Morphology control of Au@Ag nanocuboids

<sec>Au@Ag core-shell nanoparticles have emerged as a promising platform for photonic applications due to their synergistic integration of gold’s biocompatibility and silver’s exceptional plasmonic properties. And nanoparticles with sharp corners exhibit electron accumulation at the tips under electromagnetic fields, generating enhanced localized electric fields. This phenomenon facilitates their applications in fields such as surface-enhanced Raman spectroscopy and strong coupling interactions. So, when Au@Ag core-shell nanoparticles possess sharp corners, they will exhibit excellent performance in trace molecule detection, biosensing, and catalytic applications. By using gold nanorod (AuNR) seeds with different dimensions and adjusting the volume of silver precursors, the seed-mediated synthesis of Au@Ag nanocuboids with adjustable morphology, size and surface plasmon resonance is systematically investigated in this work. Key synthesis parameters, including AuNR diameters, aspect ratios, and AgNO<sub>3</sub> volumes, are modulated to realize the morphological, size and optical control. In experiments of adjusting the size of AuNR seeds for synthesizing Au@Ag nanocuboids, as the diameter of AuNR decreases from (136.5 ± 5) nm to (11.2 ± 2) nm and its aspect ratio increases from 1.39 to 8.20, the aspect ratio of Au@Ag nanocuboids increases from 1.18 to 2.69. Notably, when the diameter of AuNR is below 100 nm, the sharpness of the corners of Au@Ag nanocuboids is progressively improved with the increase of diameter and decrease of aspect ratio of the AuNRs. However, when the AuNR diameter exceeds 100 nm, the corners of the synthesized Au@Ag nanocuboids exhibit truncation. Meanwhile, the extinction spectrum reveals that apart from the broadened and indistinct peaks caused by the size effect, Au@Ag nanocuboids can primarily excite the longitudinal plasmon resonance mode, transverse plasmon resonance mode, and octupolar plasmon resonance modes. Furthermore, the plasmon resonance peaks exhibit corresponding shifts in response to changes in the size and morphology of Au@Ag nanocuboids. Meanwhile, neither the characterization results of high-resolution transmission electron microscopy nor selected area electron diffraction shows {111} crystal planes, indicating that the Au@Ag nanocuboids with the sharpest corners are not truncated and exhibits an exceptional morphology. And the results from high-angle annular dark-field scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy reveal that the silver shell exhibits anisotropic growth features on the gold core, with its transverse thickness being significantly greater than the longitudinal thickness. Besides, Au@Ag nanocuboids’ dimensions are linearly regulated by the volume of AgNO<sub>3</sub> (100 mmol/L) from 5 μL to 30 μL, yielding tunable lengths ((110.3 ± 7.8) nm to (141.3 ± 5.5) nm), widths ((59.7 ± 2.1) nm to (103.7 ± 5.6) nm), aspect ratios (1.85 to 1.36) and corresponding plasmon resonance peaks as validated by SEM and extinction spectrum. The simulation results of their extinction spectra are in better agreement with the experimental measurements. For the nanocuboid with an aspect ratio of 1.45, as the sharpness of the top corners decreases (<i>r</i>/<i>L</i> = 0.2%–11.5%), the strength of the electric field at the corners shows a trend of first increasing and then decreasing, with the maximum electric field enhancement observed at <i>r</i>/<i>L</i> = 0.5%.</sec><sec>This work synthesizes Au@Ag nanocuboids with controllable sharpness of corners and dimension by adjusting the size and aspect ratio of AuNRs or changing the quantity of silver precursors. The method proposed in this study for synthesizing sharp-cornered Au@Ag nanocuboids provides possibilities for customized fabrication of Au@Ag nanocuboids, thereby expanding their application prospects in nanophotonics, catalysis, sensing, photothermal therapy and other fields.</sec>