TEM Tips for Imaging Uranyl Acetate鈥揘egative鈥揝tained Particles
(Liposomes, extracellular vesicles, viruses, protein鈥搇ipid assemblies, etc.)
1. Grid Preparation
- Use carbon鈥慶oated 200-mesh copper grids (Formvar鈥憇upported carbon if your vesicles are very delicate).
- Glow discharge the grids ~20鈥�30 seconds before use.
- This makes the carbon film hydrophilic, improving even particle spreading and preventing beading.
Tip: Poor wetting = uneven staining + distorted vesicles.
2. Sample Application
- Apply 3鈥�5 碌L of sample to the grid.
- Allow 30鈥�60 seconds for adsorption.
- Avoid highly concentrated samples; dilute to ~10鈦糕��10鈦� particles/mL for clean, non鈥憃verlapping fields.
Tip: Too many particles cause clumping, flattening, and staining artifacts.
3. Washing Step (Optional but recommended for EVs)
- After blotting the sample, briefly touch the grid to a small drop of filtered water.
- This removes salts that can crystallize and obscure fine detail.
Tip: One quick wash is usually enough鈥攖oo much washing may dislodge vesicles.
4. Uranyl Acetate (UA) Staining
- Use 1鈥�2% aqueous uranyl acetate, freshly filtered (0.02 渭m syringe filter).
- Apply 3鈥�5 碌L to the grid for 10鈥�30 seconds, depending on particle type.
- Blot gently with filter paper from the grid edge.
Avoid common UA artifacts:
- Dry stain crystals 鈫� indicate slow or uneven blotting.
- Stain pooling 鈫� suggests a hydrophobic grid or too much residual buffer.
- Overstaining 鈫� causes high contrast but loss of membrane definition.
Tip: For delicate EVs, a short 10鈥�15 s stain often gives the most natural morphology.
5. Particle Morphology Considerations
Liposomes
- Negative stain may flatten spherical liposomes鈥攖his is normal.
- Expect 鈥渃up鈥憇haped鈥� deformation due to dehydration forces.
- Multi鈥憀amellar structures may appear as onion鈥憆ing shells in stain.
Extracellular Vesicles (EVs)
- EVs often appear as round or cup鈥憇haped, depending on fixation and drying.
- The presence of protein contaminants may appear as small dark granules around vesicles.
- If vesicles appear collapsed, lower their adsorption time or apply stain earlier.
6. Grid Drying
- Allow grids to air鈥慸ry completely, ideally 鈮� 5鈥�10 minutes, or use a heat lamp.
- Keep grids protected from dust (e.g., inside a covered petri dish).
Tip: Residual moisture produces image drift and charging.
7. Imaging Conditions
- Use low-dose TEM, if available (soft biological material is beam鈥憇ensitive).
- Recommended initial settings:
- 80鈥�120 kV acceleration voltage
- Low鈥慸ose mode if available
- Minimal beam exposure when searching for areas
- Start at lower magnification (5,000鈥�20,000脳) to locate vesicles, then move to 50,000鈥�120,000脳 for details.
Tip: Excessive beam can cause vesicle collapse, UA bubbling, and membrane tearing.
8. Avoiding Common Negative Stain Problems
Problem: "Doughnut" or 鈥渉alo鈥� appearances
Cause: Stain excluded from the center due to lipid content.
Fix: Try a shorter adsorption time or a very brief pre鈥憇tain wash.
Problem: Crystals everywhere
Cause: Salt from sample buffer.
Fix: Add a quick distilled water dip before staining.
Problem: Vesicle rupture
Cause: Harsh blotting or highly charged grids.
Fix:
- Blot more gently
- Reduce glow discharge time
- Lower sample adsorption time
9. Sample Buffer Recommendations
- Avoid buffers high in salts or divalent cations (e.g., PBS, HEPES with Mg虏鈦� or Ca虏鈦�).
- Use low鈥憇alt buffers when possible (e.g., 10 mM Tris).
- Avoid detergents鈥攖hese dissolve vesicle membranes.
10. Quantification & Image Quality Considerations
- Negative staining is excellent for morphology but unreliable for size quantification due to dehydration artifacts.
- Use 肠谤测辞鈥惯贰惭 or NTA (nanoparticle tracking analysis) for accurate size distributions.