Oxygen vacancy concentration in metal oxides is crucial to gas sensing performance. This study introduces a novel strategy of oxygen vacancies modulation on CoFe2O4, by two-step controllable incorporation of TiO2@MXene and carbon. In the first step, MXene simultaneously acts as an incorporation agent and a titanium source to grow TiO2 and regulate the oxygen vacancy concentration in CoFe2O4. The fabricated n-n heterojunctions of TiO2/CoFe2O4 further induced a gradient distribution of oxygen vacancies through energy band bending. And a second step, combining glucose as a carbon source, further increases the oxygen vacancy concentration in CoFe2O4 by chemical reduction during a hydrothermal process, leading to the formation of a CoFe2O4-TiO2@MXene-C nanostructured composite. Through controlling the glucose content, the ratio of Co3+ to Co2+ within the composite can be sequentially adjusted, which allows for the further regulation of oxygen vacancy concentration on the composite surface. The effectiveness of this two-step incorporation is demonstrated through enhanced acetone sensing performance, providing valuable insights into the fabrication of high-performance metal oxide-based gas sensors via controlled oxygen vacancy modulation.

For the practical diagnosis of inflammatory respiratory diseases, achieving sensitive and rapid NO sensing at the parts per billion level, all at room temperature, is of great significance. Herein, we developed a chemiresistor gas sensor with a sheet-on-sheet structure composed of an amorphous Cu-hemin MOF with reduced graphene oxide (rGO) nanosheets. The SEM images show that the Cu-hemin MOF/rGO composite exhibits a two-dimensional sheet-like structure. Due to its nanosized architecture, the Cu-hemin MOF exhibits a significant number of active sites for efficient NO detection. The Cu-hemin MOF/rGO composite material exhibited excellent NO sensing performance, including high sensitivity (Ra/Rg = 1.06, 50 ppb), reliable repeatability, high selectivity, and fast response/recovery (43 s/367 s, 10 ppm). The mechanism study revealed that the formation of the MOF altered the hemin dimer’s structure, resulting in the release of additional Fe(III)–N4 active sites and improved sensitivity. Moreover, the incorporation of rGO significantly boosted the conductivity of Cu-hemin MOFs. Using this two-dimensional sheet-like material, a mask-type sensor was also prepared and verified to be effective as a flexible and wearable sensing device for parts per billion level exhaled NO detection.