Human Collagen: PeakForce Tapping

The primary advantage of tapping AFM is that it is a non-contact AFM technique. While it "taps" on the sample surface, the tapping is intermittent and as such there is minimum shear force. This is ideal in imaging soft materials, including biological systems, as significant shear forces modify the sample surface while it is being imaged.

That tapping is generally done at the resonant frequency of the probe. A stiff silicon tapping probe has a resonant frequency on the order of 300 kHz, and so a tapping frequency just below resonance is selected for tapping AFM. Constraining the tapping causes the probe frequency to increase, and this will cause the probe frequency to move towards resonance not off resonance.

While this mode of imaging produces minimal shear force, one disadvantage is that it produces an indeterminate maximum normal force. While one can image in soft tapping mode by reducing the tapping amplitude, it is a non-trivial task to estimate the maximum peak normal force in tapping AFM. It is sometimes also desirable to know the maximum force exerted on a sample to quantitate sample deformation.

Bruker's PeakForce tapping combines the best of tapping and contact AFM imaging modes. What it does is perform complete force curves at 1-2 kHz at every image point. The force curve is triggered at the maximum applied normal force according to these force curves, and it is this "PeakForce" that is the set-point or parameter maintained constant during imaging. The second image, borrowed from Bruker, shows force curves as a function of time and position, and the PeakForce set-point is point C on both curves. A further innovation is ScanAsyst technology which dynamically and intelligently monitors and optimizes the scan rate, PeakForce set-point, gains, and Z-limit to produce the best image quality.

In this image human collagen from cadaver skin was imaged using a ScanAsyst Air probe in ScanAsyst mode. This image of a fairly soft biological specimen was imaged with little operator interaction beyond aligning the scanner and focusing on the sample. An additional advantage of the PeakForce tapping imaging mode is that force curves are available at every image point, and these can be used to perform nanomechanical measurements-- what is called PeakForce QNM.

Tapping AFM: Arabidopsis haliana Grana Membranes

Images of Arabidopsis thaliana grana membranes using tapping AFM. 

The Dimension 3000 AFM was used to image these membranes in air with stiff silicon probes (Olympus #OMCLAC160TS-W2, with 7 nm tip radius and 42 N/m spring constant). Arabidopsis thaliana grana membranes were isolated through treatment with 2% digitonin showing semi-crystalline array formation of photo-system II complex proteins due to changes in growth light intensity. Protrusions are the water splitting complex of photo-system II in photosynthetic membranes. Scale bar is 375 nm. 

 
Thanks to Stefanie Tietz and Dr. Helmutt Kirchoff, Washington State University Molecular Plant Sciences, as well as Dr. Steve Lenhert, FSU Department of Biology.

Lipid Grating

This image shows a dipalmitoyl phosphatidylcholine (DPPC) multilayer grating fabricated by lipid multilayer stamping. The image was taken in air using tapping mode, and is a good example of soft materials imaged in air using AFM techniques.

O. A. Nafday, T. W. Lowry, S. Lenhert, Multifunctional lipid multilayer stamping, Small 8, 1021-1028 (2012).

Tapping AFM of Buckypaper

This image is a tapping AFM image performed on a Buckypaper sample made of carbon nanotube bundles precipitated into a fibrous paper. Even though the system consists of free-standing fibers with very deep pores, it could be easily imaged.