Lateral force-distance curve technique

Nano-mechanical properties of single biomolecules and polymers as well as interaction forces between specific molecular pairs have been intensively studied during the last two decades by using atomic force microscopy with force measurement technique. In these experiments, molecules were pulled up from the substrate in vertical direction. However, it is also interesting to develop a similar lateral force-distance technique in which sample molecules stay lain on the solid surfaces during mechanical stretching. Thus, wider possibilities can be opened up to study the stretched macromolecules by other microscopy/spectroscopy methods. Since typical atomic force microscopes have the lateral scanner ranges up to ~100 μm, very long macromolecules can be studied and manipulated in this way. As an example, up to 8 μm stretching of self-polymerized poly(ethylene glycol)-bis-thiol on gold surface was observed. It was also confirmed that even in the lateral force-distance curves (50-300 nm above surface), the vertical cantilever bending, not torsion, was mainly responsible for the observed deflection signal. This made lateral force data analysis simpler since the normal force constants are easier to estimate than torsion ones. The force curves in our method were also free of any interference fringes. They had horizontal and easy identifiable baselines.