Molecular combing

Sample preparation for AFM analysis is relatively simple. Generally, a desired amount of sample is absorbed onto a smooth and clean substrate surface, for example, a freshly cleaved mica surface. For example, to prepare a food macromolecule sample for AFM imaging in air, the diluted macromolecule solution is disrupted by vortexing. Then, a small aliquot (tens of microliters) of vortexed solution is deposited onto a surface of freshly cleaved mica sheet by pipette. The mica surface is air dried before the AFM scan. A clean surrounding is required to avoid the interference of dust in the air. Molecular combing or fluid fixation may be applied to manipulate the molecule to get more information. 

We used molecular combing to determine Young s modulus for individual microfibrils, and X-ray diffraction of zonular filaments of the eye to establish the linearity of microfibril periodic extension (Sherratt et al, 2003). Microfibril periodicity is not altered at physiological zonular tissue extensions, and Young s modulus is between 78 to 96 MPa, MPa, which is two orders of magnitude stiffer than elastin. Thus, elasticity... 

Figure 11 Atomic Force Microscopy image of aligned PTAA coated insulin fibrils that have been transferred to a glass surface after molecular combing directly onto the surface of a PDMS stamp. The lines have been added to illustrate gaps within the PDMS stamp. The scale bar is 2 mm in length (Herland, Bjdrk et at, 2007). Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.

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

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 Interpretation. Stresses in a polymer melt may lead to alignment of the polymer chains along the direction of the imposed stress vector. A reduction in entanglements accompanies this orientation, so that translational motion of the polymer is enhanced. The tip is thought to behave as a molecular comb which orients the polymer chains parallel to the tip trajectory. Successive visits of the tip to the same polymer chains result in greater net orientation of the chains and hence enhanced translational mobility. 

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

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... 

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). 

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. ... 

Initially, molecular combing was used to comb a nanotube over a pair of gold electrodes. Self-assembled monolayers (SAMs) on the electrodes and the silicon substrate were used to help induce the SWCNT (dispersed via sodium dodecyl sulphate [SDS]) to deposit at the correct location the liquid-air meniscus was then used to orient the nanotube across the two electrodes (Fig. 16.2b). This method has the advantage of being easily scalable. However, contact with the nanotube is superior when the electrode is deposited with e-beam lithography over the nanotube. Because molecular combing is a very convenient way to extend nanotubes on the substrate, it is often used in this case as well. These studies have demonstrated... 

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,... 

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