The sample cells also vary in their design. It is very common for a microscope slide to be used as the basis of an experimental cell. A well is created on the slide, either by building up walls or by creating a small trough like structure. The solution with suspended particles is placed in this well and a coverslip, is placed over the top, making a sealed cell (figure Ill.i). The sample cell is usually of the order of 50 - 100 pm in depth. A drop of oil or water must be placed on top of the coverslip to match the refractive indices of the oil/w ater immersion objective lens to the coverslip. If the sample consists of water, the water immersion objective lens may sometimes be lowered straight into the specimen solution.
Laser Laser Microscope Objective Lens Coverslips Index Matching
Fluid Index MatchingFluid
Samples Microscope Slide
The sample particles are usually of the order of a few microns in size and suspended in a solution. Different solutions can be used, but water is the most common. The choice depends on the solution properties required, such as viscosity, density and the likelihood of any reaction with the particles. The particles may be synthetic such as silica or glass spheres, or of a biological nature. To achieve optical trapping in the way described earlier, the particles must be at least partially transparent to allow trapping to take place.
III. 7 Applications of optical tweezers
Optical tweezers are now quite common in the fields of cell biology, biophysics and microbiology [25, 26, 28-31]. This is the major application of tweezers to date. The first successful trapping of a biological specimen was reported in 1987 [18]. Once more it was Ashkin and Dziedzic who took the development of optical tweezers to the application stage.
III. 7.1
Biological applications
Ashkin's group trapped tobacco mosaic viruses which were suspended in an aqueous solution. These viruses were cylindrical in shape with a length of --0.3 pm, effectively asymmetric Rayleigh particles. They were successfully trapped using an argon ion laser of wavelength 515 nm operating at -150 mW with no obvious damage to the viruses themselves. In these samples, two types of bacteria were seen after a few days and were identified as rod-like motile bacteria, with rotating tails for propulsion, of lengths 0.5 and 1.5 pm. These too were successfully trapped with only a few milliWatts of laser power. Full details of this first demonstration of optical trapping of biological particles can be found in Ashkin and Dziedzic's paper, reference
[18].
However, long exposure in the optical trap, even at only a few milli Watts of the green laser radiation, still caused optical damage to the biological specimens. Ashkin termed this optical damage "opticution", the death of a biological sample by light. He therefore decided to use a near infra-red laser to reduce the harmful effects caused by absorption. He was then able to
successfully trap and manipulate red blood cells, yeast and individual organelles inside the cell walls of algae using optical tweezers [32].
Optical tweezers create forces of the order of picoNewtons. Forces of tliis size are of the correct m agnitude to move and reposition cells [33], bend membranes and stop bacteria swimming freely. The study of biological structures is now greatly enhanced by the ability to manipulate samples with the aid of optical tweezers. For example, Steven Block at Harvard University used tweezers to make measurements of the elasticity of bacterial flagella or tails [34].
The tweezers technology has also been combined w ith other laser techniques, in particular, with a pulsed ultra-violet laser to create an "'optical scalpel". This allows the cutting or ablation of biological material on a microscopic scale. The fusion of cells has been performed by using optical tweezers to bring together two cells and then cutting their membranes to allow their fusion with ultra-violet light [35]. Berns and colleagues have combined optical tweezers and optical scalpels successfully [36] and also tried to use optical tweezers for in vitro fertilisation. Recently, Bruer, Gahagan, Swartzlander Jnr and Weathers from the Worcester Polytechnic Institute in Massachusetts, reported the cutting open of a plant cell with a laser scalpel and inserting a single bacterium into the cell using optical tweezers [37].
Another useful application of the tweezers has been to use micron-sized spheres as "handles" to hold molecules and biological structures. One of the most publicised experiments in this field was by Steven Block, now at Princeton University and his colleagues, who attached microspheres to the ends of a DNA strand [38]. The molecule was stretched and then released. The relaxation was used to make detailed experimental observations of the polymer behaviour. Similar work has also been undertaken by Justin Molloy and colleagues at York University [39] and Bob Simmons and co workers at King's College, London [40-42], to measure the forces of individual muscle fibres.
Multiple optical traps are also possible. These allow particles of different shapes and sizes to be orientated at will, which is not possible with a single
trap. Double tweezers systems have been used to construct arrays of latex particles into an ordered structure [43] and also to study the resonance of thermal hopping over time-varying potential barriers [44]. Similarly, dual traps have been developed within optical tweezers forming two fully steerable 3-D traps [45]. Automated cell sorters using optical traps have also been constructed successfully [46].