The ABEL trap (“anti-brownian electrokinetc trap”) has been invented and realized by A. E. Cohen and W. E. Moerner in Stanford
(see websites of AEC in Harvard and WEM in Stanford).

It’s purpose is to immobilize single molecules, proteins or nanoparticles in solution by active canceling their threedimensional brownian motion. The nano-object is monitored by fluorescence imaging. The position of the nano-object is determined with respect to a target position and used for a feedback voltage applied to four electrodes. The electric field or the induced electroosmotic flux pushes the nano-object back and compensates the brownian motion.

A microfluidic device constricts the diffusion of the nano-object to the x and y
dimensions. The chamber is made of transparent PDMS. Four 20 µm deep channels with the electrodes are arranged perpendicular to confine the nano-object within the trap region with only 1 µm thickness.

Widefield illumination of the trap region and a fast sensitive CCD camera to detect the position of the fluorescent nano-object was sufficient to immobilize a single DNA molecule, a tabac mosaic virus (300 nm length), a liposome (100 nm radius) or even a 10 nm polystyrene bead. For trapping smaller objects like single fluorophores or soluble proteins, faster versions of the ABEL trap have been developed.

 Application to a single  F_oF₁-ATP synthase
(monitoring conformations of a single membrane transporter)

In collaboration with AEC and WEM we modify the ABEL trap to immobilize a single liposome with one embedded membrane proteine. Therefore the F_oF₁-ATP synthase will be labeled with two fluorophores to monitor the internal rotary motion by confocal single-molecule FRET. In addition the liposome will be stained with another fluorophore to be hold and transported to the target position in  the ABEL trap. The target postion is definded by the laser focus for the FRET measurement.

Actually we can trap 100 nm liposomes (see image on the left: the liposome is confined for seconds before the fluorescence falls below a trapping threshold due to photobleaching; then the liposome diffuses away in the upper right direction) and are implementing the combined FRET @ ABELtrap experiment.

New movie of an Atto680-labeled liposome trapped for several seconds in the ABEL trap (Flash file):