Gas Phase Sensor Arrays Electronic Noses

Since 1995 a sensor array based on metalloporphyrin-coated QMBs has been developed. The performance of the developed electronic nose, called LibraNose, has been tested in different fields of applications, ranging from food to clinical analysis [122-132].

For food analyses the objectives of the experiments were different, depending on the particular matrices analyzed, ranging from the recognition of storage time to assessment of food quality.

In the case of the analysis of cod fish and veal meat [122, 123], the aim of the analysis was the classification and recognition of days of storage. In both cases the electronic nose provided a good classification of the data, which was in perfect agreement with the food aging. More recently codfish fillet aging has been studied over a period of 17 days and it was demonstrated that the electronic nose followed the variation of the headspace chemical composition [129]. Furthermore it was showed that misclassification errors were due to the nonlinear evolution of the headspace chemical composition during the spoilage process and not to lack of resolution of the sensor array.

For red wine the electronic nose was exploited to monitor the aroma modification when the wine was exposed to the air; a significant modification was observed three hours after the opening of the bottle, with a qualitative agreement with the expected variation of the taste: it is well known that red wine needs contact with air to enhance its taste [123].

A significant result was obtained in the analysis of tomato paste; in this case the aim of the experiment was the classification of different tomato pastes produced by two different cultivations: biological and traditional. The data obtained with the electronic nose were compared with those obtained by a human panel of trained tasters. Data analysis indicated a good agreement between the classification obtained by the electronic nose and the human panel, but electronic nose showed a lower dispersion of the data and consequently a better definition of classes [124, 125]. A similar comparison was carried out for the analysis of UHT milk samples [123]. In this test, 13 different commercially available brands of

UHT milk were taken into consideration. Each of them was analyzed with the electronic nose, while the panel searched the presence and the intensity of a specific sensation, which is a drawback for this particular product: the aroma of burned milk. There was perfect agreement between sensorial analysis and electronic nose results and this was surprising because sensorial analysis was aimed at evaluating only one of the aroma features, while the electronic nose data were related to the whole aroma composition. The same approach was recently followed for the analysis of fruit samples, such as peaches and nectarines, with the aim to define a sensorial profile for these fruits [132].

A different experiment was aimed to measure the aroma decay of oranges over a period of time of about one month, while in the case of apples the study was devoted to the measure of mealiness and skin damage. Results obtained showed that electronic nose was able to correctly predict the amount of defects (apple) and storage days (for oranges) [128].

Electronic nose and spectroscopic techniques were recently integrated to give a virtual instrument devoted to the measurement of some analytical parameters of peach samples [127]. The fusion of visible spectra and electronic nose data gave an interesting method to probe fruit samples in a nondestructive way.

Different olive oils have also been classified by electronic nose analysis [130]. Two different electronic noses were exploited in this study and their data were compared and integrated.

Although food analysis has been the most explored field of application for electronic noses, more recently the medical field was also taken into consideration and the application of the electronic nose to detect diseases was proposed [133-136]. The hypothesis of these studies was that body odor is correlated with human health. Human odor is in fact correlated with the combined action of both gland and bacterial population and any change of the complex equilibrium regulating the organism can induce a change in the nature and amount of volatile organic compounds excreted by the organism. On the other hand medicine recognizes that some pathologies produce unpleasant characteristic odors (for example diabetes and hepatic diseases). As a consequence the exploitation of electronic nose, which is a noninvasive probe, can represent an eligible tool for a preliminary diagnosis of diseases. In particular, the sensor array was able to detect 5a-androst-16-en-3-one, a steroid present in human sweat and supposed to be a male pheromone, with a resolution comparable to that of human olfaction [133]. In a different paper correlation between electronic nose measurements and the ovulation cycle phases in fertile women has been claimed [135].

The metalloporphyrin-QMB sensor array was also exploited for the analysis of the headspace of urines belonging to patients affected by kidney diseases [134]. The electronic nose showed good performance in distinguishing samples containing blood, one of the pathologies induced by the kidney disease, also giving some indication of the blood amount.

More recently, LibraNose has been exploited for the diagnosis of lung cancer [136]. Volatile organic compounds present in expired breath may give a potential source of information about general metabolic conditions and, in particular, on lung physiology; and breath analysis carried out by combined gas chromatography-mass spectroscopy evidenced that different volatile patterns occur in diseased individuals. The exploitation of the electronic nose to analyze breath from individuals affected by lung cancer resulted in a good classification among the groups of individuals according to their known pathologies, discriminating them from a sample of healthy people measured as reference. A number of patients were measured after the surgical removal of the cancer mass. In this case the nose considered these patients to be healthy. These preliminary results demonstrated a very promising field of application for electronic noses, with an enormous impact for the early diagnosis of diseases.

Other than QMBs, the same authors proposed an electronic nose based on metalloporphyrin based optical sensors [137]. The optoelectronic nose was developed by deposition of thin films of two metalloporphyrins, CoTPP and RhTP-PCl, a Mn(III) corrole and a sapphyrin as sensing materials onto a transparent substrate. Commercial, low-price blue-LEDs and photodetectors were used to develop the sensor array. In order to reduce as much as possible the influence of LED intensity fluctuations, a differential measurement strategy was chosen and one position was left free of porphyrin and used as blank reference. In Figure 24 a sketch of the array is reported. The array was exposed to different concentrations of VOCs and was able to discriminate among them. If compared with QMB sensors, optical transduction was demonstrated to give higher resolutions for the same sensing material (Table 5).

Very recently an optical sensor array was constituted of LB films of FeTPPCl, MnTPPCl, CoTMPP, and CoOEP co-deposited with arachidic acid [138]. The system used four LEDs operating at different wavelengths (645, 615, 601, and 546 nm) as light sources. Fiber optics were used to illuminate LB films and to receive the reflected lights. The responses of such a sensors were fast and reversible and the array was able to discriminate the aroma of three different capsicum samples.

Following a different approach, Suslick and co-workers have recently reported a colorimetric electronic nose based on metalloporphyrins [139-142]. The array is formed by spots of different metalloporphyrins deposited onto a silica s1-ref s2- ref

s1-ref s2- ref

- s7- ref volume chamber: 18ml

Figure 24. Sketch of the porphyrin-based optical sensor array. Reprinted with permission from [137], C. Di Natale et al., Sens. Actuators B 65, 220 (2000). © 2000, Elsevier Science.

volume chamber: 18ml

Figure 24. Sketch of the porphyrin-based optical sensor array. Reprinted with permission from [137], C. Di Natale et al., Sens. Actuators B 65, 220 (2000). © 2000, Elsevier Science.

support. The image of the array was digitalized by a scanner before and after the interaction with VOC and color change profiles represent the response of the array and are characteristic of the analyzed VOC. In Figure 25 the colorimetric changes observed have been reported. This electronic nose approach has been called SmellSeeing™ [140, 141]. Resolutions down to 2 ppm were obtained for such an array and in some case responses in the range of 100 ppb were recorded. The inherent simplicity, the absence of complicated data analysis, the high sensitivity, and the fast and reversible sensor responses make this approach particularly attractive. Furthermore this approach did not suffer from humidity variation, a parameter of significant importance for practical applications.

Exploitation of this approach to the detection of bacterial growth has been recently reported [142]. The SmellSeeing™ array was able to detect a wide range of VOCs, which can be related to bacterial growth, in the ppb range. This results in the detection of culture of E. coli bacteria in short time; promising results were also obtained with the growth of skin and mouth bacteria, making this array particularly promising for the development of real-time, portable, and easy-to-use sensors for bacteria identification.

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