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Scanning force microscopy

Scanning Force Microscopy (SFM) is a method for imaging surfaces at micro- to nanometer scales. Our SFM is combined with an inverse light microscope and therefore, beside classical surfaces, also suitable for biological and biomimetic samples, e.g. living cells and vesicles in physiological conditions. For simultaneous SFM and light microscopy a transparent sample is required. A specific chamber (so-called Biocell) enables gas exchange as well as a temperature control (37°C for cell analysis).

The SFM consists of a cantilever with a sharp tip at its end that is used to scan the surface. A laser beam is reflected from the back of the cantilever via a mirror onto a photodiode. Minute deflections of the cantilever can be detected by the photodiode and are converted into an electronic signal. Bending of the cantilever varies according to differences in height of the sample. The signal at the photodiode is linked to the position of the cantilever on the sample and provides a three dimensional profile of sample height.

Fig.01 AFM

Different Modes

Contact Mode

In this mode the tip of the cantilever stays in contact with the sample. The cantilever scans over the surface with a default force (Constant Force Mode). To hold the force, the cantilever is moved perpendicular to the sample via a feedback mechanism. This movement in height is converted into a profile of height of the sample. It is also possible to keep the height constant while measuring cantilever bending (Constant Height Mode).

Fig.02 AFM

Non Contact Mode

In this mode the cantilever is oscillated with given amplitude. On approach of the tip to the surface the oscillation is damped. The resulting changes in amplitude correspond to the difference in distance between tip and surface and provide a height profile.

Fig.03 AFM

Force Distance Curve

Approaching the cantilever at a given position without scanning in x- and y-direction, a Force Distance Curve is obtained. Such a curve includes specific interactions between tip and surface. Thus, the shape of the Force Distance Curve provides information about surface characteristics as adhesion strength or elasticity. A series of measurements at different positions results in a map of properties of one sample.

Fig.04 AFM

Using these techniques mechanical properties, e.g. bending stiffness, elasticity or effective spring constant of protein-coated vesicles can be determined. For this a tipless cantilever is used (1).

Fig.05 AFM

Contact person: Dr. S. Dieluweit

1.Mechanical Properties of Bare and Protein-Coated Giant Unilamellar Phospholipid Vesicles. A Comparative Study of Micropipet Aspiration and Atomic Force Microscopy, Dieluweit, S , Csiszar, A, Rubner, W, et al., LANGMUIR, 26, 13, 11041-11049, 2010