Chapter 1
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

1.1 Introduction

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 [4]. 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 [10].

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 [16]. 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 [19]. 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 [20]. 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 [21].

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 nanoparticles in detail. First of all, the basic knowledge about the chiral optical spectra is briefly introduced. Next, the origin and construction of chiral metal nanoclusters are discussed. Subsequently, chiral nanoparticles or nanoparticle assemblies with characteristic optical activity are covered. Finally, the applications and perspectives of chiral noble metal nanostructures are presented. It should be noted that because the enantioselective catalysis by using chiral noble metal nanostructures has been extensively summarized in recent reviews [11, 12], herein the application of chiral noble metal nanostructures will be concentrated in the field of optics.

Among different properties, the optical activity is one of the most important features of chiral molecules, which is extensively characterized by circular dichroism (CD) spectrum [12]. The detection principle of CD spectra is as follows: When two circular polarized lights (CPLs) of same intensity and frequency but opposite direction are passed through a chiral sample, the difference in CPL absorption leads to production of the elliptically polarized light. The CD effect of a chiral molecule could be generally expressed by the following equation [27, 28]:

where and are the electric and magnetic dipole moments of a molecule, respectively.

In addition to CD spectra, other methods including vibrational circular dichroism (VCD) and optical rotatory dispersion (ORD) are also broadly used for measuring the optical activity of chiral molecules. The main difference between CD and VCD is optical wavelength. CD spectra located at the UV-vis region could be used for analysis of the second- or higher-order structures of chiral molecules, whereas VCD located at the infrared region might be adopted to determine the structure and absolute configuration of molecules. As for ORD and CD, ORD is based on the scattering difference when CPL lights are passed through a chiral medium, while CD is originated from the absorption difference. The ORD and CD spectra might be easily converted via the Kronig-Kramers equation. Notably, all the above spectra even could also be combined with other techniques such as high-performance liquid chromatography (HPLC) or synchrotron radiation system to meet more complicated needs for chiral molecules [29, 30].

1.2 Chiral Noble Metal Clusters

Clusters are a special class of nanomaterials, which contain few atoms of the characteristic sizes ranging from 0.5 to 2 nm. Different from the normal noble metal nanoparticles with the sizes of 2-100 nm, the extremely small metallic cores of the clusters are susceptible to the surrounding organic shells, resulting in chirality inside the clusters. Moreover, the quasi-continuous energy levels of noble metal clusters are opened with size shrinkage, generating the strong quantum confinement and exciton localization [31]. This is the reason why we use a separate chapter in this review to introduce the chiral noble metal clusters. The noble metal clusters are generally expressed as Mx(L)y, where M and L stand for the metal element and organic ligand, respectively, and the subscripts x and y denote the corresponding number of metal atoms and ligands. The metals refer to Au and Ag, while the ligands could be various organic molecules such as phosphine [32] and thiols [33].

1.2.1 Origin of Chiral Noble Metal Clusters

Typically, three mechanisms have been proposed on the chirality origin of metal clusters: (i) chiral metal core; (ii) dissymmetric field model; and (iii) chiral footprint model. As for the chiral cores, Garzon et al. used Hausdorff chirality measure to calculate the intrinsic structure of noble metal clusters, revealing that the lowest energy isomers of bare Au28 and Au55 clusters were chiral. Furthermore, the chirality index was found to increase after modification of cluster surfaces with achiral thiol molecules [34]. The second mechanism suggests that the chirality of the achiral cores is induced when they are placed in a chiral environment, such as a chiral adsorption pattern. The particle-in-a box module is adopted to explain the mechanism [35]. This module demonstrates that the chiral charges of the ligands could induce chiral images inside metal clusters, giving rise to the chiral electronic state of metal cores, while the structure of the cores remains achiral. Chiral footprint model is an intermediate one between the former two theories; that is, the chiral ligand absorption could lead to the surface atoms relaxation, creating a chiral footprint on the surface of metal clusters [36]. Double anchoring points of the ligand molecules seem to facilitate the formation of the foot prints on the surface of metal clusters. It should be pointed out that to separate these mechanisms in real samples is very difficult, especially when the ligands are chiral molecules. The reason is the fact that the organic ligands not only act as the stabilizers but also distort the surface of metal clusters. For instance, by using the time-dependent density functional theory (TD-DFT), Garzon et al. disclosed that the chirality of [Au25(glutathione)18] clusters simultaneously originated from the slight structural distortion of the metal cores and the dissymmetric field of the organic ligands [37].

Fast development on the experimental characterization offers direct evidences to understand the origin of the chiral noble metal clusters. The X-ray structure survey on Au102(p-mercaptobenzoic acid)44 clusters showed that they are chiral, having two enantiomers alternating in the crystal lattice [38]. In single clusters, 89 of 102 Au atoms had fivefold rotational symmetry, whereas the rest of 13 Au atoms on...