Keisuke Goda (Professor, Department of Chemistry / Adjunct Professor, University of California, Los Angeles / Adjunct Professor, Wuhan University / Cooperative Researcher, Quantum Science and Technology Agency)

Tinghui Xiao (Assistant Professor, Department of Chemistry / Technician, Japan Quantum Science and Technology Agency)

Kotaro Hiramatsu (Assistant Professor, Research Centre for Spectrochemistry / PRESTO Researcher, Japan Science and Technology Agency)

Akihiro Isozaki, Project Associate Professor, Department of Chemistry

Key points of the presentation

• Chiral molecules are molecules with "chirality," a property in which a three-dimensional object cannot be superimposed on its mirror image, and are very important for drug discovery and materials development. Raman optical activity (ROA) spectroscopy, a method for analyzing the structure of chiral molecules in aqueous solution, has advantages over X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy in terms of simplicity (sample preparation time, cost, etc.), but the extremely weak signal of ROA spectroscopy has made it impractical.
• In this study, to solve this difficult problem, we developed a theory of all-dielectric chiral field-enhanced ROA using a nanostructured plate made of silicon, "silicon nanodisk array," and by using it as a measurement substrate for ROA spectroscopy, we experimentally demonstrated 100 times stronger interaction between the photoelectric field and chiral molecules compared to conventional ROA spectroscopy. The interaction between the photoelectric field and chiral molecules was experimentally demonstrated to be 100 times stronger than that of conventional ROA spectroscopy.
• This method enables simple, rapid, inexpensive, and stable absolute structural analysis of small amounts of chiral molecules, which is impossible by X-ray crystallography or NMR spectroscopy. It is expected to be applied to various fields such as analytical chemistry, structural biology, material science, pharmaceutical science, and quantum life science.

Summary of Presentation

A research group led by Professor Keisuke Goda at the Graduate School of Science, The University of Tokyo, has developed a new spectroscopic measurement method that can sensitively detect chirality using "silicon nanodisk arrays," which have a nanostructure made of silicon. By optimizing the structure of the silicon nanodisk array, the optical chirality ^{(Note 1) of the} nearby photoelectric field was skillfully controlled, and the interaction strength between the photoelectric field and chiral molecules was successfully increased up to 100 times compared to the conventional method using circularly polarized light ^{(Note 2)}. This has realized a high sensitivity Raman Optical Activity (ROA) ^{(Note 3)} spectroscopy and extended the range of its application.

ROA spectroscopy, which was demonstrated in the 1970s, is effective for studying the torsional structure and behavior of chiral molecules in aqueous solution, and has advantages over X-ray crystallography ^{(Note 4)} and nuclear magnetic resonance (NMR) spectroscopy ^{(Note 5)} in terms of simplicity (time and cost for sample preparation). However, ROA spectroscopy has signal However, the signal of ROA spectroscopy is in principle three to five orders of magnitude weaker than the Raman signal of Raman spectroscopy ^{(Note 6} ) due to the weak interaction of light and matter in chiral molecules. Localized Surface Plasmon Resonance (LSPR) ^{(Note 7)} of metal nanoparticles has been employed to enhance the ROA signal, but it suffers from artifacts in the ROA signal. Specifically, there are two problems: optical heat generation by LSPR and the inability to efficiently transfer and enhance optical chirality from the far field to the near field.

In this study, to solve these difficulties, we developed a silicon nanodisk array and demonstrated all-dielectric chiral field-enhanced ROA by utilizing its dark modes. By using it as a substrate for ROA spectroscopy measurements, we measured pairs of chemical and biological mirror-image isomers ^{(Note 8} ), respectively, and demonstrated interactions that are 100 times stronger than those of conventional ROA spectroscopy, while keeping artifacts negligible in ROA spectroscopy measurements.

This method enables simple, rapid, inexpensive, and stable absolute structural analysis of small amounts of chiral molecules, which is impossible by X-ray crystallography or NMR spectroscopy. In addition, this method is expected to be applied in diverse fields such as analytical chemistry, structural biology, material science, pharmaceutical science, and quantum life science (Figure 1).

Figure 1: Conceptual diagram of this research. Conventional X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy are highly sensitive, but have problems in sample preparation and cost. On the other hand, Raman optical activity (ROA) spectroscopy is simple but has a low sensitivity problem. In this study, to solve this difficult problem, we developed a theory of all-dielectric chiral field-enhanced ROA using a nanostructured plate made of silicon, "silicon nanodisk array," and by using it as a measurement substrate for ROA spectroscopy, we experimentally demonstrated 100 times stronger interaction between the photoelectric field and chiral molecules compared to conventional ROA spectroscopy. The interaction between the photoelectric field and chiral molecules was experimentally demonstrated to be 100 times stronger than that of conventional ROA spectroscopy. This method enables simple, rapid, inexpensive, and stable absolute structural analysis of small amounts of chiral molecules, which is impossible by X-ray crystallography or NMR spectroscopy. It is expected to be applied to various fields such as analytical chemistry, structural biology, material science, pharmaceutical science, and quantum life science.

This research was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Flagship Program for Photonics and Quantum Leap (JPMXS0120330644), the Japan Society for the Promotion of Science (JSPS) Research Center Initiative and Grant-in-Aid for Scientific Research (JP18K13798, JP20K14785), the Murata Foundation for Science and Technology, White Rock Foundation, The University of Tokyo The project was supported by the GAP Fund Program.

The research results were published online in Nature Communications on May 24, 2021 (6:00 PM).

Publication Details

Background and History of the Research
Chiral molecules are molecules with "chirality," a property in which a three-dimensional object cannot be superimposed on its mirror image. Since most of the biomolecules that make up our bodies are chiral molecules, their effects on the body can be completely different even if they are mirror images of each other. Therefore, chirality is important structural information for drug discovery and material development. The importance of chirality can be seen from the fact that three researchers, including Ryoji Noyori, were awarded the 2001 Nobel Prize in Chemistry ^{(Note 9)} for their research on asymmetric reactions using chiral molecules as catalysts. Raman optically active (ROA) spectroscopy, a method for spectroscopically analyzing the conformation and absolute configuration of chiral molecules in aqueous solution, can identify the absolute configuration of chiral molecules in aqueous solution by measuring the difference in scattering intensity of Raman spectroscopy using right circularly polarized light (RCP) and left circularly polarized light (LCP). Since its first demonstration in the early 1970s, ROA spectroscopy has been used to estimate the steric structures of proteins, nucleic acids, carbohydrates, viruses, and biopolymers, etc. Since ROA spectroscopy can easily estimate molecular structures under bioactive conditions, it is expected to be applied in various fields such as biochemistry, analytical chemistry, structural biology, and pharmaceutical sciences. It is expected to be applied in various fields such as biochemistry, analytical chemistry, structural biology, and pharmacology.

Unfortunately, the signal from ROA spectroscopy is very weak. Typically, the ROA signal is three to five orders of magnitude weaker than the Raman spectral signal, and it takes several hours (sometimes days) to acquire ROA spectra of biomolecules. This signal weakness is the biggest obstacle to the practical application of ROA spectroscopy in various fields. To solve this problem, surface-enhanced ROA (SEROA) measurements, which enhance the ROA signal using localized surface plasmon resonance (LSPR) induced by metal nanoparticles, have been reported SEROA (Surface-enhanced ROA: SEROA) measurements have been reported. However, SEROA spectra reported so far have poor reproducibility and biocompatibility. The reasons for this were considered to be that the polarization state on the metal surface changes with time due to random movement and disorder in the arrangement of metal nanoparticles suspended in aqueous solution, and that the structure of molecules adsorbed on the particle surface is changed by photothermal conversion in the metal nanoparticles.

Description of Research
In this study, by skillfully controlling the optical chirality of silicon nanodisk arrays, the interaction between circularly polarized incident light and chiral molecules is greatly enhanced by utilizing the dark modes of silicon nanodisk arrays, and an all-dielectric (metal-free) chiral field-enhanced ROA is theoretically proposed and experimentally Demonstration. This overcomes the limitation of the LSPR-based ROA spectroscopy. The dark mode is a combination of an electric dipole and a toroidal dipole (a dipole induced by a magnetic field in the shape of a doughnut), and in the far field these dipoles partially cause destructive interference. Specifically, an optically isotropic silicon nanodisk array was designed and fabricated on a chip. This allowed us to precisely adjust the optical chirality in the near-field during the signal acquisition time and avoid artifacts in the enhanced ROA spectroscopy measurements. In addition to the physical advantages of silicon nanodisk arrays, their fabrication process uses the same equipment used in semiconductor production (Figure 2), making them applicable for use with other on-chip devices and for mass production for chiral measurements. To demonstrate the practicality of the method, ROA spectroscopic measurements of a pair of chemical and biological mirror-image isomers, (±)-α-pinene ^{(Note 10) (} Figure 3) and (±)-tartaric acid ^{(Note 11) (} Figure 4), were performed, and a two-phase virtual with very slight artifact In enantiomeric ROA optics, the enhancement of the ROA signal in the near-field was about 100-fold.

Figure 2: Silicon nanodisk arrays. a. Fabrication method of silicon nanodisk arrays. ebl: electron beam lithography. icp: inductively coupled plasma etching. b. Scanning electron microscope image of a fabricated silicon nanodisk array. c. ROA spectroscopy of silicon nanodisk arrays.

Figure 3: Demonstration of chiral field-enhanced ROA spectroscopy of all dielectrics using chiral molecules. a. Experimental system for ROA spectroscopy measurements using conventional silica and silicon nanodisk array substrates. b. Raman spectrum of (±)-α-pinene on silicon nanodisk array. c. Raman spectrum of (±)-α-pinene on silicon nanodisk array. The Raman intensity is enhanced by about 100 times by the silicon nanodisk array. c. ROA spectrum of (±)-α-pinene on silicon nanodisk array. the ROA intensity is enhanced by about 100 times by the silicon nanodisk array.

Figure 4: Demonstration of chiral field-enhanced ROA spectroscopy of all dielectrics using biochiral molecules. a. Experimental system for ROA spectroscopy measurements using conventional silica substrate and silicon nanodisk array substrate. b. Raman spectrum of (±)-tartaric acid on silicon nanodisk array. The Raman intensity is enhanced by about 100 times by the silicon nanodisk array. c. ROA spectrum of (±)-tartaric acid on silicon nanodisk array. the ROA intensity is enhanced by about 100 times by the silicon nanodisk array.

Future Development
This method enables us to analyze absolute structures of small amounts of chiral molecules easily, rapidly, inexpensively, and stably, which is impossible by X-ray crystallography or NMR spectroscopy. Therefore, this method is expected to be applied in various fields such as analytical chemistry, structural biology, and pharmaceutical sciences. For example, it can be used for antigen-antibody analysis of infectious diseases (influenza, novel coronavirus infection, etc.), cancer metabolism profiling analysis, absolute structure analysis of chiral drugs, and quantum life science research by molecular vibration measurement of biomolecules of photosynthetic organisms.

Research Team
The research team consists of Tinghui Xiao (Assistant Professor, Department of Chemistry, Graduate School of Science, The University of Tokyo / Cooperative Researcher, National Institute of Quantum Science and Technology), Zhenzhou Cheng (Assistant Professor, Department of Chemistry, Graduate School of Science, The University of Tokyo at the time of the research), Zhenyi Luo (Assistant Professor, Graduate School of Science, The University of Tokyo (at the time of the research: Master's student, Department of Chemistry, Graduate School of Science, The University of Tokyo), Akihiro Isozaki (Associate Professor, Department of Chemistry, Graduate School of Science, The University of Tokyo), Kotaro Hiramatsu (Assistant Professor, Department of Chemistry, Graduate School of Science, The University of Tokyo/ PRESTO Researcher, National Institute of Science and Technology (JST)), Tamotake Ito (Senior Project Researcher, Institute of Engineering Innovation, National Institute of Technology (NIH)), and Zhenzhou Cheng (at the time of the research: Assistant Professor, Department of Chemistry, Graduate School of Science, The University of Tokyo), Zhenyi Luo (at the time of the research: Master's student, Graduate School of Science, The University of Tokyo), Kotaro Hiramatsu (Assistant Professor, Department of Spectrochemistry, Graduate School of Science, The University of Tokyo), and Kotaro Hiramatsu (Assistant Professor, Department of Chemistry, Graduate School of Science, The University of Tokyo). Senior Staff, Department of Bioengineering), Masahiro Nomura (Associate Professor, Institute of Industrial Science, The University of Tokyo), Satoshi Iwamoto (Professor, Institute of Industrial Science, The University of Tokyo), Keisuke Goda (Professor, Department of Chemistry, Graduate School of Engineering, The University of Tokyo / Adjunct Professor, Faculty of Engineering, The University of California, Los Angeles) Keisuke Goda (Professor, Department of Bioengineering, School of Engineering, University of California, Los Angeles; Adjunct Professor, Institute of Industrial Science, Wuhan University; and Cooperating Researcher, National Institute of Quantum Science and Technology, National University Corporation).

Journals

Journal name	Nature Communications
Title of paper	All-dielectric chiral-field-enhanced Raman optical activity
Author(s)	Ting-Hui Xiao, Zhenzhou Cheng, Zhenyi Luo, Akihiro Isozaki, Kotaro Hiramatsu, Tamitake Itoh, Masahiro Nomura, Satoshi Iwamoto, and Keisuke Goda* (author)
DOI Number	10.1038/s41467-021-23364-w
URL	https://www.nature.com/articles/s41467-021-23364-w

Terminology

Note 1 Optical Chirality

Light is an electromagnetic field that propagates in spatio-temporal space. Like ordinary objects, light can define a chirality determined by the spatial structure of its electromagnetic field, which is called optical chirality. ↑up

Note 2 Circular polarization

The polarization state of light in which the direction of the electric field is rotated by propagation. Circular polarization is sometimes used in 3D displays to project different images to the right and left eyes. ↑up

Note 3 Raman Optical Activity (ROA)

One of the vibrational spectroscopic methods that can discriminate the chirality of molecules. When measuring chiral molecules, the Raman spectra measured with clockwise and counterclockwise circularly polarized light have slightly different shapes. Raman optical activity spectroscopy measures the difference. It is used to determine the absolute conformation of small molecules and to estimate the higher-order structure of proteins in aqueous solution. ↑up

Note 4 X-ray crystallography

An analytical method to determine molecular structure by analyzing diffraction patterns between electrons scattered by electrons around atomic nuclei. It is widely used to determine the structure of proteins in particular. Although precise molecular structure information can be obtained, it requires a large amount of time and labor to prepare the sample as a single crystal is necessary for the measurement. ↑up

Note 5 Nuclear magnetic resonance (NMR) spectroscopy

A spectroscopy method that measures the resonance (nuclear magnetic resonance) between the precession of nuclear spins generated at a certain frequency by an externally applied static magnetic field and a rotating magnetic field. Since the frequency of the precession varies depending on the environment around the nucleus (chemical bonding state, etc.), nuclear magnetic resonance measurements can be used to estimate the molecular structure. ↑up

Note 6 Raman spectroscopy

A molecular spectroscopy method that estimates the electronic, rotational, and vibrational states of materials by measuring inelastic scattering of light. It is widely used to estimate the vibrational state of molecules in particular. Since label-free and non-invasive molecular measurement is possible simply by irradiating a laser beam onto the measurement target, various applications such as bio-imaging and chemical analysis are being developed. ↑up

Note 7 Localized surface plasmon resonance (LSPR)

When metal nanoparticles are irradiated with light, free electrons on the surface of the particles move collectively. Localized surface plasmon resonance occurs when the eigenfrequency of the collective motion of electrons, which is determined by the shape of the nanoparticles and their constituent atoms, matches the frequency of the irradiating light, inducing a particularly large collective motion of electrons. ↑up

Note 8 Mirror-image isomers

Structures of chiral molecules that are in a mirror-image relationship with each other are called mirror isomers, and are distinguished by the prefix (+) or (-) in front of the molecular name. ↑up

Note 9: Nobel Prize in Chemistry 2001

Awarded to Ryoji Noyori, William Knowles, and Barry Sharpless for the development of chiral selective chemical reaction processes using asymmetric catalysis. ↑up

Note 10 (±)-α-pinene

A chiral molecule found in many conifers such as pine, cypress, and cedar, which is widely used as a raw material for fragrances and medicines because of its unique aroma. ↑(±)-tartaric acid

Note 11 (±)-Tartaric acid

A chiral molecule found in many sour fruits, especially grapes and wine. ↑ (±)-Tartaric acid