LbL method

Extending these ideas to enzymatic catalysis, Jiang et al. reported the use of protamine-silica hybrid microcapsules in combination with a host gel-like bead structure to encapsulate several enzymes individually in the enzymatic conversion of C02 to methanol [20]. They used a layer-by-layer (LbL) method where alternately charged layers were deposited on an enzyme-containing CaC03 core. The layers, however, were not polyelectrolytes, but protamine and silica (Scheme 5.6). 

The encapsulation process used was the LbL method, employing alternately charged polymers, namely PDA [poly(diallyldimethylammonium chloride)] and PSS... 

As mentioned in Section 5.2.2, the LbL method generally involves the addition of alternately charged layers, often on to a sacrificial solid that has a charged surface (Figure 5.4). 

Figure 5.5 Synthesis of an encapsulated metal catalyst by the LbL method with a different metal catalyst in the membrane walls. Reproduced by permission ofthe PCCP Owner Societies [58].

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Having already examined the use of the LbL method to make various nanocapsules, including polymer nanocapsules, and having already encountered the use of star polymers for catalyst encapsulation, we turn our attention to other methods for the formation of polymeric nanocapsules. Useful reviews of the formation of these capsules using various methods are available [78-84]. 

A strong surface charge on the halloysite tubules has been exploited for designing nano-organized multilayers using the layer-by-layer (LbL) method of Lvov et al. [8,13,14]. The lumen of the halloysite has been used as an enzymatic nano-reactor by Shchukin et al. [15] The biocompatible nature of the halloysite was... 

The LbL methods have been also used to prepare spatially ordered bienzymatic electrodes, two examples are shown in Figure 2.25. In the first one, glucose is aerobically oxidized by GOx in the outer layers to produce hydrogen peroxide that is thereafter reduced by soybean peroxidase (SBP) wired to the electrode with PAH-Os [182]. This system responds both to H2O2 and to glucose, but in the... 

Figure 2.24 Diagrams of bienzymatic spatially ordered electrodes built using the LbL method. SBP Soybean peroxidase, COx Glucose oxidase, GDH glucose dehydrogenase.

The major part of the reports discussed above provides only a qualitative description of the catalytic response, but the LbL method provides a unique opportunity to quantify this response in terms of enzyme kinetics and electron-hopping diffusion models. For example, Hodak et al. [77[ demonstrated that only a fraction of the enzymes are wired by the polymer. A study comprising films with only one GOx and one PAH-Os layer assembled in different order on cysteamine, MPS and MPS/PAH substrates [184[ has shown a maximum fraction of wired enzymes of 30% for the maximum ratio of mediator-to-enzyme, [Os[/[GOx[ fs 100, while the bimolecular FADH2 oxidation rate constant remained almost the same, about 5-8 x 10 s ... 

The universal character of the LbL method has catalyzed the introduction of the method for a wide range of bioapplications. Proteins (enzymes) [30-33], polypeptides [34], polysaccharides [35], lipids [36, 37], nucleic acids [38-42], viruses [43], inorganic particles, and crystals [44] have been embedded in the films. Use of these compounds makes the films attractive for biorelated applications such as biosensors, drug delivery, tissue engineering, and biocoatings. Biological [45, 46] and nonbio-logical [21, 47 19] applications of LbL films are reviewed in the literature. 

P3HT-co-P3(ODAP)HT/CdSe(MHT) (NC diameter 5.8 nm), using the LbL method. In the order of increasing absorbance, each spectrum corresponds to the addition of one bilayer, (b) Absorbance measured at 504 nm and 622 nm as a function of the number of bilayers.140 (Reprinted with permission from J. De Girolamo et al., J. Phys. Chem. C 2008, 112, 8797-8801. Copyright 2008 American Chemical Society.)... 

The alternate assembly of TALH and PDADMAC has also been performed on nickel nanorods (average diameter 65 nm, length 1.5 pm), followed by hydrolysis of TALH upon heating under reflux (Fig. 9a). Subsequent dissolution of the nickel core yielded titania-based nanotubes (Fig. 9b) [51]. This example illustrates that the LbL method offers a promising route for the preparation of nanotubes with tailored composition and wall thickness, derived from inorganic molecular precursor and polyelectrolyte assembly. Concentric nanotubes (different composition on the inside of the nanotube from that of the surface) could also be prepared via this method. 

The stable hollow microcapsules on the basis of natural polysaccharide alginic acid and protein protamine sulfate are formed via LbL method. The (PtS/AlA)4 microcapsules fabricated are promising for biomedical applications. 

The LbL methods work only with hydrophilic substrates, polyelectrolytes and surface charged NCs. Electrostatic forces and van der Waals forces dominate between these components to stabilize the system. It is not possible to generate... 

Layer-by-layer (LbL) assembly is a unique technique for the fabrication of composite films with precise thickness control at the nanometer scale [111, 112], The method is based on the alternate adsorption of oppositely charged species from their solutions. The attractive feature of this approach is its ability to assemble complex structures from modular components, and integrate them into self-assembling constructions for a wide range of applications. The LbL method has been successfully exploited in the construction of dendrimer biosensors [113,114], The LbL films provide a favorable environment for the intimate contact between the dendrimer and biomolecule (enzymes or proteins), promoting a direct electron transfer between them and the underlying electrodes. 

---