Olfaction

Senior investigator: Prof. Giorgio Sberveglieri

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From chemical gas sensors to Electronic Noses

A chemical sensor is a device that transforms chemical information (ranging from the concentration of a specific sample component to total composition analysis) into an analytically useful signal.
Due to their small dimension, low cost, low power consumption, on-line operation and high compatibility with microelectronic processing, conductometric semiconductor thin films are between the most promising devices in the field of solid state chemical sensors. The fundamental sensing mechanism of semiconductor gas sensors relies on a change in electrical conductivity due to the interaction process between the surface complexes and the gas molecules to be detected.
Unfortunately, sensors still suffer from lack of selectivity and long-term stability: they are not able to recognize single chemical compounds, their analytical approach being based on unspecific chemical interactions (redox processes on semiconductor surface). However, the mentioned limits have not restricted the interest and the use of these tools. ENs have indeed found applications in many field, from environmental monitoring to medical diagnostic and food analysis [Pearce, Shiffman, Nagle, & Gardner, 2003].

Figure 2. Typical response of a MOx gas sensor; the curve essentially consists of three intervals: (a) baseline; (b) adsorption step, during which the sensor interacts with the odour sample; (c) desorption, namely the recovery of baseline.

An array of sensors is the core of the Artificial Olfactory System (AOS) – also called “Electronic Nose” (EN).
Great effort has been put in EN applications. Below we present some recent successful applications of the EOS835.

1) Ozone detection for industrial safety [1]. We tested the detection properties of four MOX sensors toward different ozone mixtures to identify sets of sensing layers and interfering compounds concentrations most suitable for a reliable detection of ozone. The measurement campaign lasted 1 year divided in four sessions. We collected a substantial amount of measurements (more than 500) with diverse interfering gases: ammonia, ethanol, ethylene, carbon monoxide and humidity. To make sense of the data, we systematically applied the exploratory data analysis methodology. This allowed to find the causes of variation in the data, e.g. time (sensorsÕ long-term stability) or interfering effects of the different chemical compounds. All the analysis techniques employed in this work are implemented in a software package developed at our laboratory. We concluded that the two best stable and sensitive sensors are based on WO3 and SnO2 (Au catalyzed). We ranked the contributions of different gases on sensor responses, deducing that our sensors are suitable to detect steps of 50 ppb of ozone when ethylene is less than 10 ppm. Carbon monoxide does not affect the measurements. The strongest interfering compound is humidity that needs to be controlled or parallely measured.

2) A novel, wide-range sensor read out electronics [2]. We worked (in cooperation with the Dept. of Electronics, Univ. Brescia) on a low-cost electronic nose based on a resistance to period converter readout system, suitable to handle a wide range of resistance values (from k½ up to tens of G½) with high accuracy (<1%). An array composed of four metal oxide based gas sensors, with baseline resistance spreading on the above range has been used to validate the system. The electronic nose has been applied to the detection of key aromas peculiar of different stages of the bread baking process has been chosen as target application, revealing the suitability of the proposed electronic nose to distinguish these volatiles in an ordered manner reflecting the different baking step they represent.

3)  Detection of mycotoxins in corn [3]. The maize cultivation is easily spoiled by fungi with potential contamination by mycotoxins. One of the most common toxins is Fumonisin B1, mainly produced by Fusarium verticillioides and associated to the occurrence of human and animal diseases. We have demonstrated the capability of the electronic nose to detect fungal contamination of maize and to classify F. verticillioides strains in relation to their different behavior in fumonisins production. Electronic nose can represent a valid method of screening the maize bulk in order to prevent the entry of mycotoxins into the food chain.

4) Microbial contamination in canned tomatoes [4]. Microbial contamination can easily affect processed tomato, thus determining both organoleptic adulterations and potential health risks for customers. Innovative techniques for a rapid and reliable diagnose of spoilage are highly requested in order to guarantee food safety and to improve production. We artificially spoiled canned peeled tomatoes with different kinds of microbial flora and then analysed them with the Electronic Nose. Preliminary analyses by Dynamic-Headspace Gas Chromatographic-Mass Spectrometry showed significant differences in the semi-quantitative volatile compounds profile of spoiled tomato samples just after few hours from contamination, thus suggesting employing the Electronic Nose for an early diagnose of microbial presence. The Electronic Nose was indeed able to reveal contamination, even at early stages for some contaminant (e.g. for S. cerevisiae and E. coli), and to recognize spoiled tomato samples with good classification performances.

5)  Contamination in soft drinks [5]  and fruit juices [6] by Alicyclobacillus spp. (ACB). Since its first appearance in 1984 ACB has been recognized by producer companies as a major quality control target microorganism, especially because of its skill in surviving pasteurization and developing some days after product packaging, thus escaping the quality controls usually performed. Traditional analytical techniques are able to identify the ACBÕs presence only after the production of secondary metabolites (namely guaiacol and halophenols), i.e. once the products are already spoiled. The EOS835 was tested on commercial soft drinks, ready to be released on the market, naturally contaminated by ACB. It showed a noteworthy ability in early identifying contaminated beverages: real time PCR analyses, conducted in parallel with EN measurements, showed indeed a very low amount of ACB and HPLC indicated that guaiacol, recognized as the main taint matabolite, was present at very low level or not present at all. Similar results were obtained for fruit juices.

References

[1] M. Vezzoli, A. Ponzoni, M. Pardo, M. Falasconi, G. Faglia, G. Sberveglieri, Exploratory data analysis for industrial safety application, Sensors and Actuators B 2008, 131, 100Ð109
[2] A. Ponzoni, A. Depari, M. Falasconi, E. Comini, A. Flammini, D. Marioli, A. Taroni, G. Sberveglieri. Bread baking aromas detection by low-cost electronic nose, Sensors and Actuators B 2008, 130, 100Ð104.
[3] M. Falasconi et al. Detection of toxigenic strains of Fusarium verticillioides in corn by electronic olfactory system, Sensors and Actuators B 2005, 108, 250Ð257.
[4] I. Concina, et al. Early detection of microbial contamination in processed tomatoes by electronic nose, Food Control, 2009, 20, 873-880.
F. Bianchi, M. Careri, A. Mangia, M. Mattarozzi, M. Muscia, I. Concina, M. Falasconi, E. Gobbi, M. Pardo, G. Sberveglieri. Differentiation of the volatile profile of microbiologically contaminated canned tomatoes by dynamic headspace extraction followed by gas chromatographyÐmass spectrometry analysis, Talanta 2009, 962-970.
[5] I. Concina, M. Borsnek, S. Baccelliere, M. Falasconi, E. Gobbi & G. Sberveglieri. Alyciclobacillus spp.: detection in soft drinks by electronic nose. Food Res. Int., 2010, 43, 2108-2114.
[6] E. Gobbi, M. Falasconi, I. Concina, G. Mantero, F. Bianchi, et al. Electronic nose and Alicyclobacillus spp. spoilage of fruit juices: an emerging diagnostic tool. Food Control, 2010, 21 (10), 1374-1382.


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Electronic circuit interface

The research work in the experimental characterization of chemical sensor arrays needs to move towards new kind of instrumentation, able to improve the measurement metrological characteristics of the traditional electrical quantities (e.g. the conductivity). The research activity is oriented to the study of circuits and architectures for the realization of “ad hoc” instrumentation for chemical sensor array experimental characterization. A special attention will be paid to sensor dynamic characterisation, that is exciting sensor with well tailored thermal transients and excitation voltages.

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Distributed electronic noses

A better characterization of the sensor array behavior allows finding and elaborating a new sensor management method in order to reduce the power consumption. In fact, thanks to the reduction of the power consumption and by using power harvesting techniques (solar cells, em source …) it is possible to design cooperating and auto-arranging networks based on wireless chemical sensors. Such networks could furthermore be supported by traditional electronic noses, realizing a kind of distributed electronic nose to be employed in industrial, civil and territory control applications.