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The application of gas sensors in breath analysis is an important trend in the early diagnostics of different diseases including lung cancer, ulcers, and enteric infection. However, traditional methods of synthesis of metal oxide gas-sensing materials for semiconductor sensors based on wet sol-gel processes give relatively high sensitivity of the gas sensor to changing humidity. The sol-gel process leading to the formation of superficial hydroxyl groups on oxide particles is responsible for the strong response of the sensing material to this factor. In our work, we investigated the possibility to synthesize metal oxide materials with reduced sensitivity to water vapors. Dry synthesis of SnO2 nanoparticles was implemented in gas phase by spark discharge, enabling the reduction of the hydroxyl concentration on the surface and allowing the production of tin dioxide powder with specific surface area of about 40 m2/g after annealing at 610 °C. The drop in sensor resistance does not exceed 20% when air humidity increases from 40 to 100%, whereas the response to 100 ppm of hydrogen is a factor of 8 with very short response time of about 1 s. The sensor response was tested in mixtures of air with hydrogen, which is the marker of enteric infections and the marker of early stage fire, and in a mixture of air with lactate (marker of stomach cancer) and ammonia gas (marker of Helicobacter pylori, responsible for stomach ulcers).

Investigation of gas-sensitivity of sensor structures to hydrogen in a wide range of temperature, concentration and humidity of gas medium a b s t r a c t Response to H 2 of seven sensor structures of SnO 2 with various catalytic additives, made by thick-film technology, was investigated in a dry gas medium at a concentration of 200 ppm H 2 in a temperature range of 100-600 • C. Concentration dependence of six sensors (pure SnO 2 and doped with five catalytic additives: 3% Pd, 3% La 2 O 3 , 1% Pt + 3% Pd, 1% Sb 2 O 5 + 3% La 2 O 3 and 3% Pd with a support layer from Al 2 O 3 ) was investigated in a range of 1-19,700 ppm H 2 of different relative humidity (RH) in a range of 0-100% RH at two operating temperatures. The resistance and response of the sensors weakly depended on the humidity of the gas medium at RH ≥ 0 or 10%. Arithmetic mean values of response and resistance in the range of 0 (10)-100% RH were approximated by linear functions of log(S ma − 1) = B + n log C and log R ma = A + k log C, respectively.Based on the comparative analysis of experimental data (response to H 2 , reproducibility of indications, influence of humidity on the resistance and response, response threshold and a resistance drift in pure air), the best for H 2 detection were the two sensor structures of pure SnO 2 and SnO 2 + 3% La 2 O 3 , working at temperatures 500 • C and 450 • C, respectively. Formulas for resistance and response of these sensors were presented, which allowed estimation of H 2 concentration in air based on the measured parameters. Reproducibility of indications in the range of 1-19,700 ppm H 2 was within 4-10%. The response threshold was 0.08 ± 0.006 ppm H 2 in the mentioned range of humidity at the response equal to 1.1.The interpretation was given for the mechanism of interaction of H 2 with the catalytic surface of semiconductive sensors. The present sensors can be recommended as the primary transducer of H 2 concentration in air for the gas control devices.

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