**5. Biosensing with 3D chiral nanostructures**

To amplify the CD signal of a chiral molecule, the differential absorption of the chiral molecule must be increased. For an analytical estimate of the speed of light absorption, one can use the relation [72]:

$$A \propto \frac{\alpha \phi}{2} \left( \alpha'' \left| \tilde{E}" \right|^2 + \mathcal{X} \left| \tilde{B} \right|^2 \right) - G'' \alpha \text{Im} \left( \tilde{E}" \cdot \tilde{B} \right) \tag{1}$$

**49**

**Figure 8.**

*Chiral Hybrid Nanosystems and Their Biosensing Applications*

nally introduced in 1964 [73] and recently revised [72].

electric-magnetic polarizability of the molecule, and *Ẽ* and *B̃*are the complex electric and magnetic field, respectively. CD is determined by the second term, which is defined by both the intrinsic chirality of a molecule (*G*″ ≠ 0) and by local electromagnetic field ( ( ) <sup>∗</sup> Im · *E B* ), which needs to have nonorthogonal components

Thereby, to enhance the CD response of a chiral molecule one can modify the electromagnetic field to maximize the local optical chirality *C*, the quantity origi-

> ( ) <sup>∗</sup> = − <sup>0</sup> Im · 2 *C EB* ε ω

The present subsections consider the research directed at constructing plasmonic

The efforts of many groups of scientists have been expended in order to increase the optical chirality *C* (Eq. (2)) using chiral plasmonic nanostructures. One of the chiral structures that scientists have studied both theoretically and experimentally is nanohelix. The numerical modeling of such structures determined a strong CD effect [74]. However, for gold nanohelices 400 nm high, 400 nm in diameter, and 80 nm thick nanowires, the chirality enhancement factor was determined as 10, so the use of such structures in biosensing is limited [75]. This problem was solved by fabricating superstructures with several nanohelices [76] using direct laser recording methods [77, 78] or ion- and electron beam lithography [79–81]. The use of such structures made it possible to increase the optical chiral amplification factor up to two orders of magnitude and to adjust the CD signal from IR to the visible region of the spectrum. A schematic representation of the structure and the field distribution are shown in **Figure 8**. In addition, this approach made it possible to create core-

Recently, another method using three-dimensional chiral plasmonic nanohelices for probing was demonstrated [83]. Instead of using enhanced near-field chirality, the CD signal of Pd nanohelices was used to detect a (0.1–1%) hydrogen content. Large-area arrays of Pd and Pd-Au hybrid nanostructures were obtained using the angular nanoglancing deposition method [84], which combines block copolymer micellar nanolithography [85] with glancing-angle deposition (GLAD) [86].

The last considered approach for synthesis of chiral lattice is adsorption of chiral organic molecules on growing nanocrystals. The following review demonstrated

*3D chiral plasmonic nanostructures: (a, b) simulated four-helix gold nanostructure showing strongly confined optical chirality within the helices and (c) the graph presenting the enhancement of differential absorption signal depending on the radius of integration, for different length of a structure. Adapted with permission from* 

*Schäferling et al. [76], Copyright 2014 American Chemical Society.*

systems with enhanced near-field chirality and examples of biosensing based on

shell nanohelices with adjustable dissymmetry coefficients and CD [82].

(2)

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

of electric and magnetic field vectors.

these nanostructures.

where ω is the angular frequency of electromagnetic wave, *α*″, *χ*″ and *G*″ are the imaginary parts of electric polarizability, magnetic susceptibility and mixed

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

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

was reported that amyloid fibrils of α-synuclein CD were detected using helically arranged gold nanorods (see **Figure 7**) [60]. At the same time, chiro-optical activity was not detected when only α-synuclein monomers were present. This technique

*Composite α-synuclein fiber with 3D chiral arrangement of Au nanorods: (a) TEM image, (b) cryo-TEM tomography reconstruction, (c) extinction, and (d) CD spectra of Au nanorods monitored 30 min after the addition of 30 μL of purified brain homogenates from healthy (black) and Parkinson-disease-affected (red) patients. Adapted with permission from Kumar et al. [60], Copyright 2018 National Academy of Sciences.*

There are cases of aggregation of plasmonic nanoparticles, leading to the sharp optical changes upon initiation by only one enantiomer. D-glutamic acid led to the aggregation of gold nanoparticles coated with CTAB, which followed by a significant change in color, while for the L-enantiomer there were practically no changes [70]. Similarly, the enantioselective detection of D-cysteine by silver nanoparticles was recently demonstrated in the solution and in the special bacterial cellulose matrix [71].

To amplify the CD signal of a chiral molecule, the differential absorption of the chiral molecule must be increased. For an analytical estimate of the speed of light

> ( ) ( ) ∗ ∗ <sup>∝</sup> ′′ ′′ ′ + − ′ 2 2 · <sup>2</sup> *A E B G Im E B*

where ω is the angular frequency of electromagnetic wave, *α*″, *χ*″ and *G*″ are the

 ω

(1)

can be further expanded to detect infectious recombinant prions.

**5. Biosensing with 3D chiral nanostructures**

ω

αχ

imaginary parts of electric polarizability, magnetic susceptibility and mixed

absorption, one can use the relation [72]:

**48**

**Figure 7.**

electric-magnetic polarizability of the molecule, and *Ẽ* and *B̃*are the complex electric and magnetic field, respectively. CD is determined by the second term, which is defined by both the intrinsic chirality of a molecule (*G*″ ≠ 0) and by local electromagnetic field ( ( ) <sup>∗</sup> Im · *E B* ), which needs to have nonorthogonal components of electric and magnetic field vectors.

Thereby, to enhance the CD response of a chiral molecule one can modify the electromagnetic field to maximize the local optical chirality *C*, the quantity originally introduced in 1964 [73] and recently revised [72].

$$C = -\frac{\mathcal{E}\_0 \partial}{2} \text{Im} \left( \tilde{E}^\* \cdot \tilde{B} \right) \tag{2}$$

The present subsections consider the research directed at constructing plasmonic systems with enhanced near-field chirality and examples of biosensing based on these nanostructures.

The efforts of many groups of scientists have been expended in order to increase the optical chirality *C* (Eq. (2)) using chiral plasmonic nanostructures. One of the chiral structures that scientists have studied both theoretically and experimentally is nanohelix. The numerical modeling of such structures determined a strong CD effect [74]. However, for gold nanohelices 400 nm high, 400 nm in diameter, and 80 nm thick nanowires, the chirality enhancement factor was determined as 10, so the use of such structures in biosensing is limited [75]. This problem was solved by fabricating superstructures with several nanohelices [76] using direct laser recording methods [77, 78] or ion- and electron beam lithography [79–81]. The use of such structures made it possible to increase the optical chiral amplification factor up to two orders of magnitude and to adjust the CD signal from IR to the visible region of the spectrum. A schematic representation of the structure and the field distribution are shown in **Figure 8**. In addition, this approach made it possible to create coreshell nanohelices with adjustable dissymmetry coefficients and CD [82].

Recently, another method using three-dimensional chiral plasmonic nanohelices for probing was demonstrated [83]. Instead of using enhanced near-field chirality, the CD signal of Pd nanohelices was used to detect a (0.1–1%) hydrogen content. Large-area arrays of Pd and Pd-Au hybrid nanostructures were obtained using the angular nanoglancing deposition method [84], which combines block copolymer micellar nanolithography [85] with glancing-angle deposition (GLAD) [86].

The last considered approach for synthesis of chiral lattice is adsorption of chiral organic molecules on growing nanocrystals. The following review demonstrated

#### **Figure 8.**

*3D chiral plasmonic nanostructures: (a, b) simulated four-helix gold nanostructure showing strongly confined optical chirality within the helices and (c) the graph presenting the enhancement of differential absorption signal depending on the radius of integration, for different length of a structure. Adapted with permission from Schäferling et al. [76], Copyright 2014 American Chemical Society.*

#### **Figure 9.**

*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. [77], Copyright 2018 Springer Nature.*

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 the high-index planes of growing particles.
