Spatially Tailored Energy Transfer Unlocks Bright Lanthanide Ratiometric Nanothermometers

ABSTRACT Lanthanide‐doped nanocrystals are ideally suited for luminescent nanothermometry, yet their performance is often limited by deleterious cross‐relaxation and back energy transfer among lanthanide ions, which diminish both emission efficiency and thermal sensitivity. Here, we design a core–shell‐shell‐shell nanostructure in which Nd 3+ and Ho 3+ activator shells are spatially separated by a 12 nm‐thick Yb 3+ sensitizer interlayer. This spatial isolation effectively blocks cross‐relaxation between activators and suppresses back energy transfer from activators to sensitizers while enabling efficient interfacial and long‐range energy transfer. As a result, intense dual NIR emissions at 750 and 804 nm are unlocked, yielding a remarkable upconversion quantum yield of 2.29% and an unprecedented brightness of 12311.12 M −1 cm −1 under 170 W cm −2 excitation. Moreover, the Nd 3+ ‐related 804 nm emission shows thermal enhancement governed by lattice phonons, whereas the Ho 3+ ‐based 750 nm emission undergoes thermal quenching dominated by surface defects, resulting in a maximum thermal sensitivity of 3.7%°C −1 at 38°C and remaining above 3.0%°C −1 across the physiological temperature range. The designed nanoprobes also demonstrate excellent stability against variations in irradiation time, pH, and concentration, enabling precise thermal imaging at the cellular level. This work provides a general strategy for constructing high‐performance ratiometric nanothermometers.