Optical trapping method

Monodisperse spherical colloids and most of the applications derived from these materials are still in an early stage of technical development. Many issues still need to be addressed before these materials can reach their potential in industrial applications. For example, the diversity of materials must be greatly expanded to include every major class of functional materials. At the moment, only silica and a few organic polymers (e.g., polystyrene and polymethylmethacrylate) can be prepared as truly monodispersed spherical colloids. These materials, unfortunately, do not exhibit any particularly interesting optical, nonlinear optical or electro-optical functionality. In this regard, it is necessary to develop new methods to either dope currently existing spherical colloids with functional components or to directly deal with the synthesis of other functional materials. Second, formation of complex crystal structures other than closely packed lattices has been met with limited success. As a major limitation to the self-assembly procedures described in this chapter, all of them seem to lack the ability to form 3D lattices with arbitrary structures. Recent demonstrations based on optical trapping method may provide a potential solution to this problem, albeit this approach seems to be too slow to be useful in practice.181-184 Third, the density of defects in the crystalline lattices of spherical colloids must be well-characterized and kept below... 

The optical trapping method uses a highly focused laser beam to trap and manipulate particles of interest in a medium (illustrated in Figure 3). The laser is focused on a dielectric particle (e.g., a silica microscopic bead), the refractive index of which is higher than the suspension medium. This produces a light pressure (or gradient force), which moves the particle towards the focal point of the beam, that is, the beam waist (Lim et al., 2006). 

Optical trapping is a very sensitive method that is capable of the manipulation of sub-micron particles such as individual viruses and bacteria. Figure 4 is an illustration of the use of the optical trapping method to measure the elastic properties of RBCs (Mills et al., 2004). 

One of the advantages of the optical trapping method is that there is limited physical contact with the cell, although it is possible that it could be damaged by the laser (Lim et al, 2006). In addition, it is possible to measure forces in the sub pico-Newton range, which is extremely difficult to do by other methods. However, these small forces mean that it is not appropriate for investigating the response of samples to large deformations, especially if the sample has a cell wall. 

In addition to using compression testing, the Young s modulus of bacterial macrofibers of Bacillus subtilis has been determined to be 50 MPa using the optical trap method (Mendelson et al., 2000). 

Note These optical trapping methods reduce the velocity components (Vx, Vy,v ) to a small interval around v = 0. However, they do not compress the atoms into a spatial volume, except if the dispersion force for the field gradients V / 0 is... 

With the development of single molecule techniques using atomic force microscopy and/or optical trapping method, the experimental approach to the mechanical properties of helical polypeptides has been revived along with forced unfolding of globular proteins as stated earlier in this chapter. 

We have also developed a method of measurement for local temperature in microspace with a fluorescence correlation technique. Using this method, the temperature elevation at the optical trapping point due to absorption of the NIR trapping beam by solvent was quantitatively evaluated the temperature at the trapping point increased linearly with increase in the incident NIR light, and the temperature elevation coefficient was mainly dependent on two physical parameters of the solvent the absorption coefficient at 1064 nm and the thermal conductivity. 

S. B. Smith, Y. Cui, and C. Bustamante, An optical-trap force transducer that operates by direct measurement of light momentum. Methods Enzymol. 361, 134-162 (2003). 

Another approach for the formation of radical anions by LFP has been developed to overcome some of these difficulties. The approach involves the formation of radical anions by trapping a solvated electron produced by photoionization of 4,4 -dimethoxystilbene (DMS) to its cation radical (equations 31 and 32). This photoionization/electron trapping method is quite general for substrates that are transparent where DMS absorbs and that are more easily reduced than dimethoxystilbene. In many ways, this method is similar to pulse radiolysis, another useful approach used to generate radical anions for optical kinetic studies. 

Fig. 9.4.25 Optical absorption spectra of copper colloid prepared by the gas flow-solution trap method as a function of lime development. The numbers in the figure are the time after the preparation of Lhe sample. The spectrum of sodium eihoxidc in ethanol (authentic sample) is also shown in the same figure, marked by b. The insertion is the expansion of the region of the isosbestic point. The deviation from the isosbestic point at 10 h after the preparation of colloids is shown by a in the insert. (From Ref. 26.)...

Lang, M. J., Fordyce, P. M., Engh, A. M., Neuman, K. C., and Block, S. M. (2004). Simultaneous, coincident optical trapping and single-molecule fluorescence. Nat. Methods... 

Observing single molecules. A major advance in the study of molecular motors has been the development of ways to observe and study single macromolecules. The methods make use of optical traps (optical "tweezers") that can hold a very small ( 1 pm diameter) polystyrene or silica bead near the waist of a laser beam focused through a microscope objective. ° ... 

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