Tailoring of sol-gel films for optical sensing of oxygen in gas and aqueous phase

Sol-gel-based optical sensors for both gas-phase and dissolved oxygen have been developed. Both sensors operate on the principle of fluorescence quenching of a ruthenium complex which has been entrapped in a porous sol-gel silica film. A comprehensive investigation was carried out in order to establish optimal film-processing parameters for the two sensing environments. Both tetraethoxysilane and organically modified sol-gel precursors such as methyltriethoxysilane and ethyltriethoxysilane were used. Film hydrophobicity increases as a function of modified precursor content, and this was correlated with enhanced dissolved oxygen (DO) sensor performance. Extending the aliphatic group of the modified precursor further improved DO sensitivity. The influence of water/precursor molar ratio, R, on the sol-gel film microstructure was investigated. R value tailoring of the microstructure and film surface hydrophobicity tailoring were correlated with oxygen diffusion behavior in the films via the Stern-Volmer constants for both gas phase and DO sensing. Excellent performance characteristics were measured for both gas-phase and DO oxygen sensors. The long-term quenching stability of DO sensing films was established over a period of 6 months.     

The pH sensing characteristics of the extended-gate field-effect transistors of multi-walled carbon-nanotube thin film using low-temperature ultrasonic spray method

A novel, simple and low-temperature ultrasonic spray method was developed to fabricate the multi-walled carbon-nanotubes (MWCNTs) based extended-gate field-effect transistors (EGFETs) as the pH sensor. With an acid-treated process, the chemically functionalized two-dimensional MWCNT network could provide plenty of functional groups which exhibit hydrophilic property and serve as hydrogen sensing sites. For the first time, the EGFET using a MWCNT structure could achieve a wide sensing rage from pH = 1 to pH = 13. Furthermore, the pH sensitivity and linearity values of the CNT pH-EGFET devices were enhanced to 51.74 mV/pH and 0.9948 from pH = 1 to pH = 13 while the sprayed deposition reached 50 times. The sensing properties of hydrogen and hydroxyl ions show significantly dependent on the sprayed deposition times, morphologies, crystalline and chemical bonding of acid-treated MWCNT. These results demonstrate that the MWCNT-EGFETs are very promising for the applications in the pH and biomedical sensors.     

Rapid and ultrahigh ethanol sensing based on Au-coated ZnO nanorods

Rapid and ultrahigh sensing is realized from Au-coated ZnO rods with diameters down to 15?nm. Both the small diameters and the Au coating make the surface-depletion effect more pronounced for gas sensing. Such enhanced surface depletion increases the sensitivity, lowers the operation temperature and decreases the response time. A sensitivity of 89.5-100?ppm ethanol is obtained with response time shorter than 2?s at 300?°C, and the operation temperature can be as low as 150?°C. It is found that the Au coating improves the sensitivity by three times; this is much higher than that of noble metal-doped metal oxide sensors controlled by a grain-boundary barrier. Our results imply that the surface-depletion model is very helpful in fabricating high performance gas sensors.     

Nitric oxide-releasing fluorescence-based oxygen sensing polymeric films

The in vitro analytical performance of fluorescence-based oxygen sensing polymeric films prepared with silicone rubbers that spontaneously release nitric oxide (NO) is examined. The use of NO-release polymers for fabricating functional optical sensors is proposed as a potential solution to fingering biocompatibility and concomitant performance problems encountered with prototype intravascular optical oxygen sensors. Plasticized silicone rubber films formulated with two distinct types of diazeniumdiolate NO donors release NO for more than 24 h. The optical oxygen sensing films prepared by doping these NO release polymeric materials with oxygen indicators (pyrene/perylene donor/acceptor pair) display different analytical responses, as compared to controls without NO release capability. Nonlinear Stern-Volmer behavior is observed for single-layer NO release oxygen sensors owing to heterogeneous environments for the pyrene/perylene pair and a concomitant quenching of the fluorescence by excess amine sites in such films. In contrast, a dual-layer configuration using an underlying NO-release silicone rubber layer covered with a second polymeric layer containing the fluorescent indicators is shown to yield identical sensitivity and linearity toward oxygen as conventional non-NO-releasing oxygen sensing films, while still providing surface NO fluxes necessary to yield more thromboresistive devices.     

Organic thin-film transistors for chemical and biological sensing

Organic thin-film transistors (OTFTs) show promising applications in various chemical and biological sensors. The advantages of OTFT-based sensors include high sensitivity, low cost, easy fabrication, flexibility and biocompatibility. In this paper, we review the chemical sensors and biosensors based on two types of OTFTs, including organic field-effect transistors (OFETs) and organic electrochemical transistors (OECTs), mainly focusing on the papers published in the past 10 years. Various types of OTFT-based sensors, including pH, ion, glucose, DNA, enzyme, antibody-antigen, cell-based sensors, dopamine sensor, etc., are classified and described in the paper in sequence. The sensing mechanisms and the detection limits of the devices are described in details. It is expected that OTFTs may have more important applications in chemical and biological sensing with the development of organic electronics.     

Ultra-sensitive NEMS-based cantilevers for sensing, scanned probe and very high-frequency applications

Scanning probe microscopies (SPM) and cantilever-based sensors generally use low-frequency mechanical devices of microscale dimensions or larger. Almost universally, off-chip methods are used to sense displacement in these devices, but this approach is not suitable for nanoscale devices. Nanoscale mechanical sensors offer a greatly enhanced performance that is unattainable with microscale devices. Here we describe the fabrication and operation of self-sensing nanocantilevers with fundamental mechanical resonances up to very high frequencies (VHF). These devices use integrated electronic displacement transducers based on piezoresistive thin metal films, permitting straightforward and optimal nanodevice readout. This non-optical transduction enables applications requiring previously inaccessible sensitivity and bandwidth, such as fast SPM and VHF force sensing. Detection of 127 MHz cantilever vibrations is demonstrated with a thermomechanical-noise-limited displacement sensitivity of 39 fm Hz(-1/2). Our smallest devices, with dimensions approaching the mean free path at atmospheric pressure, maintain high resonance quality factors in ambient conditions. This enables chemisorption measurements in air at room temperature, with unprecedented mass resolution less than 1 attogram (10(-18) g).     

Enhancement of ethanol vapor sensing of TiO2 nanobelts by surface engineering

TiO(2) nanobelts were prepared by a hydrothermal process, and the structures were manipulated by surface engineering, including surface coarsening by an acid-corrosion procedure and formation of Ag-TiO(2) heterostuctures on TiO(2) nanobelts surface by photoreduction. Their performance in the detection of ethanol vapor was then examined and compared by electrical conductivity measurements at varied temperatures. Of the sensors based on the four nanobelt samples (TiO(2) nanobelts, Ag-TiO(2) nanobelts, surface-coarsened TiO(2) nanobelts, and surface-coarsened Ag-TiO(2) nanobelts), they all displayed improved sensitivity, selectivity, and short response times for ethanol vapor detection, in comparison with sensors based on other oxide nanostructures. Importantly, the formation of Ag-TiO(2) heterostuctures on TiO(2) nanobelts surface and surface coarsening of TiO(2) nanobelts were found to lead to apparent further enhancement of the sensors sensitivity, as well as a decrease of the optimal working temperature. That is, within the present experimental context, the vapor sensor based on surface-coarsened Ag-TiO(2) composite nanobelts exhibited the best performance. The sensing mechanism was interpreted on the basis of the surface depletion model, and the improvement by oxide surface engineering was accounted for by the chemical sensitization mechanism. This work provided a practical approach to the enhancement of gas sensing performance by one-dimensional oxide nanomaterials.     

GaN and InN nanocolumns as electrochemical sensing elements: Potentiometric response to KCl, pH and urea

GaN and InN nanocolumns have been explored as electrochemical sensing elements. It is shown that these nanostructured sensors respond to anions as their corresponding compact crystals, but with a much lower sensitivity (chloride anion detection). Moreover, even though both kinds of nanocolumns are found to be more vulnerable to HCl etching compared to their flat counterparts, they exhibit quite high pH sensitivity. This fact allows for their use as pH sensors only under mild conditions, such as in biosensing systems, as it is proved with the example of urea biosensors.     

Functionalizing nanowires with catalytic nanoparticles for gas sensing application

Metal oxide semiconducting nanowires are among the most promising materials systems for use as conductometric gas sensors. These systems function by converting surface chemical processes, often catalytic processes, into observable conductance variations in the nanowire. The surface properties, and hence the sensing properties of these devices can be altered dramatically improving the sensitivity and selectivity, by the deposition of catalytic metal nanoparticles on the nanowire's surface. This leads not only to promising sensor strategies but to a route for understanding some of the fundamental science occurring on these nanoparticles and at the metal/nanowire junction. In particular studying these systems can lead to a better understanding of the influence of the catalyst particle on the electronic structure of the nanowire and its electron transport. This report surveys results obtained so far in this area. In particular, the comparative sensing performance of single quasi-1D chemiresistors (i.e., nanowires or nanobelts) before and after surface decoration with noble metal catalyst particles show significant improvement in sensitivity toward oxidizing and reducing gases. Moreover, one finds that the sensing mechanism can depend dramatically on the degree of metal coverage of the nanowire.     

Folic acid mediated synthesis of hierarchical ZnO micro-flower with improved gas sensing properties

The microscale structure and size are extremely important factors for gas sensing materials. In this study, hierarchical flower-like ZnO architectures were synthesized by a biomolecular mediated route. The influence of various experiment parameters including reaction time, pH value, and reaction temperature on the formation of ZnO architectures was studied. When used as sensing material, this material possesses a higher sensing response towards ethanol and formaldehyde. Towards 100 ppm of ethanol and formaldehyde, the ZnO sensor can display remarkable sensing responses (R_a/R_g) of 13.6 and 16.5, respectively. These values are higher than or comparable to most of reported ZnO-based gas sensors. In addition, the sensors can show obvious sensing response to 5 ppm of ethanol and formaldehyde, indicating the lower limit of detection. It is proposed that the unique hierarchical microstructure contributes to the enhanced sensing performance.     

Recent progress in the ZnO nanostructure-based sensors

This review focuses on the sensors based on zinc oxide (ZnO) nanostructures, which have fascinating properties including large specific surface area, good biocompatibility, high electron mobility and piezoelectricity. Due to these versatile characteristics, ZnO nanostructures can be based upon to construct gas sensors, chemical sensors, biosensors, UV sensors, pH sensors and other sensors with different sensing mechanisms. The main structures of the sensors and factors influencing the sensitivity are also discussed.     

Functional,RF‐Trilayer Sensors for Tooth‐Mounted,Wireless Monitoring of the Oral Cavity and Food Consumption

Wearable devices have emerged as powerful tools for personalized healthcare in spite of some challenges that limit their widespread applicability as continuous monitors of physiological information. Here, a materials‐based strategy to add utility to traditional dielectric sensors by developing a conformal radiofrequency (RF) construct composed of an active layer encapsulated between two reverse‐facing split ring resonators is applied. These small (down to 2 mm × 2 mm) passive dielectric sensors possess enhanced sensitivity and can be further augmented by functionalization of this interlayer material. Demonstrator devices are shown where the interlayer is: (i) a porous silk film, and (ii) a modified PNIPAM hydrogel that swells with pH or temperature. In vivo use is demonstrated by adhesion of the device on tooth enamel to detect foods during human ingestion. Such sensors can be easily multiplexed and yield data‐rich temporal information during the diffusion of analytes within the trilayer structure. This format could be extended to a suite of interlayer materials for sensing devices of added use and specificity.     

Hydrogen sensing in titanate nanotubes associated with modulation in protonic conduction

Hydrogen/sodium titanate nanotubes (TNTs) were investigated as hydrogen (H(2)) sensors. TNT films exhibit good sensing properties and a large response, in particular at room temperature. Electrical conductivity measurements performed under different atmospheres from 25 to 300?°C indicate that, for T > 100?°C, conduction is thermally activated and can be attributed to electronic transport, whereas for T < 100?°C conduction is dominated by protonic transport. The T dependence of the H(2) sensitivity was determined and related to this variation in the dominant transport mechanism. For low T, H(2) sensing originates from the modulation in protonic conduction. Such modulation was attributed to the creation/destruction of surface hydroxyl groups.     

Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles

The sensing ability of individual SnO(2) nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface.     

基于ZnO复合材料的芯片式pH和温度传感器

可穿戴传感器可以方便地监测汗液pH、体表温度等信号,以此判断人体的健康状况,因而吸引了广泛注意。本研究制备了一种用于检测人体皮肤表面温度及汗液pH的芯片式传感器。pH传感器为ZnO/聚苯胺(PAni)微纳米结构,在不同pH溶液中的表面电位不同,灵敏度达120mV/pH。温度传感器为ZnO/还原氧化石墨烯(rGO)复合材料,用简单的滴落涂布法在聚对苯二甲酸乙二醇酯/氧化铟锡(PET/ITO)导电电极表面修饰一层ZnO/rGO。随着温度的升高, ZnO/rGO复合材料的电阻下降,其电阻变化量的灵敏度达–0.67%/℃。两种传感材料可以集成在一个微小的芯片上,获得的多功能传感器表现出较高的稳定性,在皮肤表面pH和温度检测方面具有潜在的应用价值。     

SnO/sub 2/ gas sensing array for combustible and explosive gas leakage recognition

A gas-sensing array with ten different SnO/sub 2/ sensors was fabricated on a substrate for the purpose of recognizing various kinds and quantities of indoor combustible gas leakages, such as methane, propane, butane, LPG, and carbon monoxide, within their respective threshold limit value (TLV) and lower explosion limit (LEL) range. Nano-sized sensing materials with high surface areas were prepared by coprecipitating SnCl/sub 4/ with Ca and Pt, while the sensing patterns of the SnO/sub 2/-based sensors were differentiated by utilizing different additives. The sensors in the sensor array were designed to produce a uniform thermal distribution along with a high and differentiated sensitivity and reproducibility for low concentrations below 100 ppm. Using the sensing signals of the array, an electronic nose system was then applied to classify and identify simple/mixed explosive gas leakages. A gas pattern recognizer was implemented using a neuro-fuzzy network and multi-layer neural network, including an error-back-propagation learning algorithm. Simulation and experimental results confirmed that the proposed gas recognition system was effective in identifying explosive and hazardous gas leakages. The electronic nose in conjunction with a neuro-fuzzy network was also implemented using a digital signal processor (DSP).     

Methanol, ethanol and hydrogen sensing using metal oxide and metal (TiO(2)-Pt) composite nanoclusters on GaN nanowires: a new route towards tailoring the selectivity of nanowire/nanocluster chemical sensors

We demonstrate a new method for tailoring the selectivity of chemical sensors using semiconductor nanowires (NWs) decorated with metal and metal oxide multicomponent nanoclusters (NCs). Here we present the change of selectivity of titanium dioxide (TiO(2)) nanocluster-coated gallium nitride (GaN) nanowire sensor devices on the addition of platinum (Pt) nanoclusters. The hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO(2) and/or Pt nanoclusters (NCs) using the sputtering technique. This paper present the sensing characteristics of GaN/(TiO(2)-Pt) nanowire-nanocluster (NWNC) hybrids and GaN/(Pt) NWNC hybrids, and compare their selectivity with that of the previously reported GaN/TiO(2) sensors. The GaN/TiO(2) NWNC hybrids showed remarkable selectivity to benzene and related aromatic compounds, with no measurable response for other analytes. Addition of Pt NCs to GaN/TiO(2) sensors dramatically altered their sensing behavior, making them sensitive only to methanol, ethanol and hydrogen, but not to any other chemicals we tested. The GaN/(TiO(2)-Pt) hybrids were able to detect ethanol and methanol concentrations as low as 100 nmol mol(-1) (ppb) in air in approximately 100 s, and hydrogen concentrations from 1 μmol mol(-1) (ppm) to 1% in nitrogen in less than 60 s. However, GaN/Pt NWNC hybrids showed limited sensitivity only towards hydrogen and not towards any alcohols. All these hybrid sensors worked at room temperature and are photomodulated, i.e. they responded to analytes only in the presence of ultraviolet (UV) light. We propose a qualitative explanation based on the heat of adsorption, ionization energy and solvent polarity to explain the observed selectivity of the different hybrids. These results are significant from the standpoint of applications requiring room-temperature hydrogen sensing and sensitive alcohol monitoring. These results demonstrate the tremendous potential for tailoring the selectivity of the hybrid nanosensors for a multitude of environmental and industrial sensing applications.     

Oxygen sensing properties of Cu(I) complex/polystyrene composite nanofibers prepared by electrospinning

The preparation and oxygen sensing properties of luminescence quenching oxygen sensing materials based on an electrospum composite nanofibers with [Cu(POP)DPPZ]BF4(POP = bis[2-(diphenylphosphino)phenyl]ether; DPPZ = dipyrido[2,3-a:3',2'-c]phenazine) complex embedded within polystyrene (PS) matrix are described in this article. The luminescence of [Cu(POP)DPPZ]BF4 complex within the electrospum nanofibers can be efficiently quenched by oxygen with good sensitivity (I0/I100 = 5.54) and rapid response (3 s), which suggests that the Cu(I) complex/polystyrene composite nanofibers presented in this article can be used for developing oxygen sensors. The downward oxygen sensing Stern-Volmer plots can be attributed to the heterogeneous environment of the Cu(I) luminophore within the nanofibers. The high sensitivity is attributed to the low-dimensional structure of the nanofiber and its large surface area-to-volume ratio, which facilitates oxygen entry and diffusion.     

Chemical sensors based on micromachined transducers with integrated piezoresistive readout

We demonstrate an approach for the development of chemical sensors utilizing silicon micromachined physical transducers with integrated piezoresistive readout. Originally, these transducers were developed and optimized as sensitive accelerometers for automotive applications. However, by applying a chemically responsive layer onto the transducer, we convert these transducers into chemical sensors. These transducers are attractive for chemical sensing applications for several key reasons. First, the required sensitivity of the chemical sensor can be achieved by choosing the right spring constant of the transducer. Second, the integrated piezoresistive readout of the transducer is already optimized and is very straightforward, providing a desired reproducibility in measurements, while not requiring bulky equipment. Third, chemically responsive film deposition is simple due to the ease of access to the transducer's surface. Fourth, such transducers are already available for another (automotive) application, making these sensors very cost-effective. The applicability of this approach is illustrated by the fabrication of highly sensitive CO2 sensors. To study hysteresis effects, we selected high CO2 concentrations (10-100% CO2) to provide the worst-case scenario for the sensor operation. These sensors demonstrate a hysteresis-free performance over the concentration range from 10 to 100% vol CO2, have detection limits of 160-370 ppm of CO2, and exhibit a relatively rapid response time, T(90) = 45 s. Importantly, we demonstrate a simple method for cancellation of vibration effects when these physical transducers, initially developed as accelerometers, are applied as chemical sensors.     

WO3 thin film prepared by PECVD technique and its gas sensing properties to NO2

Tungsten oxide films have been successfully fabricated from tungsten oxychloride (WOCl₄) precursor by using plasma enhanced vapor deposition (PECVD) technique. The films were deposited onto silicon substrates and ceramic tubes maintained at 100°C under a constant operating pressure of He-O₂ gas mixtures. The compositions and the structures of the thin films have been investigated by means of anaysis methods, such as XRD, XPS, UV and IR. The as-deposited WO₃ thin films were amorphous state and became crystalline after annealing above 400°C. The surface analysis of the films indicates that stoichiometry O/W is 2.77 : 1. The gas sensing measurements of the WO₃ thin film sensors indicate that these sensors have a high sensitivity, excellent selectivity and quick response behavior to NO₂.