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Wang, F.; Wang, X.; Lu, X.; Huang, C. Nanophotonic Platforms for Enhancing Chirality Sensing. Encyclopedia. Available online: https://encyclopedia.pub/entry/53859 (accessed on 18 May 2024).
Wang F, Wang X, Lu X, Huang C. Nanophotonic Platforms for Enhancing Chirality Sensing. Encyclopedia. Available at: https://encyclopedia.pub/entry/53859. Accessed May 18, 2024.
Wang, Fei, Xue Wang, Xinchao Lu, Chengjun Huang. "Nanophotonic Platforms for Enhancing Chirality Sensing" Encyclopedia, https://encyclopedia.pub/entry/53859 (accessed May 18, 2024).
Wang, F., Wang, X., Lu, X., & Huang, C. (2024, January 16). Nanophotonic Platforms for Enhancing Chirality Sensing. In Encyclopedia. https://encyclopedia.pub/entry/53859
Wang, Fei, et al. "Nanophotonic Platforms for Enhancing Chirality Sensing." Encyclopedia. Web. 16 January, 2024.
Nanophotonic Platforms for Enhancing Chirality Sensing
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This entry is adapted from the peer-reviewed paper 10.3390/bios14010039

Chiral sensing is crucial in the fields of biology and the pharmaceutical industry. Many naturally occurring biomolecules, i.e., amino acids, sugars, and nucleotides, are inherently chiral. Their enantiomers are strongly associated with the pharmacological effects of chiral drugs. The nanophotonic platform allows for a stronger interaction between the chiral molecules and light to enhance chiral sensing. 

chiral sensing nanophotonics circular dichroism

1. Structural Chirality Based on Self-Assembly

During the self-assembly process, the optical properties of nanoparticles are fine-tuned via the particle size, shape, and composition, which makes self-assembly one of the most promising tools for constructing chiral macroscopic materials. Owing to their inherent structural chirality, DNA [1][2][3], peptides [4][5], and proteins are usually selected as templates to assemble achiral metal nanoparticles into chiral structures. Nguyen et al. used DNA origami technology to synthesize highly stable gold-silver core-shell nanoparticles [6]. The silver shell induced more plasmonic enhancement than gold nanorods, and the bimetallic chiral assembly showed a strongly increased CD response, which holds great promise for plasmonic sensing. Wang et al. proposed self-assembled reconfigurable plasmonic diastereomers based on DNA origami nanotechnology [2]. Using a stepwise assembly strategy, they constructed plasmonic diastereomers with up to three distinguishable chiral centers.
Compared to DNA, amino acids, micelles, etc., the use of naturally “hard” inorganic templates provides more freedom for further processing steps or the functionalization of optically active nanoparticles [7]. In 2017, Cheng et al. suggested the utilization of silica nanohelices as a chiral template substrate for the self-assembly of gold nanoparticles through electrostatic adsorption, leading to a chiral three-dimensional superstructure composed of gold helices [8]. Similarly, employing the nanosilica helical structure as a template, Negrín-Montecelo et al. sequentially self-assembled gold and TiO nanoparticles onto the substrate, and finally achieved a composite chiral structure with photocatalytic properties [9]. In addition to silica templates, layered van der Waals materials (WS2) have also been used as templates for the chiral assembly of nanoparticles. Kachtík et al. found that both metallic and dielectric nanoparticles were adsorbed to the nanotube surface through the chemically active site: an unsaturated sulfur bond in WS2 [7]. 
An alternative approach for fabricating chiral plasmonic nanostructures involves directional growth during the nanoparticle synthesis process [10][11]. The synthesized chiral gold nanoparticles exhibit an asymmetric factor of up to 0.04 in the visible-light wavelength range. In addition, the disparate binding energies of various amino acid enantiomers to the gold crystal surface regulate the growth pathways of nanoparticles. Consequently, the utilization of amino acids and small peptides allows for the controlled growth of gold nanoparticles. Lee and co-workers [12] developed a method to control the chiral morphology of gold nanoparticles through the molecular interactions of amino acids or peptides with high-refractive-index surfaces. Gold nanoparticles fabricated by this method display strong chiral plasmonic optical activity (asymmetry factor of 0.2), even when randomly dispersed in solution. Halides, especially iodide and bromide ions, show preferential adsorption on specific crystal planes, thus initiating the formation of chiral structures with noble metal nanoparticles of different shapes [13]. Recently, in recognition of the influence of anisotropic seeds and the effect of halides on directional growth, Zheng et al. [14] introduced a growth strategy termed Halide-Assisted Differential Growth (HADG), which is applied to anisotropic metal nanoparticle seeds and successfully yields plasmonic metal nanocrystals characterized by distinct chiral shapes.

2. Superchiral Near-Fields

In 1964, Daniel M. Lipkin [15] first introduced a new time-even pseudoscalar, whose physical meaning was later supplemented and improved by Tang et al. [16] in 2010, i.e., optical chirality C, which is defined as:
 
where 𝜀0 and 𝜇0 are the permittivity and permeability in free space, respectively, and E and B represent the time-dependent electric and magnetic fields. The angle βiE,H indicates the phased angle between iE and H. In Equation (1), to generate a larger C, the following three conditions must be satisfied: firstly, electric field E and magnetic field H must be enhanced; secondly, both E and H must have parallel components; thirdly, E and H must have a non-zero phase difference, preferably a phase difference of π/2. For CPL in free space, the optical chirality CCPL is given by:
 
where E0 is the electric field magnitude in the free space. The sign of CCPL is contingent upon the handedness of CPL (LCP: +; RCP: −−). With 𝐶/𝐶CPL>, a “superchiral” field is generated. The relationship between the optical chirality C and the rate of absorption by a chiral molecule is typically described by [16]:
 
In Equation (3), 𝐴± is the absorption rate of chiral molecules for LCP and RCP, respectively, and 𝛼″ is the electric polarizability. 𝜒″ and 𝐺″ represent the imaginary components of the magnetic and chiral polarizability of the chiral molecule, respectively. According to the definition of CD, it can be expressed as:
 
From Equation (4), increasing the optical chirality, C, is an effective solution to enhance the CD magnitude of the chiral molecule detection.

2.1. Plasmonic Nanostructures

Owing to their intrinsic chirality, chiral plasmonic substrates are prone to generating a strong optical chirality C, which makes them competitive candidates for chiral sensing based on superchiral field. However, the left- and right-handedness exist simultaneously in many structures and cancel each other out, ultimately showing a weak overall near-field optical chirality. To overcome this challenge, Schäferling et al. [17] proposed a 3D chiral substrate composed of multiple helices, which greatly enhances optical chirality C over a larger sensing volume. Specifically, the fundamental mode of the helical plasmonic nanoantenna exhibits non-orthogonal electric and magnetic dipole moments. Inside the structure, the electric field vector (red field) and magnetic field vector (blue field) are mainly parallel, resulting in a non-zero optical chirality. Changing the chirality of the structure flips the relative direction of the field vectors, and thus shifts the chirality near field. To achieve both a strong chiral near field and good coupling to external fields, additional helices are added.
Traditional methods for constructing 3D structures often involve 3D printing or the assembly of multiple layers [18], which lead to challenges associated with large feature sizes, extended processing times, and intricate procedures. Alternatively, various origami methods that address the limitations associated with folding and bending 2D films have been developed, providing innovative solutions for complex three-dimensional structures from initially flat materials. 
Two-dimensional planar chiral nanostructures are also known as pseudochiral nanostructures but exhibit hand-dependent responses to circularly polarized light at normal incidence. These structures acquire chirality through chiral-substrate-induced symmetry breaking or manufacturing defects, which eliminates the need for 3D characteristics, thus simplifying the fabrication process [19].
In addition to the optical chiral characteristics observed in the chiral structures mentioned above, optical chirality is also generated in achiral structures. In achiral structures, chirality results from symmetry breaking, including oblique illumination, structural plane tilt, etc. Horrer et al. [20] revealed the optical chirality inside highly symmetric plasmonic metamolecules. Taking a trimer composed of three gold nanodisks arranged in an equilateral triangle, they found that the local optical chirality originates from the near-field interference coupling between plasmon modes generated by a single nanodisk.

2.2. Dielectric Nanostructures

In addition to plasmonic nanostructures, dielectric nanostructures are also used for generating superchiral fields. High-index dielectric materials, i.e., silicon, intrinsically support electrical and magnetic resonances simultaneously, which makes them attractive for chiral sensing. Compared with plasmonic metallic structures, although dielectric nanomaterials have lower optical losses [21], their local electric field enhancement is weaker. Rui’s group [22] proposed a metasurface consisting of a dimer array of silicon nanocylinders in a square lattice. Unlike traditional structures, which require circularly polarized light to excite the superchiral near field, the researchers found that illumination with off-axis polarization leads to superchiral localized hot spots in the gaps of the dimer structure. Meanwhile, the near-field electric and magnetic fields of a single resonator are significantly modified by coupling with adjacent resonators, resulting in stronger local field enhancement. The optical chirality field is usually positive on all cutting planes, which is very important for enhanced chiral sensing. As far as the fabrication is concerned, since the critical dimensions of the structure exceed 300 nm, it is expected to be compatible with wafer-level nanomanufacturing processes [23], such as deep ultraviolet lithography [24], and nanoimprinting [25].

2.3. Plasmonic and Dielectric Hybrid Platform

Since metallic and dielectric nanoparticles provide strong electric and magnetic resonance, metal–dielectric hybrid structures are expected to achieve strong near-field optical chirality. To obtain optimal optical chirality, in addition to maximizing the resonance intensity, the resonances must also spectrally coincide. Simultaneously, the components of the electric and magnetic fields must be parallel and possess a π/2 phase shift, as well as overlap in space. To satisfy the conditions for optimal optical chirality, Mohammadi et al. [26] proposed a hybrid composition of metallic (gold) and high-refractive-index dielectric (silicon) particles, in which both metallic and dielectric nanoparticles provide electric and magnetic resonances. Since the helicity of the incident field is retained, the phase coefficient is perfect, and the scattered light with a phase difference of π/2 between the electric and magnetic field components is ensured, a more perfect superchiral field is produced.

3. Plasmon-Coupled Circular Dichroism

PCCD is another important method used to identify chiral molecules, where the CD originates from the Coulomb interaction (dipole and multipole) between the chiral molecule and the achiral plasmonic structures [27]. Compared with the absorption peak of the FlgA3 peptide located in the ultraviolet band, the FlgA3 peptide–Au particle complex produced a new absorption peak at ~520 nm, resulting in the repeatable characteristic peaks in the visible light band of CD spectrum. 
Recently, the stability of PCCD-based plasmonic nanoparticles has also attracted widespread attention. In most PCCD systems, chiral molecules are gathered on the surface of plasmonic nanoparticles through chemical adsorption, and the desorption of molecules undoubtedly seriously affects the stability of the entire coupled system. Wei et al. found that this problem can be significantly improved by embedding chiral molecules into Ag NPs [28]. 

4. Surface Plasmon Resonance Platform

Recently, researchers have discovered that macroscopic platforms based on surface plasmons are also expected to recognize and detect chiral molecules. Droulias and Bougas [29] proposed an angle-resolved chiral surface plasmon resonance (SPR) scheme based on the Kretschmann configuration, which detects the absolute chirality (handedness and magnitude) of chiral samples. As SPR reflectance is sensitive to both the real and imaginary parts of the refractive index of the chiral samples, the reflection spectrum curve indicates that the excitation angle of SPR is different for various chiral layers. Therefore, the excitation angle is used to characterize the chiral molecules. Although this work only contains theoretical calculations and simulation results, it provides a simple idea for the identification of chiral enantiomers.
Recently, the concepts of transverse spin angular momentum and Bellinfant spin momentum of evanescent waves have attracted widespread attention [30][31]. As a type of evanescent wave, surface plasmons also carry an inherent transverse spin angular momentum, which is locked in its propagation direction due to the quantum spin Hall effect of light. Introducing monochirality into the dielectric medium facilitates the differentiation of chiral enantiomers. Zhang et al. [32] found that chiral molecules are subject to chiral-selective lateral forces in opposite directions, which are not only used to identify chiral enantiomers but are also expected to achieve the separation of chiral enantiomers.

5. Comparison of the Four Methods

Benefiting from the LSPR effect of single nanoparticles and the mutual coupling between adjacent nanoparticles, chiral nanostructures based on nanoparticle self-assembly indicate a greater chiral optical response compared with chiral molecules. The preparation of chiral nanoparticles is mainly based on bottom-up chemical processes with broad application prospects in biochemical fields, i.e., chemical catalysis, disease diagnosis and treatment.
The nanophotonic platform based on the superchiral field utilizes the metal micro-nano structure to generate a near-field optical chirality that is stronger than the incident light. Interacting with the superchiral field, a stronger signal from the nearby chiral molecules is obtained. Although plasmonic nanostructures exhibit strong electric dipolarization and enhance near-field electric fields, their magnetic field enhancement is very limited. Therefore, it is crucial to design structures that enhance both electric and magnetic fields, especially magnetic components. On the other hand, as the nanostructures produce a strong CD response, it is not easy to effectively distinguish the contribution from the structure itself and the chiral analytes to the CD signal. At present, the literature [33] points out that this problem can be alleviated by using racemic nanostructures to detect chiral analytes.
Unlike superchiral structures directly generating CD signals, the PCCD induces the CD signals originating from Coulomb (dipole and multipole) interactions between achiral plasmonic nanostructures and chiral molecules. There is preliminary evidence that single-molecule PCCD is achievable, indicating that PCCD is promising for high-sensitivity measurements and even single-molecule detection [34][35].
All three of the above solutions involve the precise design and fabrication of nanostructures, which are avoided for platforms by using propagating surface plasmons. This scheme is compatible with existing surface plasmon resonance platforms based on the Kretschmann configuration and can identify chiral enantiomers with simple process and low cost. However, this solution is still in the theoretical exploration stage, and has no corresponding experimental results at present.

References

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Fei Wang
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Xue Wang
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Xinchao Lu
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Chengjun Huang
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