Fig. 1. Preparation process of Au@Pd NPs nanolayer film： （a） Au@Pd NPs coated with octadecylamine；（b） Hydrophobic Au@Pd NPs toluene solution was added to the tank； （c） Toluene-water interface formed nanoparticle monolayer；（d） Complete evaporation of toluene formed a large number of nanoparticle monolayers at the air-water interface；（e） Convergence of dispersed nanoparticle island membranes

Fig. 2. Structure of transmission optical fiber sensor with gold-palladium core-shell nanoparticles

Fig. 3. Absorption spectra of Au@Pd NPs solution and Au NPs solution

Fig. 4. Transmission electron microscopy （TEM） image of nanoparticles

Fig. 5. Au@Pd NPs diffraction pattern

Fig. 6. SEM images of Au@Pd NPs film

Fig. 7. Three cycle response curves of H₂ sensor through 4%， 2%， 1%， 0.5% and 0.1% respectively

Fig. 8. Periodic response curves of H₂ sensor to H₂ of 4%， 2%， 1%， 0.5% and 0.1%， respectively

Fig. 9. Response amplitude curve at different H₂ concentrations