Ultrafast Dynamics, Optical Nonlinearities, and Chemical Sensing Application of Free-Standing Porous Silicon-Based Optical Microcavities

The present work demonstrates the ultrafast carrier dynamics and third-order nonlinear optical properties of electrochemically fabricated free-standing porous silicon (FS-PSi)-based optical microcavities via femtosecond transient absorption spectroscopy (TAS) and single-beam Z-scan techniques, respectively. The TAS (pump: 400 nm, probe: 430-780 nm, ∼70 fs, 1 kHz) decay dynamics are dominated by the photoinduced absorption (PIA, lifetime range: 4.7-156 ps) as well as photoinduced bleaching (PIB, 4.3-324 ps) for the cavity mode (λc) and the band edges. A fascinating switching behavior from the PIB (-ve) to the PIA (+ve) has been observed in the cavity mode, which shows the potential in ultrafast switching applications. The third-order optical nonlinearities revealed an enhanced two-photon absorption coefficient (β) in the order of 10-10 mW-1 along with the nonlinear refractive index (n2) in the range of 10-17 m2 W-1. Furthermore, a real-time sensing application of such FS-PSi microcavities has been demonstrated for detecting organic solvents by simultaneously monitoring the kinetics in reflection and transmission mode.

Estimation of Environmental Effects and Response Time in Gas-Phase Explosives Detection Using Photoluminescence Quenching Method

Detecting the presence of explosives is important to protect human lives during military conflicts and peacetime. Gas-phase detection of explosives can make use of the change of material properties, which can be sensitive to environmental conditions such as temperature and humidity. This paper describes a remote-controlled automatic shutter method for the environmental impact assessment of photoluminescence (PL) sensors under near-open conditions. Utilizing the remote-sensing method, we obtained environmental effects without being exposed to sensing vapor molecules and explained how PL intensity was influenced by the temperature, humidity, and exposure time. We also developed a theoretical model including the effect of exciton diffusion for PL quenching, which worked well under limited molecular diffusions. Incomplete recovery of PL intensity or the degradation effect was considered as an additional factor in the model.

Hydrogels for Chemical Sensing and Biosensing

The development and synthesis of hydrogels for chemical and biosensing are of great value. Hydrogels can be tailored to its own physical structure, chemical properties, biocompatibility, and sensitivity to external stimuli when being used in a specific environment. Herein, hydrogels and their applications in chemical and biosensing are mainly covered. In particular, it is focused on the manner in which hydrogels serve as sensing materials to a specific analyte. Different types of responsive hydrogels are hence introduced and summarized. Researchers can modify different chemical groups on the skeleton of the hydrogels, which make them as good chemical and biosensing materials. Hydrogels have great application potential for chemical and biosensing in the biomedical field and some emerging fields, such as wearable devices.

Green IoT Event Detection for Carbon-Emission Monitoring in Sensor Networks

This research addresses the intersection of low-power microcontroller technology and binary classification of events in the context of carbon-emission reduction. The study introduces an innovative approach leveraging microcontrollers for real-time event detection in a homogeneous hardware/firmware manner and faced with limited resources. This showcases their efficiency in processing sensor data and reducing power consumption without the need for extensive training sets. Two case studies focusing on landfill CO2 emissions and home energy usage demonstrate the feasibility and effectiveness of this approach. The findings highlight significant power savings achieved by minimizing data transmission during non-event periods (94.8-99.8%), in addition to presenting a sustainable alternative to traditional resource-intensive AI/ML platforms that comparatively draw and produce 20,000 times the amount of power and carbon emissions, respectively.

Developments in food neonicotinoids detection: novel recognition strategies, advanced chemical sensing techniques, and recent applications

Neonicotinoid insecticides (NEOs) are a new class of neurotoxic pesticides primarily used for pest control on fruits and vegetables, cereals, and other crops after organophosphorus pesticides (OPPs), carbamate pesticides (CBPs), and pyrethroid pesticides. However, chronic abuse and illegal use have led to the contamination of food and water sources as well as damage to ecological and environmental systems. Long-term exposure to NEOs may pose potential risks to animals (especially bees) and even human health. Consequently, it is necessary to develop effective, robust, and rapid methods for NEOs detection. Specific recognition-based chemical sensing has been regarded as one of the most promising detection tools for NEOs due to their excellent selectivity, sensitivity, and robust interference resistance. In this review, we introduce the novel recognition strategies-enabled chemical sensing in food neonicotinoids detection in the past years (2017-2023). The properties and advantages of molecular imprinting recognition (MIR), host-guest recognition (HGR), electron-catalyzed recognition (ECR), immune recognition (IR), aptamer recognition (AR), and enzyme inhibition recognition (EIR) in the development of NEOs sensing platforms are discussed in detail. Recent applications of chemical sensing platforms in various food products, including fruits and vegetables, cereals, teas, honey, aquatic products, and others are highlighted. In addition, the future trends of applying chemical sensing with specific recognition strategies for NEOs analysis are discussed.

Smart Stimulation Response of a Pyrene-Based Lanthanide(III) MOF: Fluorescence Enhancement to HX (F and Cl) or R-COOH and Artificial Applicable Film on HCl Vapor Sensing

Toxic acids produced by industries are major hazards to the environment and human health, and luminescent pyrene-based crystalline metal-organic frameworks (MOFs) demonstrate promising performance in the detection of toxic acids. Herein, two novel isostructural 3D porous lanthanide MOFs, H3O·[Ln3(TBAPy)2(μ2-H2O)2(OH)2]·2DMA·2Diox·6.5H2O (Ln = Pr (1) and Ce (2); H4TBAPy (1,3,6,8-tetrakis(p-benzoic acid)pyrene); and DMA: N,N-dimethylacetamide) were synthesized, which showed alb topology. Based on the protonation and hydrogen bond mechanism, complex 1 could be used as a fluorescence recognition sensor for HX (X = F, Cl, Br, and I) acid solutions with different luminescence behaviors. It is worth noting that complex 1 exhibited high sensitivity in the fluorescence enhancement sensing of hydrofluoric acid, oxalic acid, and trichloroacetic acid. In particular, complex 1 had a low limit of detection (LOD) for OA (0.1 μM) and was applied to real monitoring of orange fruit samples. In addition, the PVA@1 film could selectively, sensitively, and quantitatively respond to hydrochloric acid (HCl) vapor through fluorescent quenching; due to its protonation and adsorption capacity, the LOD was 0.18 ppm. Therefore, the portable optical device, the PVA@1 film, can detect HCl gas in trace amounts, achieving the ultimate goal of real-time and rapid detection, which has potential application value for industrial production safety.

Synthesis of TiO2-(B) Nanobelts for Acetone Sensing

Titanium dioxide nanobelts were prepared via the alkali-hydrothermal method for application in chemical gas sensing. The formation process of TiO2-(B) nanobelts and their sensing properties were investigated in detail. FE-SEM was used to study the surface of the obtained structures. The TEM and XRD analyses show that the prepared TiO2 nanobelts are in the monoclinic phase. Furthermore, TEM shows the formation of porous-like morphology due to crystal defects in the TiO2-(B) nanobelts. The gas-sensing performance of the structure toward various concentrations of hydrogen, ethanol, acetone, nitrogen dioxide, and methane gases was studied at a temperature range between 100 and 500 °C. The fabricated sensor shows a high response toward acetone at a relatively low working temperature (150 °C), which is important for the development of low-power-consumption functional devices. Moreover, the obtained results indicate that monoclinic TiO2-B is a promising material for applications in chemo-resistive gas detectors.

Transitioning from Supramolecular Chemistry to Molecularly Imprinted Polymers in Chemical Sensing

This perspective article focuses on the overwhelming significance of molecular recognition in biological processes and its emulation in synthetic molecules and polymers for chemical sensing. The historical journey, from early investigations into enzyme catalysis and antibody-antigen interactions to Nobel Prize-winning breakthroughs in supramolecular chemistry, emphasizes the development of tailored molecular recognition materials. The discovery of supramolecular chemistry and molecular imprinting, as a versatile method for mimicking biological recognition, is discussed. The ability of supramolecular structures to develop selective host-guest interactions and the flexible design of molecularly imprinted polymers (MIPs) are highlighted, discussing their applications in chemical sensing. MIPs, mimicking the selectivity of natural receptors, offer advantages like rapid synthesis and cost-effectiveness. Finally, addressing major challenges in the field, this article summarizes the advancement of molecular recognition-based systems for chemical sensing and their transformative potential.

Recent Advancements in Metal and Non-Metal Mixed-Doped Carbon Quantum Dots: Synthesis and Emerging Potential Applications

In nanotechnology, the synthesis of carbon quantum dots (CQDs) by mixed doping with metals and non-metals has emerged as an appealing path of investigation. This review offers comprehensive insights into the synthesis, properties, and emerging applications of mixed-doped CQDs, underlining their potential for revolutionary advancements in chemical sensing, biosensing, bioimaging, and, thereby, contributing to advancements in diagnostics, therapeutics, and the under standing of complex biological processes. This synergistic combination enhances their sensitivity and selectivity towards specific chemical analytes. The resulting CQDs exhibit remarkable fluorescence properties that can be involved in precise chemical sensing applications. These metal-modified CQDs show their ability in the selective and sensitive detection from Hg to Fe and Mn ions. By influencing their exceptional fluorescence properties, they enable precise detection and monitoring of biomolecules, such as uric acid, cholesterol, and many antibiotics. Moreover, when it comes to bioimaging, these doped CQDs show unique behavior towards detecting cell lines. Their ability to emit light across a wide spectrum enables high-resolution imaging with minimal background noise. We uncover their potential in visualizing different cancer cell lines, offering valuable insights into cancer research and diagnostics. In conclusion, the synthesis of mixed-doped CQDs opens the way for revolutionary advancements in chemical sensing, biosensing, and bioimaging. As we investigate deeper into this field, we unlock new possibilities for diagnostics, therapeutics, and understanding complex biological processes.

Lanthanide Functionalized Covalent Organic Frameworks Hybrid Materials for Luminescence Responsive Chemical Sensing

Covalent organic frameworks (COFs) possess several unique features of structural and functional chemistry, together with other modular photophysical performance, which make them candidates for luminescence responsive chemical sensing. Lanthanide (Ln3+ ) functionalized COFs hybrid materials still keep the parent COFs' virtues and also embody the abundant multiple luminescence response with both COFs and Ln3+ ions or other guest species. In this review, the summary is highlighted on the lanthanide functionalized COFs hybrid materials and their relevant systems for luminescence responsive chemical sensing. It is subdivided into five sections involving the three main topics. Firstly, the basic knowledges of COFs materials related to the luminescence responsive chemical sensing are introduced (including three sections), involving the chemistry, application and post-synthetic modification (PSM) of COFs, the luminescence and luminescence responsive chemical sensing, and the luminescence responsive chemical sensing of non-lanthanide functionalized COFs hybrids materials. Secondly, the systematic progresses are outlined on the lanthanide functionalized COFs hybrid materials in luminescence responsive chemical sensing, which is the emphasis for this review. Finally, the conclusion and prospect are given.

Electrochemically Versatile Graphite Nanoplatelets Prepared by a Straightforward, Highly Efficient, and Scalable Route

Nanostructured carbon materials with tailor-made structures (e.g., morphology, topological defect, dopant, and surface area) are of significant interest for a variety of applications. However, the preparation method selected for obtaining these tailor-made structures determines the area of application, precluding their use in other technological areas of interest. Currently, there is a lack of simple and low-cost methodologies versatile enough for obtaining freestanding carbon nanostructures that can be used in either energy storage or chemical detection. Here, a novel methodology for the development of a versatile electrochemically active platform based on freestanding graphite nanoplatelets (GNP) has been developed by exploiting the interiors of hollow carbon nanofibers (CNF) comprising nanographene stacks using dry ball-milling. Even though ball-milling could be considered as a universal method for any carbonaceous material, often, it is not as simple (one step, no purification, and no solvents), efficient (just GNP without tubular structures), and quick (just 20 min) as the sustainable method developed in this work, free of surfactants and stabilizer agents. We demonstrate that the freestanding GNP developed in this work (with an average thickness of 3.2 nm), due to the selective edge functionalization with the minimal disruption of the basal plane, can act either as a supercapacitor or as a chemical sensor, showing both a dramatic improvement in the charge storage ability of more than 30 times and an enhanced detection of electrochemically active molecules such as ascorbic acid with a 236 mV potential shift with respect to CNF in both cases. As shown here, GNP stand as an excellent versatile alternative compared to the standard commercially available carbon-based materials. Overall, our approach paves the way for the discovery of new nanocarbon-based electrochemical active platforms with a wide electrochemical applicability.

Artificial Olfactory Signal Modulation for Detection in Changing Environments

Animals have evolved to sense in complex environments through both modulation behavior including sniffing as well as sophisticated neural processing including memory and neuromodulation. Here, we explore thermal modulation of chemically diverse sensor arrays, where response patterns are based on partitioning of odorants across the array. The differential response patterns contain information about the chemical nature of the odorant for identification. By transitioning away from well-defined concentration modulation, traditionally used in the field, to thermal modulation, it is possible to capture both diagnostic patterns as well as intensity information in complex environments. This performance is demonstrated with carbon-black based, chemically diverse sensor arrays, that are thermally modulated with light at 25 mHz exposed to different analytes of varying concentrations.

Monolithic MOF-Based Metal-Insulator-Metal Resonator for Filtering and Sensing

Metal-insulator-metal (MIM) configurations based on Fabry-Pérot resonators have advanced the development of color filtering through interactions between light and matter. However, dynamic color changes without breaking the structure of the MIM resonator upon environmental stimuli are still challenging. Here, we report monolithic metal-organic framework (MOF)-based MIM resonators with tunable bandwidth that can boost both dynamic optical filtering and active chemical sensing by laser-processing microwell arrays on the top metal layer. Programmable tuning of the reflection color of the MOF-based MIM resonator is achieved by controlling the MOF layer thicknesses, which is demonstrated by simulation of light-matter interactions on subwavelength scales. Laser-processed microwell arrays are used to boost sensing performance by extending the pathway for diffusion of external chemicals into nanopores of the MOFs. Both experiments and molecular dynamics simulations demonstrate that tailoring the period and height of the microwell array on the MIM resonator can advance the high detection sensitivity of chemicals.

Progress and Perspectives of Single-Atom Catalysts for Gas Sensing

Single-atom catalysts (SACs) attract extensive attention in the field of heterogeneous catalysis in recent years due to the maximum atom utilization and unique physical and chemical properties. The gas sensing is actually a heterogeneous catalysis process but the SACs are new to this area. Although SACs show huge potential in gas sensing, the SACs gas sensing area currently is still at the infancy stage. This work critically reviews the recent advances and current status of single-atom gas sensing materials. General synthesis routes, characterization methods, and sensing performance indexes are introduced. At the end, the challenges and future prospects on SACs gas sensing are presented from the authors' perspectives. This work is anticipated to provide insights and guideline for the chemical sensing community.

Deep Learning Image Analysis of Nanoplasmonic Sensors: Toward Medical Breath Monitoring

Sensing biomarkers in exhaled breath offers a potentially portable, cost-effective, and noninvasive strategy for disease diagnosis screening and monitoring, while high sensitivity, wide sensing range, and target specificity are critical challenges. We demonstrate a deep learning-assisted plasmonic sensing platform that can detect and quantify gas-phase biomarkers in breath-related backgrounds of varying complexity. The sensing interface consisted of Au/SiO₂ nanopillars covered with a 15 nm metal-organic framework. A small camera was utilized to capture the plasmonic sensing responses as images, which were subjected to deep learning signal processing. The approach has been demonstrated at a classification accuracy of 95 to 98% for the diabetic ketosis marker acetone within a concentration range of 0.5-80 μmol/mol. The reported work provides a thorough exploration of single-sensor capabilities and sets the basis for more advanced utilization of artificial intelligence in sensing applications.

Fabrication of a Reusable Carbon Dot/Gold Nanoparticle/Metal-Organic Framework Film for Fluorescence Detection of Lead Ions in Water

Solid-state sensing platforms are desirable for the development of reusable sensors to promote public health measures such as testing for drinking water contamination. A bioinspired metal-organic framework (MOF)-based material has been developed by imitating metal-protein interactions in biological systems to attain high sensitivity and selectivity to Pb²⁺ through fluorescence sensing. A zirconium terephthalate-type framework (also known as NH₂-UiO-66) was modified with both gold nanoparticles and thiol-functionalized carbon dots to give HS-C/Au(x)/UiO-66 composites with different Au content (x) and were subsequently adapted into films that show extraordinary sensitivity to Pb²⁺. The HS-C/Au(1.4)/UiO-66 film that consists of 1.4 wt % Au shows a quenching response with the limit of detection of 80 parts per trillion and sustained performance for five cycles. Moreover, the fluorescence response of the HS-C/Au(x)/UiO-66 film to Pb²⁺ can be reversed from emission quenching to enrichment of fluorescence by increasing the Au content. The performance of the HS-C/Au(x)/UiO-66 film as a solid-state sensor demonstrates its potential for application in reusable sensing devices to ensure public safety from Pb²⁺ contamination in drinking water.

Metasurface-Enhanced Raman Spectroscopy (mSERS) for Oriented Molecular Sensing

Surface-enhanced Raman spectroscopy (SERS) is a widely used sensing technique for ultrasensitivity chemical sensing, biomedical detection, and environmental analysis. Because SERS signal is proportional to the fourth power of the local electric field, several SERS applications have focused on the design of plasmonic nanogaps to take advantage of the extremely strong near-field enhancement that results from plasmonic coupling, but few designs have focused on how SERS detection is affected by molecular orientation within these nanogaps. Here, we demonstrate a nanoparticle-on-metal metasurface designed for near-perfect optical absorption as a platform for Raman detection of highly oriented molecular analytes, including two-dimensional materials and aromatic molecules. This metasurface platform overcomes challenges in nanoparticle aggregation, which commonly leads to low or fluctuating Raman signals in other colloidal nanoparticle platforms. Our metasurface-enhanced Raman spectroscopy (mSERS) platform is based on a colloidal Langmuir-Schaefer deposition, with up to 32% surface coverage density of nanogaps across an entire sensor chip. In this work, we perform both simulations of the local electric field and experimental characterization of the mSERS signal obtained for oriented molecular layers. We then demonstrate this mSERS platform for the quantitative detection of the drinking-water toxin polybrominated diphenyl ether (BDE-15), with a limit of detection of 0.25 μM under 530 μW excitation. This detection limit is comparable to other SERS-based sensors operating at laser powers over 3 orders of magnitude higher, indicating the promise of our mSERS platform for nondestructive and low-level analyte detection.

Aggregation-Induced Luminescence Based UiO-66: Highly Selective Fast-Response Styrene Detection

One of the main pollutants in indoor air is volatile organic compounds (VOCs), which can cause great harm to human health. So the development of a VOC detection technology is of great significance. In this work, a tetraphenylethylene-functionalized UiO-66 based on aggregation-induced emission was successfully prepared. The UiO-66-TBPE structure exhibits the characteristic blue emission of TBPE ligands under UV excitation and can be used as a luminescence sensor for fast and efficient detection of VOCs. More importantly, UiO-66-TBPE has a high fluorescence sensing selectivity in p-xylene and styrene vapor. To further improve the practical performance, we combined UiO-66-TBPE with the polymer polyacrylate (PA) to obtain a flexible hybrid membrane with fast detection performance for styrene vapor within the 30 s. The deeper sensing mechanism of p-xylene and styrene inducing different fluorescence enhancement and fluorescence quenching is explained by a combination of modern characterization techniques and computer simulation. Finally, we applied UiO-66-TBPE/PA to leather and still maintained a good sensing performance. It provides a potential way for the application of fluorescent metal-organic frameworks (MOFs) to detect VOCs in daily life.

Recent Advances in Polyoxometalates with Enzyme-like Characteristics for Analytical Applications

Artificial enzymes based on inorganic solids with both enzyme-mimetic activities and the special material features has been a promising candidate to overcome many deleterious effects of native enzymes in analytical applications. Polyoxometalates (POMs) are an importance class of molecular metal-oxygen anionic clusters. Their outstanding physicochemical properties, versatility and potential applications in energy conversion, magnetism, catalysis, molecular electronics and biomedicine have long been studied. However, the analytical applications of them is limited. Recently, the intrinsic enzymatic activities of POMs have also been found and become an area of growing interest. In this review, along with other reports, we aimed to classify the enzymatic activity of POMs, summarize the construction of POMs-based enzymes, and survey their recent advances in analytical fields. Finally, the current challenges and trends of the polyoxometalates with enzymatic activity in future chemo-/bio-sensing applications are briefly discussed.

Efficient Fabrication of Organic Electrochemical Transistors via Wet Chemical Processing

A wet processing method to fabricate high-performance organic electrochemical transistors (OECTs) is reported. Wet chemical processing enables a simple and reliable patterning step, substituting several complex and expensive cleanroom procedures in the fabrication of OECTs. We fabricate depletion-mode OECTs based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and enhancement-mode OECTs based on a conjugated polyelectrolyte PCPDTBT-SO₃K on rigid and flexible substrates using this wet processing method. We show that the wet chemical processing step can also serve as a chemical treatment to enhance the electrical properties of the active material in OECTs. To highlight the potential of the fabrication process in applications, a transistor-based chemical sensor is demonstrated, capable of detecting methylene blue, a popular redox reporter in biodetection and immunoassays, with good detectivity. Given the tremendous potential of OECTs in emerging technologies such as biosensing and neuromorphic computing, this simple fabrication process established herein will render the OECT platform more accessible for research and applications.

Stratified Disk Microrobots with Dynamic Maneuverability and Proton-Activatable Luminescence for in Vivo Imaging

Microrobots can expand our abilities to access remote, confined, and enclosed spaces. Their potential applications inside our body are obvious, e.g., to diagnose diseases, deliver medicine, and monitor treatment efficacy. However, critical requirements exist in relation to their operations in gastrointestinal environments, including resistance to strong gastric acid, responsivity to a narrow proton variation window, and locomotion in confined cavities with hierarchical terrains. Here, we report a proton-activatable microrobot to enable real-time, repeated, and site-selective pH sensing and monitoring in physiological relevant environments. This is achieved by stratifying a hydrogel disk to combine a range of functional nanomaterials, including proton-responsive molecular switches, upconversion nanoparticles, and near-infrared (NIR) emitters. By leveraging the 3D magnetic gradient fields and the anisotropic composition, the microrobot can be steered to locomote as a gyrating "Euler's disk", i.e., aslant relative to the surface and along its low-friction outer circumference, exhibiting a high motility of up to 60 body lengths/s. The enhanced magnetomotility can boost the pH-sensing kinetics by 2-fold. The fluorescence of the molecular switch can respond to pH variations with over 600-fold enhancement when the pH decreases from 8 to 1, and the integration of upconversion nanoparticles further allows both the efficient sensitization of NIR light through deep tissue and energy transfer to activate the pH probes. Moreover, the embedded down-shifting NIR emitters provide sufficient contrast for imaging of a single microrobot inside a live mouse. This work suggests great potential in developing multifunctional microrobots to perform generic site-selective tasks in vivo.

Highly Selective and Sensitive Detection of Volatile Sulfur Compounds by Ionically Conductive Metal-Organic Frameworks

High selectivity to specific analyte is essential for chemical sensors but difficult to achieve. For most chemical sensors, although the response to the target analyte can be more significant than interference analytes, they still show obvious responses to the interference analytes. Here, highly selective chemical sensors are developed with negligible responses to other interference vapors. Instead of the widely investigated electronically conductive metal-organic frameworks (EC-MOFs), ionically conductive MOFs (IC-MOFs) are used as the sensing materials, and the unique interaction between the ion charge carrier and the analyte is utilized to achieve high sensing selectivity. Through the modulation of the metal nodes (Cu, Co, Ni, Zn, Mg) and organic ligands (H₂ TCPP, H₂ THPP, H₄ BTEC), sensor arrays based on a set of IC-MOFs are fabricated and achieve highly selective detection toward volatile sulfur compounds (VSCs). H₂ S and CH₃ SH can be selectively detected at concentrations down to 1 ppb and 1 ppm, respectively. The facile preparation and low cost endow the device with disposability. These results suggest new approaches for the development of highly selective chemical sensors.

Olfactory Optogenetics: Light Illuminates the Chemical Sensing Mechanisms of Biological Olfactory Systems

The mammalian olfactory system has an amazing ability to distinguish thousands of odorant molecules at the trace level. Scientists have made great achievements on revealing the olfactory sensing mechanisms in decades; even though many issues need addressing. Optogenetics provides a novel technical approach to solve this dilemma by utilizing light to illuminate specific part of the olfactory system; which can be used in all corners of the olfactory system for revealing the olfactory mechanism. This article reviews the most recent advances in olfactory optogenetics devoted to elucidate the mechanisms of chemical sensing. It thus attempts to introduce olfactory optogenetics according to the structure of the olfactory system. It mainly includes the following aspects: the sensory input from the olfactory epithelium to the olfactory bulb; the influences of the olfactory bulb (OB) neuron activity patterns on olfactory perception; the regulation between the olfactory cortex and the olfactory bulb; and the neuromodulation participating in odor coding by dominating the olfactory bulb. Finally; current challenges and future development trends of olfactory optogenetics are proposed and discussed.

Fabrication of Optical Fibers with Multiple Coatings for Swelling-Based Chemical Sensing

We discuss distributed chemical sensing based on the swelling of coatings of optical fibers. Volume changes in the coating induce strain in the fiber's glass core, provoking a local change in the refractive index which is detectable by distributed fiber optical sensing techniques. We describe methods to realize different coatings on a single fiber. Simultaneous detection of swelling processes all along the fiber opens the possibility to interrogate thousands of differently functionalized sections on a single fiber. Principal component analysis is used to enable sensors for environmental monitoring, food analysis, agriculture, water quality monitoring, or medical diagnostics.

SynCells: A 60 × 60 μm2 Electronic Platform with Remote Actuation for Sensing Applications in Constrained Environments

Autonomous electronic microsystems smaller than the diameter of a human hair (<100 μm) are promising for sensing in confined spaces such as microfluidic channels or the human body. However, they are difficult to implement due to fabrication challenges and limited power budget. Here we present a 60 × 60 μm electronic microsystem platform, or SynCell, that overcomes these issues by leveraging the integration capabilities of two-dimensional material circuits and the low power consumption of passive germanium timers, memory-like chemical sensors, and magnetic pads. In a proof-of-concept experiment, we magnetically positioned SynCells in a microfluidic channel to detect putrescine. After we extracted them from the channel, we successfully read out the timer and sensor signal, the latter of which can be amplified by an onboard transistor circuit. The concepts developed here will be applicable to microsystems targeting a variety of applications from microfluidic sensing to biomedical research.

Scalable Superior Chemical Sensing Performance of Stretchable Ionotronic Skin via a π-Hole Receptor Effect

Skin-attachable gas sensors provide a next-generation wearable platform for real-time protection of human health by monitoring environmental and physiological chemicals. However, the creation of skin-like wearable gas sensors, possessing high sensitivity, selectivity, stability, and scalability (4S) simultaneously, has been a big challenge. Here, an ionotronic gas-sensing sticker (IGS) is demonstrated, implemented with free-standing polymer electrolyte (ionic thermoplastic polyurethane, i-TPU) as a sensing channel and inkjet-printed stretchable carbon nanotube electrodes, which enables the IGS to exhibit high sensitivity, selectivity, stability (against mechanical stress, humidity, and temperature), and scalable fabrication, simultaneously. The IGS demonstrates reliable sensing capability against nitrogen dioxide molecules under not only harsh mechanical stress (cyclic bending with the radius of curvature of 1 mm and cyclic straining at 50%), but also environmental conditions (thermal aging from -45 to 125 °C for 1000 cycles and humidity aging for 24 h at 85% relative humidity). Further, through systematic experiments and theoretical calculations, a π-hole receptor mechanism is proposed, which can effectively elucidate the origin of the high sensitivity (up to parts per billion level) and selectivity of the ionotronic sensing system. Consequently, this work provides a guideline for the design of ionotronic materials for the achievement of high-performance and skin-attachable gas-sensor platforms.

A Highly Birefringent Photonic Crystal Fiber for Terahertz Spectroscopic Chemical Sensing

A photonic crystal fiber (PCF) with high relative sensitivity was designed and investigated for the detection of chemical analytes in the terahertz (THz) regime. To ease the complexity, an extremely simple cladding employing four struts is adopted, which forms a rectangular shaped core area for filling with analytes. Results of enormous simulations indicate that a minimum 87.8% relative chemical sensitivity with low confinement and effective material absorption losses can be obtained for any kind of analyte, e.g., HCN (1.26), water (1.33), ethanol (1.35), KCN (1.41), or cocaine (1.50), whose refractive index falls in the range of 1.2 to 1.5. Besides, the PCF can also achieve high birefringence (∼0.01), low and flat dispersion, a large effective modal area, and a large numerical aperture within the investigated frequency range from 0.5 to 1.5 THz. We believe that the proposed PCF can be applied to chemical sensing of liquid and THz systems requiring wide-band polarization-maintaining transmission and low attenuation.

Swelling-Based Distributed Chemical Sensing with Standard Acrylate Coated Optical Fibers

Distributed chemical sensing is demonstrated using standard acrylate coated optical fibers. Swelling of the polymer coating induces strain in the fiber’s silica core provoking a local refractive index change which is detectable all along an optical fiber by advanced distributed sensing techniques. Thermal effects can be discriminated from strain using uncoated fiber segments, leading to more accurate strain readings. The concept has been validated by measuring strain responses of various aqueous and organic solvents and different chain length alkanes and blends thereof. Although demonstrated on a short range of two meters using optical frequency-domain reflectometry, the technique can be applied to many kilometer-long fiber installations. Low-cost and insensitive to corrosion and electromagnetic radiation, along with the possibility to interrogate thousands of independent measurement points along a single optical fiber, this novel technique is likely to find applications in environmental monitoring, food analysis, agriculture, water quality monitoring, or medical diagnostics.

Interband Cascade Photonic Integrated Circuits on Native III-V Chip

We describe how a midwave infrared photonic integrated circuit (PIC) that combines lasers, detectors, passive waveguides, and other optical elements may be constructed on the native GaSb substrate of an interband cascade laser (ICL) structure. The active and passive building blocks may be used, for example, to fabricate an on-chip chemical detection system with a passive sensing waveguide that evanescently couples to an ambient sample gas. A variety of highly compact architectures are described, some of which incorporate both the sensing waveguide and detector into a laser cavity defined by two high-reflectivity cleaved facets. We also describe an edge-emitting laser configuration that optimizes stability by minimizing parasitic feedback from external optical elements, and which can potentially operate with lower drive power than any mid-IR laser now available. While ICL-based PICs processed on GaSb serve to illustrate the various configurations, many of the proposed concepts apply equally to quantum-cascade-laser (QCL)-based PICs processed on InP, and PICs that integrate III-V lasers and detectors on silicon. With mature processing, it should become possible to mass-produce hundreds of individual PICs on the same chip which, when singulated, will realize chemical sensing by an extremely compact and inexpensive package.

A bio-hybrid odor-guided autonomous palm-sized air vehicle

Biohybrid systems integrate living materials with synthetic devices, exploiting their respective advantages to solve challenging engineering problems. One challenge of critical importance to society is detecting and localizing airborne volatile chemicals. Many flying animals depend their ability to detect and locate the source of aerial chemical plumes for finding mates and food sources. A robot with comparable capability could reduce human hazard and drastically improve performance on tasks such as locating disaster survivors, hazardous gas leaks, incipient fires, or explosives. Three advances are needed before they can rival their biological counterparts: (1) a chemical sensor with a much faster response time that nevertheless satisfies the size, weight, and power constraints of flight, (2) a design, sensor suite, and control system that allows it to move toward the source of a plume fully autonomously while navigating obstacles, and (3) the ability to detect the plume with high specificity and sensitivity among the assortment of chemicals that invariably exist in the air. Here we address the first two, introducing a human-safe palm-sized air vehicle equipped with the odor-sensing antenna of an insect, the first odor-sensing biohybrid robot system to fly. Using this sensor along with a suite of additional navigational sensors, as well as passive wind fins, our robot orients upwind and navigates autonomously toward the source of airborne plumes. Our robot is the first flying biohybrid system to successfully perform odor localization in a confined space, and it is able to do so while detecting and avoiding obstacles in its flight path. We show that insect antennae respond more quickly than metal oxide gas sensors, enabling odor localization at an improved speed over previous flying robots. By using the insect antennae, we anticipate a feasible path toward improved chemical specificity and sensitivity by leveraging recent advances in gene editing.

A ppm Ethanol Sensor Based on Fabry-Perot Interferometric Surface Stress Transducer at Room Temperature

Disease screening by exhaled breath diagnosis is less burdensome for patients, and various devices have been developed as promising diagnostic methods. We developed a microelectromechanical system (MEMS) optical interferometric surface stress sensor to detect volatile ethanol gas at room temperature (26~27 °C) with high sensitivity. A sub-micron air gap in the optical interferometric sensor reduces interference orders, leading to increased spectral response associated with nanomechanical deflection caused by ethanol adsorption. The sub-micron cavity was embedded in a substrate using a transfer technique of parylene-C nanosheet. The sensor with a 0.4 µm gap shows a linear stable reaction, with small standard deviations, even at low ethanol gas concentrations of 5–110 ppm and a reversible reaction to the gas concentration change. Furthermore, the possibility of detecting sub-ppm ethanol concentration by optimizing the diameter and thickness of the deformable membrane is suggested. Compared with conventional MEMS surface stress gas sensors, the proposed optical interferometric sensor demonstrated high-sensitivity gas detection with exceeding the detection limit by two orders of magnitude while reducing the sensing area.

Ultrafast and Highly Sensitive Chemically Functionalized Graphene Oxide-Based Humidity Sensors: Harnessing Device Performances via the Supramolecular Approach

Humidity sensors have been gaining increasing attention because of their relevance for well-being. To meet the ever-growing demand for new cost-efficient materials with superior performances, graphene oxide (GO)-based relative humidity sensors have emerged recently as low-cost and highly sensitive devices. However, current GO-based sensors suffer from important drawbacks including slow response and recovery, as well as poor stability. Interestingly, reduced GO (rGO) exhibits higher stability, yet accompanied by a lower sensitivity to humidity due to its hydrophobic nature. With the aim of improving the sensing performances of rGO, here we report on a novel generation of humidity sensors based on a simple chemical modification of rGO with hydrophilic moieties, i.e., triethylene glycol chains. Such a hybrid material exhibits an outstandingly improved sensing performance compared to pristine rGO such as high sensitivity (31% increase in electrical resistance when humidity is shifted from 2 to 97%), an ultrafast response (25 ms) and recovery in the subsecond timescale, low hysteresis (1.1%), excellent repeatability and stability, as well as high selectivity toward moisture. Such highest-key-performance indicators demonstrate the full potential of two-dimensional (2D) materials when decorated with suitably designed supramolecular receptors to develop the next generation of chemical sensors of any analyte of interest.

1D Titanium Dioxide: Achievements in Chemical Sensing

For the last two decades, titanium dioxide (TiO₂) has received wide attention in several areas such as in medicine, sensor technology and solar cell industries. TiO₂-based gas sensors have attracted significant attention in past decades due to their excellent physical/chemical properties, low cost and high abundance on Earth. In recent years, more and more efforts have been invested for the further improvement in sensing properties of TiO₂ by implementing new strategies such as growth of TiO₂ in different morphologies. Indeed, in the last five to seven years, 1D nanostructures and heterostructures of TiO₂ have been synthesized using different growth techniques and integrated in chemical/gas sensing. Thus, in this review article, we briefly summarize the most important contributions by different researchers within the last five to seven years in fabrication of 1D nanostructures of TiO₂-based chemical/gas sensors and the different strategies applied for the improvements of their performances. Moreover, the crystal structure of TiO₂, different fabrication techniques used for the growth of TiO₂-based 1D nanostructures, their chemical sensing mechanism and sensing performances towards reducing and oxidizing gases have been discussed in detail.

Label-Free and Reproducible Chemical Sensor Using the Vertical-Fluid-Array Induced Optical Fiber Long Period Grating (VIOLIN)

Fiber optical refractometers have gained a substantial reputation in biological and chemical sensing domain regarding their label-free and remote-operation working mode. However, the practical breakthrough of the fiber optical bio/chemosensor is impeded by a lack of reconfigurability as well as the explicitness of the determination between bulk and surface refractive indices. In this letter, we further implement the highly flexible and reproducible long period grating called "VIOLIN" in chemical sensing area for the demonstration of moving those obstacles. In this configuration, the liquid is not only leveraged as the chemical carrier but also the periodic modulation of the optical fiber to facilitate the resonant signal. The thiol compound that is adsorbed by the fluidic substrate can be transduced to the pure alteration of the bulk refractive index of the liquid, which can be sensitively perceived by the resonant drift. Taking advantage of its freely dismantled feature, the VIOLIN sensor enables flexible reproduction and high throughput detection, yielding a new vision to the fiber optic biochemical sensing field.

Noble Metal Nanostructured Materials for Chemical and Biosensing Systems

Nanomaterials with unique physical and chemical properties have attracted extensive attention of scientific research and will play an increasingly important role in the future development of science and technology. With the gradual deepening of research, noble metal nanomaterials have been applied in the fields of new energy materials, photoelectric information storage, and nano-enhanced catalysis due to their unique optical, electrical and catalytic properties. Nanostructured materials formed by noble metal elements (Au, Ag, etc.) exhibit remarkable photoelectric properties, good stability and low biotoxicity, which received extensive attention in chemical and biological sensing field and achieved significant research progress. In this paper, the research on the synthesis, modification and sensing application of the existing noble metal nanomaterials is reviewed in detail, which provides a theoretical guidance for further research on the functional properties of such nanostructured materials and their applications of other nanofields.

Surface-Enhanced Raman Scattering-Based Odor Compass: Locating Multiple Chemical Sources and Pathogens

Olfaction is important for identifying and avoiding toxic substances in living systems. Many efforts have been made to realize artificial olfaction systems that reflect the capacity of biological systems. A sophisticated example of an artificial olfaction device is the odor compass which uses chemical sensor data to identify odor source direction. Successful odor compass designs often rely on plume-based detection and mobile robots, where active, mechanical motion of the sensor platform is employed. Passive, diffusion-based odor compasses remain elusive as detection of low analyte concentrations and quantification of small concentration gradients from within the sensor platform are necessary. Further, simultaneously identifying multiple odor sources using an odor compass remains an ongoing challenge, especially for similar analytes. Here, we show that surface-enhanced Raman scattering (SERS) sensors overcome these challenges, and we present the first SERS odor compass. Using a grid array of SERS sensors, machine learning analysis enables reliable identification of multiple odor sources arising from diffusion of analytes from one or two localized sources. Specifically, convolutional neural network and support vector machine classifier models achieve over 90% accuracy for a multiple odor source problem. This system is then used to identify the location of an Escherichia coli biofilm via its complex signature of volatile organic compounds. Thus, the fabricated SERS chemical sensors have the needed limit of detection and quantification for diffusion-based odor compasses. Solving the multiple odor source problem with a passive platform opens a path toward an Internet of things approach to monitor toxic gases and indoor pathogens.

Information processing by endoplasmic reticulum stress sensors

The unfolded protein response (UPR) is a collection of cellular feedback mechanisms that seek to maintain protein folding homeostasis in the endoplasmic reticulum (ER). When the ER is 'stressed', through either high protein folding demand or undersupply of chaperones and foldases, stress sensing proteins in the ER membrane initiate the UPR. Recently, experiments have indicated that these signalling molecules detect stress by being both sequestered by free chaperones and activated by free unfolded proteins. However, it remains unclear what advantage this bidirectional sensor control offers stressed cells. Here, we show that combining positive regulation of sensor activity by unfolded proteins with negative regulation by chaperones allows the sensor to make a more informative measurement of ER stress. The increase in the information capacity of the combined sensing mechanism stems from stretching of the active range of the sensor, at the cost of increased uncertainty due to the integration of multiple signals. These results provide a possible rationale for the evolution of the observed stress-sensing mechanism.

Signal Output of Triboelectric Nanogenerator at Oil-Water-Solid Multiphase Interfaces and its Application for Dual-Signal Chemical Sensing

A liquid-solid contact triboelectric nanogenerator (TENG) based on poly(tetrafluoroethylene) (PTFE) film, a copper electrode, and a glass substrate for harvesting energy in oil/water multiphases is reported. There are two distinctive signals being generated, one is from the contact electrification and electrostatic induction between the liquid (water/oil) and the PTFE film (V_TENG and I_TENG ); and the other is from the electrostatic induction in the copper electrode by the oil/water interfacial charges (ΔV_interface and I_interface ), which is generated only when the liquid-solid contact TENG is inserted across the oil/water interface. The two signals show interesting opposite changing trends that the V_TENG and I_TENG decrease while the oil/water interfacial signals of ΔV_interface and I_interface increase after coating a layer of polydopamine on the surfaces of PTFE and glass via self-polymerization. As an application of the observed phenomena, both the values of I_TENG and I_interface have a good linear relationship versus the natural logarithm of the concentration of the dopamine. Based on this, the first self-powered dual-signal detection of dopamine using TENG is demonstrated.

A Review of Methods for Fibre-Optic Distributed Chemical Sensing

Chemical sensing is of great importance in many application fields, such as medicine, environmental monitoring, and industrial process control. Distributed fibre-optic sensing received significant attention because of its unique feature to make spatially resolved measurements along the entire fibre. Distributed chemical sensing (DCS) is the combination of these two techniques and offers potential solutions to real-world applications that require spatially dense chemical measurements covering large length scales. This paper presents a review of the working principles, current status, and the emerging trends within DCS.

Low-Power Detection of Food Preservatives by a Novel Nanowire-Based Sensor Array

Food preservatives are compounds that are used for the treatment of food to improve the shelf life. In the food industry, it is necessary to monitor all processes for both safety and quality of the product. An electronic nose (or e-nose) is a biomimetic olfactory system that could find numerous industrial applications, including food quality control. Commercial electronic noses are based on sensor arrays composed by a combination of different sensors, which include conductometric metal oxide devices. Metal oxide nanowires are considered among the most promising materials for the fabrication of novel sensing devices, which can enhance the overall performances of e-noses in food applications. The present work reports the fabrication of a novel sensor array based on SnO₂, CuO, and WO₃ nanowires deposited on top of μHPs provided by ams Sensor Solutions Germany GmbH. The array was tested for the discrimination of four typical compounds added to food products or used for their treatment to increase the shelf life: ethanol, acetone, nitrogen dioxide, and ozone. Results are very promising; the sensors array was able to operate for a long time, consuming less than 50 mW for each single sensor, and principal component analysis (PCA) confirmed that the device was able to discriminate between different compounds.

Cylindrical Dielectric Resonator Antenna-Based Sensors for Liquid Chemical Detection

A compact, cylindrical dielectric resonator antenna (CDRA), using radio frequency signals to identify different liquids is proposed in this paper. The proposed CDRA sensor is excited by a rectangular slot through a 3-mm-wide microstrip line. The rectangular slot has been used to excite the CDRA for HEM11 mode at 5.25 GHz. Circuit model values (capacitance, inductance, resistance and transformer ratios) of the proposed CDRA are derived to show the true behaviour of the system. The proposed CDRA acts as a sensor due to the fact that different liquids have different dielectric permittivities and, hence, will be having different resonance frequencies. Two different types of CDRA sensors are designed and experimentally validated with four different liquids (Isopropyl, ethanol, methanol and water).

Transcriptional regulators involved in responses to volatile organic compounds in plants

Field studies have shown that plants growing next to herbivore-infested plants acquire higher resistance to herbivore damage. This increased resistance is partly due to regulation of plant gene expression by volatile organic compounds (VOCs) released by plants that sense environmental challenges such as herbivores. The molecular basis for VOC sensing in plants, however, is poorly understood. Here, we report the identification of TOPLESS-like proteins (TPLs) that have VOC-binding activity and are involved in VOC sensing in tobacco. While screening for volatiles that induce stress-responsive gene expression in tobacco BY-2 cells and tobacco plants, we found that some sesquiterpenes induce the expression of stress-responsive genes. These results provided evidence that plants sense these VOCs and motivated us to analyze the mechanisms underlying volatile sensing using tobacco as a model system. Using a pulldown assay with caryophyllene derivative-linked beads, we identified TPLs as transcriptional co-repressors that bind volatile caryophyllene analogs. Overexpression of TPLs in cultured BY-2 cells or tobacco leaves reduced caryophyllene-induced gene expression, indicating that TPLs are involved in the responses to caryophyllene analogs in tobacco. We propose that unlike animals, which use membrane receptors for sensing odorants, a transcriptional co-repressor plays a role in sensing and mediating VOC signals in plant cells.

Synthesis, structure and selective luminescence sensing for iron(III) ions of a three-dimensional zinc(II) (4,6)-connected coordination network

Coordination polymers constructed from conjugated organic ligands and metal ions with a d¹⁰ electronic configuration exhibit intriguing properties for chemical sensing and photochemistry. A Zn^II-based coordination polymer, namely poly[aqua(μ₆-biphenyl-3,3',5,5'-tetracarboxylato)(μ₂-4,4'-bipyridine)dizinc(II)], [Zn₂(C₁₆H₆O₈)(C₁₀H₈N₂)(H₂O)₂]_n or [Zn₂(m,m-bpta)(4,4'-bipy)(H₂O)₂]_n, was synthesized from a mixture of biphenyl-3,3',5,5'-tetracarboxylic acid [H₄(m,m-bpta)], 4,4'-bipyridine (4,4'-bipy) and Zn(NO₃)₂·6H₂O under solvothermal conditions. The title complex has been structurally characterized by IR spectroscopy, elemental analysis, single-crystal X-ray diffraction and powder X-ray diffraction analysis, and features a μ₆-coordination mode. The Zn^II ions adopt square-pyramidal geometries and are bridged by two syn-syn carboxylate groups to form [Zn₂(COO)₂] secondary buildding units (SBUs). The SBUs are crosslinked by (m,m-bpta)^4- ligands to produce a two-dimensional grid-like layer that exhibits a stair-like structure along the a axis. Adjacent layers are linked by 4,4'-bipy ligands to form a three-dimensional network with a {4⁴.6¹⁰.8}{4⁴.6²} topology. In the solid state, the complex displays a strong photoluminescence and an excellent solvent stability. In addition, the luminescence sensing results indicate a highly selective and sensitive sensing for Fe³⁺ ions.

Plasmonic Conducting Metal Oxide-Based Optical Fiber Sensors for Chemical and Intermediate Temperature-Sensing Applications

The demand for real-time sensors in harsh environments at elevated temperature is significant and increasing. In this manuscript, the chemical and temperature sensing using the optical response through the practical fiber platform is demonstrated, and principle component analysis is coupled with targeted experimental film characterization to understand the fundamental sensing layer properties, which dominate measured gas sensing responses in complex gas mixtures. More specifically, tin-doped indium oxide-decorated sensors fabricated with the sol-gel method show stable and stepwise transmission responses varying over a wide range of H₂ concentration (5-100%) at 250-350 °C as well as responses to CH₄ and CO to a lesser extent. Measured responses are attributed to modifications to the surface plasmon resonance absorption in the near-infrared range and are dominated by the highest concentrations of the most-reducing analyte based upon systematic mixed gas stream experiments. Principal component analysis is utilized for this type of sensor to improve the quantitative and qualitative understanding of responses, clearly identifying that the dominant principle component (PC #1) accounts for ∼78% of total data variance. Correlations between PC #1 and the experimentally derived free carrier concentration confirm that this material property plays the strongest role on the ITO gas sensing mechanism, while correlations between the free carrier mobility and the second most important principle component (PC #2) suggest that this quantity may play a significant but secondary role. As such, the results presented here clarify the relationship between generalized principle components and fundamental sensing materials properties thereby suggesting the pathway toward improved multicomponent gas speciation through sensor layer engineering. The work presented represents a significant step toward the ultimate objective of optical waveguide sensors integrated with multivariate data analytics for multiparameter monitoring with a single sensor element.

Injection-Molded Microfluidic Device for SERS Sensing Using Embedded Au-Capped Polymer Nanocones

To enable affordable detection and diagnostic, there is a need for low-cost and mass producible miniaturized sensing platforms. We present a fully polymeric microfluidic lab-on-a-chip device with integrated gold (Au)-capped nanocones for sensing applications based on surface-enhanced Raman spectroscopy (SERS). All base components of the device were fabricated via injection molding (IM) and can be easily integrated using ultrasonic welding. The SERS sensor array, embedded in the bottom of a fluidic channel, was created by evaporating Au onto IM nanocone structures, resulting in densely packed Au-capped SERS active nanostructures. Using a Raman active model analyte, trans-1,2-bis-(4-pyridyl)-ethylene, we found a surface-averaged SERS enhancement factor of ∼5 × 10⁶ with a relative standard deviation of 14% over the sensor area (2 × 2 mm²), and a 18% signal variation among substrates. This reproducible fabrication method is cost-effective, less time consuming, and allows mass production of fully integrated polymeric, microfluidic systems with embedded high-density and high-aspect ratio SERS sensor.

Orthogonal Ambipolar Semiconductors with Inherently Multi-Dimensional Responses for the Discriminative Sensing of Chemical Vapors

Numerous examples of field-effect transistor (FET) biosensors and chemical sensors with good sensitivity and selectivity have now been developed. However, effectively discriminating between analytes has required either the use of receptors that selectively bind specific analytes or the fabrication of an array of sensors with varying but nonspecific responses. Both approaches exhibit significant limitations. In the first case, it can be difficult to design sufficiently specific receptors for many compounds, whereas the number of receptors required scales with the number of analytes to be detected, making it impractical to recognize many different compounds. In the second case, existing approaches to FET sensor arrays are generally material-inefficient and provide modest sensitivity. Here, we demonstrate that orthogonal ambipolar semiconductors consisting of semiconducting p-type polymers and n-type small-molecule nanowires with perpendicular in-plane orientations provide a platform with high sensitivity and inherently multi-dimensional response. This allows for discrimination between even closely related derivatives such as aromatic isomers and n-alkyl alcohols varying in length by a single carbon atom resolution using only a single sensor element.

Flexible Metal-Organic Framework-Based Mixed-Matrix Membranes: A New Platform for H2 S Sensors

Metal-organic framework (MOF)-polymer mixed-matrix membranes (MMMs) have shown great potential and superior performance in gas separations. However, their sensing application has not been fully established yet. Herein, a rare example of using flexible MOF-based MMMs as a fluorescent turn-on sensor for the detection of hydrogen sulfide (H₂ S) is reported. These MOF-based MMMs are readily prepared by mixing a highly stable aluminum-based nano-MOF (Al-MIL-53-NO₂ ) into poly(vinylidene fluoride) with high loadings up to 70%. Unlike the intrinsic fragility and poor processability of pure-MOF membranes, these MMMs exhibit desirable flexibility and processability that are more suitable for practical sensing applications. The uniform distribution of Al-MIL-53-NO₂ particles combined with the permanent pores of MOFs enable these MMMs to show good water permeation flux and consequently have a full contact between the analyte and MOFs. The developed MMM sensor (70% MOF loading) thus shows a highly remarkable detection selectivity and sensitivity for H₂ S with an exceptionally low detection limit around 92.31 × 10^-9 m, three orders of magnitude lower than the reported powder-form MOFs. This work demonstrates that it is feasible to develop flexible luminescent MOF-based MMMs as a novel platform for chemical sensing applications.

PEDOT: Dye-Based, Flexible Organic Electrochemical Transistor for Highly Sensitive pH Monitoring

Organic electrochemical transistors (OECTs) are bioelectronic devices able to bridge electronic and biological domains with especially high amplification and configurational versatility and thus stand out as promising platforms for healthcare applications and portable sensing technologies. Here, we have optimized the synthesis of two pH-sensitive composites of PEDOT (poly(3,4-ethylenedioxythiophene)) doped with pH dyes (BTB and MO, i.e., Bromothymol Blue and Methyl Orange, respectively), showing their ability to successfully convert the pH into an electrical signal. The PEDOT:BTB composite, which exhibited the best performance, was used as the gate electrode to develop an OECT sensor for pH monitoring that can reliably operate in a two-fold transduction mode with super-Nernstian sensitivity. When the OECT transconductance is employed as analytical signal, a sensitivity of 93 ± 8 mV pH unit^-1 is achieved by successive sampling in aqueous electrolytes. When the detection is carried out by dynamically changing the pH of the same medium, the offset gate voltage of the OECT shifts by (1.1 ± 0.3) × 10² mV pH unit^-1. As a further step, the optimized configuration was realized on a PET substrate, and the performance of the resulting flexible OECT was assessed in artificial sweat within a medically relevant pH range.