Smart design of exquisite multidimensional multilayered sand-clock-like upconversion nanostructures with ultrabright luminescence as efficient luminescence probes for bioimaging application

A facile scalable approach is presented for the rational design of multidimensional, multilayered sand-clock-like UCNPs (denoted as UCCKs) bounded with high index facets, with a tunable Nd³⁺ content, and without a template or multiple complicated reaction steps. This was achieved using the seed-mediated growth and subsequent longitudinal direction epitaxial growth with the assistance of oleic acid and NH₄F. The as-formed UCCKs composed of an inner layer (NaYF₄:Yb,Er,Ca), an intermediate layer (NaYF₄:Yb,Ca), and an outer layer (NaNdF₄:Yb,Ca). The outer shell, enriched with Nd³⁺ sensitizer, augmented the near-infrared (NIR) photon absorption, whereas the intermediate shell, enriched with Yb³⁺, acted as a bridge for energy transfer from Nd³⁺ to Er³⁺ emitter in the inner core alongside with precluding any deleterious energy back-transfer from Er³⁺ or quenching effect from Nd³⁺. These unique structural and compositional properties of UCCKs endowed the UCL intensity of UCCKs by 22 and 10 times higher than that of hexagonal UCNP core (NaYF₄:Yb,Er,Ca) and hexagonal UCNP core-shell (NaYF₄:Yb,Er,Ca@NaYF₄:Yb,Ca), respectively. Intriguingly, the UCL intensity increased significantly with increasing the content of Nd³⁺ in the outer shell. The silica-coated UCCKs were used as excellent long-term luminescence probes for the in vitro bioimaging without any noteworthy cytotoxicity. The presented approach may pave the road for controlling the synthesis of multidimensional UCCKs for various applications.