Position-independent chemical quantitation with passive 13.56-MHz radio frequency identification (RFID) sensors

Recently, we have demonstrated an attractive approach to adapt conventional radio frequency identification (RFID) tags for multianalyte chemical sensing. These RFID sensors could be very attractive as ubiquitous distributed remote sensor networks. However, critical to the wide acceptance of the demonstrated RFID sensors is the analyte-quantitation ability of these sensors in presence of possible repositioning errors between the RFID sensor and its pickup coil. In this study, we evaluate the capability for such position-independent analyte quantification using multivariate analysis tools. By measuring simultaneously several parameters of the complex impedance from such an RFID sensor and applying multivariate statistical analysis methods, we were able to compensate for the repositioning effects such as baseline signal offset and magnitude of sensor response to an analyte.     

Use of spatiotemporal response information from sorption-based sensor arrays to identify and quantify the composition of analyte mixtures

Linear sensor arrays made from small molecule/carbon black composite chemiresistors placed in a low-headspace volume chamber, with vapor delivered at low flow rates, allowed for the extraction of new chemical information that significantly increased the ability of the sensor arrays to identify vapor mixture components and to quantify their concentrations. Each sensor sorbed vapors from the gas stream and, thereby, as in gas chromatography, separated species having high vapor pressures from species having low vapor pressures. Instead of producing only equilibrium-based sensor responses that were representative of the thermodynamic equilibrium partitioning of analyte between each sensor and the initial vapor phase, the sensor responses varied depending on the position of the sensor in the chamber and the time since the beginning of the analyte exposure. The concomitant spatiotemporal (ST) sensor array response therefore provided information that was a function of time, as well as of the position of the sensor in the chamber. The responses to pure analytes and to multicomponent analyte mixtures comprised of hexane, decane, ethyl acetate, chlorobenzene, ethanol, and/or butanol were recorded along each of the sensor arrays. Use of a non-negative least-squares (NNLS) method for analysis of the ST data enabled the correct identification and quantification of the composition of two-, three-, four-, and five-component mixtures from arrays using only four chemically different sorbent films. In contrast, when traditional time- and position-independent sensor response information was used, these same mixtures could not be identified or quantified robustly. The work has also demonstrated that, for ST data, NNLS yielded significantly better results than analyses using extended disjoint principal components modeling. The ability to correctly identify and quantify constituent components of vapor mixtures through the use of such ST information significantly expands the capabilities of such broadly cross-reactive arrays of sensors.     

Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gas chromatography

We developed and characterized a rapid, sensitive and integrated optical vapor sensor array for micro-gas chromatography (μGC) applications. The sensor is based on the Fabry-Pérot (FP) interferometer formed by a micrometre-thin vapor-sensitive polymer layer coated on a silicon wafer. The thickness and the refractive index of the polymer vary in response to the vapor analyte, resulting in a change in the reflected intensity of the laser impinged on the sensor. In our study, four different polymers were coated on four wells pre-etched on a silicon wafer to form a spatially separated sensor array. A CMOS imager was employed to simultaneously monitor the polymers' response, thus enabling multiplexed detection of a vapor analyte passing through the GC column. A sub-second detection time was demonstrated. In addition, a sub-picogram detection limit was achieved, representing orders of magnitude improvement over the on-chip vapor sensors previously reported.     

Understanding the dynamics of signal transduction for adsorption of gases and vapors on carbon nanotube sensors

Adsorption dynamics and their influence on signal transduction for carbon nanotube-based chemical sensors are explored using continuum site balance equations and a mass action model. These sensors are shown to possess both reversible and irreversible binding sites that can be modeled independently. For the case of irreversible adsorption, it is shown that the characteristic response time scales inversely with analyte concentration. It is inappropriate to report a detection limit for this type of sensor since any nonzero analyte concentration can be detected in theory but at a cost of increasing transduction time with decreasing concentration. The response curve should examine the initial rate of signal change as a function of analyte concentration. Conversely, a reversible sensor has a predefined detection limit, independent of the detector geometry with a characteristic time scaling that becomes constant in the zero analyte concentration limit. A simple analytical test is presented to distinguish between these two mechanisms from the transient response of a nanotube sensor array. Two systems appearing in the literature are shown to have an irreversible component, and regressed surface rate constants for this component are similar across different sensor geometries and analytes.     

Gas sensing mechanism in chemiresistive cobalt and metal-free phthalocyanine thin films

The gas sensing behaviors of cobalt phthalocyanine (CoPc) and metal-free phthalocyanine (H2Pc) thin films were investigated with respect to analyte basicity. Chemiresistive sensors were fabricated by deposition of 50 nm thick films on interdigitated gold electrodes via organic molecular beam epitaxy (OMBE). Time-dependent current responses of the films were measured at constant voltage during exposure to analyte vapor doses. The analytes spanned a range of electron donor and hydrogen-bonding strengths. It was found that, when the analyte exceeded a critical base strength, the device responses for CoPc correlated with Lewis basicity, and device responses for H2Pc correlated with hydrogen-bond basicity. This suggests that the analyte-phthalocyanine interaction is dominated by binding to the central cavity of the phthalocyanine with analyte coordination strength governing CoPc sensor responses and analyte hydrogen-bonding ability governing H2Pc sensor responses. The interactions between the phthalocyanine films and analytes were found to follow first-order kinetics. The influence of O2 on the film response was found to significantly affect sensor response and recovery. The increase of resistance generally observed for analyte binding can be attributed to hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands.     

聚氯乙烯/炭黑导电复合体系的压阻行为

研究了聚氯乙烯/炭黑(PVC/CB)导电复合体系在单轴压力作用下的压阻行为,发现CB含量对电阻-机械响应具有显著的影响.当CB含量低于渗流阈值时,PVC/CB复合材料呈现PPC效应;而高于渗流阈值时,呈现NPC效应.在渗流阈值附近,单轴压缩可诱导NPC效应的出现,或者抑制PPC效应.     

Interactions of DNA with graphene and sensing applications of graphene field-effect transistor devices: A review

Graphene field-effect transistors (GFET) have emerged as powerful detection platforms enabled by the advent of chemical vapor deposition (CVD) production of the unique atomically thin 2D material on a large scale. DNA aptamers, short target-specific oligonucleotides, are excellent sensor moieties for GFETs due to their strong affinity to graphene, relatively short chain-length, selectivity, and a high degree of analyte variability. However, the interaction between DNA and graphene is not fully understood, leading to questions about the structure of surface-bound DNA, including the morphology of DNA nanostructures and the nature of the electronic response seen from analyte binding. This review critically evaluates recent insights into the nature of the DNA graphene interaction and its affect on sensor viability for DNA, small molecules, and proteins with respect to previously established sensing methods. We first discuss the sorption of DNA to graphene to introduce the interactions and forces acting in DNA based GFET devices and how these forces can potentially affect the performance of increasingly popular DNA aptamers and even future DNA nanostructures as sensor substrates. Next, we discuss the novel use of GFETs to detect DNA and the underlying electronic phenomena that are typically used as benchmarks for characterizing the analyte response of these devices. Finally, we address the use of DNA aptamers to increase the selectivity of GFET sensors for small molecules and proteins and compare them with other, state of the art, detection methods.     

Toward Sol-Gel-Processed Chemical Sensing Platforms: Effects of Dopant Addition Time on Sensor Performance

The development of new chemical and biochemical sensing schemes has been a topic of growing interest. Simplicity of preparation and mild processing conditions have made sol-gel-derived composites attractive for many chemical sensing schemes. A portion of our research centers on using sol-gel-processed materials for the development of selective sensors. Over the years we have aimed to characterize the analytical performance of these types of sol-gel-based sensing platforms. In the course of this work we recently discovered that the time (prior to casting) when the sensing chemistry is actually doped into the sol-gel processing solution plays a critical role in a given sensor's analytical performance. In this paper we report on the effects of doping time on the behavior of a model organic dopant (pyrene) sequestered within sol-gel-derived microfiber tips and films. We use O₂ as the analyte and determine the sensor sensitivity and temporal response as a function of doping time. We also quantify the local dipolarity of the immediate environment surrounding the average pyrene molecule as a function of doping time.     

Acoustic Wave Mass Sensors

The application of acoustic wave microsensors for mass sensing will be reviewed with focus on the quartz crystal microbalance (QCM) and surface acoustic wave (SAW) devices. The use of QCM and SAW devices in chemical sensing as well as in the determination of solid and liquid properties will be described. In chemical sensing, it is unlikely that a single sensor with a single coating will display a selective and reversible response to a given analyte in a mixture. Alternative strategies such as the use of sensor arrays and the use of sampling devices can be used to improve performance. QCM sensors (QCMs) will oscillate under liquids; their use in under-liquid sensing will be discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective detection of vapor phase hydrogen peroxide with phthalocyanine chemiresistors

The use of hydrogen peroxide as a precursor to improvised explosives has made its detection a topic of critical importance. Chemiresistor arrays comprised of 50 nm thick films of metallophthalocyanines (MPcs) are redox selective vapor sensors of hydrogen peroxide. Hydrogen peroxide is shown to decrease currents in cobalt phthalocyanine sensors while it increases currents in nickel, copper, and metal-free phthalocyanine sensors; oxidation and reduction of hydrogen peroxide via catalysis at the phthalocyanine surface are consistent with the pattern of sensor responses. This represents the first example of MPc vapor sensors being oxidized and reduced by the same analyte by varying the metal center. Consequently, differential analysis by redox contrast with catalytic amplification using a small array of sensors may be used to uniquely identify peroxide vapors. Metallophthalocyanine chemiresistors represent an improvement over existing peroxide vapor detection technologies in durability and selectivity in a greatly decreased package size.     

Description of the response of a fiber optic velocity sensor applied to capillary gas chromatography

This paper explores the response of a novel fiber optics sensor allowing real-time determination of the migration rate of vapor zones in capillary gas chromatography. The sensitivity is related to the gradient of the vapor zone distribution in the capillary and it is highest when vapor zones show steep variations in concentration. The expected linearity between the height of the velocity peaks and the response of a thermal conductivity detector is demonstrated experimentally. The sensor can be used to infer an approximate value of the analyte diffusion coefficient from the time response. Finally, the time evolution of the envelope of the optical signal is explained with experimental evidences.     

Electrical conducting behaviors in polymeric composites with carbonaceous fillers

Several kinds of polymer composites with carbonaceous fillers such as carbon black (CB), vapor‐grown carbon fiber (CF), and carbon nanotube (CNT) are prepared by a gelation/crystallization process or a melt mixing method. The electrical phenomena, changes of electrical conductivities with different filler's type, filler's concentration and temperatures, and the mechanism of electron transport in these carbon‐filled polymer composites are directly influenced by the geometric grain shape and aggregating morphology of the fillers dispersed in the polymer matrix. For the composites of CB and CF, long‐range macroscopic conduction are governed by the percolation phenomenon, the conduction is behaved through the conductive path formed by the conductors' contacting, and the thermal expansion changes the physical dimensions of the entire electrical network and leads to the changes in the electrical phenomenon. Microscopic conduction between conductive elements is influenced by the tunneling barrier or tunneling voltage, which varies with the temperature change, explaining the apparent observation of the temperature dependence of the composites. In comparison with fillers of CB and CF, the CNT performs unique electric properties for their nonspherical geometry and morphology as a three‐dimensional network (high structures), which has been visually proved by SEM photos in our former research, leading to the percolation threshold lower than 1% in the volume fraction and much less temperature dependence in its composites. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1037–1044, 2007     

Survey on mass determination with oscillating systems: Part III. Acoustic wave mass sensors for chemical and biological sensing

We will review the application of acoustic wave mass sensors in chemical and biological sensing with focus on quartz crystal microbalance and surface acoustic wave devices. In chemical sensing, it is unlikely that a single sensor will display a selective and reversible response to a given analyte in a mixture. Alternative strategies such as use of sensor arrays and sampling devices will be discussed to improve performance. We will also discuss applications of quartz crystal microbalance as biosensor in the liquid phase.This revised version was published online in November 2005 with corrections to the Cover Date.     

A Multi‐Component Sensor System for Detection of Amphiphilic Compounds

Many biologically important compounds are amphiphilic in character. Glycolipids, for example, represent a biologically important class of amphiphiles. Receptors and sensors for such compounds must also be amphiphilic making them a challenge to prepare. Here, a cucurbit[8]uril (CB[8])‐based sensor system has been prepared and tested for detection of amphiphilic compounds. This multi‐component system consists: a CB[8], which acts as a hydrophobic lipid receptor, a hydrophilic pyridinium‐based carbohydrate receptor, and a fluorescent indicator. The system self‐assembles in aqueous solution. The pyridinium quenches the fluorescence of the indicator giving a strong turn‐on response when it is displaced by the analyte. The sensor system was characterized by NMR, X‐ray crystallography, and fluorescence titrations.     

A NOVEL GAS SENSOR FROM POLYMER-GRAFTED CARBON BLACK: EFFECTS OF POLYMER,CRYSTALLINE ORGANIC COMPOUND,AND CARBON BLACK ON ELECTRIC RESPONSE TO TETRAHYDROFURAN VAPOR

The responsiveness of electric resistance of three types of composites, polyethylene (PE)/PE-grafted carbon black (CB) (CB-g-PE), N,N'-dicyclohexylcarbodiimide (DCC)/CB-g-PE and PE/DCC/CB-g-PE, to THF vapor was investigated. To modify the CB compatibility in the composite, PE was grafted onto a CB surface. Although responsiveness of composite DCC/CB-g-PE was the most sensitive to THF vapor among the three composites, the composite film is too weak to be of practical use. On the contrary, the responsiveness and the sensitivity of PE/CB-g-PE were too low to be used in many fields. However, through modification by the addition of DCC, responsiveness was greatly improved, and the sensitivity was considerably higher than that of PE/CB-g-PE. The effects of CB-g-PE and DCC content in the composites on the responsiveness to THF vapor was studied in detail. It was found that the responsiveness of electric resistance to THF vapor was caused by the change of the crystalline structure of the composite. A crystalline model was proposed to explain the response of the composite to solvent vapor.     

Micro- and nanomechanical sensors for environmental, chemical, and biological detection

Micro- and nanoelectromechanical systems, including cantilevers and other small scale structures, have been studied for sensor applications. Accurate sensing of gaseous or aqueous environments, chemical vapors, and biomolecules have been demonstrated using a variety of these devices that undergo static deflections or shifts in resonant frequency upon analyte binding. In particular, biological detection of viruses, antigens, DNA, and other proteins is of great interest. While the majority of currently used detection schemes are reliant on biomarkers, such as fluorescent labels, time, effort, and chemical activity could be saved by developing an ultrasensitive method of label-free mass detection. Micro- and nanoscale sensors have been effectively applied as label-free detectors. In the following, we review the technologies and recent developments in the field of micro- and nanoelectromechanical sensors with particular emphasis on their application as biological sensors and recent work towards integrating these sensors in microfluidic systems.     

纤维状填料/HDPE复合体系的导电渗流与流变渗流行为的关系

研究了纤维状导电材料不锈钢纤维(SSF)填充高密度聚乙烯(HDPE)导电复合体系的导电渗流与流变渗流行为之间的关系,并与颗粒状导电颗粒炭黑(CB)/HDPE导电复合体系进行了比较.发现当SSF含量极低(0.3vol%)时,SSF/HDPE体系即发生导电渗流现象,且导电渗流转变区域极窄;而仅当SSF含量达到4.8vol%时,该复合体系才表现出流变渗流现象,这一结果与CB/HDPE体系及纳米级导电纤维填充体系截然不同.此外,通过正温度系数效应的研究发现SSF形成的导电通路稳定性高于CB/HDPE体系.我们认为,SSF/HDPE体系呈现的这些特点均与SSF较大的直径及长径比且其导电通路及流变渗流网络的形成机理不同有关.     

Synergistic effect of carbon nanotubes and carbon black as nanofillers of silicone rubber pressure sensors

Pressure sensors based on nitrogen-doped bamboo-shaped carbon nanotubes (N-BCNT) and carbon black (CB) as nanofillers, polyurethane foam (PU) as supporting substrate, and silicone rubber (SR) as a matrix were prepared. Dip coating was used to coat PU with 0.44 wt% nanofiller, including different mixing ratios of N-BCNT and CB (5:5; 6:4; 7:3; 8:2; 9:1). Then, the coated PU is impregnated in SR to fill the pores. Due to the higher aspect ratio of the N-BCNT, it contributes more to improving the electrical conductivity in the composites, while the CB fills the smaller gaps. The prepared sensors were tested in various applications, and it was found that the optimal mixing ratio of nanofillers was 7:3 N-BCNT:CB. Thus, a multifunctional pressure sensor has been developed successfully with excellent flexibility and good resilience, suitable for motion detection and finger touch applications. The pressure sensor showed high sensitivity, and the ability to detect a wide range of pressures. The sensor exhibited success in a range of applications, paving the way for its potential use in various fields in the future, such as wearable devices, prosthetics, robotic devices, and medical devices.     

Selective modified SnO2-based materials for gas sensors arrays

An enhancement of selectivity of semiconductor gas sensors, based on nanocrystalline SnO2 is reported. It is shown that modification of the surface of crystallites, forming thick films of conductive sensor materials, with catalytic clusters of gold or oxides of ruthenium, nickel, copper and iron allows selective response of sensors to different gases, such as carbon monoxide, ammonia, hydrogen sulfide, nitrogen dioxide and acetone vapor. These selective sensor responses can be obtained in the ranges of gas concentrations close to or below threshold limit values while the working temperature of sensors can be kept below 300 °C. The described approach for modification of selectivity of sensor materials could be used as perspective route in developingselective gas sensors. These results allow us to propose application of obtained materials in electronic nose sensor systems.     

Microparticle ratiometric oxygen sensors utilizing near-infrared emitting quantum dots

Luminescent sensors incorporating two luminophores, an indicator and a reference, offer many advantages over intensity measurements from sensors made with one indicator dye. Quantum dots have yet to be widely employed as insensitive reference luminophores in such systems. This work describes the use of near-infrared emitting quantum dots in conjunction with a long-lifetime platinum(II) porphyrin phosphor in a microsphere-based, ratiometric oxygen sensor. The process for self-assembly of the nanocomposite system was developed, and the response and photostability of the prototypes were investigated. Results indicate the sensors possess excellent sensitivity (K(SV) = 0.00826 μM(-1)) at oxygen concentrations below 300 μM and were resistant to photobleaching. The sensor luminophores displayed minimal spectral overlap and little interference from excitation light, preventing the need for optical filters. A reversible photoenhancement of the quantum dot signal was also observed when exposed for extended periods of time. This work demonstrates the advantages of incorporating long-wavelength quantum dots into ratiometric intensity sensing schemes and highlights some key limitations that must be considered in their use.