Choosing an afm microscope: why the imaging mode and the probe matter more than the instrument, what an atomic force microscope measures that an optical or electron instrument cannot, where afm microscopy is the wrong tool, and the sample preparation that decides whether any of it works

Specifying the instrument and the probe

1. Choose the mode from the sample's fragility. Contact mode drags the tip across the surface and damages soft material. Tapping or intermittent contact oscillates the cantilever and touches briefly, which is what makes biological samples imageable. Force spectroscopy holds position and measures the force curve, which is how stiffness is obtained.
2. Match the cantilever spring constant to the sample. A stiff cantilever on a soft cell deforms it; a soft cantilever on a hard surface gives no contrast. The spring constant is the single most consequential consumable choice, and it has to be calibrated on the instrument rather than taken from the box.
3. Decide whether you need liquid operation. Imaging in buffer is what allows living cells and hydrated biomolecules to be measured in a meaningful state. It requires a liquid cell, a different cantilever holder and careful attention to drift, and it is not a trivial add-on.
4. Prepare the sample so it stays put. Whatever you image must be firmly attached to a flat substrate, because the tip will otherwise push it around. Mica and functionalised glass are the usual choices, and immobilisation chemistry is most of the method development on a new sample type.
5. Site it away from vibration and acoustic noise. Vertical resolution is below a nanometre, so building vibration, footsteps and air currents all appear in the image. An acoustic enclosure and an isolation table are part of the instrument rather than accessories, and a basement room beats a fifth floor one.

Calibrating so the numbers mean something

Height is calibrated against a step height standard and the cantilever spring constant against a known method, because a force curve reported from an uncalibrated cantilever is an arbitrary number. Both should be done at intervals and recorded with the data.

Lateral dimensions are convolved with the tip shape, so a feature narrower than the tip is reported wider than it is. Tip characterisation matters whenever lateral size is being quoted rather than height.

Throughput and where it hurts

An image takes minutes rather than milliseconds and covers a field far smaller than an optical one. Statistics are therefore expensive, and a study needing hundreds of cells is a very different proposition from one needing ten.

Plan the sampling before the session. Atomic force microscopy datasets are frequently too small to support the conclusion drawn from them, and that is a design problem rather than an instrument one.

Buying it against buying access

Atomic force microscopes need skilled operators and are frequently underused when bought by a group with one application. A core facility with an experienced operator produces better data and costs less for intermittent work.

Buy when the application is continuous and someone's job includes running it. That person is the real purchase, and an instrument without one becomes an expensive surface in a basement.

Common questions

What does an afm microscope measure?

Surface height, by touching the sample with a sharp tip on a flexible cantilever and tracking the deflection. It gives true three dimensional topography and, in force spectroscopy mode, mechanical properties such as stiffness and adhesion.

Can it image living cells?

Yes, in buffer, in a gentle oscillating mode with a soft cantilever. It is one of the few techniques that measures the mechanics of a living cell, and it is slow compared with optical imaging, which is the trade.

How does it compare with electron microscopy?

An electron microscope gives higher lateral resolution and needs vacuum and usually a coating. Atomic force microscopy gives true height data, works in air or liquid on unstained material, and has far lower lateral resolution. They answer different questions.

What limits the resolution in practice?

The tip radius laterally, and vibration vertically. A blunt or contaminated tip is the commonest cause of a disappointing image, and tips are consumables that degrade during use rather than lasting.

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