General Procedure

A cleaned substrate of any shape and dimension is immersed into a dilute solution of a cationic polyelectrolyte, for a time optimized for the adsorption of a single monolayer

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (23-51)

(ca 1 nm thick), and then it is rinsed and dried. The next step is the immersion of the polycation-covered substrate into a dilute dispersion of polyanions or negatively charged nanoparticles (or any other nanosize-charged species) also for a time optimized for the adsorption of a monolayer; then it is rinsed and dried. These operations complete the self-assembly of a polyelectrolyte monolayer and monopar-ticulate layer sandwich unit onto the substrate (Fig. 1). Subsequent sandwich units are self-assembled analogously. Different nanoparticles, enzymes, and polyions may be assembled in a preplanned order in a single film.

The forces between nanoparticles and binder layers govern the spontaneous layer-by-layer self-assembly of ultra-thin films. These forces are primarily electrostatic and cova-lent in nature, but they can also involve hydrogen bonding, hydrophobic, and other types of interactions. The properties of the self-assembled multilayers depend on the choice of building blocks used and their rational organization and integration along the axis perpendicular to the substrate.

The sequential adsorption of oppositely charged colloids was reported in a seminal paper in 1966 by Iler [2]. The electrostatic self-assembly was subsequently "rediscovered" in the mid-nineties and extended to the preparation of multilayers of polycations and phosphonate ions, as well as to the layering of linear polyions, proteins, and nanoparticles by Decher, Mallouk, Mohwald, Lvov, Rubner, Fendler, Hammond, Kunitake, Schlenoff, Kotov, and others. This self-assembly is now employed in the fabrication of ultra-thin films from charged polymers (polyions) [3-47], dyes [48-51], nanoparticles (metallic, semiconducting, magnetic, insulating) and clay nanoplates [52-75], proteins [76-96], and other supramolecular species [79]. The greatest advantage of this self-assembly is that any of these species can be absorbed layer-by-layer in any order. The oppositely charged species

Layer-by-layer electrostatic assembly by alternate adsorption of oppositely charged nanoparticles, polyions and proteins t

Polycation/polyanion bilayer, D = 1-2 nm

Polycation/polyanion bilayer, D = 1-2 nm

Nanoparticle/polyion (or protein) bilayer, D = 5-50 nm are held together by strong ionic bonds and they form long-lasting, uniform, and stable films. Self-assembly is economical and readily amenable to scaling-up for the fabrication of large-area, defect-free devices on any kind and shape of surfaces.

The main idea of this method is the resaturation of polyion adsorption, which results in the alternation of the terminal charge after each layer is deposited. This idea is general and implies that there are no major restrictions in the choice of polyelectrolytes. It is possible to design composite polymeric films in the range of 5 to 1000 nm, with a definite knowledge of their composition. For the successful assembly of nanoparticle or protein multilayers, the alternation with linear polyion layers is important. Flexible linear polyions penetrate between nanoparticles and act as electrostatic glue. The concept of "electrostatic polyion glue," which keeps together neighboring arrays of nanoparticles, is central to this approach [77, 79]. The self-assembled film contains amorphous polyion interlayers, and this organization "heals" defects that arise because of the introduction of foreign particles during the process of film formation (dust, microbes) [13, 79].

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