# Fluctuation-Enhanced Sensing (FES): A Promising Sensing Technique
###### Abstract  
Fluctuation-enhanced sensing (FES) is a very powerful odor and gas sensing technique and as such it can play a fundamental role in the control of environments and, therefore, in the protection of health. For this reason, we conduct a comprehensive survey on the state-of-the-art of the FES technique, highlighting potentials and limits. Particular attention is paid to the dedicated instrumentation necessary for the application of the FES technique and also in this case limits and possible future developments are highlighted. In particular, we address resolution, measurement speed, reproducibility, memory, noise, and other problems such as the influence of humidity. A number of techniques and guidelines are proposed to overcome these problems. Circuit solutions are also discussed. fluctuation-enhanced sensing; noise; gas sensing; low noise instrumentation +
Footnote †: journal: Applied  
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may contain a wide variety of volatile organic compounds (VOC) that may cause, in case of exposure to them, mucous irritation, neurotoxic effects, and nonspecific reactions [14; 15; 16]. Several monitoring techniques are being developed for the study of indoor environment, such as gas distribution mapping (GDM), which can be used to evaluate the efficiency of environmental control systems and identify pollutant sources [17]. Obviously, a fundamental role is played by the quality of the sensors used, in terms of sensitivity, selectivity, and fast response. The classical Taguchi-type semiconductor gas sensor bases its principle of operation on the change of its resistance as the gas to be sensed is adsorbed onto the sensor surface [18] and it represents a low cost solution. Metal oxides are the most commonly used sensing materials [19]. Nanostructured materials offer some distinctive advantages in this area, such as small grain size, controlled growth of morphology and heterojunction effect provided by compositing techniques, making them good candidates for CO\({}_{2}\) gas sensors [20]. The porous structure of one class of these materials, by offering a high surface area, increases the sensitivity of the gas sensors [21; 22]. Not only the high surface area, but also the actual nanostructure plays a key role for sensitivity enhancement of nanostructured materials [23]. Sensitivity can also be improved by doping the oxide materials [19; 24]. The alarming rise of indoor pollution has promoted, thanks to appealing physical, chemical, optical, and electronic properties of new materials, increasing effort in advanced sensing materials. The state-of-art gas-sensing nanomaterials and their future perspectives are well-documented and summarized in [25]. However, improvements in all the above-mentioned directions cannot be achieved without further research on the mechanisms of the proceeding reactions and phenomena and on other impacting factors. Indeed, much work is still required for the application of new materials in the real life devices [26]. To improve the performance of gas sensors, in parallel with the research in the field of new materials, it is possible to explore new advanced sensing techniques, even exploiting standard sensors. For example, chemical selectivity of semiconductor gas sensors can be boosted by operating an array of sensors, each of them having different sensitivity to different gases (the so called Electronic Nose) [27]. A pattern recognition method must be employed to analyze the output of the sensors array [28] and machine learning technique can also be used [29]. Chemical selectivity can also be improved by investigating the dynamic response of temperature modulated sensors [30]. However, almost all of the investigated gas sensors still suffer from lack of selectivity. Coupling smart gas sensors with artificial intelligence algorithms could be a solution to empower conventional gas sensing technologies and increase accuracy in gas detection [31].