Compare fluorescence, confocal and electron microscopy, and choose the right method for a question.
Why it matters
Ordinary light microscopes show cells and tissues, but many lab questions need more. Where in the cell is this protein? Is this bacterium alive? What does a virus or a crack in a metal grain boundary look like at 10 nanometres? Fluorescence and electron microscopy answer those questions, and they are central to biochemistry, cell biology, clinical microbiology and materials science. You may operate these instruments as a technician or depend on core facilities that do.
The resolution limit of light
Magnification makes an image bigger. Resolution is the smallest distance at which two points still look separate. Magnifying beyond the resolution limit only makes a blur bigger.
For a light microscope, the Abbe limit is approximately:
d = wavelength / (2 x NA)
where NA is the numerical aperture of the objective. With green light (about 500 nm) and a good oil-immersion objective (NA 1.4), d is about 500 / 2.8 = about 180 nm. Most viruses and all individual proteins are smaller than that.
Fluorescence microscopy
A fluorophore absorbs light of one wavelength and emits light of a longer wavelength. The difference is the Stokes shift. A fluorescence microscope uses this:
- A bright light source (LED, mercury or xenon lamp, or a laser) passes through an excitation filter that selects the wavelength the fluorophore absorbs.
- A dichroic mirror reflects that light onto the sample.
- The fluorophore emits longer-wavelength light, which passes through the dichroic mirror and an emission filter to the camera or eyepiece.
- Only the labelled structures glow against a dark background.
The excitation filter, dichroic and emission filter are often mounted together as a filter cube matched to a fluorophore.
Common labels:
- DAPI and Hoechst bind DNA and stain nuclei blue.
- Green fluorescent protein (GFP) can be genetically fused to a protein so living cells show where it goes.
- Antibodies tagged with fluorophores (immunofluorescence) label specific proteins in fixed cells or tissue.
Confocal and beyond
A widefield fluorescence image includes out-of-focus glow from above and below the focal plane. A confocal microscope scans a laser point and uses a pinhole to block out-of-focus light, giving sharp optical sections that can be stacked into 3D images. Super-resolution methods (such as STED, PALM and STORM) use clever tricks with fluorophores to get below the Abbe limit, to tens of nanometres.
Limits
- Photobleaching: fluorophores fade with exposure. Keep light exposure short and image consistently.
- Phototoxicity: strong light damages living cells.
- Autofluorescence: some tissues glow on their own; controls without the label reveal it.
- You only see what you labelled.
Electron microscopy
Electrons have a far shorter wavelength than visible light, so electron microscopes resolve down to the nanometre scale or below. The beam must travel in a vacuum, so samples must be dry or frozen and stable in vacuum.
| Transmission EM (TEM) | Scanning EM (SEM) | |
|---|---|---|
| How it works | Electrons pass through a very thin section | A focused beam scans the surface; detectors collect emitted electrons |
| Image | 2D internal structure (organelles, viruses, crystal defects) | 3D-looking surface topography |
| Sample preparation | Fix, embed, cut ultrathin sections (tens of nm), stain with heavy metals | Fix, dry, coat non-conductive samples with a thin metal layer (sputter coating) |
| Typical resolution | Down to below 1 nm | About 1 to 10 nm |
SEM instruments often add energy-dispersive X-ray spectroscopy (EDS), which identifies elements in a spot on the sample. That makes SEM a workhorse in materials science for fracture surfaces, coatings and particles.
Cryo-electron microscopy (cryo-EM) freezes samples so fast that water forms glass-like ice instead of crystals, preserving molecules close to their natural state. It is now a major method for determining protein structures.
Choosing a method
| Question | Better choice |
|---|---|
| Where is protein X in living cells, over time? | Fluorescence (GFP), confocal for 3D |
| What is the internal fine structure of a mitochondrion or virus? | TEM |
| What does the fracture surface of a steel part look like, and what elements are present? | SEM with EDS |
| What is the 3D structure of a purified protein complex? | Cryo-EM |
Common mistakes
- Confusing magnification with resolution.
- Interpreting a fluorescence image without no-label or no-primary-antibody controls.
- Comparing images taken with different exposure settings.
- Forgetting that EM sample preparation (drying, staining, coating) can create artefacts.
Quick recap
- Light microscopes are limited to about 200 nm resolution.
- Fluorescence uses excitation and emission filters with labels such as DAPI, GFP and antibodies; confocal adds sharp optical sections.
- TEM looks through thin sections; SEM scans surfaces; both need vacuum and careful preparation.
- Pick the method by the question: living or fixed, inside or surface, molecules or materials.
These instruments are operated only after training by the facility that runs them.
Try it
This lesson is quiz-only. Open the Do tab to check your understanding.