Single upconversion nanoparticle imaging at sub-10 W cm(-2) irradiance

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 10(6) variation in incident power, down to 8 W cm(-2). A core-shell-shell (CSS) structure (NaYF4@NaYb1-xF4:Er-x@NaYF4) is shown to be significantly brighter than the commonly used NaY0.78F4:Yb0.2Er0.02. 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.