Chemical images by arti®cial olfactory epithelia Ja Ânos Mizsei * , Sa Ândor Ress Department of Electron Devices, Budapest University of Technology and Economics, Goldmann Gy. te Âr 3, H 1521 Budapest XI, Hungary Abstract Creating olfactory images is an attractive method for selective gas analysis. These images can be considered as response patterns of a sensor matrix. The scanning version of the vibrating capacitor has been used as a simple and accurate tool for measuring the adsorption induced change of the work function at the surface of different gas sensing layers. The work function maps olfactory images) characterise the gas compositions H 2 in air, alcohol vapour, ammonium, and chloroform) over the receptor area Pd-Ag-Au-Pt-V-Pt-SnO 2 and Pt-Pd-Cu-Fe-Ni-Pt strips at different temperatures). The present paper details this new analytical tool and different olfactory images obtained by the scanning Kelvin method. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Sensor array; Chemical pictures; Work function; Vibrating capacitor 1. Introduction Chemical imaging is a very old principle of the analysis. It has been used for many million years in biological tasting and smelling systems: different receptors are separated and located in different places. We sense the sweet, bitter and sour at different areas of our tongue. Another example is the traditional paper chromatography in the chemical analysis, which results in real chemical images. Selective gas analysis usually needs a multi-sensor system composed of sensors having different sensing characteristics [1,2]. Different sensor temperatures and different activation doping of sensing layers) are the most common methods for building multi-sensor systems. Some of these can be con- sidered as distributed chemical sensors [3,4]. There are many different methods for reading out the information from the highly integrated surface potential sensors [5]. The multi-sensor system together with a computer hardware and software yield an electronic nose [6,7]. Chemical images can be composed from the output signals of a multi-sensor system discrete sensor array, based on four resistive type semiconductor gas sensors, [8]) by converting the different sensor responses to pixel elements see Fig. 1, for very simple examples). The logarithmic ratio of resistances ln R=R g values) have been used for the conversion, these numbers are related to the normalised work function chemical potential) changes at the surface [9]. There is another way to produce a lot of sensors with different characteristics which is the use of several different sensing and adsorbing materials on an asymmetrically heated substrate, as it has been demonstrated in our earlier preliminary work on olfactory pictures [10]. In the recent article, we go over the simple demonstration, using wider variation of sensing materials and gases saturated vapours), better substrata and better control of the gas/vapour con- centrations. 2. Experimental Our recent arti®cial olfactory epithelia hold even more different ``receptor'' materials Pd-Ag-Au-Pt-V-Pt-SnO 2 and Pt-Pd-Cu-Fe-Ni-Pt strips) on ceramic substrata, which are heated asymmetrically by thin platinum resistor layers on the backside. This kind of heating results in a continu- ously changing surface temperature, i.e. temperature gradi- ent, which is perpendicular to the material composition gradient, as it can be seen in Figs. 2 and 3. Thus, the surface can be considered as an integrated and distributed sensor system containing different adsorbing materials at different temperatures. The gas adsorption depends on both the composition of the surface material and the temperature, and it brings about changes in the surface parameters i.e. shifts the work functions) of adsorbing layers. We developed a new slipping box system connected with a vibration pump and a 2500 cm 3 reservoir. This system keeps the constant gas composition over the sensing surface during the vibrating capacitor scan, see Fig. 2. The reservoir Sensors and Actuators B 83 2002) 164±168 * Corresponding author. Tel.: 36-1-463-2715; fax: 36-1-463-2973. E-mail address: mizsei@eet.bme.hu J. Mizsei). 0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0925-400501)01035-8