**2. Chiral molecules coupled to a single plasmonic nanoantenna**

The absorbance of an achiral excitonic system can be strongly enhanced via the interaction with plasmonic particles due to the antenna effect [13]. Similar phenomena can amplify the chiroptical response of a chiral molecular system coupled to a plasmon, which is highly desirable for (bio)sensing, as it can increase the sensitivity and lower the detection limit of a sensor.

It was experimentally demonstrated that silver nanoparticles induce nearly two orders of magnitude amplification of chromophore's CD. Besides, the CD signal appeared at the same wavelength as the Localized Surface Plasmon Resonance (LSPR) of the nanoparticles [14]. These results initiated thorough theoretical studies of the nature of plasmon-induced CD amplification [15–18]. It has been found that binding of a chiral molecule to a small achiral plasmonic nanoparticle (particle size ≈ 10 nm) can enhance the molecular CD signal in conjunction with the appearance of a new CD signal at the plasmon resonance wavelength. The suggested model is presented in **Figure 1**.

The charge is localized in the small gap between the molecule and the nanoparticle and the Coulomb interactions between their electronic systems can be very strong. As a result, the CD signal induced by molecular transition can be boosted by plasmonic

#### **Figure 1.**

*The model of the metal nanoparticle-dye molecule assembly (left) and the scheme of excitation-deexcitation processes (right) with solid vertical (horizontal) arrows representing light (Coulomb)-induced transitions. The dotted vertical arrows show the relaxation processes. Adapted with permission from Govorov et al. [15], Copyright 2010 American Chemical Society.*

**43**

**Figure 2.**

*Copyright 2011 American Chemical Society.*

*Chiral Hybrid Nanosystems and Their Biosensing Applications*

nanoparticles with 10 nm radius, is presented in **Figure 2** [19].

tures with plasmon resonance in UV for biosensing [24, 25].

resonance, and, at the same time, the surface current in plasmonic nanoparticle becomes chiral due to the chiral molecule, resulting in a new CD signal [15]. With increasing distance (d) between the metal nanoparticle and the molecule, the near field intensity decreases as d−3. Since most biomolecules have CDs in the UV range, the appearance of the CD peak in the visible range is important for practical purposes

A number of experimental studies support these findings. The CD peak in the visible range, generated by an adsorption of peptide molecules on spherical gold

Similar results were obtained for a bilayer of riboflavin 50-monophosphate and polylysine molecules adsorbed on gold island films [20]. For cysteine and its derivatives in the presence of 45 nm spherical silver nanoparticles [21], for glutathione molecules adsorbed on 45 nm silver nanocubes [22] and for tobacco mosaic virus uniformly covered by spherical 5 nm gold nanoparticles [23] the enhanced chiroptical signals in the visible spectral range were also detected. The authors of the latter work achieved near-field amplification of the induced CD signal controlling by the separation between the molecule and the metal nanoparticle. The adsorbed chiral molecules were found to be in resonance with plasmon for the new appeared CD peak. Though, matching the wavelengths of molecular absorption and plasmon resonance intensified the effect, which suggests the use of nanostruc-

Thus, to amplify CD signal of small chiral molecules a plasmonic nanoantenna can be used. But practical application of this approach in biosensing is limited by strong sensitivity to separation distance. It was shown that the induced CD can be sensitive to the orientation of the transition dipole moments of the adsorbed molecule relative to the local plasmon polarization of the nanoparticle [26]. Two orders of magnitude enhancement coefficients of CD were observed for 42 ± 2 nm gold/silver core-shell nanocubes coated with DNA molecules [24]. Additionally, the induced CD signal can be sensitive to the orientation of the transition dipole moments of the adsorbed molecule relatively to the local plasmonic polarization of

*CD spectra of E5 peptide (black line), gold nanoparticles (green line), and their assembly (red line). The inset shows the appearance of the CD peak in the visible range. Reproduced with permission from Slocik et al. [19].* 

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

as it can be easily detected.

the nanoparticle [26].

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

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

orientation and high-order structure.

sensitivity and lower the detection limit of a sensor.

suggested model is presented in **Figure 1**.

spectroscopy [8, 9]. Many chiral plasmonic nanostructures are studied both experimentally and theoretically. The nature of plasmonic chirality and the chiroptical effects in plasmonic nanostructures are described in several extensive reviews [10–12]. This chapter is dedicated to the applications of different plasmonic metal nanostructures and their hybrid nanosystems with optically active organic and biomolecules in chiral biosensing. We focus on the recently published results of using plasmon-induced evanescent fields and consider the main types of molecular plasmonic systems capable of generating amplified chiroptic signal in order to detect the presence of certain biomolecules and (in some cases) to determine their

**2. Chiral molecules coupled to a single plasmonic nanoantenna**

The absorbance of an achiral excitonic system can be strongly enhanced via the interaction with plasmonic particles due to the antenna effect [13]. Similar phenomena can amplify the chiroptical response of a chiral molecular system coupled to a plasmon, which is highly desirable for (bio)sensing, as it can increase the

It was experimentally demonstrated that silver nanoparticles induce nearly two orders of magnitude amplification of chromophore's CD. Besides, the CD signal appeared at the same wavelength as the Localized Surface Plasmon Resonance (LSPR) of the nanoparticles [14]. These results initiated thorough theoretical studies of the nature of plasmon-induced CD amplification [15–18]. It has been found that binding of a chiral molecule to a small achiral plasmonic nanoparticle (particle size ≈ 10 nm) can enhance the molecular CD signal in conjunction with the appearance of a new CD signal at the plasmon resonance wavelength. The

The charge is localized in the small gap between the molecule and the nanoparticle and the Coulomb interactions between their electronic systems can be very strong. As a result, the CD signal induced by molecular transition can be boosted by plasmonic

*The model of the metal nanoparticle-dye molecule assembly (left) and the scheme of excitation-deexcitation processes (right) with solid vertical (horizontal) arrows representing light (Coulomb)-induced transitions. The dotted vertical arrows show the relaxation processes. Adapted with permission from Govorov et al. [15],* 

**42**

**Figure 1.**

*Copyright 2010 American Chemical Society.*

resonance, and, at the same time, the surface current in plasmonic nanoparticle becomes chiral due to the chiral molecule, resulting in a new CD signal [15]. With increasing distance (d) between the metal nanoparticle and the molecule, the near field intensity decreases as d−3. Since most biomolecules have CDs in the UV range, the appearance of the CD peak in the visible range is important for practical purposes as it can be easily detected.

A number of experimental studies support these findings. The CD peak in the visible range, generated by an adsorption of peptide molecules on spherical gold nanoparticles with 10 nm radius, is presented in **Figure 2** [19].

Similar results were obtained for a bilayer of riboflavin 50-monophosphate and polylysine molecules adsorbed on gold island films [20]. For cysteine and its derivatives in the presence of 45 nm spherical silver nanoparticles [21], for glutathione molecules adsorbed on 45 nm silver nanocubes [22] and for tobacco mosaic virus uniformly covered by spherical 5 nm gold nanoparticles [23] the enhanced chiroptical signals in the visible spectral range were also detected. The authors of the latter work achieved near-field amplification of the induced CD signal controlling by the separation between the molecule and the metal nanoparticle. The adsorbed chiral molecules were found to be in resonance with plasmon for the new appeared CD peak. Though, matching the wavelengths of molecular absorption and plasmon resonance intensified the effect, which suggests the use of nanostructures with plasmon resonance in UV for biosensing [24, 25].

Thus, to amplify CD signal of small chiral molecules a plasmonic nanoantenna can be used. But practical application of this approach in biosensing is limited by strong sensitivity to separation distance. It was shown that the induced CD can be sensitive to the orientation of the transition dipole moments of the adsorbed molecule relative to the local plasmon polarization of the nanoparticle [26]. Two orders of magnitude enhancement coefficients of CD were observed for 42 ± 2 nm gold/silver core-shell nanocubes coated with DNA molecules [24]. Additionally, the induced CD signal can be sensitive to the orientation of the transition dipole moments of the adsorbed molecule relatively to the local plasmonic polarization of the nanoparticle [26].

#### **Figure 2.**

*CD spectra of E5 peptide (black line), gold nanoparticles (green line), and their assembly (red line). The inset shows the appearance of the CD peak in the visible range. Reproduced with permission from Slocik et al. [19]. Copyright 2011 American Chemical Society.*
