Gas sensitive single-walled carbon nanotubes layers for environmental monitoring of ammonia

Low-cost chemiresistor gas sensors (CGS) based on single-walled carbon nanotubes (SWCNT) layers have been prepared on different substrates, including flexible plastic foils. These CGS operate in air at room temperature, displaying an enhanced sensitivity to NH₃ with a detection limit in air down to 3 ppb. When functionalized with ITO nanoparticles, a sensitivity increase is detected, along with an opposite response towards moisture.

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. Retrieve articles - Enhancing the sensitivity of chemiresistor gas sensors based on pristine carbon nanotubes to detect low-ppb ammonia concentrations in the environment Federica Rigoni, Silvia Tognolini, Patrizia Borghetti, Giovanni Drera, Stefania Pagliara, Andrea Goldoni and Luigi Sangaletti Analyst, 2013, 138, 7392-7399 - High sensitivity, moisture selective, ammonia gas sensors based on single-walled carbon nanotubes functionalized with indium tin oxide nanoparticles Federica Rigoni, Giovanni Drera, Stefania Pagliara, Andrea Goldoni, Luigi Sangaletti Carbon 2014 (in press)