Junction-Amplified Porous SnO2-Co3O4 Nanospheres for ppb-Level Low-Temperature Acetone Detection and Wearable-Integrated Breath Monitoring

Porous oxide heterostructures are attractive for breath acetone sensing, yet low-temperature operation in humidity-rich environments is often constrained by insufficient signal gain and interfacial transport loss. Here, we report junction-rich porous SnO2-Co3O4 nanospheres synthesized via a glucose-templated route, in which the Sn:Co precursor ratio is systematically programmed to tune phase composition, heterointerface density, and the dominant carrier type across an n-to-p transition. The optimized sample (SnCo-3) exhibits the lowest optimal operating temperature and the highest acetone response, enabling ppb-level detection at 125 °C with a theoretical limit of detection of 43 ppb. Beyond an adsorption-only interpretation, we build a correlative mechanistic framework connecting the electronic structure and transport kinetics to the chemiresistive output. UV-vis spectroscopy, Mott-Schottky analysis, and valence-band XPS jointly constrain band positions and Fermi-level evolution, while electrochemical impedance spectroscopy reveals reduced interfacial transport loss for SnCo-3. Together with O 1s oxygen-chemistry analysis, these results indicate that the superior performance arises from an electronically well-coupled p-n junction network that enhances band-bending tunability while maintaining continuous charge-transport pathways. In this optimized regime, the junction barriers serve as modulation-sensitive transduction sites rather than transport-blocking interfaces, allowing surface redox events to be converted into amplified resistance modulation with reduced interfacial transport loss. The sensor further shows robust repeatability, long-term stability, and humidity-dependent response trends relevant to breath conditions, and SnCo-3 is integrated on a flexible platform for wearable-oriented evaluation. This work offers a mechanism-guided strategy to design low-temperature chemiresistive breath sensors by jointly optimizing junction gain and interfacial transport.