Self-assembly confinement

Several synthetic strategies to control the sizes of mesoporous nanoparticles have been reported. Lu[272] reported a rapid, aerosol-based process for synthesizing solid, well ordered spherical particles with stable pore mesostructures of hexagonal and cubic topology, as well as layered (vesicular) structures. This method relies on evaporation-induced interfacial self-assembly confined to spherical aerosol droplets. This simple, generalizable process can be modified for the formation of ordered mesostructured thin films. 

Figure 31 Aerosol-assisted self-assembly of mesostructured spherical silica nanoparticles with hexagonal and cubic morphology as well as layered stractures. The method relies of evaporation-induced interfacial self-assembly confined to spherical aerosol droplets. Source From Ref. 130.

In this chapter we describe the basic principles involved in the controlled production and modification of two-dimensional protein crystals. These are synthesized in nature as the outermost cell surface layer (S-layer) of prokaryotic organisms and have been successfully applied as basic building blocks in a biomolecular construction kit. Most importantly, the constituent subunits of the S-layer lattices have the capability to recrystallize into iso-porous closed monolayers in suspension, at liquid-surface interfaces, on lipid films, on liposomes, and on solid supports (e.g., silicon wafers, metals, and polymers). The self-assembled monomolecular lattices have been utilized for the immobilization of functional biomolecules in an ordered fashion and for their controlled confinement in defined areas of nanometer dimension. Thus, S-layers fulfill key requirements for the development of new supramolecular materials and enable the design of a broad spectrum of nanoscale devices, as required in molecular nanotechnology, nanobiotechnology, and biomimetics [1-3]. 

Confinement effects may also be employed to characterize the nucleation and growth of porous materials [211]. The underlying mechanisms of self-assembly and crystallization of these complex heterogeneous systems may be traced by solid state NMR methods well before their detection by diffraction methods. 

Fig. 14. Hyperbranched polymer grafts prepared on a mercaptoundecanoic acid (MUA) self assembled monolayer confined to a gold substrate. PAAM-c-PAA represents a random copolymer of poly(acrylamide) and poly(acrylic acid) prepared from the poly(acrylic acid) carboxylic acid groups and an amine [129]...

The first report of surface-immobilized dendrimers was in 1994 [54]. Subsequently, our research group showed that the amine-terminated PAMAM and PPl dendrimers could be attached to an activated mercaptoimdecanoic acid (MUA) self-assembled monolayer (SAM) via covalent amide linkages [55, 56]. Others developed alternative surface immobilization strategies involving metal com-plexation [10] and electrostatic binding [57]. These surface-confined dendrimer monolayers and multilayers have found use as chemical sensors, stationary phases in chromatography, and catalytic interfaces [41,56,58,59]. Additional applications for surface-confined dendrimers are inevitable, and are dependent only on the synthesis of new materials and the development of clever, new immobilization strategies. 

For many applications such as catalysis and possible functional devices, SAMs are simply too thin, the organized structure not flexible enough or the sterical situation within the layer too confined in order to incorporate a desired function or respond to changes in the environment in a dynamic and reversible way. One approach to increase the layer thickness of well-ordered self-assembled stractures of up to 100 nm is the formation of SAM and LB multilayers by means of consecutive preparation steps (Fig. 9.1 (3)) [5, 108]. This strategy was successfully applied by several research groups, but requires the constant intervention of the experimenter to put one type of monomolecular layer on top of the other. The dynamic behavior of the layer is limited by the crystal-like organization of the system and the extreme confinement of all surface-bonded molecules. Hence, surface... 

Quantum dots are the engineered counterparts to inorganic materials such as groups IV, III-V and II-VI semiconductors. These structures are prepared by complex techniques such as molecular beam epitaxy (MBE), lithography or self-assembly, much more complex than the conventional chemical synthesis. Quantum dots are usually termed artificial atoms (OD) with dimensions larger than 20-30 nm, limited by the preparation techniques. Quantum confinement, single electron transport. Coulomb blockade and related quantum effects are revealed with these OD structures (Smith, 1996). 2D arrays of such OD artificial atoms can be achieved leading to artificial periodic structures. 

Spatially confined self-assembly has been further confirmed by localizing ther-molysin on certain areas of a PEGylated surface. Upon immersion of this modified surface into a solution containing self-assembling precursors, nanostructures were formed in the vicinity of the enzyme, as observed through congo-red staining (Fig. 7c) [21]. Thus, enzyme-assisted self-assembly allows for construction of supramolecular polymers with spatiotemporal control, i.e. where and when they are required. 

A number of dynamic supramolecular polymers control vital functions in biology. These are tightly regulated by highly selective and spatially confined catalytic mechanisms whereby non-assembling precursors are catalytically activated to produce self-assembling components. 

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