Ultimately Sensitive Organic Bioelectronic Transistor Sensors by Materials and Device Structure Design

Organic bioelectronic sensors are gaining momentum as they can combine high‐performance sensing level with flexible large‐area processable materials. This opens to potentially highly powerful sensing systems for point‐of‐care health monitoring and diagnostics at low cost. Prominent to detect biochemical recognition events, are electrolyte‐gated organic field‐effect transistors (EGOFETs) and organic electrochemical transistors (OECTs) as they are easily fabricated and operated. EGOFETs are recently shown to be capable of label‐free single‐molecule detections, even in serum. This progress report aims to provide a critical perspective through a selected overview of the literature on both EGOFET and OECT biosensors. Attention is paid to correctly attribute them to the potentiometric and amperometric biosensor categories, which is important to set the right conditions for quantification purposes. Moreover, to deepen the understanding of the sensing mechanisms, with the support of unpublished data, focus is put on two among the most critical aspects, namely, the capacitance interplay and the role of Faradaic currents. The final aim is to provide a rationale of the functional mechanisms encompassing both EGOFET and OECT sensors, to improve materials and devices' designs taking advantage of the processes that enhance the sensing response enabling the extremely high‐performance level resulting in ultimate sensitivity, selectivity, and fast response.     

Selective Determination of Dopamine on a Boron‐Doped Diamond Electrode Modified with Gold Nanoparticle/Polyelectrolyte‐coated Polystyrene Colloids

Negatively charged gold nanoparticles (AuNPs) and a polyelectrolyte (PE) have been assembled alternately on a polystyrene (PS) colloid by a layer‐by‐layer (LBL) self‐assembly technique to form three‐dimensional (Au/PAH)₄/(PSS/PAH)₄ multilayer‐coated PS spheres (Au/PE/PS multilayer spheres). The Au/PE/PS multilayer spheres have been used to modify a boron‐doped diamond (BDD) electrode. Cyclic voltammetry is utilized to investigate the properties of the modified electrode in a 1.0 M KCl solution that contains 5.0 × 10^?3 M K₃Fe(CN)₆, and the result shows a dramatically decreased redox activity compared with the bare BDD electrode. The electrochemical behaviors of dopamine (DA) and ascorbic acid (AA) on the bare and modified BDD electrode are studied. The cyclic voltammetric studies indicate that the negatively charged, three‐dimensional Au/PE/PS multilayer sphere‐modified electrodes show high electrocatalytic activity and promote the oxidation of DA, whereas they inhibit the electrochemical reaction of AA, and can effectively be used to determine DA in the presence of AA with good selectivity. The detection limit of DA is 0.8 × 10^?6 M in a linear range from 5 × 10^?6 to 100 × 10^?6 M in the presence of 1 × 10^?3 M AA.     

Biosensing Properties of TitanateNanotube Films: Selective Detection of Dopamine in the Presence of Ascorbate and Uric Acid

A novel strategy based on titanate nanotubes (TNTs) for developing an electrochemical biosensor is proposed. Stable TNT films are fabricated on glassy carbon (GC) electrodes by a casting technique. Cyclic voltammetry, electrochemical impedance spectrometry, and linear‐sweep voltammetry are used to characterize the TNT membrane‐covered GC electrodes (TNT/GCs). The TNT film is shown to demonstrate selectivity by charge exclusion. The TNT film is also shown to be capable of improving the mass transport to the electrode surface and electron transfer between dopamine (DA) and the electrode. Therefore, DA exhibits a quasireversible electrochemical reaction at the TNT/GC electrode. The voltammetric signal of DA is well resolved from those of ascorbate (AA) and uric acid (UA) at the TNT/GC electrode; therefore, DA can be selectively detected in the presence of a large excess of AA and UA at physiological pH. The linear calibration curve for DA is obtained over the concentration range 0.1–30 μM in a physiological solution that contains 0.1 mM AA and 0.3 mM UA.     

Poisson Ratio and Piezoresistive Sensing: A New Route to High‐Performance 3D Flexible and Stretchable Sensors of Multimodal Sensing Capability

The performance of flexible and stretchable sensors relies on the optimization of both the flexible substrate and the sensing element, and their synergistic interactions. Herein, a novel strategy is reported for cost‐effective and scalable manufacturing of a new class of porous materials as 3D flexible and stretchable piezoresistive sensors, by assembling carbon nanotubes onto porous substrates of tunable Poisson ratios. It is shown that the piezoresistive sensitivity of the sensors increases as the substrate's Poisson's ratio decreases. Substrates with negative Poisson ratios (auxetic foams) exhibit significantly higher piezoresistive sensitivity, resulting from the coherent mode of deformation of the auxetic foam and enhanced changes of tunneling resistance of the carbon nanotube networks. Compared with conventional foam sensors, the auxetic foam sensor (AFS) with a Poisson's ratio of –0.5 demonstrates a 300% improvement in piezoresistive sensitivity and the gauge factor increases as much as 500%. The AFS has high sensing capability, is extremely robust, and capable of multimodal sensing, such as large deformation sensing, pressure sensing, shear/torsion sensing, and underwater sensing. AFS shows great potential for a broad range of wearable and portable devices applications, which are described by reporting on a series of demonstrations.     

Highly Sensitive Glucose Biosensors Based on Organic Electrochemical Transistors Using Platinum Gate Electrodes Modified with Enzyme and Nanomaterials

Organic electrochemical transistors with glucose oxidase‐modified Pt gate electrodes are successfully used as highly sensitive glucose sensors. The gate electrodes are modified with nanomaterials (multi‐wall carbon nanotubes or Pt nanoparticles) for the first time, which results in a dramatic improvement in the sensitivity of the devices. The detection limit of the device modified with Pt nanoparticles on the gate electrode is about 5 nM, which is three orders of magnitude better than a device without the nanoparticles. The improvement of the device performance can be attributed to the excellent electrocatalytic properties of the nanomaterials and more effective immobilization of enzyme on the gate electrodes. Based on the same principle, many other types of enzyme sensors with high sensitivity and low cost are expected to be realized by modifying the gate electrodes of organic electrochemical transistors with specific enzymes and nanomaterials.     

Stretchable,Skin‐Mountable,and Wearable Strain Sensors and Their Potential Applications: A Review

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin‐mountable, and wearable strain sensors are needed for several potential applications including personalized health‐monitoring, human motion detection, human‐machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin‐mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.     

High‐Performance Dopamine Sensors Based on Whole‐Graphene Solution‐Gated Transistors

Solution‐gated graphene transistors with graphene as both channel and gate electrodes are fabricated for the first time and used as dopamine sensors with the detection limit down to 1 nM, which is three orders of magnitude better than that of conventional electrochemical measurements. The sensing mechanism is attributed to the change of effective gate voltage applied on the transistors induced by the electro‐oxidation of dopamine at the graphene gate electrodes. The interference from glucose, uric acid, and ascorbic acid on the dopamine sensor is characterized. The selectivity of the dopamine sensor is dramatically improved by modifying the gate electrode with a thin Nafion film by solution process. This work paves the way for developing many other biosensors based on the solution‐gated graphene transistors by specifically functionalizing the gate electrodes. Because the devices are mainly made of graphene, they are potentially low cost and ideal for high‐density integration as multifunctional sensor arrays.     

A Mediator‐Free Electroenzymatic Sensing Methodology to Mitigate Ionic and Electroactive Interferents' Effects for Reliable Wearable Metabolite and Nutrient Monitoring

Wearable electroenzymatic sensors enable monitoring of clinically informative biomolecules in epidermally retrievable biofluids. Conventional wearable enzymatic sensors utilize Prussian Blue (a redox mediator) to achieve selectivity against electroactive interferents. However, the use of Prussian Blue presents fundamental challenges including: 1) the susceptibility of the sensor response to dynamic concentration variation of ionic species and 2) the poor operational stability due to the degradation of its framework. As an alternative wearable electroenzymatic sensor development methodology to bypass the aforementioned limitations, a mediator‐free sensing interface is devised, comprising of a coupled platinum nanoparticle/multiwall carbon nanotube layer and a permselective membrane. The interface is adapted to develop sensors targeting glucose, lactate, and choline (as examples of informative metabolites and nutrients), showing high degrees of sensitivity, selectivity (against a wide panel of naturally present and diverse interfering species), stability (<6.5% signal drift over 20 h operation), and reliability of sensing operation in sweat samples. By integration within a readout board, a wireless sample‐to‐answer system is realized for on‐body sweat biomarker analysis. This methodology can be adapted to target a wide panel of biomarkers in various biofluids, introducing a new sensor development direction for personal health monitoring.     

An Organic Electrochemical Transistor Integrated Photodetector for High Quality Photoplethysmogram Signal Acquisition

The organic photodiode (OPD) is a promising building block for solution-processable, flexible, lightweight, and miniaturized photodetectors, ideal for wearable applications. Despite the advances in materials used in OPDs, their photocurrent and light responsivity are limited, and alternative methods are required to boost the signal response. Herein, a miniaturized organic electrochemical transistor (OECT) is integrated with an OPD module to unlock the potential of OPDs to acquire physiological signals. In this integrated photodetector (IPD) system, the light intensity regulates the OPD voltage output that modulates the OECT channel current. The high transconductance of the OECT provides efficient voltage-to-current conversion, enhancing the signal-to-noise ratio on the sensing site. A microscale, p-type enhancement-mode OECT with high g_m and fast switching speed performs better in this application than depletion-mode OECT of the same geometry. The IPD achieves a photocurrent and responsivity 318 and 140 times higher than the standalone OPD, respectively. It is shown that with the IPD, the amplitude of the photoplethysmogram signals detected by the OPD is enhanced by a factor of 2.9 × 10³, highlighting its potential as a wearable biosensor and to detect weak, often uncaptured, light-based signals from living systems.     

Highly Aligned,Anisotropic Carbon Nanofiber Films for Multidirectional Strain Sensors with Exceptional Selectivity

Realization of sensing multidirectional strains is essential to understanding the nature of complex motions. Traditional uniaxial strain sensors lack the capability to detect motions working in different directions, limiting their applications in unconventional sensing technology areas, like sophisticated human–machine interface and real‐time monitoring of dynamic body movements. Herein, a stretchable multidirectional strain sensor is developed using highly aligned, anisotropic carbon nanofiber (ACNF) films via a facile, low‐cost, and scalable electrospinning approach. The fabricated strain sensor exhibits semitransparency, good stretchability of over 30%, outstanding durability for over 2500 cycles, and remarkable anisotropic strain sensing performance with maximum gauge factors of 180 and 0.3 for loads applied parallel and perpendicular to fiber alignment, respectively. Cross‐plied ACNF strain sensors are fabricated by orthogonally stacking two single‐layer ACNFs, which present a unique capability to distinguish the directions and magnitudes of strains with a remarkable selectivity of 3.84, highest among all stretchable multidirectional strain sensors reported so far. Their unconventional applications are demonstrated by detecting multi‐degrees‐of‐freedom synovial joint movements of the human body and monitoring wrist movements for systematic improvement of golf performance. The potential applications of novel multidirectional sensors reported here may shed new light into future development of next‐generation soft, flexible electronics.     

Dye Modification of Nanofibrous Silicon Oxide Membranes for Colorimetric HCl and NH3 Sensing

Colorimetric sensors for monitoring and visual reporting of acidic environments both in water and air are highly valuable in various fields, such as safety and technical textiles. Until now sol‐gel‐based colorimetric sensors are usually nonflexible bulk glass or thin‐film sensors. Large‐area, flexible sensors usable in strong acidic environments are not available. Therefore, in this study organically modified silicon oxide nanofibrous membranes are produced by combining electrospinning and sol‐gel technology. Two pH‐indicator dyes are immobilized in the nanofibrous membranes: methyl yellow via doping, methyl red via both doping, and covalent bonding. This resulted in sensor materials with a fast response time and high sensitivity for pH‐change in water. The covalent bond between dye and the sol‐gel network showed to be essential to obtain a reusable pH‐sensor in aqueous environment. Also a high sensitivity is obtained for sensing of HCl and NH₃ vapors, including a memory function allowing visual read‐out up to 20 min after exposure. These fast and reversible, large‐area flexible nanofibrous colorimetric sensors are highly interesting for use in multiple applications such as protective clothing and equipment. Moreover, the sensitivity to biogenic amines is demonstrated, offering potential for control and monitoring of food quality.     

A Flexible and Biomimetic Olfactory Synapse with Gasotransmitter-Mediated Plasticity (Adv. Funct. Mater. 18/2023)

Neuromorphic electronics has demonstrated great promise in mimicking the sensory and memory functions of biological systems. However, synaptic devices with desirable sensitivity, selectivity, and operational voltage imitating the olfactory system have rarely been reported. Here, a flexible and biomimetic olfactory synapse based on an organic electrochemical transistor (OECT) coupled with a breath-figure derived porous solid polymer electrolyte (SPE) is proposed. The device demonstrates excellent sensitivity with a ppb-level response limit and desirable selectivity toward hydrogen sulfide (H₂S) over other gases, and successfully achieves wireless real-time detection of excessive concentration of H₂S from rotten eggs. H₂S-mediated synaptic plasticity is accomplished with the device and typical synaptic behaviors are realized, including short-term memory (STM), long-term memory (LTM), transition from STM to LTM, etc., enabling the imitation of potential cumulative damages upon H₂S exposure. The proposed device paves new ways toward next-generation olfactory systems capable of sensing and memorizing functionalities mimicking neurobiological systems, offering critical materials strategies to accomplish intelligent artificial sensory systems.     

Light‐Boosting Highly Sensitive Pressure Sensors Based on Bioinspired Multiscale Surface Structures

Pressure sensors have attracted tremendous attention because of their potential applications in the fields of health monitoring, human–machine interfaces, artificial intelligence, and so on. Improving pressure‐sensing performances, especially the sensitivity and the detection limit, is of great importance to expand the related applications, however it is still an enormous challenge so far. Herein, highly sensitive piezoresistive pressure sensors are reported with novel light‐boosting sensing performances. Rose petal–templated positive multiscale millimeter/micro/nanostructures combined with surface wrinkling nanopatterns endow the assembled pressure sensors with outstanding pressure sensing performance, e.g. an ultrahigh sensitivity (70 KPa^?1, <0.5 KPa), an ultralow detection limit (0.88 Pa), a wide pressure detect ion range (from 0.88 Pa to 32 KPa), and a fast response time (30 ms). Remarkably, simple light illumination further enhances the sensitivity to 120 KPa^?1 (<0.5 KPa) and lowers the detection limit to 0.41 Pa. Furthermore, the flexible light illumination offers unprecedented capabilities to spatiotemporally control any target in multiplexed pressure sensors for optically enhanced/tailorable sensing performances. This light‐control strategy coupled with the introduction of bioinspired multiscale structures is expected to help design next generation advanced wearable electronic devices for unprecedented smart applications.     

Syntheses,Properties and Electrochemical Activity of Carbon Microtubes Modified with Amino Groups

Amine‐modified carbon micro/sub‐microtubes that encapsulate magnetite cores have been synthesized by decomposing a ferrocene/hexabromobenzene mixture in the presence of ammonia under solvothermal conditions at 250 °C for 24 h in a one‐step process. The as‐prepared carbon microstructures (NH₂‐ME‐CMTs) were 90–2000 nm in diameter and from tens to hundreds of micrometers in length that could be tuned in various solvents. The surface of the carbon microtubes can be modified with amino groups by the synthetic process, as confirmed by infra‐red (IR) spectroscopy and X‐ray photoelectron spectroscopy (XPS). Ammonia in the reaction system plays a key role in the formation of the microtube morphology and was the source of the surface functionalization groups. Fluorescent fluorescein isothiocyanate (FITC) was selected as a model compound and successively attached to the amino groups of the carbon microtubes. This result confirms the reactivity of the amino groups on the surface of the carbon microtubes. The inner magnetite cores were removed after immersion in 1 M HCl solution at room temperature over two months, and hollow carbon microtubes (NH₂‐H‐CMTs) were obtained. The magnetite‐encapsulated carbon microtubes and the hollow carbon microtubes were coated on gold electrodes to prepare carbon microtube‐modified gold electrodes. The two electrodes have been used to investigate the oxidative properties of dopamine (DA) and ascorbic acid (AA). Different from the magnetite‐encapsulated microtube‐modified electrode, the hollow microtube‐modified electrode can be utilized in the selective detection of DA in the presence of a large excess of AA. The electrochemical behaviour of DA and AA on the hollow carbon microtubes modified with amino groups is similar to that on carbon nanotubes. This result suggests that the one‐step synthesis method will not change the electrochemical properties or break the backbone structure of the carbon microtubes.     

Carbonized Silk Nanofiber Membrane for Transparent and Sensitive Electronic Skin

Recent years have witnessed the explosive development of electronic skin. Highly sensitive pressure sensing is one of the primary abilities of electronic skin. To date, most of the reported skin‐like pressure sensors are based on nanomaterials and microstructured polydimethylsiloxane (PDMS) films, limiting their wide practical applications due to the unknown biotoxicity and the redundant fabrication procedure. A cost‐effective, large‐area‐capable, and biocompatible approach for fabrication of high‐performance skin‐like pressure sensors is highly desired. Silk fibroin (SF) is a natural protein that has recently drawn great attention due to its application as the substrate for flexible electronics. Here, the fabrication of skin‐like pressure sensors is demonstrated using SF‐derived active materials. Flexible and conformal pressure sensors can be fabricated using transparent carbonized silk nanofiber membranes (CSilkNM) and unstructured PDMS films through a cost‐effective and large‐scale capable approach. Due to the unique N‐doped carbon nanofiber network structure of CSilkNM, the obtained pressure sensor shows superior performance, including ultrahigh sensitivity (34.47 kPa^?1) for a broad pressure range, an ultralow detection limit (0.8 Pa), rapid response time (<16.7 ms), and high durability (>10 000 cycles). Based on its superior performance, its applications in monitoring human physiological signals, sensing subtle touch, and detecting spatial distribution of pressure are demonstrated.     

Recent Advances in Sensing Applications of Graphene Assemblies and Their Composites

Development of next‐generation sensor devices is gaining tremendous attention in both academia and industry because of their broad applications in manufacturing processes, food and environment control, medicine, disease diagnostics, security and defense, aerospace, and so forth. Current challenges include the development of low‐cost, ultrahigh, and user‐friendly sensors, which have high selectivity, fast response and recovery times, and small dimensions. The critical demands of these new sensors are typically associated with advanced nanoscale sensing materials. Among them, graphene and its derivatives have demonstrated the ideal properties to overcome these challenges and have merged as one of the most popular sensing platforms for diverse applications. A broad range of graphene assemblies with different architectures, morphologies, and scales (from nano‐, micro‐, to macrosize) have been explored in recent years for designing new high‐performing sensing devices. Herein, this study presents and discusses recent advances in synthesis strategies of assembled graphene‐based superstructures of 1D, 2D, and 3D macroscopic shapes in the forms of fibers, thin films, and foams/aerogels. The fabricated state‐of‐the‐art applications of these materials in gas and vapor, biomedical, piezoresistive strain and pressure, heavy metal ion, and temperature sensors are also systematically reviewed and discussed, and their sensing performance is compared.     

Inkjet Printing of a Highly Loaded Palladium Ink for Integrated,Low‐Cost pH Sensors

An inkjet printing process for depositing palladium (Pd) thin films from a highly loaded ink (>14 wt%) is reported. The viscosity and surface tension of a Pd‐organic precursor solution is adjusted using toluene to form a printable and stable ink. A two‐step thermolysis process is developed to convert the printed ink to continuous and uniform Pd films with good adhesion to different substrates. Using only one printing pass, a low electrical resistivity of 2.6 μΩ m of the Pd film is obtained. To demonstrate the electrochemical pH sensing application, the surfaces of the printed Pd films are oxidized for ion‐to‐electron transduction and the underlying layer is left for electron conduction. Then, solid‐state reference electrodes are integrated beside the bifunctional Pd electrodes by inkjet printing. These potentiometric sensors have sensitivities of 60.6 ± 0.1 and 57 ± 0.6 mV pH^?1 on glass and polyimide substrates, and short response times of 11 and 6 s, respectively. Also, accurate pH values of real water samples are obtained by using the printed sensors with a low‐cost multimeter. These results indicate that the facile and cost‐effective inkjet printing and integration techniques may be applied in fabricating future electrochemical monitoring systems for environmental parameters and human health conditions.     

Porous Silicon‐Based Optical Microsensors for Volatile Organic Analytes: Effect of Surface Chemistry on Stability and Specificity

Sensing of the volatile organic compounds (VOCs) isopropyl alcohol (IPA) and heptane in air using sub‐millimeter porous silicon‐based sensor elements is demonstrated in the concentration range 50–800 ppm. The sensor elements are prepared as one‐dimensional photonic crystals (rugate filters) by programmed electrochemical etch of p⁺⁺ silicon, and analyte sensing is achieved by measurement of the wavelength shift of the photonic resonance. The sensors are studied as a function of surface chemistry: ozone oxidation, thermal oxidation, hydrosilylation (1‐dodecene), electrochemical methylation, reaction with dicholorodimethylsilane and thermal carbonization with acetylene. The thermally oxidized and the dichlorodimethylsilane‐modified materials show the greatest stability under atmospheric conditions. Optical microsensors are prepared by attachment of the porous Si layer to the distal end of optical fibers. The acetylated porous Si microsensor displays a greater response to heptane than to IPA, whereas the other chemical modifications display a greater response to IPA than to heptane. The thermal oxide sensor displays a strong response to water vapor, while the acetylated material shows a relatively weak response. The results suggest that a combination of optical fiber sensors with different surface chemistries can be used to classify VOC analytes. Application of the miniature sensors to the detection of VOC breakthrough in a full‐scale activated carbon respirator cartridge simulator is demonstrated.     

Large‐Area Compliant,Low‐Cost,and Versatile Pressure‐Sensing Platform Based on Microcrack‐Designed Carbon Black@Polyurethane Sponge for Human–Machine Interfacing

It is a challenge to manufacture pressure‐sensing materials that possess flexibility, high sensitivity, large‐area compliance, and capability to detect both tiny and large motions for the development of artificial intelligence products. Herein, a very simple and low‐cost approach is proposed to fabricate versatile pressure sensors based on microcrack‐designed carbon black (CB)@polyurethane (PU) sponges via natural polymer‐mediated water‐based layer‐by‐layer assembly. These sensors are capable of satisfying the requirements of ultrasmall as well as large motion monitoring. The versatility of these sensors benefits from two aspects: microcrack junction sensing mechanism for tiny motion detecting (91 Pa pressure, 0.2% strain) inspired by the spider sensory system and compressive contact of CB@PU conductive backbones for large motion monitoring (16.4 kPa pressure, 60% strain). Furthermore, these sensors exhibit excellent flexibility, fast response times (<20 ms), as well as good reproducibility over 50 000 cycles. This study also demonstrates the versatility of these sensors for various applications, ranging from speech recognition, health monitoring, bodily motion detection to artificial electronic skin. The desirable comprehensive performance of our sensors, which is comparable to the recently reported pressure‐sensing devices, together with their significant advantages of low‐cost, easy fabrication, especially versatility, makes them attractive in the future of artificial intelligence.