Lanthanide-based upconverting nanoparticles exhibit significant promise for solar energy generation, biological imaging, and security technologies but have not seen widespread adoption due to the prohibitively low efficiencies of current materials. Weak transition dipole moments between 4f orbitals hinder both photon absorption and emission. Here, we introduce a novel way to increase the radiative transition rates in Yb,Er-based upconverting nanoparticles based on local symmetry distortion. Beginning from a host matrix of the well-studied hexagonal (β)-phase NaYF4, we incrementally remove Y3+ ions and cosubstitute for them a 1:1 mixture of Gd3+ and Lu3+. These two ions act to expand and contract the lattice, respectively, inducing local-level distortion while maintaining the average host structure. We synthesize a range of β-NaY0.8-2xGdxLuxF4:Yb0.18Er0.02 nanoparticles and experimentally confirm that particle size, phase, global structure, and Yb3+ and Er3+ concentrations remain constant as x is varied. Upconversion quantum yield is probed as the degree of cosubstitution is varied from x = 0 to x = 0.24. We achieve a maximum quantum yield value of 0.074% under 63 W/cm2 of excitation power density, representing a 1.6× enhancement over the unmodified particles and the highest measured value for near-infrared-to-visible upconversion in sub-25 nm unshelled nanoparticles. We also investigate upconversion emission at the single-particle level and report record improvements in emission intensity for sub-50 nm particles. Radiative rate enhancements are confirmed by measuring excited-state lifetimes. The approach described herein can be used in combination with more established methods of efficiency improvement, such as adding passivating shells or coupling to plasmonic nanoattenas, to further boost the upconversion quantum yield.

Electron microscopy has been instrumental in our understanding of complex biological systems. Although electron microscopy reveals cellular morphology with nanoscale resolution, it does not provide information on the location of different types of proteins. An electron-microscopy-based bioimaging technology capable of localizing individual proteins and resolving protein-protein interactions with respect to cellular ultrastructure would provide important insights into the molecular biology of a cell. Here, we synthesize small lanthanide-doped nanoparticles and measure the absolute photon emission rate of individual nanoparticles resulting from a given electron excitation flux (cathodoluminescence). Our results suggest that the optimization of nanoparticle composition, synthesis protocols and electron imaging conditions can lead to sub-20-nm nanolabels that would enable high signal-to-noise localization of individual biomolecules within a cellular context. In ensemble measurements, these labels exhibit narrow spectra of nine distinct colours, so the imaging of biomolecules in a multicolour electron microscopy modality may be possible.