Correlative Cathodoluminescence Electron Microscopy: Immunolabeling Using Rare-Earth Element Doped Nanoparticles

Cathodoluminescent and Characteristic X-Ray-Emissive Rare-Earth-Doped Core/Shell Protein Labels for Spectromicroscopic Analysis of Cell Surface Receptors

Understanding the localization and the interactions of biomolecules at the nanoscale and in the cellular context remains challenging. Electron microscopy (EM), unlike light-based microscopy, gives access to the cellular ultrastructure yet results in grey-scale images and averts unambiguous (co-)localization of biomolecules. Multimodal nanoparticle-based protein labels for correlative cathodoluminescence electron microscopy (CCLEM) and energy-dispersive X-ray spectromicroscopy (EDX-SM) are presented. The single-particle STEM-cathodoluminescence (CL) and characteristic X-ray emissivity of sub-20 nm lanthanide-doped nanoparticles are exploited as unique spectral fingerprints for precise label localization and identification. To maximize the nanoparticle brightness, lanthanides are incorporated in a low-phonon host lattice and separated from the environment using a passivating shell. The core/shell nanoparticles are then functionalized with either folic (terbium-doped) or caffeic acid (europium-doped). Their potential for (protein-)labeling is successfully demonstrated using HeLa cells expressing different surface receptors that bind to folic or caffeic acid, respectively. Both particle populations show single-particle CL emission along with a distinctive energy-dispersive X-ray signal, with the latter enabling color-based localization of receptors within swift imaging times well below 2 min perμ m $\umu\text{m}$ 2 while offering high resolution with a pixel size of 2.78 nm. Taken together, these results open a route to multi-color labeling based on electron spectromicroscopy.

In situ observation of thermal-driven structural transitions of a β-NaYF4 single nanoparticle aided with correlative cathodoluminescence electron microscopy

NaYF4 systems have been widely studied as up-conversion host matrices, and their phase transitions are flexible and worth investigating in great detail. Herein, the evolution of morphology and crystal structure of a Eu3+-doped β-NaYF4 single nanoparticle heated in an air atmosphere was investigated using in situ transmission electron microscopy (TEM). The annealing process revealed that the hexagonal β-NaYF4 phase undergoes sequential transformations into high-temperature cubic phases at both 350 °C and 500 °C. The emission characteristics of Eu3+ in the single nanoparticle after heating treatment were also analyzed using Correlative Cathodoluminescence Electron Microscopy (CCLEM). The results of CCLEM suggest a gradual decrease followed by a subsequent increase in structural symmetry. A comprehensive spectroscopic and structural analysis encapsulates the entire transformation process as NaYF4 → YOF → Y2O3. In situ energy dispersive spectroscopy analyses (EDS) support this reaction process. The aforementioned technique yields correlative lattice-resolved TEM images and nanoscale spectroscopic information, which can be employed to assess the structure-function relationships on the nanoscale.

One for All, All for One: A Close Look at In-Resin Fluorescence Protocols for CLEM

Sample preparation is the novel bottleneck for high throughput correlative light and electron microscopy (CLEM). Protocols suitable for both imaging methods must therefore balance the requirements of each technique. For fluorescence light microscopy, a structure of interest can be targeted using: 1) staining, which is often structure or tissue specific rather than protein specific, 2) dye-coupled proteins or antibodies, or 3) genetically encoded fluorescent proteins. Each of these three methods has its own advantages. For ultrastructural investigation by electron microscopy (EM) resin embedding remains a significant sample preparation approach, as it stabilizes the sample such that it withstands the vacuum conditions of the EM, and enables long-term storage. Traditionally, samples are treated with heavy metal salts prior to resin embedding, in order to increase imaging contrast for EM. This is particularly important for volume EM (vEM) techniques. Yet, commonly used contrasting agents (e.g., osmium tetroxide, uranyl acetate) tend to impair fluorescence. The discovery that fluorescence can be preserved in resin-embedded specimens after mild heavy metal staining was a game changer for CLEM. These so-called in-resin fluorescence protocols present a significant leap forward for CLEM approaches towards high precision localization of a fluorescent signal in (volume) EM data. Integrated microscopy approaches, combining LM and EM detection into a single instrument certainly require such an “all in one” sample preparation. Preserving, or adding, dedicated fluorescence prior to resin embedding requires a compromise, which often comes at the expense of EM imaging contrast and membrane visibility. Especially vEM can be strongly hampered by a lack of heavy metal contrasting. This review critically reflects upon the fundamental aspects of resin embedding with regard to 1) specimen fixation and the physics and chemistry underlying the preservation of protein structure with respect to fluorescence and antigenicity, 2) optimization of EM contrast for transmission or scanning EM, and 3) the choice of embedding resin. On this basis, various existing workflows employing in-resin fluorescence are described, highlighting their common features, discussing advantages and disadvantages of the respective approach, and finally concluding with promising future developments for in-resin CLEM.

Two d10 2D Cathode-Ray Scintillation Coordination Polymers with High Efficiency and High-Voltage Stability

Two examples of efficient cathode-ray scintillation coordination polymers with good stability at high voltage were prepared by conjugating luminescent groups with d¹⁰ metal ions. The synergistic effect of inorganic metal and organic ligand suppresses the self-quenching of the conjugated luminescent groups and enhances the scintillation performance. This work provides new ideas for the design of new field-emission displays and cathode-ray scintillation materials.

Liquid-phase scanning electron microscopy for single membrane protein imaging

Liquid-phase electron microscopy is highly desirable for observing biological samples in their native liquid state at high resolution. We developed liquid imaging approaches for biological cells using scanning electron microscopy. Novel approaches included scanning transmission electron imaging using a liquid-cell apparatus (LC-STEM), as well as correlative cathodoluminescence and electron microscopy (CCLEM) imaging. LC-STEM enabled imaging at a ∼2 nm resolution and excellent contrast for the precise recognition of localization, distribution, and configuration of individually labeled membrane proteins on the native cells in solution. CCLEM improved the resolution of fluorescent images down to 10 nm. Liquid SEM technologies will bring unique and wide applications to the study of the structure and function of cells and membrane proteins in their near-native states at the monomolecular level.

Electron-Beam Induced Luminescence and Bleaching in Polymer Resins and Embedded Biomaterial

Electron microscopy is crucial for imaging biological ultrastructure at nanometer resolution. However, electron irradiation also causes specimen damage, reflected in structural and chemical changes that can give rise to alternative signals. Here, luminescence induced by electron-beam irradiation is reported across a range of materials widely used in biological electron microscopy. Electron-induced luminescence is spectrally characterized in two epoxy (Epon, Durcupan) and one methacrylate resin (HM20) over a broad electron fluence range, from 10^-4 to 10³ mC cm^-2 , both with and without embedded biological samples. Electron-induced luminescence is pervasive in polymer resins, embedded biomaterial, and occurs even in fixed, whole cells in the absence of resin. Across media, similar patterns of intensity rise, spectral red-shifting, and bleaching upon increasing electron fluence are observed. Increased landing energies cause reduced scattering in the specimen shifting the luminescence profiles to higher fluences. Predictable and tunable electron-induced luminescence in natural and synthetic polymer media is advantageous for turning many polymers into luminescent nanostructures or to fluorescently visualize (micro)plastics. Furthermore, these findings provide perspective to direct electron-beam excitation approaches like cathodoluminescence that may be obscured by these nonspecific electron-induced signals.

Highly Enhanced OER Performance by Er-Doped Fe-MOF Nanoarray at Large Current Densities

Great expectations have been held for the electrochemical splitting of water for producing hydrogen as a significant carbon-neutral technology aimed at solving the global energy crisis and greenhouse gas issues. However, the oxygen evolution reaction (OER) process must be energetically catalyzed over a long period at high output, leading to challenges for efficient and stable processing of electrodes for practical purposes. Here, we first prepared Fe-MOF nanosheet arrays on nickel foam via rare-earth erbium doping (Er_0.4 Fe-MOF/NF) and applied them as OER electrocatalysts. The Er_0.4 Fe-MOF/NF exhibited wonderful OER performance and could yield a 100 mA cm⁻² current density at an overpotential of 248 mV with outstanding long-term electrochemical durability for at least 100 h. At large current densities of 500 and 1000 mA cm⁻², overpotentials of only 297 mV and 326 mV were achieved, respectively, revealing its potential in industrial applications. The enhancement was attributed to the synergistic effects of the Fe and Er sites, with Er playing a supporting role in the engineering of the electronic states of the Fe sites to endow them with enhanced OER activity. Such a strategy of engineering the OER activity of Fe-MOF via rare-earth ion doping paves a new avenue to design other MOF catalysts for industrial OER applications.