Adsorbed film /layer bilayer

In order to verify that the adsorbed lipid membrane indeed forms a bilayer film, another experiment is conducted with an aim to detect the formation of a monolayer lipid. It starts with a piranha-cleaned micro-tube treated with silane to render its inner surface hydrophobic. POPC liposome is then injected into the microtube. It is known that POPC lipid will form a monolayer to such a surface by orienting their hydrophobic tails toward the hydrophobic wall. The experimental results using a mode with similar sensitivity as the previous experiment are shown in Fig. 8.39. The resonance shift in this case is 22 pm, which is about half of that observed for the adsorption of a lipid bilayer. These two experiments suggest that the microtube resonator is capable of accurately determining an adsorbed biomolecular layer down to a few nm thicknesses. 

The buildup of multilayers (HRP/PSS) on (PEI/PSS)2-modified surfaces using the common LbL assembly technique that includes washing and drying the whole film after adsorption of each layer [1-3] was also investigated by the QCM method (Fig. 2). The thickness of the adsorbed film, estimated by means of the Sauerbrey equation in dry state, is 0.77 0.7 nm for one HRP/PSS bilayer. 

Finally, we note that a growing body of evidence shows that the stability of a planar membrane can be enhanced by spreading it across a small aperture [97], For example, a DiPhyPC bilayer suspended across a 150nm radius orifice in a glass pipet remains intact when removed from buffer [150], This suggests that it may be possible to form arrays in which fluid, stable bilayer patches are surrounded by a patterned substrate that anchors the membrane. Air stability can also be achieved by coating a PSLB with a hydrophilic polymer film (e.g., a biospecifically adsorbed protein layer [23,149]). Both of these approaches maintain some degree of lateral lipid mobility in the membrane. 

Hemimicelle An aggregate of adsorbed surfactant molecules that may form beyond monolayer coverage, the enhanced adsorption being due to hydrophobic interactions between surfactant tails. Hemimicelles (halfmicelles) have been considered to have the form of surface aggregates or of a second adsorption layer with reversed orientation, somewhat like a bimolecular film. In bilayer surfactant adsorption, the term ad-micelles has also been used (20). 

The process of formation of a multilayer film on the ITO coated glass from sequential addition of PABA/RNA bilayers was observed with UV-Vis Spectroscopy as shown in Figure 3.34. The film growth observed with the deposition of additional bilayers suggests that the multilayer formation of PABA/RNA is reproducible with sequential deposition. All spectra exhibit an intense and sharp peak attributed to the w-tt and bipolaron band transitions. The bipolaron absorption band at 800 nm, associated with complexation of RNA with PABA, increases linearly with the number of PABA/RNA bilayers (Figure 3.34 inset). The linear relationship between absorbance and the number of deposited bilayers indicates that the deposition was reproducible from layer to layer, i.e., the amount of PABA adsorbed in each bilayer was the same. In addition to these results, multilayer formation was observed with ellipsometric and X-ray photoelectronic spectroscopy. The linear increase in film thickness with number of PABA/RNA bilayers was observed using ellipsometry. The average thickness of the PABA/RNA bilayer built up on a silicon substrate was approximately 10 nm. Additionally, X-ray photoelectron... 

AFM is particularly sensitive to the lateral structure in the film, and this has yielded some surprising results, showing that adsorbed surfactant layers contain aggregates that are strikingly similar to elements of complex fluid phases including spherical or globular and cylindrical micelles, branched or mesh structures, as well as (occasionally) conventional bilayers. ... 

The popular applications of the adsorption potential measurements are those dealing with the surface potential changes at the water/air and water/hydrocarbon interface when a monolayer film is formed by an adsorbed substance. " " " Phospholipid monolayers, for instance, formed at such interfaces have been extensively used to study the surface properties of the monolayers. These are expected to represent, to some extent, the surface properties of bilayers and biological as well as various artificial membranes. An interest in a number of applications of ordered thin organic films (e.g., Langmuir and Blodgett layers) dominated research on the insoluble monolayer during the past decade. 

Fig. 4.13 General scheme of dendritic macromolecules within molecular layers at interfaces (a) compressed Langmuir bilayer at air-water interface (b) adsorbed and self-assembled monolayers of neutral dendrimers (left) and dendrimers with sticky surface groups (right) (c) multilayer self-assembled films obtained by layer-by-layer deposition of dendrimers low molar mass ions (left) and two adjacent dendrimer generations (right). (From ref. [106])...

Fig. 3 Vesicle stabilization by PLL covering (a, b), followed by separation of well-covered single vesicles from excess of nonbound PLL (b, c). Native vesicles are ruptured upon adsorption on a (PLL/HA)i2/PLL film, forming a lipidic bilayer (a, d). Free non-bound PLL is preferably adsorbed on a (PLL/HA)i2 film rather than on PLL-covered vesicles (b, e). Liposome-containing film (PLL/HA)i2/Lip-PLL/HA/PLL/HAis formed by adsorption of PLL-covered liposomes (Lip-PLL) on a (PLL/HA)i2 film, followed by additional coating with HA/PLL/HA layers (c, f). Reproduced from [82]...

Lalchev et. al. [491-493] have reported results employing the FRAP method for the recovery half times (tm) and the lateral diffusion coefficients (D) of fluorophore molecules in lecithin foam films of different type. Significant differences between the values of D were obtained for very thick foam films (h 100 nm) and for grey foam films (h 30 nm) showing D values of 2210 8 and 8-10 8 cm2 s 1, respectively. A further decrease in D was observed in CBF (D = 51 O 8 cm2 s 1) and in NBF (D = 2.2-10 8 cm2 s 1) (Fig. 3.111). The CBFs have an equivalent water thickness of approximately 13 nm and consist of a free water layer between the two adsorbed layers according the three-layer model (see Chapter 2). The value of the lateral diffusion coefficient in NBF, characterised by an equivalent water thickness of approximately 7 to 8 nm (the thinnest foam bilayers in this case) and which contains no free water layer between the monolayers, was twice lower (D 210 8 cm2 s 1), than in the CBF (Fig. 3.111). Since the decrease of the film thickness reflects the decrease of the free-water... 

In 1991, Johnson et al. reported one of the first NR studies of phospholipid bilayers at the solid-solution interface [46]. Although these measurements were not the first to employ NR to study molecules adsorbed at the solid-hquid interface, they did constitute the first measurements of a supported bilayer using NR. A bilayer of dimyristoylphosphatidylcholine (DMPC) was spread on a quartz surface by the fusion and rupturing of smaU unilamellar vesicles. The very smooth, singlecomponent substrate aUowed a complex model of the interface to be constructed from layers corresponding to (i) the quartz, (ii) a thin film of water on the quartz... 

A related, but alternative, approach uses layer-by-layer adsorption driven by hydrogen-bonding interactions [51]. This has been accomplished with polyvinylpyrrohdone, polyvinyl alcohol, polyacrylamide, and polyethylene oxide. In the case of polyaniline, comparisons with films assembled via the electrostatic mechanism, using sulphonated polystyrene, indicate that the nonionic polymers adsorb onto polyanihne with a greater density of loops and tails and form highly interpenetrated bilayers with high polyaniline content. 

Investigated examples include the determination of the spatial distribution of a polymer, solvent and mobile species in poly( -toluidine) [983, 984] and polybithiophene [989] films, film swelling and solvent content in electroactive films containing transition metal complexes [988, 985], postdeposition modified electroactive polymers [986] and organic adsorbate layers [987]. The method allows also the investigation of buried interfaces in bilayer systems of various polymers [988]. 

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