**6. Planar chiral nanostructures and metasurfaces**

Next perspective structures are planar plasmonic nanostructures with thickness less than the wavelength of the incident light. They are very attractive from a technological point of view because of a possibility of mass production using conventional lithographic methods. As well as for truly chiral forms with enantiomers that cannot be superimposed by any rotation in 3D space, the left and right enantiomers of a flat chiral shape are not superimposed on each other by the rotation in a plane. Handling in the latter case has been determined by the viewing side, which leads to inverted CD spectra when the planar structure is illuminated from the opposite normal direction. In contrast to three-dimensional spiral structures, the differential light absorption of LCP and RCP by planar plasmon nanostructures is due to the difference in LSPR-induced near-field distributions [88]. Similar effects were observed in the study of 2D chiral plasmon nanostructures (see **Figure 10**), such as G- [89], S- [90], L-shaped [91] nanostructures, gammadions [92, 93], nanohelix [75] asymmetric nanoparticles [94], checkerboard nanorods [95] and also for flat metamaterials, including thin metal films with two-dimensional chiral holes, such as dimers with nanogap [96, 97].

Chiroptical response of planar chiral plasmonic nanostructures can be highly sensitive to the presence and to the specifics of tertiary structure of biomolecules, as in the case of arrays of gold gammadion nanostructures [98]. These effects can be viewed in plasmonic peak shifts for RCP and LCP light upon adsorption of proteins and in the difference of the values ∆∆λ = ∆λRCP − ∆λLCP, and can been used as an analytic signal. The parameter ∆∆λ becomes zero: ∆∆λ = 0 for achiral adsorbed molecules. In this way, the picogram levels of proteins were detected.

**51**

proteins [101].

**Figure 10.**

order in complex biointerfaces [99].

*Chiral Hybrid Nanosystems and Their Biosensing Applications*

Furthermore, the proteins with different content of β-sheets give different CD response, so, providing information about the structure of the adsorbed biomolecules. This feature has been studied in detail using chiral "shuriken"-like goldcovered indentations created on polycarbonate templates using injection molding method and gold deposition [100]. The chiroptic properties of the far-field were characterized by the collection of ORD spectra in the reflection mode for linearly polarized incident light. The observed peak shifts demonstrated picogram limit of detection of protein. Conformational changes associated with protein binding to ligands can be accompanied by the changes in the asymmetry factor ∆∆λ, demonstrating sensitivity to the tertiary and domain (quaternary) structure of

*Adapted with permission from Kelly et al. [99], Copyright 2018 American Chemical Society.*

*Examples of planar chiral nanostructures: (a) arrays of G-shaped gold nanoparticles. Adapted with permission from Valev et al. [89], Copyright 2009 American Chemical Society. (b) Arrays of L-shaped gold nanoparticles. Adapted with permission from Ye et al. [91], Copyright 2017 American Physical Society. (c) Short-ordered arrays of comma-shaped gold nanoparticles [94]. (d) Gold metafilm with arrays of "shuriken" nanostructures.* 

More recent studies have shown that these chiral plasmon substrates can distinguish between proteins that have similar structures but have primary sequences that differ in one amino acid [102]. So, it can provide information about the structural

Thus, the "superchiral" fields generated by 2D chiral nanostructures can provide a unique opportunity to probe the chirality of adsorbed biomolecules at such levels, as conformation, orientation, molecular structure, and supramolecular ordering.

*DOI: http://dx.doi.org/10.5772/intechopen.93661*

*Chiral Hybrid Nanosystems and Their Biosensing Applications DOI: http://dx.doi.org/10.5772/intechopen.93661*

#### **Figure 10.**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

the high-index planes of growing particles.

*[77], Copyright 2018 Springer Nature.*

**Figure 9.**

as dimers with nanogap [96, 97].

**6. Planar chiral nanostructures and metasurfaces**

the formation of chiral nanocrystals of Te and Se, which can be used as a matrix for growing nanostructures of gold and silver tellurides [87]. Also, there was presented the growth of chiral gold nanoparticles in the opposite direction induced by amino acids and peptides [77]. The effect of different growth rates on the chiral morphology of gold nanocrystals in the presence of L-Cys or D-Cys was observed. The structure and CD spectra of these nanocrystals are shown in **Figure 9**. It has been revealed that in the presence of L-glutathione (GSH) growing nanocrystals have different morphologies. The proposed mechanism involves specific adsorption of Cys or GSH on

*Three-dimensional plasmonic helicoids controlled by cysteine chirality transfer: (a) CD spectra and (b–c) SEM images of chiral nanoparticles synthesized using L-Cys and D-Cys. The insets highlight the tilted edges (solid lines), cubic outline (dashed lines), and tilt angles (−φ and +φ). Reproduced with permission from Lee et al.* 

Next perspective structures are planar plasmonic nanostructures with thickness less than the wavelength of the incident light. They are very attractive from a technological point of view because of a possibility of mass production using conventional lithographic methods. As well as for truly chiral forms with enantiomers that cannot be superimposed by any rotation in 3D space, the left and right enantiomers of a flat chiral shape are not superimposed on each other by the rotation in a plane. Handling in the latter case has been determined by the viewing side, which leads to inverted CD spectra when the planar structure is illuminated from the opposite normal direction. In contrast to three-dimensional spiral structures, the differential light absorption of LCP and RCP by planar plasmon nanostructures is due to the difference in LSPR-induced near-field distributions [88]. Similar effects were observed in the study of 2D chiral plasmon nanostructures (see **Figure 10**), such as G- [89], S- [90], L-shaped [91] nanostructures, gammadions [92, 93], nanohelix [75] asymmetric nanoparticles [94], checkerboard nanorods [95] and also for flat metamaterials, including thin metal films with two-dimensional chiral holes, such

Chiroptical response of planar chiral plasmonic nanostructures can be highly sensitive to the presence and to the specifics of tertiary structure of biomolecules, as in the case of arrays of gold gammadion nanostructures [98]. These effects can be viewed in plasmonic peak shifts for RCP and LCP light upon adsorption of proteins and in the difference of the values ∆∆λ = ∆λRCP − ∆λLCP, and can been used as an analytic signal. The parameter ∆∆λ becomes zero: ∆∆λ = 0 for achiral adsorbed molecules. In this way, the picogram levels of proteins were detected.

**50**

*Examples of planar chiral nanostructures: (a) arrays of G-shaped gold nanoparticles. Adapted with permission from Valev et al. [89], Copyright 2009 American Chemical Society. (b) Arrays of L-shaped gold nanoparticles. Adapted with permission from Ye et al. [91], Copyright 2017 American Physical Society. (c) Short-ordered arrays of comma-shaped gold nanoparticles [94]. (d) Gold metafilm with arrays of "shuriken" nanostructures. Adapted with permission from Kelly et al. [99], Copyright 2018 American Chemical Society.*

Furthermore, the proteins with different content of β-sheets give different CD response, so, providing information about the structure of the adsorbed biomolecules. This feature has been studied in detail using chiral "shuriken"-like goldcovered indentations created on polycarbonate templates using injection molding method and gold deposition [100]. The chiroptic properties of the far-field were characterized by the collection of ORD spectra in the reflection mode for linearly polarized incident light. The observed peak shifts demonstrated picogram limit of detection of protein. Conformational changes associated with protein binding to ligands can be accompanied by the changes in the asymmetry factor ∆∆λ, demonstrating sensitivity to the tertiary and domain (quaternary) structure of proteins [101].

More recent studies have shown that these chiral plasmon substrates can distinguish between proteins that have similar structures but have primary sequences that differ in one amino acid [102]. So, it can provide information about the structural order in complex biointerfaces [99].

Thus, the "superchiral" fields generated by 2D chiral nanostructures can provide a unique opportunity to probe the chirality of adsorbed biomolecules at such levels, as conformation, orientation, molecular structure, and supramolecular ordering.
