Selective detection of sub-1-ppb level isoprene using Pd-coated In₂O₃ thin film integrated in portable gas chromatography

Monitoring breath isoprene concentration plays a role in non-invasive diagnosis of blood-cholesterol levels. In this study, we developed a highly sensitive and selective isoprene analyzer based on a Pd-coated In₂O₃ thin film integrated in a miniaturized gas chromatography column. To improve the sensing performance, the In₂O₃ thin films were fabricated with high uniformity and crystallinity using a dual-ion-beam sputtering system, and various metal catalysts (Au, Pt, and Pd) were loaded on the film surface. Among them, the Pd catalyst afforded the highest sensing reaction and the lowest detection limit (approximately 0.4 ppb), which is the best performance ever reported. It also significantly lowered the optimal operating temperature of the sensor from 432 °C to 196 °C. The excellent isoprene sensing performance of the Pd-coated In₂O₃ film can be attributed to the high density of oxygen vacancies, efficient reduction-reoxidation of PdO, and the Mars-van-Krevelen catalytic reaction. We further optimized the isoprene sensing performance of a Pd-coated In₂O₃ film by varying the Pd thickness and discovered that 1 nm of Pd deposition showed the optimal status of discontinuous islands for sensing. We expect that our device can be applied to a portable breath isoprene analyzer.