Application of metalloporphyrins-based gas and liquid sensor arrays to the analysis of red wine

Abstract

Electronic noses and electronic tongues have been introduced in the past decade as technological attempts to mimic the functions of the human chemical senses. Beside this scientifically challenging objective, they have also shown to be practical instruments to analyse samples characterised by a complex composition. In this paper, two arrays of metalloporphyrins-based gas and liquid sensors are applied to the analysis of a red wine. The scope of the experiment here illustrated was to reproduce the classification properties of sensorial analysis and to quantify, from the sensor arrays data, both sensorial descriptors and chemical parameters. Results demonstrate the capability of such systems to be trained according to the behaviour of a practical panel of tasters. The analysis of data also revealed that the combination of the two arrays enhances the prediction properties both for qualitative and quantitative analysis.

Introduction

Since the 1980s, the integration of chemical sensors in arrays has shown to be an advantageous solution to measure environments characterised by the simultaneous presence of several compounds [1], [2]. In this configuration, sensors should not necessarily be specific for a target analyte, but they can rather be broadly selective or even not selective at all, with the only necessary requisite to behave differently from one each other i.e. to provide partially correlated responses. For the analogy of this approach with the working mechanism of the mammalian “chemical” senses, olfaction and taste, these sensor arrays are generally called either electronic noses (vapour phase sensors) or electronic tongues (liquid phase sensors). After the first approach of Persaud and Dodd [3], the development of electronic noses has become a consolidated field of research, several examples of such arrays have been reported, and some instruments are commercially available [4].

In spite of the widely accepted term “electronic nose”, current systems are still far from the structure and the functions of natural olfaction. The only point of common between artificial and natural systems is the strategy of measurement based on arrays of non-selective sensors. The concept underlying electronic nose systems has been demonstrated to be independent on the particular sensor mechanism, indeed during the last decade almost all the available sensor technologies have been utilised for electronic noses. Clearly, these sensors are very different from the natural receptors. These dissimilarities make the perception of electronic nose very different from that of natural olfaction but, in spite of these, several examples of correlations between sensorial analysis and electronic nose have been demonstrated.

While electronic noses are an established research and development topic, surprisingly only a few examples of the relative concept applied to liquid samples have been developed. Nonetheless multivariate analysis with arrays of ion selective electrodes were proposed in the late 1980s [5], and the first electronic tongue, based on ion selective electrodes functionalised with lipid membranes, was proposed at the beginning of the 1990s by Toko and co-workers [6]. An electronic tongue based on potentiometric electrodes was successively elaborated by a co-operation between the University of St. Petersburg and the University of Rome “Tor Vergata”, using a set of chalcogenide glasses-based electrodes [7]. As for electronic noses, various electronic tongues exploiting different sensor technologies have been investigated [4].

The simultaneous utilisation of electronic noses and electronic tongues has been also suggested on the basis of the idea that the co-operation of both systems is expected to increase the amount of information extracted from a sample. It is, indeed, a common experience that olfaction and taste co-operate in the formation of the judgement over, for instance, a certain food. It is worth mentioning that the two sensor systems, when applied to the same liquid sample, do not sense the same features. Electronic tongue electrodes are immersed in the sample while the electronic nose sensors are in contact with its headspace. For this reason, an electronic nose and electronic tongue are expected to provide an increase of information with respect to the use of only one of the two systems.

Beside the great challenge to develop artificial systems mimicking the chemical senses of humans, research on electronic noses and tongues immediately has addressed practical utilisation. Opportunities for industrial applications arise from the fact that human senses are of primary importance in many fields where sensory analysis is used to handle the information extracted from human perceptions.

Among these fields, those related to food and beverage are certainly the most travelled roads towards the definition of practically usable applications.

The analysis of foods and beverages involves methods developed in many different basic sciences. Most of these measurements are concerned with safety (e.g. the search for contaminants), composition (to individuate the basic constituents), effects of food treatment and processing and so on. For each of these aspects, a plethora of techniques are being studied and developed [8]. Beside the objectives mentioned above, recently there has been a strong increase for the certification of quality. In an epoch of strong globalisation of production and market quality, it is becoming a key issue in food marketing.

Quality is a global character of foods; it concerns all the aspects of the interaction between food and consumers. Thus, the approach “par excellance” to determine quality is sensory analysis. Although sensory analysis has been studied for since many years, panels are still affected by imprecision and scarce reproducibility makes it difficult to compare analyses done at different times.

In terms of quality, one of the best-characterised beverage products is wine. Mostly due to cultural aspects, sensory analysis of wine has been significantly developed. This gives an almost unique opportunity to compare electronic and natural senses, and to investigate to what extent artificial senses can provide the same sensorial evaluation of trained humans. For this reason, wines have been investigated in the past with both electronic noses [9] and electronic tongues [10], [11].

In this paper the application of electronic nose and tongues to the characterization of red wine is illustrated. Signals from the instruments are used to build predictive models of both chemical and sensorial parameters. Furthermore, data from both the instruments are fused in order to increase the prediction accuracy.

As has been mentioned before, several sensor technologies have been utilised to resemble artificial senses. From the point of view of sensors it is important to tailor the sensitivity of a single sensor, still maintaining non-selectivity, in order to endow the array with the necessary sensitivity to discriminate between different samples. Among the investigated sensor materials, metalloporphyrins have been studied as the basis of solid-state sensors for electronic nose and tongue applications [12]. These molecules are well known for their important role played in many biological processes. In particular, solid-state metalloporphyrins show great selectivity which can be oriented towards different classes of species by changing the molecular constituents: pyrrole ring, central metal atom and peripheral substituents [13].

Furthermore, metalloporphyrins, because of their organometallic nature, are particularly eligible for artificial olfaction, indeed most of the odorous compounds are excellent ligands for metal ions. Although very little is known about the structures of the olfactory receptor proteins, it may be argued that many of the receptors are likely to contain metal ions at their active sites [14].

In this paper the simultaneous application of a porphyrin-based quartz piezoelectric microbalance gas sensor array and potentiometric electrode sensor for solutions is presented. The choice of the same material gives the possibility of a better control of the sensors characteristics.