Structure-directed assembly

Fig. 2 A quantum dot transport structure, consisting of a source, a drain, and a gate, with gold nanoparticles surrounded by DNA (the bright white dots). The transport through these structures can be fitted well to a simple Coulomb blockade limit description. From S.-W. Chung et al. Top-Down Meets Bottom-Up Dip-Pen Nanolithography and DNA-Directed Assembly of Nanoscale Electrical Circuits Small (2005) 1, 64-69. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

Caspar and Klug (1962) made an important distinction between two fundamental types of assembly processes. True self-assembly was conceptualized as a series of reactions relying on the propensity of subunits to condense and form assembled structures strictly as a result of the information encoded in the architecture of the components. On the other hand, template-directed assembly may be considered as a process depending on the presence of a separate template that imparts structural constraints on the pathway for constructing the final assembled structure. True self-assembly is observed, for example, in the formation of many oligomeric proteins. Indeed, Friedman and Beychok (1979) have re-... 

Figure 13. Self-assembled organic template (micellar or liquid crystalline) that directs the assembly of the inorganic phase about the structure-directing organic phase. (Courtesy of Prof. Sarah Tolbert, UCLA.)...

The direct assembly of nitrile ylides by the reaction of carbenes (e.g., 84) with nitriles has, in principle, great potential for structural variation as both components are easily accessible. The first reports of such reactions appeared in the early 1980s and were concerned with the reaction of nitriles with stabilized carbenes such as 84 and 86 (41 3). 

Generally, the mechanism proposed for structure direction and self-assembly in the synthesis of Si-ZSM-5 involves the formation of an ordered, hydrophobic hydration sphere around the TPA cation [11]. The assembly process of MCM-41 is controlled by electrostatic interaction between silicate species and charged surfactant head groups [12]. 

Many of the same ionic surfactants used for the assembly of mesostructured molecular sieve catalysts [1-4] and related bulk phases [5] can be intercalated in a variety of layered host structures [6]. We have recently demonstrated that some of these mesostructure - forming surfactants retain their structure directing properties when intercalated in the galleries of smectite clays. In a manner quite analogous to bulk mesostructure formation, the intercalated surfactants direct the assembly of an open framework metal oxide (silica) structure within the constrained gallery regions of the layered host (7). The resulting porous intercalates are referred to as porous clay heterostructures (PCH). 

An overview of the structure is located in Figure 3b. The structure is assembled from chains of hydrogen-bonded molecules that stretch along the 6-axis. The unit cell contains two types of non-equivalent molecules, each of which form their own chain structures and are inter-related only by a pseudocentering in the a and b directions. 

The strong and specific biotin-streptavidin binding was used to assemble biomolecule-functionalized nanoparticles in multilayered structures.67 Application of an electrical field allowed the assembly of multilayer structures by using extremely low concentrations of nanoparticles with minimal nonspecific binding. A microelectrode array was used to facilitate the rapid parallel electrophoretic transport and binding of biotin- and streptavidin-functionalized fluorescent nanoparticles to specific sites. By controlling the current, voltage, and activation time at each nanoparticle adsorption step, the directed assembly of more than 50 layers of nanoparticles was accomplished within an hour. 

An example of the first approach is the integration of hydrogels into nanostructured silica films by addition of a suitable monomer (e.g., methyl methacrylate, /V-isopropyl acrylamide, etc.) and an initiator for radical polymerization to a solution containing a structure-directing surfactant and a prehydrolyzed silica precursor. During self-assembly, the monomers partition within the hydrophobic core of the surfactant mesophase postsynthesis polymerization (for instance, by UV treatment) followed by solvent washing to remove the surfactant template yields a polymer-silica nanohybrid. 

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