Optically trapped aqueous droplets for single molecule studies

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Original entry: Zach Wissner-Gross, APPHY 226, Spring 2009

Second Entry: Nick Chisholm, AP 225, Fall 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

Figure 1: Photobleaching of one (left), two (middle), and three (right) fluorescent molecules in a single aqueous droplet.
Figure 2: Sequence of video images showing the merging of two water droplets held by optical tweezers. Scale bar: 1 <math>\mu</math>m.

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. What about the capillary forces?

Well, these should be on the scale of the surface tension (about 70 mN/m) multiplied by the size of the drops (again, about 2 <math>\mu</math>m), or roughly 140 nN. So in this example, the capillary forces are much, much greater than the optically-generated forces, explaining why the droplets do not appear deformed (but are rather quite spherical) as they are manipulated by the optical traps (see Figure 2).

Colin Bain (who gave a seminar after class earlier this semester) and researchers at Durham University in England found that by adding a particular surfactant to heptane droplets they were able to sufficiently reduce the surface tension of the droplets so that they were much less than optical forces [1]. This allowed them to shape droplets with lasers, and even to pull them apart, thereby creating nanofluidic channels between daughter droplets.




General Information

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

Publication: Applied Physics Letters 89, 013904 (2006)

Soft Matter Keywords

Fluorescence, surfactant, concentration, optical tweezers

Summary

This paper revolves around the creation and manipulation of femtoliter aqueous droplets in an immiscible fluorocarbon liquid. Within these femtoliter aqueous droplets, individual molecules (or molecular complexes) can be confined, isolated, and observed, the latter by using fluorescence imaging. The droplets with molecules are formed by ultrasonic mixing of the aqueous solution with the molecules in question and the matrix fluorocarbon; the molecules naturally go into the aqueous droplets because they are hydrophilic. The aqueous droplets can then be optically trapped using optical tweezers, and manipulated so that two droplets can fuse together to induce mixing of molecules from separate droplets.

Although the optical set-up is not directly of soft matter importance, some (like myself!) will find it interesting. See Figure 1 for the schematic diagram of the optical set-up [Note: In the paper, a very good caption is provided, however it is too large to include here; see the paper for a very good description of the set-up in the schematic's caption].

Figure 1: Optical set-up, taken from [1].

Soft Matter Discussion

In order to optically trap the aqueous droplets inside the fluorocarbon medium, optical tweezers are used. Optical tweezers are made from a single focus laser trap, and rely on the difference in polarizability between the droplets and the fluorocarbon medium. Equivalently, optical tweezers rely on the difference in the index of refraction between the droplets (<math>n = 1.33</math>) and surrounding medium (<math>n = 1.29</math>) in order to achieve trapping.

The medium (with droplets) was contained in a drilled hole of a microscope slide, sandwiched between two cover slips. The slide was manually translated until a droplet was trapped by the optical tweezers. When imaging was to be done, the green light path was unblocked, and fluorescence occurred for five to twenty seconds (until all the dye molecules in the droplet were photobleached; i.e. until they stop fluorescing). Figure 2 shows detection counts for a droplet with (a) one molecule, (b) two molecules, and (c) three molecules; each sharp decrease in imaging counts corresponds to one molecule photobleaching.

Figure 2, taken from [1].

It is important to note that the shape of the droplets is insignificantly altered by the optical tweezers that trap them. This is due to the fact that the force that the optical tweezers apply (on the order of piconewtons) is significantly smaller than the surface tension of the aqueous droplets in the fluorocarbon medium.

In Figure 3, a sequence of video images shows the fusion of two droplets. Both droplets are trapped using optical tweezers; the bottom trap is fixed, whereas the top trap is mobile. The droplets spontaneously fuse when brought together, and as long as no leaking occurs, the molecules contained in each droplet will both be contained in the newly fused droplet. In this new droplet, the molecules are able to interact with one another.

Figure 3, taken from [1].

Reference

[1] J. E. Reiner, A. M. Crawford, R. B. Kishore, Lori S. Goldner, K. Helmerson, and M. K. Gilson, "Optically trapped aqueous droplets for single molecule studies," Applied Physics Letters 89, 013904 (2006)