Graphene Solution-gated Field-effect Transistors Research Articles

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.

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.

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.

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.

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.

Optimization of graphene transistor sensors based on quantum capacitance and charge carrier mobility analysis

Charge density of molecules (Nm) in hybrid nanostructures that is formed at the interface of graphene and liquid in solution gated graphene field effect transistors (SGFETs) determines the selective response of chemical and biological sensors based on these SGFETs. For optimization of this response it is important to determine how it depends on characteristics of SGFETs such as quantum capacitance (Cq) and charge mobility (μ) which are functionally linked to Nm. The proposed model shows that when the gate voltage (Vgate) is near the minimum point of graphene conductivity (Dirac point) the sensor response is low and increases with gate voltage until Cq is approximately equal to the capacitance of the formed double layer (Cdl) in SGFETs. A decrease in sensor response is predicted upon further increase of Vgate in cases where there is a stronger dependence of μ on Nm than μ propto 1/Nm. A comparison of the predicted results of the model and literature data obtained in SGFET sensors for lysine in an aqueous solution are in agreement with the assumption that the optimal condition of Cq~ Cdl is reached approximately in the Vgate region of (0.5-1.4) V from the Dirac Point. Keywords: graphene, hybrid nanostructures, transistor sensor, charge mobility, interface.

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Charge density of molecules (Nm) in hybrid nanostructures that is formed at the interface of graphene and liquid in solution gated graphene field effect transistors (SGFETs) determines the selective response of chemical and biological sensors based on these SGFETs. For optimization of this response it is important to determine how it depends on characteristics of SGFETs such as quantum capacitance (Cq) and charge mobility (µ) which are functionally linked to Nm . The proposed model shows that when the gate voltage (Vgate) is near the minimum point of graphene conductivity (Dirac point) the sensor response is low and increases with gate voltage until Cq is approximately equal to the capacitance of the formed double layer (Cdl) in SGFETs. A decrease in sensor response is predicted upon further increase of Vgate in cases where there is a stronger dependence of µ on Nm than µ ∝ 1/Nm. A comparison of the predicted results of the model and literature data obtained in SGFET sensors for lysine in an aqueous solution are in agreement with the assumption that the optimal condition of Cq ≈ Cdl is reached approximately in the region of (0.5−1.4) V from the Dirac Point.

Highly Sensitive and Selective Detection of Enrofloxacin Residues in Chicken Based on Solution-Gated Graphene Field-Effect Transistors

Highly Sensitive and Selective Detection of Enrofloxacin Residues in Chicken Based on Solution-Gated Graphene Field-Effect Transistors

Highly Sensitive and Selective Detection of Enrofloxacin Residues in Chicken Based on Solution-Gated Graphene Field-Effect Transistors

Highly Sensitive and Selective Detection of Enrofloxacin Residues in Chicken Based on Solution-Gated Graphene Field-Effect Transistors

Drift Suppression of Solution-Gated Graphene Field-Effect Transistors by Cation Doping for Sensing Platforms.

Solution-gated graphene field-effect transistors (SG-GFETs) provide an ideal platform for sensing biomolecules owing to their high electron/hole mobilities and 2D nature. However, the transfer curve often drifts in an electrolyte solution during measurements, making it difficult to accurately estimate the analyte concentration. One possible reason for this drift is that p-doping of GFETs is gradually countered by cations in the solution, because the cations can permeate into the polymer residue and/or between graphene and SiO2 substrates. Therefore, we propose doping sufficient cations to counter p-doping of GFETs prior to the measurements. For the pre-treatment, GFETs were immersed in a 15 mM sodium chloride aqueous solution for 25 h. The pretreated GFETs showed that the charge neutrality point (CNP) drifted by less than 3 mV during 1 h of measurement in a phosphate buffer, while the non-treated GFETs showed that the CNP was severely drifted by approximately 50 mV, demonstrating a 96% reduction of the drift by the pre-treatment. X-ray photoelectron spectroscopy analysis revealed the accumulation of sodium ions in the GFETs through pre-treatment. Our method is useful for suppressing drift, thus allowing accurate estimation of the target analyte concentration.

Surface-tailored graphene channels

The detection of ions and molecules in liquids has been receiving considerable attention for the realization of the electronic tongue. Solution-gated field-effect transistors (SFETs) with high sensitivity are useful for detecting ions and molecules by reading electrical transconductance. However, to date, ionic and molecular sensors that employ SFETs have limitations, such as the lack of a dynamic on–off function and low selectivity. In this study, we evaluate rationally designed graphene SFETs as pH and glucose-selective sensors. The integration of the microfluidic channel to the graphene SFET exhibits dynamic on–off functions by controlling injection and withdrawal of solutions. The graphene SFET device exhibits high pH and glucose selectivity when coated with Nafion as a molecular sieve and Au-decorated nanoparticles as receptors, respectively. The dynamic on–off functions and high selectivity of SFETs with tailored graphene channels have a high potential for advancing as a platform for electronic tongues by integrating the separate SFETs as an array for simultaneous sensing of multiple targets.

Multiplexed neural sensor array of graphene solution-gated field-effect transistors

Electrocorticography (ECoG) is a well-established technique to monitor electrophysiological activity from the surface of the brain and has proved crucial for the current generation of neural prostheses and brain–computer interfaces. However, existing ECoG technologies still fail to provide the resolution necessary to accurately map highly localized activity across large brain areas, due to the rapidly increasing size of connector footprint with sensor count. This work demonstrates the use of a flexible array of graphene solution-gated field-effect transistors (gSGFET), exploring the concept of multiplexed readout using an external switching matrix. This approach does not only allow for an increased sensor count, but due to the use of active sensing devices (i.e. transistors) over microelectrodes it makes additional buffer transistors redundant, which drastically eases the complexity of device fabrication on flexible substrates. The presented results pave the way for upscaling the gSGFET technology towards large-scale, high-density μECoG–arrays, eventually capable of resolving neural activity down to a single neuron level, while simultaneously mapping large brain regions.

Distortion-Free Sensing of Neural Activity Using Graphene Transistors.

Graphene solution-gated field-effect transistors (g-SGFETs) are promising sensing devices to transduce electrochemical potential signals in an electrolyte bath. However, distortion mechanisms in g-SGFET, which can affect signals of large amplitude or high frequency, have not been evaluated. Here, a detailed characterization and modeling of the harmonic distortion and non-ideal frequency response in g-SGFETs is presented. This accurate description of the input-output relation of the g-SGFETs allows to define the voltage- and frequency-dependent transfer functions, which can be used to correct distortions in the transduced signals. The effect of signal distortion and its subsequent calibration are shown for different types of electrophysiological signals, spanning from large amplitude and low frequency cortical spreading depression events to low amplitude and high frequency action potentials. The thorough description of the distortion mechanisms presented in this article demonstrates that g-SGFETs can be used as distortion-free signal transducers not only for neural sensing, but also for a broader range of applications in which g-SGFET sensors are used.

Ion Sensing with Solution-Gated Graphene Field-Effect Sensors in the Frequency Domain.

Here, we examine the concept of frequency domain sensing with solution-gated graphene field-effect transistors, where a sine wave of primary frequency (1f) was applied at the gate and modulation of the power spectral density (PSD) of the drain-source current at 1f, 2f, and 3f was examined as the salt in the gate electrolyte was switched from KCl to CaCl2, and their concentrations were varied. The PSD at 1f, 2f, and 3f increased with the concentration of KCl or CaCl2, with the PSD at 1f being the most sensitive. We further correlated these changes to the shift in Dirac point. Switching the graphene substrate from oxide to hexagonal boron nitride, led to an improved device-to-device reproducibility and a significant reduction of noise, which translated to a higher signal-to-noise ratio and resolution in sensing salt concentrations. The signal-to-noise ratio at 1f was found to be a logarithmic function of KCl or CaCl2 concentration in the 0.1 to 1000 mM range.

Sensitivity enhancement of potassium ion (K+) detection based on graphene field-effect transistors with surface plasma pretreatment

Potassium (K+) ions are essential minerals that are important in maintaining fluid and electrolyte balance in biological systems and regulating bodily functions. However, abnormal K+ concentration level will cause numerous health problems or diseases. Therefore, the effective detection of physiological K+ level is very important in the healthcare field. In this work, we designed, fabricated and characterized solution-gated field effect transistors (FETs) based on high quality CVD graphene for highly sensitive and selective detection of K+ ions. O2 plasma pre-processing for 10 s yielded a limit of detection (LOD) down to 0.058 pM, and a linear dynamic range spanning from sub-0.1 pM to 100 nM. Meanwhile, as the G-quadruplex conformation possesses strong affinity for K+, the FETs exhibit excellent discrimination against other common cations, such as Na+ and Fe3+. As a result, the solution-gated graphene FETs with enhanced sensitivity and selectivity for K+ can be applied to health monitoring and disease prevention.

High-resolution mapping of infraslow cortical brain activity enabled by graphene microtransistors.

Recording infraslow brain signals (<0.1 Hz) with microelectrodes is severely hampered by current microelectrode materials, primarily due to limitations resulting from voltage drift and high electrode impedance. Hence, most recording systems include high-pass filters that solve saturation issues but come hand in hand with loss of physiological and pathological information. In this work, we use flexible epicortical and intracortical arrays of graphene solution-gated field-effect transistors (gSGFETs) to map cortical spreading depression in rats and demonstrate that gSGFETs are able to record, with high fidelity, infraslow signals together with signals in the typical local field potential bandwidth. The wide recording bandwidth results from the direct field-effect coupling of the active transistor, in contrast to standard passive electrodes, as well as from the electrochemical inertness of graphene. Taking advantage of such functionality, we envision broad applications of gSGFET technology for monitoring infraslow brain activity both in research and in the clinic.