High sensitivity, moisture selective, ammonia gas sensors based on single-walled carbon nanotubes functionalized with indium tin oxide nanoparticles Federica Rigoni a , Giovanni Drera a , Stefania Pagliara a , Andrea Goldoni b , Luigi Sangaletti a, * a Interdisciplinary Laboratory for Advanced Materials Physics and Dipartimento di Matematica e Fisica, Universita ` Cattolica del Sacro Cuore, via dei Musei 41, I-25121 Brescia, Italy b Elettra Sincrotrone Trieste S.C.p.A., s.s. 14 Km. 163.5, 34149 Trieste, Italy ARTICLE INFO Article history: Received 3 April 2014 Accepted 20 August 2014 Available online 28 August 2014 ABSTRACT An effective monitoring of the air quality in an urban environment requires the capability to measure polluting gas concentrations in the low-ppb range, a limit so far virtually neglected in most of the novel carbon nanotube (CNT)-based sensors, as they are usually tested against pollutant concentrations in the ppm range. We present low-cost gas sensors based on single-walled CNT (SWCNT) layers prepared on plastic substrates and operating at room temperature, displaying a high sensitivity to [NH 3 ]. Once combined with the low noise, the high sensitivity allowed us to reach an ammonia detection limit of 13 ppb. This matches the requirements for ammonia monitoring in the environment, disclosing the possibility to access the ppt detection limit. Furthermore, a blend of SWCNT bundle layers with indium-tin oxide (ITO) nanoparticles resulted in a threefold sensitivity increase with respect to pristine CNT for concentrations above 200 ppb. Finally, the peculiar response of the ITO-SWCNT blend to water vapor provides a way to tailor the sensor selectivity with respect to the relevant interfering effects of humidity expected in outdoor environmental monitoring. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Novel perspectives in the development of chemiresistor gas sensors (CGS) are determined by the capability to meet the requirements of environmental monitoring, in particular (i) the sensitivity in the low ppb range and (ii) the capability to monitor the polluting molecules on a background of water molecules brought about by the humidity conditions. Among the chemiresistor gas sensors, devices based on nanostruc- tured materials have become one of the most explored alternatives to thin films, due to the high surface/volume ratio and to the manifold of morphologies that are expected to offer several alternatives for the improvement of sensitiv- ity and selectivity. As nanostructured materials, carbon nano- tubes (CNT) play a central role in the development of novel devices aimed to monitor the environmental conditions. So far, CNTs have been tested against many polluting gases, as they are known to interact with many gas molecules that, depending on their reducing or oxidizing properties, may inject or extract electrons from the CNTs, resulting in a http://dx.doi.org/10.1016/j.carbon.2014.08.074 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved. * Corresponding author. E-mail address: sangalet@dmf.unicatt.it (L. Sangaletti). CARBON 80 (2014) 356 – 363 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon