Electrophoresis is the underlying mechanism for a broad range of essential analysis techniques in colloid science, biochemistry, and biotechnology. The electrophoretic mobility (drift velocity in a viscous medium under application of external electric field) of proteins and other macromolecules is a very sensitive indicator of their internal charge state, their interaction with the surrounding environment, and their hydrodynamic radius. As such, it has been used for sorting a mixture of macromolecules or studying their conformational changes due to chemical reactions or physical adsorption to other molecules. Capillary- and gel- electrophoresis are two of the most widely used methods of measuring electrophoretic mobility in ensembles of molecules.
In a glorified experiment more than a century ago, Robert Millikan and Harvey Fletcher mastered measuring the drift velocity of a floating charged particle continuously to a level that they could set an accuracy record for the value of the elementary charge. The challenge we still face is how to perform that measurement on a single particle or even a single molecule, rapidly enough for resolving the reaction steps. The significance of reaching this goal is self-evident: if a researcher can monitor the charge (or the electrophoretic mobility) of a single solute rapidly enough, she or he will be able to study kinetic interactions such as ionization, hydrolysis, or charge transfer at the single particle level. In such a measurement, one can directly "track" the intermediate steps of a reaction, which are smeared out in bulk experiments because each molecule follows its own pathway stochastically.
Our nanoEPics team uses various trapping methods to confine the motion of single nanoparticles and monitor their charge state for an extended period of time. For more detailed information about the different projects please check the nanoEPics website.
The Millikan / Fletcher apparatus
The apparatus used by Robert Millikan and Harvey Fletcher to measure the electrostatic charge of oil droplets.
The optical tweezers setup
Our optical tweezers setup for monitoring the electrophoretic mobility of single trapped nanoparticles.