Structural Control over Bimetallic Core–Shell\nNanorods for Surface-Enhanced Raman Spectroscopy

Abstract

Bimetallic nanorods\nare important colloidal nanoparticles for optical\napplications, sensing, and light-enhanced catalysis due to their versatile\nplasmonic properties. However, tuning the plasmonic resonances is\nchallenging as it requires a simultaneous control over the particle\nshape, shell thickness, and morphology. Here, we show that we have\nfull control over these parameters by performing metal overgrowth\non gold nanorods within a mesoporous silica shell, resulting in Au–Ag,\nAu–Pd, and Au–Pt core–shell nanorods with precisely\ntunable plasmonic properties. The metal shell thickness was regulated\nvia the precursor concentration and reaction time in the metal overgrowth.\nControl over the shell morphology was achieved via a thermal annealing,\nenabling a transition from rough nonepitaxial to smooth epitaxial\nPd shells while retaining the anisotropic rod shape. The core–shell\nsynthesis was successfully scaled up from micro- to milligrams, by\ncontrolling the kinetics of the metal overgrowth via the pH. By carefully\ntuning the structure, we optimized the plasmonic properties of the\nbimetallic core–shell nanorods for surface-enhanced Raman spectroscopy.\nThe Raman signal was the most strongly enhanced by the Au core–Ag\nshell nanorods, which we explain using finite-difference time-domain\ncalculations.