Biophotonics_Concepts_to_Applications

excited on the coated metal layer. Through this process, a strong evanescent
electromagneticfield is induced on the metal surface, as shown in Fig.9.18.
Thereby the Raman modes of molecules that are close to the metallic surface are
enhanced dramatically, because the Raman intensity is proportional to the square of
the incident electromagneticfield amplitude.
Thus SERS now is widely used for biomedical research, because this technique
provides narrow Raman spectra (*1 nm full width at half-maximum) that serve as
unique biologicalfingerprints to identify different chemical species. These specific
enhanced Raman spectra therefore can be used to identify a particular molecular
species in a biological sample and to quantify distinct components in a mixture of
different types of molecules. Because the SERS spectra have strong distinct spectral
lines, this technique can be used for the simultaneous detection and analysis of
multiple diseases that have different lines [ 46 ].

Example 9.11As an alternative to attaching target molecules onto a

nanoparticle roughened substrate, another technique is to immobilize

biomarkers on the inside core surface of a hollow-core photonic crystalfiber

(HCPCF). How would such a setup be realized?

Solution: The preparation and use of three types of Raman-reporter biomarkers

immobilized inside a HCPCF is described in [ 46 ]. The biomarkers were cyanine

5 (Cy5), malachite green isothiocyanate (MGICT), and naphthalenethiol (NT).

Each of these SERS nanotags have at least two distinct spectral peaks. This

situation allows for the simultaneous multiplex detection of three different

diseased conditions. The peaks are Raman shifts of 1120 and 1595 cm−^1 for

Cy5, 1175 and 1616 cm−^1 for MGICT, and 1066 and 1378 cm−^1 for NT.

9.8 Coherent Anti-stokes Raman Scattering Spectroscopy.

Coherent anti-Stokes Raman scattering(CARS) spectroscopy is a nonlinear optical
four-wave-mixing process [ 47 – 49 ].Nonlinear opticsdeals with the interaction of
light with matter in a manner in which the amplitude response of the material to the

Laser

Raman scattering

Photodetector

Substrate for metal layer

Noble metal

roughened with

nanoparticles

Target

Laser molecules

beam

Fig. 9.18 Basic
plasmonic-based setup for
SERS

280 9 Spectroscopic Methodologies