An Introduction to Chiral Nanomaterials: Origin, Construction, and Optical Application
Zhengtao Li, Lin Shi and Zhiyong Tang
The National Center for Nanoscience & Technology, No. 11 Beiyitiao, Zhongguancun, Beijing, 100190, China
Chirality has aroused extensive interest in both science and technology since its first observation in the early nineteenth century [1-3]. Generally speaking, chirality is related to a structure without Sn symmetry elements, such as mirror plane (s), and inversion ( i ) symmetry. For instance, an organic molecule with chiral carbon atom that is connected with four different functional groups is a typical chiral system ( Figure 1.1 ). Understanding chirality at molecular level has led to enormous growth in multidisciplinary fields. In biology, it is believed to be one of the keys for understanding the life origin and evolution . Two basic biomolecule building blocks, amino acid (l-form) and nucleotide (d-form) of homochirality, assemble into second- or higher-order structures, which could further evolve into different functional organisms. In medicine, many synthetic drugs of the specific chirality could be used to cure disease, whereas its isomer acts in the opposite way [5, 6]. Accordingly, chiral organic synthesis based on catalysis and postseparation has become one of the hottest research topics in chemistry [7-9]. Tremendous advances have been achieved in preparation of chiral drugs, and even the full synthesis of chiral biomacromolecules is available .
Figure 1.1 Scheme of chiral organic molecules.
Extending the chirality from molecules to nanomaterials is bringing many new opportunities for the chiral study [11, 12]. Nanomaterials of the sizes ranging from 0.5 to 100 nm actually act as a bridge for the chiral study between molecules and bulk materials. The unique physical and chemical properties of nanomaterials could be easily tuned by altering their size, shape, or ingredient, providing a powerful platform for exploring the chiral properties [13-15]. For example, we can manipulate the chiral optical activity to any target wavelength just by controlling the size of nanomaterials, which is difficult and troublesome for organic molecules . Furthermore, additional action modes such as multipole-multipole coupling [17, 18], which are normally ignored in small molecules and become increasingly important in nanoscale objects, are bringing new insights into conventional chiral optics mainly based on dipole-dipole interaction. Except for fundamental research, chiral nanomaterials offer potential novel applications . As an example, grafting chiral biomolecules onto the nanomaterial surfaces might generate multifunctional binding sites, which are more efficient to crosslink with surface receptors. Therefore, nanomaterials not only act as the simple carriers of chiral biomolecules but also play an active role in biomedical applications . Our recent work has distinguished the obvious difference in the interaction efficiency between the living cells and the nanoparticles modified with biomolecules of the opposite chirality .
In the last five years, we witnessed many outstanding works about synthesis, property, and application of the chiral nanomaterials, and some excellent reviews related to this topic have been published [3, 11, 12, 22, 23]. It should be noted that most previous publications are focused on introduction of the chiral properties of the nanostructures obtained with the help of organic molecular assemblies [1, 24-26], and nevertheless there is absence of systematic summary on chiral noble metal structures, especially Au and Ag nanoparticles, though they have been proved to possess very specific optical activity. Here, we will summarize the-state-of-art progress of chiral noble metal na