Exhaled breath analysis technology enables the preliminary diagnosis of certain diseases via detecting the concentration of volatile organic compounds (VOCs) in human breath. Some studies indicate that acetone can serve as a biomarker to assist in the diagnosis of diabetes. Developing acetone gas sensors with low detection limits and high selectivity is thus crucial for achieving early screening of diabetes. In recent years, spinel ferrites, as an important class of metal oxides, have attracted much attention in the gas sensing field due to their low cost and excellent stability. Among them, ZnFe₂O₄ can improve the number of active sites on the material surface via tailoring its microstructure, thereby exhibiting a high sensitivity to VOCs like acetone. However, single-component ZnFe₂O₄ gas sensors suffer from issues such as high operating temperatures and low gas-sensing selectivity in practical detection. The existing research shows that ion doping is a modification method that induces lattice distortion in metal oxides, thereby regulating surface defects on the material. The generation of surface defects can provide more active centers for gas-sensing reactions and promote the adsorption of oxygen molecules on the material surface, thus improving its gas-sensing properties. Furthermore, doping rare-earth elements into MOS materials can enhance their catalytic activity and increase electron migration rates, leading to an improved sensing performance. This study was to prepare a series of La³⁺-doped ZnFe₂O₄ composites via electrospinning. Their morphology, surface chemical composition, and elemental valence states were characterized, and the gas-sensing properties of the composites were analyzed.

0.808 g Fe(NO₃)₃·9H₂O (AR, Sinopharm Chemical Reagent Co., Ltd., China), 0.297 g Zn(NO₃)₂·6H₂O (AR, Sinopharm Chemical Reagent Co., Ltd., China), and varying mass ratios of La(NO₃)₃·nH₂O (AR, Sinopharm Chemical Reagent Co., Ltd., China) were accurately weighed. Each mixture was put into respective 10 mL beakers, add 5 mL of DMF (AR, Sinopharm Chemical Reagent Co., Ltd., China), and stir to dissolve. 0.85 g of PVP (AR, Shanghai Aladdin Biochemical Technology Co., Ltd., China) was added slowly to each solution and continue stirring for over 12 h until complete dissolution of PVP. After stirring no visible bubbles remained in the solution. The solutions were transferred into 10 mL syringes and mounted onto a horizontal electrospinning apparatus. The needle tip was set as an anode (11-13 kV) and the aluminum foil collector as a cathode, with an electrode distance of 15 cm. The ambient humidity was maintained at 30%-50% and the solution was fed at a rate of 3 μL/min. The electrospun fibers collected were calcined in a muffle furnace at 600 ℃ for 2 h (at a heating rate of 1 ℃/min). The phase composition was analyzed by X-ray diffraction (XRD, Bruker D8 Advance, 40 kV, 40 mA, Cu-K_α, 5(°)/min, 10°-80°). The microstructure of the samples was determined by scanning electron microscope (SEM) with an accelerating voltage of 15 kV. The elemental valence states of the materials were analyzed by a X-ray photoelectron spectroscopy (XPS, Al K_α, hν=1486 eV).

For the determination of the gas-sensitive properties of La doping ZnFe₂O₄ composite material, the sample was placed in a mortar, and 1-2 drops of turpentine penetration alcohol were added. The mixture was ground until a uniform paste was formed. The paste was then evenly applied onto the surface of an alumina ceramic tube using a small brush. Afterwards, a nickel-chromium heating wire was passed through the ceramic tube and welded to the base of the gas sensor. Finally, the gas sensor was aged at 200 ℃ for 12 h before conducting gas-sensing performance tests.

The XRD patterns show that the grain size is significantly reduced and the gas-sensing performance is improved, indicating that La³⁺ doping inhibits the crystallization process and hinders crystal growth. A reduction in grain size leads to an increase in the specific surface area of the sensor, providing more acetone active sites on the surface of the material. The SEM images show that the surface of La³⁺-doped ZnFe₂O₄ nanomaterials is relatively smooth and cross-linked to form a network structure. The EDS results indicate that the composite material is composed of elements La, Zn, Fe, and O. According to the XPS analysis, La³⁺doping causes a lattice distortion, resulting in an increase in adsorbed oxygen on the material surface. The more oxygen is adsorbed on the surface of the material, which in turn improves the gas-sensing performance. Based on the analysis of UV-visible absorption spectroscopy and the corresponding band gap, the sample LZF-8 has a narrower band gap, indicating that the energy required for electron transition is lower, which is beneficial to electron migration, thereby shortening the response/recovery time. The response value of sample LZF-8 to 0.1 g/L acetone at room temperature is 21.3, which is 11.5 times greater than that of pure ZnFe₂O₄. Meanwhile, the selectivity and sensitivity of the material to different gases at room temperature are investigated. The results show that the sample LZF-8 has good selectivity and detection limit for acetone, with the minimum detection limit of 0.00001 g/L, proving that the prepared sensor LZF-8 has a great application potential.

ZnFe₂O₄ composites doped with different proportions of La³⁺ were prepared via electrospinning. Based on the analysis of the XRD patterns, SEM images, and XPS spectra, La³⁺ was successfully doped into ZnFe₂O₄. The results of gas-sensing test showed that as 8% La³⁺ was doped, the response value (i.e., R_a/R_g = 21.3) to 0.1 g/L acetone at room temperature was approximately 11.5 times greater than that of pure ZnFe₂O₄ (R_a/R_g = 1.7), with the response/recovery time of 27.0 s/10.7 s, and the minimum detection limit of acetone concentration of 0.00001 g/L. This composite gas sensor could have a great potential for efficient detection of acetone gas under room temperature conditions.