Expanding Chemical Space in the Synthesis of Gold Bipyramids

Gold bipyramids (AuBPs), despite having superior properties compared to their spectroscopically similar counterparts, gold nanorods, have found comparatively limited applications. This discrepancy is primarily due to the lack of protocols to tailor their dimensions. Typically, concentration of Au seeds is virtually the sole factor that determines aspect ratio and thus, optical properties of AuBPs. As a result, varying the volumes of AuBPs while incurring minimal changes to their optical spectra remains a synthetically non-trivial task. Here, we expand the chemical space in the seeded growth of AuBPs, by exploiting the interplay between bromide, silver ions, and seed concentration for tuning the final dimensions and optical properties of AuBPs. Specifically, we achieved a 6-fold change in volumes of AuBPs while maintaining the fixed plasmon band position. Further overgrowth of as-prepared bipyramids broadens the realizable dimensions without compromising quality and initial morphology. Overall, our results expand the chemical toolbox in the wet-chemistry synthesis of anisotropic gold nanoparticles, which is relevant for health, colorimetric sensors, and energy applications.