Self-assembly multilayers

Clearly if a = l (or, in traditional chemical terminology, if the reaction has a 100% yield) the layers will build up in a satisfactory manner but, for all realistic lesser values of a, the numbers of molecules successfully deposited in a layer will decrease with n. For values of a very nearly equal to unity and fairly thin multilayers, reasonable success is 

Putvinski et al. [395] have proposed a variant of the zirconium process [392, 393] in which a succession of three different reagents are applied and it is possible to form non-centrosymmetric films. They produced an initial surface of OH groups by reacting HS(CH2)MOH with a gold 

Jin and Johnstone [396] have originated a method of forming multilayers by self-assembly which makes use of the fact that isocyanates react with alcohols to produce a very high yield. The initial step makes use of one of the various methods discussed above to form a substrate surface covered with OH groups. The groups on this surface can be viewed as the alcohols in the reaction shown below and will be denoted by ROH. 

Tredgold et at. [397] adopted a rather different approach to self-assembly. In Equation (1.11) in Section 1.3 it was shown that, when London forces predominate, materials having different high frequency permittivities, and hence different refractive indices, will tend to separate. 

It is possible for the long axes of the molecules to have a uniform tilt over macroscopic distances, as is also true in the cases of Langmuir-Blodgett films and films at the air/water interface. 

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Fig. 8. Construction of self-assembled multilayers from methyl 23-tticlilorosilyltticosanoate.

Self-assembled multilayers can be prepared simply by alternating adsorption of Zr" " ions and a,CO-alkyhdenediphosphate (15) on a phosphorylated surface (336,337). Other diphosphates have also been investigated (237,325,326,338—340) (Fig. 15). These are all centrosymmetric multilayers. 

Fig. 15. Diphosphonic acids used in self-assembled multilayer preparation.

C. Fou, O. Onitsuka, M. Ferreira, M.F. Rubner, and B.R. Hsieh, Fabrication and properties of light-emitting diodes based on self-assembled multilayers of poly(phenylene vinylene), J. Appl. Phys., 79 7501-7509, 1996. 

Li, Z., et al., Electrostatic layer-by-layer self-assembly multilayer films based on graphene and manganese dioxide sheets as novel electrode materials for supercapacitors. Journal of Materials Chemistry, 2011. 21(10) p. 3397-3403. 

Figure 2.26 (a) Scheme for an electrostatically self-assembled multilayer (PAHOs)4(Apo-COx)3(COx) electrode, (b) Catalytic current response to p-d-glucose concentration for self-assembled nanostructured thin films of PAH-Os/COx/(PAH-Os/(ApoGOx)3 and (PAH-Os/ApoCOx)3/PAH-Os/COx, where ApoCOx is FAD-free glucose oxidase. Taken from Ref [219]. 

Figure 8.11. A rigid to-biotinylated molecule as employed by Morgan et at. [462] to bring about formation of a self-assembled multilayer.

Extended pyridine derivatives have also been considered in the development of new materials as they present rigid systems with different electronic character than benzene. Self-assembled multilayers (SAMs) of stilbazolium derivatives, such as those based on compound 63, have been investigated <1996JA8034> and shown to have very high structural regularity and subsequently be very smooth. These show promise for nonlinear optical materials, as they have very high second-order susceptibility. 

Figure 5.56 Schematic illustrating the self-assembled multilayers of sodium 3-mercaptoethane-sulfonate (layer 1 ), hexacationic homooxacalix[3]arene-fullerene (2 1) complex (layer 2 ), with either the anionic porphyrin polymer ( 3 ) or its zinc porphyrin analogue ( 4 ) forming the third layer. Reprinted with permission from A. Ikeda, T. Hatano, S. Shinkai, T. Akiyama and S. Yamada, /. Am. Chem. Soc., 123,4855 (2001). Copyright (2001) American Chemical Society...

Luminescent multilayer systems have been reported extensively in Langmuir-Blodgett studies however, reports of intrinsically luminescent self-assembled multilayers are far rarer. Bard has reported on an intriguing luminescent multilayer comprised of alternate layers of 1,6-hexane dithiols and Cu ions (see Figure 5.59),... 

Tskruk, V. V., et al. (1997), Self-assembled multilayer films from dendrimers, Langmuir, 13(8), 2171-2175. 

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. 

Amperometric biosensors based on self-assembled multilayered structures... 

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. 

Wang Zh, Heflin JR, Stolen RH, Ramachandran S (2005) Highly Sensitive Optical Response of Optical Fiber Long Period Gratings to Nanometer-thick Ionic Self-assembled Multilayers. Appl Phys Lett 86(1-3) 223104... 

Thierry B, Kujawa P, Tkaczyk C, Winnik FM, Bilodeau L, Tabrizian M. Delivery platform for hydrophobic drugs prodrug approach combined with self-assembled multilayers. J Am Chem Soc 2005 127 1626-1627. 

Chromophoric self-assembled multilayers. Organic superlattice approaches to thin-film nonlinear optical materials, J. Amer. Chem. Soc. 112 7389 (1990). 

Advincula RC. 2002. Polyelectrolyte layer by layer self assembled multilayers containing azobenzene dyes. In Tripathy SK, Kumar J, Nalwa HS, editors. Handbook of Polyelectrolytes and Their Applications. Stevenson Ranch (CA) American Scientific Publishers, p. 65 97. 

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