Abstract
Currently, gas detection using the mass spectrometer is a common method because of its high resolution and good sensitivity. However, due to the bulky equipment, high cost and complicate procedure, it is difficult to use as a breath sensing tool for on-site rapid screening or real-time diagnostics. In this study, we simulated, fabricated, and characterized a disposable lab-in-a-tube gas sensors based on single TiO2 nanowire and single Ag nanowire fabricated and integrated on a flexible plastic substrate, which can detect 2-propyl-1-pentanol (2-pp, one of lung cancer biomarkers), humidity, flow rate, and temperature conditions in flowing gas. In design of the lab-in-a-tube gas sensor, we optimized the tube size of 1.91 cm in diameter and 30 cm in length by CFD ACE+ simulation package. The simulation results showed that the variation of flow boundary layer thickness in sensor array region (25~30 cm from the tube entrance) is within 3 % at constant flow-rate. And when further simulated with dynamic boundary conditions from human expiratory flow curve, the variation of flow boundary layer thickness decreases from 79.56 % (1~5 cm from the tube entrance) to 2.88 % (26~30 cm from the tube entrance) in comparsion between 5 cm-long and 30 cm-long tubes. In our designs, we investigated the gas sensing characteristics of the developed nanowire sensors. First, the sensitivity of the Ag nanowire-based flow sensors is 3.29 mV/(L/s) (1~8 L/s, 25 °C, 20 % RH). The response time is 560 ms, and the recovery time is 1.12 seconds. The S/N ratio is 28.63 dB. The flow sensors, which have the improvements in resolution, sensitivity up to 93.02 % and 37.66 %, respectively. Second, the sensitivity of our Ag nanowire-based temperature sensor is 0.26 Ω/°C in the range of 25 ~ 85 °C (20 % RH). The response time is 1.79 seconds, and the recovery time is 1.64 seconds. The S/N ratio is 42.92 dB. Third, the humidity sensing with single TiO2 nanowire showed the maximum sensitivity of 41.98% in the RH range of 20~87 % (25 °C). The response time is 2.14 seconds, and the recovery time is 14.86 seconds. The S/N ratio is 28.63 dB. Fourth, the response and recovery time of our single TiO2 nanowire sensor for 2-propyl-1-pentanol (2-pp, one of lung cancer biomarkers) sensing are 3.64 and 8.79 seconds at the concentration of 100 ppb at the flow rate of 1 ~ 8 L/s (25 °C, 20 % RH). The S/N ratio is 15.85 dB. Moreover, the maximun selectivity of the biosensor is 6.38. Meanwhile, our design decrease the sensing varation and mositure interference of the lung cancer biomarker by 43.28 % and 78.77 %, respectively. In summary, we simulated, fabricated, and characterized a novel exhaled breath sensing tube with on-tube nanowire sensor array, which showed high performance in a tube. In future, our developed exhaled breath sensing tube can be combined with embedded systems, which has the potential for use in several disease screening from exhaled breath, lung function diagnostics, and enviorment safety.