Highly sensitive and selective isoprene sensing performance of ZnO quantum dots for a breath analyzer

We report the quantum-size effect on the sensing properties of ZnO nanoparticles for the detection of isoprene. For this purpose, two types of ZnO particles with different sizes, including nanoparticles (NPs, ˜25 nm) and quantum dots (QDs, ˜5 nm), were prepared by a wet chemical method. The ZnO QDs exhibited excellent sensing performance to 1 ppm isoprene compared to NPs. The maximum sensing response and response time were ˜42 and 8 s at 350 °C for the QDs, while ˜5.6 and 40 s at 500 °C for the NPs, respectively. The higher sensing response, rapid response, and lower optimal working temperature of the ZnO QDs can be attributed to an increase in oxygen vacancies, band gap, and specific surface area owing to the small size effect. Both ZnO NPs and QDs showed the lower limit of 0.01 ppm for detecting isoprene. It is noted that the sensing properties of ZnO QDs to 1 ppm isoprene are superior to previously reported isoprene sensors that are based on semiconducting metal oxides. Furthermore, we demonstrated that the low concentration (0.4–6.2 ppm) of isoprene can be selectively detected within ˜83 s using a recently developed miniaturized gas chromatography integrated with the ZnO QD sensor.