Trapping and manipulating single molecules in solution

This thesis describes the development, characterization, and application of the Anti-Brownian Electrokinetic trap (ABEL trap): a device for trapping and manipulating individual nanoscale objects in solution. The ABEL trap works by tracking the Brownian motion of a single fluorescent object of interest, and then applying a feedback voltage to the solution to induce an electrokinetic drift that cancels this Brownian motion.; The ABEL trap is capable of trapping objects far smaller than can be trapped by any other means under comparable conditions. It provides a way to study individual molecules in their native environment, bypassing the perils of surface attachment chemistry. The feedback voltages themselves also provide information along a dimension not usually accessible to single molecule experiments. By analyzing the feedback voltages one can extract time-dependent information about the diffusion coefficient and electrokinetic mobility of the trapped object. Finally, the ABEL trap has potential for use as a small-scale fabrication technology. Trapped objects may be photochemically immobilized, providing a means to form permanent nanoscale structures.; Chapter 1 provides a background on previous trapping technology, electrokinetic phenomena, and the theory of feedback, including some new results on particle-tracking. Chapter 2 describes an implementation of the ABEL trap that uses a high speed camera and real-time image processing to implement the feedback. In Chapter 3 I describe trapping of single molecules of lambda-DNA and analyze their mechanical properties. Chapter 4 is a theoretical diversion inspired by the experiments on DNA. It addresses the question, what are the natural ways to describe the shape of a random walk? Chapter 5 is again on instrumentation, describing a scheme that performs the feedback entirely in hardware. The advantage of this scheme is its increased ability to trap very small particles, even down to individual fluorophores. Chapter 6 discusses outstanding theoretical questions and directions for future research.