Surface-Enhanced Raman Scattering Based on Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 251

11.1.1 Electromagnetic Enhancement Mechanism

The collective excitation of the conduction electron is known as
surface plasmon. A small spherical NP whose dimension is much
smaller than the wavelength of incident radiation will sustain
oscillating surface plasmon multipole. For a suficiently small particle
(i.e., on the scale of the wavelength of the incident radiation), the
electric ield is approximately uniform across the entire particle and
the LSPR can be approximated as a dipole, p 0 . When the excitation
radiation matches the energy of the plasmon dipole, the nanoparticle
(NP) will radiate characteristic plasmonic radiation which can
be easily monitored by absorption and scattering spectroscopic
techniques. The following description of the EM mechanism is
adapted from the theoretical model presented in Kerker et al. 6

• r E R ( r,Ȧ )

E p ( d , Ȧ 0 )

d

İ 0

E d ω = E d ω + E d ω

E i ( Ȧ 0 ) E sc ( Ȧ )

a

p i LM

E r ω = E r ω + E r ω

R DIP sc

İ

E LM ( Ȧ 0 )

E DIP ( Ȧ )

Figure 11.1 A schematic view of the SERS relevant electric ields near a
spherical NP of radius a , and dielectric function ε immersed
in a medium with a dielectric constant ε 0 . An adsorbate
molecule is placed at a distance d away from the sphere. An
observation point r located in the far-ield is associated with
ield E R . At the incident frequency ω 0 , there are the incident,
Lorenz-Mie and primary ield denoted as E i ( ω 0 ), E LM ( ω 0 ),
E p ( ω 0 ) respectively. At the Raman shifted frequency, ω , there
are the dipole, scattered, and Raman ields denoted as E DIP ( ω ),
E SC ( ω ), E R ( ω ), respectively. 6 Reprinted with permission from
Ref. 6. See also Color Insert.

It is instructive to consider this enhancement mechanism in
the framework of a molecule situated at a distance d away from a
spherical metal NP as depicted in Fig. 11.1. The particle has a radius
a and dielectric function of ε ( ω ) and is surrounded by a medium
with dielectric constant ε 0 . Upon irradiation with a plane wave of