Anal. Chem. 2009, 81, 16–19
Optical tweezers: not just for physicists anymore Christine Piggee A new generation of commercial instruments makes optical tweezers more accessible. New technologies such as genetic engineering are often foreshadowed in science fiction long before they are actualized.1 Another case in point: optical tweezers (also known as an optical trap), which use laser light to move small particles very precisely within a three-dimensional space. “It’s like Star Trek with the tractor beam,” says Keir Neuman of the U.S. National Institutes of Health. “It really is quite astonishing.” Initially a tool for physicists, the optical trap was later adopted by researchers who worked on biological and materials-science questions. Now, the technique has spread to many fields. Indeed, Neuman has observed that researchers are publishing increasing numbers of optical trapping results in traditional biology and chemistry journals. “What that tells me is that it’s being adopted as a tool, and it’s no longer just a neat technique.... It’s a real workhorse,” he says. This Product Review gives an overview of optical tweezers and some of their applications. Table 1 presents some basic specifications of commercial optical trapping systems. This article is not intended to be an exhaustive analysis of all optical trapping instruments; interested readers should contact the vendors for more information about their products and services. APPLICATIONS Researchers have used optical tweezers in diverse fields such as biophysics, cell biology, and materials science. Nevertheless, all optical trapping applications can be divided into the broad categories of manipulation and force measurement. In the former category, investigators have used optical tweezers to sort cells, move organelles within cells, and track the movement of bacteria.2 For such experiments, optical traps have the advantage of permitting noninvasive, sterile manipulation of objects in typical buffers. In addition, these instruments can perform very sensitive force measurements in ranges well suited to the study of molecular motors and other important biological processes. For force measurements, optical trapping is considered complementary to magnetic tweezers and atomic force microscopy, because the three techniques operate in different force ranges.3 Researchers have used optical trap force measurements in fields 16
Analytical Chemistry, Vol. 81, No. 1, January 1, 2009
such as single-molecule biophysics and microrheology.4 Nancy Forde of Simon Fraser University (Canada) says that one of the more fruitful applications of optical tweezers has been the study of molecular motors. “I think that we, as a community, have learned a lot about how molecular motors operate,” she says. Optical tweezers could detect the base-pair stepping of RNA polymerase during transcription and, more recently, the singlecodon stepping of ribosomes during translation.5,6 In addition, Forde says that researchers have learned a great deal about the structure of DNA, RNA, and proteins by studying their generic stretching behavior and how they unfold. Despite these successes, optical traps have potential disadvantages, including their high cost, low throughput, and lack of selectivity. The technique provides limited throughput. “You could study tens of molecules a day if things were working well,” says Forde. In addition, the lasers used in the systems raise some concerns. For safety reasons, optical tweezers that are not enclosed in a box usually require the users to wear laser goggles. Also, the near-IR lasers commonly found in optical tweezers do cause some photodamage and local heating at the power levels 10.1021/ac8023203 2009 American Chemical Society Published on Web 12/31/2008
Table 1. Commercially available optical tweezers.1 Product
BioRyx 200
E3400
PALM MicroTweezers IV Carl Zeiss MicroImaging GmbH +49-89-90-9000-800 www.zeiss.de/ microdissection
mmi CellManipulator
NanoTracker
Company
Arryx, Inc. 773-726-6675 www.arryx.com
Elliot Scientific +41-1582-766300 www.elliotscientific. com
Molecular Machines and Industries +41-44-809-1010 www.molecular-machines. com
JPK Instruments AG +49-30-5331-12070 www.jpk.com
Primary end use
Biophysics research
Biophysics research
Biology/molecular biology research
Biology/molecular biology research
Biophysics research
Cost (U.S.D.)2
230,000-500,000
20,000-80,000
150,000-230,000
150,000-320,0003
250,000-450,000
Maximum power in specimen plane Trap moved in how many dimensions? Stage moved in how many dimensions? Particle position measured in how many dimensions? Image analysis for position measurements? Laser deflection for position measurements? Maximum resolution of position detection Maximum bandwidth of position detection Sensitivity and resolution of force measurements Maximum force
450 mW
N/A
∼750 mW
1.5 W
1W
3
2
3
2
3
2 or 3, depending on stage chosen 2
N/A
2
2
3
N/A
2
2
3
No
No
Yes
No
Yes
Yes
No
No
Yes
Yes
2 nm
N/A
Not yet defined
3 nm
