Sensors and Sensory Systems for an Electronic Nose

Abstract

A novel sensing system is proposed based on the multi-dimensional information contained in a dynamic nonlin-ear response. A sinusoidal temperature change was applied to a SnO 2 semiconductor gas sensor, and the resulting output conductance of the sensor was analyzed by fast Fourier transformation (FFT). The higher harmonics of the FFT characterized the nonlinear properties of the response. The amplitudes of the higher harmonics of the FFT exhibit characteristic changes which depend on the chemical structure, concentration, and the kinetics of adsorption and the reaction of hydrocarbon gases and aromatic vapors on the sensor surface. In addition, it is possible to distinguish between gases in a gaseous mixture with a single detector using this dynamic nonlinear response. Nonlinear responses are discussed in relation to the kinetics of the reaction at the sensor surface and the temperature-dependent barrier potential of the semiconductor. Although gas sensors have been developed previously in order to achieve high selectivity for a particular chemical species, it is difficult to distinguish chemical species on the basis of the static information obtained with a single detector, such as the conduc-tance of a semiconductor and the frequency of a surface acoustic wave (SAW). 1-6 The difficulty is attributed to the fact that gas sensors are usually responding to interferants coexisting in a gas sample. To overcome this difficulty, the combination of different types of sensors (a multiarray sensor) has been expected to be useful, including different kinds of semiconductor detectors and different surface modifications for SAW. 3,7 In this case, the distinction among components and quantification of the components are considered to be possible, based on the assumption of a linear relationship between the output for the detectors and the concentrations of individual species. It is obvious that this assumption is generally not valid because, in practice, the sensors usually have nonlinear characteristics: i.e., (1) the dependence of the response on concentration is usually nonlinear due to the saturation effect at high concentrations, (2) the response cannot be represented as merely the sum of the responses to each chemical species due to the competition among the chemical species at the surface of the sensor, and (3) the sensor response is intrinsically time-dependent and exhibits hysteresis and aging effects. Thus, it may be useful to develop a strategy in addition to the idea of using multiarray sensing systems that are based on the assumption of a "linear response". In contrast to artificial sensory systems, living organisms can detect and quantify chemical stimuli even in complex states, such as natural odors in the environment. 2 In living organisms, information regarding the chemical structure and concentration is transformed into nervous impulses. Since excitation and pulse generation in biomembranes are typical nonlinear phenomena, it may be worthwhile to investigate how the dynamic nonlinear information in the impulses is transformed into information regarding chemical species. Recently, we have described a new gas sensing method which could quantitatively characterize the dynamic nonlinear responses of a semiconductor gas sensor using a sinusoidal varying input voltage to the sensor heater. 8-12 We found that the waveform of the sensor resistance changes characteristically, depending on the composition of the gas species. The physicochemical significance of higher harmonics in the FFT signal was discussed in relation to the nonlinear characteristics of the temperature-dependent conductance and the kinetics of the adsorption of gas molecules on the sensor surface. In the present paper, we report that the amplitudes of the higher harmonics of the FFT signal exhibit characteristic changes depending on the chemical structure and concentration of hydrocarbon gases and aromatic vapors and demonstrate that it is possible to distinguish among these gases in a gaseous mixture with a single detector by using the nonlinear response. The