Solution-gated Field-effect Transistors Research Articles

Diamond FET Biosensor Fabrication and Application

Diamond is renowned as the ultimate semiconductor thanks to its exceptional physical properties, including unmatched hardness, exceptional wear resistance, superior mechanical and tribological characteristics, and high fracture strength. Diamond solution-gate field-effect transistors (D-SGFETs) leverage these advantages, along with their outstanding high-power and high-frequency performance, excellent thermal conductivity, wide bandgap, high carrier mobility, and rapid saturation speed. These features make D-SGFETs highly promising for fast and precise biomedical detection applications. This paper provides a comprehensive review of the fabrication techniques for diamond SGFETs, encompassing diamond film synthesis, surface conduction layer formation, source/drain fabrication, and FET packaging. Furthermore, the study delves into the surface functionalization of diamond SGFETs and their diverse applications in biomedical detection. Finally, the paper discusses the future outlook of diamond SGFETs in advancing biomedical detection technologies.

Integration of highly sensitive large-area graphene-based biosensors in an automated sensing platform

Graphene-based biosensors, featuring exceptional electronic, mechanical, and surface properties, have emerged as frontrunners in advanced sensing technologies. However, to achieve widespread industrial adoption, advancements in the fabrication and integration of large-area graphene devices are essential. Critical parameters such as enhanced sensitivity, scalable production methods, economic viability, integration capabilities, and consistent uniformity must be meticulously addressed. In this work, we demonstrate that our ultra-clean, chemical wet transfer protocol of large-area graphene enables a scalable, smooth integration of graphene into an established assay platform for transporter protein drug discovery. Furthermore, we demonstrate sensitive detection of electrolytic buffers, varying pH, bovine serum albumin (BSA) and single-stranded DNA (ssDNA) adsorption, using our large-area graphene solution-gated field-effect transistor (SGFET) sensors, thereby proving their robust and reliable performance. The sensors’ biocompatibility and ion sensitivity, down to the picomolar range, substantiate their suitability for the investigation of electroactive transport in ion channels and membrane transporters.

Highly Sensitive DNA Sensing in Subthreshold Regime with ITO-Based One-Piece Thin Film Transistor

INTRODUCTION Solution-gated field-effect transistors (FETs) with silicon or other semiconductive materials as channels can specifically and selectively detect ions and biomolecules in biological samples by chemically modifying the gate electrode surface, and these are known as biologically coupled FETs (Bio-FETs) [1]. Since Bio-FETs can directly detect the charges of ions and biomolecules, they do not require to label fluorescent dyes and to induce redox reactions based on enzymes. Therefore, Bio-FETs are expected to be used as in vitro diagnostic devices for a label-free monitoring of health conditions in daily life.Our research group is investigating the application of thin-film transistors (TFTs) composed of indium tin oxide (ITO), which has been widely used as an electrode material for displays owing to its transparency and conductivity, to Bio-FETs. Considering the fact that the ITO thin film is not only conductive but also semiconductive, depending on the thickness, we have found that solution-gated FETs can be fabricated by one-step sputtering of ITO [2]. Moreover, by etching a part of the conductive ITO film with the thickness of 100 nm) to the optimal thickness (< 30 nm), which induces semiconductive characteristics, a solution-gated ITO-TFT can be fabricated, which is composed of one piece of ITO thin film and whose shape and properties can be precisely controlled [3]. The ITO-based one-piece TFT has no interface among the source/channel/drain electrodes and exhibits a steep subthreshold slope (SS) of about 80 mV/decade, which is based on the large capacitance of the electric double layer formed by the direct contact of electrolyte solutions with the ITO channel surface,. Therefore, the ITO-based one-piece TFT is expected to be applied to biosensors with high sensitivity.In this study, we demonstrate the possibility of a highly sensitive DNA detection in the steep SS with the ITO-based one-piece TFT for the biosensing application. In particular, DNA probes are chemically modified on the ITO channel surface, and then DNA hybridization is detected in the near subthreshold regime with the ITO-based one-piece TFT.EXPERIMENTAL First, photoresist was patterned on a glass substrate by photolithography. ITO thin film (100 nm thickness) was deposited on the patterned area by sputtering, and photoresist was removed using a solvent. Next, as shown in Figure 1, photolithography was used again to deposit the resist film so that the channel portion was exposed. Subsequently, a drop of 0.1 M hydrochloric acid was dropped onto the exposed channel area, and then the exposed area was etched. Etching was controlled by monitoring the conductivity between the source and drain over time. Moreover, thiolated DNA probes were immobilized on the channel surfaces of the fabricated devices using m-maleimidobenzonyl-N-hydroxysuccinimide ester (MBS) as a crosslinker to the amino groups at the poly-serotonin surface as the anchor layer (Figure 1). Thus, DNA probes were fixed on the surface to detect the target DNA, and various bioaffinity parameters such as detection limit and sensitivity were investigated in the near-subthreshold region of ITO-based one-piece TFTs.A semiconductor parameter analyzer was used to monitor conductivity during etching and to evaluate electrical properties such as gate voltage (VG)-drain current (ID) transfer characteristics of the fabricated ITO-based one-piece TFTs. AFM (Bruker) was used to evaluate the shape of the surface chemical modification.RESULTS and DISCUSSION The ITO-based one-piece TFT functionalized with DNA probes was used to evaluate the potential response to the addition of target DNA. The measurement was conducted in the near-subthreshold regime. As a result, the DNA probe-modified device showed the larger electrical signals than the device without the modification of DNA probes. This suggests that the functionalized ITO-based one-piece TFT enables the highly sensitive detection of biomolecules in the subthreshold regime.CONCLUSIONS In this study, we demonstrated the relatively highly sensitive DNA detection even in the near subthreshold regime with the ITO-based one-piece TFT with DNA probes. This indicates that the functionalized ITO-based one-piece TFT would allow the specific detection of various biomarkers in the subthreshold regime, which has the potential to be applied to biosensors with high sensitivity in the future.REFERENCES[1] Sakata, T. ACS Omega 2019, 4, 11852.[2] Sakata, T.; Nishitani, S.; Saito, A.; Fukasawa, Y. ACS Appl. Mater. Interfaces 2021, 13, 38569.[3] Katayama, R.; Sakata, T. ECS Trans. 2023, 111, 37. Figure 1

Photoelectrochemical Solution Gated Graphene Field-Effect Transistor Functionalized by Enzymatic Cascade Reaction for Organophosphate Detection.

Solution Gated Graphene Field-Effect Transistors (SGGT) are eagerly anticipated as an amplification platform for fabricating advanced ultra-sensitive sensors, allowing significant modulation of the drain current with minimal gate voltage. However, few studies have focused on light-matter interplay gating control for SGGT. Herein, this challenge is addressed by creating an innovative photoelectrochemical solution-gated graphene field-effect transistor (PEC-SGGT) functionalized with enzyme cascade reactions (ECR) for Organophosphorus (OPs) detection. The ECR system, consisting of acetylcholinesterase (AChE) and CuBTC nanomimetic enzymes, selectively recognizes OPs and forms o-phenylenediamine (oPD) oligomers sediment on the PEC electrode, with layer thickness related to the OPs concentration, demonstrating time-integrated amplification. Under light stimulation, the additional photovoltage generated on the PEC gate electrode is influenced by the oPD oligomers sediment layer, creating a differentiated voltage distribution along the gate path. PEC-SGGT, inherently equipped with built-in amplification circuits, sensitively captures gate voltage changes and delivers output with an impressive thousandfold current gain. The seamless integration of these three amplification modes in this advanced sensor allows a good linear range and highly sensitive detection of OPs, with a detection limit as low as 0.05pm. This work provides a proof-of-concept for the feasibility of light-assisted functionalized gate-controlled PEC-SGGT for small molecule detection.

Ethanol Solvation of Polymer Residues in Graphene Solution-Gated Field Effect Transistors.

The persistence of photoresist residues from microfabrication procedures causes significant obstacles in the technological advancement of graphene-based electronic devices. These residues induce undesired chemical doping effects, diminish carrier mobility, and deteriorate the signal-to-noise ratio, making them critical in certain contexts, including sensing and electrical recording applications. In graphene solution-gated field-effect transistors (gSGFETs), the presence of polymer contaminants makes it difficult to perform precise electrical measurements, introducing response variability and calibration challenges. Given the absence of viable short to midterm alternatives to polymer-intensive microfabrication techniques, a postpatterning treatment involving THF and ethanol solvents was evaluated, with ethanol being the most effective, environmentally sustainable, and safe method for residue removal. Employing a comprehensive analysis with XPS, AFM, and Raman spectroscopy, together with electrical characterization, we investigated the influence of residual polymers on graphene surface properties and transistor functionality. Ethanol treatment exhibited a pronounced enhancement in gSGFET performance, as evidenced by a shift in the charge neutrality point and reduced dispersion. This systematic cleaning methodology holds the potential to improve the reproducibility and precision in the manufacturing of graphene devices. Particularly, by using ethanol for residue removal, we align our methodology with the principles of green chemistry, minimizing environmental impact while advancing diverse graphene technology applications.

Drift suppression of solution-gated graphene field-effect transistors through electrolyte submersion

In solution-gated graphene FETs (SG-GFETs), cations in electrolyte solutions can intercalate between graphene and SiO2. Such permeation affects substrate-induced hole doping effects, resulting in drifts in the charge neutrality point (CNP) of SG-GFETs. In this study, we investigated the effect of submerging GFETs in electrolyte solutions on CNP values. The results revealed that the CNP decreased considerably from approximately 180 mV to nearly zero with the increase in the immersion period. The CNP drifts during electrical measurements were also suppressed by the prolonged submersion. These insights can be used for developing improved SG-GFETs.

Comparing solution-gate and bottom-gate nanowire field-effect transistors on pH sensing with different salt concentrations and surface modifications

Field-effect transistors (FETs) have been developed as pH sensors by using various device structures, fabrication technologies, and sensing film materials. Different transistor structures, like extended-gate (EG) FETs, floating-gate FET sensors, and dual-gate (DG) FETs, can enhance the sensor performance. In this article, we report the effects of using solution-gate and bottom-gate FET configurations on pH sensing and investigate the influence of different ionic concentrations of buffers in the measured signals. The surface charge of hafnium dioxide (HfO2) affected by the buffer pH, with/without the modification of polyethylene glycol (PEG) terminated with hydroxyl groups, and the location of applied gate voltage are vital factors to the sensor performance in pH sensing. Based on the results, the solution-gate FET exhibits good pH sensitivity even in the high ionic strength solutions of bis-tris propane (BTP), and these values of pH sensitivity are close to the Nernst limit (59.2 mV/pH). In general, silane-PEG-OH modification can reduce the deviations of measured signals in pH sensing. The performance of bottom-gate FET is inferior in the BTP buffers with high ionic solutions but suitable to be operated in low ionic concentrations, such as 0.1, 1, and 10 mM BTP buffers. The size of the ions was also studied and discussed. The solution-gate FET demonstrates excellent performance under high ionic strengths, meaning a more significant potential for detecting biological molecules under physiological conditions.

Impact of gate electrode on free chlorine sensing performance in solution-gated graphene field-effect transistors.

Free chlorine is widely used to disinfect water used for drinking and food processing. This requires highly sensitive, simple, and capable continuous-measurement sensors to enable the concentration of free chlorine in water to be monitored and controlled. Free chlorine sensors based on solution-gated graphene field-effect transistors (GFETs) are a suitable platform for highly sensitive and continuous measurements. However, their sensing mechanisms require further elucidation to improve their performance. In this study, we focused on the gate electrode and investigated its influence on the sensing performance. Using the free chlorine sensor based on the solution-gate GFET, we showed that the Dirac point voltage in the transfer curve changed significantly as the free chlorine concentration changed, and the electric double-layer capacitance of the gate electrode decreased. Furthermore, we demonstrated that a solution-gated GFET using graphene or boron-doped diamond as the gate electrode could be used to detect changes in the free chlorine concentration in the concentration range of tap water. The sensing performance in the low concentration range benefits from the wide potential window of carbon-based electrodes, which do not have electrochemically active sites. Using these carbon-based materials as gate electrodes, GFETs have the potential to be used as durable sensors that are resistant to surface fouling and oxidation.

Recording physiological and pathological cortical activity and exogenous electric fields using graphene microtransistor arrays in vitro.

Graphene-based solution-gated field-effect transistors (gSGFETs) allow the quantification of the brain's full-band signal. Extracellular alternating current (AC) signals include local field potentials (LFP, population activity within a reach of hundreds of micrometers), multiunit activity (MUA), and ultimately single units. Direct current (DC) potentials are slow brain signals with a frequency under 0.1 Hz, and commonly filtered out by conventional AC amplifiers. This component conveys information about what has been referred to as "infraslow" activity. We used gSGFET arrays to record full-band patterns from both physiological and pathological activity generated by the cerebral cortex. To this end, we used an in vitro preparation of cerebral cortex that generates spontaneous rhythmic activity, such as that occurring in slow wave sleep. This examination extended to experimentally induced pathological activities, including epileptiform discharges and cortical spreading depression. Validation of recordings obtained via gSGFETs, including both AC and DC components, was accomplished by cross-referencing with well-established technologies, thereby quantifying these components across different activity patterns. We then explored an additional gSGFET potential application, which is the measure of externally induced electric fields such as those used in therapeutic neuromodulation in humans. Finally, we tested the gSGFETs in human cortical slices obtained intrasurgically. In conclusion, this study offers a comprehensive characterization of gSGFETs for brain recordings, with a focus on potential clinical applications of this emerging technology.

Chronic full-band recordings with graphene microtransistors as neural interfaces for discrimination of brain states.

Brain states such as sleep, anesthesia, wakefulness, or coma are characterized by specific patterns of cortical activity dynamics, from local circuits to full-brain emergent properties. We previously demonstrated that full-spectrum signals, including the infraslow component (DC, direct current-coupled), can be recorded acutely in multiple sites using flexible arrays of graphene solution-gated field-effect transistors (gSGFETs). Here, we performed chronic implantation of 16-channel gSGFET arrays over the rat cerebral cortex and recorded full-band neuronal activity with two objectives: (1) to test the long-term stability of implanted devices; and (2) to investigate full-band activity during the transition across different levels of anesthesia. First, we demonstrate it is possible to record full-band signals with stability, fidelity, and spatiotemporal resolution for up to 5.5 months using chronic epicortical gSGFET implants. Second, brain states generated by progressive variation of levels of anesthesia could be identified as traditionally using the high-pass filtered (AC, alternating current-coupled) spectrogram: from synchronous slow oscillations in deep anesthesia through to asynchronous activity in the awake state. However, the DC signal introduced a highly significant improvement for brain-state discrimination: the DC band provided an almost linear information prediction of the depth of anesthesia, with about 85% precision, using a trained algorithm. This prediction rose to about 95% precision when the full-band (AC + DC) spectrogram was taken into account. We conclude that recording infraslow activity using gSGFET interfaces is superior for the identification of brain states, and further supports the preclinical and clinical use of graphene neural interfaces for long-term recordings of cortical activity.

Single-Step Functionalization Strategy of Graphene Microtransistor Array with Chemically Modified Aptamers for Biosensing Applications.

Graphene solution-gated field-effect transistors (gSGFETs) offer high potential for chemical and biochemical sensing applications. Among the current trends to improve this technology, the functionalization processes are gaining relevance for its crucial impact on biosensing performance. Previous efforts are focused on simplifying the attachment procedure from standard multi-step to single-step strategies, but they still suffer from overreaction, and impurity issues and are limited to a particular ligand. Herein, a novel strategy for single-step immobilization of chemically modified aptamers with fluorenylmethyl and acridine moieties, based on a straightforward synthetic route to overcome the aforementioned limitations is presented. This approach is benchmarked versus a standard multi-step strategy using thrombin as detection model. In order to assess the reliability of the functionalization strategies 48-gSGFETs arrays are employed to acquire large datasets with multiple replicas. Graphene surface characterization demonstrates robust and higher efficiency in the chemical coupling of the aptamers with the single-step strategy, while the electrical response evaluation validates the sensing capability, allowing to implement different alternatives for data analysis and reduce the sensing variability. In this work, a new tool capable of overcome the functionalization challenges of graphene surfaces is provided, paving the way toward the standardization of gSGFETs for biosensing purposes.

Optimization of diamond sensor for trace detection of SARS-CoV-2 N-protein using Au nanoparticles

In this study, Au nanoparticles were introduced to functionalize hydrogen-terminated diamond (H-diamond) to optimize surface modification and improve the performance of diamond sensor for trace detection of SARS-CoV-2 N-protein. The Au nanoparticles were obtained by an electron beam evaporation system and annealing. Atom force microscope and scanning electron microscope were utilized to characterize the structure, morphology, and distribution of fabricated Au nanoparticles. The concentration of SARS-CoV-2 N-protein was determined by the investigation of transfer characteristics of a diamond solution gate field effect transistor. Sensitivity of this sensor was obtained as 27.43 mV/lg (N-protein concentration), which was higher than a regular H-diamond sensor. In addition, it had a wider linear detection range of 10−15–10−5 mg/ml and a lower limit of detection of 10−15 mg/ml. Accordingly, the detection performance of diamond sensor has been greatly improved due to the functionalization of Au nanoparticles.

Dual-Needle Field-Effect Transistor Biosensor for In Vivo pH Monitoring.

Local pH of the brain microenvironment is a prominent indicator for assessing health status and is closely related to many diseases; therefore, the development of effective in vivo pH methods is of great importance. This work demonstrates a dual-needle biosensor based on a solution-gate field-effect transistor (FET) for selective and sensitive monitoring of pH in cerebrospinal fluid in the central nervous system. The sensor consists of two parts: a needle FET modified with high-purity carbon nanotubes for electrical signal conduction and a needle gate modified with polyaniline for specific pH response. Based on the device's specific shape and sensing characteristics, the dual-needle sensor is sensitive to the measurement of pH in the living brain while maintaining excellent stability. The prepared dual-needle biosensor exhibits a high Nernstian response of 53.7 mV/pH over a wide pH range from 4.0 to 9.0 and excellent selectivity toward pH against other potential interfering species in the brain. Either in the case of directly injecting weak acids and bases into the rat brain or in the constructed acute acid-base poisoning model, the dual-needle biosensor can respond sensitively to the pH changes of the rat brain. This work has produced a unique dual-needle FET biosensor with high reliability and stability, which provides a new method for real-time monitoring of dynamic pH changes in the body.

Ion-Sensitive Stainless Steel Vessel for All-Solid- State pH Sensing System Incorporating pH-Insensitive Diamond Solution Gate Field-Effect Transistors

An all-solid-state pH sensing system utilizing a stainless steel vessel (SUS304) and a pH-insensitive diamond solution-gate field-effect transistor (SGFET) is presented here to explain the interaction between stainless steel vessel and field-effect transistor (FET) pH sensors for the first time. A pH sensitive ion-sensitive field-effect transistor (ISFET) was first used to show the change of the sensing behavior from 47.78 mV/pH to -4.73 mV/pH when using an Ag/AgCl electrode and stainless steel vessel as the gate, respectively. This intriguing sensing behavior was investigated by developing large and small-signal equivalent circuit models in a transistor circuit for both the Ag/AgCl and the stainless steel vessel gate. The result shows that the targeted ion change ΔQ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</sub> corresponding to the pH sensitivity has been offset which explains the phenomenon observed when using the pH sensitive ISFET with the stainless steel vessel. We then hypothesize that combining a pH insensitive device with the stainless steel vessel gate should show a pH sensitivity close to the Nernst response. To validate this, a pH insensitive diamond SGFET was then fabricated and combined with the stainless steel vessel for pH measurements. The system demonstrates a high pH sensitivity at -54.18 mV/pH across a wide range of pH solutions (pH 2-12) and remains stable in elevated temperatures when measured with a potentiostat setup at 80 °C. The results also suggest that this all-solid-state sensing system has great potential to be used in the food and beverage industry where stainless steel is widely employed due to its excellent corrosion resistance, low cost, and high sensing capabilities.

Trace detection of SARS-CoV-2 N-protein by diamond solution-gate field-effect transistor.

In this study, we introduced H-terminated diamond solution-gate field-effect transistor (H-diamond SGFET) to detect trace SARS-CoV-2 N-protein, which plays an important role in replication and transcription of viral RNA. 1-Pyrenebutyric acid-N-hydroxy succinimide ester (Pyr-NHS) was modified on H-diamond surface as linker, on which the specific antibody of SARS-CoV-2 N-protein was catenated. Fourier transform infrared spectrum, scanning electron microscope and energy dispersive spectrum were utilized to demonstrate the modification of H-diamond with Pyr-NHS and antibody. Shifts of IDS(max) at VGS=-500mV in transfer characteristics of H-diamond SGFET was observed to determine N-protein concentration in phosphate buffer solution. Good linear relationship between IDS(max) and log10(N-protein) was observed from 10-14 to 10-5g/mL with goodness of fit R2=0.90 and sensitivity of 1.98μA/Log10 [concentration of N-protein] at VDS=-500mV, VGS=-500mV. Consequently, this prepared H-diamond SGFET biosensor may provide a new idea for diagnosis of SARS-CoV-2 due to a wide detection range from 10-14 to 10-5g/mL and low limit of detection 10-14g/mL.

Highly sensitive and selective detection of enrofloxacin residues in chicken based on solution-gated graphene field-effect transistors

Enrofloxacin (ENR) has been widely used in poultry farming because of its low cost and high efficiency, yet ENR residues due to improper use may cause severe damage to human body. Herein for effective control of ENR residues, we report an ultra-sensitive ENR sensor based on a solution-gated graphene field-effect transistor (SGGT) through modification of the gate with Au nanoparticles and multi-walled carbon nanotube, dispersed by poly diallyl-dimethyl ammonium chloride (PDDA). The detection limit of the optimized SGGT-based sensor was as low as 10 nM (3.59 ng/mL) for ENR, and the linear range was 300 nM ∼ 30 μM (107.7 ng/mL ∼ 107.7 μg/mL).The prepared sensor was applied to ENR-spiked chicken samples, and the detection recoveries were 94.47% ∼ 105.95%. Considering the low-cost and easy manufacturing features of the SGGT-based sensor, we expect that it offers a highly sensitive sensing platform for on-site and real-time detection of ENR residues in many other food samples.

Sensitive label-free detection of the biomarker phosphorylated tau−217 protein in Alzheimer's disease using a graphene-based solution-gated field effect transistor

Alzheimer's disease (AD) is generally diagnosed using advanced imaging, but recent research suggests early screening using biomarkers in peripheral blood is feasible; among them, plasma tau proteins phosphorylated at threonine 231, threonine 181, and threonine 217 (p-tau217) are potential targets. A recent study indicates that the p-tau217 protein is the most efficacious biomarker. However, a clinical study found a pg/ml threshold for AD screening beyond standard detection methods. A biosensor with high sensitivity and specificity p-tau217 detection has not yet been reported. In this study, we developed a label-free solution-gated field effect transistor (SGFET)-based biosensor featuring a graphene oxide/graphene (GO/G) layered composite. The top layer of bilayer graphene grown using chemical vapor deposition was functionalized with oxidative groups serving as active sites for forming covalent bonds with the biorecognition element (antibodies); the bottom G could act as a transducer to respond to the attachment of the target analytes onto the top GO conjugated with the biorecognition element via π−π interactions between the GO and G layers. With this unique atomically layered G composite, we obtained a good linear electrical response in the Dirac point shift to p-tau217 protein concentrations in the range of 10 fg/ml to 100 pg/ml. The biosensor exhibited a high sensitivity of 18.6 mV/decade with a high linearity of 0.991 in phosphate-buffered saline (PBS); in human serum albumin, it showed approximately 90% of the sensitivity (16.7 mV/decade) in PBS, demonstrating high specificity. High stability of the biosensor was also displayed in this study.

Partly-O-Diamond Solution-Gate Field-Effect Transistor as an Efficient Biosensor of Glucose

Assessment of glucose concentration is vital for diagnosis and treatment of diabetes mellitus. Since the electrochemical advantages of diamond for biosensing, we introduced partly-oxygen-diamond solution-gate field-effect transistor (SGFET) to detect glucose. Partly-O-diamond was obtained from H-diamond by ultraviolet ozone treatment. Then, 1-pyrenebutyric acid–N-hydroxy succinimide ester and glucose oxidase were modified on diamond surface. At last, shifts of transfer characteristics were applied to determine glucose concentration. The as-prepared biosensor exhibited a wide linear response from 10−5 to 10−1 M with high sensitivity of −53 mV/log10 [glucose concentration]. Furthermore, this biosensor showed good repeatability and stability in multiple detections.

Covalent functionalisation controlled by molecular design for the aptameric recognition of serotonin in graphene-based field-effect transistors.

In the last decade, solution-gated graphene field effect transistors (GFETs) showed their versatility in the development of a miniaturized multiplexed platform for electrophysiological recordings and sensing. Due to their working mechanism, the surface functionalisation and immobilisation of receptors are pivotal to ensure the proper functioning of devices. Herein, we present a controlled covalent functionalisation strategy based on molecular design and electrochemical triggering, which provide a monolayer-like functionalisation of micro-GFET arrays retaining the electronic properties of graphenes. The functionalisation layer as a receptor was then employed as the linker for serotonin aptamer conjugation. The micro-GFET arrays display sensitivity toward the target analyte in the micromolar range in a physiological buffer (PBS 10 mM). The sensor allows the in-flow real-time monitoring of serotonin transient concentrations with fast and reversible responses.

A Comprehensive Study on the Theory of Graphene Solution‐Gated Field Effect Transistor: Simulations and Experiments

Graphene solution-gated field effect transistors (G-SgFETs) have been widely developed in the field of biosensors, but deficiencies in their theories still exist. A theoretical model for G-SgFET, including the three-terminal equivalent circuit model and the numerically calculating method, is proposed by the comprehensive analyses of the graphene-liquid interface and the FET principle. Not only the applied voltages on the electrode-pairs of gate-source and drain-source, but also the nature of graphene and its derivatives are considered by analysing their influences on the Fermi level, the carriers’ concentration and mobility, which may consequently affect the output drain-source current. To verify whether it is available for G-SgFETs based on different method prepared graphene, three kinds of graphene materials which are liquid-phase exfoliated graphene, reduced graphene oxide (rGO), and tetra (4-aminophenyl) porphyrin hybridized rGO are used as examples. The coincidences of calculated output and transfer feature curves with the measured ones are obtained to confirm its adaptivity for simulating the basic G-SgFETs’ electric features, by modulating Fermi level and mobility. Furthermore, the model is exploited to simulate G-SgFETs’ current responding to the biological functionalization with aptamer and the detections for circulating tumor cells, as a proof-of-concept. The calculated current changes are compared with the experimental results, to verify the proposed G-SgFETs’ model is also suitable for mimicking the bio-electronic responding, which may give a preview of some conceived G-SgFETs’ biosensors and improve the design efficiency.