-weight biomolecule for interference. The denser the surface coverage of the grafted monolayer, the smaller the potential response of the EG-Au-FET because the redox reaction of uric acid with the Au gate surface is suppressed. On the other hand, the effect of the aryl-derivative multilayer on the suppression of the potential response was smaller than that of the monolayer because the electrogenerated aryl radicals did not react with the Au surface but with the grafted species, resulting in an exposed part of the Au surface among the grafted aryl molecules. Thus, a platform based on such a monolayer film electrografted via diazonium chemistry is suitable for controlling the potential response based on the interference of low-molecular-weight biomolecules in biosamples.

Polymeric Nanofilter Biointerface for Potentiometric Small-Biomolecule Recognition

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.

Chemistry integrated circuit: chemical system on a complementary metal oxide semiconductor integrated circuit

By integrating chemical reactions on a large-scale integration (LSI) chip, new types of device can be created. For biomedical applications, monolithically integrated sensor arrays for potentiometric, amperometric and impedimetric sensing of biomolecules have been developed. The potentiometric sensor array detects pH and redox reaction as a statistical distribution of fluctuations in time and space. For the amperometric sensor array, a microelectrode structure for measuring multiple currents at high speed has been proposed. The impedimetric sensor array is designed to measure impedance up to 10 MHz. The multimodal sensor array will enable synthetic analysis and make it possible to standardize biosensor chips. Another approach is to create new functional devices by integrating molecular systems with LSI chips, for example image sensors that incorporate biological materials with a sensor array. The quantum yield of the photoelectric conversion of photosynthesis is 100%, which is extremely difficult to achieve by artificial means. In a recently developed process, a molecular wire is plugged directly into a biological photosynthetic system to efficiently conduct electrons to a gold electrode. A single photon can be detected at room temperature using such a system combined with a molecular single-electron transistor.

Nanowire BioFETs: An Overview

In this chapter, the biosensing as a key element of nanotechnology and commanding a wide range of applications is discussed, e.g., fast and efficient clinical diagnostics, health care, security, environmental monitoring, etc. The operation and sensing mechanism of BioFETs and ion-sensitive FETs are elaborated on a molecular level, based upon the molecular recognition between target and probe molecules and the input gate voltage and output ON current of the conventional FETs. In particular, the extended roles of the gate electrode of BioFETs as the probing surface are highlighted, in comparison with the conventional gate electrode, together with the physical and biological processes for detecting target molecules. Moreover, the bottom-up syntheses of vertical and horizontal nanowires are presented and the ensuing nanowires are characterized. Also, the top-down and bottom-up approaches for processing nanowires are compared by taking as criteria the process complexity and quality of the nanowires produced. Finally, the future prospects of bio-sensing are presented.

Integrating solid-state sensor and microfluidic devices for glucose, urea and creatinine detection based on enzyme-carrying alginate microbeads

A solid-state sensor embedded microfluidic chip is demonstrated for the detection of glucose, urea and creatinine in human serum. In the presented device, magnetic powder-containing enzyme-carrying alginate microbeads are immobilized on the surface of an electrolyte-insulator-semiconductor (EIS) sensor by means of a step-like obstacle in the microchannel and an external magnetic force. The sample is injected into the microchannel and reacts with the enzyme contained within the alginate beads; prompting the release of hydrogen ions. The sample concentration is then evaluated by measuring the resulting change in the voltage signal of the EIS sensor. The reaction time and alginate bead size are optimized experimentally using a standard glucose solution. The experimental results show that the device has a detection range of 2-8mM, 1-16mM and 10(-2)-10mM for glucose, urea and creatinine, respectively. Furthermore, it is shown that the device is capable of sequentially measuring all three indicators in a human serum sample. Finally, it is shown that the measured values of the glucose, urea and creatinine concentrations obtained using the device deviate from those obtained using a commercial kit by just 5.17%, 6.22% and 13.53%, respectively. This method can be extended to sequentially measure multiple blood indicators in the sample chip by replacing different types of enzyme in alginate bead and can address the enzyme preservation issue in the microfluidic device. Overall, the results presented in this study indicate that the microfluidic chip has significant potential for blood monitoring in point-of-care applications.

Interpretation of protein adsorption through its intrinsic electric charges: a comparative study using a field-effect transistor, surface plasmon resonance, and quartz crystal microbalance

We describe the highly sensitive detection of the nonspecific adsorption of proteins onto a 1-undecanethiol self-assembled monolayer (SAM)-formed gold electrode by parallel analysis using field effect transistor (FET), surface plasmon resonance (SPR), and quartz crystal microbalance (QCM) sensors. The FET sensor detects the innate electric charges of the adsorbed protein at the electrode/solution interface, transforming the change in charge density into a potentiometric signal in real time, without the requirement for labels. In particular, using the Debye-Huckel model, the degree of potential shift was proportional to the dry mass of adsorbed albumin and β-casein. A comparison of the FET signal with SPR and QCM data provided information on the conformation and orientation of the surface-bound protein by observing characteristic break points in the correlation slopes between the signals. These slope transitions reflect a multistage process that occurs upon protein adsorption as a function of protein concentration, including interim coverage, film dehydration, and monolayer condensation. The FET biosensor, in combination with SPR and QCM, represents a new technology for interrogating protein-material interactions both quantitatively and qualitatively.

Optical and Electric Multifunctional CMOS Image Sensors for On-Chip Biosensing Applications

In this review, the concept, design, performance, and a functional demonstration of multifunctional complementary metal-oxide-semiconductor (CMOS) image sensors dedicated to on-chip biosensing applications are described. We developed a sensor architecture that allows flexible configuration of a sensing pixel array consisting of optical and electric sensing pixels, and designed multifunctional CMOS image sensors that can sense light intensity and electric potential or apply a voltage to an on-chip measurement target. We describe the sensors' architecture on the basis of the type of electric measurement or imaging functionalities.

Chemical-to-Electrical-Signal Transduction Synchronized with Smart Gel Volume Phase Transition

A stimulus-responsive polymer gel designed on a field-effect transistor gate undergoes a reversible volume phase transition in response to a specific biomolecule. An abrupt permittivity change at the gel/gate interface during the transition gives rise to a chemical to electrical signal conversion; the signal is thus detectable via a transistor without the limit of the Debye length.

An Integrated ISFET Sensor Array

A monolithically integrated ISFET sensor array and interface circuit are described. A new high-density, low-power source-drain follower was developed for the sensor array. ISFETs were formed by depositing Au/Ti extended-gate electrodes on standard MOSFETs, then thin silicon nitride layers using catalytic chemical vapor deposition and/or SU-8 protective layers were formed on the extended-gate electrodes. Applications for the array include: (1) pH detection by statistical distribution observing time and space fluctuations; (2) DNA detection using thiol-modified or silane-coupled oligonucleotides; (3) bio-image sensing by converting photons to electrons using Photosystem I of Thermosynechococcus elongatus, and sensing the converted electric charges by ISFETs.