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

Electrochemically Active Materials for Tissue-Interfaced Soft Biochemical Sensing

Tissue-interfaced soft biochemical sensing represents a crucial approach to personalized healthcare by employing electrochemically active materials to monitor biochemical signals at the tissue interface in real time, either noninvasively or through implantation. These soft biochemical sensors can be integrated with various biological tissues, such as neural, gastrointestinal, ocular, cardiac, skin, muscle, and bone, adapting to their unique mechanical and biochemical environments. Sensors employing materials like conductive polymers, composites, metals, metal oxides, and carbon-based nanomaterials have demonstrated capabilities in applications, such as continuous glucose monitoring, neural activity mapping, and real-time metabolite detection, enhancing diagnostics and treatment monitoring across a range of medical fields. Next-generation tissue-interfaced biosensors that enable multimodal and multiplexed measurement of biochemical markers and physiological parameters could be transformative for personalized medicine, allowing for high-resolution, time-resolved historical monitoring of an individual's health status. In this review, we summarize current trends in the field to provide insights into the challenges and future trajectory of tissue-interfaced soft biochemical sensors, highlighting their potential to revolutionize personalized medicine and improve patient outcomes.

Ready-to-Use OECT Biosensor toward Rapid and Real-Time Protein Detection in Complex Biological Environments

Organic electrochemical transistor (OECT) sensors are a promising approach for point-of-care testing (POCT) thanks to their high sensitivity and ability to operate in an aqueous environment. However, OECTs suffer from biological fouling at the gate and channel interfaces when exposed to complex biological samples. These nonspecific interactions can often obscure the weak signal from the trace biomarker, compromising the accuracy and sensitivity of measurements and even leading to false detection outcomes. In this study, we developed an intrinsically antifouling OECT by modifying both the gate and channel with phosphorylcholine-functionalized poly(3,4-ethylenedioxythiophene) (PEDOT-PC). This modification notably enhances the OECT performance by leveraging the material's inherent mixed electron-ion conductivity, which increases transconductance and decreases gate voltage. Additionally, the zwitterionic nature of the device enables its ultrasensitive detection of C-reactive protein (CRP) with a limit of detection of 0.11 pg/mL, mediated by calcium ions. This exceptional sensitivity arises from the device's enhanced transconductance and ability to sense through the gate and channel interfaces simultaneously. Furthermore, the zwitterionic OECT sensor has demonstrated the fastest sample-to-result time for protein detection (≤60 s), making it highly suitable for real-time CRP monitoring. Importantly, it provides precise real-time detection of CRP without interference from nonspecific proteins such as bovine serum albumin, fibrinogen, lysozyme, and fetal bovine serum. We envision this intrinsically antifouling OECT device offering a robust biosensing platform for the rapid and convenient detection of biomarkers in complex biological environments, providing a reliable and efficient solution for POCT diagnostics.

Stretchable semiconducting polymer aerogel transistors for high-performance biosensors and artificial synapses

Stretchable organic electrochemical transistors (OECTs) are attractive for high-performance flexible electronics. Poly(3-hexylthiophene) (P3HT) and poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPPDTT) are commonly used for OECTs because of their excellent semiconducting properties. However, it is challenging to achieve stretchable and high-performance OECTs based on hydrophobic P3HT and DPPDTT because of their limited ion penetration. Here, unprecedented stretchable high-performance OECTs based on P3HT and DPPDTT aerogels with crimpled porous structures are developed. They are achieved by a pre-stretching strategy combined with sol-gel and template methods. The porous structures of the aerogels with high porosities facilitate ion penetration and transport, leading to the enhanced transconductance of the aerogel-based OECTs compared with those of the dense counterparts. The crimpled porous structures endow the aerogels and OECTs with good stretchability and stretching stability. The stretchable OECT-based biosensors can detect trace amounts of ascorbic acid in complex samples such as sweat, saliva, serum, and fruit juice in real time. Besides, the OECTs can be applied as stretchable artificial synapses for neuromorphic simulation. This work provides a powerful strategy toward stretchable high-performance transistors and flexible electronics.

Ni/Fe-MOF Electrochemical Transistor Biosensors with 3D Debye Space for Ultrasensitive Detection of Coronavirus Nucleocapsid Protein

Electrochemical transistor (ECT) biosensors can achieve ultrahigh sensitivity, while keeping challenges for large biomolecules due to their sizes over the thickness of the electric double layer (EDL) causing a failure of sensing reaction. Here, wafer-level Ni/Fe-metal-organic framework (MOF) films and corresponding ECT biosensors are fabricated for the detection of coronavirus nucleocapsid protein, which realize an ultralow detection limit of 1 fg mL-1 and a wide detection range of 1 fg mL-1 to 100 pg mL-1. A 3D Debye space, originating from the unique stacking structure of nanosheets and nanoparticles in Ni/Fe-MOF film, is proposed to explain such ultrahigh sensitivity, which ensures the immune-binding and sensing process of the nucleocapsid protein large molecules located in the effective EDL space. Moreover, an appropriate Ni/Fe molar ratio is optimized to enhance the transmission of carriers, thus improving the signal-to-noise ratio and the sensing response. The Ni/Fe-MOF-ECT biosensors will find applications in highly sensitive detection for large biomolecules, and the 3D Debye space may be a design model for constructing other biosensors.

Extracellular vesicle-based point-of-care testing for diagnosis and monitoring of Alzheimer's disease

Extracellular vesicles (EVs) show potential for early diagnosis of Alzheimer's disease (AD) and monitoring of its progression. However, EV-based AD diagnosis faces challenges due to the small size and low abundance of biomarkers. Here, we report a fully integrated organic electrochemical transistor (OECT) sensor for ultrafast, accurate, and convenient point-of-care testing (POCT) of serum EVs from AD patients. By utilizing acoustoelectric enrichment, the EVs can be quickly propelled, significantly enriched, and specifically bound to the OECT detection area, achieving a gain of over 280 times response in 30 s. The integrated POCT sensor can detect serum EVs from AD patients with a limit of detection as low as 500 EV particles/mL and a reduced detection time of just two minutes. Furthermore, the integrated POCT sensors were used to monitor AD progression in an AD mouse model by testing the mouse Aβ EVs at different time courses (up to 18 months) and compared with the Aβ accumulation using high-resolution magnetic resonance imaging (MRI). This innovative technology has the potential for accurate and rapid diagnosis of Alzheimer's and other neurodegenerative diseases, and monitoring of disease progression and treatment response.

Advancing transistor-based point-of-care (POC) biosensors: additive manufacturing technologies and device integration strategies for real-life sensing

Infectious pathogens pose a significant threat to public health and healthcare systems, making the development of a point-of-care (POC) detection platform for their early identification a key focus in recent decades. Among the numerous biosensors developed over the years, transistor-based biosensors, particularly those incorporating nanomaterials, have emerged as promising candidates for POC detection, given their unique electronic characteristics, compact size, broad dynamic range, and real-time biological detection capabilities with limits of detection (LODs) down to zeptomolar levels. However, the translation of laboratory-based biosensors into practical applications faces two primary challenges: the cost-effective and scalable fabrication of high-quality transistor sensors and functional device integration. This review is structured into two main parts. The first part examines recent advancements in additive manufacturing technologies-namely in screen printing, inkjet printing, aerosol jet printing, and digital light processing-and evaluates their applications in the mass production of transistor-based biosensors. While additive manufacturing offers significant advantages, such as high quality, cost-effectiveness, rapid prototyping, less instrument reliance, less material waste, and adaptability to diverse surfaces, challenges related to uniformity and yield remain to be addressed before these technologies can be widely adopted for large-scale production. The second part focuses on various functional integration strategies to enhance the practical applicability of these biosensors, which is essential for their successful translation from laboratory research to commercialization. Specifically, it provides a comprehensive review of current miniaturized lab-on-a-chip systems, microfluidic manipulation, simultaneous sampling and detection, wearable implementation, and integration with the Internet of Things (IoT).

Effect of channel patterning precision on the performances of vertical OECTs

Precise patterning of electronic functional layers is vital for integrated electronics, where high integration density is required. Similarly, for organic electrochemical transistors (OECTs), the patterning precision of the channel layer is essential for device miniaturization, parasitic capacitance reduction, and accurate performance evaluation. In particular, for an emerging OECT architecture, vertical OECT (vOECT), the effect of patterning precision on key device parameters (such as transconductance (gm) and transient time (τ)) remains unclear. Here, controllable patterning of vOECT channel regions is realized by direct laser etching, where 2-100 μm margin lengths (lM) are left beyond the vertical channel area. By quantitatively analyzing the impact of margin areas on device performance (including drain currents (ID), gm, and τ), it has been found that a larger lM leads to significantly increased ID and gm in both n- and p-type OECTs (106.94% and 61.46% enhancement of ID and 102.92% and 92.59% enhancement of gm in n- and p-type OECTs, respectively, are observed as lM increases), which saturate under an lM of ∼60 μm. Nevertheless, linearly increasing τ (from hundreds of microseconds to a few milliseconds) is observed with increasing lM, revealing that parasitic capacitance outside the channel would result in a longer redox reaction time but not always higher ID and gm. It is revealed that the patterning precision of active layers alters the OECT performances tremendously and can be designed to meet different application requirements (either high amplification capability, high integrating density, or fast response time) in OECT-based electronics.

Engineering 3D microtip gates of all-polymer organic electrochemical transistors for rapid femtomolar nucleic-acid-based saliva testing

Point-of-care testing (POCT) of trace amount of biomarkers in biofluids is critical towards health monitoring and early diagnosis. In particular, to facilitate non-invasive saliva testing, the development of low-cost, lightweight and disposable biosensors is in urgent need, while the ultrahigh sensitivity beyond conventional clinical tests remains a great challenge. Herein, we demonstrate a simple and fully printable all-polymer organic electrochemical transistor (OECT) biosensor to detect femtomolar (fM)-level biomolecules in saliva within a few minutes by employing highly conducting lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-doped poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) serving as both the channel and gate. A 3D microtip-shaped and Au nanoparticle-decorated multiscale gate interface facilitates the sub-fM-level sensing of female hormones (e.g., progesterone) and oligonucleotide cancer biomarkers by aptamers and DNA probes, respectively. Compared to a planar gate, the micro-engineered interface endows the OECT biosensor with significantly lower detection limit by 10-100 times down to <0.1 fM and faster response of <5 min, accomplishing unprecedentedly high sensitivity while maintaining outstanding mechanical flexibility. Consequently, such microtip-gate all-polymer OECT (MAOECT) enables POCT directly in 1000-fold diluted human saliva samples without centrifugation or redox probes, benefiting female fertility monitoring and oral cancer diagnosis as proof-of-concept demonstrations. This straightforward approach presents great potentials in low-cost wearable health management, at-home monitoring and personalized medicine.

Poly(3,4-ethylenedioxythiophene) Nanorod Arrays-Based Organic Electrochemical Transistor for SARS-CoV-2 Spike Protein Detection in Artificial Saliva

The outbreak and continued spread of coronavirus disease 2019 (COVID-19) have significantly threatened public health. Antibody testing is essential for infection diagnosis, seroepidemiological analysis, and vaccine evaluation. However, achieving convenient, fast, and accurate detection remains challenging in this prolonged battle. This study reports a highly sensitive severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein detection platform based on organic electrochemical transistors (OECTs) for biosensing applications. We developed a nanostructured poly(3,4-ethylenedioxythiophene) (PEDOT) conductive polymer with the carboxylic acid functional group (PEDOTAc) for modifying specific antibodies on an OECT channel for the detection of the COVID-19 spike protein. The OECT device features a channel composed of a PEDOT:polystyrenesulfonate (PEDOT:PSS) bottom layer, with the upper layer decorated with PEDOTAc nanorod arrays via the oxidative polymerization and a trans-printing method. Our novel PEDOTAc nanorod array-based OECT device exhibits promising potential for future healthcare and point-of-care sensing due to its rapid response, high sensitivity, and high accuracy. Through optimization, we achieved specific detection of the SARS-CoV-2 spike protein within minutes, with a detectable region from 10 fM to 100 nM. These biosensors hold significant promise for use in the diagnosis and prognosis of COVID-19.

Vertically Phase-Separated PEDOT:PSS Film via Solid-Liquid Interface Doping for Flexible Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are seen as some of the most promising devices in organic bioelectronics. Recent interest in OECTs is sparked by the high performance of an organic semiconductor channel material, i.e., poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The capability of ion penetration and charge transport of the channel determines the performance of the OECTs. However, the uniform structure of the PEDOT:PSS channel always makes it difficult to achieve a well-balanced between the two functions. Here, we report a novel PEDOT:PSS film with a vertical phase separation structure (VPSS-P), where PSS accumulates at the surface, and PEDOT enriches at the bottom of the film. Such a unique structure improves the electrochemical stability and reduces the contact resistance, significantly enhancing OECT performance with high transconductance (70.5 mS), product of mobility (μ) and volumetric capacitance (C*) (μC* ∼ 479 F cm-1 V-1 s-1), and ultralow contact resistance (∼0.79 Ω cm). Flexible OECT devices with VPSS-P show robust performance against deformation. Our findings highlight a new class of high-performance transistors and provide guidelines for designing state-of-the-art channel materials.

Detection of IgG Antibodies Against COVID-19 N-Protein by Hybrid Graphene-Nanorod Sensor

The COVID-19 pandemic highlighted the global necessity to develop fast, affordable, and user-friendly diagnostic alternatives. Alongside recognized tests such as ELISA, nanotechnologies have since been explored for direct and indirect diagnosis of SARS-CoV-2, the etiological agent of COVID-19. Accordingly, in this work, we report a method to detect anti-SARS-CoV-2 antibodies based on graphene-based field-effect transistors (GFETs), using a nanostructured platform of graphene with added gold nanorods (GNRs) and a specific viral protein. To detect anti-N-protein IgG antibodies for COVID-19 in human sera, gold nanorods were functionalized with the nucleocapsid (N) protein of SARS-CoV-2, and subsequently deposited onto graphene devices. Our test results demonstrate that the sensor is highly sensitive and can detect antibody concentrations as low as 100 pg/mL. Using the sensor to test human sera that were previously diagnosed with ELISA showed a 90% accuracy rate compared to the ELISA results, with the test completed in under 15 min. Integrating graphene and nanorods eliminates the need for a blocker, simplifying sensor fabrication. This hybrid sensor holds robust potential to serve as a simple and efficient point-of-care platform.

Organic photoelectrochemical transistor biosensor based on BiVO4-ZnIn2S4 material for efficient and sensitive detection of MCF-7 cells

Recently, organic photoelectrochemical transistor (OPECT) has become a very interesting biological measurement method in photoelectrochemical (PEC) bioanalysis and future bio-related applications. OPECT is expected to be a powerful tool for disease detection and early warning. Circulating tumor cells (CTCs) are generally deemed to be the dominant factor of tumor metastasis, and 90 % of cancer patients die from this metastatic disease. Therefore, there is an imminent need to develop a highly sensitive CTCs detection sensing system to improve the survival rate of cancer patients. Here, we use a DNA tetrahedrons (DNA NTH) with an aptamer at the top to immobilize on the surface of the photoelectric material to capture cells (MCF-7). Specifically, the BiVO4-ZnIn2S4 hybrid was synthesized by a simple hydrothermal method, which can effectively modulated devices with high current gain. Au NPs were directly integrated on the electrode surface to construct an OPECT photoelectric sensing platform. Subsequently, the aptamer which is thiol-functionalized (SH-Apt) was immobilized on the electrode surface. Because of the overexpression of MUC1 protein on the cell membrane, it can specifically capture MCF-7 cells. The introduction of MCF-7 cells resulted in a significant decrease in the current signal. There is a relationship between the change of photocurrent and the logarithm of MCF-7 cell concentration, which is a good linear relationship ranging from 50 to 5 × 105 cell mL-1. The obtained detection limit is 43 cell mL-1. The biosensor has high selectivity and sensitivity, and achieves sensitive detection of MCF-7.

Graphene-Based Opto-Electronic Platform for Ultra-Sensitive Biomarker Detection at Zeptomolar Concentrations

A ground-breaking graphene-based biosensor designed for label-free detection of immunoglobulin M (IgM) achieving a remarkable concentration of 100 zeptomolar (10-19 m), is reported. The sensor is a two-terminal device and incorporates a millimeter-wide gold interface, bio-functionalized with ≈1012 anti-IgM antibodies and capacitively coupled to a bare graphene electrode through a water-soaked paper strip. In this configuration, few affinity binding events trigger a collective electrostatic reorganization of the protein layer, leading to an extended surface potential (SP) shift of the biofunctionalized Au surface. The SP shift, mediated by electrolyte capacitive coupling, induces a corresponding shift in the Fermi level of graphene. This shifts the graphene phonon frequencies, which are measured by Raman spectroscopy. Decoupling the sensing interface from the transducing graphene layer provides flexibility in surface chemistry modifications, while preserving the graphene integrity. A key aspect of this biosensor is its ability to precisely determine the graphene charge neutrality point from the voltage dependence of phonon frequency shifts, enabling detections of biomarker at unprecedented low concentrations. The integration of graphene with optical probing demonstrates a proof-of-concept and establishes a ground-breaking approach to in situ biomarker detection, setting the stage for a future generation of portable opto-electronic high-performance diagnostic tools for single-marker detection.

Vertical-Structure Overcomes the Strain Limit of Stretchable Organic Electrochemical Transistors

Intrinsically stretchable organic electrochemical transistors (IS-OECTs), utilizing organic mixed ionic-electronic conductors (OMIECs) as their channel materials, have drawn great attention recently because of their potential to enable seamless integration between bioelectronic devices and living systems. However, the fabrication of IS-OECTs presents challenges due to the limited availability of OMIEC materials that possess the desired combination of mechanical and electrical properties. In this work, 1) we report the first successful fabrication of a vertical intrinsically stretchable OECT (VIS-OECT), achieved by using elastoadhesive electrodes; 2) we experimentally proved that vertical architecture can push the strain limit of an IS-OECT from 20% to 50%; and 3) the above finding introduces an unconventional design concept: the strain limit of an IS-OECT can surpass the intrinsic stretchability of the constituent OMIECs by employing vertical structure.

Sensors for surveillance of RNA viruses: a One Health perspective

RNA viruses, especially those capable of cross-species transmission, pose a serious threat to human, animal, and environmental health, as exemplified by the 2024 outbreak of the highly pathogenic avian influenza H5N1 virus in cattle, unpasteurised milk, and workers on dairy farms in the USA. This escalating risk of a new RNA virus pandemic highlights the urgent need to implement One Health strategies. However, the centralised virus detection systems currently in use fall short of meeting the required level of virus surveillance and infection diagnosis, particularly in resource-limited regions. In this context, the latest advancements in RNA virus-sensing technologies offer promising solutions. Through interdisciplinary collaboration, these sensors can achieve sensitivity and reliability similar to that of standard laboratory equipment and offer several advantages, such as compact size, affordability, and operational simplicity. In this Review, we highlight the latest advances in sensing technologies for detecting different biomarkers of viral infections (RNA, antigens, and antibodies). We further compare the sensing principles and performances of these technologies and discuss the possibility of deployment of these sensors in the One Health approach and the challenges expected in this pursuit. In conclusion, the widespread use of RNA virus sensors is expected to enhance the effectiveness of surveillance systems for infectious diseases.

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.

Sensing Interfaces Engineering for Organic Thin Film Transistors-Based Biosensors: Opportunities and Challenges

Organic thin film transistors (OTFTs) enable rapid and label-free high-sensitivity detection of target analytes due to their low cost, large-area processing, biocompatibility, and inherent signal amplification. At the same time, the freedom of synthesis, tailorability, and functionalization of organic semiconductor materials and their ability to be combined with flexible substrates make them one of the ideal platforms for biosensing. However, OTFTs-based biosensors still face significant challenges, such as unexpected surface adsorption, disordered conformation, inhomogeneous graft density, and flexibility of probe molecules that biological sensing probes would face during immobilization. In this review, efficient immobilization strategies based on OTFTs biological sensing probes developed in the last 5 years are highlighted. First, the biosensors are classified according to their sensing interface. Second, a comprehensive discussion of the types of biological sensing probes is presented. Third, three commonly used methods for immobilizing biological sensing probes and their challenges are briefly described. Finally, the applications of OTFTs-based biosensors for liquid phase detection are summarized. This review provides a comprehensive and timely review of optimization in sensing interface engineering so that efficient immobilization of biological sensing probes with sensing interfaces will contribute to the development of high-performance OTFTs-based biosensors.

Tri-signal CdS@SiO2 nanoprobes for accurate and sensitive detection of human immunoglobulin G with enhanced flexibility and internal validation

Accurate and sensitive determination of human immunoglobulin G (HIgG) level is critical for diagnosis and treatment of various diseases, including rheumatoid arthritis, humoral immunodeficiencies, and infectious disease. In this study, versatile tri-signal probes were developed by preparing CdS@SiO2 nanorods that integrate photoluminescence (PL), multi-phonon resonant Raman scattering (MRRS) and infrared absorption (IRA) properties. Through the coating of multiple CdS nanoparticles as cores within SiO2 shells, the PL and MRRS properties of CdS were improved, resulting in a significantly lowered limit of detection (LOD), with the lowest LOD of 12.37 ag mL-1. Integration with the distinctive IRA property of SiO2 shells widened the detection range towards higher concentrations, establishing a final linear range of 50 ag mL-1 to 10 μg mL-1. The remarkable consistency among the three signals highlighted the robust internal verification capability for accurate detection. This approach enhances flexibility in selecting detection methodologies to suit diverse scenarios, facilitating HIgG detection. The tri-signal nanoprobes also exhibited excellent detection selectivity, specificity and repeatability. This study presents a fresh idea for developing high-performance detection strategies.

A Photoelectrochemical Retinomorphic Synapse

Reproducing human visual functions with artificial devices is a long-standing goal of the neuromorphic domain. However, emulating the chemical language communication of the visual system in fluids remains a grand challenge. Here, a "multi-color" hydrogel-based photoelectrochemical retinomorphic synapse is reported with unique chemical-ionic-electrical signaling in an aqueous electrolyte that enables, e.g., color perception and biomolecule-mediated synaptic plasticity. Based on the specific enzyme-catalyzed chromogenic reactions, three multifunctional colored hydrogels are developed, which can not only synergize with the Bi2S3 photogate to recognize the primary colors but also synergize with a given polymeric channel to promote the long-term memory of the system. A synaptic array is further constructed for sensing color images and biomolecule-coded information communication. Taking advantage of the versatile biochemistry, the biochemical-driven reversible photoelectric response of the cone cell is further mimicked. This work introduces rich chemical designs into retinomorphic devices, providing a perspective for replicating the human visual system in fluids.

Reconfiguration of organic electrochemical transistors for high-accuracy potentiometric sensing

Organic electrochemical transistors have emerged as a promising alternative to traditional 2/3 electrode setups for sensing applications, offering in-situ transduction, electrochemical amplification, and noise reduction. Several of these devices are designed to detect potentiometric-derived signals. However, potentiometric sensing should be performed under open circuit potential conditions, allowing the system to reach thermodynamic equilibrium. This criterion is not met by conventional organic electrochemical transistors, where voltages or currents are directly applied to the sensing interface, that is, the gate electrode. In this work, we introduce an organic electrochemical transistor sensing configuration called the potentiometric‑OECT (pOECT), which maintains the sensing electrode under open circuit potential conditions. The pOECT exhibits a higher response than the 2-electrode setup and offers greater accuracy, response, and stability compared to conventional organic electrochemical transistors. Additionally, it allows for the implementation of high-impedance electrodes as gate/sensing surfaces, all without compromising the overall device size.

Transient Response and Ionic Dynamics in Organic Electrochemical Transistors

The rapid development of organic electrochemical transistors (OECTs) has ushered in a new era in organic electronics, distinguishing itself through its application in a variety of domains, from high-speed logic circuits to sensitive biosensors, and neuromorphic devices like artificial synapses and organic electrochemical random-access memories. Despite recent strides in enhancing OECT performance, driven by the demand for superior transient response capabilities, a comprehensive understanding of the complex interplay between charge and ion transport, alongside electron-ion interactions, as well as the optimization strategies, remains elusive. This review aims to bridge this gap by providing a systematic overview on the fundamental working principles of OECT transient responses, emphasizing advancements in device physics and optimization approaches. We review the critical aspect of transient ion dynamics in both volatile and non-volatile applications, as well as the impact of materials, morphology, device structure strategies on optimizing transient responses. This paper not only offers a detailed overview of the current state of the art, but also identifies promising avenues for future research, aiming to drive future performance advancements in diversified applications.

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 Mixed Ionic-Electronic Conductors for Bioelectronic Sensors: Materials and Operation Mechanisms

The field of organic mixed ionic-electronic conductors (OMIECs) has gained significant attention due to their ability to transport both electrons and ions, making them promising candidates for various applications. Initially focused on inorganic materials, the exploration of mixed conduction has expanded to organic materials, especially polymers, owing to their advantages such as solution processability, flexibility, and property tunability. OMIECs, particularly in the form of polymers, possess both electronic and ionic transport functionalities. This review provides an overview of OMIECs in various aspects covering mechanisms of charge transport including electronic transport, ionic transport, and ionic-electronic coupling, as well as conducting/semiconducting conjugated polymers and their applications in organic bioelectronics, including (multi)sensors, neuromorphic devices, and electrochromic devices. OMIECs show promise in organic bioelectronics due to their compatibility with biological systems and the ability to modulate electronic conduction and ionic transport, resembling the principles of biological systems. Organic electrochemical transistors (OECTs) based on OMIECs offer significant potential for bioelectronic applications, responding to external stimuli through modulation of ionic transport. An in-depth review of recent research achievements in organic bioelectronic applications using OMIECs, categorized based on physical and chemical stimuli as well as neuromorphic devices and circuit applications, is presented.

SpyDirect: A Novel Biofunctionalization Method for High Stability and Longevity of Electronic Biosensors

Electronic immunosensors are indispensable tools for diagnostics, particularly in scenarios demanding immediate results. Conventionally, these sensors rely on the chemical immobilization of antibodies onto electrodes. However, globular proteins tend to adsorb and unfold on these surfaces. Therefore, self-assembled monolayers (SAMs) of thiolated alkyl molecules are commonly used for indirect gold-antibody coupling. Here, a limitation associated with SAMs is revealed, wherein they curtail the longevity of protein sensors, particularly when integrated into the state-of-the-art transducer of organic bioelectronics-the organic electrochemical transistor. The SpyDirect method is introduced, generating an ultrahigh-density array of oriented nanobody receptors stably linked to the gold electrode without any SAMs. It is accomplished by directly coupling cysteine-terminated and orientation-optimized spyTag peptides, onto which nanobody-spyCatcher fusion proteins are autocatalytically attached, yielding a dense and uniform biorecognition layer. The structure-guided design optimizes the conformation and packing of flexibly tethered nanobodies. This biolayer enhances shelf-life and reduces background noise in various complex media. SpyDirect functionalization is faster and easier than SAM-based methods and does not necessitate organic solvents, rendering the sensors eco-friendly, accessible, and amenable to scalability. SpyDirect represents a broadly applicable biofunctionalization method for enhancing the cost-effectiveness, sustainability, and longevity of electronic biosensors, all without compromising sensitivity.

Nanomaterials-based electrochemical biosensors for diagnosis of COVID-19

Since the outbreak of corona virus disease 2019 (COVID-19), this pandemic has caused severe death and infection worldwide. Owing to its strong infectivity, long incubation period, and nonspecific symptoms, the early diagnosis is essential to reduce risk of the severe illness. The electrochemical biosensor, as a fast and sensitive technique for quantitative analysis of body fluids, has been widely studied to diagnose different biomarkers caused at different infective stages of COVID-19 virus (SARS-CoV-2). Recently, many reports have proved that nanomaterials with special architectures and size effects can effectively promote the biosensing performance on the COVID-19 diagnosis, there are few comprehensive summary reports yet. Therefore, in this review, we will pay efforts on recent progress of advanced nanomaterials-facilitated electrochemical biosensors for the COVID-19 detections. The process of SARS-CoV-2 infection in humans will be briefly described, as well as summarizing the types of sensors that should be designed for different infection processes. Emphasis will be supplied to various functional nanomaterials which dominate the biosensing performance for comparison, expecting to provide a rational guidance on the material selection of biosensor construction for people. Finally, we will conclude the perspective on the design of superior nanomaterials-based biosensors facing the unknown virus in future.

Sensitivity-Enhancing Strategies of Graphene Field-Effect Transistor Biosensors for Biomarker Detection

The ultrasensitive recognition of biomarkers plays a crucial role in the precise diagnosis of diseases. Graphene-based field-effect transistors (GFET) are considered the most promising devices among the next generation of biosensors. GFET biosensors possess distinct advantages, including label-free, ease of integration and operation, and the ability to directly detect biomarkers in liquid environments. This review summarized recent advances in GFET biosensors for biomarker detection, with a focus on interface functionalization. Various sensitivity-enhancing strategies have been overviewed for GFET biosensors, from the perspective of optimizing graphene synthesis and transfer methods, refinement of surface functionalization strategies for the channel layer and gate electrode, design of biorecognition elements and reduction of nonspecific adsorption. Further, this review extensively explores GFET biosensors functionalized with antibodies, aptamers, and enzymes. It delves into sensitivity-enhancing strategies employed in the detection of biomarkers for various diseases (such as cancer, cardiovascular diseases, neurodegenerative disorders, infectious viruses, etc.) along with their application in integrated microfluidic systems. Finally, the issues and challenges in strategies for the modulation of biosensing interfaces are faced by GFET biosensors in detecting biomarkers.

Rapid and on-site wireless immunoassay of respiratory virus aerosols via hydrogel-modulated resonators

Rapid and accurate detection of respiratory virus aerosols is highlighted for virus surveillance and infection control. Here, we report a wireless immunoassay technology for fast (within 10 min), on-site (wireless and battery-free), and sensitive (limit of detection down to fg/L) detection of virus antigens in aerosols. The wireless immunoassay leverages the immuno-responsive hydrogel-modulated radio frequency resonant sensor to capture and amplify the recognition of virus antigen, and flexible readout network to transduce the immuno bindings into electrical signals. The wireless immunoassay achieves simultaneous detection of respiratory viruses such as severe acute respiratory syndrome coronavirus 2, influenza A H1N1 virus, and respiratory syncytial virus for community infection surveillance. Direct detection of unpretreated clinical samples further demonstrates high accuracy for diagnosis of respiratory virus infection. This work provides a sensitive and accurate immunoassay technology for on-site virus detection and disease diagnosis compatible with wearable integration.

A portable transistor immunosensor for fast identification of porcine epidemic diarrhea virus

Widespread distribution of porcine epidemic diarrhea virus (PEDV) has led to catastrophic losses to the global pig farming industry. As a result, there is an urgent need for rapid, sensitive and accurate tests for PEDV to enable timely and effective interventions. In the present study, we develop and validate a floating gate carbon nanotubes field-effect transistor (FG CNT-FET)-based portable immunosensor for rapid identification of PEDV in a sensitive and accurate manner. To improve the affinity, a unique PEDV spike protein-specific monoclonal antibody is prepared by purification, and subsequently modified on FG CNT-FET sensor to recognize PEDV. The developed FET biosensor enables highly sensitive detection (LoD: 8.1 fg/mL and 100.14 TCID50/mL for recombinant spike proteins and PEDV, respectively), as well as satisfactory specificity. Notably, an integrated portable platform consisting of a pluggable FG CNT-FET chip and a portable device can discriminate PEDV positive from negative samples and even identify PEDV and porcine deltacoronavirus within 1 min with 100% accuracy. The portable sensing platform offers the capability to quickly, sensitively and accurately identify PEDV, which further points to a possibility of point of care (POC) applications of large-scale surveillance in pig breeding facilities.

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.

An Organic Electrochemical Transistor-Based Sensor for IgG Levels Detection of Relevance in SARS-CoV-2 Infections

Organic electrochemical transistors appear as an alternative for relatively low-cost, easy-to-operate biosensors due to their intrinsic amplification. Herein, we present the fabrication, characterization, and validation of an immuno-detection system based on commercial sensors using gold electrodes where no additional surface treatment is performed on the gate electrode. The steady-state response of these sensors has been studied by analyzing different semiconductor organic channels in order to optimize the biomolecular detection process and its the application to monitoring human IgG levels due to SARS-CoV-2 infections. Detection levels of up to tens of μgmL-1 with sensitivities up to 13.75% [μg/mL]-1, concentration ranges of medical relevance in seroprevalence studies, have been achieved.

Coin-sized, fully integrated, and minimally invasive continuous glucose monitoring system based on organic electrochemical transistors

Continuous glucose monitoring systems (CGMs) are critical toward closed-loop diabetes management. The field's progress urges next-generation CGMs with enhanced antinoise ability, reliability, and wearability. Here, we propose a coin-sized, fully integrated, and wearable CGM, achieved by holistically synergizing state-of-the-art interdisciplinary technologies of biosensors, minimally invasive tools, and hydrogels. The proposed CGM consists of three major parts: (i) an emerging biochemical signal amplifier, the organic electrochemical transistor (OECT), improving the signal-to-noise ratio (SNR) beyond traditional electrochemical sensors; (ii) a microneedle array to facilitate subcutaneous glucose sampling with minimized pain; and (iii) a soft hydrogel to stabilize the skin-device interface. Compared to conventional CGMs, the OECT-CGM offers a high antinoise ability, tunable sensitivity and resolution, and comfort wearability, enabling personalized glucose sensing for future precision diabetes health care. Last, we discuss how OECT technology can help push the limit of detection of current wearable electrochemical biosensors, especially when operating in complicated conditions.

Extracellular vesicles-based point-of-care testing for the diagnosis and monitoring of Alzheimer's disease

Alzheimer's disease (AD) is a debilitating condition that affects millions of people worldwide. One promising strategy for detecting and monitoring AD early on is using extracellular vesicles (EVs)-based point-of-care testing; however, diagnosing AD using EVs poses a challenge due to the low abundance of EV-biomarkers. Here, we present a fully integrated organic electrochemical transistor (OECT) that enables high accuracy, speed, and convenience in the detection of EVs from AD patients. We incorporated self-aligned acoustoelectric enhancement of EVs on a chip that rapidly propels, enriches, and specifically binds EVs to the OECT detection area. With our enhancement of pre-concentration, we increased the sensitivity to a limit of detection of 500 EV particles/μL and reduced the required detection time to just two minutes. We also tested the sensor on an AD mouse model to monitor AD progression, examined mouse Aβ EVs at different time courses, and compared them with intraneuronal Aβ cumulation using MRI. This innovative technology has the potential to diagnose Alzheimer's and other neurodegenerative diseases accurately and quickly, enabling monitoring of disease progression and treatment response.

A hybrid transistor with transcriptionally controlled computation and plasticity

Organic electrochemical transistors (OECTs) are ideal devices for translating biological signals into electrical readouts and have applications in bioelectronics, biosensing, and neuromorphic computing. Despite their potential, developing programmable and modular methods for living systems to interface with OECTs has proven challenging. Here we describe hybrid OECTs containing the model electroactive bacterium Shewanella oneidensis that enable the transduction of biological computations to electrical responses. Specifically, we fabricated planar p-type OECTs and demonstrated that channel de-doping is driven by extracellular electron transfer (EET) from S. oneidensis. Leveraging this mechanistic understanding and our ability to control EET flux via transcriptional regulation, we used plasmid-based Boolean logic gates to translate biological computation into current changes within the OECT. Finally, we demonstrated EET-driven changes to OECT synaptic plasticity. This work enables fundamental EET studies and OECT-based biosensing and biocomputing systems with genetically controllable and modular design elements.

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.

Recent Development of Implantable Chemical Sensors Utilizing Flexible and Biodegradable Materials for Biomedical Applications

Implantable chemical sensors built with flexible and biodegradable materials exhibit immense potential for seamless integration with biological systems by matching the mechanical properties of soft tissues and eliminating device retraction procedures. Compared with conventional hospital-based blood tests, implantable chemical sensors have the capability to achieve real-time monitoring with high accuracy of important biomarkers such as metabolites, neurotransmitters, and proteins, offering valuable insights for clinical applications. These innovative sensors could provide essential information for preventive diagnosis and effective intervention. To date, despite extensive research on flexible and bioresorbable materials for implantable electronics, the development of chemical sensors has faced several challenges related to materials and device design, resulting in only a limited number of successful accomplishments. This review highlights recent advancements in implantable chemical sensors based on flexible and biodegradable materials, encompassing their sensing strategies, materials strategies, and geometric configurations. The following discussions focus on demonstrated detection of various objects including ions, small molecules, and a few examples of macromolecules using flexible and/or bioresorbable implantable chemical sensors. Finally, we will present current challenges and explore potential future directions.

Healthcare Monitoring Sensors Based on Organic Transistors: Surface/Interface Strategy and Performance

Organic transistors possess inherent advantages such as flexibility, biocompatibility, customizable chemical structures, solution-processability, and amplifying capabilities, making them highly promising for portable healthcare sensor applications. Through convenient and diverse modifications at the material and device surfaces or interfaces, organic transistors allow for a wide range of sensor applications spanning from chemical and biological to physical sensing. In this comprehensive review, the surface and interface engineering aspect associated with four types of typical healthcare sensors is focused. The device operation principles and sensing mechanisms are systematically analyzed and highlighted, and particularly surface/interface functionalization strategies that contribute to the enhancement of sensing performance are focused. An outlook and perspective on the critical issues and challenges in the field of healthcare sensing using organic transistors are provided as well.

Low-Cost Biosensor Technologies for Rapid Detection of COVID-19 and Future Pandemics

Many systems have been designed for the detection of SARS-CoV-2, which is the virus that causes COVID-19. SARS-CoV-2 is readily transmitted, resulting in the rapid spread of disease in human populations. Frequent testing at the point of care (POC) is a key aspect for controlling outbreaks caused by SARS-CoV-2 and other emerging pathogens, as the early identification of infected individuals can then be followed by appropriate measures of isolation or treatment, maximizing the chances of recovery and preventing infectious spread. Diagnostic tools used for high-frequency testing should be inexpensive, provide a rapid diagnostic response without sophisticated equipment, and be amenable to manufacturing on a large scale. The application of these devices should enable large-scale data collection, help control viral transmission, and prevent disease propagation. Here we review functional nanomaterial-based optical and electrochemical biosensors for accessible POC testing for COVID-19. These biosensors incorporate nanomaterials coupled with paper-based analytical devices and other inexpensive substrates, traditional lateral flow technology (antigen and antibody immunoassays), and innovative biosensing methods. We critically discuss the advantages and disadvantages of nanobiosensor-based approaches compared to widely used technologies such as PCR, ELISA, and LAMP. Moreover, we delineate the main technological, (bio)chemical, translational, and regulatory challenges associated with developing functional and reliable biosensors, which have prevented their translation into the clinic. Finally, we highlight how nanobiosensors, given their unique advantages over existing diagnostic tests, may help in future pandemics.

Portable and Rapid Dual-Biomarker Detection Using Solution-Gated Graphene Field Transistors in the Accurate Diagnosis of Prostate Cancer

Prostate-specific antigen (PSA) is the common serum-relevant biomarker for early prostate cancer (PCa) detection in clinical diagnosis. However, it is difficult to accurately diagnose PCa in the early stage due to the low specificity of PSA. Herein, a new solution-gated graphene field transistor (SGGT) biosensor with dual-gate for dual-biomarker detection is designed. The sensing mechanism is that the designed aptamers immobilized on the surface of the gate electrodes can capture PSA and sarcosine (SAR) biomolecules and induce the capacitance changes of the electric double layers of SGGT. The limit of detections of PSA and SAR biomarkers can reach 0.01 fg mL-1 , which is three-to-four orders of magnitude lower than previously reported assays. The detection time of PSA and SAR is ≈4.5 and ≈13 min, which is significantly faster than the detection time (1-2 h) of conventional methods. The clinical serum samples testing demonstrates that the biosensor can distinguish the PCa patients from the control group and the diagnosis accuracy can reach 100%. The SGGT biosensor can be integrated into the portable platform and the diagnostic results can directly display on the smartphone/Pad. Therefore, the integrated portable platform of the biosensor can distinguish cancer types through the dual-biomarker detection.

Biosensors based on potent miniprotein binder for sensitive testing of SARS-CoV‑2 variants of concern

The miniprotein binder TRI2-2 was employed as an antibody alternative to build a single antibody-coupled TRI2-2 based gold nanoparticle-based lateral flow immunoassay (AT-GLFIA) biosensor. The biosensor provides high specificity and affinity binding between TRI2-2 and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOCs) spike antigen receptor binding domain (S-RBD). It also enables rapid testing of wild-type (WT), B.1.1.7 (Alpha), B.1.351 (Beta), B.1.617.2 (Delta), P.1 (Gamma), and B.1.1.529 (Omicron) SARS-CoV-2 S-RBD and is at least ~ 16-fold more sensitive than conventional antibody pair-based GLFIA (AP-GLFIA). Besides, we developed a wireless micro-electrochemical assay (WMECA) biosensor based on the TRI2-2, which demonstrates an excellent VOCs testing capability at the pg mL-1 level. Overall, our results demonstrate that integrating miniprotein binders into conventional immunoassay systems is a promising design for improving the testing capabilities of such systems without hard-to-obtain antibody pair, complex reporter design, laborious signal amplification strategies, or specific instrumentation.

Interface-Engineered Field-Effect Transistor Electronic Devices for Biosensing

Promising advances in molecular medicine have promoted the urgent requirement for reliable and sensitive diagnostic tools. Electronic biosensing devices based on field-effect transistors (FETs) exhibit a wide range of benefits, including rapid and label-free detection, high sensitivity, easy operation, and capability of integration, possessing significant potential for application in disease screening and health monitoring. In this perspective, the tremendous efforts and achievements in the development of high-performance FET biosensors in the past decade are summarized, with emphasis on the interface engineering of FET-based electrical platforms for biomolecule identification. First, an overview of engineering strategies for interface modulation and recognition element design is discussed in detail. For a further step, the applications of FET-based electrical devices for in vitro detection and real-time monitoring in biological systems are comprehensively reviewed. Finally, the key opportunities and challenges of FET-based electronic devices in biosensing are discussed. It is anticipated that a comprehensive understanding of interface engineering strategies in FET biosensors will inspire additional techniques for developing highly sensitive, specific, and stable FET biosensors as well as emerging designs for next-generation biosensing electronics.

Duplex Electrochemical Microfluidic Sensor for COVID-19 Antibody Detection: Natural versus Vaccine-Induced Humoral Response

The rapid transmission and resilience of coronavirus disease 2019 (COVID-19) have led to urgent demands in monitoring humoral response for effective vaccine development, thus a multiplex co-detection platform to discriminate infection-induced from vaccine-induced antibodies is needed. Here a duplex electrochemical immunosensor for co-detection of anti-nucleocapsid IgG (N-IgG) and anti-spike IgG (S-IgG) is developed by using a two-working electrode system, via an indirect immunoassay, with antibody quantification obtained by differential pulse voltammetry. The screen-printed electrodes (SPEs) are modified by carbon black and electrodeposited gold nanoflowers for maximized surface areas, enabling the construction of an immunological chain for S-IgG and N-IgG electrochemical detection with enhanced performance. Using an optimized immunoassay protocol, a wide linear range between 30-750 and 20-1000 ng mL-1 , and a limit of detection of 28 and 15 ng mL-1 are achieved to detect N-IgG and S-IgG simultaneously in serum samples. This duplex immunosensor is then integrated in a microfluidic device to obtain significantly reduced detection time (≤ 7 min) while maintaining its analytical performance. The duplex microfluidic immunosensor can be easily expanded into multiplex format to achieve high throughput screening for the sero-surveillance of COVID-19  and other infectious diseases.

Wireless, battery-free, multifunctional integrated bioelectronics for respiratory pathogens monitoring and severity evaluation

The rapid diagnosis of respiratory virus infection through breath and blow remains challenging. Here we develop a wireless, battery-free, multifunctional pathogenic infection diagnosis system (PIDS) for diagnosing SARS-CoV-2 infection and symptom severity by blow and breath within 110 s and 350 s, respectively. The accuracies reach to 100% and 92% for evaluating the infection and symptom severity of 42 participants, respectively. PIDS realizes simultaneous gaseous sample collection, biomarker identification, abnormal physical signs recording and machine learning analysis. We transform PIDS into other miniaturized wearable or portable electronic platforms that may widen the diagnostic modes at home, outdoors and public places. Collectively, we demonstrate a general-purpose technology for rapidly diagnosing respiratory pathogenic infection by breath and blow, alleviating the technical bottleneck of saliva and nasopharyngeal secretions. PIDS may serve as a complementary diagnostic tool for other point-of-care techniques and guide the symptomatic treatment of viral infections.

Electrochemical Detection of Viral Nucleic Acids by DNA Nanolock-Based Porous Electrode Device

Developing rapid, sensitive, and facile nucleic acid detection technologies is of paramount importance for preventing and controlling infectious diseases. Benefiting from the advantages such as rapid response, low cost, and simple operation, electrochemical impedance spectroscopy holds great promise for point-of-care nucleic acid detection. However, the sensitivity of electrochemical impedance spectroscopy for low molecular weight nucleic acids testing is still limited. This work presents a DNA nanolock-based porous electrode to improve the sensitivity of electrochemical impedance spectroscopy. Once the target nucleic acids are recognized by the DNA probes, the pore-attached DNA nanolock caused remarkable impedance amplification by blocking the nanopores. Taking SARS-CoV-2 nucleic acid as a model analyte, the detection limit of the porous electrode was as low as 0.03 fM for both SARS-CoV-2 RNA and DNA. The integration of a porous electrode with a wireless communicating unit generates a portable detection device that could be applied to direct SARS-CoV-2 nucleic acid testing in saliva samples. The portable device could effectively distinguish the COVID-19 positive and negative samples, showing a sensitivity of 100% and a specificity of 93%. Owing to its rapid, ultrasensitive, specific, and portable features, the as-designed DNA nanolock and porous electrode-based portable device holds great promise as a point-of-care platform for real-time screening of COVID-19 and other epidemics.

Redefining serological diagnostics with immunoaffinity proteomics

Serological diagnostics is generally defined as the detection of specific human immunoglobulins developed against viral, bacterial, or parasitic diseases. Serological tests facilitate the detection of past infections, evaluate immune status, and provide prognostic information. Serological assays were traditionally implemented as indirect immunoassays, and their design has not changed for decades. The advantages of straightforward setup and manufacturing, analytical sensitivity and specificity, affordability, and high-throughput measurements were accompanied by limitations such as semi-quantitative measurements, lack of universal reference standards, potential cross-reactivity, and challenges with multiplexing the complete panel of human immunoglobulin isotypes and subclasses. Redesign of conventional serological tests to include multiplex quantification of immunoglobulin isotypes and subclasses, utilize universal reference standards, and minimize cross-reactivity and non-specific binding will facilitate the development of assays with higher diagnostic specificity. Improved serological assays with higher diagnostic specificity will enable screenings of asymptomatic populations and may provide earlier detection of infectious diseases, autoimmune disorders, and cancer. In this review, we present the major clinical needs for serological diagnostics, overview conventional immunoassay detection techniques, present the emerging immunoassay detection technologies, and discuss in detail the advantages and limitations of mass spectrometry and immunoaffinity proteomics for serological diagnostics. Finally, we explore the design of novel immunoaffinity-proteomic assays to evaluate cell-mediated immunity and advance the sequencing of clinically relevant immunoglobulins.

Highly Effective and Efficient Self-Assembled Multilayer-Based Electrode Passivation for Operationally Stable and Reproducible Electrolyte-Gated Transistor Biosensors

To ensure the operational stability of transistor-based biosensors in aqueous electrolytes during multiple measurements, effective electrode passivation is crucially important for reliable and reproducible device performances. This paper presents a highly effective and efficient electrode passivation method using a facile solution-processed self-assembled multilayer (SAML) with excellent insulation property to achieve operational stability and reproducibility of electrolyte-gated transistor (EGT) biosensors. The SAML is created by the consecutive self-assembly of three different molecular layers of 1,10-decanedithiol, vinyl-polyhedral oligomeric silsesquioxane, and 1-octadecanethiol. This passivation enables EGT to operate stably in phosphate-buffered saline (PBS) during repeated measurements over multiple cycles without short-circuiting. The SAML-passivated EGT biosensor is fabricated with a solution-processed In2O3 thin film as an amorphous oxide semiconductor working both as a semiconducting channel in the transistor and as a functionalizable biological interface for a bioreceptor. The SAML-passivated EGT including In2O3 thin film is demonstrated for the detection of Tau protein as a biomarker of Alzheimer's disease while employing a Tau-specific DNA aptamer as a bioreceptor and a PBS solution with a low ionic strength to diminish the charge-screening (Debye length) effect. The SAML-passivated EGT biosensor functionalized with the Tau-specific DNA aptamer exhibits ultrasensitive, quantitative, and reliable detection of Tau protein from 1 × 10-15 to 1 × 10-10 M, covering a much larger range than clinical needs, via changes in different transistor parameters. Therefore, the SAML-based passivation method can be effectively and efficiently utilized for operationally stable and reproducible transistor-based biosensors. Furthermore, this presented strategy can be extensively adapted for advanced biomedical devices and bioelectronics in aqueous or physiological environments.

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.

Intermolecular 3D-MoRSE Descriptors for Fast and Accurate Prediction of Electronic Couplings in Organic Semiconductors

The theoretical rational design of organic semiconductors faces an obstacle in that the performance of organic semiconductors depends very much on their stacking and local morphology (for example, phase domains), which involves numerous molecules. Simulation becomes computationally expensive as intermolecular electronic couplings have to be calculated from density functional theory. Therefore, developing fast and accurate methods for intermolecular electronic coupling estimation is essential. In this work, by developing a series of new intermolecular 3D descriptors, we achieved fast and accurate prediction of electronic couplings in both crystalline and amorphous thin films. Three groups of developed descriptors could perform faster and higher accuracy prediction on electronic couplings than the most advanced state-of-the-art descriptors. This work paves the way for large-scale simulations, high-throughput calculations, and screening of organic semiconductors.

Soft, flexible conductivity sensors for ocean salinity monitoring

Flexible electrochemical sensors that measure the concentrations of specific analytes (e.g., ions, molecules, and microorganisms) provide valuable information for medical diagnosis, personal health care, and environmental monitoring. However, the conductive electrodes of such sensors need to be exposed to the surrounding environments like chloride-containing aqueous solutions during their operation, where chloride ions (Cl-) can potentially cause corrosion and dissolution of the sensors, negatively impacting their performance and durability. In this work, we develop soft, flexible conductivity sensors made of gold (Au) electrodes and systematically study their electrochemical behaviors in sodium chloride (NaCl) solutions to prevent chloride-induced corrosion and enhance their sensitivity for marine environmental monitoring. The causes of gold chlorination reactions and polarization effects are identified and effectively prevented by analyzing the effects of direct current (DC) and alternating current (AC) voltages, AC frequencies, and exposed sensing areas of the conductivity (salinity) sensors. Accordingly, a performance diagram is constructed to provide guidance for the selection of operation parameters for the salinity sensor. We also convert the varying impedance values of salinity sensors at different salinity levels into output voltage signals using a voltage divider circuit with an AC voltage (0.6 V) source. The results offer an assessment of the accuracy and response time of the salinity sensors, as well as their potential for integration with data transmission components for real-time ocean monitoring. This study has important implications for the development of soft, flexible, Au-based electrochemical sensors that can operate efficiently in diverse biological fluids and marine environments.

Metallointercalated-DNA Nanotubes as Functional Light Antenna for Organic Photoelectrochemical Transistor Biosensor with Minimum Background

Organic photoelectrochemical transistor (OPECT) biosensor with a removed background is desired but remains challenging. So far, scientists still lack a solution to this issue. The light-matter interplay is expected to achieve an advanced OPECT with unknown possibilities. Here, we address this challenge by tailoring a unique heterogeneous light antenna as the functional gating module and its cascade interaction with a proper channel, which is exemplified by bioinduced [Ru(bpy)2dppz]2+-intercalated DNA nanotubes (NTs)/NiO heterojunction and its modulation against a diethylenetriamine-treated poly(ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) channel. Light stimulation of the antenna can generate the obvious cathodic photocurrent and, hence, modulate the channel, accomplishing OPECT with a minimal background and the hitherto highest current gain of 19 000. Linking with nucleic acid hybridization using microRNA-155 as the representative target, the device achieves sensitive biosensing down to 5.0 fM.

Scalable biomimetic sensing system with membrane receptor dual-monolayer probe and graphene transistor arrays

Affinity-based biosensing can enable point-of-care diagnostics and continuous health monitoring, which commonly follows bottom-up approaches and is inherently constrained by bioprobes' intrinsic properties, batch-to-batch consistency, and stability in biofluids. We present a biomimetic top-down platform to circumvent such difficulties by combining a "dual-monolayer" biorecognition construct with graphene-based field-effect-transistor arrays. The construct adopts redesigned water-soluble membrane receptors as specific sensing units, positioned by two-dimensional crystalline S-layer proteins as dense antifouling linkers guiding their orientations. Hundreds of transistors provide statistical significance from transduced signals. System feasibility was demonstrated with rSbpA-ZZ/CXCR4QTY-Fc combination. Nature-like specific interactions were achieved toward CXCL12 ligand and HIV coat glycoprotein in physiologically relevant concentrations, without notable sensitivity loss in 100% human serum. The construct is regeneratable by acidic buffer, allowing device reuse and functional tuning. The modular and generalizable architecture behaves similarly to natural systems but gives electrical outputs, which enables fabrication of multiplex sensors with tailored receptor panels for designated diagnostic purposes.