Single Particle Cathodoluminescence Spectroscopy with Sub-20 nm, Electron-Stable Phosphors

Lanthanide-doped nanophosphors have emerged as promising optical labels for high-resolution, "multicolor"electron microscopy. Here, we develop a library of 11 unique lanthanide-doped nanophosphors with average edge lengths of 15.2 ± 2.0 nm (N = 4284). These nanophosphors consist of an electron-stable BaYF5 host lattice doped at 25% atomic concentration with the lanthanides Pr<sup>3+</sup>, Nd<sup>3+</sup>, Sm<sup>3+</sup>, Eu<sup>3+</sup>, Tb<sup>3+</sup>, Dy<sup>3+</sup>, Ce<sup>3+</sup>, Ho<sup>3+</sup>, Er<sup>3+</sup>, Tm<sup>3+</sup>, and Yb<sup>3+</sup>. Under ~100 pA/nm<sup>2</sup> beam beam current in a transmission electron microscope, each nanophosphor species exhibits strong cathodoluminescence spectra with sharp characteristic emission lines for each lanthanide. The bright emission and stability of these nanoparticles enable not only ensemble, but also single-particle cathodoluminescence spectroscopy, which we demonstrate with BaYF<sub>5</sub>:Ln<sup>3+</sup>, where Ln<sup>3+</sup> = Tb<sup>3+</sup>, Ho<sup>3+</sup>, Er<sup>3+</sup>, Sm<sup>3+</sup>, Eu<sup>3+</sup>, or Pr<sup>3+</sup>. Single-particle cathodoluminescence corresponds directly with HAADF intensity across nanoparticles, confirming high spatial localization of the measured cathodoluminescence signal of lanthanide-doped nanophosphors. Furthermore, our synthesis and characterization of sub-20 nm, electron-stable nanophosphors provides a robust material platform to achieve single-molecule labeled correlative cathodoluminescence electron microscopy, a critical foundation for high-resolution correlation of single molecules within the context of cellular ultrastructure.