1：Experimental Design and Method Selection:

A one-pot hydrothermal method was used to synthesize WO3 nanoparticles with polyethyleneimine as a mediator to prevent agglomeration and enhance gas sensing properties. The method was chosen for its simplicity and efficiency in producing nanostructured materials with high surface area.

2：Sample Selection and Data Sources:

Chemical reagents including PEI, Na2WO4·2H2O, and HCl of analytical purity were used without further purification. Deionized water was employed throughout the experiment.

3：List of Experimental Equipment and Materials:

Equipment included a Teflon-lined stainless-steel autoclave, quartz tube furnace, agate mortar, ceramic tube with Au electrodes and Pt lead wires, Ni-Cr alloy coil, aging equipment, X-ray diffractometer (XRD, PANalytical X'Pert Pro), field-emission scanning electron microscope (FESEM, ZEISS Ultra Plus), transmission electron microscope (TEM, FEIG2-20), X-ray photoelectron spectroscope (XPS, ESCALAB 250 Xi), Fourier transform infrared spectroscope (FTIR, NICOLET 380), and gas-sensing measurement system (WS-30A, China). Materials included PEI, Na2WO4·2H2O, HCl, absolute ethanol, and various gases for testing.

4：Experimental Procedures and Operational Workflow:

WO3 nanoparticles were synthesized by dissolving PEI in deionized water, adding HCl and Na2WO4 solution, hydrothermal treatment at 160°C for 12 hours, centrifugation, washing, drying at 60°C, and annealing at 400°C for 4 hours. The sensor was prepared by coating WO3 paste on a ceramic tube, aging at 300°C for 24 hours. Gas sensing measurements were conducted at 40% relative humidity with NO2 concentrations from 50 ppb to 5 ppm at operating temperatures from 50°C to 175°C.

5：Data Analysis Methods:

Sensor response was defined as Rg/Ra for oxidizing gases. Response and recovery times were measured. Data were analyzed using power law relationships for sensor response vs. concentration, and statistical methods for repeatability and stability.