Nanoparticles with Chiroptical Responses

Chirality is a fundamental and pervasive concept in chemistry and physics. Chirality refers to the property of an object or a molecule that cannot be superimposed onto its mirror image through any rotations or translations. Chiral materials have been found to exhibit unique chiroptical properties when interacting with left- and right-handed circularly polarized (LCP and RCP) light. The difference in the real part of the complex refractive index of a material for incident LCP and RCP light causes optical rotation or circular birefringence (CB).[1-3] The difference in the imaginary part of the complex refractive index of a material induces circular dichroism (CD).[4] The CD effect causes one component of circular polarization to be absorbed more strongly than the other. Both individual chiral molecules and chiral nanoparticles can exhibit chiroptical responses. The CD responses of chiral molecules arise from differential absorption under the excitation of LCP and RCP light. The CD signals of chiral molecules are usually small because the characteristic length of the chiral bonding structure is much smaller than the helical pitch length of circularly polarized light (CPL).[4] Increasing the pitch length of a chiral material to match that of CPL is an effective way to enhance chiroptical responses. The generation of twisting features in chiral nanoparticles is therefore highly desirable for attaining strong chiroptical responses.[5-7] Similar to the case of light absorption, a chiral nanoparticle also often exhibits different scattering signals under LCP and RCP light excitation. The differential scattering of a chiral nanoparticle under the excitation of LCP and RCP light becomes more significant as the nanoparticle increases in size. The differential extinction of a chiral nanoparticle is the sum of the differential absorption and differential scattering. Advanced fabrication techniques have been developed to create complex three-dimensional twisting structures.[8-10] A prototypical design is helical structures. The glancing angle deposition technique has been widely used to fabricate metal helices. The length and number of the helical pitches can be readily controlled. Helical structures have been found to not only exhibit far-field chiroptical responses in the mid-infrared wavelength range but also give rise to strong near-field optical chirality.[11] Miniaturizing twisting structures to the nanoscale is necessary for chiroptical responses at optical and telecommunication wavelengths. Creating helical nanostructures by physical fabrication techniques is challenging because of the limited resolution. Chiral assemblies constructed from achiral objects can be utilized to overcome the challenge. Various methods have been developed to assemble molecules or nanoparticles, including chemical, biological, and physical means. Chemical self-assembly of amphiphilic molecules has been used to arrange molecules into helical structures,[12] while biological molecules such as deoxyribonucleic acids (DNAs) can be used as scaffolds for assembling achiral nanoparticles.[13] Physical methods such as layer-by-layer deposition allow different materials to be stacked layer-by-layer, which gives rise to chiral stacked structures.[14] The chiroptical responses of chiral molecules and nanoparticles were first studied independently. Molecules and nanoparticles were later assembled together to create hybrid chiral nanostructures. The localized electromagnetic field enhancement around achiral plasmonic nanoparticles can amplify the CD signals of nearby chiral molecules in the hybrid structures. On the other hand, chiral nanoparticles generate strong optical chirality in the near-field owing to their unique geometric configurations and plasmonic properties. The strong optical chirality can induce chiroptical responses from nearby achiral molecules in the hybrid structures. Recent studies have demonstrated the immense potential of hybrid chiral nanomaterials for generating chiral fluorescence. The examples include up- and down-shifting CPL emissions and enantioselective surface-enhanced Raman scattering (SERS). Chiral fluorescence offers a wide range of applications because of its unique properties. The polarization sensitivity of chiral fluorescence makes it suitable for use in displays, optical devices, sensors, and spintronics.[15] Chiral fluorescence exhibits angular momentum selection rules, making it sensitive to the coherence and entanglement properties of quantum systems.[16] Such properties are highly desirable for driving progress in quantum computing and communication. The use of chiral nanostructures in biomedicine also holds great promise for targeted drug delivery and biomedical imaging, as well as for the development of new diagnostic tools and therapeutic agents. Upconversion nanoparticles, such as lanthanide nanoparticles, can convert low-energy photons into high-energy ones, resulting in up-shifting fluorescence. Down-shifting materials, such as quantum dots, organic dyes, fluorescent proteins, and two-dimensional fluorescent materials, absorb high-energy photons and emit lower-energy photons through relaxation processes.[17-19] The degree of circular polarization of the chiral fluorescence from individual nanoparticles and molecules is typically weak. Upconversion nanoparticles and down-shifting materials have been assembled with chiral nanostructures to enhance the degree of circular polarization of the chiral fluorescence.[20] Enantioselective SERS is another chiroptical response that relies on the interaction between chiral molecules and plasmonic nanostructures.[21] This effect results in changes in the spectral features of SERS that depend on the handedness of molecules. Combining organic–inorganic hybrid chiral nanostructures with other components to produce functional materials and devices is challenging due to the fragility and large footprints of the hybrid chiral nanostructures. A simple preparation method to create nanoscale twisting features is therefore very appealing.[5] The seed-mediated growth method provides a promising means for the synthesis of chiral metal nanoparticles.[22, 23] A seeded-growth approach was developed in 2018 to synthesize colloidal chiral Au nanoparticles using chiral amino acids and small peptides as the structure-directing agents.[22] The amino-acid- and peptide-directed overgrowth evolves Au seeds with low-index facets into high-index-facet-exposed chiral Au nanoparticles. Plasmonic nanoparticles with different chiral geometric configurations have been synthesized by the aforementioned seeded-growth method. They bring many advantages and enable many potential applications (Figure 1). First, chiral plasmonic nanoparticles can produce enhanced optical chirality owing to their chiral geometric configurations with nanoscale sizes. This property can lead to high sensitivity in sensing and improved resolution in imaging. Second, the plasmonic properties of chiral plasmonic nanoparticles can be tailored by their shapes, bringing more control over chiroptical responses. Chiral plasmonic nanoparticles can also be used for tailoring the angular momentum of light and creating functional optical devices. Third, chiral plasmonic nanoparticles are promising for achieving strong light–matter interactions, which will be highly useful for energy harvesting and conversion. Asymmetric absorption of LCP and RCP light by chiral nanoparticles can be harnessed to generate exotic electrical and mechanical responses. Chiral nanoparticles can therefore be used as building blocks for advanced nanorobots to manipulate objects at the nanoscale. In addition, chiral nanoparticles can play a key role in the development of quantum optics, data storage, and secure communication technologies. The future of chiral nanoparticles is bright, with many exciting potential applications on the horizon. This special issue of Advanced Optical Materials showcases cutting-edge research works on nanoparticles with chiroptical responses. The issue covers a broad range of topics related to chiral nanomaterials, including the synthesis and characterization of new chiral nanomaterials, the optimization of chiral morphologies, and the chiroptical properties of chiral nanomaterials. Various approaches for assembling chiral nanostructures and synthesizing colloidal chiral Au nanoparticles have been demonstrated. X. C. Wu et al. prepared chiral assemblies with strong chiroptical responses by adding chiral thiol molecules on Au nanorods (article number 2202804). Controlled assemblies of chiral core@shell nanostructures without the use of any chiral templating agent were explored by A. Désert et al. (article number 2300119). Three research works (article numbers 2300037, 2202858, and 2203119) were dedicated to the synthesis of chiral Au nanoparticles by seed-mediated growth methods. Theoretical modeling plays an important role in optimizing the chiroptical responses of chiral plasmonic nanoparticles. Computer-aided-design models and full-wave electrodynamic simulations were used by L. M. Liz-Marzán et al. to study the effect of the geometrical parameters on chiroptical responses (article number 2203090). H. J. Chen et al. explored the relationship between the geometrical chirality, far-field chirality, and near-field chirality by the use of a quasinormal mode expansion method (article number 2300599). Theoretical understanding of chiroptical responses can guide experimental developments to optimize chiral nanoparticles for potential applications in various fields. Chiroptical responses are not limited to noble metal nanoparticles, as evidenced by three studies. A review article (article number 2202859) summarized recent research works on chiral magnetic oxide nanomaterials. G. Markovich et al. demonstrated the chiroptical response of chiral tellurium nanorods (article number 2203142). Z. F. Huang et al. developed a layer-by-layer glancing angle deposition technique to fabricate ternary- and quaternary-alloy chiral nanoparticles (article number 2300696). The issue also features works on the applications of chiral nanoparticles in up- and down-shifting CPL emissions and second-order nonlinear optical responses. There are three articles that focus on the CPL emissions of chiral nanostructures. A review article (article number 2300337) presented the up- and down-shifting emissions of nanomaterials with chiroptical responses. Another work (article number 2203068) showed the CPL emissions of chiral copper-penicillamine nanosheets. The third work (article number 2300001) presented multicolor upconverted circularly polarized fluorescence from lanthanide-doped upconversion nanoparticles in chiral nematic liquid crystals. The second-order nonlinear optical response of chiral perovskite bulk crystals were studied by T. Y. Zhang et al. (article number 2203078). One research interest in chiral nanoparticles involves their abilities to create chiral photonic films with fascinating chiroptical properties. Understanding the difference between the chiroptical responses of nanoparticles dispersed in solutions or supported on substrates is important to the applications of the aforementioned chiral films, which was discussed by A. Govorov et al. (article number 2300013). Four more works explored the different aspects of chiral photonic films, including the fabrication methods, dynamic switching of light polarization (article number 2300618), broadband control of polarization in a low THz regime (article number 2300238), chiral plasmon resonance of a (chiral Au nanoparticle)-on-mirror structure (article number 2202865), and chiral silver mirrors (article number 2202831). The authors declare no conflict of interest. Jianfang Wang received his B.S. degree in inorganic chemistry and computer software design in 1993 from the University of Science and Technology of China and his M.S. degree in inorganic chemistry in 1996 from Peking University. After obtaining his Ph.D. degree in physical chemistry in 2002 from Harvard University, he carried out postdoctoral research at the University of California, Santa Barbara from 2002 to 2005. He became an assistant professor in the Department of Physics at the Chinese University of Hong Kong in 2005 and was promoted to an associate professor in 2011 and a full professor in 2015. His current research interests include colloidal metal nanocrystals, nanoplasmonics, nanophotonics, and plasmonic photocatalysis.