Mauro Epifani

Monitoring of rancidity of milk by means of an electronic nose and a dynamic PCA analysis

Abstract This paper reports the use of a semiconductor thin films based electronic nose to recognise the rancidity of two different kinds of milk (UHT and pasteurised) during their ageing days. The employed sensor array consists of five different SnO 2 thin films prepared by means of the sol–gel technology. The data coming from the response of the sensors have been elaborated by principal component analysis (PCA) in order to obtain a classification of the data clusters related to different milk ageing days and so track the dynamic evolution of milk rancidity. The obtained results could lead to industrial applications in dairying for the quality control of starting milk and the rancidity check during the different stages of production.

Analysis of vapours and foods by means of an electronic nose based on a sol–gel metal oxide sensors array

Abstract In this paper, a novel electronic nose based on a metal oxide gas sensors array, prepared by means of the sol–gel method in the form of thin films, is described, including the fabrication process. Two different configurations of the array were developed: one consisting of sensors with the same sensing layer (SnO2) but different geometry and material for the electrical electrodes, and another consisting of chemically modified SnO2 thin films. Examples of application to the analysis of air pollutants (CO, NO2, CH4, C2H5OH and CH3OH) and different foods (oil, milk, tomato, wine, ground coffee). Recognition of different air contaminants and of the aroma of foods is achieved by using the principal component analysis (PCA). Good discrimination among the clusters has been obtained indicating promising results for the application of our sensor array as electronic nose, in particular in the food industry for the classification of different olive oils.

A novel gas sensor based on SnO2/Os thin film for the detection of methane at low temperature

Abstract Osmium-doped tin oxide thin films were prepared by means of sol–gel technique. Their methane sensing properties were studied and compared to undoped SnO 2 thin films. The good sensitivity to methane at a low working temperature makes them very attracting active materials to be used in domestic CH 4 leak-detectors.

AIR QUALITY MONITORING BY MEANS OF SOL-GEL INTEGRATED TIN OXIDE THIN FILMS

Abstract Sol–gel pure and Pd-doped SnO2 thin films on integrated substrates were developed to detect NO2 and CO. PdO particles are included in the film matrix and their presence enhances the CO gas sensitivity and response time. Measurements of electrical conductivity showed that these films have a response variation higher than 1000% for NO2 at temperatures in the range of about 200°C with a detection limit near 0.1 ppm, good for air quality monitoring. The response to CO is below 100% of the electrical variation but also in this case the sensor can detect very low concentration level.

Solvothermal, Chloroalkoxide-based Synthesis of Monoclinic WO3 Quantum Dots and Gas-Sensing Enhancement by Surface Oxygen Vacancies

We report for the first time the synthesis of monoclinic WO3 quantum dots. A solvothermal processing at 250 °C in oleic acid of W chloroalkoxide solutions was employed. It was shown that the bulk monoclinic crystallographic phase is the stable one even for the nanosized regime (mean size 4 nm). The nanocrystals were characterized by X-ray diffraction, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis, Fourier transform infrared and Raman spectroscopy. It was concluded that they were constituted by a core of monoclinic WO3, surface covered by unstable W(V) species, slowly oxidized upon standing in room conditions. The WO3 nanocrystals could be easily processed to prepare gas-sensing devices, without any phase transition up to at least 500 °C. The devices displayed remarkable response to both oxidizing (nitrogen dioxide) and reducing (ethanol) gases in concentrations ranging from 1 to 5 ppm and from 100 to 500 ppm, at low operating temperatures of 100 and 200 °C, respectively. The analysis of the electrical data showed that the nanocrystals were characterized by reduced surfaces, which enhanced both nitrogen dioxide adsorption and oxygen ionosorption, the latter resulting in enhanced ethanol decomposition kinetics.

Innovative aspects in thin film technologies for nanostructured materials in gas sensor devices

Abstract This work reports on the use of two innovative techniques in the field of gas sensors for preparing nano-structured materials: sol–gel and supersonic cluster beam deposition. By means of sol–gel, nano-structured In 2 O 3 thin films have been prepared and deposited under different deposition parameters on silicon wafer. In this way the results have shown a good compatibility of the method with silicon technology, then potentially suitable to be used in the fabrication of integrated devices. The second technique has been applied for the preparation of nano-structured TiO 2 thin films showing its capability to be used in the fabrication of gas sensor devices, mainly when a good control of the grain dimension is required.

In2O3 Thin Films Obtained Through a Chemical Complexation Based Sol-Gel Process and Their Application as Gas Sensor Devices

A new approach is presented for preparing In2O3 thin films starting from inorganic precursors, based on a very simple but effective modification of the usual precipitation-peptization process. Indium nitrate was dissolved in methanol and In3+ ions were chelated with acetylacetone before adding concentrated base to the resulting solution. Such a route allowed obtaining long-term stable sols, from which films could be deposited on glass substrates by spin-coating. Furthermore, the films exhibited a very good adhesion and uniformity, without any need for adding additives to the solution. The modification of the In precursor was confirmed by thermal analysis, while XRD studies revealed that the films prepared with the modified route result in smaller In2O3 grains compared to the traditional precipitation-peptization process. Optical reflectance measurements on the films further highlighted the difference between the two processes. Gas-sensing tests carried out on the films deposited onto alumina substrates in the temperature range between 100 and 400°C showed that faster responses are obtained at temperatures higher than 250°C. The response value (R/R0, where R is the electrical resistance of the sensor in the test gas and R0 that in dry air) to 100 ppb ozone is remarkably high: it is equal to 1500 for In2O3 with a response time of about 1 minute. The recovery time is about 10 minutes.

Recognition of olive oils by means of an integrated sol–gel SnO2 Electronic Nose

In this paper, we report a specific application of a semiconductor thin films based sensor array for the discrimination of different olive oils. The active elements of the sensor array were undoped and Ni, Pd, Pt, Os-doped SnO thin films prepared by 2 sol–gel technique. Olive oil samples were randomly selected. They included commercial olive oils of different qualities (extravirgin, virgin and husk oils) and non-commercial local olive oils (deriving from different zone of the Salento region in Apulia) with different degrees of acidity. The sensor array coupled with a principal component analysis allows discrimination between different olive oils. In particular good separation between commercial and local products was obtained. 2002 Elsevier Science B.V. All rights reserved.

Capping ligand effects on the amorphous-to-crystalline transition of CdSe nanoparticles.

Amorphous CdSe nanoparticles were prepared by a base-catalyzed room-temperature reaction between cadmium nitrate and selenourea, with dodecanethiol as a capping ligand. The nanoparticle size could be controlled from 1.9 to 3.6 nm by increasing the water concentration in the reaction. When the nanoparticles were heated in a pyridine suspension, excitonic peaks appeared in the initially featureless optical absorption spectra. By changing the suspension solvent and the capping ligand and its concentration, it was shown that the dynamic surface exchange between the ligand and pyridine controls the crystallization process. This phenomenon was interpreted as a surface rigidity effect imposed by the ligand, whose importance was separately evidenced on the dried nanoparticles by the evolution of X-ray diffraction patterns and Raman spectra. In particular, both techniques showed that a threshold temperature is needed before crystallization occurs, and such a threshold was related to ligand desorption. The surface effect was directly visualized by high-resolution transmission electron microscopy observations of the amorphous particles, where crystallization under the electron beam was observed to start by the formation of a crystalline nucleus in the nanoparticle interior and then to extend to the whole structure.

Surface modification of metal oxide nanocrystals for improved supercapacitors

TiO2 (anatase) nanocrystals were prepared and their surface was modified by sol–gel deposition of vanadium oxide species. The resulting surface-modified TiO2 combines the good properties of both materials and new, synergistic properties arise, resulting in an increased electrical conductivity, voltage window, specific capacitance, and cycling stability.