Showing papers in "Sensors and Actuators B-chemical in 1997"

On the sensitivity of surface plasmon resonance sensors with spectral interrogation

Abstract: Theoretical analysis of the sensitivity of surface plasmon resonance (SPR) sensors with spectral interrogation is presented. Two basic configurations of SPR sensors with spectral interrogation are investigated—for the measurement of variations in the refractive index of bulk media, and for the monitoring of variations in the thickness of thin films. In both cases, analytical expressions allowing the sensitivity of SPR sensors to be calculated are derived and validated. On the basis of the theoretical analysis, the optimization of a spectral SPR sensor in terms of the operation wavelength and the choice of metal layer is carried out. It is demonstrated that spectral SPR sensors may attain higher sensitivity if operating at longer wavelengths and if using metal layers with a higher modulus of the real part of the dielectric constant.

Gas identification by modulating temperatures of SnO2-based thick film sensors

Abstract: A new method is presented to identify the presence of two gases in the ambient atmosphere. The method employs only one SnO2-based gas sensor in a sinusoidal temperature mode to perform the quantitative analysis of a binary gas mixture (CO/NO2) in air.

Grain size control in nanocrystalline In2O3 semiconductor gas sensors

Abstract: In2O3 thin films prepared by sol–gel method make it possible to detect low levels (several hundreds ppb) of nitrogen dioxide in air. The possibility of grain size control in indium oxide-sensing layers has been established by using of two preparation methods—electron beam evaporation (EB) and sol–gel technique (SG). SG-prepared samples show smaller particles sizes (down to 5 nm), higher state of agglomeration, higher sensor resistance in air and higher response to NO2 in comparison to EB samples. Sol–gel technique leads to the preparation of polycrystalline indium oxide with particle sizes of about 5–6 nm after calcination at 400°C and 20 nm after calcination at 700°C. The initial state of particle agglomeration in initial indium hydroxide sol (IHS), stabilized with nitric acid, influences the structure and surface morphology of the resulting indium oxide. While the In2O3 layer prepared by using low agglomerated IHS is smooth and porous, In2O3 layers prepared from highly agglomerated IHS consist of two regions—thin layer and crystallite agglomerates in cubic and rectangular parallelepiped form. The last shows the best results in terms of NO2 sensitivity. Sensor resistance and NO2 sensitivity increase with the decrease of the grain sizes in In2O3.

Tasting of beverages using an electronic tongue

Abstract: An electronic tongue based on the sensor array of non-specific solution sensors together with pattern recognition tools has been applied to qualitative analysis of different beverages. It has been found that it is capable both to discriminate reliably between various sorts of the same type of beverages (tea, coffee, beer, soft drinks, juice, etc.) and to monitor the process of aging of juice. Correlations have been found between integral parameters produced by electronic tongue and quality of juice. Some conceptions and backgrounds used for electronic tongue development have been put forward and discussed.

Humidity sensing properties of Nation and sol-gel derived SiO2/Nafion composite thin films

Abstract: Nafion, sol-gel derived SiO2 and SiO2/Nafion composite films were prepared through casting and dip-coating methods. Impedance response of the films to relative humidity and the stability of the response against different aging environments were examined. The response to relative humidity of each film was analyzed with their complex impedance spectra at different humidities.

Qualitative and quantitative analysis of volatile organic compounds using transient and steady-state responses of a thick-film tin oxide gas sensor array

Abstract: Quantitative analysis of gases, by means of semiconductor sensor arrays and pattern-recognition techniques such as artificial neural networks, has been the goal of a great deal of work over the last few years. However, the lack of selectivity, repeatability and drifts of the sensors, have limited the applications of these systems to qualitative or semi-quantitative gas analysis. While the steady-state response of the sensors is usually the signal to be processed in such analysis systems, our method consists of processing both, transient and steady-state information. The sensor transient behaviour is characterised through the measure of its conductance rise time (Tr), when there is a step change in the gas concentration. Tr is characteristic of each gas/sensor pair, concentration-independent and shows higher repeatability than the steady state measurements. An array of four thick-film tin oxide gas sensors and pattern-recognition techniques are used to discriminate and quantify among ethanol, toluene and o-xylene [concentration range: 25, 50 and 100 ppm]. A principal component analysis is carried out to show qualitatively that selectivity improves when the sensor behaviour is dynamically characterised. The steady-state and transient conductance of the array components are processed with artificial neural networks. In a first stage, a feed-forward back-propagation-trained ANN discriminates among the studied compounds. Afterwards, three separate ANN (one for each vapour) are used to quantify the previously identified compound. Processing data from the dynamic characterisation of the sensor array, considerably improves its identification performance, rising the discrimination success rate from a 66% when only steady-state signals are used up to 100%.

Optical measurement of oxygen and temperature in microscale: strategies and biological applications

Abstract: Sediments, microbial mats, biofilms and other microbial communities are characterized by steep gradients of physical and chemical parameters. Microsensors are powerful tools to measure these parameters with a sufficient spatial resolution and with a small disturbance of the micro-environment in natural systems. Recently, fiber-optical microsensors have been introduced in the field of aquatic biology as an alternative to existing electrochemical microsensors. Such micro-optodes have already been developed for high-resolution measurement of dissolved oxygen and for temperature measurements. They are easy to fabricate and show an improved long-term and storage stability. An overview is given on the development and characterization of different types of micro-optodes for oxygen and temperature. A luminescence lifetime-based device has been developed which is portable and enables microsensing both in the laboratory and under field conditions. Limitations in practical work with optical microsensors are demonstrated, and strategies to overcome them briefly discussed. A micro-optode array as well as a method for high-resolution oxygen imaging in sediments are presented as two different ways to investigate the two-dimensional oxygen distribution in heterogeneous living systems. Future applications and developments in micro-optode research will be discussed briefly.

Detection and simultaneous identification of microorganisms from headspace samples using an electronic nose.

Abstract: The detection and simultaneous identification of a range of microorganisms by measuring the volatile compounds produced from plate cultures has been carried out using an electronic nose and a neural network classifier. Headspace samples were taken from static atmospheres formed from inoculated agar plates after a suitable growth period at 37°C and analysed using a standard 16 sensor array operating in transient flow mode. The response of the sensor array to water and to the control media in the absence of microbial growth was also determined, allowing greater discrimination of microbial volatiles. The response curves produced were processed using standard back propagation neural network techniques to provide identification. The overall classification rate for 12 different bacteria and one pathogenic yeast was 93.4%. Data for a sub-set of seven bacteria gave 100% classification using the same methods. In a second experiment three similar yeast cultures were compared and correctly classified at a level of 96.3% with no pre-processing to remove the sample signal generated by the media. Principal component analysis on selected data gave clear discrimination between water vapour and the test samples.

Preparation and characterization of nanocrystalline α-Fe2O3 by a sol-gel process

Abstract: α -Fe 2 O 3 ultra-fine powder with an average particle size of 6–26nm has been prepared by a sol-gel process. Thermal analysis, X-ray diffraction and transmission electron microscope were used to study its formation process and micro-structure. The temperature dependence of the electric conductance of the elements made of nanocrystalline α -Fe 2 O 3 shows that the gas-sensing properties are strongly related to its surface. The elements exhibited good sensitivity and selectivity to ethyl alcohol, indicating it is a promising alcohol-sensing material.

An electronic nose for food analysis

Abstract: Since the first developments of electronic noses, food analysis has been considered as one of its most useful applications. In this paper an electronic nose based on quartz microbalances coated with metallo-porphyrins and related compounds is presented and illustrated. Extensive tests on various substances playing key roles in food analysis show that sensing properties of the sensing materials (in terms of sensitivity and selectivity) can be exploited for electronic nose applications devoted to the analysis of various kinds of foods. The versatility of this system has been successfully tested on different kinds of foods, such as fish, meat, vegetable and wine for which results are shown.

A very large integrated pH-ISFET sensor array chip compatible with standard CMOS processes

Abstract: The development and preliminary evaluation of a very large pH-sensitive ISFET sensor array chip is reported in this paper. The sensor array chip boasts a 15×16 array of pH-ISFETs with on-chip control and readout circuits. It was designed and fabricated using a novel process which is compatible with standard CMOS technology. This process only required four new mask layers in the design stage and four extra standard processing steps in the fabrication stage. A novel signal processing technique was also employed.

Application of conducting polyaniline as sensor material for ammonia

Abstract: The change in resistance of polyaniline on exposure to aqueous ammonia has been utilized for the study of a prototype chemical sensor. This paper discusses various aspects of polyaniline and its suitability as a chemical sensor, particularly with reference to aqueous ammonia solution.

Multicomponent analysis on polluted waters by means of an electronic tongue

Abstract: In this paper the simultaneous measurements of the concentrations of a number of chemical species in solutions performed by a sensor array of ion sensitive electrodes are presented and discussed By analogy with the well known electronic nose this sensor array operating in solutions, will be here called electronic tongue In order to extract optimized information from the electronic tongue output data, many different techniques have been applied; they were based on chemometrics, non-linear least squares and neural networks The best results have been achieved by the introduction of modular models which make use, at the same time, of both qualitative and quantitative information © 1997 Elsevier Science SA

Prospects for fibre-optic evanescent-field gas sensors using absorption in the near-infrared

Abstract: In this paper we investigate the prospects for realization of evanescent-wave gas sensors using gas absorption lines within the transmission window of silica fibres. Our work focuses on methane and uses D-fibre as the evanescent-field sensor. We discuss the three major obstacles, namely, low sensitivity, high background signal levels from interference effects and system degradation through surface contamination. We explain how sensitivity may be improved through sol-gel coatings, how background signal levels may be reduced by use of polarization-maintaining fibre and suggest a way of compensating for surface contamination by continuous monitoring of the birefringence of the D-fibre. Improved performance can be obtained through use of a distributed-feedback laser instead of a light-emitting diode, but at much greater cost. In the long term, the likely application for the evanescent-field sensor is in distributed measurement systems.

Miniature integrated optical modules for chemical and biochemical sensing.

Abstract: An overview is given on the principal ideas, new concepts and practical issues for accomplishing complete miniature sensor modules for chemical and biochemical applications, which are at the same time small, highly sensitive and stable The novelty of the approach is mainly based on using different types of smart planar optical transducer (SPOT) chips, which not only carry out the task of conventional integrated optical transducers, but also perform additional functions for which peripheral equipment is otherwise needed The emphasis is on pointing out and discussing the most important aspects for refractometric modules Different approaches and sensor types are compared with respect to fulfilling the requirements for practical applications New results concerning the use of multilayer waveguides and the implementation of novel modules are also presented Key issues discussed are the importance of a proper theoretical modelling of the sensor chips and the (bio-) chemical sensing layers; the availability of micro- and nanofabrication technologies suitable for realizing sensors based on a variety of substrate materials, such as glasses, polymers and semiconductors; the possibility of mass-producing low-cost SPOT chips; using standardized waveguides for the implementation of miniature integrated optical chemical benches; employing planar micro-optical elements for accomplishing illumination and detection sub-modules; the advantages of surface-emitting chip configurations with respect to miniaturization and adaptation to different applications; schemes for single-chip multicomponent analysis based on sensor-pad arrays; and the significance of using on-chip measuring variables and true differential on-chip referencing for achieving both high sensitivity and stability

Low temperature bonding for microfabrication of chemical analysis devices

Abstract: A low temperature bonding process was developed for the fabrication of microchip devices for liquid and heterogeneous phase chemical analysis. Photolithographically etched microchannels on glass substrates were closed by bonding a glass cover plate using a spin-on sodium silicate layer as an adhesive. Good channel sealing was achieved by curing at 90°C for 1 h or room temperature overnight. The fluidic performance of the device was evaluated by monitoring the electroosmotic flow on the chip. The results compared well with those obtained from devices made by high temperature direct bonding of the substrate and cover plate. The dielectric and mechanical strength for bonds, created using the low and high temperature methods, were compared. A dielectric strength of 400 kV cm−1 was obtained for the sodium silicate bonding and 1100 kV cm−1 for the high temperature bonding. Mechanical strength measurements gave a surface energy value of ≈2.7 J m−2 for sodium silicate bonding, compared to 6.5 J m−2 for direct bonding. The mechanical strength of glass bonds obtained with sodium silicate at low temperature was comparable to that reported for the sodium silicate bonding of silicon wafers at >200°C or by conventional direct bonding of oxidized silicon at 1400°C. The low temperature bonding performance is adequate for microfabricated fluidic devices that employ electrokinetic transport phenomena. The reduced temperature of the bonding process will allow chemical surface modification prior to bonding.

Mechanism of highly sensitive and fast response Cr doped TiO2 oxygen gas sensor

Abstract: The recent trend in the field of gas sensor research is to develop new sensing materials with higher sensitivity, selectivity and shorter response time. Titania (TiO2) doped with chromium has been investigated for its oxygen sensing properties. At an optimum Cr concentration of 0.4 wt.% the sensor shows a high sensitivity and shorter response time at 700°C as compared to undoped material. This material also shows n to p-type transition above oxygen partial pressure of 1100 ppm. The x-ray photoelectron spectroscopy (XPS) results indicate changes in the oxidation states of Ti and Cr. The charge transfer mechanism at Ti and Cr sites has been discussed. A correlation between sensitivity and response time with shifts in binding energy (B.E.) of Ti 2p peak has been obtained. On the basis of these results, a sensitization mechanism in these sensors has been proposed.

A sol–gel based fiber optic sensor for local blood pH measurements

Abstract: This paper describes a fiber optic pH sensor based upon sol–gel encapsulation of a self-referencing dye, seminaphthorhodamine-1 carboxylate (SNARF-1C). The simple sol–gel fabrication procedure is ideal for encapsulation and immobilization of dye molecules onto the end of an optical fiber. A miniature bench-top fluorimeter system was developed for use with the optical fiber to obtain pH measurements. Linear and reproducible responses were obtained in human blood in the pH range 6.8–8.0, which encompasses the clinically-relevant range. The sensor was also shown to have low coating leachability. This sensor will be used as part of an array of sensors being developed to monitor stroke patients.

The realization of an integrated Mach-Zehnder waveguide immunosensor in silicon technology

Abstract: We describe the realization of a symmetric integrated channel waveguide Mach-Zehnder sensor which uses the evanescent field to detect small refractive-index changes (?nmin ? 1 × 10?4) near the guiding-layer surface. This guiding layer consists of ridge structures with a height of 3 nm and a width of 4 ?m made in Si3N4. This layer has a thickness of 100 nm. The sensor device has been tested with glucose solutions of different bulk refractive indices. Results of a slab-model calculation are in good agreement with obtained experimental results. The feasibility of applying this sensor for immunosensing, detecting directly the binding of antigen to an antibody receptor surface, is shown with antibody-antigen binding experiments.

Cross-sensitivity evaluation of chemical sensors for electronic tongue: determination of heavy metal ions

Abstract: The development of the promising new field of sensor applications, multisensor arrays for liquid analysis based on the principles of the electronic tongue, implies some new demands on sensor material research and development. Stable and reproducible sensors with partial specificity and considerable cross-sensitivities to different components in solutions are of primary interest. Solid-state potentiometric sensors, both crystalline and vitreous, are likely to be the most promising ones for multisensor devices designed for long-term analytical application in natural and artificial complex media. The present paper deals with development of a method of evaluation of integral heavy metal cation sensitivity of solid-state sensors with special consideration of cross-sensitivity features. The method involves a comparative study of different sensor materials in individual component solutions, using several criteria based on integral response parameters. The procedure scheme can be applied to evaluate cross-sensitivity of any kind of potentiometric sensors for liquid media.

A novel thick-film ceramic humidity sensor

Abstract: The paper describes the results of studies on the fabrication and characterisation of a thick-film humidity sensor based on the semiconducting metal oxide MnWO 4 . The sensor element possesses a novel ‘sandwich’-configuration with a 40 μm porous MnWO 4 ceramic layer sandwiched by two 10 μm polarity-reversed, interdigitated metal films. Instead of traditional glass frits, LiCl powders are used as adhesion promoters for sintering the sensor paste. With this method, MnWO 4 powders with an average particle size of 3.0 μm are sintered at the standard thick-film firing temperature of 850°C. The sintered ceramic layer exhibits a porous structure. The novel electrode arrangement combines the advantages of humidity sensors in the form of a parallel capacitor with those in the form of an interdigital capacitor, permitting a high sensitivity and a fast response. The influence of temperature on the sensor characteristics has been compensated for by integrating a thick-film NTC resistor. The humidity sensor shows no cross-sensitivity to organic vapour. The organic contamination on the sensor surface can be burned out by heating the sensor element at about 400°C with the refresh heater printed on the back side of the substrate.

Odours and flavours identified with hybrid modular sensor systems1

Abstract: Hybrid sensor systems contain different types of chemical sensors whereby each type (transducer principle) contains an array of individual sensors. This leads to a large flexibility in the choice of transducers and sensor materials with the general aim of optimising the analytical performance of the total system. This concept makes it possible to optimise the quantitative analysis of mixtures of known gases as it will be demonstrated for mixtures of volatile organic compounds (VOCs). Alternatively this makes it possible to optimise the system for characterising odours and flavours. This will be demonstrated for different plastic as well as textile materials used in car industries and for different products of food industries, i.e. coffees, tobaccos, whiskeys, and olive oils. In our modular sensor systems we used arrays of different semiconductor gas sensors (based on metal oxides), of polymer coated quartz microbalance (QMB) sensors, of calorimetric sensors and of electrochemical sensors, with an option to add metal oxide semiconductor field effect transistor (MOSFET) sensors. These arrays are arranged as separate components in a modular sensor system ‘MOSES’. For the qualitative discrimination of different odour samples a headspace-autosampler was added and transient sensor signals were monitored. The use of different transducer principles is shown to be essential for an unequivocal identification of odours and flavours.

A surface plasmon resonance based integrated optical sensor

Abstract: An integrated optical sensor based on the resonant coupling between a surface plasmon wave and a guided mode of an integrated optical waveguide is reported. Detailed theoretical analysis of the proposed sensor based upon the modal approach and upon the mode expansion and propagation method is presented. The performance of a realized sample of the waveguide sensor is investigated. It is demonstrated that by measuring the optical power transmitted through the sensing element, variations in the refractive index of the analyte as small as 2×10 −5 may be resolved.

Ph-isfet based urea biosensor

Abstract: A new method of direct immobilization of urease on a hydrated silicon nitride surface with the use of glutaraldehyde is presented. The main parameters characterizing the developed urea biosensor based on pH-ion-selective field effect transistor (pH-ISFET) with directly immobilized urease are: (1) maximal analytical signal, 120–140 mV in 10 mM phosphate buffer solution; (2) linear range of ΔUgs=f(log Curea):pCurea(2–3.5) in 10 mM phosphate buffer; (3) response time, 80 s; and (4) lifetime of 35 days with a stable analytical signal. The biosensor was used for urea determination in blood serum and in hemodialysis fluid with satisfactory results.

Optical assay for glucose based on the luminescnence decay time of the long wavelength dye Cy5

Abstract: An optical assay for glucose is described based on the luminescence decay time of a long wavelength dye (Cy5) which can be excited with currently available red laser diodes. Concanavalin A was covalently labeled with Cy5 which served as the donor in an assay based on fluorescence resonance energy transfer (FRET). The acceptor was Malachite Green which was covalently linked to insulin which served as a carrier protein. To provide binding affinity for ConA Malachite Green insulin was also covalently labeled with maltose (MIMG). Binding of Cy5ConA to MIMG resulted in a decreased intensity and decay time of Cy5 as observed by time-correlated single photon counting. Glucose was detected by competitive displacement of MIMG from Cy5ConA, resulting in increased intensity and decay time. This glucose assay has several features which can result in practical real world assays for glucose. The long absorption wavelength of Cy5 allows excitation with red laser diodes, which can be readily pulsed or amplitude-modulated for time-domain or frequency-domain decay time measurements. Additionally, decay times can be measured through skin using long wavelength excitation and emission, suggesting the possibility of an implanted glucose sensor. And finally, the assay affinity and reversibility can in principle be adjusted by controlling the extent and type of sugar labeling of the carrier protein.

Novel bioaffinity sensors for trace analysis based on luminescence excitation by planar waveguides

Abstract: We have developed a novel generation of optical bioaffinity sensors for ultra trace analysis. These sensors are based on luminescence generation in the evanescent field of high-refractive-index single-mode planar waveguides, With the waveguiding layers and the grating parameters chosen, very sharp discrimination of bulk against surface-confined excitation, in combination with high excitation intensities in the evanescent field, can be achieved, leading to unprecedented sensitivity. Experimental data of the optimization of the transducer parameters will be presented. Incoupling of excitation light is performed using diffractive gratings. Different methods for the detection of both transmitted and luminescence light will be presented. The transmitted excitation light can be detected either at the distal waveguide chip end or using a second outcoupling grating. Both isotropically emitted luminescence, collected by a lens located below the transducer substrate (‘volume detection’), and emission coupled back into the waveguiding layer can be monitored, the latter via a second outcoupling grating. First experimental results obtained in model bioaffinity assays will be presented, demonstrating the feasibility of the different detection methods mentioned above, as well as the superior sensitivity of our novel sensor configuration. In still preliminary experiments, 100 attomoles of fluorescently labelled DNA (16-mer oligonucleotide), applied at 100 femtomolar concentration, can be detected.

Thermal modelling and characterisation of micropower chemoresistive silicon sensors

Abstract: There is considerable interest in the development of low-cost, low-power resistive sensors for possible application in hand-held gas monitors. In this paper we describe the fabrication of a high-temperature, resistive sensor using silicon microtechnology which lies on a sub-micron thick membrane with an embedded platinum resistance heater. A thermal model of this micro-hotplate is constructed and compared with its observed behaviour. The microsensor can be operated at temperatures of up to 600°C and has a low d.c. power consumption per sensor of 40 mW at 300°C. As the thermal time-constant of the microsensor is only about 5 ms, its average power consumption can be reduced by a factor of at least ten through an a.c. mode of operation, thus making a single device or even an array device suitable for battery-powered instruments.

Microstructure and CO gas sensing properties of porous ZnO produced by starch addition

Abstract: Effects of corn starch addition on the microstructure and CO gas response were studied by sintering ZnO at 600∼900°C for 3 h in air. The addition of 5 wt% corn starch as the fugitive phase decreased both the grain size and the sintered density of ZnO at all sintering temperatures and thus increased the sensitivity to 200 ppm CO. The increasing relative density and grain size with sintering temperature was accompanied by a decreasing CO gas sensitivity after maximum at 700°C. The temperature showing the maximum CO gas sensitivity decreased with decreasing grain size. Humidity decreased both the hysteresis of electrical conductivity and CO gas sensitivity.

An optical gas sensor based on polyaniline Langmuir-Blodgett films

Abstract: The technique of surface plasmon resonance has been used to characterize Langmuir-Blodgett films of polyaniline. The thickness per layer obtained by curve fitting was 5.5 ± 0.6 nm. The surface plasmon resonance curves have been shown to be influenced by both NO 2 and H 2 S. The effects were partly reversible, with lower detection limits of about 50 vapour parts per million at room temperature.

Drift counteraction in odour recognition applications: lifelong calibration method

Abstract: Sensor drift is addressed as one of the most serious impairments afflicting chemical and biochemical sensors. One possible solution to this problem is to view sensor arrays as time-varying dynamic systems, whose variations have to be tracked by adaptive estimation algorithms. A theory of hidden variable dynamics for the rejection of common mode drifting of sensors has previously been developed and is here coupled with a recursive least squares algorithm. In Section 7 a smart solution is provided for a difficult vapour recognition problem vexed by drift that have failed with traditional pattern recognition techniques. Among the many advantages we distinguish that model adaptation to changes in the sensor array makes lifelong calibration possible without interrupting the operation of the array.