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Enhancing the sensitivity of chemiresistor gas sensors based on pristine carbon nanotubes to detect low-ppb ammonia concentrations in the environment
Crucial features in gas sensors for environmental monitoring are the sensitivity in the low ppb range and the ability to discriminate variations of a certain polluting gas from other interfering gases. Monitoring of ammonia concentrations in the environment is mandatory because, besides being a toxic molecule, NH3 is one of the main precursors of secondary fine particulate (PM10, PM2.5). In spite of this urgency, the detection of ammonia concentrations in urban areas with CGS has been so far widely overlooked, since the average levels are usually low, i.e. in the 20-30 ppb range and the CGSs are usually tested by exposure to ammonia in the ppm range.
SWCNTs can provide a high physical and chemical stability and a wide range of possible operational features, that make them a unique system for gas detection. Several CNT-based architectures have been proposed and ppt sensitivity of pristine CNTs to ammonia has been demonstrated in inert Ar atmosphere under UV irradiation. Though results so far reported are encouraging, the use of inert atmosphere represents a limitation for environmental monitoring. To overcome this limitation, the present study was carried out in the lab air environment, which is much closer to the final destination of the CNTs sensors. |
Among the several kind of gas sensors, chemiresistor gas sensors are the simplest to produce, being composed of a substrate, a sensitive element deposited on top, and the electrodes to contact the device to a readout circuit. By measuring the variation of the current (or resistance) induced by the presence of gas, the sensor response ΔR/R0 is obtained and after a sequence of exposure to different gas concentrations it is possible to draw the response curve. Finally the detection limit (DL), defined as the limit of detectable concentration, which corresponds to the minimum detectable signal above 5σ, can be estimated by considering the baseline R0, the noise σ, and the fit parameters of the response curve. For the present layers deposited on ceramic substrates a DL down to 3 ppb was estimated, well below the expected average ammonia concentration values in a urban area.  |
Further possibilities to improve the CNT-based gas sensor performances are expected from CNT-oxide hybrid systems, in particular from the blend of CNT with nanoparticle metal oxides such as TiO2, SnO2, WO3, indium-tin oxide (ITO). This is the case of the SWCNT-ITO sensing layers prepared at the MNC-lab on plastic substrates. Plastic substrates were chosen in view of possible device development with printable electronics methods.
These layers displayed a three-fold sensitivity increase with respect to pristine CNT for concentrations above 200 ppb. Moreover,the opposite resistance variation of SWCNT and functionalized ITO-SWCNT when exposed to water vapor (inset of Fig.2b) discloses the possibility to tailor the sensor selectivity with respect to the relevant interfering effects of humidity, expected in outdoor environmental monitoring. |
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Enhancing the sensitivity of chemiresistor gas sensors based on pristine carbon nanotubes to detect low-ppb ammonia concentrations in the environment
Federica Rigoni,ab Silvia Tognolini,ab Patrizia Borghetti,bc Giovanni Drera,ab Stefania Pagliara,ab Andrea Goldonid and Luigi Sangaletti*ab
a Interdisciplinary Laboratories for Advanced Materials Physics (I-LAMP), via dei Musei 41, Brescia, Italy
b Dipartimento di Matematica e Fisica, Università Cattolica del Sacro Cuore, via dei Musei 41, Brescia, Italy
c Centro de Física de Materiales (CSIC/UPV-EHU) – Materials Physics Center, Paseo Manuel de Lardizabal 5, E-20018 San Sebastián, Spain
d Elettra Sincrotrone Trieste S.C.p.A., s.s.14 Km. 163.5, Trieste, Italy
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Analyst, 2013,138, 7392-7399
DOI: 10.1039/C3AN01209C
Received 19 Jun 2013, Accepted 03 Oct 2013
First published online 04 Oct 2013
