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@NaYb1−xF4:Erx@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.
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S.C. acknowledges financial support from the Moore Foundation (no. 4309) and the Stanford Neurosciences Institute (no. 119600). TEM and SEM imaging were performed at the Stanford Microscopy Facility (National Institutes of Health grant SIG number 1S10RR02678001). EDX mapping and high-resolution TEM were performed at the TEM facility of Nanjing University of Posts and Telecommunications. The authors thank A. Brunger, E. Chan, B. Cohen, J. Collins, J. Dionne, D. Jin and X. Liu for helpful discussions.
The authors declare no competing interests.
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Liu, Q., Zhang, Y., Peng, C.S. et al. Single upconversion nanoparticle imaging at sub-10 W cm−2 irradiance. Nature Photon 12, 548–553 (2018). https://doi.org/10.1038/s41566-018-0217-1
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