Optical tweezers principles

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. 

To begin with it is best to place the field of nanolithography and its companion nanomanipulation in perspective. This is shown in Figure 21.2. Nanomanipulation, in principle, should also include manipulation using forces of self-assembly or other chemical forces and manipulations using optical tweezers. However, the word nanomanipulation is often used in a limited context where a SPM tip is used for manipulation of a nano-object. We stick to this definition, partly to reduce the scope of the review and partly because other manipulations are not in the area of expertise of the author. 

Figure 12.7 CaLibration of trap stiffness by thermal noise analysis. A single 1.1 jim bead is held in the optical tweezers and data is collected at 2 kHz. (a) The graph shows the bead position vs time -solid lines denote 1 standard deviation of bead position, (b) The same data plotted as a histogram. The mean displacement is 0 nm and the variance is determined by the vibration due to brownian noise, (c) The trap stiffness can be determined from this information using the equipartition principle lf2Ktrsj, x ) = l/2ksT, where Ktrap trap stiffness, (x ) = variance, /fB= Boltzman constant and T= absolute temperature...

Molecular probes, such as optical or magnetic tweezers,64-71 micropipets,72 and microfibers,73-74 have been developed to manipulate single molecules and to measure their response to mechanical actions such as stretching, torsion, and compression. A force resolution down to 0.1 pN enabled quantitative measurement of the molecular forces and provided novel information on the basic principles of folding, motion, and interactions of individual molecules. Complementary to the local mechanical probes, actions of external fields were monitored on individual polymer molecules.75 77... 

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