Abstract
We present an Anti-Brownian Electrokinetic trap (ABEL trap) capable of trapping individual fluorescently labeled protein molecules in aqueous buffer. The ABEL trap operates by tracking the Brownian motion of a single fluorescent particle in solution, and applying a time-dependent electric field designed to induce an electrokinetic drift that cancels the Brownian motion. The trapping strength of the ABEL trap is limited by the latency of the feedback loop. In previous versions of the trap, this latency was set by the finite frame rate of the camera used for video-tracking. In the present system, the motion of the particle is tracked entirely in hardware (without a camera or image-processing software) using a rapidly rotating laser focus and lock-in detection. The feedback latency is set by the finite rate of arrival of photons. We demonstrate trapping of individual molecules of the protein GroEL in buffer, and we show confinement of single fluorophores of the dye Cy3 in water.
Highlights
The invention of traps for individual atoms and molecules in the gas phase led to new physical measurements, and new analytical techniques
One output of the fanout goes to a PicoHarp 300 Time-Correlated Single Photon Counting (TCSPC) module (Picoquant, Berlin, Germany), and the other output goes to the demodulation electronics
A pattern of channels (Fig. 4(a), dark in the inset) connecting the feedback electrodes to the trapping region was created in photoresist, transferred into the Si via a SF6 reactive ion etch (RIE), and ∼ 20 μm into the silica via a 6:1 buffered oxide etch (BOE) lasting several hours
Summary
The invention of traps for individual atoms and molecules in the gas phase led to new physical measurements (e.g. of the anomalous magnetic moment of the electron[1]), and new analytical techniques (e.g. ion cyclotron mass spectrometry[2]). Molecules in solution show much more complex behavior than molecules in the gas phase, but until now there have not existed devices capable of trapping single molecules in aqueous buffer To address this issue we designed and built an Anti-Brownian Electrokinetic trap (ABEL trap) that grabs and holds single proteins in water at room temperature. An earlier version of the ABEL trap used video tracking and computer-controlled feedback to achieve a latency of 4.5 ms, principally due to the 300 Hz maximum frame-rate of the camera This latency set a lower bound of 20 nm on the diameter of objects that could be trapped in water ( smaller objects could be trapped by increasing the viscosity of the solution[14]).
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