**2. Materials and Methods**

#### *2.1. Preparation*

GO aqueous solution (0.5 mg/mL, Hangzhou Gaoxi Tech Co., Ltd., Hangzhou, China), WSe2 nanosheets dispersion (0.5 mg/mL, Nanjing MKNANO Tec Co., Ltd., Nanjing, China), and Pd nanoparticles (NPs, 99.9% metals basis, ≤1 μm, Shanghai Aladdin Biochemical Tech Co., Ltd., Shanghai, China) were prepared. Polyelectrolytes poly (diallyldimethylammonium chloride) (PDDA, 200,000–350,000, 20 wt. % aqueous solution, polycation), and poly (sodium-pstyrenesulfonate) (PSS, 70,000, polyanion) were purchased from Sigma-Aldrich. PDDA and PSS were diluted and dissolved in deionized (DI) water to 1 wt. % and

2 wt. %, respectively. Pd NPs were then dispersed in PSS solution with a concentration of 0.5 mg/mL. All reagents were directly used without further purification.

### *2.2. Sensor Fabrication*

Ethylene sensitive films were deposited on SiO2/Si substrates pre-patterned with Ti/Au (20 nm/50 nm) interdigital electrodes (IDEs) by the layer-by-layer self-assembling method. The cleaned substrate was vertically immersed in PDDA solution for 15 min and dried with nitrogen, followed by immersing in DI water for removal of excessive PDDA molecules. Therefore, positive charges were fixed on the surface of substrates. Since the GO, WSe2, and PSS-Pd solutions were negatively charged, the positive charged substrate was then vertically immersed in negatively charged solution for 15 min for self-assembling of nanostructures on IDEs due to the electrostatic adsorption effects. Finally, the sensing films were annealed at 200 ◦C for 1 h to reduce GO.
