Radiopaque fluorescence-transparent TaO_x decorated upconversion nanophosphors for in vivo CT/MR/UCL trimodal imaging

To address the intractable issues such as the low performance or biocompatibility frequently encountered in previous CT, magnetic resonance (MR) and fluorescence trimodal imaging nanoprobes, a nanocomposite has been constructed by decorating gadolinium ions doped upconversion nanoparticle (Gd-doped UCNP) with radiopaque but fluorescence-transparent tantalum oxide (TaO_x, x ≈ 1). The as-synthesized water-soluble nanoparticle showed a litchi-like shape with an average size of ∼30 nm and demonstrated extraordinarily high longitudinal and transverse relaxivity values (r₁ = 11.45 mM^-1s^-1 and r₂ = 147.3 mM^-1s^-1) compared with the reported Gd-doped UCNPs to date. Obvious CT contrast enhancement was obtained by the combined effect between the radiopaque TaO_x shell and the Gd-doped UCNP inner core. Strong upconversion luminescence (UCL) signal could unobstructedly penetrate out in virtue of high transparency of the TaO_x shell. No mutual interference among different modalities of the upconversion nanolitchi (UCNL) was found, which ensured that the individual merits of every imaging modality could be brought into full play, demonstrated by in vitro and in vivo imagings. Furthermore, UCNLs showed only a slight effect on macrophages and RBCs in vitro and tissue in vivo.