Solution-gated Field-effect Transistors Research Articles

An All-Solid-State pH Sensor Employing Fluorine-Terminated Polycrystalline Boron-Doped Diamond as a pH-Insensitive Solution-Gate Field-Effect Transistor.

A fluorine-terminated polycrystalline boron-doped diamond surface is successfully employed as a pH-insensitive SGFET (solution-gate field-effect transistor) for an all-solid-state pH sensor. The fluorinated polycrystalline boron-doped diamond (BDD) channel possesses a pH-insensitivity of less than 3mV/pH compared with a pH-sensitive oxygenated channel. With differential FET (field-effect transistor) sensing, a sensitivity of 27 mv/pH was obtained in the pH range of 2–10; therefore, it demonstrated excellent performance for an all-solid-state pH sensor with a pH-sensitive oxygen-terminated polycrystalline BDD SGFET and a platinum quasi-reference electrode, respectively.

Graphene-based field effect transistors as biosensors

Graphene and graphene-related materials exhibit switchable charge-carrier mobility induced by applying a potential or by interaction with molecules. The short review summarizes concepts for the development of solution-gated field effect transistors using graphene or related substances as gate material focusing on applications in biosensors.

Mapping brain activity with flexible graphene micro-transistors

Establishing a reliable communication interface between the brain and electronic devices is of paramount importance for exploiting the full potential of neural prostheses. Current microelectrode technologies for recording electrical activity, however, evidence important shortcomings, e.g. challenging high density integration. Solution-gated field-effect transistors (SGFETs), on the other hand, could overcome these shortcomings if a suitable transistor material were available. Graphene is particularly attractive due to its biocompatibility, chemical stability, flexibility, low intrinsic electronic noise and high charge carrier mobilities. Here, we report on the use of an array of flexible graphene SGFETs for recording spontaneous slow waves, as well as visually evoked and also pre-epileptic activity in vivo in rats. The flexible array of graphene SGFETs allows mapping brain electrical activity with excellent signal-to-noise ratio (SNR), suggesting that this technology could lay the foundation for a future generation of in vivo recording implants.

Characterization and electrolytic cleaning of poly(methyl methacrylate) residues on transferred chemical vapor deposited graphene

Poly(methyl methacrylate) (PMMA) residue has long been a critical challenge for practical applications of the transferred chemical vapor deposited (CVD) graphene. Thermal annealing is empirically used for the removal of the PMMA residue; however experiments imply that there are still small amounts of residues left after thermal annealing which are hard to remove with conventional methods. In this paper, the thermal degradation of the PMMA residue upon annealing was studied by Raman spectroscopy. The study reveals that post-annealing residues are generated by the elimination of methoxycarbonyl side chains in PMMA and are believed to be absorbed on graphene via the π–π interaction between the conjugated unsaturated carbon segments and graphene. The post-annealing residues are difficult to remove by further annealing in a non-oxidative atmosphere due to their thermal and chemical stability. An electrolytic cleaning method was shown to be effective in removing these post-annealing residues while preserving the underlying graphene lattice based on Raman spectroscopy and atomic force microscopy studies. Additionally, a solution-gated field effect transistor was used to study the transport properties of the transferred CVD graphene before thermal annealing, after thermal annealing, and after electrolytic cleaning, respectively. The results show that the carrier mobility was significantly improved, and that the p-doping was reduced by removing PMMA residues and post-annealing residues. These studies provide a more in-depth understanding on the thermal annealing process for the removal of the PMMA residues from transferred CVD graphene and a new approach to remove the post-annealing residues, resulting in a residue-free graphene.

Polycrystalline boron-doped diamond with an oxygen-terminated surface channel as an electrolyte-solution-gate field-effect transistor for pH sensing

A polycrystalline boron-doped diamond (BDD) electrolyte solution-gate field effect transistor (SGFET) for use as a pH sensor was developed. The polycrystalline diamond films with a boron-doped layer possessed semiconducting properties that were comparable to hydrogen-terminated non-doped diamond. The hydrogen-terminated BDD surface was successfully transferred to a partially oxygen-terminated surface by ozone exposure, and its SGFET current–voltage (I–V) characteristics were evaluated with bias voltages within the potential window of diamond. The drain-source current(Ids)–drain-source voltage(Vds) characteristics showed pinch-off and saturation. In addition, they stably operated in electrolyte solutions with pH values from 2 to 12. The transfer characteristics exhibited a pH sensitivity of approximately 30mV/pH, which is comparable with the pH sensitivity of the conventional oxygen-terminated non-doped SGFET and the single-crystal BDD SGFET investigated in our previous work. Furthermore, the BDD SGFET exhibited improved long-term stability, and the coefficient of variation (CV) of Ids for 10 months was up to 10%.

Influence of non-adherent yeast cells on electrical characteristics of diamond-based field-effect transistors

Diamond thin films provide unique features as substrates for cell cultures and as bio-electronic sensors. Here we employ solution-gated field effect transistors (SGFET) based on nanocrystalline diamond thin films with H-terminated surface which exhibits the sub-surface p-type conductive channel. We study an influence of yeast cells (Saccharomyces cerevisiae) on electrical characteristics of the diamond SGFETs. Two different cell culture solutions (sucrose and yeast peptone dextrose–YPD) are used, with and without the cells. We have found that transfer characteristics of the SGFETs exhibit a negative shift of the gate voltage by −26mV and −42mV for sucrose and YPD with cells in comparison to blank solutions without the cells. This effect is attributed to a local pH change in close vicinity of the H-terminated diamond surface due to metabolic processes of the yeast cells. The pH sensitivity of the diamond-based SGFETs, the role of cell and protein adhesion on the gate surface and the role of negative surface charge of yeast cells on the SGFETs electrical characteristics are discussed as well.

Flexible graphene transistors for recording cell action potentials

Graphene solution-gated field-effect transistors (SGFETs) are a promising platform for the recording of cell action potentials due to the intrinsic high signal amplification of graphene transistors. In addition, graphene technology fulfills important key requirements for in-vivo applications, such as biocompability, mechanical flexibility, as well as ease of high density integration. In this paper we demonstrate the fabrication of flexible arrays of graphene SGFETs on polyimide, a biocompatible polymeric substrate. We investigate the transistor’s transconductance and intrinsic electronic noise which are key parameters for the device sensitivity, confirming that the obtained values are comparable to those of rigid graphene SGFETs. Furthermore, we show that the devices do not degrade during repeated bending and the transconductance, governed by the electronic properties of graphene, is unaffected by bending. After cell culture, we demonstrate the recording of cell action potentials from cardiomyocyte-like cells with a high signal-to-noise ratio that is higher or comparable to competing state of the art technologies. Our results highlight the great capabilities of flexible graphene SGFETs in bioelectronics, providing a solid foundation for in-vivo experiments and, eventually, for graphene-based neuroprosthetics.

PH에 민감한 그래핀 전계효과 트랜지스터(FET)

최근 환경, 의료분야에서 실시가 검출 및 인체 삽입형 pH 센서에 대한 요구가 증가하고 있다. 이에 본 연구에서는 생체적합성이 우수한 그래핀을 이용하여 실시간 pH 검출이 가능한 센서를 개발하였다. Polyethylene terephthalate(PET) 기판에 전사된 그래핀 표면에 이온 용액속에서 동작하는 전계효과 트랜지스터(solution-gated field-effect transistors; SGFETs)를 제작하였으며 이를 이용하여 이온 용액의 pH를 검출하였다. 제작한 트랜지스터의 게이트 채널 길이는 500 ㎛, 게이트 채널 폭은 8 ㎜이다. 이온 용액속에서 트랜지스터 동작특성 및 pH 감도를 평가하기 위하여 드레인-소스 전압(VDS)에 따른 드레인-소스 전류(IDS) 및 게이트-소스 전압(VGS)에 따른 드레인-소스 전류(IDS)를 측정하였다. PET기판에 전사된 그래핀 위에 제작한 그래핀 SGFETs의 전류-전압 특성은 이온 용액내에서 매우 안정적으로 동작하였으며 그래핀 SGFETs의 Dirac point는 이온 용액의 pH값이 증가함에 따라 양의 방향으로 19.32 ㎷/pH씩 증가하였다.

α,ω-dihexyl-sexithiophene thin films for solution-gated organic field-effect transistors

While organic semiconductors are being widely investigated for chemical and biochemical sensing applications, major drawbacks such as the poor device stability and low charge carrier mobility in aqueous electrolytes have not yet been solved to complete satisfaction. In this work, solution-gated organic field-effect transistors (SGOFETs) based on the molecule α,ω-dihexyl-sexithiophene (DH6T) are presented as promising platforms for in-electrolyte sensing. Thin films of DH6T were investigated with regard to the influence of the substrate temperature during deposition on the grain size and structural order. The performance of SGOFETs can be improved by choosing suitable growth parameters that lead to a two-dimensional film morphology and a high degree of structural order. Furthermore, the capability of the SGOFETs to detect changes in the pH or ionic strength of the gate electrolyte is demonstrated and simulated. Finally, excellent transistor stability is confirmed by continuously operating the device over a period of several days, which is a consequence of the low threshold voltage of DH6T-based SGOFETs. Altogether, our results demonstrate the feasibility of high performance and highly stable organic semiconductor devices for chemical or biochemical applications.

Graphene-Based FET Detector forE. coliK12 Real-Time Monitoring and Its Theoretical Analysis

This paper presents a theoretical analysis for a graphene-based FET real-time detector of the target bacteriaE. coliK12. The motivation for this study is to design a sensor device for detection of bacteria in food and water in order to guarantee food safety. Graphene is chosen as our material for sensor design, which has outstanding electrical, physical, and optical performance. In our sensor structure, graphene-based solution gate field effect transistor (FET) is the device model; fabrication and functionalization protocol are presented together in this paper. What is more, a real-time signal display system is the accompanied equipment for our designed biosensor device. In this system, the sensor bias current signalIdswould change obviously when the target bacteria are attached to the sensor surface. And the bias currentIdsincreases when theE. coliconcentration increases. In the latter part, a theoretical interpretation of the sensor signal is to explain the bias currentIdsincreasing after theE. coliK12 attachment.

Characterization and preliminary application of top-gated graphene ion-sensitive field effect transistors

Graphene, a 2-dimensional material, has received increasing attention due to its unique physicochemical properties (high surface area, excellent conductivity, and high mechanical strength). Field-effect transistor is shown to be a very promising candidate for electrically detecting chemical and biological species. Most of the reports on graphene field-effect transistors show that solution-gated graphene field effect transistors have been used so far. Although the traditional solution-gated graphene field effect transistor has high sensitivity, but the graphene channel is contaminated easily. The stability of the device is reduced so that the device cannot be reused. Only very recently, has the top-gated graphene, which is potentially used for pH sensors, been reported. In the top-gated graphene the dielectrics is deposited at the top of graphene. However, the sensitivity is lower than other sensors. To improve the properties, we design and fabricate a top-gated graphene ion-sensitive field effect transistor by using large-area graphene synthesized by chemical vapor deposition. At the top of graphene, HfO2/Al2O3 thin film is deposited by atomic layer deposition. The Al2O3 film plays a role of sensitive membrane, and the HfO2/Al2O3 thin film protects the graphene from contamination of the solution. After depositing the top-gate, because of the shield of the insulation, the boundary between the graphene and the substrate is not clear. And the Raman spectrum indicates the presence of a defective top layer accompanied by an increase in the Raman D peak. After a series of electrical characterizations, compared with solution-gated graphene field effect transistor which directly contacts the graphene channel with the solution, the top-gated graphene ion-sensitive field effect transistor has a high resistance. This increase relative to uncovered grapheme, is attributed to the participation of the top -orbitals in van der Waals bonds to the insulation. The graphene -orbitals contributing to van der Waals bonds have less overlaps and thus result in reduced conductivity. However the output curves and transfer curves show that the top-gated graphene ion-sensitive field effect transistor has higher signal-to-noise ratio and better stability. In view of the biochemical detection, in this paper we also examine the adsorption of single-stranded DNA. Silane functionalization of metal oxide system is a versatile technique that can be used in DNA microarray and nanotechnology. The DNA immobilization process we have developed contains several steps: silanization (APTES), crosslinker attachment (EDC and NHS), reaction with carboxyl-DNA and removal of non-covalently bound DNA. We characterize the process with carboxyl-quantum dots. We also measure the transfer curves before and after the adsorption of DNA, and demonstrate the effectiveness of the functionalized process and the feasibility that the top-gated graphene ion-sensitive field effect transistor is used as the biosensor.

Real-time Monitoring of Cell Activities by Diamond Solution-gated Field Effect Transistors

In this study, we present basic aspects for real-time monitoring of cell growth of human cell lines. We successfully fabricated fully optically transparent diamond-based solution-gated field effect transistor (SG-FET) on glass substrates. The diamond transistor functionalities are demonstrated by the sensitivity of electrical characteristics to solutions with different Na+ and K+ ion concentrations, and pH. Moreover, the diamond SG-FETs exhibited sensitivities to the adhesion of proteins and cells, to the cell delamination, or even to the cell death.

In situ Raman spectroscopy of the graphene/water interface of a solution-gated field-effect transistor: electron–phonon coupling and spectroelectrochemistry

We present a novel measurement approach which combines the electrical characterization of solution-gated field-effect transistors based on epitaxial bilayer graphene on 4H-SiC (0001) with simultaneous Raman spectroscopy. By changing the gate voltage, we observed Raman signatures related to the resonant electron–phonon coupling. An analysis of these Raman bands enabled the extraction of the geometrical capacitance of the system and an accurate calculation of the Fermi levels for bilayer graphene. An intentional application of higher gate voltages allowed us to trigger electrochemical reactions, which we followed in situ by Raman spectroscopy. The reactions showed a partially reversible character, as indicated by an emergence/disappearance of peaks assigned to C-H and Si-H vibration modes as well as an increase/decrease of the defect-related Raman D band intensity. Our setup provides a highly interesting platform for future spectroelectrochemical research on electrically-induced sorption processes of graphene on the micrometer scale.

In situ monitoring of myenteric neuron activity using acetylcholinesterase-modified AlGaN/GaN solution-gate field-effect transistors

The response characteristics of acetylcholinesterase-modified AlGaN/GaN solution-gate field-effect transistors (AcFETs) are quantitatively analyzed by means of a kinetic model. The characterization shows that the covalent enzyme immobilization process yields reproducible AcFET characteristics with a Michaelis constant KM of (122±4) μM for the immobilized enzyme layer. The increase of KM by a factor of 2.4 during the first four measurement cycles is attributed to partial denaturation of the enzyme. The AcFETs were used to record the release of acetylcholine (ACh) by neuronal tissue cultivated on the gate area upon stimulation by rising the extracellular K+ concentration. The neuronal tissue constituted of isolated myenteric neurons from four to 12 days old Wistar rats, or sections from the muscularis propria containing the myenteric plexus from adult rats. For both cases the AcFET response was demonstrated to be related to the activity of the immobilized acetylcholinesterase using the reversible acetylcholinesterase blocker donepezil. A concentration response curve of this blocking agent revealed a half maximal inhibitory concentration of 40nM which is comparable to values measured by complementary in vitro methods.

Gamma radiation effects on hydrogen-terminated nanocrystalline diamond bio-transistors

Diamond is considered as a promising tissue equivalent material in radiation therapies as well as for bio-electronic sensors due to its unique set of properties. These features are combined in this work where effects of gamma irradiation (60Co, up to 300Gy) on function and stability of microscopic (60×20μm2) hydrogen-terminated diamond (H-diamond) solution-gated field effect transistors (SG-FETs) are studied. The H-diamond SG-FETs were prepared using 300nm thin diamond films deposited on glass from methane and hydrogen gas mixture by microwave plasma. Prior to gamma irradiation they were interfaced to proteins (fetal bovine serum) and cells (human sarcoma osteogenic cell line — SAOS2) in cell growth medium. Blank H-diamond SG-FETs did not degrade after the irradiation. With adsorbed proteins and cells they showed specific changes in gate current characteristics (about 100% increase) after the irradiation. These current changes are attributed to modified protein layer and cell morphology on the diamond surface. The presented results establish a first step towards real-time electronic monitoring of cell growth during the irradiation by therapeutically relevant doses.

Creation of reduced graphene oxide based field effect transistors and their utilization in the detection and discrimination of nucleoside triphosphates.

Two low-cost, micropatterned, solution-gated field effect transistors (modified FET and unmodified FET) based on reduced graphene oxide (RGO) were developed and used for detection and discrimination of nucleoside triphosphates (NTPs). The modified FET was realized by simple deposition of a positively charged bis-pyrenyl derivative, py-diIM-py, onto the conducting RGO strips of the unmodified FET. The electrical properties and sensing behaviors of the as-prepared devices were studied comprehensively. Electrical transfer property tests revealed that both of the two FETs exhibit V-shaped ambipolar field effect behavior from p-type region to n-type region. Sensing performance studies demonstrated that modification of the native FET with py-diIM-py improves its sensing ability to NTPs-GTP and ATP in particular. The detection limit of GTP and ATP was as low as 400 nM, which is the lowest value for graphene-based electronic sensors reported so far. Furthermore, based on the cross-reactive responses of the two devices to NTPs, NTPs can be conveniently distinguished via combining use of the two devices. The enhancement of the modifier (py-diIM-py) to the sensing performance of the FET is tentatively attributed to its possible mediation role in sticking onto RGO strips and accumulating analytes by electrostatic association with the relevant species. Because they are sensitive and fast in response, simple and low-cost in preparation, and possibly useful in sensor-array fabrication, the developed sensors show great potential in real-life application.

Osteoblastic cells trigger gate currents on nanocrystalline diamond transistor

We show the influence of osteoblastic SAOS-2 cells on the transfer characteristics of nanocrystalline diamond solution-gated field-effect transistors (SGFET) prepared on glass substrates. Channels of these fully transparent SGFETs are realized by hydrogen termination of undoped diamond film. After cell cultivation, the transistors exhibit about 100× increased leakage currents (up to 10nA). During and after the cell delamination, the transistors return to original gate currents. We propose a mechanism where this triggering effect is attributed to ions released from adhered cells, which depends on the cell adhesion morphology, and could be used for cell culture monitoring.

Flexible Solution-Gated Graphene Field Effect Transistor for Electrophysiological Recording

Graphene represents a high-performance material for field effect transistor (FET) manufacture. As a monolayer of carbon atoms, graphene offers flexibility, high carrier mobility, and transparency. Solution-gated graphene FETs based on flexible substrates have been developed for various applications. However, none of these FETs offer both flexibility and good insulation from the electrolyte simultaneously. This would restrict their applicability in certain areas, such as implantation. To solve this problem, we used commercially available graphene and polyimide to fabricate a graphene FET. Photosensitive polyimide was used as both substrate and insulator for our device. Two Au electrodes were used separately as source and drain, and the graphene was exposed to an electrolytic solution to form a solution-gated structure. In this paper, we have studied the resistance variation between source and drain during specific fabrication steps to optimize the fabrication process. By analyzing the FET performance after repeated bending tests, our device's performance reliability was also confirmed. When compared with similar published flexible devices, our FET showed higher transconductance and lower noise levels. Also, neural activities were recorded from cortex neurons cultured on graphene FET for the first time, which proved our device's biocompatibility and demonstrated its potential for electrophysiological applications.

Electrical coupling between cells and graphene transistors.

In this work, both experimental data and a model are presented on the coupling between living cells and graphene solution-gated field-effect transistors. Modified HEK 293 cells are successfully cultured on graphene transistor arrays and electrically accessed by the patch clamp method. Transistor recordings are presented, showing the opening and closing of voltage-gated potassium ion channels in the cell membrane. The experimental data is compared with the broadly used standard point-contact model. The ion dynamics in the cell-transistor cleft are analyzed to account for the differences between the model and the experimental data revealing a significant increase in the total ionic strength in the cleft. In order to describe the influence of the ion concentration resulting from the cell activity, the ion-sensitivity of graphene solution-gated field-effect transistors is investigated experimentally and modelled by considering the screening effect of the ions on the surface potential at the graphene/electrolyte interface. Finally, the model of the cell-transistor coupling is extended to include the effect of ion accumulation and ion sensitivity. The experimental data shows a very good agreement with this extended model, emphasizing the importance of considering the ion concentration in the cleft to properly understand the cell-transistor coupling.

Single layer graphene as an electrochemical platform.

Over the past decade, there has been a great deal of interest in graphene with regards to its electrochemical behavior. Previous studies have focused on understanding fundamental processes such as charge transfer and molecular transport at the graphene-electrolyte interface as well as on applications of graphene in electronic, optical, and mechanical systems. We present illustrative examples of large area, single layer graphene platforms for applications such as optical and sensing devices as well as microfluidic systems. Three examples of graphene modified with thin polymer films are discussed. We have explored the use of graphene as an electrochemical platform for surface-generated electrogenerated chemiluminescence (ECL) using poly-[Ru(v-bpy)3](2+), where v-bpy is 4-vinyl, 4'-methyl 2,2'-bipyridine, as a model system. We found that while graphene can sustain ECL conditions, there was film degradation during ECL, as demonstrated by a decrease in ECL intensity upon potential cycling even in the presence of a graphene coating ("graphene blanket"). Using poly 3,4-ethylenedioxythiophene (EDOT), we demonstrate a facile method of fabricating electrochromic electrodes from large area graphene. The oxidation of NADH at graphene was catalyzed using an electrodeposited layer of 3,4-dihydroxybenzaldehyde as an effective redox mediator. In addition, we describe the fabrication and characterization of a microfluidic device based on a solution-gated field effect transistor which was able to detect changes of 60 mV per pH unit change in an inverted cell design. On the other hand, a 29 mV shift in the Dirac point per unit pH change was measured with our microfluidic devices, and a ca. 10% FET conductance change was measured when we continuously changed the pH in solution from 6.91 to 7.64 in the microfluidic channel, demonstrating local microfluidic pH sensing (albeit non-Nerstian) in real time.