Chiral nanostructuring

Fig. 31 Self-assembly of chiral nanostructures on hopg. Melamine-calix[4]arene units self-assemble with diethylbarbiturate (DEB) into chiral tetrarosettes (a) [95]. The nanostructures form different 2D domains with unit cells being mirror images of each other (b). Reprinted with permission from Wiley...

PAIs with a chiral nanostructure have been synthesized by this method. To assess the efficiency of microwave irradiation method together with ionic liquids, the method was compared with a polycondensation reaction using the conventional heating method. The microwave irradiation method has the advantage of safe operation, low pollution, rapid access to products, and simple work-up. 

In another work, silica nano helices have been prepared from the organic self-assemblies of -tartrate amphiphiles by sol-gel transcription. Then this chiral nanostructures were functionalized with (3-aminopropyl)-trietho)y-silane (APTES) or (3-mercaptopropyl)triethoxysilane (MPTES) and decorated with gold nanoparticles of various diameters (1-15 nm) resulting in nanohelix hybrid structures (Fig. 25). ° It was found that the surface plasmon resonance intensity of these nanohybrid systems increased with gold particle size. Gold nanoparticles of 10-14 nm diameter have clearly showed a surface enhanced effect on Raman spectroscopy. This sj tem is a unique example of the 3D hybrid network that could be used as ultrasensitive chemical and biological sensors for detection of molecules of interest in liquids by accumulation under flow. 

Another area which was initiated during last year is development of chiral metal oxide based nanomaterials such as chiral Ti02 nanofibres and chiral ZrOj nanotubes. It is anticipated that these chiral metal oxide nanostructures will find very important applications as asymmetric catalysts. In addition the progress in the fabrication of mesoporous silica based chiral nanostructures e.g. helical architectures) should open new opportunities in chiral separation of enantiomeric compounds. 

In general, we expect that further uses for chiral nanomaterials will continue to develop rapidly and the demand for these materials will grow in the near future. However, further systematic experimental and theoretical studies will be needed for fundamental understanding of the properties and behaviour of these new chiral nanostructures for the development of their appropriate applications. 

Electro-optic composite nanostructures or chiral nanostructures of conducting polymers have received great attention in the application of electro-optic nanodevices [100]. Coordination of micelle soft-template with functional dopant induced method is a simple and an efficient approach to prepare electro-optical... 

Recently, aerogels based on peptides were synthesized [41] as well. Self-assembling of the fibril network was observed and chiral, nanostructured low-density aerogels out of the resulting gel. This product can be used for sensing, tissue engineering, and drug release. 

Figure 5 Variable tonpCTature CD experiment dranonstrating disassembly of chiral nanostructure on application of heat. (Reproduced from Ref. 33. Amaican Chanical Society, 2005.)...

Figure 9.8) [70]. This work is part of a broader activity in the field of binaphthyl-based chiral macrocycles for chiroptical sensing and chiral nanostructuring [71-87]. The macrocycle 23 incorporates a Binol unit. In order to counterbalance the inherent distortion brought about by the binaphthyl units into the macro-cyclic framework, sp carbon atoms, imparting a higher conformational freedom with respect to sp- or sp -hybridized carbon atoms, have been introduced. 

The study hinted that the presence of at least one double bond is required in the lipophilic side chain to form twisted nanostructures and impart supramolecular chirality. Studies on the formation of helical nanostructures are limited to the presence of chiral carbon centers and have been well established by Furhop et al. [73]. Various hydrophilic chiral moieties such as glucose, galactose, open-chain... 

Introducing chirality into polymers has distinctive advantages over the use of nonchiral or atactic polymers because it adds a higher level of complexity, allowing for the formation of hierarchically organized materials. This may have benefits in high-end applications such as nanostructured materials, biomaterials, and electronic materials. Synthetically, chiral polymers are typically accessed by two methods. Firstly, optically active monomers - often obtained from natural sources - are polymerized to afford chiral polymers. Secondly, chiral catalysts are applied that induce a preferred helicity or tacticity into the polymer backbone or activate preferably one of the enantiomers [59-64]. 

Finally, we mention a remarkable example of lateral resolution reported for supramolecular nanostructures on hopg [96]. Held together by 72 hydrogen bonds, the molecular nanostructure is formed from three melamine-substituted calix[4]arene units and twelve 5,5-diethylbarbiturate molecules (Fig. 31a). The nanostructure, basically a stack of four rosettes, has chiral symmetry. With its components all being achiral, both enantiomers are formed upon self-assembly in solution. Deposition of the tetrarosettes on hopg leaves this nanostructure intact and allows surface self-assembly. AFM studies revealed close-packed 2D lattices formed by the tetrarosettes on hopg... 

In a further step, Yam and coworkers constructed helical superstructures emplo5nng chiral alkynylplatiniun(II)-terpyridyl complexes, obtaining metallogels that show helical fibrous nanostructures. The chiral supramolecular structiue and the spectroscopic properties depend on the extent of aggregation through Pt- Pt and n-n interactions, which can be influenced by varying the nature of counteranions, as revealed by UV/vis, circular dichroism, and luminescence studies (221). 

Jorio A, Saito R, Dresselhauss G, Dresselhaus MS (2004) Determination of nanotube properties by Raman spectroscopy. Phil Trans R Soc Lond A 362 2311-2336 Kobayashi K, Takagi D, Ueno Y, Homma Y (2004) Characterization of carbon nanotubes suspended between nanostructures using micro-Raman spectroscopy. Physica E 24 26-31 Dresselhaus MS, Eklund P (2000) Phonons in carbon naotubes. Adv Phys 49 705-814 Anderson N, Hartschuh A, Novotny L (2007) Chirality changes in carbon nanotubes studied with near-field Raman spectroscopy. Nano Lett 7 577-582... 

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