Please cite this article in press as: C. Wongchoosuk, et al., Portable electronic nose based on carbon nanotube-SnO 2 gas sensors and its application for detection of methanol contamination in whiskeys, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.03.072 ARTICLE IN PRESS G Model SNB-12243; No. of Pages 8 Sensors and Actuators B xxx (2010) xxx–xxx Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Portable electronic nose based on carbon nanotube-SnO 2 gas sensors and its application for detection of methanol contamination in whiskeys Chatchawal Wongchoosuk a , Anurat Wisitsoraat b , Adisorn Tuantranont b , Teerakiat Kerdcharoen a,c,∗ a Department of Physics and Center of Nanoscience and Nanotechnology, Faculty of Science, Mahidol University, Bangkok 10400, Thailand b Nanoelectronic and MEMS Lab, National Electronic and Computer Technology Center, Pathumthani 12120, Thailand c NANOTEC Center of Excellence at Mahidol University, National Nanotechnology Center, Thailand article info Article history: Received 11 March 2009 Received in revised form 16 March 2010 Accepted 24 March 2010 Available online xxx Keywords: E-nose Carbon nanotube Metal oxide Gas sensor Feature extraction techniques abstract In this paper, a portable electronic nose (E-nose) based on hybrid carbon nanotube-SnO 2 gas sensors is described. The hybrid gas sensors were fabricated using electron beam (E-beam) evaporation by means of powder mixing. The instrument employs feature extraction techniques including integral and primary derivative, which lead to higher classification performance as compared to the classical features (R and R/R 0 ). It was shown that doping of carbon nanotube (CNT) improves the sensitivity of hybrid gas sensors, while quantity of CNT has a direct effect on the selectivity to volatile organic compounds, i.e., methanol (MeOH) and ethanol (EtOH). The real-world applications of this E-nose were also demonstrated. Based on the proposed methods, this instrument can monitor and classify 1 vol% of MeOH contamination in whiskeys. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Nowadays, electronic nose (E-nose) has become a powerful tool to evaluate the aroma compounds during the quality con- trol process of foods and beverages [1–3]. Besides, E-noses have also been employed for public safety [4], environment protection [5,6], disease diagnostics [7], etc. E-nose is composed of an array of gas sensors made from various materials that display distinct gas-sensing behaviors of which differentiation can be combined and interpreted via pattern recognition techniques [8]. Among the available sensing materials, metal oxide semiconductors (MOS), such as SnO 2 and WO 3 , have been the most popular due to their high sensitivity to a rich set of volatile compounds. Doping has long been used as a traditional mean to obtain new MOS gas sensors that exhibit gas-sensing properties differentiated from the original ones. Recently, much interest has been focused on carbon nanotube (CNT) as potential dopant, due to its special electronic properties and high specific surface area that can boost catalytic reactions occurring at the metal oxide surface. The hybrid CNT–SnO 2 gas sensors prepared by different techniques have been reported to have excellent responses to NO 2 [9,10], CO [10], NH 3 ∗ Corresponding author at: Department of Physics and Center of Nanoscience and Nanotechnology, Faculty of Science, Mahidol University, Bangkok 10400, Thailand. Tel.: +66 866037395; fax: +66 22015843. E-mail address: sctkc@mahidol.ac.th (T. Kerdcharoen). [11],H 2 [12], CHOH [13] and indoor air pollutants [14]. Among such techniques, co-evaporation of SnO 2 /CNT is a relatively new concept to form hybrid CNT–SnO 2 gas sensors [15,16]. It offers exten- sive possibilities for controlling the film structure and morphology with high deposition rates, low contamination, high reliability and high productivity. However, there have been very few reports on CNT–SnO 2 gas sensors prepared by this technique, and this sensor system has not been applied for E-nose applications. In this work, we report on an E-nose based on hybridized CNT–SnO 2 gas sensors prepared by electron beam (E-beam) evapo- ration, which is inexpensive, fast, portable, reliable and suitable for use for the detection and classification of both solid and liquid sam- ples. In addition, feature extraction techniques including integral and primary derivative are proposed for improving classification performance by principal component analysis (PCA). This E-nose was tested in a real-world application, i.e., for detecting methanol (MeOH) contaminant in whiskeys. This system will be a useful tool for quality assurance of whiskey produced by village industries. 2. Experimental 2.1. Fabrication of gas sensors The gas sensors were fabricated by E-beam evaporation. Top and cross-sectional views of sensor structure are shown in Fig. 1a and b, respectively. First, Cr/Au interdigitated electrodes on alumina substrates were prepared. Prior to deposition of the electrodes, 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.03.072