Metal-Oxide Semiconductor Sensing for Volatile Compounds

A set of twelve metal-oxide semiconducting films has been selected for the purpose. The materials chosen are five solid solutions of SnO₂ and TiO₂ (ST20 650, ST25 650, ST25 + Au1%, ST30 650, ST50 650), a solid solution of WO₃ and SnO₂ (WS30), a solution of TiO₂, Ta₂O₅ and vanadium oxide (TiTaV) and, finally, a solution of SnO₂, TiO₂ and Nb₂TiO₇ (STN). For a better understanding, the metal oxide composition of sensors is reported in Table 1.
Generally, the presence of SnO₂ makes the active material sensitive to a wide range of gases, so the addition of other oxides is fundamental to refine the selectivity of the sensors. Functional materials were prepared by the sol-gel technique [17 ], then fired at the temperatures indicated at the end of their names and used to screen-print sensing layers onto miniaturized alumina substrates [18 ]. Afterwards, they were characterized with the X-ray diffraction technique (XRD), thermo-gravimetry/differential thermal analysis (TG/DTA) and scanning electron microscopy (SEM). Details about the synthesis of these materials and the deposition technique have been reported in previous works of our group [19 –22 ]. Sensors were positioned inside a sealed test chamber (Figure 1), and conductance measurements were performed with the so-called “flow-through technique” [23 ,24 ]. The flow-rate is measured in sccm (standard cubic centimeters per minute), and it affects the rate of the surface reactions between the gas and the surface of the sensitive material.
To identify the best detecting temperature for each type of sensing material, we tested the response at several working temperatures (300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650°C). Temperatures lower than 300°C are not taken into account, because, at low temperatures, the metal-oxide sensors used in this work do not show a stable response. The working temperatures are set by applying an external voltage V_h to the heating circuit of each sensor, whose resistance is indicated here with R_h. Therefore, it is possible to control and directly modify R_h, which determines the sensor's temperature. The temperature of the chamber (36–37°C) is directly influenced by the sensors working temperatures and remains almost constant. We performed this temperature analysis in dry conditions (synthetic dry air with 20% of O₂ and 80% of N₂) with the following target gases: C₆H₆ (2 ppm), CH₄ (50 ppm), NO (5 ppm), and we made some interfering tests also with H₂ (60 ppm) and humidity. CO₂ is not considered, because it is well known that it is hardly detected by chemoresistive sensors [19 ]. For methane T=300°C was not considered, because is too low for sensors to generate a response. Dry conditions were chosen to show the absolute response to the gases of interest; in fact, even if humidity is present in our intestine, generally diminishing the sensors response, it does not conspicuously change the response ratios between benzene and its interferers. Measurements in wet conditions are in progress and not presented here. The results are summarized in Figures 2, 3 and 4.
The concentrations chosen for interferers are based on the fact that we want to be able to detect benzene even in the unfortunate case that the gut is filled with fermentation products. Therefore, CH₄ and NO are tested with a greater concentration than that of benzene. On the other hand, when a gastrointestinal exam occurs, normally, the patient has to take a particular diet some days before the test, in order to reduce the amount of disturbing gaseous compounds inside the intestine. Then, some interference tests were made. We tested C₆H₆ + CH₄ at the best temperature for benzene, derived from the previous analysis.
After selecting the most sensitive materials for benzene, we tested them in dry conditions with C₆H₆ + H₂ (2, 60 ppm) C₆H₆ + H₂ + CH₄ (2, 60, 10 ppm) and C₆H₆ + H₂ + CH₄ (2, 60, 10 ppm) using humidity as an interferer (RH = 37%).
In the second part, the same analysis is performed with the VOC, 1-iodo-nonane (chemical formula: C₉H₁₉I [25 ]), with the apparatus shown in Figure 5. We obtained the responses as a function of the temperature shown in Figure 6.
Tests were done in wet conditions (with a constant relative humidity (RH) ∼18% inside the volume of the chamber) to reproduce the intestinal environment. A fixed fraction of the total flux came from the gas bubbler filled with distilled water, while the remaining fraction was composed of two lines, one of synthetic dry air and the other passing through a second gas bubbler in which there were some drops of 1-iodononane. After the stabilization of sensors in wet air, a drop of 1-iodo-nonane is put inside the gas bubbler after being weighed with a precision balance (accuracy of 10⁻⁵g). After the measurement, the concentration has been calculated, dividing the quantity of 1-iodo-nonane, just measured before, by the evaporation time, taking into account also the volume of 1-iodo-nonane in the chamber and the volume of the chamber itself. The characteristics of 1-iodo-nonane used for the tests are listed in Table 2.