Evidence of a correlation between the non-linearity of chemical sensors and the asymmetry of their response and recovery curves

The experimental response/recovery time of a chemical sensor is considered as the sum of an intrinsic time, independent of experimental conditions and supposing an instantaneous change of the fluid surrounding the sensor, and of an extrinsic time, depending on the transient concentration of target species, linked to the fluid delivery system. The transient concentration in the test cell is assumed to follow the exponential variations described by the well-stirred tank model. The application of the law governing the sensor signal to the transient concentration yields the extrinsic response/recovery times and accounts well for the symmetry or asymmetry between the response and recovery curves, according to the linearity or non-linearity of the sensor. All major trends derived from this theoretical approach are verified experimentally with various types of sensors, and this is shown to have major implications in many practical cases, including microsensors and microfluidic or car exhaust sensors.