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Photon upconversion generally results from a series of successive electronic transitions within complex energy levels of lanthanide ions that are embedded in the lattice of a crystalline solid. In conventional lanthanide-doped upconversion nanoparticles, the dopant ions homogeneously distributed in the host lattice are readily accessible to surface quenchers and lose their excitation energy, giving rise to weak and susceptible emissions. Therefore, present studies on upconversion are mainly focused on core–shell nanoparticles comprising spatially confined dopant ions. By doping upconverting lanthanide ions in the interior of a core–shell nanoparticle, the upconversion emission can be substantially enhanced, and the optical integrity of the nanoparticles can be largely preserved. Optically active shells are also frequently employed to impart multiple functionalities to upconversion nanoparticles. Intriguingly, the core …