Sensors and Actuators B 43 (1997) 180–185 Chemoresistive conducting polymer-based odour sensors: influence of thickness changes on their sensing properties Elisa Stussi *, Rita Stella, Danilo De Rossi Centro ‘E. Piaggio’, Facolta ` di Ingegneria, Via Diotisali, 2, I -56126 Pisa, Italy Accepted 8 April 1997 Abstract Conducting polymer films are widely used in the detection of odours. The change of resistance in the presence of odorants depends on the polymer thickness, and affects the sensitivity properties of the device. Using the vapour phase polymerization technique it is possible to fabricate polymer layers of controllable, uniform thickness. Aiming at the characterization of polymeric sensors as a function of their thickness, we implemented sensors of different thicknesses and carried out sensitivity measurements using toluene as an analyte. Sensitivity in terms of percentage variation of resistance per unitary odour concentration change was found to decrease with increasing thickness. Data on sensitivity as a function of thickness is presented and a possible explanation is proposed to account for the experimentally observed behaviour. © 1997 Elsevier Science S.A. Keywords: Polymer-based odour sensors; Polymer thickness; Vapour phase polymerization 1. Introduction Conducting polymers are being widely used for odour sensing in the form of arrays made up of highly sensitive, scarcely selective, chemoresistive sensors char- acterized by different sensitivity spectra. Such sensing layers are usually formed by electro- chemical polymerization from a monomer solution across two electrodes until a bridge of polymer is formed establishing an electrical contact. The final product is usually a polymer of irregular thickness. Vapour phase polymerization (VPP) is an innovative technological method [1] that allows formation of lay- ers of conducting polymers of any desired form and thickness in a uniform way on an insulating substrate. This technique has been recently extended to solid monomers [2], greatly increasing its range of applicabil- ity and making the use of VPP really appealing for the fabrication of odour sensors. The interaction with volatile compounds affects sev- eral properties of conducting polymers such as conduc- tivity, work function, optical absorption spectra and others. Among these, conductivity variations can be easily detected, are a linear function of the analyte concentration and are therefore usually used as actual signals in conducting polymer-based odour sensing devices. Since all these changes are largely reversible, interactions between the polymer and the volatile chem- ical are supposed to be weak [3]. In sensor applications the signal to be processed usually consists of the steady state value of the resis- tance change that can be expressed in various equiva- lent forms (e.g. R /R 0 , 100 ×(R -R 0 )/R 0 ). Response and recovery time are usually quite short (of the order of a few seconds) in comparison to the dynamics of the flow system and the performance needed, and do not therefore represent a critical point, whereas the sensitiv- ity of the sensor, i.e. the resistance change as a function of concentration, is a very important feature. This parameter is a function of the film thickness of the sensing layer, and some experimental measurements have been carried out in order to investigate the diffu- sion and sorption mechanisms between the polymer and the volatile chemicals [3 – 6]. In particular, Topart and Josowicz [4] found that the methanol mass uptake of PPy increases with the mass of conductive polymer, showing that a bulk sorption takes place. * Corresponding author. Present address: Universita ¨t Tu ¨ bingen, Inst. f. Physikalische und Theoretische Chemie, Auf der Morgenstelle 8, 72076 Tu ¨ bingen, Germany. Tel.: +49 7071 2977633; +49 7071 2978766; e-mail: elisa.stussi@ipc.uni-tuebingen.de 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S09 2 5 -4 005(97)00 14 7 - 0