Monolayer Two‐dimensional Molecular Crystals for an Ultrasensitive OFET‐based Chemical Sensor

The sensitivity of conventional thin‐film OFET‐based sensors is limited by the diffusion of analytes through bulk films and remains the central challenge in sensing technology. Now, for the first time, an ultrasensitive (sub‐ppb level) sensor is reported that exploits n‐type monolayer molecular crystals (MMCs) with porous two‐dimensional structures. Thanks to monolayer crystal structure of NDI3HU‐DTYM2 (NDI) and controlled formation of porous structure, a world‐record detection limit of NH₃ (0.1 ppb) was achieved. Moreover, the MMC‐OFETs also enabled direct detection of solid analytes of biological amine derivatives, such as dopamine at an extremely low concentration of 500 ppb. The remarkably improved sensing performances of MMC‐OFETs opens up the possibility of engineering OFETs for ultrasensitive (bio)chemical sensing.     

Inverted Opal Fluorescent Film Chemosensor for the Detection of Explosive Nitroaromatic Vapors through Fluorescence Resonance Energy Transfer

This paper reports an inverted opal fluorescence chemosensor for the ultrasensitive detection of explosive nitroaromatic vapors through resonance‐energy‐transfer‐amplified fluorescence quenching. The inverted opal silica film with amino ligands was first fabricated by the acid–base interaction between 3‐aminopropyltriethoxysilane and surface sulfonic groups on polystyrene microsphere templates. The fluorescent dye was then chemically anchored onto the interconnected porous surface to form a hybrid monolayer of amino ligands and dye molecules. The amino ligands can efficiently capture vapor molecules of nitroaromatics such as 2,4,6‐trinitrotoluene (TNT) through the charge‐transfer complexing interaction between electron‐rich amino ligands and electron‐deficient aromatic rings. Meanwhile, the resultant TNT–amine complexes can strongly suppress the fluorescence emission of the chosen dye by the fluorescent resonance energy transfer (FRET) from the dye donor to the irradiative TNT–amino acceptor through intermolecular polar–polar resonance at spatial proximity. The quenching response of the highly ordered porous films with TNT is greatly amplified by at least 10‐fold that of the amorphous silica films, due to the interconnected porous structure and large surface‐to‐volume ratio. The inverted opal film with a stable fluorescence brightness and strong analyte affinity has lead to an ultrasensitive detection of several ppb of TNT vapor in air.     

Porous reduced graphene oxide for ultrasensitive detection of nitrogen dioxide

The defect engineering in graphene plays a significant role for the application of gas sensors. In this work, we proposed an efficient method to prepare ultrasensitive gas sensors based on the porous reduced graphene oxide (PRGO). Photo-Fenton etching was carried out on GO nanosheets in a controlled manner to enrich their vacancy defects. The resulting porous graphene oxide (PGO) was then drop-coated on interdigital electrodes and hydrothermal reduced at 180 °C. Controllable reduction was achieved by varying the water amount. The gas sensor based on PRGO-5 min-6 h exhibited superior sensing and selective performance toward nitrogen dioxide (NO₂), with an exceptional high sensitivity up to 12 ppm^?1. The theoretical limit of detection is down to 0.66 ppb. The excellent performance could be mainly attributed to the typical vacancy defects of PRGO. Some residue carboxylic groups on the edges could also facilitate the adsorption of polar molecules. The process has a great potential for scalable fabrication of high-performance NO₂ gas sensors.     

An ultrasensitive fluorescent probe for hydrazine detection and its application in water samples and living cells

Hydrazine, as a strong reducing agent, has been extensively used in many industrial manufactures. However, it is a potential human carcinogen and an environmental contaminant due to its high toxicity. Therefore, developing an ultrasensitive method for determining hydrazine in real water and biosystems is of great significance. Herein, based on coumarin dye, a turn-on fluorescent probe Cou-1-N₂H₄, which contains an acetyl group as the trigger unit and the fluorescence quencher, is developed. The probe can achieve a rapid (3min) and colorimetric sensing detection for hydrazine with an extremely low limit detection (11.9?nM or 0.38?ppb). More importantly, the practical utilities of probe have been successfully proved through quantitative N₂H₄ detection in environmental water samples and bioimaging of N₂H₄ in living cells.     

Crack‐Free Periodic Porous Thin Films Assisted by Plasma Irradiation at Low Temperature and Their Enhanced Gas‐Sensing Performance

Homogenous thin films are preferable for high‐performance gas sensors because of their remarkable reproducibility and long‐term stability. In this work, a low‐temperature fabrication route is presented to prepare crack‐free and homogenous metal oxide periodic porous thin films by oxygen plasma irradiation instead of high temperature annealing by using a sacrificial colloidal template. Rutile SnO₂ is taken as an example to demonstrate the validity of this route. The crack‐free and homogenous porous thin films are successfully synthesized on the substrates in situ with electrodes. The SnO₂ porous thin film obtained by plasma irradiation is rich in surface OH groups and hence superhydrophilic. It exhibits a more homogenous structure and lower resistance than porous films generated by annealing. More importantly, such thin films display higher sensitivity, a lower detection threshold (100 ppb to acetone) and better durability than those that have been directly annealed, resulting in enhanced gas‐sensing performance. The presented method could be applied to synthesize other metal oxide homogenous thin films and to fabricate gas‐sensing devices with high performances.     

Hybrid ZnO/SWNT Nanostructures Based Gas Sensor

Zinc oxide (ZnO) nanoparticles decorated single walled carbon nanotubes (SWNTs) were electrochemically synthesized where the deposition conditions were systematically explored to tailor the size, density, and microstructure of the ZnO nanoparticles and correlated to the gas sensing performance. Room temperature conductometric detection of various analytes including CO, CO₂, NO₂, NH₃, SO₂, H₂S with ZnO/SWNT hybrid nanostructures demonstrated uncharacteristic selectivity towards H₂S with little to no response for the other analytes examined. Optimal ZnO/SWNTs gas sensor devices showed a significantly increased in H₂S sensitivity over unfunctionalized SWNT networks (i.e. 4.96 % per ppm_V vs. 0.225 % ppm_V) with a lower detection limit in the ppb range. Additionally, the H₂S sensing performance was greatly improved by enhancing the crystallinity of ZnO nanoparticles.     

A novel power-controlling approach for integrated,conductometric gas sensors

An original power controlling driving/reading circuit for Porous Silicon JFET (PSJFET) gas sensors is presented. The PSJFET is an integrated p-channel JFET with two independent gates: a meso-structured PS layer, acting as a sensing, floating gate, which modulates the JFET current upon adsorption/desorption of specific analytes, and a high-impedance electric gate, which allows the JFET current tuning independently from analytes in the environment. The circuit exploits the independence of the sensing and electrical gate terminals to set/control the sensor power-dissipation, which is kept almost constant independently from adsorption/desorption-induced effects, while simultaneously carrying out a current-voltage conversion. For such a purpose, a negative feedback loop is used to modulate the PSJFET electric gate voltage, which becomes the output signal, while keeping constant the source-drain sensor current and, hence, the power dissipation. The proposed approach is validated by performing time-resolved measurements on PSJFET sensors under different NO₂ concentrations (100ppb, 300ppb, 500ppb), at room temperature.     

Rational construction and triethylamine sensing performance of foam shaped α-MoO3@SnS2 nanosheets

Owing to their high surface area, stable structure and easy fabrication, composite nanomaterials with encapsulation structures have attracted considerable research interest as sensing materials to detect volatile organic compounds. Herein, a hydrothermal route is designed to prepare foam shaped α-MoO₃@SnS₂ nanosheets that exhibit excellent sensing performance for triethylamine（TEA）. The developed sensor,based on α-MoO₃@SnS₂ nanosheets, displays a high ...     

Hierarchical NiCo2O4 microspheres assembled by nanorods with p-type response for detection of triethylamine

The morphological and structural design provides an efficient protocol to optimize the performance of gas sensing materials. In this work, a gas sensor with high sensitivity for triethylamine (TEA) detection is developed based on p-type NiCo₂O₄ hierarchical microspheres. The NiCo₂O₄ microspheres, synthesized by a hydrothermal route, have a three-dimensional (3D) urchin-like structure assembled by nanorod building blocks. The structure-property correlation has been investigated by powder X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscope, scanning electron microscope, N₂ adsorption-desorption tests and comprehensive gas sensing experiments. The influence of calcination temperature on the morphological structure and sensing performances has been investigated. Results reveal that the material annealed at 300 °C has a very large specific surface area of 125.27 m²/g, thereby demonstrating the best TEA sensing properties including high response and low limit of detection (145 ppb), good selectivity and stability. The further increase of the calcination temperature leads to the collapse of the 3D hierarchical structure with significantly decreased surface area, which is found to decline the sensing performances. This work indicates the promise of ternary p-type metal oxide nanostructures for application in highly sensitive gas sensors.     

Plasmonic Liquid Marbles: A Miniature Substrate‐less SERS Platform for Quantitative and Multiplex Ultratrace Molecular Detection

Inspired by aphids, liquid marbles have been studied extensively and have found application as isolated microreactors, as micropumps, and in sensing. However, current liquid‐marble‐based sensing methodologies are limited to qualitative colorimetry‐based detection. Herein we describe the fabrication of a plasmonic liquid marble as a substrate‐less analytical platform which, when coupled with ultrasensitive SERS, enables simultaneous multiplex quantification and the identification of ultratrace analytes across separate phases. Our plasmonic liquid marble demonstrates excellent mechanical stability and is suitable for the quantitative examination of ultratrace analytes, with detection limits as low as 0.3 fmol, which corresponds to an analytical enhancement factor of 5×10⁸. The results of our simultaneous detection scheme based on plasmonic liquid marbles and an aqueous–solid–organic interface quantitatively tally with those found for the individual detection of methylene blue and coumarin.     

Naked eye sensing of melamine using rationally tailored gold nanoparticles: hydrogen-bonding and charge-transfer recognition

A highly sensitive analytical method based on Au nanoparticles rationally tailored with recognition elements uracil-5-carboxylic acid (UCA) and 2,4,6-trinitrobenzenesulfonic acid (TNBS) for the visual sensing of melamine at the parts-per-billion (ppb) level is described. The tailored Au nanoparticles function as an excellent color indicating reporter and it recognizes the target analytes by triple hydrogen-bonding or charge-transfer interaction in aqueous solution. The interaction of melamine with UCA- or TNBS-tailored reporters induces a rapid visible color change due to the aggregation of reporters. The color change was spectrally monitored to precisely quantify the amount of melamine. The charge-transfer interaction of melamine with TNBS-tailored reporter brings a remarkable change in the spectral signature even at the ppb level. Such an interaction paves the way for the detection of melamine at the 5 ppb level, which is well below the safety limit set by UN food standard commission. This method is highly selective and the common interfering analytes such as cyanuric acid, cytosine, glucose, thymine, uracil, etc., do not interfere in the sensing of melamine. The practical utility of the method is demonstrated by quantifying the amount melamine in real samples.     

Ultrasensitive NOX Detection in Simulated Exhaled Air: Enhanced Sensing via Alumina Modification of In‐Situ Grown WO3 Nanoblocks

Seedless growth of vertically aligned nanostructures, which can induce smoother transport and minimize Ohmic contact between substrate and semiconductor, can be fabricated by in situ growth utilizing modified hydrothermal methods. Such devices can be useful in designing non‐invasive ultrasensitive hand‐held sensors for diagnostic identification of volatile organic compounds (VOCs) in exhaled air, offering pain‐free and easier detection of long‐term diseases such as asthma. In the present work, WO₃ nanoblocks, with a high surface area and porosity, have been grown directly over transparent conducting oxide to minimize Ohmic resistance, facilitating smoother electron transfer and enhanced current response. Further modification with porous alumina (γ‐Al₂O₃), by electrodeposition, resulted in the selective and ultrasensitive detection of NO_X in simulated exhaled air. Crystal phase purity of as‐fabricated pristine as well modified samples is validated with X‐ray diffraction analysis. Morphological and microstructural analyses reveal the successful deposition of porous alumina over the surface of WO₃. Improved surface area and porosity is presented by porous alumina in the modified WO₃ device, suggesting more active sites for the gas molecules to get adsorbed and diffuse through the pores. Oxygen vacancies, which are detrimental in the transport phenomenon in the presented sensors, have been studied using X‐ray photoelectron spectroscopic (XPS) analysis. Gas sensing studies have been performed by fabricating chemiresistor devices based on bare WO₃ and Al₂O₃‐modified WO₃. The higher sensitivity for NO_X gas in case of γ‐Al₂O₃‐modified WO₃ based devices, as compared to bare WO₃‐based devices, is attributed to the better surface area and charge transport kinetics. The presented device strategy offers crucial understanding in the design and development of non‐invasive, hand‐held devices for NO gas present in the human breath, with potential application in medical diagnostics.     

Raman spectroscopy probing of self-assembled monolayers inside the pores of gold nanotube membranes

Electroless deposition was used to coat porous alumina membranes with gold. This process reduced the pore diameters and provided a platform suitable for surface modifications with self assembled monolayers (SAMs). The surface enhanced Raman scattering (SERS) effect was employed in order to confirm and characterise the formation of SAMs of 3-mercaptobenzoic acid (mMBA) inside the pores of gold nanotube membranes prepared using porous alumina (PA) templates. The investigation of the coverage and reproducibility of SAMs within porous matrices is of utmost importance in the design of filtration membranes and sensing platforms. Raman spectroscopy is capable of spatially resolved techniques such as mapping which was used to characterise the distribution of mMBA assembly within the pores. Due to the highly ordered structure of porous alumina and well controlled electroless gold deposition, these gold coated membranes have the potential to develop into SERS active substrates for ultrasensitive sensing technologies.     

Photocatalytically Powered Matchlike Nanomotor for Light‐Guided Active SERS Sensing

Surface enhanced Raman spectroscopy (SERS) is a powerful optical sensing technique that can detect analytes of extremely low concentrations. However, the presence of enough SERS probes in the detection area and a close contact between analytes and SERS probes are critical for efficient acquisition of a SERS signal. Presented here is a light‐powered micro/nanomotor (MNM) that can serve as an active SERS probe. The matchlike AgNW@SiO₂ core–shell structure of the nanomotors work as SERS probes based on the shell‐isolated enhanced Raman mechanism. The AgCl tail serves as photocatalytic nanoengine, providing a self‐propulsion force by light‐induced self‐diffusiophoresis. The phototactic behavior was utilized to achieve enrichment of the nanomotor‐based SERS probes for on‐demand biochemical sensing. The results demonstrate the possibility of using photocatalytic nanomotors as active SERS probes for remote, light‐controlled, and smart biochemical sensing on the micro/nanoscale.     

Boosting the Dynamic Range for Electrochemical Sensing of Hydrogen Peroxide by Enhanced Integration of Pd Nanoparticles in 3D Porous Si Framework

Three-dimensional porous silicon framework (3D-pSi) integrated with various nanostructures is highly potential for functional devices usage such as micro fuel cells or sensing chips. For noble metal deposition in highly directional Si nanowire array or porous Si with large aspect ratios, one difficulty is the restriction on the depth of deposition available. Herein, we would like to introduce a facile route to enhance the integration of Pd nanoparticles with anisotropic Si porous structure. By converting Si nanowire array into 3D-pSi, the surface coverage of Pd nanoparticles is effectively improved as shown by scanning electron micrographs and cross-sectional element mapping data. The relative electrochemical active surface area is increased by 3.5 folds. In order to demonstrate the merits brought by this morphological evolution, the electrochemical sensing devices are prepared for detecting H₂O₂ in PBS solution. As shown by differential pulse voltammetry, the upper limit of linear range of detection can be raised from 6.30 mM to 14.95 mM. This approach may indicate an alternative for boosting the performance of future sensing chips with progressively limited die area, especially attractive for those scenarios where large dynamic range is favourable.     

Molecularly Imprinted Photonic Polymers as Sensing Elements for the Creation of Cross‐Reactive Sensor Arrays

By combining molecular imprinting and colloidal crystal templating, molecularly imprinted inverse‐opal photonic polymers (MIPPs) acting as sensing elements have been exploited to create sensor arrays for the first time. With this new strategy, abundant sensing elements with differential sensing abilities were easily accessible. Because of the unique hierarchical porous structure integrated in each sensing element, high sensitivity and selectivity, fast response and self‐reporting (label‐free) detection could be simultaneously achieved. All these fascinating features indicate that MIPPs are ideal sensing elements for creating sensor arrays. By integrating the individual sensing elements on a substrate, the formed array chip delivers better portability and high‐throughput capability. As a demonstration, six kinds of contaminants were selected as analytes. The detection and discrimination of these analytes and even their mixtures in a wide range of concentrations, particularly trace amounts of analyte against a high background of other components, could be achieved, indicating the powerful capability of MIPPs‐based sensor array for sensing. These results suggest that the described strategy opens a new route for sensor array creation and should find important applications in a wide range of areas.     

Reversing current rectification to improve DNA‐sensing sensitivity in conical nanopores

Herein, we report the ultrasensitive DNA detection through designing an elegant nanopore biosensor as the first case to realize the reversal of current rectification direction for sensing. Attributed to the unique asymmetric structure, the glass conical nanopore exhibits the sensitive response to the surface charge, which can be facilely monitored by ion current rectification curves. In our design, an enzymatic cleavage reaction was employed to alter the surface charge of the nanopore for DNA sensing. The measured ion current rectification was strongly responsive to DNA concentrations, even reaching to the reversed status from the negative ratio (?6.5) to the positive ratio (+16.1). The detectable concentration for DNA was as low as 0.1 fM. This is an ultrasensitive and label‐free DNA sensing approach, based on the rectification direction‐reversed amplification in a single glass conical nanopore.     

Mesoporous materials in sensing: morphology and functionality at the meso-interface

Mesoporous materials are finding increasing utility in sensing applications. These applications can benefit from a surface area that may exceed 1,000 m² g⁻¹ and fast diffusion of analytes through a porous structure. This article reviews recent developments in mesoporous materials-based sensing and provides examples of the impact of different surface functionality, pore structure, and macro-morphology in an attempt to illustrate the contribution of these factors to the selectivity and sensitivity of a sensor response. The materials discussed include ordered mesoporous silicates synthesized with surfactants, hard templated ordered mesoporous carbons, and metal oxides with porous textures which have been applied to advantage in various detection schemes. Chemical functionalization of mesoporous materials through silane grafting, co-condensation, and adsorption are also addressed.     

H2S Sensors: Fumarate‐Based fcu‐MOF Thin Film Grown on a Capacitive Interdigitated Electrode

Herein we report the fabrication of an advanced sensor for the detection of hydrogen sulfide (H₂S) at room temperature, using thin films of rare‐earth metal (RE)‐based metal–organic framework (MOF) with underlying fcu topology. This unique MOF‐based sensor is made via the in situ growth of fumarate‐based fcu ‐MOF (fum‐ fcu ‐MOF) thin film on a capacitive interdigitated electrode. The sensor showed a remarkable detection sensitivity for H₂S at concentrations down to 100 ppb, with the lower detection limit around 5 ppb. The fum‐ fcu ‐MOF sensor exhibits a highly desirable detection selectivity towards H₂S vs. CH₄, NO₂, H₂, and C₇H₈ as well as an outstanding H₂S sensing stability as compared to other reported MOFs.     

Enzyme spheres as novel tracing tags coupled with target-induced DNAzyme assembly for ultrasensitive electrochemical microRNA assay

In this work, an ultrasensitive electrochemical microRNA detection strategy was developed based on porous palladium-modified horseradish peroxidase sphere (Pd@HRP) and target-induced assembly of DNAzyme. A highly loaded HRP sphere was prepared by covalent layer-by-layer assembly with CaCO₃ as sacrificial template for the first time, and was further modified with porous Pd. Notably, Pd@HRP composite showed a good redox activity of HRP and electrocatalytic activity toward H₂O₂. The utilization of Pd@HRP as electrochemical signal indicator and enhancer to fabricate biosensor could avoid the need for additional redox mediator and amplify the detection sensitivity. Moreover, target recycling amplification was achieved by Pb²⁺-induced cleavage of ternary “Y” structure, circumventing the use of labile nuclease. Subsequent DNA concatamer synthesized through rolling circle amplification (RCA) reaction with cleaved hairpin probe as primer, hybridized with plentiful Pd@HRP-DNA probes, which led to the increased loading of redox-active and electrocatalytic Pd@HRP for sensitivity improvement. So the proposed electrochemical biosensor detected miRNA-24 down to 0.2 fM (S/N = 3) with a wide linear range from 3 fM to 1 nM. With bifunctional Pd@HRP tag, DNAzyme-aided target recycle and programmable junction probe, this strategy possessed the advantages of high efficiency, high sensitivity, low cost and versatility, and thus held great promise for other low-abundance nucleic acids determination.