High-purity gold nanobipyramids can be obtained by an electrolyte-assisted and functionalization-free separation route

Gold nanobipyramids (NBPs) have attracted intensive attentions because they exhibit more advantageous plasmonic properties than comparable gold nanorods. However, unlike that short gold nanorods can be synthesized routinely in a high yield (around 99% of the total particles), current syntheses of gold NBPs generally receive a low yield (around 30% of the total particles) and co-produce spherical impurities difficult to separate. Thus an effective purification route of gold NBPs is desirable for optimizing their performances. In this study, we demonstrated that the spherical nanoparticles with smaller inter-particle contact area can be separated from the NBPs that undergo gradual precipitation by electrolyte-induced electrostatic screening. During this procedure, no special surface-functionalization of the NBPs was needed. As a result of this simple separation strategy, NBPs at a level of purity to above 90% is achieved in a single purification round. In particularly, the precipitates of cetyltrimethylammonium bromide (CTAB)-capped NBPs can be easily converted into colloidal state due to the strong steric constraint of CTA+ bilayer, facilitating further investigations.