This work describes the preparation of gold/polypyrrole nanorods (AuPPy NRs) using anodized alumina oxide (AAO) template and both pulsed galvanic deposition and electropolymerization for the deposition of Au and polypyrrole (PPy) nanorods (NRs), respectively. Characterization of the whole structure after AAO etching revealed the formation of a high density of NRs along the substrate with uniform diameters of approximately 50 nm and total lengths of 700 nm, the last corresponding to 1/3 and 2/3 of the length of the Au and PPy NRs, respectively. These structures are provided of bottom/top electrodes and a heating element coupled to the backside of the substrate, and their gas sensing properties towards various concentrations of NO2 in resistive configuration are presented.
The reliable and low-cost quantitative detection of ethylene for food/fruit applications remains an unsolved problem.
Existing commercial systems are able to quantify ethylene (at sub ppm levels) but either they are off-line: require
periodic sample collection and use of reagents or high-cost. We will report on the development of an RFID reader with
onboard micro-machined metal oxide gas sensors aimed at monitoring climacteric fruit during transport and vending.
The developed platform integrates a commercial off the shelf inductive coupling RF transceiver in the 13.56MHz band,
fully compliant with the ISO15693 standard, micro-hotplate gas sensors, driving and readout electronics. If the sensors
are operated at a fixed temperature, the reader could work as an alarm level monitor able to assess the conservation stage
of apples. On the other hand, when the sensors are operated under an optimised temperature-modulation mode, accurate
calibration models for the species that are relevant to assess the conservation stage of apples (i.e., ethylene, acetaldehyde
and ethanol) can be built. Finally, different feature extraction techniques such as the FFT and the Energy Vector will be
used in combination with pattern recognition tools like PLS and PLS-DA to show that our system is able to identify and
quantify the species that are relevant for the application considered.
Nanoparticle films of crystalline WO3, designed for gas sensing applications, were deposited on alumina substrates by reactive gas deposition. H2S, ethanol vapour, and binary mixtures of ethanol/H2S, ethanol/NO2 and H2S/NO2 were used in different concentrations for testing the performance of the sensor device. The sensor was operated in dynamic mode by modulating its temperature between 150 and 250 °C. Coefficients were extracted by applying Fast Fourier Transform (FFT) and Discrete Wavelet Transform (DWT) methods to the dynamic resistance response of the sensor. These coefficients were then used as inputs for pattern recognition methods to extract both quantitative (concentration) and qualitative (chemical selectivity) information about the test gases. After sensor calibration, it was possible to detect as little as 200 ppb of ethanol and 20 ppb of H2S with good accuracy. Furthermore, ethanol and H2S could be detected with good sensitivity and selectivity in the presence of both reducing and oxidising gases.
Metal oxide gas sensors suffer from lack of selectivity and response drift. The use of sensor dynamics has been introduced to ameliorate sensor performance. The usual approach consists of modulating the operating temperature of gas sensors. Temperature modulation alters the kinetics of the adsorption and reaction processes taking place at sensors' surface. This results in response patterns that are characteristic of gas/sensor pairs. Despite the fact that a great deal of work has been done, the selection of the modulating frequencies remains an obscure and non-systematic method. A new approach to systematically select frequencies is discussed. The method is based on the use of pseudo-random binary sequences (MLS) to modulate the working temperature of gas sensors in a wide frequency range. The impulse response of a pair sensor-gas can be estimated from the circular cross-correlation of the MLS and the sensor response sequences. From the study of the impulse response in the frequency domain, an identification of the modulating frequencies that
convey important information to both identify and quantify gases is obtained.
The analyses of air quality using chromatography or optical methods are very accurate but rather expensive. A cheaper solution consists of using solid state chemical sensors in easy-to- use portable and rugged equipment or distributed systems to achieve a continuous monitoring of pollution. Nevertheless it is difficult to discriminate among a set of gases because these sensors have poor selectivity. Measurements on thick-film tin oxide gas sensors have shown that their conductance transient, when they are exposed to a step change in gas concentration, is diffusion limited. The analysis and characterization of this transient enables a rise time to be obtained, which is independent of the final conductance value and depends, among other parameters, on the diffusion coefficient of gases into the porous structure of the sensor. This parameter is easily measurable and gives useful information for gas/vapor recognition. In particular in this work we have shown that it is possible to discriminate among a set of three volatile organic compounds VOCs (benzene, toluene and o-xylene) using a commercial Taguchi Inc. semiconductor gas sensor.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.