Detection of volatile organic compounds using surface acoustic wave sensors with different polymer coatings M.C. Horrillo a, * , M.J. Ferna ´ndez a , J.L. Fontecha a , I. Sayago a , M. Garcı ´a a , M. Aleixandre a , J.P. Santos a , L. Are ´s a , J. Gutie ´rrez a , I. Gra `cia b , C. Cane ´ b a Laboratorio de Sensores (IFA-CSIC), Serrano, 144, 28006 Madrid, Spain b Centro Nacional de Microelectro ´nica (IMB-CSIC), Campus UAB, 08193 Bellaterra, Spain Received 2 September 2003; received in revised form 6 April 2004; accepted 6 April 2004 Available online 14 May 2004 Abstract This paper compares the obtained experimental results with surface acoustic wave devices coated with three different polymers, polydimethylsiloxane (PDMS), polybutadiene (PBD) and polyetherurethane (PEUT), by detecting low concentrations of three volatile organic compounds (VOCs): octane, toluene and propanal. The polymer type and thickness and so as the test gas nature have a great influence on the sensitivity and selectivity of this sensor type. The different coatings have been prepared with the same thicknesses and they have been tested with the same gas concentrations. It has been possible to measure low concentrations of volatile organic compounds obtaining very good results of sensitivity and very short response and recovery times. There is difference of response among the different tested concentrations and the responses have been linear. D 2004 Elsevier B.V. All rights reserved. PACS: 8385Gk Keywords: Sensors; Polymers; Surface acoustic waves; Organic substances; Quartz 1. Introduction For many years, different devices have been designed and used to detect volatile organic compound (VOC) emissions from industrial and commercial processes. These devices manufactured since the 1920s convert a physical or chemical quantity into a measurable electrical signal. It is very important to develop chemical sensors that allow the measurement of volatile organic compounds in a fast and continuous way and in situ, since these compounds are very dangerous for human health and environment. Up to now the classical techniques of analysis are the most used to measure such compounds in air, water and soils, resulting in an expensive and very large analysis, and in addition, they are difficult of carrying out in situ. Therefore, a huge need exists to carry out analysis in real time; that is to say, we have to be able to perform the testing at the site where the pollutants are produced. Mass-sensitive devices can usually work as chemical sensors for detecting odorants. Polymeric materials are used to detect organic vapours since the vapour sorption is fast and reversible. In this work, surface acoustic wave sensors (SAW) of small size have been developed on ST-cut quartz to obtain a frequency of 157 MHz, since it is well known that this sensor type shows good characteristics for VOCs detection [1–5]. In using the gas sensors, the surface of the substrate is coated with chemically active thin layers, which are capable of sorbing gas specific molecules, and therefore, a change of mass is produced. The sensitivity, response time, selectivity and reversibility depend on the thermodynamics and kinetics of the sensor material/vapour interactions. In this paper, experimental results have been obtained for hydrocarbons detection, such as octane, toluene, and for aldehydes as propanal with sensors coated with three highly permeable different polymer types: polydimethylsi- loxane (PDMS), viscous liquid rubber; polybutadiene (PBD), solid rubber; and polyetherurethane (PEUT), solid rubber; since in this way, the processes of sorption and 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.04.012 * Corresponding author. Tel.: +34-91-5618806; fax: +34-91-4117651. E-mail address: carmenhorrillo@ifa.cetef.csic.es (M.C. Horrillo). www.elsevier.com/locate/tsf Thin Solid Films 467 (2004) 234 – 238