Optimized multi-shell engineering for enhanced luminescence in core@shell@shell upconversion nanorods for dual-channel imaging

In this study, core@shell (NaYF₄:Yb,Er@NaYbF₄) and core@shell@shell (NaYF₄:Yb,Er@NaYbF₄@NaYF₄:Yb,Tm) upconversion nanorods (UCNRs) were synthesised using a seeded growth strategy. The NaYF₄:Yb,Er core, with a diameter of 129 ± 4 nm, served as a template for the epitaxial growth of subsequent shells. Structural and imaging studies confirmed the precise shell formation, enhancing light-harvesting capabilities, particularly in the near-infrared range, thereby facilitating superior energy transfer between the layers. Power-dependent upconversion luminescence (UCL) studies demonstrated the presence of two-photon and three-photon upconversion processes, leading to emissions across the red, and green regions, ideal for bioimaging applications. Optimisation of the first-shell thickness (15 nm) significantly enhanced the red and green emissions by 1.3 and 1.1 times respectively, while the core@shell@shell design further boosted these emissions by 2.3 and 1.5 times respectively. These UCNRs were tested in vitro using WRL-68 cells, maintaining over 95 % cell viability up to a concentration of 100 µg mL⁻¹, demonstrating excellent biocompatibility. The internalisation of the UCNRs by WRL-68 cells was confirmed via fluorescence imaging, which captured strong red and green fluorescence signals, highlighting their potential as effective dual-channel bioimaging agents. These findings suggest that the optimised UCNRs are promising candidates not only for high efficiency bioimaging but also for applications in photocatalysis and photonic devices, owing to their enhanced UCL and stability.