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Extended chiral metamaterials using vibrating circular dichroism to extract fingerprints


Recently, Chengkuo Lee’s team from the Department of Electrical and Computer Engineering, National University of Singapore proposed an enhanced vibrating circular dichroic (VCD) sensing platform based on plasma chiral metamaterials, which has 6 orders of magnitude signal enhancement and chiral molecular selectivity.

Under the guidance of coupling mode theory, the team used the in-plane and out-of-plane symmetrical breaking structures to achieve chiral metamaterial design through a two-step photolithography process, which increased the near-field coupling intensity and changed the ratio between absorption and radiation loss, thereby improving the interaction between chiral light and matter and enhancing the molecular VCD signal. In addition, the team demonstrated the thin-film sensing process for BSA and β-lactoglobulin, which contain secondary structures ɑ-helix and β-fold with detection limits up to zeptomole levels.

The team also explored, for the first time, the potential to enhance VCD spectra by demonstrating the selective sensing process of chiral mixtures, where mixing ratios can be successfully distinguished by the chiral metamaterials proposed in the paper. This discovery improves the sensing signal of the molecules, expands the information that can be extracted, and paves the way for label-free, compact, small-volume chiral molecular detection for stereochemical and clinical diagnostic applications.

Research background

The main limitation of vibrational circular dichroism (VCD) spectroscopy is the naturally weak signal, typically around 10-5 levels, which is 3 orders of magnitude smaller than the UV-visible circular dichroism (CD) molecular signal. Therefore, there is a great desire to develop enhanced VCD sensors to better calibrate the signal with a smaller sample volume while exploring its potential for selective detection of chiral mixtures.

One of the best candidates to bridge the gap in VCD signal detection limitations is the MIR nanophotonic platform, which utilizes ultra-confined light fields and resonant coupling. In addition, in combination with an unbalanced design framework, chiral nanostructures, also known as chiral metamaterials, can present anisotropic interactions with circularly polarized light. At the same time, due to the interesting coupling phenomenon between enantiomers and chiral metamaterials, enhanced optical chirality in the local near-field region also enables stronger chiral light-matter interactions, thereby enhancing research interest in valley-polarized optical luminescence, drug screening and sensing applications. However, little attention has been paid to enhancing weak VCD signals. These works lack design methods and optimization processes for microwave chiral metamaterials. Although previous work has demonstrated the relationship between nonradiative dissipation of CD and MIR metamolecules using time-coupled mode theory (TCMT), near-field hand-sign photo-matter interactions remain unexplored.

Regarding the manufacturing process, chemical synthesis and self-assembly techniques have been widely used to fabricate chiral nanostructures to achieve larger molecular CD signals. But these methods can be time-consuming and less efficient for fabricating MIR chiral structures because the size of the resonant structures increases to match longer wavelengths. In addition, lithography methods have also been used to pattern chiral metamaterials, but also require complex processes to break off-plane symmetry, which promises better performance. Although previous work has proposed VCD sensing cavities that break out-of-plane symmetry, the fabrication process can become a challenge. Therefore, there is an urgent need to develop a well-established and simple MIR chiral metamaterial to establish a feasible VCD sensing platform to expand its application by retrieving molecular fingerprints.

Innovative research

Here, the researchers propose vertically positioned nanorods with metal insulator metal (MIM) structures based on infrared chiral plasma metamaterials (IRCPMs) as sensing platforms. Like SEIRA, the team proposed surface enhanced vibrational circular dichroism (SEVCD) spectroscopy for enhanced molecular IR and CD signal sensing, as shown in Figure 1-2. As a design framework, the team proposes a loss engineering method based on time-coupled mode theory (TCMT) to design and optimize chiral metamaterials by studying the influence of loss ratio and near-field coupling coefficient on the difference of electromagnetic wave absorption between LCP and RCP. Chiral metamaterials are optimized by customizing these parameters and adjusting in-plane and out-of-plane asymmetry factors to achieve larger VCD signals.

This proposed chiral metamaterial not only provides near-field enhancement through in-plane clearance, but also establishes a 2.5D configuration to break out-of-plane symmetry. In addition, the IRCPMs proposed by the team require only a two-step electron beam lithography and metal stripping process.

First, the team developed an enhanced VCD sensing platform composed of chiral metamaterials that achieved 6 orders of magnitude enhancement compared to traditional VCD spectra. Secondly, the method of designing and optimizing MIR chiral metamaterials using time-coupled mode theory is described, which shows the importance of near-field coupling coefficient and loss ratio, which leads to the design of in-plane and out-of-plane symmetry-breaking dimensions. Third, using the sensing platform described above, the team demonstrated the protein thin-film sensing process and achieved a minimum detection limit of about 23 zeptomole levels. Fourth, the team reported for the first time an enhanced VCD sensing process with a mixed protein secondary structure with high selectivity for vibration transitions. Compared to previous chiral mixture sensors, the method in this paper also allows the selection of molecules from the vibrational transitions of the absorption spectrum, as shown in Figure 3.

The findings in this paper show a promising SEVCD chiral detection platform for molecular recognition, biomolecules, and pharmacological analysis from a variety of species and low concentrations on a chip, opening up a new avenue for chemical or biomedical applications, such as the study and analysis of chiral nanostructures in dynamic reactions.

Figure 1 How SEVCD spectroscopy using IRCPMs works

Figure 2 CD and IR-enhanced near-field plot

Figure 3 Experimental demonstration of chiral mixture sensing

The article was recently published in the top international academic journal Light: Science & Applications, entitled “Expanding chiral metamaterials for retrieving fingerprints via vibrational circular dichroism”, with Cheng Xu as the first author of the paper, Chengkuo Professor Lee is the corresponding author of the paper. (Source: LightScience Applications WeChat public account)

Related paper information:https://www.nature.com/articles/s41377‍-023-0‍1186-3

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