Flexible Metal–Organic Framework‐Based Mixed‐Matrix Membranes: A New Platform for H2S 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 (H₂S) 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 H₂S with an exceptionally low detection limit around 92.31 × 10^?9m , 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.     

Trace-Level,Multi-Gas Detection for Food Quality Assessment Based on Decorated Silicon Transistor Arrays

Multiplexed gas detection at room temperature is critical for practical applications, such as for tracking the complex chemical environments associated with food decomposition and spoilage. An integrated array of multiple silicon-based, chemical-sensitive field effect transistors (CSFETs) is presented to realize selective, sensitive, and simultaneous measurement of gases typically associated with food spoilage. CSFETs decorated with sensing materials based on ruthenium, silver, and silicon oxide are used to obtain stable room-temperature responses to ammonia (NH₃), hydrogen sulfide (H₂S), and humidity, respectively. For example, one multi-CSFET sensor signal changes from its baseline by 13.34 in response to 1 ppm of NH₃, 724.45 under 1 ppm H₂S, and 23.46 under 80% relative humidity, with sensitive detection down to 10 ppb of NH₃ and H₂S. To demonstrate this sensor for practical applications, the CSFET sensor array is combined with a custom-printed circuit board into a compact, fully integrated, and portable system to conduct real-time monitoring of gases generated by decomposing food. By using existing silicon-based manufacturing methodologies, this room-temperature gas sensing array can be fabricated reproducibly and at low cost, making it an attractive platform for ambient gas measurement needed in food safety applications.     

Development of Polymeric Nanoprobes with Improved Lifetime Dynamic Range and Stability for Intracellular Oxygen Sensing

A class of core‐shell nanoparticles possessing a layer of biocompatible shell and hydrophobic core with embedded oxygen‐sensitive platinum‐porphyrin (PtTFPP) dyes is developed via a radical‐initiated microemulsion co‐polymerization strategy. The influences of host matrices and the PtTFPP incorporation manner on the photophysical properties and the oxygen‐sensing performance of the nanoparticles are investigated. Self‐loading capability with cells and intracellular‐oxygen‐sensing ability of the as‐prepared nanoparticle probes in the range 0%–20% oxygen concentration are confirmed. Polymeric nanoparticles with optimized formats are characterized by their relatively small diameter (<50 nm), core‐shell structures with biocompatible shells, covalent‐attachment‐imparted leak‐free construction, improved lifetime dynamic range (up to 44 μs), excellent storage stability and photostability, and facile cell uptake. The nanoparticles’ small sensor diameter and core‐shell structure with biocompatible shell make them suitable for intracellular detection applications. For intracellular detection applications, the leak‐free feature of the as‐prepared nanoparticle sensor effectively minimizes potential chemical interferences and cytotoxicity. As a salient feature, improved lifetime dynamic range of the sensor is expected to enable precise oxygen detection and control in specific practical applications in stem‐cell biology and medical research. Such a feature‐packed nanoparticle oxygen sensor may find applications in precise oxygen‐level mapping of living cells and tissue.     

An evanescent field absorption gas sensor at mid-IR 3.39 μm wavelength

Trace gases such as H₂O, CO, CO₂, NO, N₂O, NO₂ and CH₄ strongly absorb in the mid-IR (>2.5 μm) spectral region due to their fundamental rotational and vibrational transitions. CH₄ gas is relatively non-toxic, however, it is extremely explosive when mixed with other chemicals in levels as low as 5% and it can cause death by asphyxiation. In this work, we propose a silicon strip waveguide at 3.39 μm for CH₄ gas sensing based on the evanescent field absorption. These waveguides can provide the highest evanescent field ratio (EFR)>55% with adequate dimensions. Moreover, EFR and sensitivity of the sensor are highly dependent on the length of the waveguide up to a certain limit. Therefore, it is always a compromise between the length of the waveguide and EFR in order to obtain greater sensitivity.     

Silicon on silicon dioxide slot waveguide evanescent field gas absorption sensor

Several trace gases such as H₂O, CO, CO₂, NO, N₂O, NO₂ and CH₄ strongly absorb in the mid-IR spectral region due to their fundamental rotational and vibrational transitions. In this work, we propose an evanescent field absorption gas sensor based on silicon/silicon dioxide slot waveguide at 3.39 μm for sensing of methane gas. These waveguides can provide the highest evanescent field ratio (EFR) > 47% with adequate dimensions. Higher EFR values often come at an expense of higher propagation losses. These waveguides have relatively higher losses as compared to conventional waveguides, such as rib and slab waveguides, as these fundamental losses are static and the proposed sensing mechanism is established on the incremental loss due to the absorption of the gas. Therefore, incident power can always be incremented to compensate the waveguide losses.     

In Situ Two‐Step Photoreduced SERS Materials for On‐Chip Single‐Molecule Spectroscopy with High Reproducibility

A method is developed to synthesize surface‐enhanced Raman scattering (SERS) materials capable of single‐molecule detection, integrated with a microfluidic system. Using a focused laser, silver nanoparticle aggregates as SERS monitors are fabricated in a microfluidic channel through photochemical reduction. After washing out the monitor, the aggregates are irradiated again by the same laser. This key step leads to full reduction of the residual reactants, which generates numerous small silver nanoparticles on the former nanoaggregates. Consequently, the enhancement ability of the SERS monitor is greatly boosted due to the emergence of new “hot spots.” At the same time, the influence of the notorious “memory effect” in microfluidics is substantially suppressed due to the depletion of surface residues. Taking these advantages, two‐step photoreduced SERS materials are able to detect different types of molecules with the concentration down to 10^?13m . Based on a well‐accepted bianalyte approach, it is proved that the detection limit reaches the single‐molecule level. From a practical point of view, the detection reproducibility at different probing concentrations is also investigated. It is found that the effective single‐molecule SERS measurements can be raised up to ≈50%. This microfluidic SERS with high reproducibility and ultrasensitivity will find promising applications in on‐chip single‐molecule spectroscopy.     

Plasmonic Metasurfaces Based on Nanopin‐Cavity Resonator for Quantitative Colorimetric Ricin Sensing

In view of the toxic potential of a bioweapon threat, rapid visual recognition and sensing of ricin has been of considerable interest while remaining a challenging task up to date. In this study, a gold nanopin‐based colorimetric sensor is developed realizing a multicolor variation for ricin qualitative recognition and analysis. It is revealed that such plasmonic metasurfaces based on nanopin‐cavity resonator exhibit reflective color appearance, due to the excitation of standing‐wave resonances of narrow bandwidth in visible region. This clear color variation is a consequence of the reflective color mixing defined by different resonant wavelengths. In addition, the colored metasurfaces appear sharp color difference in a narrow refractive index range, which makes them especially well‐suited for sensing applications. Therefore, this antibody‐functionalized nanopin‐cavity biosensor features high sensitivity and fast response, allowing for visual quantitative ricin detection within the range of 10–120 ng mL^?1 (0.15 × 10^?9–1.8 × 10^?9 m ), a limit of detection of 10 ng mL^?1, and the typical measurement time of less than 10 min. The on‐chip integration of such nanopin metasurfaces to portable colorimetric microfluidic device may be envisaged for the quantitative studies of a variety of biochemical molecules.     

Chemically Tailoring Coal to Fluorescent Carbon Dots with Tuned Size and Their Capacity for Cu(II) Detection

The desired control of size, structure, and optical properties of fluorescent carbon dots (CDs) is critical for understanding the fluorescence mechanism and exploring their potential application. Herein, a top‐down strategy to chemically tailor the inexpensive coal to fluorescent CDs by a combined method of carbonization and acidic oxidation etching is reported. The size and optical properties of the as‐made CDs are tuned by controlling the structures of graphitic crystallites in the starting precursor. The coal‐derived CDs exhibit two different distinctive emission modes, where the intensity of the short‐wavelength emission is significantly enhanced by partial reduction treatment. The evolution of the electronic structure and the surface states analysis show that two different types of fluorescence centers, nano‐sized sp² carbon domains and surface defects, are responsible for the observed emission characteristics. The reduced CDs are demonstrated as an effective fluorescent sensing material for label‐free and selective detection of Cu(II) ions with a detection limit as low as 2.0 nm , showing a great promise for real‐world sensor applications.     

Planar Optical Waveguide Evanescent Wave ${rm CO}_{2}$ Sensor Based on a Clad of Alstonia Scholaris Leaf Extract

An evanescent wave biosensor is designed and developed using a single mode planar optical waveguide based on a spin coated clad of leaf extract of Alstonia Scholaris. The fabricated sensor showed ${rm CO}_{2}$ concentration dependent response. The specialty of this sensor is that it can be used when stored at room temperature (25$~^{circ}{rm C}$) up to a maximum of 25–30 days with 90% retention of original sensitivity. These ${rm CO}_{2}$ sensing biochips showed good operational efficiency for 10 cycles. The planar optical waveguide is versatile, easy to fabricate and can be used for ppm level ${rm CO}_{2}$ measurement with good sensitivity. Cross sensitivity with respect to humidity is studied. The sensor exhibited a short response time of 4–5 s and recovery time of 25 s with good repeatability and reproducibility.      

An integrated metal clad leaky waveguide sensor for detection of bacteria

An integrated optical metal clad leaky waveguide (MCLW) sensor device has been developed for the detection of bacteria. This is more sensitive than waveguide sensors currently in use. The MCLW device has been fabricated to extend the evanescent field to provide significant light intensity over the entire volume of the bacteria bound on the chip surface within this field. This in turn increases the interaction of the light with the entire volume of the bacteria. MCLW devices have been used for detecting refractive index changes, scattering, and fluorescence from bacterial spores captured on an immobilized antibody. The detection limit of Bacillus subtilis var. niger bacterial spores using refractive index detection was 8 x10(4) spores/mL. The scattering intensity of the BG spores was found to be three times greater than the scattering intensity generated using surface plasmon resonance. The extended light propagation along the direction of flow for a few millimeters provides an effective interrogation approach to increase the area of detection to detect low concentrations down to 1 x 10(4) spores/mL. The sensor was then optimized by studying the key factors affecting sensor performance including changing the pH of the medium, type of antibody immobilization matrix, sensor surface regeneration approaches, and longevity of the sensor.     

Modelling nanofiber Mach–Zehnder interferometers for refractive index sensors

In a single-mode silica nanofibre a large amount of the energy of the guided light is in the form of evanescent waves, making it possible to develop a novel sensing element with high sensitivity. Based on theoretical modelling, a highly-sensitive sensor employing a nanofibre-assembled Mach–Zehnder structure is suggested and investigated here. The sensor is used to measure the refractive indices of isopropyl alcohol (IPA) solutions of different concentrations. A phase shift of the guided mode, originating from the change of refractive index of the ambient medium, is obtained. In addition, the important parameters, including sensitivity and detection limit, are also estimated. The results show that Mach–Zehnder interferometric sensor based on nanofibres exhibits the capability of measuring an index variation of ～10^?6. Our simulations are helpful for studying and developing new miniaturised high-performance sensors with high sensitivity.     

Sensitivity enhancement of integrated optical sensors by use of thin high-index films

The proportion of power carried in the superstrate medium by the guided modes of integrated optical waveguides can be increased by the addition of a thin high-index film. Enhanced refractive-index sensing is demonstrated with channel waveguide Mach-Zehnder interferometers with Ta(2)O(5) overlays. Sensitivity increases by a factor greater than 50, and a detection limit better than 5 x 10(-7) is obtained. This approach is broadly applicable to sensing at waveguide surfaces where the strength of evanescent fields dictates performance.     

Controlling DNA Tug‐of‐War in a Dual Nanopore Device

Methods for reducing and directly controlling the speed of DNA through a nanopore are needed to enhance sensing performance for direct strand sequencing and detection/mapping of sequence‐specific features. A method is created for reducing and controlling the speed of DNA that uses two independently controllable nanopores operated with an active control logic. The pores are positioned sufficiently close to permit cocapture of a single DNA by both pores. Once cocapture occurs, control logic turns on constant competing voltages at the pores leading to a “tug‐of‐war” whereby opposing forces are applied to regions of the molecules threading through the pores. These forces exert both conformational and speed control over the cocaptured molecule, removing folds and reducing the translocation rate. When the voltages are tuned so that the electrophoretic force applied to both pores comes into balance, the life time of the tug‐of‐war state is limited purely by diffusive sliding of the DNA between the pores. A tug‐of‐war state is produced on 76.8% of molecules that are captured with a maximum two‐order of magnitude increase in average pore translocation time relative to the average time for single‐pore translocation. Moreover, the translocation slow‐down is quantified as a function of voltage tuning and it is shown that the slow‐down is well described by a first passage analysis for a 1D subdiffusive process. The ionic current of each nanopore provides an independent sensor that synchronously measures a different region of the same molecule, enabling sequential detection of physical labels, such as monostreptavidin tags. With advances in devices and control logic, future dual‐pore applications include genome mapping and enzyme‐free sequencing.     

The Semiconductor/Conductor Interface Piezoresistive Effect in an Organic Transistor for Highly Sensitive Pressure Sensors

The piezoresistive pressure sensor, a kind of widely investigated artificial device to transfer force stimuli to electrical signals, generally consists of one or more kinds of conducting materials. Here, a highly sensitive pressure sensor based on the semiconductor/conductor interface piezoresistive effect is successfully demonstrated by using organic transistor geometry. Because of the efficient combination of the piezoresistive effect and field‐effect modulation in a single sensor, this pressure sensor shows excellent performance, such as high sensitivity (514 kPa^?1), low limit of detection, short response and recovery time, and robust stability. More importantly, the unique gate modulation effect in the transistor endows the sensor with an unparalleled ability—tunable sensitivity via bias conditions in a single sensor, which is of great significance for applications in complex pressure environments. The novel working principle and high performance represent significant progress in the field of pressure sensors.     

A Superhydrophobic Smart Coating for Flexible and Wearable Sensing Electronics

Superhydrophobic surfaces have shown versatile applications in waterproofing, self‐cleaning, drag reduction, selective absorption, etc. The most convenient and universally applicable approach to forming superhydrophobic surfaces is by coating; however, currently, superhydrophobic, smart coatings with flexibility and multiple functions for wearable sensing electronics are not yet reported. Here, a highly flexible multifunctional smart coating is fabricated by spray‐coating multiwalled carbon nanotubes dispersed in a thermoplastic elastomer solution, followed by treatment with ethanol. The coatings not only endow various substrate materials with superhydrophobic surfaces, but can also respond to stretching, bending, and torsion—a property useful for flexible sensor applications. The coatings show superior sensitivity (gauge factor of 5.4–80), high resolution (1° of bending), a fast response time (<8 ms), a stable response over 5000 stretching–relaxing cycles, and wide sensing ranges (stretching: over 76%, bending: 0°–140°, torsion: 0–350 rad m^?1). Moreover, multifunctional coatings with thicknesses of only 1 µm can be directly applied to clothing for full‐range and real‐time detection of human motions, which also show extreme repellency to water, acid, and alkali, which helps the sensors to work under wet and corrosive conditions.     

In situ trace analysis of oil in water with mid-infrared fiberoptic chemical sensors

The determination of trace amounts of oil in water facilitates the forensic analysis on the presence and origin of oil in the aqueous environment. To this end, the present study focuses on direct sensing schemes for quantifying trace amounts of oil in water using mid-infrared (MIR) evanescent field absorption spectroscopy via fiberoptic chemical sensors. MIR transparent silver halide fibers were utilized as optical transducer for interrogating oil-in-water emulsions via the evanescent field emanating from the waveguide surface, and penetrating the surrounding aqueous environment by a couple of micrometers. Unmodified fibers and fibers surface-modified with grafted epoxidized polybutadiene layers enabled the direct detection of crude oil in a deionized water matrix at the ppm level to ppb concentration level, respectively. Thus, direct chemical sensing of crude oil IR signatures without any sample preparation as low as 46 ppb was achieved with a response time of a few seconds.     

Spectral resolution analysis of resonant grating waveguides

A high spectral resolution analysis of narrowband reflection filters based on resonant grating waveguide structures is presented. A tunable high‐performance dye laser with ～0.15 cm^‐1 line width and a beam analyzing system consisting of three simultaneously controlled CCD cameras were used to investigate grating waveguide resonances at wavelengths ～694 nm and ～633 nm. A reflectivity of 91 % and a line width of ～0.3 nm were measured and theoretically modeled for a resonant reflection filter specifically designed for the ruby laser wavelength 694.2 nm. A resonance shift of several nanometers was observed for a second grating waveguide structure in the region of the helium‐neon laser emission wavelength 632.8 nm by changing the sample temperature. We discuss the potential of grating waveguide devices combining the narrow line width and the tunability of the resonant response as promising candidates for implementation in innovative concepts for reflection filter and sensor applications.     

Capillary waveguide optrodes: an approach to optical sensing in medical diagnostics

Glass capillaries with a chemically sensitive coating on the inner surface are used as optical sensors for medical diagnostics. A capillary simultaneously serves as a sample compartment, a sensor element, and an inhomogeneous optical waveguide. Various detection schemes based on absorption, fluorescence intensity, or fluorescence lifetime are described. In absorption-based capillary waveguide optrodes the absorption in the sensor layer is analyte dependent; hence light transmission along the inhomogeneous waveguiding structure formed by the capillary wall and the sensing layer is a function of the analyte concentration. Similarly, in fluorescence-based capillary optrodes the fluorescence intensity or the fluorescence lifetime of an indicator dye fixed in the sensing layer is analyte dependent; thus the specific property of fluorescent light excited in the sensing layer and thereafter guided along the inhomogeneous waveguiding structure is a function of the analyte concentration. Both schemes are experimentally demonstrated, one with carbon dioxide as the analyte and the other one with oxygen. The device combines optical sensors with the standard glass capillaries usually applied to gather blood drops from fingertips, to yield a versatile diagnostic instrument, integrating the sample compartment, the optical sensor, and the light-collecting optics into a single piece. This ensures enhanced sensor performance as well as improved handling compared with other sensors.     

Quantum Dot–Based FRET Immunoassay for HER2 Using Ultrasmall Affinity Proteins

Engineered scaffold affinity proteins are used in many biological applications with the aim of replacing natural antibodies. Although their very small sizes are beneficial for multivalent nanoparticle conjugation and efficient Förster resonance energy transfer (FRET), the application of engineered affinity proteins in such nanobiosensing formats has been largely neglected. Here, it is shown that very small (≈6.5 kDa) histidine‐tagged albumin‐binding domain‐derived affinity proteins (ADAPTs) can efficiently self‐assemble to zwitterionic ligand–coated quantum dots (QDs). These ADAPT–QD conjugates are significantly smaller than QD‐conjugates based on IgG, Fab', or single‐domain antibodies. Immediate applicability by the quantification of the human epidermal growth factor receptor 2 (HER2) in serum‐containing samples using time‐gated Tb‐to‐QD FRET detection on the clinical benchtop immunoassay analyzer KRYPTOR is demonstrated here. Limits of detection down to 40 × 10^?12m (≈8 ng mL^?1) are in a relevant clinical concentration range and outperform previously tested assays with antibodies, antibody fragments, and nanobodies.