Hydrogen bond acidic polymers for surface acoustic wave vapor sensors and arrays

Four hydrogen bond acidic polymers are examined as sorbent layers on acoustic wave devices for the detection of basic vapors. A polysiloxane polymer with pendant hexafluoro-2-propanol groups and polymers with hexafluorobisphenol groups linked by oligosiloxane spacers yield sensors that respond more rapidly and with greater sensitivity than fluoropolyol, a material used in previous SAW sensor studies. Sensors coated with the new materials all reach 90% of full response within 6 s of the first indication of a response. Unsupervised learning techniques applied to pattern-normalized sensor array data were used to examine the spread of vapor data in feature space when the array does or does not contain hydrogen bond acidic polymers. The radial distance in degrees between pattern-normalized data points was utilized to obtain quantifiable distances that could be compared as the number and chemical diversity of the polymers in the array were varied. The hydrogen bond acidic polymers significantly increase the distances between basic vapors and nonpolar vapors when included in the array.     

Frequency encoding of resonant mass sensors for chemical vapor detection

Chemical vapors can be detected by a resonant mass sensor array with selective absorption coatings implementing a frequency encoding method. The sensor array consists of sensor elements with different frequencies for their identifications in the frequency response obtained with a pulse Fourier transform detection scheme. Zero-loading resonance frequencies are chosen so that frequency shift due to absorption is bounded within a predefined region so that there is no overlap of peaks and all peaks can be assigned to the correct elements at any operation conditions. Mechanical oscillations of all or selected numbers of the sensor elements are excited by application of an excitation signal. Free oscillation decay signals from all or selectively excited sensor elements are detected and digitized. The free oscillation decay signal is subjected to a spectral analysis routine converting into a frequency spectrum, in which frequency shifts due to absorption of chemical vapors can be obtained. The implementation of the frequency encoding method with pulse Fourier transform detection to resonant mass sensors allows simultaneous multisensor detection, fast data acquisition speed, high signal-to-noise ratio by coaddition of raw data, flexible excitation, reduced complexity of electronic hardware, application of advanced data/spectral analysis algorithms, and realization of many other advantages by the introduction of the pulse Fourier transform method. A practical chemical vapor sensing system is demonstrated experimentally by use of nine frequency-encoded and polymer-coated sensors.     

Hazardous industrial gases identified using a novel polymer/MWNT composite resistance sensor array

Hazardous industrial chemical gases pose a significant threat to the environment and human life. Therefore, there is an urgent need to develop a reliable sensor for identifying these hazardous gases. In this work, a silicon wafer microelectrode substrate for a resistance sensor was fabricated using the semiconductor manufacturing process. Conductive carbon nanotubes were then mixed with six different polymers with different chemical adsorption properties to produce a composite thin film for the fabrication of a chemical sensor array. This array was then utilized to identify three hazardous gases at different temperatures. Experimental results for six polymers for chemical gases, such as tetrahydrofuran (THF), chloroform (CHCl₃) and methanol (MeOH) at different temperatures, indicate that the variation in sensitivity resistance increased when the sensing temperature increased. The poly(ethylene adipate)/MWNT sensing film had high sensitivity, excellent selectivity, and good reproducibility in detecting all chemical agent vapors. Additionally, this study utilized a bar chart and statistical methods in principal component analysis to identify gases with the polymer/MWNT sensor.     

Multianalyte chemical identification and quantitation using a single radio frequency identification sensor

We demonstrate an approach for multianalyte chemical identification and quantitation using a single conventional radio frequency identification (RFID) tag that has been adapted for chemical sensing. Unlike other approaches of using RFID sensors, where a special tag should be designed at a much higher cost, we utilize a conventional RFID tag and coat it with a chemically sensitive film. As an example, we demonstrate detection of several vapors of industrial, health, law enforcement, and security interest (ethanol, methanol, acetonitrile, water vapors) with a single 13.56-MHz RFID tag coated with a solid polymer electrolyte sensing film. By measuring simultaneously several parameters of the complex impedance from such an RFID sensor and applying multivariate statistical analysis methods, we were able to identify and quantify several vapors of interest. With a careful selection of the sensing film and measurement conditions, we achieved parts-per-billion vapor detection limits in air. These RFID sensors are very attractive as ubiquitous multianalyte distributed sensor networks.     

Ordered‐Assembly Conductive Nanowires Array with Tunable Polymeric Structure for Specific Organic Vapor Detection

An artificial organic vapor sensor based on a finite number of 1D nanowires arrays can provide a strategy to allow classification and identification of different analytes with high efficiency, but fabricating a 1D nanowires array is challenging. Here, a coaxial Ag/polymer nanowires array is prepared as an organic vapor sensor with specific recognition, using a strategy combining superwettability‐based nanofabrication and polymeric swelling‐induced resistance change. Such organic vapor sensor containing commercial polymers can successfully classify and identify various organic vapors with good separation efficiency. An Ag/polymer nanowires array with synthetic polyethersulfone polymers is also fabricated, through molecular structure modification of the polymers, to distinguish the similar organic vapors of methanol and ethanol. Theoretical simulation results demonstrate introduction of specific molecular interaction between the designed polymers and organic vapors can improve the specific recognition performance of the sensors.     

用于压/拉检测的机器人皮肤柔/弹性传感器阵列设计

基于炭黑/硅橡胶复合材料的压力以及拉伸敏感特性,设计了一种可用于检测机器人皮肤接触压力和拉伸特性的新型传感器阵列。该传感器阵列的弹性电极结构克服了传统传感器不可拉伸的缺点,实现了传感器阵列的柔/弹性;阵列中设计了9个检测压力的传感器单元和2个检测拉伸的传感器单元,通过传感器结构设计和拉/压干扰特性分析以及补偿算法解决了拉伸和压力同时测量时的干扰问题,并构建了求解压力与拉伸的数学模型。实验结果表明:该传感器阵列实现了对压力和拉伸的同步检测,可用于机器人柔性皮肤中关节等部位。     

Identification of Different Vapors Using a Single Temperature Modulated Polymer Sensor With a Novel Signal Processing Technique

This paper reports on a novel temperature modulation technique to improve the ability of a single carbon black/polymer resistive sensor to discriminate, identify and characterize environmental pollutants. Such a sensor can be low cost, low-power consumption, highly reliable, and thus ideal for indoor gas monitoring. Here, a carbon black/polyvinylpyrrolidone film was deposited onto an SOI-CMOS microhotplate. A novel temperature modulation technique is proposed that allows the detection of vapors using a single chemoresistor. Identification and quantification of water, methanol and ethanol vapors with different concentrations (water: 0–6000 ppm, methanol: 1360–5430 ppm and ethanol: 2270–9080 ppm) are shown.      

High-speed fluorescence detection of explosives-like vapors

In this paper, we report on the preparation of novel cross-reactive optical microsensors for high-speed detection of low-level explosives and explosives-like vapors. Porous silica microspheres with an incorporated environmentally sensitive fluorescent dye are employed in high-density sensor arrays to monitor fluorescence changes during nitroaromatic compound (NAC) vapor exposure. The porous silica-based sensor materials have good adsorption characteristics, high surface areas, and surface functionality to help maximize analyte-dye interactions. These interactions occur immediately upon vapor exposure, i.e., in less than 200 ms and are monitored with a high-speed charge-coupled device camera to produce characteristic and reproducible vapor response profiles for individual sensors within an array. Employing thousands of identical microsensors permits sensor responses to be combined, which significantly reduces sensor noise and enhances detection limits. Normalized response profiles for 1,3-dinitrobenzene (1,3-DNB) are independent of analyte concentration, analyte exposure time, or sensor age for an array of one sensor type. Explosives-like NACs such as 2,4-dinitrotoluene and DNB are detected at low part-per-billion levels in seconds. Sensor-analyte profiles of some sensor types are more sensitive to low-level NAC vapor even when in a higher organic vapor background. We show that single-element arrays permit the detection of low-level nitroaromatic compound vapors because of sensor-to-sensor reproducibility and signal averaging.     

Biocompatible force sensor with optical readout and dimensions of 6 nm3

We have developed a nanoscopic force sensor with optical readout. The sensor consists of a single-stranded DNA oligomer flanked by two dyes. The DNA acts as a nonlinear spring: when the spring is stretched, the distance between the two dyes increases, resulting in reduced F?rster resonance energy transfer. The sensor was calibrated between 0 and 20 pN using a combined magnetic tweezers/single-molecule fluorescence microscope. We show that it is possible to tune the sensor's force response by varying the interdye spacing and that the FRET efficiency of the sensors decreases with increasing force. We demonstrate the usefulness of these sensors by using them to measure the forces internal to a single polymer molecule, a small DNA loop. Partial conversion of the single-stranded DNA loop to a double-stranded form results in the accumulation of strain: a force of approximately 6 pN was measured in the loop upon hybridization. The sensors should allow measurement of forces internal to various materials, including programmable DNA self-assemblies, polymer meshes, and DNA-based machines.     

The fabrication of a MWNTs–polymer composite chemoresistive sensor array to discriminate between chemical toxic agents

In this study, a chemoresistive sensor was fabricated by the chemical polymerization and coating of either polyaniline (PANI), poly[2-methoxy-5-(2-ethyloxy)-p-phenylenevinylene], or commercial poly(methyl methacrylate) on MWNTs. We investigated the resistance responsiveness of the multilayer samples to simulated chemical warfare agents, including dimethyl methyl phosphonate (DMMP) and dichloromethane (DCM), as well as to organic agents, such as chloroform, tetrahydrofuran, methyl-ethyl ketone, and xylene. The MWNTs–PANI film was characterized by SEM and FT-IR, and the resistivity values for the six solvents were measured at different temperatures. We observed that the MWNTs-PANI sensing film exhibited a high sensitivity, excellent selectivity, and good reproducibility to the detection of all of the aforementioned agent vapors. In addition, we used atomic force microscopy to demonstrate the MWNTs–PANI absorption of DMMP vapor, wherein the sensing film exhibited a swelling phenomenon, such that the film thickness increased from 0.8 to 1.3 μm. In addition, we used principal component analysis to evaluate the performance of the sensor in detecting DMMP, DCM, and the aforementioned organic agent vapors.     

A carbon nanotube needle biosensor

A carbon nanotube needle biosensor was developed to provide fast, cost effective and highly sensitive electrochemical detection of biomolecules. The sensor was fabricated based on an array of aligned multi-wall carbon nanotubes synthesized by chemical vapor deposition. A bundle of nanotubes in the array was welded onto the tip of a tungsten needle under a microscope. The needle was then encased in glass and a polymer coating leaving only the tip of the needle exposed. Cyclic voltammetry was performed to examine the redox behavior of the nanotube needle. The cyclic voltammetry results showed a steady-state response attributable to radial diffusion with a high steady-state current density. An amperometric sensor was then developed for glucose detection by physically attaching glucose oxidase on the nanotube needle. The amperometric response of these nanotube needles showed a high sensitivity with a low detection limit. It is expected that the nanotube needle can be sharpened to increase the sensitivity to the point where the current is almost too small to measure. The simple manufacturing method should allow commodity level production of highly sensitive electronic biosensors.     

Detection of organic chemical vapors with a MWNTs-polymer array chemiresistive sensor

Organic chemical hazardous gases pose a significant threat to human life and the environment. An urgent need exists for the development of reliable chemical sensors that would be able to identify these hazardous gases. In a recent study, conductive carbon nanotubes were mixed with six polymers with various chemical adsorption properties to produce a composite thin film for the fabrication of a chemical sensor array. A silicon wafer was used as a microelectrode substrate for a resistance sensor fabricated using a typical semiconductor manufacturing process. This sensor array was then used to identify hazardous chemical gases at various temperatures. Results for two hazardous gases, ammonia (NH₃) and chloroform (CHCl₃), tested with the six polymers at different temperatures, indicated that the variation in sensitivity/resistance increased when the temperature increased. It was found that the MWNTs-PVP and MWNTs-PMVEMA sensing films had high sensitivity, excellent selectivity, and favorable reproducibility in detecting the two chemical agent vapors. In addition, we derived the solubility parameter (Δδ) to demonstrate the sensitivity of the polymers to ammonia (NH₃). The results showed that smaller solubility parameter corresponds to a stronger interaction between NH₃ gas and polymers, and increased sensitivity. Additionally, we used the statistical methods of principal component analysis to identify the interaction of hazardous gases with the MWNTs-polymer sensor.     

Sensitive Chemical Optic Sensor Using Birefringent Porous Glass for the Detection of Volatile Organic Compounds

A simple design involving a birefringent porous glass oriented between two crossed polarizers serves as the foundation for an optically based sensitive broad-spectrum chemical sensor. Volatile organic compounds (VOCs) such as acetonitrile vapors can be readily detected at concentrations of as low as 50 ppm. Changes are observed in polarized light transmitted by the anisotropic porous material constituting the sensor, upon exposure to VOC-bearing air, as intensity changes at a defined wavelength or as changes in spectral content (color) detectable by the eye. The optical effects resulting from exposure to various vapors are reversible and may result from adsorption of solvent vapors with attendant reduction of anisotropy. The microporous structure as well as the surface chemistry of the sensor may be controlled for tuning the response to VOCs for industrial applications. Miniaturization of the sensor using low-cost materials such as plastic or glass optical fibers, Polaroid films, and birefringent porous glass is demonstrated. The sensor described in this paper could use ambient light as source and the eye as detector (color change) or electronically controlled light emission and detection for better sensitivity and real time monitoring of VOCs. Such intrinsic explosion proof sensors could be used to safely monitor VOC levels in remote environments.     

Single-walled carbon nanotube paper as a sorbent for organic vapor preconcentration

Single-walled carbon nanotubes were examined as an adsorptive material for a thermally desorbed preconcentrator for organic vapors. The nanotubes were processed into a paper form and packed into a metal tube for flow-through sampling. Adsorbed vapors were released by a temperature-programmed desorption method and detected downstream with a flexural plate wave vapor sensor. The tested vapors, methyl ethyl ketone, toluene, and dimethyl methylphosphonate, were released from the packed column at different temperatures. The vapors were retained more strongly than previously observed for the widely used Tenax porous polymer, indicating a significant affinity of the single walled nanotubes for organic vapors.     

Measuring the Prong Velocity of Quartz Tuning Forks Used to Probe Quantum Fluids

Recently, quartz tuning forks have been used to probe the dynamics of quantum fluids. For many of these measurements it is important to know the velocity amplitude of the tips of the vibrating fork prongs. We have used different techniques to establish, with an accuracy of a few percent, the relationship between the electrical and mechanical properties of several commercial quartz tuning forks with fundamental resonant frequency ～32 kHz. The velocity is usually inferred from an electro-mechanical calibration that models a quartz prong as a clamped, rectangular cantilever beam. We have tested the accuracy of this calibration using three methods: measurement of the amplitude at which the fork prongs touch each other; direct optical measurement of the moving fork prongs using strobe microscopy; and a Michelson interferometry technique operating with a 670 nm laser. All three methods yield consistent results. The velocity so determined is found to be 10% lower than that of the standard electro-mechanical calibration.     

Fiber-optic chemical sensor using polyaniline as modified cladding material

An intrinsic fiber-optic chemical sensor is developed by replacing a certain portion of the original cladding with a chemically sensitive material, specifically, polyaniline. Both the light absorption coefficient and the refractive index of the polymer change upon its exposure to different chemical vapors. These changes induce the optical intensity modulation of the transmitted optical signal. Therefore, the developed structure is proposed for optical sensor applications. The performance of the sensor depends on the optical properties of the replaced cladding material, the cladding modification process, and the properties of the transmitted light within the optical fiber.     

Rapid analyte recognition in a device based on optical sensors and the olfactory system

We report here the development of a new vapor sensing device that is designed as an array of optically based chemosensors providing input to a pattern recognition system incorporating artificial neural networks. Distributed sensors providing inputs to an integrative circuit is a principle derived from studies of the vertebrate olfactory system. In the present device, primary chemosensing input is provided by an array of fiber-optic sensors. The individual fiber sensors, which are broadly yet differentially responsive, were constructed by immobilizing molecules of the fluorescent indicator dye Nile Red in polymer matrices of varying polarity, hydrophobicity, pore size, elasticity, and swelling tendency, creating unique sensing regions that interact differently with vapor molecules. The fluorescent signals obtained from each fiber sensor in response to 2-s applications of different analyte vapors have unique temporal characteristics. Using signals from the fiber array as inputs, artificial neural networks were trained to identify both single analytes and binary mixtures, as well as relative concentrations. Networks trained with integrated response data from the array or with temporal data from a single fiber made numerous errors in analyte identification across concentrations. However, when trained with temporal information from the fiber array, networks using "name" or "characteristic" output codes performed well in identifying test analytes.     

Volatile organic compound detection using nanostructured copolymers

Regioregular polythiophene-based conductive copolymers with highly crystalline nanostructures are shown to hold considerable promise as the active layer in volatile organic compound (VOC) chemresistor sensors. While the regioregular polythiophene polymer chain provides a charge conduction path, its chemical sensing selectivity and sensitivity can be altered either by incorporating a second polymer to form a block copolymer or by making a random copolymer of polythiophene with different alkyl side chains. The copolymers were exposed to a variety of VOC vapors, and the electrical conductivity of these copolymers increased or decreased depending upon the polymer composition and the specific analytes. Measurements were made at room temperature, and the responses were found to be fast and appeared to be completely reversible. Using various copolymers of polythiophene in a sensor array can provide much better discrimination to various analytes than existing solid state sensors. Our data strongly indicate that several sensing mechanisms are at play simultaneously, and we briefly discuss some of them.