ACS Nano http://doi.org/bdfh (2016)
Particles attached to a surface by a molecular tether move around under the effect of Brownian motion. In an ideal system, the particles would conform to a spherically symmetric disc-shaped pattern. Experimentally, however, they exhibit a range of patterns, the origin of which remains largely unexplained. Emiel Visser and co-workers at Eindhoven University of Technology have now used a motion pattern analysis technique to shed light on this seemingly irregular behaviour.
The researchers attached colloidal particles to a glass surface with short (40 nm) DNA tethers. They then optically tracked the particles with high spatial accuracy and, with the help of Monte Carlo simulations, assigned motion patterns to specific particle–tether–surface systems. The analysis showed, for example, that a bell-shaped pattern results from the presence of a small protrusion near the tether attachment: the particle prefers to move about its centre, as it is farthest from the surface and therefore can freely wiggle around without interacting with the surface. Conversely, if the protrusion is large, the particle is pulled to one side and spends less time near its centre, giving a ring-shaped pattern. Non-spherical symmetric patterns such as stripes and triangles originate from doubly and triply tethered particles, respectively. Visser and colleagues also used a biosensing assay to analyse the behaviour of functionalized particles, which were attached to a surface with one or more bonds, in the presence of a target analyte.