Innovative metal oxide chemical sensing devices
Giorgio Sberveglieri
SENSOR Lab., CNR-INO & University of Brescia, Dept. of Information Engineering, Via
Branze 38, 25133 Brescia, Italy
E-mail
Conductometric chemical sensors based on metal oxides semiconductors are known since
1954-1962, when the effects of reaction of metal oxides with the surrounding atmosphere were
discovered and the first commercial gas sensor was developed. Since then they attracted the attention
of researchers working on sensors thanks to low costs of fabrication, simple preparation and
operation, large number of potential detections and applications together with the possible device
miniaturization. The challenges remain the improvement of their sensitivity, selectivity, stability and
speed (i.e. response and recovery rates). The interaction between gas molecules and active oxide takes
place at its surface, therefore the atoms of the surface are crucial in chemical sensing performances.
Nanowires in particular present a high surface to volume ratio that provide a way to enhance the
effects of surface phenomena (that are at the base of chemical sensing mechanism), moreover they
may be single crystalline and have well defined crystal orientations, leading to controlled reactions
and increased stability. Moreover, new interesting effects or transduction mechanism may be
exploited in chemical sensing such as opto- and magneto-optical signals.
Concerning metal oxides materials, easy and cheap growth techniques for the production of
nanostructures in a variety of morphologies are constantly proposed by the research community.
Thermal evaporation, oxidation and anodization methods have been used to prepare a variety of
oxides such as tin, zinc, copper, nickel, tungsten, titanium oxides and will be extensively described
since they are among the most explored in the recent papers and the cheapest for nanocrystal
production. Moreover, results on heterostructured nanomaterials (such as ZnO/NiO), and their
functional properties will be discussed together with the exploitation of innovative transduction
principles (optical and magneto-optical chemical sensors).
Concerning new operation principles, magneto-optical chemical sensing tests, using a MOKE
magnetometer, have proven the possibility to exploit a new transduction mechanism for sensing
devices. The sensing layer, based on Co layer covered by ZnO nanorods, showed very good H
2
detection at room temperature.
Moreover, metal oxide gas sensors have been integrated in a flexible instrument, often referred as
electronic-nose (EN), based on an array of different sensors, whose collective response is handled by
means of a pattern recognition software. In particular, the authors have developed an EN called S3
(Small Sensor System) mini. This allows to obtain selectivity and analysis capability also working
with atmospheres as complex as those encountered in security ad food safety applications. In these
fields, targets are not single molecules, but smells composed by hundreds (if not thousands) of
compounds, whose analysis through analytical chemical techniques remain a challenge.
The S3-mini EN has been successfully employed in various areas of food quality control and
traceability, including the diagnosis of microbial pathogens contamination. This type of innovative
technology adapts brilliantly for the use in several projects related to security, control, respect of
wholesomeness in the product and the consumer. Real product monitoring from "farm to table" for
many products such as extra virgin olive oil, tomato-based products, the whole chain of coffee,
cheeses, fresh meat, juices fruit, etc. will be reviewed.
Moreover, concerning security, we’ll show the capability of metal oxides sensors and S3 mini
technology to detect human odour (sweat), working with both real samples and artificial sweat
samples generated in laboratory through a microbiological approach.
Acknowledgements
The research leading to these results has received funding from NATO, under the project n° 9085043 NANEOS, from the
European Community’s FP7-ICT-2013-10, MSP— Multi-Sensor-Platform for Smart Building Management under the
project n° 611887 and from SNOOPY project FP7-SEC-2012.3.4-4 " Sniffer for concealed people discovery " n°
313110.
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