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 (NaYF 4 @NaYb 1−x F 4 :Er x @NaYF 4 ) is shown to be significantly brighter than the commonly used NaY 0.78 F 4 :Yb 0.2 Er 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.