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Lanthanide-doped upconversion materials, capable of converting low-density (< 1000 W cmÀ2) near-infrared (NIR) excitation to ultraviolet (UV) and visible emissions, have generated a large amount of interests in the areas of information technology, biotechnology, energy, and photonics.[1] Significantly, recent developments in the synthetic and multicolor tuning methods have allowed easy access to upconversion nanoparticles with well-defined phase and size, core–shell structure, optical emission, and surface properties.[2–5] The technological advances provide promising applications in sensitive biodetection and advanced bioimaging without many of the constraints associated with conventional optical biolabels.[6] Despite the attractions, further progress in using upconversion processes has been largely hindered because upconversion nanoparticles are typically sensitized by Yb3+ ions that only respond to …