The mapping of water molecules is still one of the most challenging tasks in AFM and only a few groups have been successful so far in this area. The fundamental work on hydration layer mapping has been previously done using a frequency-modulation AFM (FM-AFM): a challenging technique that uses three feedback loops to control the phase, the amplitude and the tip-sample distance while the frequency of the cantilever oscillation is precisely measured.

Easy to use

Now, researchers at the Johannes Gutenberg University have shown how to map hydration layers with a commercial instrument in the AM-AFM mode. Using only one feedback loop to control the tip-sample distance – while the amplitude and the phase are directly used as imaging signals – means that this method is easier to use. This allows a much larger group of AFM users to investigate water structures above pristine surfaces. It also allows for the use of 3D-mapping techniques for a broad range of applications and scientific questions.

Testing on calcite

The experimental set-up is benchmarked by mapping hydration layers above the calcite surface. This is the most common modification of calcium carbonate, which is an essential building block in biomineralization, and has relevance in seawater desalination and oil production. The experimental data give insights into the water arrangement above the calcite surface at the molecular scale. The characteristic "checkerboard" pattern is imaged and three distinct water layers commensurate to the underlying calcite surface can be identified.

Further details can be found in the journal Nanotechnology 25 335703.

Further reading

Scientists crack oyster's secret of strength (April 2014)
AFM model evaluates true height of water layers (Oct 2011)
Tip measures nanochannel kinetics in thin water films (April 2011)