Optical tweezers applications

Experimental techniques based on the application of mechanical forces to single molecules in small assemblies have been applied to study the binding properties of biomolecules and their response to external mechanical manipulations. Among such techniques are atomic force microscopy (AFM), optical tweezers, biomembrane force probe, and surface force apparatus experiments (Binning et al., 1986 Block and Svoboda, 1994 Evans et ah, 1995 Israelachvili, 1992). These techniques have inspired us and others (see also the chapters by Eichinger et al. and by Hermans et al. in this volume) to adopt a similar approach for the study of biomolecules by means of computer simulations. 

We have applied FCS to the measurement of local temperature in a small area in solution under laser trapping conditions. The translational diffusion coefficient of a solute molecule is dependent on the temperature of the solution. The diffusion coefficient determined by FCS can provide the temperature in the small area. This method needs no contact of the solution and the extremely dilute concentration of dye does not disturb the sample. In addition, the FCS optical set-up allows spatial resolution less than 400 nm in a plane orthogonal to the optical axis. In the following, we will present the experimental set-up, principle of the measurement, and one of the applications of this method to the quantitative evaluation of temperature elevation accompanying optical tweezers. 

Seeger, S., Monajembashi, S., Hutter, K.-J., Futterman, G., Wolfrum, J and Greulich, K. O. (1991) Application of laser optical tweezers in immunology and molecular genetics. Cytometry 12,497-504. 

Near-field Raman imaging with a scanned probe has been reported [18, 19]. However, the technique is painfully slow (5-10 h, even for strong scatterers) and it has found very little use. Acquisition time can be decreased by using a polystyrene bead as a very high numerical aperture immersion lens. Working at 532 nm, Kasim et al. used a 60x/1.2 immersion aperture as an optical tweezer to simultaneously position the bead and operate it as a high NA lens [20]. They obtained a spatial resolution of about 80 nm on doped silicon with a few minutes scan time (Fig. 5.1). However, because of the need for a relatively smooth surface and a very intense scatterer, this technique is not likely to find much application in biomedical or pharmaceutical applications [21, 22],... 

Raman probes. SERS can then be performed on optically induced aggregates of the trapped particles. Alternatively, metal nanoparticles can also be attached on micron-sized dielectric beads, which are much easier to trap. Raman probes can be adsorbed on the surface of the metal nanoparticles. In addition, combined with other techniques, such as microfluidics, the applicability of optical tweezers for SERS can be even more expanded. 

S.M. Block, Making light work with optical tweezers. Nature 360, 493-495 (1992) K. Svoboda, S.M. Block, Biological applications of optical forces. Annu. Rev. Biophys. Biomem. 23, 247-285 (1994)... 

E. Fallman, S. Schedin, J. Jass, M. Andersson, B.E. Uhhn, O. Axner, Optical tweezers based force measurement system for quantitating binding interactions system design and application for the study of bacterial adhesion. Biosens. Bioelectron. 19, 1429-1437 (2004)... 

The optical tweezers technique has been a powerful tool for biological application. We believe that precise control of the cellular microenvironment and single-cell analysis provide opportunities to predict the effects of external stimuli including cell-cell, cell-ECM and cell-soluble factor interaction on the cell behavior and fate, which are link to revealing the internal cellular signaling system. There still exists a broad distribution of cell responses even by single-cell analysis. Researchers need to improve and develop the technique to one utilizable for a precise analysis. The... 

Stokes calibration involves applying a known viscous drag force to a bead held in the optical tweezer and recording how far it is displaced from the tweezer centre. Application of a triangle wave oscillation of known size and frequency to the specimen chamber with the piezoelectric substage produces a viscous drag force given by Stoke s law... 

Optical tweezers have since been used to stretch and even unwind DNA unfold the tertiary structure of proteins pull on DNA processing enzymes like RNA polymerase as it walks along DNA resist the force produced by viral capsid packaging motors and manipulate entire mammalian cells, sperms and sub-cellular organelles and vesicles. The applications are manifold and the future possibilities are great. 

As amphiphilic block copolymers tend easily to form vesicles rather than tubes, Helmerson et al. reported another strategy for the formation of long polymer nanotubes. They started with polymersomes, which are cross-linked vesicles of amphiphilic block copolymers filled with water. Pulling on the membrane of those polymersomes with a micropipette or with optical tweezers led to the formation of water containing nanotubes stretched from the vesicles (Figure 5.46). However, this process required the application of a lot of force. To overcome this problem, they incorporated a detergent-like triblock copolymer in the formation of the polymersomes, with the effect that the membranes were rendered more fluid. 

The high costs associated with specialist ultrafast laser techniques can make their purchase prohibitive to many university research laboratories. However, centralised national and international research infrastructures hosting a variety of large scale sophisticated laser facilities are available to researchers. In Europe access to these facilities is currently obtained either via successful application to Laser Lab Europe (a European Union Research Initiative) [35] or directly to the research facility. Calls for proposals are launched at least annually and instrument time is allocated to the research on the basis of peer-reviewed evaluation of the proposal. Each facility hosts a variety of exotic techniques, enabling photoactive systems to be probed across a variety of timescales in different dimensions. For example, the STFC Central Laser Facility at the Rutherford Appleton Laboratory (UK) is home to optical tweezers, femtosecond pump-probe spectroscopy, time-resolved stimulated and resonance Raman spectroscopy, time-resolved linear and non-linear infrared transient spectroscopy, to name just a few techniques [36]. 

Since their invention, optical tweezers have proved themselves to be very powerful interdisciplinary tools. Today they are used extensively in biophysics, as they serve as delicate tools to manipulate and study single molecules of DNA [8, 9]. Optically trapped beads have been successfully used to measure local elasticities and viscosities, for example inside cells. Ashkin [5] provides an overview of the diverse uses of optical traps as an important tool in the important areas of research. Isolation and detection of sparse cells concentration of cells from dilute suspensions separation of cells according to specific properties and trapping and positioning of individual cells for characterization are the key areas of research due to their possible impact. The non-invasive nature of particle manipulation being a key requirement, forces like hydrodynamic, optical, ultrasonic and electromagnetic have been employed for such purposes. Successful examples of the use of these optical forces for biological applications are determination... 

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