Upconversion (UC) process in lanthanide-doped nanomaterials has attracted great research interest for its extensive biological applications in vitro and in vivo, benefiting from the high tissue penetration depth of near-infrared excitation light and low autofluorescence background. However, the 980 nm laser, typically used to trigger the Yb(3+)-sensitized UC process, is strongly absorbed by water in biological structures and could cause severe overheating effect. In this article, we report the extension of the UC excitation spectrum to shorter wavelengths, where water has lower absorption. This is realized by further introducing Nd(3+) as the sensitizer and by building a core/shell structure to ensure successive Nd(3+) → Yb(3+) → activator energy transfer. The efficacy of this Nd(3+)-sensitized UC process is demonstrated in in vivo imaging, and the results confirmed that the laser-induced local overheating effect is greatly minimized.

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