Molecular transport microscopy

Figure 1 shows two things a number of sketches of possible geometries for solid-state molecular transport junctions, and some electron microscopy images of actual functional transport junctions. There are two striking features to note first, the... 

Fig. 1 Sketches of break junction-type test beds for molecular transport. On the far left is a tunneling electron microscopy (TEM) image of the actual metallic structure in (mechanical) break junctions from the nanoelectronics group at University of Basel. The sketches in the middle (Reprinted by permission from Macmillan Publishers Ltd Nature Nanotechnology 4, 230-234 (2009), copyright 2009) and right (reproduced from Molecular Devices, A.M. Moore, D.L. Allara, and P.S. Weiss, in NNIN Nanotechnology Open Textbook (2007) with permission from the authors) show possible geometries for molecules between two gold electrodes, and (on the upper right) a molecule that has only one end attached across the junction...

One of the innovative applications of scanning probe microscopy for nanolithography is dip pen nanolithography (DPN). In this special technique the water meniscus formed between the tip and the substrate acts as a medium for molecular transport. The technique depends on the key phenomenon that the molecule to be deposited on the substrate (which is referred as the ink ) can be transported in a controlled way from the tip (which is initially coated with the ink) to the substrate. The molecule (the ink) to be deposited on the substrate should interact with the substrate to form a chemical bond, leading to a stable structure [82]. 

The core concept that the water meniscus at the tip-substrate contact can indeed be controlled and can be used as the molecular transport medium came from basic investigations of water meniscus on lateral force microscopy (LFM) [83]. It was... 

Finally, an example of what is not included in this contribution is, for instance, the use of scaiming electrochemical microscopy (SECM) for imaging pathways of molecular transport across skin tissues. Another topic not included, but one that has been often discussed in the literature, is the topic of electrochemical sensors in medical devices. 

SECM has also been used to measure transport dynamics in membranes (10). In this application the current at the tip is recorded as a function of time following a chemical or physical perturbation that alters the rate of transport across the membrane (e.g., a change in the electric field across the membrane). No other analytical technique or microscopy provides a comparable level of direct information about the pathways and dynamics of molecular transport in membranes. 

Watkins, A. W. Anseth, K. S. Investigation of molecular transport and distributions in polyfethylene glycol) hydrogels with confocal laser scanning microscopy. Macromolecules. 2005, 38, 1326-1334. 

Williams, M.E. Hupp, J.T. Scanning electrochemical microscopy assessment of rates of molecular transport through thin films of mesoporous films of porphyrinic molecular squares. J. Phys. Chem.. B 2001. 105 (37). 8944-8950. 

Bath, B. D., H. S. White, and E. R. Scott, Imaging molecular transport across membranes, in Scanning Electrochemical Microscopy, Bard A. J. and M. V. Mirkin, Eds. 2001, Marcel Dekker, New York, pp. 343-395. 

McKelvey, K., M. E. Snowden, M. Peruffo, and P. R. Unwin, Quantitative visualization of molecular transport through porous membranes Enhanced resolution and contrast using intermittent contact-scanning electrochemical microscopy. Anal. Chem., Vol. 83, 2011 pp. 6447-6454. 

Kim, J., A. Izadyar, N. Nioradze, and S. Amemiya, Nanoscale mechanism of molecular transport through the nuclear pore complex as studied by scanning electrochemical microscopy, J. Am. Chem. Soc., Vol. 135, 2013 pp. 2321-2329. 

Much of the difficulty in demonstrating the mechanism of breakaway in a particular case arises from the thinness of the reaction zone and its location at the metal-oxide interface. Workers must consider (a) whether the oxide is cracked or merely recrystallised (b) whether the oxide now results from direct molecular reaction, or whether a barrier layer remains (c) whether the inception of a side reaction (e.g. 2CO - COj + C)" caused failure or (d) whether a new transport process, chemical transport or volatilisation, has become possible. In developing these mechanisms both arguments and experimental technique require considerable sophistication. As a few examples one may cite the use of density and specific surface-area measurements as routine of porosimetry by a variety of methods of optical microscopy, electron microscopy and X-ray diffraction at reaction temperature of tracer, electric field and stress measurements. Excellent metallographic sectioning is taken for granted in this field of research. 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

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