Atomic force microscopy (AFM)

Information obtained

  • Physical properties: surface topography, roughness, adhesion forces, elasticity, surface charge, surface composition
  • Lateral resolution from micrometers to fractions of Ångströms
  • Images collected in situ, in solution or gas
  • Mass change with picogram resolution

Principle

The sample, mounted on a piezoelectric scanner, moves with subnanometre resolution. A sharp tip, mounted on a cantilever, feels the forces between itself and the sample. Deflections in the cantilever are monitored with a laser and a position sensitive detector to make maps of physical and chemical properties.

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Effect of bleaching agents on hair. The untreated hair fiber (left) has a more rough surface and a higher height difference between the cuticle joints than the treated hair (right)
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Precipitation monitored by AFM (in a fluid cell) of the surface of a calcite ( CaCO3) crystal during exposure to a solution containing Ca2+ and CO32- . The new layer of CaCO3 slowly grows and covers the entire analyzed surface after 2 hours. Left are the height images, right are the tip deflection images.
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Dissolution of a 60 million year old coccolith, monitored using oscillation of the AFM to measure mass loss at picogram (10-12 g). The 5 μm coccolith is glued to an AFM tip (initial SEM image E). It remained intact when exposed to a Ca-free solution made to reflect the composition of sea water (ASW); only loose, presumably non biogenic calcite particles were removed (SEM image F) which causes minor weight loss. Switch from the Ca free artificial sea water to pure water led to the complete dissolution of the coccolith. The unstructured relic of associated organic material remained.

T. Hassenkam, A. Johnsson, K. Bechgaard and S. L. S. Stipp, Tracking single coccolith dissolution with picogram resolution and implications for CO2 sequestration and ocean acidification, Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 8571–8576

Atomic force spectroscopy (AFS) or Chemical force mapping (CFM)

Information obtained

  • Adhesion force and elasticity maps with nanometre lateral resolution
  • Unraveling of single molecules, proteins
  • Properties of interfaces

Principle

A set of force-distance curves is collected as the tip is rastered over the sample. This probing of the surface forces is carried out in liquid. We often collect 10,000 curves over areas of 5 μm x 5 μm, to produce maps of 100 x 100 pixel information.

Special feature

The reaction of the tip with the surface provides spectra and maps of how the adhesion forces are distributed over the mapped area.  By functionalising the AFM tip, we can give it a specific character that can then be used to probe surface properties, such as hydrophobicity, negative or positive charge, elasticity and so on, allowing us to can make maps of surface forces and chemical properties of  the sample.

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Maps showing height, adhesion and elasticity, made with an AFM in chemical force mapping (CFM) on a coccolith fragment (C) and a calcite crystal (R) found in a chalk sample. Maps were generated from 10,000 curves such as D and E, which were taken on a 100×100 grid over a 5×5-μm2 area. We used a tip functionalised with a hydrophobic molecule, with a -CH3 end group. These maps indicate that the coccolith elements, of calcite, are less hydrophobic and less elastic than the sutures, where organic material probably remains, and they show heterogeneity in the surface properties of chalk at the nanometre scale.

Hassenkam, T., L. L. Skovbjerg, and S. L. S. Stipp, (2009) Probing the intrinsically oil-wet surfaces of pores in North Sea chalk at subpore resolution, Proc. Natl. Acad. Sci. U. S. A., 106, 6071–6076

Chemical mapping with IR-AFM