EM Grids: The Essential Tool for TEM Sample Support

In transmission electron microscopy (TEM), samples must be placed in a high-vacuum environment and withstand intense electron beam bombardment. Since most samples (e.g., biological macromolecules, nanomaterials, thin-film cross-sections) cannot stably exist in a vacuum chamber on their own, EM grids (Electron Microscopy Grids) serve as the critical support structure. They secure the sample, maintain structural stability, and ensure electron beam penetration for imaging. This article systematically introduces the common types, materials, structures, and typical applications of EM grids.

I. Basic Structure and Core Functions of EM Grids

EM grids are typically 3 mm diameter circular thin discs (compatible with standard TEM sample holders). Their main body is made of highly conductive and stable materials, with a grid-like structure (array of holes) that immobilizes the sample while exposing observation areas. Their core functions include:

• Physical Support:      Holding ultra-thin samples (typically <100 nm thick for electron beam      penetration);

• Structural Stability: Preventing sample deformation, drift, or detachment under      vacuum or electron beam exposure;

• Electron Transparency: Grid holes allow the electron beam to directly traverse the      sample, generating high-resolution images;

• Compatibility:      Adapting to various sample preparation techniques (e.g., negative      staining, cryo-EM, ultramicrotomy).

II. Classification by Material: Common Grid Materials

Grid materials must meet requirements such as high conductivity (to minimize charge buildup-induced image distortion), chemical stability (resistance to solutions/reagents during preparation), and low background interference (avoiding self-signals that affect observations). Common materials include:

1. Copper Grids

• Most Common Baseline Option: Copper offers excellent conductivity and low cost, making it      suitable for routine samples (e.g., negatively stained biological      specimens, metal nanoparticles).

• Features:      Easy to process and affordable, but prone to oxidation (requires      moisture-proof storage) and may interfere with magnetic samples.

2. Nickel Grids

• For Magnetic Samples: Nickel has higher magnetic permeability than copper, reducing      drift of strongly magnetic samples (e.g., ferromagnetic nanoparticles,      magnetic thin films) under the electron beam, thus improving image      stability.

• Note:      Slightly more expensive than copper grids; compatibility with non-magnetic      samples should be considered.

3. Gold Grids

• For High-Stability Needs: Gold is chemically inert, rarely reacting with samples,      making it ideal for sensitive specimens (e.g., biomolecular complexes,      catalysts where metal contamination is a concern).

• Features:      Good conductivity but higher cost; potential risk of mechanical damage to      ultra-thin samples due to hardness.

4. Platinum/Palladium Alloy Grids (Pt/Pd Grids)

• For Ultra-High Resolution: Platinum-palladium alloys (e.g., Pt₃₀Pd₇₀) offer a more stable lattice structure with low electron beam      scattering background, commonly used for atomic-resolution studies (e.g.,      single-atom catalysts, 2D materials).

• Features:      Expensive, primarily used in cutting-edge research.

5. Carbon-Coated or Pure Carbon Grids

• Functional Extensions: Some grids feature an additional ultra-thin carbon film (5–10      nm thick) deposited on the metal base to enhance sample support uniformity      (e.g., for cryo-EM virus particles) or reduce metal background      interference.

III. Classification by Structure: Grid Designs and Functional Extensions

The "grid structure" (pattern of holes on the 3 mm disc) directly impacts sample immobilization and observation field exposure. Based on hole size, density, and added features, common types include:

1. Standard Grids

• Typical Hole Size/Density: The most widely used general-purpose option includes 100-mesh (Mesh 100, ~100 grid cells per square inch, hole size      ~150 μm) and 200-mesh (Mesh 200, hole size ~75 μm).

• Applications:      Suitable for most negatively stained samples (e.g., proteins, nucleic      acids) or larger nanoparticles (>10 nm). Samples are typically applied      by dropping a solution onto the grid and drying naturally.

2. Lacey Carbon Grids (Microgrids)

• Key Feature: A      standard metal grid (e.g., copper) is coated with an ultra-thin, disordered carbon film (1–5 nm thick), which has randomly distributed micro-holes forming a      "lace-like" pattern.

• Advantages:      The carbon film’s flexibility helps secure fragile samples (e.g.,      biomolecules, liposomes), while the micro-holes immobilize samples via      capillary action when a solution is applied. Ideal for negative staining      (e.g., protein crystals, virus particles).

• Typical Uses:      Biological samples (e.g., bacteriophages, exosomes), nanoparticle      dispersions (e.g., carbon nanotubes).

3. Ultra-Thin Carbon Grids

• Upgraded Design:      Built upon microgrids with an even thinner carbon film (as low as 1 nm) or      a pure carbon grid (no metal base), providing more uniform support and      reduced metal background interference.

• Applications:      High-resolution analysis (e.g., single-particle cryo-EM reconstruction) or      electron-beam-sensitive soft materials (e.g., polymer films).

4. Holey Grids / Quantifoil Grids

• Specialized Hole Pattern: Features a regular      array of precisely sized holes      (e.g., circular or hexagonal, with diameters of 5–200 μm and open area      fractions up to 50%+).

• Core Advantage:      Exposes sample regions directly through holes, avoiding obstruction from      metal grid lines. Ideal for cryo-EM (e.g., frozen virus particles) and atomic-thickness 2D      materials (e.g., graphene, MoS₂).

• Typical Uses:      Cryo-EM 3D reconstruction (e.g., SARS-CoV-2 spike protein structures), 2D      materials (single layers).

IV. Specialized Functional Grids: Customized for Complex Samples

To address specific experimental needs, advanced grids have been developed:

1. Cryo-Protected Grids

• Design Improvement:      Pre-coated with a thin carbon film (2–5 nm) on holey grids (e.g.,      Quantifoil) to stabilize vitrified samples (e.g., biological cells,      viruses) by preserving their glassy water layer and preventing ice crystal      formation.

• Usage:      Requires cryo-preparation equipment (e.g., plunge freezers) and is      essential for cryo-EM.

2. Reinforced Support Grids

• Solution for Fragile Samples: For ultra-thin or brittle samples (e.g., single-layer 2D      materials), grids with additional carbon layers or denser mesh structures      (e.g., high-mesh microgrids) enhance stability and reduce drift.

3. Magnetic Shielding Grids

• Specialized Need:      For strongly magnetic samples (e.g., magnetic nanoparticle arrays), nickel      or special alloy grids are used with structural designs to minimize      magnetic field interference with the electron beam, ensuring imaging      clarity.

V. Grid Selection Guide: Matching Samples and Experimental Goals

Conclusion

Though small in size (just 3 mm in diameter), EM grids are the indispensable "bridge" in TEM analysis—connecting samples to the high-vacuum environment and enabling structural insights from the nanoscale to atomic level. As TEM technology advances (e.g., atomic-resolution cryo-EM, in-situ dynamic imaging), grid materials, structures, and functionalities continue to evolve, providing increasingly powerful tools for research in life sciences, materials science, and nanotechnology.