Electrolyte-gated Organic Field-effect Transistors Research Articles

Charge Carrier Density in Organic Semiconductors Modulates the Effective Capacitance: A Unified View of Electrolyte Gated Organic Transistors.

A framework for electrolyte-gated organic transistors (EGOTs) that unifies the view of interfacial capacitive coupling of electrolyte-gated organic field-effect transistors (EGOFETs) with the volumetric capacitive coupling in organic electrochemical transistors (OECTs) is proposed. The EGOT effective capacitance arises from in-series capacitances of the electrolyte/gate electrode and electrolyte/channel interfaces, and the chemical capacitance of the organic semiconductor channel whose weight with respect to the interfacial capacitance is modulated by the charge carrier density, hence by the gate voltage. The expression for chemical capacitance is derived from the DOS of the organic semiconductor, which it is assumed to exhibit exponential energy disorder in the HOMO-LUMO gap. The analytical expression of the EGOT current is assessed on experimental data and shown to accurately predict the shape of the whole transfer curve of an EGOT thus allowing to extract accurate values for the switch-on voltage and the interfacial transconductance, without assumptions on specific response regime and, in OECT, without invoking the volumetric capacitance. Interestingly, the EGOT model recovers EGOFET and OECT as limit cases and, in the latter case, explicitly represents the volumetric capacitance in terms of the energy disorder and the bandgap of the organic semiconductor.

Electrolyte‐gated organic field‐effect transistors with high operational stability and lifetime in practical electrolytes

AbstractA key component of organic bioelectronics is electrolyte‐gated organic field‐effect transistors (EG‐OFETs), which have recently been used as sensors to demonstrate label‐free, single‐molecule detection. However, these devices exhibit limited stability when operated in direct contact with aqueous electrolytes. Ultrahigh stability is demonstrated to be achievable through the utilization of a systematic multifactorial approach in this study. EG‐OFETs with operational stability and lifetime several orders of magnitude higher than the state of the art have been fabricated by carefully controlling a set of intricate stability‐limiting factors, including contamination and corrosion. The indacenodithiophene‐co‐benzothiadiazole (IDTBT) EG‐OFETs exhibit operational stability that exceeds 900 min in a variety of widely used electrolytes, with an overall lifetime exceeding 2 months in ultrapure water and 1 month in various electrolytes. The devices were not affected by electrical stress‐induced trap states and can remain stable even in voltage ranges where electrochemical doping occurs. To validate the applicability of our stabilized device for biosensing applications, the reliable detection of the protein lysozyme in ultrapure water and in a physiological sodium phosphate buffer solution for 1500 min was demonstrated. The results show that polymer‐based EG‐OFETs are a viable architecture not only for short‐term but also for long‐term biosensing applications.

Multisensors based on electrolyte-gated organic field-effect transistors (EGOFETs) with aptamers as recognition elements: current state of research

The review gives a systematic account of published data devoted to multisensor technologies for manufacturing effective miniature devices capable of simultaneous accurate determination of several analytes and/or characteristics of biological fluids. The use of electrolyte-gated field-effect transistors as a biosensing platform is considered in detail. The devices based on these transistors demonstrate record-low limits of detection and are easy for mass production. The use of electrolyte-gated field-effect transistors in combination with aptamers capable of specifically binding to various analytes forms the grounds for innovations in early medical diagnosis (label-free biomarker detection). The most successful examples of multisensor devices using these transistors are presented, and the prospects of using aptamers as a recognition element in these devices are demonstrated.<br>Bibliography — 154 references.

Nanoscale Operando Characterization of Electrolyte-Gated Organic Field-Effect Transistors Reveals Charge Transport Bottlenecks.

Charge transport in electrolyte-gated organic field-effect transistors (EGOFETs) is governed by the microstructural property of the semiconducting thin film that is in direct contact with the electrolyte. Therefore, a comprehensive nanoscale operando characterization of the active channel is crucial to pinpoint various charge transport bottlenecks for rational and targeted optimization of the devices. Here, the local electrical properties of EGOFETs are systematically probed by in-liquid scanning dielectric microscopy (in-liquid SDM) and a direct picture of their functional mechanism at the nanoscale is provided across all operational regimes, starting from subthreshold, linear to saturation, until the onset of pinch-off. To this end, a robust interpretation framework of in-liquid SDM is introduced that enables quantitative local electric potential mapping directly from raw experimental data without requiring calibration or numerical simulations. Based on this development, a straightforward nanoscale assessment of various charge transport bottlenecks is performed, like contact access resistances, inter- and intradomain charge transport, microstructural inhomogeneities, and conduction anisotropy, which have been inaccessible earlier. Present results contribute to the fundamental understanding of charge transport in electrolyte-gated transistors and promote the development of direct structure-property-function relationships to guide future design rules.

Stability of thiol-based self-assembled monolayer functionalized electrodes in EG-OFET-based applications

The surface passivation of thermally deposited Au using n-alkanethiol self-assembled monolayers (SAMs) as insulating monolayers in electrolyte-gated organic-field effect transistor (EG-OFET) and its impact on EG-OFET operation has been clarified. We used three different n-alkanethiols derivates, namely propanethiol (P-SAM), hexanethiol (H-SAM), and octanethiol (O-SAM), with different chain lengths (C3, C6, and C8). The non-uniform distribution of the used SAMs on the inhomogeneous Au surface significantly affects the net capacitance, i.e., the serial capacitance of the double layer (CDL) and blocking layer (CBL) capacitance of the functionalized gate. The SAM-functionalized gates were exposed to cyclic electrical stress (forward and reverse) for 128 cycles with gate voltage (VG) sweep from 0.1 to −0.4 V at constant drain voltage (VD = -0.4 V) and compared to a bulk-gold reference gate electrode. The registered transfer curves showed increased drain currents that saturated during prolonged cycling. Two figures of merit, i.e., threshold voltage (Vth) and hysteresis, were extracted from the recorded transfer curves, and their responses were studied separately. We found that both Vth and hysteresis increase with cycling. The change is small but constantly increases for the short-chain P-SAM, while for the longer-chain H- and O-SAM, the initial change is more prominent, reaching saturation after approximately 25 cycles. We have investigated the surface roughness of different gate electrodes through Atomic Force Microscopy (AFM) to confirm the packing density of thiol molecules. We also performed 80 h long-term stability data using cyclic voltammetry measurements for each thiol-functionalized electrode. No signs of desorption could be found, as evidenced by XPS. The results are consistent with previously suggested models for electrical transport across such SAMs, confirming that these monolayers restrict faradic processes at the electrodes by passivating the Au surface.

Quantitative Detection of the Influenza a Virus by an EGOFET-Based Portable Device

Elaboration of biosensors on the base of organic transistors with embedded biomolecules which can operate in an aqueous environment is of paramount importance. Electrolyte-gated organic field-effect transistors demonstrate high sensitivity in detection of various analytes. In this paper, we demonstrated the possibility of quantitative fast specific determination of virus particles by an aptasensor based on EGOFET. The sensitivity and selectivity of the devices were examined with the influenza A virus as well as with control bioliquids like influenza B, Newcastle disease viruses or allantoic fluid with different dilutions. The influence of the semiconducting layer thickness on EGOFETs sensory properties is discussed. The fabrication of a multi-flow cell that simultaneously registers the responses from several devices on the same substrate and the creation of a multi-sensor flow device are reported. The responses of the elaborated bioelectronic platform to the influenza A virus obtained with application of the portable multi-flow mode are well correlated with the responses obtained in the laboratory stationary mode.

Analytical Physical Model for Electrolyte Gated Organic Field Effect Transistors in the Helmholtz Approximation

AbstractThe analytical physical modeling of undoped electrolyte gated organic field effect transistors (EGOFETs) in the Helmholtz approximation is presented. A compact analytical model for the current–voltage (I–V) characteristics, which includes the effects of the access series resistance, has been derived and validated by means of 2D finite element numerical calculations. The model describes all operating regimes continuously (subthreshold, linear, and saturation regimes), covers channel lengths down to a few micrometres and only includes physical device parameters. From the model, analytical expressions have been proposed for all the phenomenological parameters (e.g., capacitance, threshold voltage, sub‐threshold slope voltage, and sub‐threshold capacitance) appearing in the commonly used ideal FET model. The derived analytical physical model provides a simple and quantitative way to analyze the electrical characteristics of EGOFETs and EGOFET biosensors beyond the use of the oversimplified and phenomenological ideal FET model.

Applying of C8-BTBT-Based EGOFETs at Different pH Values of the Electrolyte

Electrolyte-gated organic field-effect transistors (EGOFETs) is a popular platform for numerous sensing and biosensing applications in aqueous media. In this work, the variation of electrical characteristics of EGOFETs based on small-molecule organic semiconductor C8-BTBT and polystyrene blend in water solutions at different pH values was investigated. A positive shift of the threshold voltage with near-Nernstian pH sensitivity was demonstrated in the pH range from 4.9 to 2.8, while no measurable pH dependence in the range from 4.9 to 8.6 pH was registered. These results indicate chemical doping of the molecules of organic semiconductors by protons from the electrolyte in the acidic region. In order to check the applicability of the EGOFETs in a flow mode, a flow chamber was designed and assembled. The preliminary results obtained in the flow mode measurements showed a fast response to pH variation.

Magnetic carbon gate electrodes for the development of electrolyte-gated organic field effect transistor bio-sensing platforms

An electrolyte-gated organic field-effect transistor that uses a magnetic carbon gate electrode to collect magnetic nanoparticles properly modified with a bio-receptor is reported as a novel platform to develop sensitive bio-sensors.

Analytical Physical Model for Organic Metal‐Electrolyte‐Semiconductor Capacitors

AbstractThis work presents the analytical physical modeling of undoped organic metal‐electrolyte‐semiconductor (OMES) capacitors in the framework of the Nernst–Planck–Poisson theory, including the presence of compact interfacial layers. This work derives an exact analytical solution, up to a quadrature, for the stationary electric potential and charge density distributions in both the semiconductor film and the electrolyte solution, and from them the sheet semiconductor charge and the stationary differential capacitance are obtained as a function of the applied voltage. The dependence of these magnitudes on the physical device parameters, like the ionic concentration of the electrolyte, the capacitance of the interfacial compact layers and the injected hole density is then analyzed. This work shows that ionic diffusive effects in the electrolyte can play an important role in the device response, inducing a broadening of the transition from the weak to the strong accumulation regimes. This fact can make that the strong accumulation regime is not achieved in OMES within the usual voltage operation range of these devices. The analytical solution is validated by means of finite element numerical calculations. The implications of the results obtained on the physics of electrolyte gated organic field effect transistors (EGOFETs) are discussed.

Electrolyte-gated organic field-effect transistors based on 2,6-dioctyltetrathienoacene as a convenient platform for fabrication of liquid biosensors

Electrolyte-gated organic field-effect transistors based on 2,6-dioctyltetrathienoacene as a convenient platform for fabrication of liquid biosensors

Direct Comparison of the Effect of Processing Conditions in Electrolyte-Gated and Bottom-Gated TIPS-Pentacene Transistors

Among the plethora of soluble and easy processable organic semiconductors, 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-P5) is one of the most promising materials for next-generation flexible electronics. However, based on the information reported in the literature, it is difficult to exploit in field-effect transistors the high-performance characteristics of this material. This article correlates the HMDS functionalization of the silicon substrate with the electrical characteristics of TIPS-P5-based bottom gate organic field-effect transistors (OFETs) and electrolyte-gated organic field-effect transistors (EGOFETs) fabricated over the same platform. TIPS-P5 transistors with a double-gate architecture were fabricated by simple drop-casting on Si/SiO2 substrates, and the substrates were either functionalized with hexamethyldisilazane (HMDS) or left untreated. The same devices were characterized both as standard bottom-gate transistors and as (top-gate) electrolyte-gated transistors, and the results with and without HMDS treatment were compared. It is shown that the functionalization of the silicon substrate negatively influences EGOFETs performance, while it is beneficial for bottom-gate OFETs. Different device architectures (e.g., bottom-gate vs. top-gate) require specific evaluation of the fabrication protocols starting from the effect of the HMDS functionalization to maximize the electrical characteristics of TIPS-P5-based devices.

Real-time threshold voltage compensation on dual-gate electrolyte-gated organic field-effect transistors

Electrolyte-Gated Organic Field-Effect Transistors (EGOFETs) offer many opportunities for the development of low-cost and low-power electronics suitable for applications like sensors and point-of-care tests; however, EGOFETs can be affected by the drift of their operative point that causes signals distortion and loss of information during sensing applications. Here, a blend of 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) and polystyrene (PS) is used as the active material for the fabrication of dual-gate EGOFETs. We exploited the dual-gate architecture to improve EGOFETs stability by implementing digital feedback that uses the back-gate electrode to compensate dynamically for the transistor threshold voltage allowing us to fix its operative point for prolonged tests (>10 h) with different aqueous solutions (Milli-Q water, NaCl 0.1 M and a physiological solution). The presented real-time threshold voltage compensation does not only allow to steady EGOFETs DC output current, but it also preserves EGOFETs sensing capability for the detection of signals with frequencies as low as 1 Hz. • Design and characterization of a dual-gate EGOFET based on diF-TES-ADT:PS: extraction of the top capacitance. • Real-time compensation of the threshold voltage on a dual-gated EGOFET. • Real-time monitoring of action potentials (frequency 1 Hz) applied to the top gate.

Biorecognition Layer Based On Biotin-Containing [1]Benzothieno[3,2-b][1]benzothiophene Derivative for Biosensing by Electrolyte-Gated Organic Field-Effect Transistors.

Requirements of speed and simplicity in testing stimulate the development of modern biosensors. Electrolyte-gated organic field-effect transistors (EGOFETs) are a promising platform for ultrasensitive, fast, and reliable detection of biological molecules for low-cost, point-of-care bioelectronic sensing. Biosensitivity of the EGOFET devices can be achieved by modification with receptors of one of the electronic active interfaces of the transistor gate or organic semiconductor surface. Functionalization of the latter gives the advantage in the creation of a planar architecture and compact devices for lab-on-chip design. Herein, we propose a universal, fast, and simple technique based on doctor blading and Langmuir-Schaefer methods for functionalization of the semiconducting surface of C8-BTBT-C8, allowing the fabrication of a large-scale biorecognition layer based on the novel functional derivative of BTBT-containing biotin fragments as a foundation for further biomodification. The fabricated devices are very efficient and operate stably in phosphate-buffered saline solution with high reproducibility of electrical properties in the EGOFET regime. The development of biorecognition properties of the proposed biolayer is based on the streptavidin-biotin interactions between the consecutive layers and can be used for a wide variety of receptors. As a proof-of-concept, we demonstrate the specific response of the BTBT-based biorecognition layer in EGOFETs to influenza A virus (H7N1 strain). The elaborated approach to biorecognition layer formation is appropriate but not limited to aptamer-based receptor molecules and can be further applied for fabricating several biosensors for various analytes on one substrate and paves the way for "electronic tongue" creation.

Electrolyte-Gated Organic Field-Effect Transistors for Quantitative Monitoring of the Molecular Dynamics of Crystallization at the Solid–Liquid Interface

Quantitative measurements of molecular dynamics at the solid–liquid interface are of crucial importance in a wide range of fields, such as heterogeneous catalysis, energy storage, nanofluidics, biosensing, and crystallization. In particular, the molecular dynamics associated with nucleation and crystal growth is very challenging to study because of the poor sensitivity or limited spatial/temporal resolution of the most widely used analytical techniques. We demonstrate that electrolyte-gated organic field-effect transistors (EGOFETs) are able to monitor in real-time the crystallization process in an evaporating droplet. The high sensitivity of these devices at the solid–liquid interface, through the electrical double layer and signal amplification, enables the quantification of changes in solute concentration over time and the transport rate of molecules at the solid–liquid interface during crystallization. Our results show that EGOFETs offer a highly sensitive and powerful, yet simple approach to investigate the molecular dynamics of compounds crystallizing from water.

Receptor Induced Doping of Conjugated Polymer Transistors: A Strategy for Selective and Ultrasensitive Phosphate Detection in Complex Aqueous Environments

AbstractPhosphate oxyanions play central roles in biological, agricultural, industrial, and ecological processes. Their high hydration energies and dynamic properties present a number of critical challenges limiting the development of sensing technologies that are cost‐effective, selective, sensitive, field‐deployable, and which operate in real‐time within complex aqueous environments. Here, a strategy that enables the fabrication of an electrolyte‐gated organic field‐effect transistor (EGOFET) is demonstrated, which overcomes these challenges and enables sensitive phosphate quantification in challenging aqueous environments such as seawater. The device channel comprises a composite layer incorporating a diketopyrrolopyrrole‐based semiconducting polymer and a π‐conjugated penta‐t‐butylpentacyanopentabenzo[25]annulene “cyanostar” receptor capable of oxyanion recognition and embodies a new concept, where the receptor synergistically enhances the stability and transport characteristics via doping. Upon exposure of the device to phosphate, a current reduction is observed, consistent with dedoping upon analyte binding. Sensing studies demonstrate ultrasensitive and selective phosphate detection within remarkably low limits of detection of 178 × 10−12 m (17.3 parts per trillion) in buffered samples and stable operation in seawater. This receptor‐based doping strategy, in conjunction with the versatility of EGOFETs for miniaturization and monolithic integration, enables manifold opportunities in diagnostics, healthcare, and environmental monitoring.

Detection of Neurofilament Light Chain with Label‐Free Electrolyte‐Gated Organic Field‐Effect Transistors

AbstractNeurofilaments are structural scaffolding proteins of the neuronal cytoskeleton. Upon axonal injury, the neurofilament light chain (NF‐L) is released into the interstitial fluid and eventually reaches the cerebrospinal fluid and blood. Therefore, NF‐L is emerging as a biomarker of neurological disorders, including neurodegenerative dementia, Parkinson's disease, and multiple sclerosis. It is challenging to quantify NF‐L in bodily fluids due to its low levels. This work reports the detection of NF‐L in aqueous solutions with an organic electronic device. The biosensor is based on the electrolyte‐gated organic field‐effect transistor (EGOFET) architecture and can quantify NF‐L down to sub‐pM levels; thanks to modification of the device gate with anti‐NF‐L antibodies imparted with potentially controlled orientation. The response is fitted to the Guggenheim–Anderson–De Boer adsorption model to describe NF‐L adsorption at the gate/electrolyte interface, to consider the formation of a strongly adsorbed protein layer bound to the antibody and the formation of weakly bound NF‐L multilayers, an interpretation which is also backed up by morphological characterization via atomic force microscopy. The label‐free, selective, and rapid response makes this EGOFET biosensor a promising tool for the diagnosis and monitoring of neuronal damages through the detection of NF‐L in physio‐pathological ranges.

Electrochemically activated polyaniline based ambipolar organic electrochemical transistor

AbstractThe organic electrochemical transistors (OECTs) are largely applied as bio‐ and ion‐sensor devices due to their performance in an aqueous environment with electrochemical doping/dedoping (reversible redox reactions) of the semiconducting layer through ion injection and also low (around 1 V) operation potentials. Aiming for the complementary circuits’ development and improvement of the sensors device, we report the application of spin coated polyaniline (PANI) in ambipolar OECT. The OECT operation is based on electrochemical protonation/deprotonation of PANI upon applied gate potential that was verified by cyclic voltammograms (CV) of PANI sample. The device operates in accumulation mode with on‐currents on the milliampere range due to the use of interdigitated array (IDA) microelectrodes architecture with a larger active area. Acid (HCl) doped PANI were applied in OECT device for comparison, and it presented improved performance with increasing drain‐source current up to 38%, reaching 4.8 mA for the n‐type device. This current increase is due to the narrower bandgap of doped PANI that affects the injection barrier. The HCl doped PANI was evaluated by UV‐Vis spectrophotometry and CV analysis. Using deionized water as electrolyte defunctionalized the PANI‐based OECT (no current modulation observed) because the absence of ions in the electrolyte interrupts the electrochemical doping. This is the opposite for the poly(3‐hexylthiophene) (P3HT)‐based electrolyte‐gated organic field‐effect transistor (EGOFET) due to the polymeric hydrophobicity nature that allows for field‐effect modulation in the electrolyte/semiconductor interface. The results presented may open new possibilities for bio‐ and ion‐sensors devices.

Affinity driven ion exchange EG-OFET sensor for high selectivity and low limit of detection of cesium in seawater

The detection and quantification of Cs+ in aquatic media are an environmental safety and public health matter, so far limited by the lack of rapid, low cost, low limit of detection and selective analytical tools. Herein we demonstrate the efficient fabrication of a novel electrolyte-gated organic field-effect transistor sensor for the Cs+ detection in seawater based on the combination of two ultra-thin layers namely poly(3-hexylthiophene) as semiconductor and a single lipids monolayer as dielectric. The latter is end-capped with a specific Cs+ probe based on a calix[4]arene benzocrown ether to ensure the selectivity. Interestingly, we clearly evidence that by controlling affinity driven guest/host ion exchange, one can lift the general problem of selectivity encountered in all FET-based ion sensors, reaching a nearly perfect selectivity even in highly complex analyte solutions containing competitive ions, such as phosphate buffered saline solution or seawater. Such ultra-thin transistor structure exhibits, a limit of detection at the sub-femtomolar level which corresponds to a 5 folds’ magnitude lower than Inductively Coupled Plasma Mass Spectroscopy, the most commonly used technic today. These results pave the way to a generalized monitoring of Cs+ in complexed analytes.

Sensing Inflammation Biomarkers with Electrolyte-Gated Organic Electronic Transistors.

An overview of cytokine biosensing is provided, with a focus on the opportunities provided by organic electronic platforms for monitoring these inflammation biomarkers which manifest at ultralow concentration levels in physiopathological conditions. Specifically, two of the field's state-of-the-art technologies-organic electrochemical transistors (OECTs) and electrolyte gated organic field effect transistors (EGOFETs)-and their use in sensing cytokines and other proteins associated with inflammation are a particular focus. The overview will include an introduction to current clinical and "gold standard" quantification techniques and their limitations in terms of cost, time, and required infrastructure. A critical review of recent progress with OECT- and EGOFET-based protein biosensors is presented, alongside a discussion onthe future of these technologies in the years and decades ahead. This is especially timely as the world grapples with limited healthcare diagnostics during the Coronavirus disease (COVID-19)pandemic where one of the worst-case scenarios for patients is the "cytokine storm." Clearly, low-cost point-of-care technologies provided by OECTs and EGOFETs can ease the global burden on healthcare systems and support professionals by providing unprecedented wealth of data that can help to monitor disease progression in real time.