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 Nd 3+ 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 NH 4 F. The as-formed UCCKs composed of an inner layer (NaYF 4 :Yb,Er,Ca), an intermediate layer (NaYF 4 :Yb,Ca), and an outer layer (NaNdF 4 :Yb,Ca). The outer shell, enriched with Nd 3+ sensitizer, augmented the near-infrared (NIR) photon absorption, whereas the intermediate shell, enriched with Yb 3+ , acted as a bridge for energy transfer from Nd 3+ to Er 3+ emitter in the inner core alongside with precluding any deleterious energy back-transfer from Er 3+ or quenching effect from Nd 3+ . 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 (NaYF 4 :Yb,Er,Ca) and hexagonal UCNP core-shell (NaYF 4 :Yb,Er,Ca@NaYF 4 :Yb,Ca), respectively. Intriguingly, the UCL intensity increased significantly with increasing the content of Nd 3+ 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. Graphical abstract We developed novel multidimensional multilayered sand-clock-like upconversion nanostructures composed of a spherical inner core (NaYF 4 :Yb,Er,Ca), hexagonal intermediate shell (NaYF 4 :Yb,Ca) and two up-down outer shell (NaNdF 4 :Yb,Ca) with controllable Nd 3+ as an efficient and safe probe for bioimaging applications without any quenching effect.