Fabrication of Bimetallic Au-Pd-Au Nanobricks as an Archetype of Robust Nanoplasmonic Sensors

Conventional gas sensors work upon changes in mechanical or conductive properties of sensing materials during a chemical process, which may limit availabilities of size miniaturization and design simplification. However, fabrication of miniaturized sensors with superior sensitivities in real-time and label-free probing of chemical reactions or catalytic processes remains highly challenging, in particular with regard to integration of materials into a desired smaller volume without losing the recyclability of sensing properties. Here, we demonstrate a unique bimetallic nanostructure, the Au-Pd-Au core-shell-frame nanobrick, as a promising archetype for fabrication of miniaturized sensors at nanoscale. Upon analysis of the aqueous synthesis, both ex situ and in situ, the formation of Au frames is consistent with selective deposition and aggregation of NaBH₄-reduced Au nanoparticles at the corners and edges of cubic Pd shells, where the {100} surfaces, capped by iodide ions, are growth-limited. By virtue of the thin Pd shell (∼3.5 nm) sandwiched in-between the two Au layers of the core and the frame, the Au-Pd-Au nanobrick yields excellent optical sensitivity in hydrogen gas sensing, leading to a large 13 nm spectral shift of light scattering between Pd and PdH_x. The composite nanostructure with a size of ∼60 nm offers an archetype for miniaturized sensors possessing label-free, real-time, and high-resolution probing abilities and hence paves the way for fabrication of highly efficient nanosensors via sustainable methods.