Biodetection Based on Resonance Light Scattering of Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 159

in more detail. The roles that the nature, size, and shape of Au and
Ag NPs play in determining RLS signals are emphasized here. The
advantages and limitations of RLS techniques for bioassays are
discussed using several representative application examples. Finally,
the developed RLS techniques are summarized and their future trend
is highlighted.

8.2 Basic Theory of RLS

In general, two processes (absorption and scattering) occur when
light passes through a solution of aggregates. If the solvent itself is
nonabsorbing, then energy is removed from the incident light through
absorption and scattering by the aggregates. The light scattering
component is a consequence of differences in polarizability between
the aggregates and the solvent. The incident electromagnetic wave
induces an oscillating dipole in the assembly, which radiates light in
all directions. To understand the polarizability of the aggregates, two
terms, absorption cross section ( C abs ) and scattering cross section
( C scat ), are presented. The ratio of the rate of energy absorption from
the incident beam to the intensity of the incident beam is called C abs .
The ratio of the rate of energy scattering out of the incident beam
(in all directions) to the intensity of the incident beam is called C scat .
If the induced dipole can be considered ideal — which is usually a
valid assumption if the size of the aggregate is small compared to the
wavelength ( λ m ) of the light in the solvent — both cross sections are
related to the polarizability of the aggregates in the following simple
formulae 16
abs

C = α

(8.1)

m i

k

4

k

4

( )

2

(8.2) = α = α +α

π

m m 2 2

C

scat r i

6 6

π

where k m is the wave vector of light in the solvent, k m = 2 π / λ m , and
α r and α i are the real and imaginary parts of the polarizability of the
aggregates. Absorption at a certain wavelength band by a solution of
aggregates can be understood to be the result of a maximum in the
imaginary part of the polarizability in that region of the spectrum.
The absorbance ( A ) of a sample of thickness ( L ) is presented as 16

− ⎛⎞
= ⎜ ⎝⎠

N

(8.3)

1

A 2.3 C L

abs

V