Scheme 1.

A schematic illustration of functionalized upconversion nanoparticle (f-UCNP). A core@multi-shell UCNP is designed as the starting point for integrated multicolor UCL imaging and MRI. The inner-most core is NaYF₄:Er³⁺/Yb³⁺ (for NIR-to-Vis optical imaging); the first shell is NaYF₄:Tm³⁺/Yb³⁺ (for NIR-to-NIR optical imaging); the second shell is NaGdF₄ (for MRI). With the presence of designed active sites on the surface of UCNP, polymer coating (such as a PEG layer for stealth capability), radioisotopes (such as ⁶⁴Cu for highly sensitive PET imaging), (light cleavable) anticancer drugs/genes for cancer cell killing, single or combined targeting ligands (for multiple receptor targeting) can be conjugated to yield a multifunctional nanoplatform for molecular imaging and therapy applications.