Lateral Diffusion Process Research Articles

Lateral diffusion in organized bilayer assemblies of electroactive amphiphiles: Influence of the oxidation state of the amphiphile investigated by steady-state methods involving an interdigitated micro-electrode array device

Interdigitated micro-electrode array (IMA) devices were used to investigate lateral diffusion processes in amphiphilic bilayer assemblies. Photolithographically fabricated arrays consisted of 50 pairs of 800 μm long, 4 μm wide electrodes spaced by 4 μm. The bilayer formation involved two self-assembly steps in which a monomolecular layer of octadecyltrichlorosilane (OTS) is first covalently anchored to the IMA surface, and then a monolayer of octadecylviologen (C 18MV 2+) is formed. The typical coverage of the viologen amphiphile was 2–3 × 10 −10 mol/cm 2 in 0.5 M KCl electrolyte, and was demonstrated to be uniform across the glass inter-electrode gap and Au micromelectrode surfaces of IMA devices. Lateral charge transport dynamics were investigated by generator/collector voltammetry. Computer simulations showed how the shape of the generator/collector current depends on the ratio of the microelectrode and the gap width ( w/d), the ratio of the diffusion coefficients D o/ D r, and the scan rate. Fast scan cyclic voltammetry was used to resolve spatially the coverage of C 18MV 2+ and C 18MV .+ at the electrodes and in the inter-electrode gaps. This technique was used to interpret the steady-state generator/collector current in terms of the individual fluxes of the oxidized and reduced octadecylviologen. The dissimilarity of the diffusion coefficients ( D o > D r) leads, at steady-state, to a net migration of octadecylviologen from the anode to the cathode so that a larger gradient of C 18MV .+ compensates for the lower value of D r. The diffusion coefficients D o and D r were determined as a function of the mole fraction of the reduced octadecylviologen ( x r ) in the bilayer. The magnitude of D o decreases linearly from 7.1 × 10 −8 cm 2/s to 1.2 × 10 −8 cm 2/s as x r increases from 0 to 1. The value of D r = 1.2 × 10 −8 cm 2/s is constant over the entire range of x r. These data reveal the dependence of D o on the micro-fluidity of the octadecylviologen assembly which decreases linearly with the increasing concentration of C 18MV +. The independence of D r on the redox composition of the assembly suggests that low effective charge in the head group region of the reduced species is responsible for their low mobility which is, apparently, limited by drag interactions with the underlying OTS layer. Computer simulations of chronocoulometric experiments were also used to probe the effect of changing diffusion coefficient with the redox composition in the diffusion layer throughout the progress of an experiment. The simulation results demonstrated that it is difficult to differentiate between the negative curvature in Anson plots caused by the decreasing diffusion coefficient, and a similar curvature expected due to the progressive break-down of the semi-infinite character of diffusion in an electrode film of finite thickness. This highlights the utility of the steady-state approach developed in this work in obtaining a more detailed understanding of the molecular dynamics involved in lateral charge transport in bilayer assemblies.

Vertical and lateral turbulent dispersion: some experimental results

The results of an experimental programme designed to investigate turbulent dispersion of a continuous contaminant source in a wide channel are presented. Both two-dimensional vertical dispersion and the determination of the lateral diffusion coefficient are described. The eigenfunction solution to the turbulent diffusion equation, presented in Nokes et al. (1984) and discussed in greater detail in Nokes (1985), is strongly supported by the results of vertical mixing described here. A variety of source locations are examined in this study and the location of the ideal source, predicted by theory, is verified by the experimental results. For the two smooth-bed flows investigated the depth-averaged values of εz, deduced from the rates of lateral spreading of the plume, lie at the lower end of the range of values obtained by other researchers. Considering only the results obtained in wide channels, the authors demonstrate that previously published values of the lateral diffusion coefficient, non-dimensionalized by the shear velocity u* and the flow depth d are independent of all flow parameters except the friction factor f = 8u*/ū where ū is the mean velocity in the flow. Indeed, above a value of f = 0.055 εz/u*d is also found to be independent of f, and takes a value of 0.134. A brief mathematical analysis of the three-dimensional mixing processes in the near-source region is presented, and utilized to investigate the coupling between the lateral and vertical diffusion processes in this region. Based on these mathematical arguments the experimental results imply that the vertical and lateral diffusion processes are essentially uncoupled in the near-source zone, and thus the lateral diffusivity and longitudinal velocity have similar vertical dependence.

Mobility of enzymes on insoluble substrates: The β‐amylase–starch gel system

AbstractThe movement of enzymes along the surfaces of biopolymers containing enzyme‐susceptible sites can be described as a lateral diffusion process characterized by an apparent diffusion coefficient [E. Katchalski‐Katzir, J. Rishpon, E. Sahar, R. Lamed, and Y. I. Henis (1985) Biopolymers 24, 257–277]. Studies on the diffusion of enzymes on biopolymer substrates can therefore provide important information on the mechanism of enzyme–biopolymer interaction. For this reason, the motion of fluorescently labeled β‐amylase [α‐D‐(1 → 4)glucan maltohydrolase; E.C. 3.2.1.2] on the surface of starch gels was studied by fluorescence photobleaching recovery. The results indicate that the motion of β‐amylase on the surface of the gel substrate occurs by both lateral diffusion along the surface (over micron distances) and exchange between bound and free enzyme molecules in the solution covering the gel, and that the two processes occur concomitantly and in a random manner. Surface diffusion also appears an important process with respect to the action of the enzyme on the substrate sites, since this component of the motion disappears upon inactivation of the enzyme, leaving only exchange to contribute to the measured motion.

Inorganic resist phenomena and their applications to projection lithography

The mechanisms responsible for latent image formation in Ge <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Se <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> resist are studied using computer simulation and laboratory experiments. The conditions for the occurrence of contrast enhancement, edge sharpening, feature-dependent amplification, and feature-dependent photodoping suppression are identified. Their role in optical projection lithography is characterized in terms of the Ag diffusion length, exposure time, and Ag <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Se layer thickness. By appropriate selection of exposure and film thickness to control the lateral diffusion and photobleaching process, proximity effects caused by light diffraction can be compensated, small conventionally unresolved features can be printed, and the printability of open defects can be reduced. In the high-throughput regime, however, compensation cannot be obtained and the resolution is limited by the aerial image quality of the projection printer.

Interpretation of fluorescence photobleaching recovery experiments on oriented cell membranes

Both quantitative and qualitative aspects of the interpretation of fluorescence photobleaching recovery experiments as typically practised to obtain information on lateral diffusion processes in cell membranes are called into question in view of the polarized nature of the laser light sources routinely employed. Protocols which will eliminate the effects elicited under these conditions by any concurrent slow rotational diffusion are delineated.

Membrane specializations of neuritic growth cones in vivo: A quantitative IMP analysis

The internal structure of the membranes of axonal and Schwann cell growth processes was examined by freeze-fracture in the growing olfactory nerve, a simple in vivo system consisting of a homogeneous neuronal population. Excision of the mature nerve of adult bullfrogs provides well-synchronized primary neuritic outgrowth that is highly enriched in growth cones at its distalmost segment. The extreme uniformity of olfactory axons in terms of their diameter and their intramembrane particle (IMP) composition permits clear identification of the cellular origin of the growth cone structures observed in replicas. In vivo, growth cones of the olfactory nerve appear as irregularly shaped enlargements of the distal tip; filopodia are only infrequently exposed by the fracture plane. Axonal and Schwann cell growth cones are distinguished by 1) the larger size of the Schwann cell growth cone and the smaller diameter of its attached processes, and 2) the distinct differences in IMP composition of Schwann cell and axonal growth cones and cell processes. Schwann cell growth cones display a uniformly high IMP density on their P-face leaflet, with the exception of circumscribed moundlike protrusions that are relatively free of IMPs. In contrast, axonal growth cones display sharp regional variation in IMP density on their P-face: broad regions almost devoid of IMPs are interspersed with zones of high IMP density. Cytotic profiles occur within high IMP density zones located, most often, at the base of the axonal growth cone. A comparison of IMP size histograms of both high and low-density regions of axonal growth cones and that of the neighboring distal shaft of the axon indicates a strict partitioning of membrane components between these two regions. The IMP profile of the axonal growth cone, notable for its relative enrichment in large-diameter particles, suggests that IMP components of the growth cone are delivered to the distal tip by a mechanism that is distinct from the lateral diffusion process described for particles of the growing axon's shaft [cf. Small and Pfenninger, 1984]. The IMP profile of the concave P-face leaflet of the internal vesicles found clustered at the base of the growth cone is more similar in composition to the profile of the neuronal shaft than that of the growth cone.

Three-channel buried-crescent InGaAsP laser with 1.51 μm wavelength on semi-insulating InP

Laser diodes with current threshold as low as 16 mA for lasers of 300 μm length and channel width of 2-3 μm were obtained with operational wavelength of 1.51 μm. These lasers are fabricated with one liquid-phase-epitaxy growth step and a lateral zinc diffusion process. Current confinement is achieved by use of the semi-insulating substrate material.

33] Pyrenedecanoic acid and pyrene lecithin

Publisher Summary This chapter describes the synthesis and biophysical properties of pyrenedecanoic acid and pyrene lecithin. The excimer-forming lipids pyrenedecanoic acid and pyrene lecithin are used to investigate the dynamic properties and the structural organization of both artificial and biological membranes. Pyrenedecanoic acid and pyrene lecithin have been used to investigate lipid lateral diffusion and lipid exchange processes, as well as phase separation phenomena in artificial and natural membranes. The lipids to be studied and a given amount of excimer-forming probe were dissolved in chloroform and evaporated under a stream of nitrogen to yield a film on the wall of the glass vessel. The excimer formation technique is not restricted to the measurement of lateral mobility in membranes. It can also be used to determine the transverse mobility— that is, the lipid exchange between the lipid layers of one bilayer or between bilayers of different vesicles. It is found that because of the changes in fluidity and a modified solubility of the pyrene probes within different membrane regions, this method can also be applied to the investigation of phase separation phenomena and to lipid-protein interactions.

Phospholipids and membrane transport.

The ability of membrane lipids to adopt nonbilayer configurations suggests dynamic roles for lipids in many functional abilities of biological membranes. In this work evidence supporting the involvement of lipids in three types of membrane transport process is presented and discussed. These transport processes include facilitated transbilayer transport of polar molecules, transport mechanisms involving fusion events, and transport possibilities arising from alternative membrane morphology. In particular it is shown that lipids such as cardiolipin, which adopt the hexagonal HII phase in the presence of Ca2+, may be logically proposed to facilitate Ca2+ transport across membranes via an inverted micellar intermediate. Alternatively, in transport processes such as exocytosis the ability of Ca2+ to generate membrane instabilities favouring nonbilayer alternatives suggests a crucial role of phospholipid in the fusion event vital to exocytotic release. Finally, nonbilayer lipid structures may be suggested to favour formation of isolated compartments connected by a continuous membrane where lateral diffusion processes can lead to transport. These various possibilities are summarized in a "metamorphic mosaic" model of biological membranes.

A further analysis of reciprocating flow in phloem tubes

A more stringent mathematical treatment of reciprocating flow in phloem tubes has shown that the original model (Miller 1973) would be too inefficient to provide a quantitative explanation for the rate of solute dispersion within the tube. A modification is suggested in which the mobile phase consists of the sieve tube sap, and the companion cells constitute the static phase. Lateral diffusion of the solute between these two phases could occur through the cell wall barrier separating them via plasmodesmata and would follow first-order kinetics. It is shown that for optimum dispersion in such a model, the period of oscillation should be about 3.6 times the half life of the lateral diffusion process. Estimates of the various parameters involved indicate that such a mechanism, if driven by protoplasmic streaming, predicts a dispersion coefficient of the same order as those found experimentally.

Steric exclusion and lateral diffusion in gel‐permeation chromatography

AbstractThe peak separation in gel‐permeation chromatography (GPC) is attributed to the contributions of the steric exclusion and the lateral diffusion processes. The advantage of using the distribution coefficient KGPC of the solute molecule in interpreting the GPC separation mechanism is assessed. The physical significance of KGPC and its relation to measurable GPC parameters are examined in detail. A simple mixing experiment for determining the exclusion effect is described. The results of this experiment, as well as those of the flow rate study, show that the exclusion effect plays the primary role in GPC peak separation. For a column packed with Bio‐Rad porous glass of 200 Å designation, the diffusion effect does not contribute significantly to peak separation. However, for the case of a Waters Associates column packed with polystyrene gel of 104 Å designation, both the exclusion and the diffusion effects are shown to be important. A diffusion theory which includes the concept of a restricted diffusion coefficient is proposed to interpret the diffusion effect observed in the polystyrene gel column. The results of the theoretical calculation are found to agree with the observed flow rate dependence of the calibration curve.