Macroscale self-assembly

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

Self-assembly is essentially chemical fabrication. Like macroscale fabrication techniques, self-assembly allows a great deal of design flexibility in that it affords the opportunity to prepare materials with custom shapes or morphologies. The advantages of self-assembly include an increased level of architecture control and access to types of functionality unobtainable by most other types of liquid-phase techniques. For example, it has been demonstrated that materials with nonlinear optical properties (e.g., second harmonic generation), which require noncen-trosymmetric structures, can be self-assembled from achiral molecules. 

The spontaneous formation of structures by self-assembly relies on a delicate balance of relatively weak intermolecular forces such as the hydrophobic force, electrostatic and van der Waals interactions, and hydrogen bonding. Building blocks for self-assembly can range from small atoms to much larger macroscale objects. A wide variety of two- and three-dimensional... 

Schacht S, Huo Q, VoigtMartin IG et al (1996) Oil-water interface templating of mesoporous macroscale structures. Science 273(5276) 768-771 Madou M (1998) Eundamentals of microfabrication. CRC, Boca Raton Cui TH, Hua E, Lvov Y (2004) Lithographic approach to pattern multiple nanoparticle thin films prepared by layer-by-layer self-assembly for microsystems. Sens Actuators A 14(2-3) 501-504... 

Regardless of the type of targeting agent, carrier molecules can be subdivided into three classes on the basis of their characteristic length [243] nano-, micro-, and macroscale vectors. In general, nanoscale vectors are represented by polycationic polymers or lipids that self-assemble with NABD to form polyelectrolyte complexes. Microscale vectors usually consist of NABD entrapped within a polymeric matrix, and macroscale vectors are 2D/3D scaffolds or matrices (mainly polymeric but not only) hosting the desired NABD. Of course, it is possible to embed nano-/microscale vectors inside macroscale vectors. Nano-/microscale vectors protect NABD and favor the cellular internalization, while macroscale vectors can modulate nano-/microscale vector release kinetics at the site of action (Figure 15.24). 

Self-organization of amphiphilic (co)polymers has resulted in assemblies such as micelles, vesicles, fibers, helical superstructures, and macroscopic tubes [174, 175]. These nanoscale to macroscale morphologies are of interest in areas ranging from material science to biology [176]. Stimuli-responsive versions of these assemblies are likely to further enhance their scope as smart materials. Thermo- or pH-sensitive polymer micelles [177] and vesicles [178] have been reported in which the nature of the functionality at the corona changes in response to the stimulus. Some attention has been also paid to realize an environment-dependent switch from a micelle-type assembly with a hydrophilic corona to an inverted micelle-type assembly with a lipophilic corona [179]. 

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