Electron microscopy (EM) entered a new era with the emergence of direct electron detectors and new nanocrystal electron diffraction methods. However, sample preparation techniques have not progressed and still suffer from extensive blotting steps leading to a massive loss of sample. Here, we present a simple but versatile method for the almost lossless sample conditioning and preparation of nanoliter volumes of biological samples for EM, keeping the sample under close to physiological condition. A microcapillary is used to aspirate 3-5 nL of sample. The microcapillary tip is immersed into a reservoir of negative stain or trehalose, where the sample becomes conditioned by diffusive exchange of salt and heavy metal ions or sugar molecules, respectively, before it is deposited as a small spot onto an EM grid. We demonstrate the use of the method to prepare protein particles for imaging by transmission EM and nanocrystals for analysis by electron diffraction. Furthermore, the minute sample volume required for this method enables alternative strategies for biological experiments, such as the analysis of the content of a single cell by visual proteomics, fully exploiting the single molecule detection limit of EM.

High-resolution structural information is essential to understand protein function. Protein-structure determination needs a considerable amount of protein, which can be challenging to produce, often involving harsh and lengthy procedures. In contrast, the several thousand to a few million protein particles required for structure determination by cryogenic electron microscopy (cryo-EM) can be provided by miniaturized systems. Here, we present a microfluidic method for the rapid isolation of a target protein and its direct preparation for cryo-EM. Less than 1 μL of cell lysate is required as starting material to solve the atomic structure of the untagged, endogenous human 20S proteasome. Our work paves the way for high-throughput structure determination of proteins from minimal amounts of cell lysate and opens more opportunities for the isolation of sensitive, endogenous protein complexes.

With recent FDA approval of two recombinant adeno‐associated virus (rAAV)‐based gene therapies, these vectors have proven that they are suitable to address monogenic diseases. However, rAAVs are relatively new modalities, and their production and therapy costs significantly exceed those of conventional biologics. Thus, significant efforts are made to improve the processes, methods, and techniques used in manufacturing and quality control (QC). Here, we evaluate transmission electron microscopy (TEM), analytical ultracentrifugation (AUC), and two modes of capillary electrophoresis (CE) for their ability to analyze the DNA encapsidated by rAAVs. While TEM and AUC are well‐established methods for rAAV, capillary gel electrophoresis (CGE) has been just recently proposed for viral genome sizing. The data presented reflect that samples are very complex, with various DNA species incorporated in the virus, including small fragments as well as DNA that is larger than the targeted transgene. CGE provides a good insight in the filling of rAAVs, but the workflow is tedious and the method is not applicable for the determination of DNA titer, since a procedure for the absolute quantification (e.g., calibration) is not yet established. For estimating the genome titer, we propose a simplified capillary zone electrophoresis approach with minimal sample preparation and short separation times (<5 min/run). Our data show the benefits of using the four techniques combined, since each of them alone is prone to delivering ambiguous results. For this reason, a clear view of the rAAV interior can only be provided by using several analytical methods simultaneously.

This review compares and discusses conventional versus miniaturized specimen preparation methods for transmission electron microscopy (TEM). The progress brought by direct electron detector cameras, software developments and automation have transformed transmission cryo-electron microscopy (cryo-EM) and made it an invaluable high-resolution structural analysis tool. In contrast, EM specimen preparation has seen very little progress in the last decades and is now one of the main bottlenecks in cryo-EM. Here, we discuss the challenges faced by specimen preparation for single particle EM, highlight current developments, and show the opportunities resulting from the advanced miniaturized and microfluidic sample grid preparation methods described, such as visual proteomics and time-resolved cryo-EM studies.

Due to recent technological progress, cryo-electron microscopy (cryo-EM) is rapidly becoming a standard method for the structural analysis of protein complexes to atomic resolution. However, protein isolation techniques and sample preparation methods for EM remain a bottleneck. A relatively small number (100,000 to a few million) of individual protein particles need to be imaged for the high-resolution analysis of proteins by the single particle EM approach, making miniaturized sample handling techniques and microfluidic principles feasible.A miniaturized, paper-blotting-free EM grid preparation method for sample pre-conditioning, EM grid priming and post processing that only consumes nanoliter-volumes of sample is presented. The method uses a dispensing system with sub-nanoliter precision to control liquid uptake and EM grid priming, a platform to control the grid temperature thereby determining the relative humidity above the EM grid, and a pick-and-plunge-mechanism for sample vitrification. For cryo-EM, an EM grid is placed on the temperature-controlled stage and the sample is aspirated into a capillary. The capillary tip is positioned in proximity to the grid surface, the grid is loaded with the sample and excess is re-aspirated into the microcapillary. Subsequently, the sample film is stabilized and slightly thinned by controlled water evaporation regulated by the offset of the platform temperature relative to the dew-point. At a given point the pick-and-plunge mechanism is triggered, rapidly transferring the primed EM grid into liquid ethane for sample vitrification. Alternatively, sample-conditioning methods are available to prepare nanoliter-sized sample volumes for negative stain (NS) EM.The methodologies greatly reduce sample consumption and avoid approaches potentially harmful to proteins, such as the filter paper blotting used in conventional methods. Furthermore, the minuscule amount of sample required allows novel experimental strategies, such as fast sample conditioning, combination with single-cell lysis for "visual proteomics," or "lossless" total sample preparation for quantitative analysis of complex samples.