Magnetically Characterized Molecular Lubrication between Biofunctionalized Surfaces.

We demonstrate an efficient approach for quantifying frictional forces (sub-piconewton) at nano-bio interfaces by controlled magnetic forces, which is based on simultaneous measurements of critical frequencies for streptavidin-coupled magnetic particles. The maximum phase angle, being corresponded with the critical frequency, is formulated in terms of magnetic, frictional, and viscous forces of the particles on DNA- and SiO2 -functionalized micromagnet arrays. The streptavidin/DNA interface shows lower friction as an enhanced lubrication than the streptavidin/SiO2 interface, which is indicated by the lower transition field of quasi-static motion, the larger ratio of dynamic particles, and also the higher velocity of the particles. The friction coefficients at the streptavidin/DNA and streptavidin/SiO2 interfaces are evaluated numerically as 0.07 and 0.11, respectively, regardless of the vertical force and the velocity. The proposed method would open up new possibilities to study mechanical interactions at biological surfaces.