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lows you to look at rare phenomena that are otherwise hidden." The technique combines optical trapping—"laser tweezers"—with a tapered capillary that has an opening that is the same scale as the item being analyzed. Through the use of a vesicle as an example, the beam from a laser is brought to a focus on the target. The electric field induces a dipole on the vesicle and holds it RESEARCH where the electric field is strongest, UPDATE namely the center of the focused laser beam. Zare likens the phenomenon to Moving analytes moths being attracted to theflameof a candle. The vesicle can then be moved to one at a time the inlet of a tapered CE capillary with the Recently, much effort has been placed optical trap. into single-molecule detection, but The usual way to make capillaries with that's only the back half of the probnarrow openings is to etch the tip with lem. The question at the front end of hydrofluoric acid. However, etching the single-molecule (or single subcelmakes the tips jagged and actually inlular organelle, potentially more intercreases the inner diameter. Pulling a thinesting) problem is how to manipulate walled 100-um i.d. capillary resulted in a the vesicle, mitochondrion, or DNA smaller tip than etching a thick-walled molecule in question. 10-um i.d. capillary. In addition, the taIn the May 15 issue of Analytical pered capillary was smooth and rounded. Chemistry, Richard dare end doAn added advantage of the tapered capilworkers at Stanford University delaries was that pulling took less time than scribed a sample manipulation and etching. injection method for CE that allows them to selectively inject subcellular The ultrasmall tips are especially imcomponents or even single molecules portant for measuring subcellular cominto a capillary (Anal. Chem. 1197, ponents. To ensure measuring only one 69,1801-07). "We think we have a vesicle, the tip diameter and vesicle very important advance for the detec- must be approximately the same size. tion of single molecules and ultraThey applied suction to the capillary small volumes," Zare says. The samwith a precision syringe and could inject ples that they are dealing with are and eject a single (10-pm diameter) vesitypically on the order of femtoliters cle, without damaging the membrane. or attoliters. "An advantage of lookElectrophoretic analysis of a smaller vesing at single molecules is that it reicle containing carboxyrhodamine 6G moves the ensemble and aland fluorescein yielded only one peak indicating that the vesicle had not been lysed. The electropherogram, however, did give rise to a broad peak envelope that might indicate slight leakage of the components. In addition to maneuvering organelles, Zare and co-workers have also used the trapping technique to manipulate a single DNA molecule. The DNA molecule was trapped with the laser A 1.5-p.m vesicle (indicated by the arrow) is optically trapped beam; an ultrasmall and placed inside a capillary tip of ~ 6 pm.
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Analytical Chemistry News & Features, June 1, 1997
A schematic of the optical trapping, fluorescence imaging, and micromanipulation system.
tip, on the order of 1 um, was brought next to it, and a slight controlled suction was applied to load the molecule into the capillary. Now that they've shown that they can make and manipulate vesicles without damaging the membranes, it's time to put them to work. At the ACS meeting in San Francisco, Zare showed a video sequence of two vesicles fusing. By enclosing chemicals in the vesicles, it is possible to control the mixing of reactants. These vesicles have the capability of becoming the "world's smallest test tubes," says Zare. The manipulation procedure will be of interest to anyone who wants to deal with small volumes, says Zare. Such applications include the study of the concentration and distribution of neurotransmitters during exocytosis at synapses. Zare says, "We hope to develop what might be called 'solution nanochemistry' (as opposed to microchemical analysis). The ability to trap, manipulate, combine, separate, and analyze nanoenvironments in solution at reasonable temperatures opens new vistas. If we are to deliver or withdraw chemicals from single cells or subcellular regions we must develop these new techniques of solution nanochemistrv " Allen J. Bard of the University of Texas at Austin says, "Zare has made an important advance by adapting the laser tweezers concept to manipulate vesicles that can be used to trap molecules and move them to a location where detection byfluorescencecan be carried out." Celia Henry
