Single-Molecule Manipulation Using Optical Traps

Abstract

One of the most sensitive tools for manipulating single molecules and measuring their properties is the optical trap, also known as optical tweezers. Consisting essentially of a strongly focused light beam, optical traps were first developed and demonstrated in the 1970s and 1980s by Arthur Ashkin and colleagues (Ashkin et al. 1986). These early pioneers showed that micron-sized dielectric particles could be held and manipulated in solution by using optical forces to create a stable, three-dimensional potential well. Since then, optical trapping instrumentation has been refined and developed such that piconewton forces are now routinely applied, while at the same time measuring the resultant displacements to nanometre or even angstrom resolution. As a result of these advances, optical traps have been applied widely, from cytometry to the study of mesoscopic colloids and polymers and of course the properties of single biological macromolecules. This chapter begins with a description of the theory and design of optical traps, followed by an illustrative discussion of applications to the study of structure formation and molecular motors, a description of typical “tricks of the trade” for using optical traps, and a brief look at techniques for extending the capabilities of traps.