The application of organic electrochemical transistors in cell-based biosensors

High-performance electrolyte-gated amorphous InGaZnO field-effect transistor for label-free DNA sensing

Accurate, convenient, label-free, and cost-effective biomolecules detection platforms are currently in high demand. In this study, we showcased the utilization of electrolyte-gated InGaZnO field-effect transistors (IGZO FETs) featuring a large on-off current ratio of over 106 and a low subthreshold slope of 78.5 mV/dec. In the DNA biosensor, the modification of target DNA changed the effective gate voltage of IGZO FETs, enabling an impressive low detection limit of 0.1 pM and a wide linear detection range from 0.1 pM to 1 μM. This label-free detection method also exhibits high selectivity, allowing for the discrimination of single-base mismatch. Furthermore, the reuse of gate electrodes and channel films offers cost-saving benefits and simplifies device fabrication processes. The electrolyte-gated IGZO FET biosensor presented in this study shows great promise for achieving low-cost and highly sensitive detection of various biomolecules.

Research Progress of Biosensor Based on Organic Photoelectrochemical Transistor

In order to solve the food safety problem better, it is very important to develop a rapid and sensitive technology for detecting food contamination residues. Organic photoelectrochemical transistor (OPECT) biosensor rely on the photovoltage generated by a semiconductor upon excitation by light to regulate the conductivity of the polymer channels and realize biosensor analysis under zero gate bias. This technology integrates the excellent characteristics of photoelectrochemical (PEC) bioanalysis and the high sensitivity and inherent amplification ability of organic electrochemical transistor (OECT). Based on this, OPECT biosensor detection has been proven to be superior to traditional biosensor detection methods. In this review, we summarize the research status of OPECT biosensor in disease markers and food residue analysis, the basic principle, classification, and biosensing mechanism of OPECT biosensor analysis are briefly introduced, and the recent applications of biosensor analysis are discussed according to the signal strategy. We mainly introduced the OPECT biosensor analysis methods applied in different fields, including the detection of disease markers and food hazard residues such as prostate-specific antigen, heart-type fatty acid binding protein, T-2 toxin detection in milk samples, fat mass and objectivity related protein, ciprofloxacin in milk. The OPECT biosensor provides considerable development potential for the construction of safety analysis and detection platforms in many fields, such as agriculture and food, and hopes to provide some reference for the future development of biosensing analysis methods with higher selectivity, faster analysis speed and higher sensitivity.

Organic Electrochemical Transistors for Biomarker Detections

The improvement of living standards and the advancement of medical technology have led to an increased focus on health among individuals. Detections of biomarkers are feasible approaches to obtaining information about health status, disease progression, and response to treatment of an individual. In recent years, organic electrochemical transistors (OECTs) have demonstrated high electrical performances and effectiveness in detecting various types of biomarkers. This review provides an overview of the working principles of OECTs and their performance in detecting multiple types of biomarkers, with a focus on the recent advances and representative applications of OECTs in wearable and implantable biomarker detections, and provides a perspective for the future development of OECT-based biomarker sensors.

Organic photoelectrochemical transistor aptasensor for dual-mode detection of DEHP with CRISPR-Cas13a assisted signal amplification

Emerging organic photoelectrochemical transistors (OPECTs) with inherent amplification capabilities, good biocompatibility and even self-powered operation have emerged as a promising detection tool, however, they are still not widely studied for pollutant detection. In this paper, a novel OPECT dual-mode aptasensor was constructed for the ultrasensitive detection of di(2-ethylhexyl) phthalate (DEHP). MXene/In2S3/In2O3 Z-scheme heterojunction was used as a light fuel for ion modulation in sensitive gated OPECT biosensing. A transistor system based on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) converted biological events associated with photosensitive gate achieving nearly a thousand-fold higher current gain at zero bias voltage. This work quantified the target DEHP by aptamer-specific induction of CRISPR-Cas13a trans-cutting activity with target-dependent rolling circle amplification as the signal amplification unit, and incorporated the signal changes strategy of biocatalytic precipitation and TMB color development. Combining OPECT with the auxiliary validation of colorimetry (CM), high sensitivity and accurate detection of DEHP were achieved with a linear range of 0.1 pM to 200 pM and a minimum detection limit of 0.02 pM. This study not only provides a new method for the detection of DEHP, but also offers a promising prospect for the gating and application of the unique OPECT.

Flexible Organic Transistors for Biosensing: Devices and Applications

Flexible and stretchable biosensors can offer seamless and conformable biological-electronic interfaces for continuously acquiring high-fidelity signals, permitting numerous emerging applications. Organic thin film transistors (OTFTs) are ideal transducers for flexible and stretchable biosensing due to their soft nature, inherent amplification function, biocompatibility, ease of functionalization, low cost, and device diversity. In consideration of the rapid advances in flexible-OTFT-based biosensors and their broad applications, herein, a timely and comprehensive review is provided. It starts with a detailed introduction to the features of various OTFTs including organic field-effect transistors and organic electrochemical transistors, and the functionalization strategies for biosensing, with a highlight on the seminal work and up-to-date achievements. Then, the applications of flexible-OTFT-based biosensors in wearable, implantable, and portable electronics, as well as neuromorphic biointerfaces are detailed. Subsequently, special attention is paid to emerging stretchable organic transistors including planar and fibrous devices. The routes to impart stretchability, including structural engineering and material engineering, are discussed, and the implementations of stretchable organic transistors in e-skin and smart textiles are included. Finally, the remaining challenges and the future opportunities in this field are summarized.

A miniaturized biosensor for rapid detection of tetracycline based on a graphene field-effect transistor with an aptamer modified gate

Tetracycline is a broad-spectrum antibiotic for human, poultry and livestock that may cause health damage when enriched in humans. Therefore, it is essential to create a rapid tetracycline assay with high sensitivity, specificity and portability. In this study, a miniaturized tetracycline biosensor based on aptamer-modified graphene field-effect transistor (Apt-SGGT) was fabricated and two detection strategies using transfer characteristic curves and real-time channel current were established for different circumstances. The detection limits of the two strategies were 2.073 pM and 100 pM, respectively. The biosensor also demonstrated outstanding stability, anti-interference and specificity ability. Finally, the biosensor was employed to detect the content of tetracycline in Skim Milk with outstanding recovery rate. We believe that the miniaturized Apt-SGGT biosensor with appropriate detection strategies will provide an ideal portable sensing platform for many important analytes in food with superior selectivity and sensitivity.

Extended Review Concerning the Integration of Electrochemical Biosensors into Modern IoT and Wearable Devices

Electrochemical biosensors include a recognition component and an electronic transducer, which detect the body fluids with a high degree of accuracy. More importantly, they generate timely readings of the related physiological parameters, and they are suitable for integration into portable, wearable and implantable devices that are significant relative to point-of-care diagnostics scenarios. As an example, the personal glucose meter fundamentally improves the management of diabetes in the comfort of the patients' homes. This review paper analyzes the principles of electrochemical biosensing and the structural features of electrochemical biosensors relative to the implementation of health monitoring and disease diagnostics strategies. The analysis particularly considers the integration of the biosensors into wearable, portable, and implantable systems. The fundamental aim of this paper is to present and critically evaluate the identified significant developments in the scope of electrochemical biosensing for preventive and customized point-of-care diagnostic devices. The paper also approaches the most important engineering challenges that should be addressed in order to improve the sensing accuracy, and enable multiplexing and one-step processes, which mediate the integration of electrochemical biosensing devices into digital healthcare scenarios.

Hydrogel-Gated FETs in Neuromorphic Computing to Mimic Biological Signal: A Review

Hydrogel-gated synaptic transistors offer unique advantages, including biocompatibility, tunable electrical properties, being biodegradable, and having an ability to mimic biological synaptic plasticity. For processing massive data with ultralow power consumption due to high parallelism and human brain-like processing abilities, synaptic transistors have been widely considered for replacing von Neumann architecture-based traditional computers due to the parting of memory and control units. The crucial components mimic the complex biological signal, synaptic, and sensing systems. Hydrogel, as a gate dielectric, is the key factor for ionotropic devices owing to the excellent stability, ultra-high linearity, and extremely low operating voltage of the biodegradable and biocompatible polymers. Moreover, hydrogel exhibits ionotronic functions through a hybrid circuit of mobile ions and mobile electrons that can easily interface between machines and humans. To determine the high-efficiency neuromorphic chips, the development of synaptic devices based on organic field effect transistors (OFETs) with ultra-low power dissipation and very large-scale integration, including bio-friendly devices, is needed. This review highlights the latest advancements in neuromorphic computing by exploring synaptic transistor developments. Here, we focus on hydrogel-based ionic-gated three-terminal (3T) synaptic devices, their essential components, and their working principle, and summarize the essential neurodegenerative applications published recently. In addition, because hydrogel-gated FETs are the crucial members of neuromorphic devices in terms of cutting-edge synaptic progress and performances, the review will also summarize the biodegradable and biocompatible polymers with which such devices can be implemented. It is expected that neuromorphic devices might provide potential solutions for the future generation of interactive sensation, memory, and computation to facilitate the development of multimodal, large-scale, ultralow-power intelligent systems.

Carboxyl graphene modified PEDOT:PSS organic electrochemical transistor for in situ detection of cancer cell morphology

Circulating tumor cells in human peripheral blood play an important role in cancer metastasis. In addition to the size-based and antibody-based capture and separation of cancer cells, their electrical characterization is important for rare cell detection, which can prove fatal in point-of-care testing. Herein, an organic electrochemical transistor (OECT) biosensor made of solution-gated carboxyl graphene mixed with PEDOT:PSS for the detection of cancer cells in situ is reported. Carboxyl graphene was used in this work to modulate cancer cell morphology, which differs significantly from normal blood cells, to achieve rare cancer cell detection. When the concentration of carboxyl graphene mixed in PEDOT:PSS was increased from 0 to 5 mg mL-1, the cancer cell surface area increased from 218 μm2 to 530 μm2, respectively. A change in cell morphology was also detected by the OECT. Negative charges in the cancer cells induced a positive shift in gate voltage, which was approximately 40 mV for spherical-shaped cells. When the cell surface area increased, transfer curves of transistor revealed a negative shift in gate voltage. Therefore, the sensor can be used for in situ detection of cancer cell morphology during the cell capture process, which can be used to identify whether the captured cells are deformable.

MXene-Enhanced Bi2S3/CdIn2S4 Heterojunction Photosensitive Gate for DEHP Detection in a Signal-On OPECT Aptamer Biosensor

Organic electrochemical transistors with signal amplification and good stability are expected to play a more important role in the detection of environmental pollutants. However, the bias voltage at the gate may have an effect on the activity of vulnerable biomolecules. In this work, a novel organic photoelectrochemical transistor (OPECT) aptamer biosensor was developed for di(2-ethylhexyl) phthalate (DEHP) detection by combining photoelectrochemical analysis with an organic electrochemical transistor, where MXene/Bi2S3/CdIn2S4 was employed as a photoactive material, target-dependent DNA hybridization chain reaction was used as a signal amplification unit, and Ru(NH3)63+ was selected as a signal enhancement molecule. The poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)-based OPECT biosensor modulated by the MXene/Bi2S3/CdIn2S4 photosensitive material achieved a high current gain of nearly a thousand times at zero bias voltage. The developed signal-on OPECT sensing platform realized sensitive and specific detection of DEHP, with a detection range of 1-200 pM and a minimum detection limit of 0.24 pM under optimized experimental conditions, and its application to real water samples was also evaluated with satisfactory results. Hence, the construction of this OPECT biosensing platform not only provides a promising tool for the detection of DEHP but also reveals the great potential of the OPECT application for the detection of other environmental toxins.

Fabrication of PEDOT:PSS-based solution gated organic electrochemical transistor array for cancer cells detection

Organic electrochemical transistor (OECT) was applied in chemical and biological sensing. In this work, we developed a simple and repeatable method to fabricate OECT array, which had been successfully used to detect cancer cells. PEDPT:PSS conductive film between source and drain electrodes were patterned through photolithography, which can achieve uniform devices with same electrical characterization. When MCF-7 cancer cells are captured on the PEDOT:PSS surface via specifical antibody, the transfer characteristic of OECT shifts to higher gate electrode voltage due to the electrostatic interaction between cancer cells and device. The effective gate voltage shift can reach about 63 mV when the concentration of cancer cells increased to 5000. The shift of effective gate voltage is related to the cancer cell morphology, which is increased in the first 1 h and decreased when the capture time was larger than 1 h. The device of OECT array can increase the sample flux and make the detection result more accurate. It is expected that OECT array will have promising practical applications in single cancer cell detection in the future.

Phosphorylcholine-Functionalized PEDOT-Gated Organic Electrochemical Transistor Devices for Ultra-Specific and Sensitive C-Reactive Protein Detection

Affinity-based organic electrochemical transistor (OECT) sensors offer an attractive approach to point-of-care diagnostics due to their extreme sensitivity and easy operation; however, their application in the real world is frequently challenged by the poor storage stability of antibody proteins and the interference from biofouling in complex biofluids. In this work, we developed an antibody-free and antifouling OECT biosensor to detect C-reactive protein (CRP) at ultra-high specificity and sensitivity. The key to this novel biosensor is the gate coated by phosphorylcholine-functionalized poly (3,4-ethylene dioxythiophene) (PEDOT-PC), which possesses large capacitance and low impedance, prevents biofouling of bovine serum albumin (BSA) and the fetal bovine serum (FBS), and interacts specifically with CRP molecules in the presence of calcium ions. This PEDOT-PC-gated OECT biosensor demonstrated exceptional sensitivity when detecting the CRP molecules at 10 pg/mL, while significantly depressing the signal from the nonspecific binding. This indicates that this biosensor could detect the CRP molecules directly without nonspecific binding blocking, the usual process for the earlier transistor sensors before detection. We envision that this PEDOT-PC-gated OECT biosensor platform may offer a potentially valuable tool for point-of-care diagnostics as it alleviates concerns about poor antibody stability and BSA blocking inconstancy.

Flexible and Stretchable Organic Electrochemical Transistors for Physiological Sensing Devices

Flexible and stretchable bioelectronics provides a biocompatible interface between electronics and biological systems and has received tremendous attention for in situ monitoring of various biological systems. Considerable progress in organic electronics has made organic semiconductors, as well as other organic electronic materials, ideal candidates for developing wearable, implantable, and biocompatible electronic circuits due to their potential mechanical compliance and biocompatibility. Organic electrochemical transistors (OECTs), as an emerging class of organic electronic building blocks, exhibit significant advantages in biological sensing due to the ionic nature at the basis of the switching behavior, low driving voltage (<1 V), and high transconductance (in millisiemens range). During the past few years, significant progress in constructing flexible/stretchable OECTs (FSOECTs) for both biochemical and bioelectrical sensors has been reported. In this regard, to summarize major research accomplishments in this emerging field, this review first discusses structure and critical features of FSOECTs, including working principles, materials, and architectural engineering. Next, a wide spectrum of relevant physiological sensing applications, where FSOECTs are the key components, are summarized. Last, major challenges and opportunities for further advancing FSOECT physiological sensors are discussed.

Bionic Tactile-Gustatory Receptor for Object Identification Based on All-Polymer Electrochemical Transistor

Human sensory receptors enable the real world to be perceived effortlessly. Hence, massive efforts have been devoted to the development of bionic receptors capable of identifying objects. Unfortunately, most of the existing devices are limited to single sensory emulation and are established on solid-state electronic technologies, which are incompatible with the biological reactions occurring in electrolyte media. Here, an iontronic tactile-gustatory receptor using an all-polymer electrochemical transistor (AECT) is presented. The sensor is biocompatible with the operation voltage of 0.1 V, which is 1 to 2 orders lower than those of reported values. By this study, one receptor is able to accurately recognize various objects perceived by the human tactile and gustatory system without complex circuitry. Additionally, to promote its further application, flexible AECT arrays with channel length of 2 µm and density of 104 167 transistors cm^-2 (yield of 97%) are fabricated, 1 to 5 orders higher than those of related works. Finally, a flexible integrated network for electrocardiogram recording is successfully constructed. This study moves a step forward toward state-of-the-art bionic sensors.

Aqueous electrolyte-gated solution-processed metal oxide transistors for direct cellular interfaces

Biocompatible field-effect-transistor-based biosensors have drawn attention for the development of next-generation human-friendly electronics. High-performance electronic devices must achieve low-voltage operation, long-term operational stability, and biocompatibility. Herein, we propose an electrolyte-gated thin-film transistor made of large-area solution-processed indium-gallium-zinc oxide (IGZO) semiconductors capable of directly interacting with live cells at physiological conditions. The fabricated transistors exhibit good electrical performance operating under sub-0.5 V conditions with high on-/off-current ratios (>107) and transconductance (>1.0 mS) over an extended operational lifetime. Furthermore, we verified the biocompatibility of the IGZO surface to various types of mammalian cells in terms of cell viability, proliferation, morphology, and drug responsiveness. Finally, the prolonged stable operation of electrolyte-gated transistor devices directly integrated with live cells provides the proof-of-concept for solution-processed metal oxide material-based direct cellular interfaces.

A miniaturized organic photoelectrochemical transistor aptasensor based on nanorod arrays toward high-sensitive T-2 toxin detection in milk samples

Detection of T-2 toxin is of great significance to environment and human health, as T-2 toxin is one of the main toxins that contaminate crops, stored grain and other food. Herein, a zero-gate-bias organic photoelectrochemical transistor (OPECT) sensor was proposed based on nanoelectrode arrays as gate photoactive materials which can result in the accumulation of photovoltage and preferable capacitance leading to better sensitivity of the OPECT. For comparison, the channel current of OPECT was 100 times higher than photocurrent of conventional photoelectrochemical (PEC) attributing to remarkable signal amplification of OPECT. It was also found that the detection limit of OPECT aptasensor was as low as 28.8 pg/L, lower than 0.34 ng/L of the conventional PEC method, further indicating the advantage of the OPECT devices in T-2 toxin determination. This research has been successfully applied in real sample detection which provided a general platform of OPECT for food safety analysis.

A Mixed Protonic-Electronic Conductor Base on the Host-Guest Architecture of 2D Metal-Organic Layers and Inorganic Layers

The key to designing and fabricating highly efficient mixed protonic-electronic conductors materials (MPECs) is to integrate the mixed conductive active sites into a single structure, to break through the shortcomings of traditional physical blending. Herein, based on the host-guest interaction, an MPEC is consisted of 2D metal-organic layers and hydrogen-bonded inorganic layers by the assembly methods of layered intercalation. Noticeably, the 2D intercalated materials (≈1.3 nm) exhibit the proton conductivity and electron conductivity, which are 2.02 × 10^-5 and 3.84 × 10^-4 S cm^-1 at 100 °C and 99% relative humidity, much higher than these of pure 2D metal-organic layers (>>1.0 × 10^-10 and 2.01×10^-8 S cm^-1 ), respectively. Furthermore, combining accurate structural information and theoretical calculations reveals that the inserted hydrogen-bonded inorganic layers provide the proton source and a networks of hydrogen-bonds leading to efficient proton transport, meanwhile reducing the bandgap of hybrid architecture and increasing the band electron delocalization of the metal-organic layer to greatly elevate the electron transport of intrinsic 2D metal-organic frameworks.

Cell-Based Sensors for the Detection of EGF and EGF-Stimulated Ca2+ Signaling

Epidermal growth factor (EGF)-mediated activation of EGF receptors (EGFRs) has become an important target in drug development due to the implication of EGFR-mediated cellular signaling in cancer development. While various in vitro approaches are developed for monitoring EGF-EGFR interactions, they have several limitations. Herein, we describe a live cell-based sensor system that can be used to monitor the interaction of EGF and EGFR as well as the subsequent signaling events. The design of the EGF-detecting sensor cells is based on the split-intein-mediated conditional protein trans-cleavage reaction (CPC). CPC is triggered by the presence of the target (EGF) to activate a signal peptide that translocates the fluorescent cargo to the target cellular location (mitochondria). The developed sensor cell demonstrated excellent sensitivity with a fast response time. It was also successfully used to detect an agonist and antagonist of EGFR (transforming growth factor-α and Cetuximab, respectively), demonstrating excellent specificity and capability of screening the analytes based on their function. The usage of sensor cells was then expanded from merely detecting the presence of target to monitoring the target-mediated signaling cascade, by exploiting previously developed Ca²⁺-detecting sensor cells. These sensor cells provide a useful platform for monitoring EGF-EGFR interaction, for screening EGFR effectors, and for studying downstream cellular signaling cascades.

Device integration of electrochemical biosensors

Electrochemical biosensors incorporate a recognition element and an electronic transducer for the highly sensitive detection of analytes in body fluids. Importantly, they can provide rapid readouts and they can be integrated into portable, wearable and implantable devices for point-of-care diagnostics; for example, the personal glucose meter enables at-home assessment of blood glucose levels, greatly improving the management of diabetes. In this Review, we discuss the principles of electrochemical biosensing and the design of electrochemical biosensor devices for health monitoring and disease diagnostics, with a particular focus on device integration into wearable, portable and implantable systems. Finally, we outline the key engineering challenges that need to be addressed to improve sensing accuracy, enable multiplexing and one-step processes, and integrate electrochemical biosensing devices in digital health-care pathways.

Unamplified and Real-Time Label-Free miRNA-21 Detection Using Solution-Gated Graphene Transistors in Prostate Cancer Diagnosis

The incidence of prostate cancer (PCa) in men globally increases as the standard of living improves. Blood serum biomarker prostate-specific antigen (PSA) detection is the gold standard assay that do not meet the requirements of early detection. Herein, a solution-gated graphene transistor (SGGT) biosensor for the ultrasensitive and rapid quantification detection of the early prostate cancer-relevant biomarker, miRNA-21 is reported. The designed single-stranded DNA (ssDNA) probes immobilized on the Au gate can hybridize effectively with the miRNA-21 molecules targets and induce the Dirac voltage shifts of SGGT transfer curves. The limit of detection (LOD) of the sensor can reach 10^-20  M without amplification and any chemical or biological labeling. The detection linear range is from 10^-20 to 10^-12  M. The sensor can realize real-time detection of the miRNA-21 molecules in less than 5 min and can well distinguish one-mismatched miRNA-21 molecule. The blood serum samples from the patients without RNA extraction and amplification are measured. The results demonstrated that the biosensor can well distinguish the cancer patients from the control group and has higher sensitivity (100%) than PSA detection (58.3%). Contrastingly, it can be found that the PSA level is not directly related to PCa.

Bioderived establishment of three-dimensional type-I Ag2S/ZnIn2S4 heterojunction for high-efficacy organic photoelectrochemical transistor biomolecular detection

Advanced optoelectronic devices have attracted extensive interdisciplinary interest but lags far behind in biomolecular detection. The nascent organic photoelectrochemical transistor (OPECT) is expected to become a versatile platform to this end. Herein, using biological derivation of type-I Ag₂S/ZnIn₂S₄ heterojunction, a light-fueled high-efficacy OPECT system with zero-gate-biased operation is successfully developed for biomolecular detection. Exemplified by a sandwich immunocomplexing towards mouse IgG (MIgG) with Ag nanoparticles (Ag NPs) as the label, steering the acidolysis-release of Ag⁺ toward ZnIn₂S₄ could induce the in-situ formation of type-I Ag₂S/ZnIn₂S₄ heterojunction, increasing the recombination of light-activated excitons and thus inhibiting the photo-responsibility of ZnIn₂S₄, as sensitively monitored by the amplified OPECT response. The proposed device could achieve good analytical performance in terms of high specificity and sensitivity, with a detection limit as low as 33.7 fg mL^-1. This OPECT device based on bio-induced formation of type-I heterojunction can provide a novel route to biomolecular detection, and offered a new perspective for the optoelectronic sensors to be used in futuristic physiological and pathological detection.

High-Performance Organic Electrochemical Transistor Based on Photo-annealed Plasmonic Gold Nanoparticle-Doped PEDOT:PSS

Organic electrochemical transistors (OECTs), particularly the ones based on PEDOT:PSS, are excellent candidates for chemical and biological sensing because of their unique advantages. Improving the sensitivity and stability of OECTs is crucially important for practical applications. Herein, the transconductance of OECT is improved by 8-fold to 14.9 mS by doping the PEDOT:PSS channel with plasmonic gold nanoparticles (AuNPs) using a solution-based process followed by photo annealing. In addition, the OECT also possesses high flexibility and cyclic stability. It is revealed that the doping of AuNPs increases the conductivity of PEDOT:PSS and the photo annealing improves the crystallinity of the PEDOT:PSS channel and the interaction between AuNPs and PEDOT:PSS. These changes lead to the increase in transconductance and cyclic stability. The prepared OECTs are also demonstrated to be effective in sensitive detection of glucose within a wide concentration range of 10 nM-1 mM. Our OECTs based on photo-annealed plasmonic AuNP-doped PEDOT:PSS may find great applications in chemical and biological sensing, and this strategy may be extended to prepare many other high-performance OECT-based devices.

Rapid and Unamplified Detection of SARS-CoV-2 RNA via CRISPR-Cas13a-Modified Solution-Gated Graphene Transistors

The disease caused by severe acute respiratory syndrome coronavirus, SARS-CoV-2, is termed COVID-19. Even though COVID-19 has been out for more than two years, it is still causing a global pandemic. Due to the limitations of sample collection, transportation, and kit performance, the traditional reverse transcription-quantitative polymerase chain reaction (RT-qPCR) method has a long detection period and high testing costs. An increased risk of infection is inevitable, since many patients may not be diagnosed in time. The CRISPR-Cas13a system can be designed for RNA identification and knockdown, as a promising platform for nucleic acid detection. Here, we designed a solution-gated graphene transistor (SGGT) biosensor based on the CRISPR-Cas13a system. Using the gene-targeting capacity of CRISPR-Cas13a and gate functionalization via multilayer modification, SARS-CoV-2 nucleic acid sequences can be quickly and precisely identified without the need for amplification or fluorescence tagging. The limit of detection (LOD) in both buffer and serum reached the aM level, and the reaction time was about 10 min. The results of the detection of COVID-19 clinical samples from throat swabs agree with RT-PCR. In addition, the interchangeable gates significantly minimize the cost and time of device fabrication. In a nutshell, our biosensor technology is broadly applicable and will be suitable for point-of-care (POC) testing.

Electrochemical biosensors based on saliva electrolytes for rapid detection and diagnosis

In recent years, electrochemical biosensors (ECBSs) have shown significant potential for real-time disease diagnosis and in situ physical condition monitoring. As a multi-constituent oral fluid comprising various disease signaling biomarkers, saliva has drawn much attention in the field of point-of-care (POC) testing. In particular, during the outbreak of the COVID-19 pandemic, ECBSs which hold the simplicity of a single-step assay compared with the multi-step assay of traditional testing methods are expected to relieve the human and economic burden caused by the massive and long-term sample testing process. Noteworthily, ECBSs for the detection of SARS-CoV-2 in saliva have already been developed and may replace current testing methods. Furthermore, the detection scope has expanded from routine indices such as sugar and uric acid to abnormal biomarkers for early-stage disease detection and drug level monitoring, which further facilitated the evolution of ECBSs in the last 5 years. This review is divided into several main sections. First, we discussed the latest advancements and representative research on ECBSs for saliva testing. Then, we focused on a novel kind of ECBS, organic electrochemical transistors (OECTs), which hold great advantages of high sensitivity and signal-to-noise ratio and on-site detection. Finally, application of ECBSs with integrated portable platforms in oral cavities, which lead to powerful auxiliary testing means for telemedicine, has also been discussed.

Organic Mixed Ion-Electron Conductivity in Polymer Hybrid Systems

Recently, organic materials with mixed ion/electron conductivity (OMIEC) have gained significant interest among research communities all over the world. The unique ability to conduct ions and electrons in the same organic material adds to their use in next generation electrochemical, biotechnological, energy generation, energy storage, electrochromic, and sensor devices. Semiconducting conjugated polymers are well-known OMIECs due to their feasibility for both ion and electron transport in the bulk region. In this mini-review, we have shed light on conjugated polymers with ionic pendent groups, block copolymers of electronically and ionic conducting polymers, polymer electrolytes, blends of conjugated polymers with polyelectrolyte/polymer electrolytes; blends of conducting polymer with small organic molecules including conducting polymer-peptide conjugates; and blends of nonconjugated polymers as mixed conducting systems. These systems not only include the well-studied OMEIC systems, but also include some new systems where the OMEIC property has been predicted from the typical current-voltage (I-V) plots. The conduction mechanism of ions and electrons, ion-electron coupling, directionality, and dimensionality of these OMEIC materials are discussed in brief. The different properties of OMEIC materials and their applications in diverse fields like energy, electrochromic, biotechnology, sensing, and so forth are enlightened together with the perspective for future improvement of OMEIC materials.

Cyano-Functionalized n-Type Polymer with High Electron Mobility for High-Performance Organic Electrochemical Transistors

n-Type organic mixed ionic-electronic conductors (OMIECs) with high electron mobility are scarce and highly challenging to develop. As a result, the figure-of-merit (µC*) of n-type organic electrochemical transistors (OECTs) lags far behind the p-type analogs, restraining the development of OECT-based low-power complementary circuits and biosensors. Here, two n-type donor-acceptor (D-A) polymers based on fused bithiophene imide dimer f-BTI2 as the acceptor unit and thienylene-vinylene-thienylene (TVT) as the donor co-unit are reported. The cyanation of TVT enables polymer f-BTI2g-TVTCN with simultaneously enhanced ion-uptake ability, film structural order, and charge-transport property. As a result, it is able to obtain a high volumetric capacitance (C*) of 170 ± 22 F cm^-3 and a record OECT electron mobility (μ_e,OECT ) of 0.24 cm² V^-1 s^-1 for f-BTI2g-TVTCN, subsequently achieving a state-of-the-art µC* of 41.3 F cm^-1 V^-1 s^-1 and geometry-normalized transconductance (g_m,norm ) of 12.8 S cm^-1 in n-type accumulation-mode OECTs. In contrast, only a moderate µC* of 1.50 F cm^-1 V^-1 s^-1 is measured for the non-cyanated polymer f-BTI2g-TVT. These remarkable results demonstrate the great power of cyano functionalization of polymer semiconductors in developing n-type OMIECs with substantial electron mobility in aqueous environment for high-performance n-type OECTs.

Realizing Ultrahigh Transconductance in Organic Electrochemical Transistor by Co-Doping PEDOT:PSS with Ionic Liquid and Dodecylbenzenesulfonate

Organic electrochemical transistors (OECTs), especially the ones with high transconductance, are highly promising in sensitive detection of chemical and biological species. However, it is still a great challenge to design and fabricate OECTs with very high transconductance. Herein, an OECT with ultrahigh transconductance is reported by introducing ionic liquid and dodecylbenzenesulfonate (DBSA) simultaneously in poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS) as the semiconductive channel. Compared with the OECT based on pristine PEDOT:PSS, the OECT based on co-doped PEDOT:PSS demonstrates a significant enhancement of transconductance from 1.85 to 22.7 mS, because of the increase in volumetric capacitance and conductivity. The enhanced transconductance is attributed to the DBSA-facilitated phase separation between the ionic liquid and PEDOT:PSS, which helps to form conductive domains of ionic liquid in PEDOT:PSS matrix, and the partial dispersion of ionic liquid in the PEDOT:PSS phase. Furthermore, by using the interdigitated electrodes as the source and drain electrodes, an ultrahigh transconductance of 180 mS is obtained, which is superior to that of the state-of-the-art OECTs. Because of the ultrahigh transconductance, the obtained OECT demonstrates sensitive detection of hydrogen peroxide and glucose, making it promising in clinical diagnosis, health monitoring, and environmental surveillance.

Synthetic Nuances to Maximize n-Type Organic Electrochemical Transistor and Thermoelectric Performance in Fused Lactam Polymers

A series of fully fused n-type mixed conduction lactam polymers p(g₇NC_nN), systematically increasing the alkyl side chain content, are synthesized via an inexpensive, nontoxic, precious-metal-free aldol polycondensation. Employing these polymers as channel materials in organic electrochemical transistors (OECTs) affords state-of-the-art n-type performance with p(g₇NC₁₀N) recording an OECT electron mobility of 1.20 × 10^–2 cm² V^–1 s^–1 and a μC* figure of merit of 1.83 F cm^–1 V^–1 s^–1. In parallel to high OECT performance, upon solution doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelectric performance is observed for p(g₇NC₄N), with a maximum electrical conductivity of 7.67 S cm^–1 and a power factor of 10.4 μW m^–1 K^–2. These results are among the highest reported for n-type polymers. Importantly, while this series of fused polylactam organic mixed ionic–electronic conductors (OMIECs) highlights that synthetic molecular design strategies to bolster OECT performance can be translated to also achieve high organic thermoelectric (OTE) performance, a nuanced synthetic approach must be used to optimize performance. Herein, we outline the performance metrics and provide new insights into the molecular design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time the results of a single polymer series within both OECT and OTE applications.

Intrinsically Stretchable Block Copolymer Based on PEDOT:PSS for Improved Performance in Bioelectronic Applications

The conductive polyelectrolyte complex poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is ubiquitous in research dealing with organic electronic devices (e.g., solar cells, wearable and implantable sensors, and electrochemical transistors). In many bioelectronic applications, the applicability of commercially available formulations of PEDOT:PSS (e.g., Clevios) is limited by its poor mechanical properties. Additives can be used to increase the compliance but pose a risk of leaching, which can result in device failure and increased toxicity (in biological settings). Thus, to increase the mechanical compliance of PEDOT:PSS without additives, we synthesized a library of intrinsically stretchable block copolymers. In particular, controlled radical polymerization using a reversible addition-fragmentation transfer process was used to generate block copolymers consisting of a block of PSS (of fixed length) appended to varying blocks of poly(poly(ethylene glycol) methyl ether acrylate) (PPEGMEA). These block copolymers (PSS₍₁₎-b-PPEGMEA_(x), where x ranges from 1 to 6) were used as scaffolds for oxidative polymerization of PEDOT. By increasing the lengths of the PPEGMEA segments on the PEDOT:[PSS₍₁₎-b-PPEGMEA_(1-6)] block copolymers, ("Block-1" to "Block-6"), or by blending these copolymers with PEDOT:PSS, the mechanical and electronic properties of the polymer can be tuned. Our results indicate that the polymer with the longest block of PPEGMEA, Block-6, had the highest fracture strain (75%) and lowest elastic modulus (9.7 MPa), though at the expense of conductivity (0.01 S cm^-1). However, blending Block-6 with PEDOT:PSS to compensate for the insulating nature of the PPEGMEA resulted in increased conductivity [2.14 S cm^-1 for Blend-6 (2:1)]. Finally, we showed that Block-6 outperforms a commercial formulation of PEDOT:PSS as a dry electrode for surface electromyography due to its favorable mechanical properties and better adhesion to skin.

Ascorbic acid-mediated organic photoelectrochemical transistor sensing strategy for highly sensitive detection of heart-type fatty acid binding protein

Heart-type fatty acid binding protein (H-FABP) has been regarded as a promising biomarker for early diagnosis of acute myocardial infarction (AMI). Developing fast and reliable method for H-FABP detection is still highly desirable but challenging. Herein, an ascorbic acid (AA)-mediated organic photoelectrochemical transistor (OPECT) sensing strategy was reported for the detection of H-FABP in phosphate buffer saline (PBS) solution and human serum. A primary antibody/H-FABP/secondary antibody-Au NPs-alkaline phosphatase (ALP) sandwich immunorecognition structure was constructed. The modified ALP could catalytically convert ascorbic acid-2-phosphate to AA, which was then analyzed by OPECT. As a result, the AA-mediated OPECT sensing strategy realized highly sensitive detection of H-FABP with a detection limit of 3.23 × 10^-14 g/mL which is two orders of magnitude lower than that of PEC method. Under optimal experimental conditions, H-FABP concentration could be obtained in ∼90 min. Importantly, the analysis of H-FABP was resistant to the interference from immunoglobulin G, bovine serum albumin, cysteine, AA and human serum. The proposed AA-mediated OPECT sensing strategy provides a simple, fast, and accurate way for H-FABP detection in AMI suspected patients.

Organic Bioelectronics for In Vitro Systems

Bioelectronics have made strides in improving clinical diagnostics and precision medicine. The potential of bioelectronics for bidirectional interfacing with biology through continuous, label-free monitoring on one side and precise control of biological activity on the other has extended their application scope to in vitro systems. The advent of microfluidics and the considerable advances in reliability and complexity of in vitro models promise to eventually significantly reduce or replace animal studies, currently the gold standard in drug discovery and toxicology testing. Bioelectronics are anticipated to play a major role in this transition offering a much needed technology to push forward the drug discovery paradigm. Organic electronic materials, notably conjugated polymers, having demonstrated technological maturity in fields such as solar cells and light emitting diodes given their outstanding characteristics and versatility in processing, are the obvious route forward for bioelectronics due to their biomimetic nature, among other merits. This review highlights the advances in conjugated polymers for interfacing with biological tissue in vitro, aiming ultimately to develop next generation in vitro systems. We showcase in vitro interfacing across multiple length scales, involving biological models of varying complexity, from cell components to complex 3D cell cultures. The state of the art, the possibilities, and the challenges of conjugated polymers toward clinical translation of in vitro systems are also discussed throughout.

Ultrasensitive Detection of Ribonucleic Acid Biomarkers Using Portable Sensing Platforms Based on Organic Electrochemical Transistors

The analysis of ribonucleic acid (RNA) plays an important role in the early diagnosis of diseases and will greatly benefit patients with a higher cure rate. However, the low abundance of RNA in physiological environments requires ultrahigh sensitivity of a detection technology. Here, we construct a portable and smart-phone-controlled biosensing platform based on disposable organic electrochemical transistors for ultrasensitive analysis of microRNA (miRNA) biomarkers within 1 h. Due to their inherent amplification function, the devices can detect miRNA cancer biomarkers from little-volume solutions with concentrations down to 10^-14 M. The devices can distinguish blood miRNA expression levels at different cancer stages using a 4T1 mouse tumor model. The technique for ultrasensitive and fast detection of RNA biomarkers with high selectivity opens a window for mobile diagnosis of various diseases with low cost.

Tuning Organic Electrochemical Transistor (OECT) Transconductance toward Zero Gate Voltage in the Faradaic Mode

In this work, we investigate material design criteria for low-powered/self-powered and efficient organic electrochemical transistors (OECTs) to be operated in the faradaic mode (detection at the gate electrode occurs via electron transfer events). To rationalize device design principles, we adopt a Marcus-Gerischer perspective for electrochemical processes at both the gate and channel interfaces. This perspective considers density of states (DOS) for the semiconductor channel, the gate electrode, and the electrolyte. We complement our approach with energy band offsets of relevant electrochemical potentials that can be independently measured from transistor geometry using conventional electrochemical methods as well as an approach to measure electrolyte potential in an operating OECT. By systematically changing the relative redox property offsets between the redox-active electrolyte and semiconducting polymer channel, we demonstrate a first-order design principle that necessary gate voltage is minimized by good DOS overlap of the two redox processes at the gate and channel. Specifically, for p-type turn-on OECTs, the voltage-dependent, electrochemically active semiconductor DOS should overlap with the oxidant form of the electrolyte to minimize the onset voltage for transconductance. A special case where the electrolyte can be used to spontaneously dope the polymer via charge transfer is also considered. Collectively, our results provide material design pathways toward the development of simple, robust, power-saving, and high-throughput OECT biosensors.

Aptamer-Based Solution-Gated Graphene Transistors for Highly Sensitive and Real-Time Detection of Thrombin Molecules

Thrombin is an important biomarker for various diseases and biochemical reactions. Rapid and real-time detection of thrombin that quickly neutralizes in early coagulation in the body has gained significant attention for its practical applications. Solution-gated graphene transistors (SGGTs) have been widely studied due to their higher sensitivity and low-cost fabrication for chemical and biological sensing applications. In this paper, the ssDNA aptamer with 29 bases was immobilized on the surface of the gate electrode to specifically recognize thrombin. The SGGT sensor achieved high sensitivity with a limit of detection (LOD) up to fM. The LOD was attributed to the amplification function of SGGTs and the suitable aptamer choice. The ssDNA configuration folding induced by thrombin molecules and the electropositivity of thrombin molecules could arouse the same electrical response of SGGTs, helping the device obtain a high sensitivity. The channel current variation of sensors had a good linear relationship with the logarithm of thrombin concentration in the range of 1 fM to 10 nM. The fabricated device also demonstrated a short response time to thrombin molecules, and the response time to the 1 fM thrombin molecules was about 150 s. In summary, the sensing strategy of aptamer-based SGGTs with high sensitivity and high selectivity has a good prospect in medical diagnosis.

Ultrafast, sensitive, and portable detection of COVID-19 IgG using flexible organic electrochemical transistors

The outbreak of COVID-19 and its continued spread have seriously threatened public health. Antibody testing is essential for infection diagnosis, seroepidemiological analysis, and vaccine evaluation. However, convenient, fast, and accurate antibody detection remains a challenge in this protracted battle. Here, we report an ultrafast, low-cost, label-free, and portable SARS-CoV-2 immunoglobulin G (IgG) detection platform based on organic electrochemical transistors (OECTs), which can be remotely controlled by a mobile phone. To enable faster detection, voltage pulses are applied on the gate electrode of the OECT to accelerate binding between the antibody and antigen. By optimizing ion concentrations and pH values of test solutions, we realize specific detection of SARS-CoV-2 IgG in several minutes with a detectable region from 10 fM to 100 nM, which encompasses the range of serum SARS-CoV-2 IgG levels in humans. These portable sensors show promise for use in diagnosis and prognosis of COVID-19.

A bio-syncretic phototransistor based on optogenetically engineered living cells

Human eyes rely on photosensitive receptors to convert light intensity into action potentials for visual perception, and thus bio-inspired photodetectors with bioengineered photoresponsive elements for visual prostheses have received considerable attention by virtue of superior biological functionality and better biocompatibility. However, the current bioengieered photodetectors based on biological elements face a lot of challenges such as slow response time and lack of effective detection of weak bioelectrical signals, resulting in difficulty to perform imaging. Here, we report a human eye-inspired phototransistor by integrating optogenetically engineered living cells and a graphene-based transistor. The living cells, engineered with photosensitive ion channels, channelrhodopsin-2 (ChR2), and thus endowed with the capability of transducing light intensity into bioelectrical signals, are coupled with the graphene layer of the transistor and can regulate the transistor's output. The results show that the photosensitive ion channels enable the phototransistor to output stronger photoelectrical currents with relatively fast response (~25 ms) and wider dynamic range, and demonstrate the transistor owns optical and biological gating with a significant large on/off ratio of 197.5 and high responsivity of 1.37 mA W^-1. An artificial imaging system, which mimics the pathway of human visual information transmission from the retina through the lateral geniculate nucleus to the visual cortex, is constructed with the transistor and demonstrate the feasibility of imaging using the bioengineered cells. This work shows a potential that optogenetically engineered cells can be used to develop novel visual prostheses and paves a new avenue for engineering bio-syncretic sensing devices.

Interfacing cells with organic transistors: a review of in vitro and in vivo applications

Recently, organic bioelectronics has attracted considerable interest in the scientific community. The impressive growth that it has undergone in the last 10 years has allowed the rise of the completely new field of cellular organic bioelectronics, which has now the chance to compete with consolidated approaches based on devices such as micro-electrode arrays and ISFET-based transducers both in in vitro and in vivo experimental practice. This review focuses on cellular interfaces based on organic active devices and has the intent of highlighting the recent advances and the most innovative approaches to the ongoing and everlasting challenge of interfacing living matter to the "external world" in order to unveil the hidden mechanisms governing its behavior. Device-wise, three different organic structures will be considered in this work, namely the organic electrochemical transistor (OECT), the solution-gated organic transistor (SGOFET - which is presented here in two possible different versions according to the employed active material, namely: the electrolyte-gated organic transistor - EGOFET, and the solution gated graphene transistor - gSGFET), and the organic charge modulated field effect transistor (OCMFET). Application-wise, this work will mainly focus on cellular-based biosensors employed in in vitro and in vivo cellular interfaces, with the aim of offering the reader a comprehensive retrospective of the recent past, an overview of the latest innovations, and a glance at the future prospects of this challenging, yet exciting and still mostly unexplored scientific field.

Investigation of electronic properties of chemical vapor deposition grown single layer graphene via doping of thin transparent conductive films

It is a crucial challenge to obtain the desired electronic properties of two-dimensional materials for various ubiquitous applications and improvements in the existing technology. In this article, we have demonstrated the modulation in electronic features of the chemical vapor deposition (CVD) grown single-layer graphene (SLG) via wet doping of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The PEDOT:PSS is well known as conducting polymer and used as transparent conducting electrode in flexible organic electronic devices. The effect of doping on SLG samples were examined by Raman spectroscopy, electrical transport measurement, atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM). The Raman peaks position of doped samples provided sought evidence of p-type doping of SLG after the deposition of PEDOT:PSS films. The electrical measurement confirmed the p-type doping of SLG and also revealed enhanced carrier density and mobility of SLG after the deposition of PEDOT:PSS films. AFM micrographs revealed the homogeneous loading of PEDOT:PSS particles over the SLGs. Further, KPFM technique was used to estimate the work function modulation of SLG after PEDOT:PSS film deposition. Our investigation will be useful for understanding the device physics as well as improvement of photovoltaic devices based on PEDOT:PSS coated graphene.

100th Anniversary of Macromolecular Science Viewpoint: Soft Materials for Microbial Bioelectronics

Bioelectronics brings together the fields of biology and microelectronics to create multifunctional devices with the potential to address longstanding technological challenges and change our way of life. Microbial electrochemical devices are a growing subset of bioelectronic devices that incorporate naturally occurring or synthetically engineered microbes into electronic devices and have broad applications including energy harvesting, chemical production, water remediation, and environmental and health monitoring. The goal of this Viewpoint is to highlight recent advances and ongoing challenges in the rapidly developing field of microbial bioelectronic devices, with an emphasis on materials challenges. We provide an overview of microbial bioelectronic devices, discuss the biotic-abiotic interface in these devices, and then present recent advances and ongoing challenges in materials related to electron transfer across the abiotic-biotic interface, microbial adhesion, redox signaling, electronic amplification, and device miniaturization. We conclude with a summary and perspective of the field of microbial bioelectronics.

Self-Healable Organic Electrochemical Transistor with High Transconductance, Fast Response, and Long-Term Stability

The major challenges in developing self-healable conjugated polymers for organic electrochemical transistors (OECTs) lie in maintaining good mixed electronic/ionic transport and the need for fast restoration to the original electronic and structural properties after the self-healing process. Herein, we provide the first report of an all-solid-state OECT that is self-healable and possesses good electrical performance, by utilizing a matrix of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a nonionic surfactant, Triton X-100, as a channel and an ion-conducting poly(vinyl alcohol) hydrogel as a quasi-solid-state polymer electrolyte. The fabricated OECT exhibits high transconductance (maximum 54 mS), an on/off current ratio of ∼1.5 × 10³, a fast response time of 6.8 ms, and good operational stability after 68 days of storage. Simultaneously, the OECT showed remarkable self-healing and ion-sensing behaviors and recovered ∼95% of its ion sensitivity after healing. These findings will contribute to the development of high-performance and robust OECTs for wearable bioelectronic devices.

Finding the equilibrium of organic electrochemical transistors

Organic Electrochemical Transistors are versatile sensors that became essential for the field of organic bioelectronics. However, despite their importance, an incomplete understanding of their working mechanism is currently precluding a targeted design of Organic Electrochemical Transistors and it is still challenging to formulate precise design rules guiding materials development in this field. Here, it is argued that current capacitive device models neglect lateral ion currents in the transistor channel and therefore fail to describe the equilibrium state of Organic Electrochemical Transistors. An improved model is presented, which shows that lateral ion currents lead to an accumulation of ions at the drain contact, which significantly alters the transistor behavior. Overall, these results show that a better understanding of the interface between the organic semiconductor and the drain electrode is needed to reach a full understanding of Organic Electrochemical Transistors.

Though existing capacitive models describe the electronic transport in organic electrochemical transistors, these models do not provide an accurate steady-state solution. Here, the authors report a numerical model that describes the distribution and accumulation of ions in the transistor channel.

Enhancement-Mode PEDOT:PSS Organic Electrochemical Transistors Using Molecular De-Doping

Organic electrochemical transistors (OECTs) show great promise for flexible, low-cost, and low-voltage sensors for aqueous solutions. The majority of OECT devices are made using the polymer blend poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), in which PEDOT is intrinsically doped due to inclusion of PSS. Because of this intrinsic doping, PEDOT:PSS OECTs generally operate in depletion mode, which results in a higher power consumption and limits stability. Here, a straightforward method to de-dope PEDOT:PSS using commercially available amine-based molecular de-dopants to achieve stable enhancement-mode OECTs is presented. The enhancement-mode OECTs show mobilities near that of pristine PEDOT:PSS (≈2 cm² V^-1 s^-1 ) with stable operation over 1000 on/off cycles. The electron and proton exchange among PEDOT, PSS, and the molecular de-dopants are characterized to reveal the underlying chemical mechanism of the threshold voltage shift to negative voltages. Finally, the effect of the de-doping on the microstructure of the spin-cast PEDOT:PSS films is investigated.

Humidity-Controlled Water Uptake and Conductivities in Ion and Electron Mixed Conducting Polythiophene Films

Mixed conducting polymer films are of great interest in applications where an interface between electronic and ionic charge transport is needed, e.g., in bioelectronics, electrochemical energy applications, and photovoltaic device interfaces. The role of water on charge transport is of high relevance not only for aqueous environments but also for devices that are manufactured at ambient conditions with varying relative humidities. In this contribution, we present our results on the influence of controlled humidity changes on the mixed conductivity and correlation to the concomitant water uptake in the films. Two sulfonate-bearing polythiophene systems are studied: a self-made conjugated polyelectrolyte, poly(6-(thiophen-3-yl)hexane-1-sulfonate)-sodium (PTS-Na), and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) with different ratios of PEDOT and the polyelectrolyte PSS. Our data give clear evidence of the similarities between the aforementioned polythiophene systems and pure ionic membranes such as Nafion used in fuel cells. As such, a phase separation between the hydrophobic electronically conducting polythiophene phase and the hydrophilic water-swellable ion-conducting phase is proposed. Changing the humidity from dry conditions up to ∼90% relative humidity results in extremely high water uptakes of more than 90 wt %, which corresponds to ∼13 water molecules per sulfonate unit at maximum water uptake. Conversely, the electronic conductivity is less sensitive to increasing humidity, which is due to percolation pathways. The ionic conductivity strongly increases from 10^-10 S/cm at dry conditions to 10^-3 S/cm at around 30 wt % water content and then levels off at maximum conductivities of 10^-3-10^-2 S/cm up to water contents of 90 wt %.