Phosphotungstic Acid Negative Staining Method for TEM Sample Preparation
Phosphotungstic Acid Negative Staining Method for TEM Sample Preparation
1 Overview
1.1 Purpose and Scope
This protocol establishes a routine operating procedure for the phosphotungstic acid negative staining method. It is suitable for negative staining observation of viruses, bacteriophages, exosomes, liposomes, protein complexes, ribosomes, nucleic acid-protein complexes, bacterial flagella/pili, and nanoparticles under transmission electron microscopy (TEM). It can also be used for rapid morphological evaluation of some isolated and purified organelles.
1.2 Method Principle
(1) Negative staining uses high-electron-density heavy metal salts such as phosphotungstic acid to surround the sample, causing the background to show higher electron scattering and a darker tone under electron beam irradiation.
(2) The target sample itself is usually not fully penetrated by the stain and has relatively low electron density; therefore, it appears as a bright outline or electron-transparent structure against a dark background.
(3) This method is mainly used for rapid visualization of particle size, external morphology, surface contours, and partial internal contrast. It is not suitable as the sole basis for fine ultrastructural resolution.
1.3 Method Features
(1) The procedure is simple, and the imaging contrast is obvious.
(2) It is suitable for rapid screening of sample quality and dispersion status at low to medium-high magnification.
(3) It is sensitive to sample purity, salt ion background, pH, and the drying process.
2 Materials and Reagents
2.1 Sample Types
(1) Virus suspension.
(2) Exosome or extracellular vesicle suspension.
(3) Protein complex suspension.
(4) Liposomes, nanoparticles, or other micro-/nanostructure suspensions.
2.2 Main Reagents
(1) 2% phosphotungstic acid negative staining solution.
(2) Deionized water or distilled water.
(3) Optional prefixative, such as glutaraldehyde or osmium tetroxide, when required.
2.3 Consumables and Equipment
(1) Copper grids or support-film grids.
(2) Filter paper strips or absorbent paper.
(3) Pipettes and compatible tips.
(4) Forceps.
(5) Wax film or sealing film.
(6) Centrifuge.
(7) Transmission electron microscope.
3 Sample and Staining Solution Preparation
3.1 Sample Requirements
(1) Samples should be as fresh as possible, and repeated freeze-thaw cycles should be avoided.
(2) Samples should be uniformly dispersed, without obvious aggregation or precipitation.
(3) Sample purity and concentration should be appropriate. Excessive salt ions, sugars, or contaminating proteins may affect negative stain spreading and background quality.
3.2 Staining Solution Requirements
(1) The commonly used final concentration of negative staining solution is 2%.
(2) The staining effect of phosphotungstic acid is strongly affected by pH. Overall staining contrast is generally more stable at pH 6.0–7.0.
(3) If the sample is sensitive to pH, small-scale condition optimization should be performed first.
3.3 Grid Preparation
(1) The grid should be clean and intact, and the support film should show no obvious damage.
(2) During operation, confirm the membrane side to avoid inversion or contamination.
(3) If carbon-coated copper grids are used, gently hold the edge with forceps to avoid damaging the membrane surface.
4 Experimental Procedure
4.1 Drop Staining Method
(1) Sample preparation: The sample should be uniformly dispersed. If necessary, low-speed centrifugation (2,000 × g for 10 min) may first be performed to remove large particulate impurities or obvious sediment; if the sample concentration is insufficient, other appropriate methods may be used according to the sample type to concentrate the sample, prepare it as a suspension, and bring it to an appropriate concentration and purity.
(2) Sample loading: Place the grid with the membrane side facing upward on clean wax film or sealing film. Add 10–20 μL of sample suspension and let it stand at room temperature for 3–5 min to allow the sample to adsorb onto the support film surface.
(3) Washing: If the sample contains a high salt concentration or buffer components, after removing the sample liquid, add one drop of deionized water for quick washing. The washing step should be kept as brief as possible to avoid sample loss.
(4) Staining: After gently removing excess liquid from the edge of the grid with a filter paper strip, immediately add one drop, approximately 10 μL, of phosphotungstic acid negative staining solution and let it stand for 1–2 min.
(5) Drying: Use a filter paper strip to gently remove excess staining solution from the edge of the grid. Allow the grid to air-dry naturally at room temperature before TEM observation.
4.2 Floating Method
(1) Sample preparation: If necessary, the sample may first be centrifuged at low speed (2,000 × g for 10 min) to remove large particulate impurities or obvious sediment; other appropriate methods may also be used according to the sample type to concentrate the sample, prepare a uniform suspension, and bring it to an appropriate concentration and purity.
(2) Adsorption: Float the support-film grid on the sample droplet with the membrane side facing downward, allowing the sample to contact and adsorb onto the membrane surface.
(3) Staining: Transfer the grid to the surface of a droplet of negative staining solution and float for 1–2 min.
(4) Drying: Remove the grid, absorb excess staining solution, and air-dry naturally at room temperature before TEM observation.
5 Prefixation and Special Treatment
5.1 Treatment of Sensitive Samples
(1) Some samples sensitive to surface tension may collapse, deform, or break during natural drying.
(2) For such samples, mild prefixation may be performed after sample adsorption.
5.2 Prefixation Recommendations
(1) Glutaraldehyde or osmium tetroxide may be selected for short-term prefixation.
(2) Prefixation conditions should be optimized separately according to sample type to avoid excessive crosslinking that may affect negative staining results.
6 Staining Results
6.1 Target Samples
(1) Under TEM, target samples usually appear as electron-transparent or brighter outlines.
(2) When particle boundaries are clear, relatively complete morphology and local fine structures can be observed.
6.2 Background
(1) The background usually appears dark gray or black.
(2) A uniform background facilitates recognition of sample edges and structures.
7 Key Points for Result Interpretation
7.1 Morphological Interpretation
(1) Target particles should show relatively complete outlines, clear boundaries, and relatively uniform dispersion.
(2) If obvious collapse, rupture, or blurred edges are observed, drying damage and staining conditions should be considered together.
7.2 Background Interpretation
(1) A uniform and dense background usually indicates good spreading of the staining solution.
(2) If large areas of crystals, disordered particles, or local deposits appear in the background, this usually suggests interference from salts, sugars, or the condition of the staining solution.
7.3 Sample Purity Interpretation
(1) If many non-target particles are present in the field of view or obvious protein aggregates appear in the background, this suggests insufficient sample purification.
(2) Before negative staining, exosome, virus, or protein complex samples should have salt and contaminating protein levels reduced as much as possible.
8 Common Problems and Cause Analysis
8.1 Excessive Background Crystals
(1) The salt concentration in the sample is too high.
(2) The sample contains excessive phosphate or sugar components.
(3) Washing is insufficient, or the buffer system is not suitable for direct negative staining.
8.2 Blurred Sample Outlines
(1) The sample concentration is too low.
(2) Adsorption time is insufficient.
(3) Staining time is too long, or the staining solution spreads unevenly.
8.3 Sample Aggregation
(1) The sample is not sufficiently dispersed.
(2) Sample purity is insufficient.
(3) The concentration is too high, or local evaporation of liquid on the grid surface occurs too quickly.
8.4 Sample Structural Damage
(1) Surface tension is too strong during natural drying.
(2) The sample itself is fragile and sensitive to pH or heavy metal salts.
(3) If necessary, a prefixation step should be considered, and staining conditions should be re-optimized.
9 Application Scope
9.1 Virus and Bacteriophage Observation
(1) Used for rapid screening and morphological evaluation of viral particles and bacteriophages.
(2) Suitable for observing particle size, external boundaries, and overall dispersion status.
9.2 Exosome and Vesicle-Like Particle Observation
(1) Used to display the outlines of exosomes, liposomes, and other vesicle-like nanoparticles.
(2) Suitable for preliminary sample quality screening and comparison of preparation conditions.
9.3 Biomacromolecule and Complex Observation
(1) Used for negative staining observation of protein complexes, ribosomes, nucleic acid-protein complexes, and fibrous macromolecules.
(2) Suitable for evaluating aggregation status, dispersion, and overall morphology.
9.4 Bacterial Appendages and Organelle Observation
(1) Used to display the outlines of slender surface structures such as bacterial flagella and pili.
(2) Rapid morphological evaluation can be performed for some isolated and purified organelles.
10 Safety and Operating Guidelines
10.1 Personal Protection
(1) Wear a lab coat, mask, and disposable gloves during the experiment.
(2) When handling viral samples or high-risk biological samples, follow the corresponding biosafety level requirements.
10.2 Reagent Safety
(1) Phosphotungstic acid has certain corrosive properties and heavy metal toxicity.
(2) Avoid direct contact with skin and eyes during operation, and perform the procedure under well-ventilated conditions.
10.3 Waste Disposal
(1) Waste liquid containing phosphotungstic acid should be collected as heavy metal-related chemical waste.
(2) Sample residues, contaminated filter paper, grids, and other materials should be disposed of according to laboratory biological and chemical contamination handling regulations.
11 Selection of Related Reagents and Materials
Table 1 Selection of Reagents and Materials Related to Phosphotungstic Acid Negative Staining
Cat. No. | Name | Grade and Purity/Specification | Corresponding Step | Use |
Hosphotungstic Acid Negative Staining Solution (2%) | BioReagent, Biological Stain, for microscopy, 2% | Negative staining | Can be used directly for both the drop staining method and floating method | |
Hosphotungstic Acid Negative Staining Solution (3%) | BioReagent,Biological Stain,for microscopy,3% | Negative staining | Can be used as an alternative higher-concentration negative staining system | |
Hosphotungstic Acid Negative Staining Solution (5%) | BioReagent,Biological Stain,for microscopy,5% | Negative staining | Suitable for condition optimization when stronger background contrast is required | |
Phosphotungstic Acid Ethanol Solution (3%) | BioReagent,Biological Stain,for microscopy,3% | Negative staining/condition optimization | Belongs to a phosphotungstic acid negative staining-related system and can be used as an alternative for ethanol-based negative staining or special spreading conditions | |
Phosphotungstic acid hydrate | AR | Self-prepared negative staining solution | Suitable for preparing phosphotungstic acid negative staining solution | |
Phosphotungstic acid hydrate | ≥99.995% metals basis, Purity excludes up to 300 ppm Si | Self-prepared negative staining solution | Suitable for preparing high-purity negative staining systems | |
Sodium phosphotungstate hydrate | AR | Self-prepared negative staining solution | Can be used as an alternative raw material for preparing phosphotungstic acid salt-based negative staining systems | |
Sodium phosphotungstate hydrate | ≥99.9% metals basis | Self-prepared negative staining solution | Suitable for preparing high-purity phosphotungstate-based systems | |
Ammonium phosphotung |
| Self-prepared negative staining solution | Can be used as an alternative raw material for phosphotungstate salt-based self-prepared systems | |
Glutaraldehyde Fixative (2.5%) | BioReagent, ready-to-use | Optional prefixation | Best matched with negative staining TEM sample pretreatment; suitable for prefixation of sensitive samples | |
Glutaraldehyde Fixative (4%) | BioReagent, ready-to-use | Optional prefixation | Suitable for sample pretreatment requiring stronger fixation conditions | |
Glutaraldehyde Fixative (2.5%) | BioReagent, ready-to-use | Optional prefixation | Can be used for routine prefixation steps | |
Glutaraldehyde Fixative (4%) | BioReagent, ready-to-use | Optional prefixation | Can be used as an alternative higher-concentration glutaraldehyde prefixative | |
Glutaraldehyde | for electron microscope,50% in H2O | Self-prepared prefixative | Suitable for preparing glutaraldehyde fixatives at different concentrations | |
Osmium(IV) oxide | Os 83% | Optional prefixation/post-fixation | Can be used to explore osmium fixation conditions for special sensitive samples | |
Water | Ultra pure | Solution preparation/quick washing | Suitable for TEM negative staining systems requiring high background cleanliness | |
Water | for biotechnology nuclease-free, sterile | Solution preparation/sample processing | Can be used in systems requiring high sample cleanliness |