Multilayers electrostatic deposition

Fabrication of organic thin films based on sponfaneous molecular assembly has been considered as one of fhe powerful approaches to create novel supramolecular systems. In this context, multilayer films were fabricated by layer-by-layer electrostatic deposition techniques based on the electrostatic interaction between dsDNA and the positively charged polymer poly(diallyldimethylammonium chloride) (PDDA) on GC surfaces. A uniform assembly of PDDA/DNA multilayer films was achieved, based on the adsorption of the negatively charged DNA molecules on the positively charged substrate [55]. 

Aoki, T., Decker, E.A., McClements, D.J. (2005). Influence of environmental stresses on stability of O/W emulsions containing droplets stabilized by multilayered membranes produced by a layer-by-layer electrostatic deposition technique. Food Hydrocolloids, 19, 209-220. 

Structural Features of Electrostatically Deposited Multilayer Assemblies... 

An alternate route to assembly of nanoparficles as hollow spheres that does not require a polymerization reaction step is sequential electrostatic assembly. Electrostatic-mediated multilayer assembly of charged particles was first demonstrated by R. Her on planar surfaces, wherein he established the proof-of-principle to deposit particles sequentially onto soUd substrates [25]. Decher advanced this scheme by assembling alternately charged PEs (e.g., polycations and polyanions) onto solid supports. Ever since, this scheme has been used to form capsules by sequential electrostatic deposition of single or multiple coatings of materials on preformed colloidal templates and subsequent removal of the template by calcination or solvent dissolution. This constitutes the LBL method for assembly of hoUow spheres. 

Liposome-pectin complexes have been formed through self-assembly techniques [95], They have also been manufactured by coating by electrostatic deposition using an oppositely charged polymer to the liposome [41, 66], In a report by Alund et al., the adsorption of pectin in the liposome was studied [1]. It was found that the adsorption in the form of multilayer is unlikely due to the repulsive forces between the first layer of pectin and the second one, both have a negative charge, so adsorption is proposed as monolayers. 

Polyelectrolyte is described as a polymer carrying (an) electrolyte(s) group in its repeating units. Polyelectrolyte presents charge property when it dissociates in water or an aqueous solution. The technique of multilayer polyelectrolyte deposition on membrane siu faces is based on the electrostatic interaction of oppositely charged molecules. A polyelectrolyte membrane can be fabricated by sequential deposition of aqueous polyelectrolyte solutions on a porous substrate, as illustrated in Figure 15.22 [115]. 

There are a number of ways of attaching a monolayer of redox molecules to an electrode surface.10 Multilayered films can be obtained by deposition of a polymer containing redox centers. These may be attached to the polymer backbone covalently, electrostatically, or coordinatively if the redox center contains a transition metal. 

Auer et al. [134] presented an example for multilayer formation and controlled deposition of functionalized nanoparticles on SAM of mercaptohexadecanoic acid (MHA) using electrostatic interactions. As a pH-sensitive switchable linker between the SAM of MHA and negatively charged gold nanoparticles, bis-benzami-dine bolaamphiphiles having different alkyl spacers were used [135]. This strategy resulted in a potentially tunable and switchable property of the entire assembly. For example, the kinetics of adsorption as well as the final particle layer thickness can be controlled by the kind of bis-benzamidine used as the linker (Fig. 9.16). 

Another way to produce polymeric multilayer systems is through the use of electrostatic self-assembly. With this technique, alternating positively and negatively charged layers can be deposited. This method leads to well-ordered multilayer systems, but when only polymeric components are used a substantial amount of interpenetration occurs. This problem can be reduced by the use of inorganic materials. 

Layer-by-layer (LBL) deposition has become one of the major techniques to fabricate multilayer films. It was introduced by Decher in the form of alternate deposition of cationic and anionic polyelectrolytes [56-58]. This so-called electrostatic self-assembly process dominates the research in this area. However, more recently alternative driving forces have been employed, such as hydrogen bonding, coordination bonding, charge-transfer interactions, and covalent bonding. Since this review concerns the formation, functionaliza-... 

In certain cases, self-assembly methods can be employed to prepare multilayered thin films analogous to LB films. Typically, once the surface has been primed with a molecular adhesion layer, subsequent layers are assembled in a layer-by-layer fashion where the end group of the previously deposited layer directs the assembly of the next layer. Strong electrostatic or covalent interactions between the layers serve to stabilize the assemblies. The most notable examples of self-assembled multilayered films are those based upon metal phosphonates [21]. Although these multilayers are structurally analogous to LB films, their thermal and solvent stability makes them potentially more useful in many applications, including electron-transfer studies. 

Scheme 3. Idealized sketch of a gold surface modified with a multilayered assembly prepared by (a) alternate deposition of layers of poly(acryhc acid) and poly-L-lysine on a positively charged cystamine SAM (the multilayer assembly is stabilized by electrostatic interactions between adjacent layers, [136]) and by (b) covalent attachment of a layer of G4 poly(amidoamine) dendrimer to a 11-mercapto-l-undecanoic acid SAM after activation of the surface-bound cEirboxylic groups with pentafiuorophenol and EDC. The dendrimer monolayer is afterwards biotinylated by covalent binding of desthiobiotin amidocaproate (not shown) to assemble an overlayer of avidin by affinity interactions [133].

Electrode surfaces modified with a multilayered surface architecture prepared by a layer-by-layer repeated deposition of several enzyme mono-layers show a modulated increase of surface-bound protein with a subsequent increase in output current, which is directly correlated with the number of deposited protein layers. The versatility of this approach allows alternate layers of different proteins for the manufacture of electrode surfaces, which are the basis for multianalyte sensing devices with multiple substrate specificities. The surface chemistry used for the manufacture of multilayered electrode surfaces is similar to that previously described for the preparation of affinity sensors, and is based on the stabilization of self-assembled multilayer assemblies by specific affinity interactions, electrostatic attraction, or covalent binding between adjacent monolayers. 

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