Inkjet Production of Innovative Metal Oxide Sensors for e-Nose Systems
Authors: Bax Carmen , Bernasconi Roberto , Amoruso Michele , Oliaro Federico , Capelli Laura Maria , Sironi Selena , Magagnin Luca 
Published: July 2021

In recent years, electronic noses have been successfully adopted for odour detection and characterization in different fields, as environmental monitoring, safety, food quality control and clinical diagnosis [1-4]. However, industrial applications are still limited [5]. One of the major criticalities associated to e-nose adoption is sensor drift, which consists in small temporal variations of the sensor response exposed to the same analyte under identical conditions [6]. Indeed, signal variations over time result in a decrease of sensor sensitivity and selectivity that affects measurement reproducibility and limits the translation of e-nose laboratory results to industrial applications [5]. For the purpose of improving e-nose stability over time, many researchers started working on the development of drift mitigation algorithms to counteract drift effects and make sensor responses comparable over time and many different methods have been proposed in the scientific literature [6]. This paper proposes an alternative approach to the problem, which involves the development of an innovative technique for the production of Metal Oxide Sensors MOS, based on inkjet print. This technology is particularly suitable for microfabrication applications requiring high patterning precision and controlled deposition of functional materials. In particular, both ZnO sol-gel solutions and ZnO nanoparticles suspensions were developed and subsequently printed on alumina substrates. Inks physical properties were tailored to allow efficient jetting from a Dimatix materials printer. Annealing conditions were optimized and active films with different morphological and crystallographic characteristics obtained from the printed layers. Innovative sensors performance was tested by means of specific tests with a reference compound and evaluated in terms of efficiency towards reference compound:
ε = (R0 - R)/R0

where R0 is the resistance at the beginning of the measurement and R the minimum resistance value recorder during the analysis of the reference compound. Innovative sensors were tested for a period of 9 months and n-butanol was used as reference compound. For the entire period of testing, sensor efficiencies toward n-butanol were above 90%, highlighting very good sensibility of inkjet sensors towards the reference compound and great stability of responses over time. These results proved the feasibility of the adoption of inkjet print for the production of MOS sensors, encouraging future investigations in this field.

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