Single molecules can be trapped inside a microfluidic chamber by an electrokinetic potential that counteracts the Brownian motion. A fluorescence signal determines the position of the particle, and by measuring the feedback forces researchers can determine the translational diffusion coefficient, which is related to the hydrodynamic radius of the trapped particle. However, a parameter that is more sensitive to the size of an object is the rotational diffusivity. Yang and Moerner have now shown how to measure the rotational diffusivity of a trapped object, and evaluate its volume.
The researchers use an electrokinetic trap, but they modify it to detect the time-resolved fluorescent signal from two polarization channels. As the molecule freely rotates, the two polarized signals will decay over time with kinetics that depend, among other parameters, on the rotational correlation time. By fitting the decay curves, the researchers can determine the rotational diffusivity and hence the volume of the object, and can recognize single- from double-stranded DNA from a mixture of the two with 80% accuracy.
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Moscatelli, A. A single object rotating. Nature Nanotech 13, 769 (2018). https://doi.org/10.1038/s41565-018-0265-1