First Layers and Precursor Film

At the very beginning of the alternate assembly process, one often sees nonlinear film growth [13, 23, 41]. At the first 2-3 layers, smaller amounts of polyion are adsorbed as compared with further assembly, when the film mass and thickness increase linearly with the number of adsorption cycles. Tsukruk et al. [41] explained this as an island-type adsorption of the first polyion layer on a weakly charged solid support. In the following two-three adsorption cycles, these islands spread and cover the entire surface, and further multilayer growth occurs linearly. If a substrate is well charged, then a linear growth with repeatable steps begins earlier.

In studying the possibility of using new compound in approach [6, 13, 23]. On a substrate (silver electrode of quartz crystal microbalance (QCM) resonator or quartz slide), we deposited 2-3 layers of polyions, and on this "polyion blanket," with a well-defined charge of the outermost layer, an assembly of proteins, nanoparticles, or other compounds was produced. In a typical procedure, precursor films were assembled by repeating two or three alternate adsorptions of PEI and PSS. The outermost layer became "negative," or "positive," respectively.

Quartz crystal microbalance monitoring of multilayer growth was often the first stage of the assembly procedure elaboration. Initially, we estimated the time needed for a component's saturated adsorption in a kinetic experiment. Then we performed the assembly typically with 10 min alternate adsorption. After every other adsorption step, a layer was dried by a nitrogen stream and the QCM resonator frequency was registered. The frequency shift with adsorption cycles gave us the adsorbed mass at every assembly step. A linear film mass increase with the number of assembly steps indicated a successful procedure.

The following relationship is obtained between adsorbed mass M (g) and frequency shift AF (Hz) by taking into account the characteristics of the 9 MHz quartz resonators used [23]: AF = -1.83 x 108 M/A, where A = 0.16 ± 0.01 cm2 is the surface area of the resonator. One finds that a 1 Hz change in AF corresponds to 0.9 ng, and the thickness of a film may be calculated from its mass. The adsorbed film thickness at both faces of the electrodes (d) is obtainable from the density of the protein/polyion film (ca 1.3 g/cm3) and the real film area: d (nm) = -(0.016 ± 0.02)AF (Hz). The scanning electron microscopy data from a number of protein/polyion and linear polycation/polyanion film cross-sections permitted us to confirm the validity of this equation. Another powerful method for polyion film characterization was small-angle X-ray and neutron reflectivity.

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