Biomedical Engineering Reference
In-Depth Information
The scattering of light by very small (subwavelength) sized
particles is well described by Rayleigh's theory.
23,24
For incident light
horizontally polarized and observed in the same plane, the intensity
of light scattered (
I
scat
) in the direction
θ
by a homogeneous spherical
particle with radius (
a
), which is much smaller than the wavelength
(
λ
) of the incident beam, is given by the Rayleigh expression
23,24
2
π
464
2
16
an
I m
-1
(8.4)
med 0
2
θ
I
=
cos
scat
24
2
r
λ
m
+2
where
I
0
is the incident intensity of monochromatic light,
n
med
is
the refractive index surrounding the particle,
m
is the refractive
index of the bulk particle material, and
r
is the distance between the
particle and where the scattered light is detected. From Eq. (8.4),
the scattered light in the horizontal plane is 100% polarized. The
scattered light intensities are highest at the observation angles
θ
= 0° and 180°, zero at
θ
= 90° and 270°, and proportional to cos
2
θ
at all other angles. This spatial or angular distribution of plasmon
scatter is characteristic of an electric dipole emitter.
23,24
For light
vertically polarized and perpendicular to the scattering plane, the
intensity of scatter is given by the well-known form of the Rayleigh
expression
23
2
π
464
2
16
an
I m
-1
(8.5)
I
=
med 0
scat
24
λ
2
r
m
+2
8.3
Applications of NP-Based RLS Techniques
NP-based RLS probes are simple and sensitive techniques that
can be performed using a commercial spectroluorometer. When
compared to luorescence, RLS techniques face less problems of
photobleaching and quenching due to changes in temperature, pH,
and ionic strength.
13
As a result of these advantages, NP-based RLS
techniques have become practical for the quantiication of a number
of analytes, including drugs, carbohydrates, antibodies, nucleic acids,
and proteins in artiicial and real samples. In addition, NP-based RLS
techniques have also proven to be useful for the characterization of
aggregation and assembly of biological and chemical species.
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