Wednesday, December 18, 2013

Tensile testing a single molecule

Nanotechnology has enabled things like Atomic Forced Microscopy (AFM) to benefit materials scientists and engineers but there's a use of the same equipment a materials engineer would find extremely amazing and would probably not have thought of using AFM for; tensile testing a single molecule.

This can be done by combining AFM with a piezoelectric positioner. under the cantilever probe of the microscope. The molecule once attached / bound (chemically or otherwise), at one side to the probe and the other side to the positioner, can be stressed by the piezoelectric material which will change height with application of electricity and the tensile testing data will be recorded by AFM's sensors. Now how much information can be acquired by tensile testing a single molecule alone puts materials engineering to a nano scale perspective against the bulk and the macro properties which are often completely different.

Basic operation of the AFM. As the AFM cantilever probes the surface by moving its tip along its contours, or when it pulls on a protein, the movement of the cantilever is detected with a laser beam that is focused to beam to the head of the cantilever and refracts into a photodetecter. The movement, or deflection, of the cantilever deflects the laser correspondingly, and this creates an image or produces other data about the surface or the sample (which may be a molecule). In single molecule force spectroscopy, the cantilever is pressed against a layer of proteins attached to a substrate, and the tip adsorbs a single protein molecule, which is then extended. Extension of the molecule by retraction of the piezoelectric positioner results in deflection of the cantilever. [Oberhauser et al. PNAS (January 2001), Vol. 98 (2): 468-472]

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