We report the fabrication and characterization of a surface plasmon resonance (SPR)-based fiber optic sensor that uses coatings of silver and aluminum (Al)-zinc oxide (ZnO) core–shell nanostructure (Al@ ZnO) for the detection of phenyl hydrazine (Ph-Hyd). To optimize the volume fraction (f) of Al in ZnO and the thickness of the core–shell nanostructure layer (d), the electric field intensity along the normal to the multilayer system is simulated using the two-dimensional multilayer matrix method. The Al@ ZnO core–shell nanostructure is prepared using the laser ablation technique. Various probes are fabricated with different values of f and an optimized thickness of core–shell nanostructure for the characterization of the Ph-Hyd sensor. The performance of the Ph-Hyd sensor is evaluated in terms of sensitivity. It is found that the Ag/Al@ ZnO nanostructure core–shell-coated SPR probe with f= 0.25 and d= 0.040 μm possesses the maximum sensitivity towards Ph-Hyd. These results are in agreement with the simulated ones obtained using electric field intensity. In addition, the performance of the proposed probe is compared with that of probes coated with (i) Al@ ZnO nanocomposite,(ii) Al nanoparticles and (iii) ZnO nanoparticles. It is found that the probe coated with an Al@ ZnO core–shell nanostructure shows the largest resonance wavelength shift. The detailed mechanism of the sensing (involving chemical reactions) is presented. The sensor also manifests optimum performance at pH 7.