Thin films protein-stabilized

We have observed that such proteins as CaM and bovine serum albumin (BSA) can be developed at the air-water interface to form monolayer protein films. In previous works, the developed BSA monolayer was stabilized by cross-linking with a bifunctional reagent immediately after the preparation of protein monolayer. The BSA thin film thus prepared can be employed as a passive material, e.g., an ultrathin protein film for a matrix of enzyme-linked immunosorvent assays. 

The higher surface area of CNTs can support a much higher density of enzymes than previous approaches such as thin films. Their high aspect ratio aids in the retention of enzyme-CNT conjugates in the matrix. CNTs also enhance the stability of adsorbed proteins relative to micro- or macro-scale supports, thereby helping to preserve or enhance enzyme bioactivity in the nanocomposites (Wang,... 

In contrast, the adsorbed layer in protein-stabilized thin films is much stiffer and often has viscoelastic properties [3]. These derive from the protein-protein interactions that form in the adsorbed layer (Figure 1(b)). These interactions result in die formation of a gel-like adsorbed layer in which lateral diffusion of molecules in the adsorbed layer is inhibited. Multilayer formation can also occur. This serves to further mechanically strengthen the adsorbed layer. 

Figure 1. Schematic diagram showing the possible mechanisms of thin film stabilization, (a) The Marangoni mechanism in surfactant films (b) The viscoelastic mechanism in protein-stabilized films (c) Instability in mixed component films. The thin films are shown in cross section and the aqueous interlamellar phase is shaded.

The solution diffusion properties of FITC-labelled BSA were measured by FRAP [12], The results showed that the protein diffused freely in solution with a diffusion coefficient of approximately 3xl0 7 cm2/s. This was in reasonable agreement with previously published values [36]. FRAP measurements were also made on thin films stabilized by FITC-BSA. The films were allowed to drain to equilibrium thickness before measurements were initiated. Thin films covering a range of different thicknesses were studied by careful adjustment of solution conditions. BSA stabilized films that had thicknesses up to 40 nm showed no evidence of surface diffusion as there was no return of fluorescence after the bleach pulse in the recovery part of the FRAP curve (Figure 14(c)). In contrast, experiments performed with thin films that were > 80 nm thick showed partial recovery (55%) of the prebleach level of fluorescence (Figure 14(b)). This suggested the presence of two classes of protein in the film one fraction in an environment where it was unable to diffuse laterally, as seen with the films of thicknesses < 45 nm, and a second fraction that was able to diffuse with a calculated diffusion coefficient of lxlO 7 cm2/s. This latter diffusion coefficient was 3 times slower than that... 

Figure 17. A summary of the bulk foam stability ( ), equilibrium thin film thickness (o), and FITC-a-la surface diffusion (A) as a function of molar ratio of Tween 20 to protein (R). The concentration of a-la was 0.5 mg/ml (35.4 piM). Reproduced from reference [41] with the permission of VCH Verlagsgesellschaft.

An improvement in foam stability was observed as R was increased to >0.15 (Figure 17). This was accompanied by the onset of surface diffusion of a-la in the adsorbed protein layer. This is significantly different compared to our observations with /8-lg, where the onset and increase in surface diffusion was accompanied with a decrease in foam stability. Fluorescence and surface tension measurements confirmed that a-la was still present in the adsorbed layer of the film up to R = 2.5. Thus, the enhancement of foam stability to levels in excess of that observed with a-la alone supports the presence of a synergistic effect between the protein and surfactant in this mixed system (i.e., the combined effect of the two components exceeds the sum of their individual effects). It is important to note that Tween 20 alone does not form a stable foam at concentrations <40 jtM [22], It is possible that a-la, which is a small protein (Mr = 14,800), is capable of stabilizing thin films by a Marangoni type mechanism [2] once a-la/a-la interactions have been broken down by competitive adsorption of Tween 20. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

Clark, D.C., Coke, M., Mackie, A.R., Finder, A.C., and Wilson, D.R. Molecular-diffusion and thickness measurements of protein-stabilized thin liquid-films, /. Colloid Interface Sci, 138, 207,1990. 

J.A. Molina-Bolivar, F. Galisteo-Gonzalez, R. Hidalgo-Alvaiez, Colloidal stability of protein-polymer systems a possible explanation by hydration forces. Phys. Rev. E 55(4), 4522-4530 (1997). doi 10.1103/PhysRevE.55.4522 R.M. Pashley, J.N. Istaelachvili, Molecular laycaing of watCT in thin films between mica surfaces and its relation to hydration forces. J. Colloid Interface Sci. 101(2), 511-523 (1984). doi 10. 1016/0021 -9797(84)90063-8... 

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