Molecular Combing of DNA

Here we outline the work that has been carried on molecular combing of DNA and single-walled CNTs (SWCNTs). These studies describe many useful methods and present applications and move toward understanding the mechanism (see also Table 16.1 for a summary of molecular combing studies). 

DNA (Fig. 16.1d) was shown to be a feasible means of learning the general location of genes. Many insights into the relative locations of genes have been produced for a detailed review see Herrick and Bensimon.  

Later investigations aimed to perfect the technique of molecular combing through improvements such as combing on unmodified hydrophilic surfaces/ inside microchannels (Fig. 16.1e)/ and, together with soft lithography, in precisely defined arrays (Fig. 16.1f. Such precise placement of DNA has also become important for microelectronics applications, since chemical methods have been devised to transform combed DNA to nanowires of various materials, such as copper and platinum for a review, see Stoltenberg and Woolley.  

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Liquid deposition of SWNT networks presents another major step toward the inclusion of SWNTs into manufacturable device structures. This is due to the fact that liquid deposition, combined with a laminar flow drying technique allows the formation of highly aligned SWNTs electrical networks (15, 16). In a maimer similar to molecular combing of DNA, high purity air is applied to the air/liquid interface in a manner which aligns the SWNTs in aqueous suspension and then deposits them on a desired substrate. 

Molecular Combing of DNA and Carbon Nanotubes by a Moving Meniscus... 

We consider two models for deposition kinetics. The first (case I) assumes that depositing nanotubes increase surface charge homogeneously. The corresponding surface potential and potential barrier, AE, also increase. As AE increases, the deposition rate is reduced. This model is useful for cases where deposition is extensive or the salt concentration is very low, although as we will see below, it was not applicable to molecular combing of DNA-SWCNTs. 

The phenomenon of molecular combing of SWCNTs dispersed with SDS was studied further by Ko et a/. They investigated how surface preparation and solution conditions can be used to control the density of nanotubes and demonstrated that, just like DNA,... 

The second kinetics formulation is applicable to cases where deposition density is low. In this case (case 11), we assume that the surface has a maximum number of sites that the DNA-SWCNT can occupy and that deposition gradually slows as sites are Tilled. The number of such sites is limited because deposited molecules, by virtue of not aggregating in solution, must necessarily repel other molecules and thus block part of the surface by depositing. Such processes are governed by random sequential absorption (RSA) theory. Kinetics formulations have been developed for some special geometries, but these are not applicable to molecular combing of long molecules. 

Fig. 3 Schematic mechanism of DNA molecular combing. The meniscus generates a surface tension during evaporation, which stretches DNA. Based on [4]...

Although molecular combing has been demonstrated with a variety of high-aspect-ratio molecules, here we focus on the combing of two such molecules, DNA and carbon nanotubes (CNTs). The stretching of DNA molecules is attractive for sequencing applications... 

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