We want to observe single biological nanoparticles in solution-phase conditions, but their fast and random Brownian motion means that we only get short observation windows in our microscope (a few milliseconds). Solution-phase conditions are closer to the native environment of these particles than on a glass surface, and the statistics of the particle’s Brownian motion can be used to extract its hydrodynamic diameter, a useful measurement of particle size.

We employ active feedback to trap the particle in solution, counteracting the Brownian motion and holding the particle for many seconds to minutes. By doing so, we can gather enough information so that we can extract multiple parameters from each object.

For more information, please see:

WB Carpenter, AA Lavania, AH Squires, and WE Moerner, “Label-Free Anti-Brownian Trapping of Single Nanoparticles in Solution,” J. Phys Chem. C, 2024, 128, 20275. 

AH Squires, AE Cohen, and WE Moerner, "Anti-Brownian Traps," In Encyclopedia of Biophysics. Springer: Berlin, Heidelberg, 2018.