Single molecule/particle manipulation

One of the big problems in single-molecule experiments is how to keep the molecule in place long enough to make observations on it, as well as how to grab hold of different parts of the molecule in order to manipulate it. In AFM and similar techniques the molecules are immobilized by being adsorbed on or attached to macroscopic surfaces. Another method relies on the properties of finely focused laser beams to act as optic il twcezersor optical trap-s to manipulate microscopic particles suspended in solution. 

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... 

Bacteriophage. A small particle, composed of protein and RNA or DNA, that can infect, replicate in, and release from bacterial cells. Bacteriophage particles can be used to package DNA molecules into single-stranded forms for further manipulation and modification. 

Artificial lipid bilayer membranes can be made [22,23] either by coating an orifice separating two compartments with a thin layer of dissolved lipid (which afterwards drains to form a bilayered structure—the so-called black film ) or by merely shaking a suspension of phospholipid in water until an emulsion of submicroscopic particles is obtained—the so-called liposome . Treatment of such an emulsion by sonication can convert it from a collection of concentric multilayers to single-walled bilayers. Bilayers may also be blown at the end of a capillary tube. Such bilayer preparations have been very heavily studied as models for cell membranes. They have the advantage that their composition can be controlled and the effect of various phospholipid components and of cholesterol on membrane properties can be examined. Such preparations focus attention on the lipid components of the membrane for investigation, without the complication of protein carriers or pore-forming molecules. Finally, the solutions at the two membrane interfaces can readily be manipulated. Many, but not all, of the studies on artificial membranes support the view developed in the previous sections of this chapter that membranes behave in terms of their permeability properties as fairly structured and by no means extremely non-polar sheets of barrier molecules. 

Flow-Based Particle Trapping and Manipulation, Fig. 7 Time-lapse image of the relaxation dynamics of an individual single-stranded DNA molecule (ssDNA, L = 18 jm) studied by the hydrodynamic trap based on a planar extensional flow. A single fluorescently labeled... 

The Stark decelerator (or accelerator) for neutral polar molecules is the equivalent of a linear decelerator (or accelerator) for charged particles. The Stark decelerator exploits the quantum-state specific force that a polar molecule is subjected to in an electric field. This force is rather weak, typically some eight to ten orders of magnitude weaker than the force that the molecule, when singly ionized, would experience in an equivalent electric field. Nevertheless, this force suffices to exert a complete control over the motion of polar molecules using principles akin to those developed to manipulate charged particles. 

The relationship between macroscopic properties and molecular properties is a major area of interest, since it is through manipulation of the molecular structure of me-sogens, that the macroscopic liquid crystal properties can be adjusted towards paricu-lar values which optimize performance in applications. The theoretical connection between the tensor properties of molecules and the macroscopic tensor properties of liquid crystal phases provides a considerable challenge to statistical mechanics. A key factor is of course the molecular orientational order, but interactions between molecules are also important especially for elastic and viscoelastic properties. It is possible to divide properties into two categories, those for which molecular contributions are approximately additive (i.e. they are proportional to the number density), and those properties such as elasticity, viscosity, thermal conductivity etc. for which intermolecular forces are responsible, and so have a much more complex dependence on number density. For the former it is possible to develop a fairly simple theory using single particle orientational order parameters. 

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