Understanding of relationships between structure and function in biological specimen strongly depends on the imaging method. Achieving the proper balance between luminescence and high contrast to simultaneously study specific proteins but also to acquire ultrastructural information is one of the challenges of bioimaging.
Correlative light and electron microscopy (CLEM) is an imaging technique for optoelectronic investigation of structure-function relationships at the nanoscale, which has gained importance in recent years. However, the strong mismatch between optical and electron imaging resolutions is still a limiting factor for CLEM.
One technique that could be an attractive alternative is correlative cathodoluminescence electron microscopy (CCLEM). This bioimaging technique can be successfully used for studying the structure and composition of cells and tissues: with cathodoluminescence generated by an electron beam, it is possible to obtain luminescence data with a much higher spatial resolution than is attainable in a conventional optical microscope.
CCLEM allows to not only overcome a mismatch between optical and electron imaging resolutions, it also opens new possibilities for using luminescent labels. Commonly, organic fluorophores are used for labelling the biological specimen. However, with CCLEM technique inorganic materials, which emit photons upon electron beam exposure and are more stable, can be a good alternative.
CCLEM is currently studied and applied by a research group at EMPA in Switzerland. The group has already published two papers [1,2] on CCLEM imaging, which investigate the performance of various labels for this bioimging. Since organic proteins and dyes are rather unstable under electron irradiation and produce low CL emission, the authors of the papers investigate various exogenous luminescent labels and their feasibility for photoluminescence and cathodoluminescence bioimaging setting. The most recent paper  concluded that compared to organic fluorophores and semiconductor quantum dots, two types of RE3+ doped nanocrystals (Y2O3:Tb3+ and YVO4:Bi3+,Eu3+) showed the best results in terms of emission stability and brightness.
Figure 3. (a) Backscattering electron micrograph of YVO4:Bi3+,Eu3+ nanocrystal-containing human vascular endothelial cells and corresponding (b) CL map and (c) SE image acquired with 2 kV.
EMPA’s studies show that CCLEM can be a powerful tool for gaining insights into the interaction of engineered nanoparticles with cells, which could be potentially applied in pathophysiological research and molecular biology.
To learn more about the basics of cathodoluminescence imaging, make sure to join upcoming webinar "Introduction to cathodoluminescence imaging".
 Ultrabright and Stable Luminescent Labels for Correlative Cathodoluminescence Electron Microscopy Bioimaging Keevend, L. Puust, K. Kurvits, L. R. H. Gerken, F. H. L. Starsich, J.-H. Li, M. T. Matter, A. Spyrogianni, G. A. Sotiriou, M. Stiefel, I. K. Herrmann, Nano Lett. (2019)
 Tb3+-doped LaF3nanocrystals for correlative cathodoluminescence electron microscopy imaging with nanometric resolution in focus ion beam-sectioned biological samples Keevend, M. Stiefel, A. L. Neuer, M. T. Matter, A. Neels, S. Bertazzo, and I. K. Herrmann
Nanoscale 9, 4383-4387 (2017)