Lanthanide-doped upconversion nanoparticles (UCNPs) are promising single-molecule probes given their non-blinking, photobleaching-resistant luminescence on infrared excitation. However, the weak luminescence of sub-50 nm UCNPs limits their single-particle detection to above 10 kW cm⁻², which is impractical for live cell imaging. Here, we systematically characterize single-particle luminescence for UCNPs with various formulations over a 10⁶ variation in incident power, down to 8 W cm⁻². A core–shell–shell (CSS) structure (NaYF₄@NaYb_1−xF₄:Er_x@NaYF₄) is shown to be significantly brighter than the commonly used NaY_0.78F₄:Yb_0.2Er_0.02. At 8 W cm⁻², the 8% Er³⁺ CSS particles exhibit a 150-fold enhancement given their high sensitizer Yb³⁺ content and the presence of an inert shell to prevent energy migration to defects. Moreover, we reveal power-dependent luminescence enhancement from the inert shell, which explains the discrepancy in enhancement factors reported by ensemble and previous single-particle measurements. These brighter probes open the possibility of cellular and single-molecule tracking at low irradiance.

掺杂镧系元素的上转换纳米粒子（UCNP）是有前途的单分子探针，因为它们在红外激发下具有非闪烁，耐光漂白的发光特性。但是，亚50纳米UCNP的弱发光将其单粒子检测限制在10 kW cm ^-2以上，这对于活细胞成像是不切实际的。在这里，我们系统地描述了各种形式的UCNPs的单粒子发光，其入射功率低至8 W cm ^-2超过10 ⁶。核-壳-壳（CSS）结构（NaYF ₄ @NaYb _{1− x} F ₄：Er _x @NaYF ₄）显示比通常使用的NaY明亮得多。_0.78 F ₄：Yb _0.2 Er _0.02。在8 W cm ^-2处，8％的Er ³⁺ CSS颗粒由于其高敏化剂Yb ³⁺含量和惰性壳层的存在而表现出150倍的增强，以防止能量迁移至缺陷。此外，我们从惰性外壳中揭示了与功率有关的发光增强，这解释了整体和先前单颗粒测量报告的增强因子之间的差异。这些更亮的探针打开了在低辐照度下进行细胞和单分子跟踪的可能性。