Optically trapped aqueous droplets for single molecule studies

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Zach Wissner-Gross (May 2, 2009)

Information

Optically trapped aqueous droplets for single molecule studies

J. E. Reiner, A. M. Crawford, R. B. Kishore, Lori S. Goldner, K. Helmerson

Applied Physics Letters, 2006, 89, 013904

Soft matter keywords

Emulsion, surface tension, optical trapping

Summary

To optically trap microparticles, one requirement is that the particle have a higher index of refraction than its surrounding medium. While small plastics beads are easy to trap, water droplets are more difficult to to water's relatively low refractive index of 1.33. Reiner et al. overcome this difficulty by mixing water with a low-index (1.29) hydrophobic fluorocarbon medium, followed by ultrasonication to produce droplets approximately 2 <math>\mu</math>m in diameter.

This paper is constructed around several applications of the optical trapping system. First, the authors use it to detect single molecules by trapping a droplet containing a handful of fluorescent molecules, and then photobleaching them, a stochastic process that reveals integer multiples of fluorescence (Figure 1). The authors also demonstrate fluorescence resonance energy transfer with a single molecule, contained, of course, in an aqueous droplet. Finally, they fuse together two droplets by bringing them together with two different laser beams (Figure 2).

Soft matter discussion

In this system, there are really two types of forces at work: there are optical forces, but then there are also forces created by the surface tension of the aqueous droplets. Optical traps generate forces that are typically on the order of piconewtons, or <math>10^{-12}</math> N.