Biomedical Engineering Reference
In-Depth Information
11.3
Label-free Optical Methods for ORs and GPCRs
Optical techniques have been developed using a wide range of platforms and are
potentially useful for high-throughput ligand screening and functional analyses of
hundreds of ORs. This section provides detailed label-free optical techniques for
receptor-ligand assays.
11.3.1
Surface Plasmon Resonance (SPR) Based Odorant
Sensing
SPR is a well-known and powerful technique that can measure molecular interac-
tions on the surface of a sensor chip in real-time without any labeling. SRP detec-
tion relies on the measurement of a reflective index change at a metal surface which
has been functionalized with probe molecules [
97
,
98
]. Basically, a surface plasmon
(SP) wave is an electromagnetic wave which propagates along the boundary be-
tween a dielectric and a metal interface [
99
-
101
].
An SPW is a transverse-magnetic ™ wave (magnetic vector is parallel to the plane
of interface) and is characterized by the propagation constant and electromagnetic
field distribution. The propagation constant of an SP wave, can be depicted as below:
εε
·
dm
ω
k
=
SP
c
εε
+
d
m
where,
ω
is the angular frequency of the SP wave,
c
is the velocity of light in
vacuum, and
ε
d
and
ε
m
are dielectric functions of the dielectric material and metal,
respectively (see Fig.
11.8
). The electromagnetic field of an SP wave is confined at
the metal-dielectric boundary and decreases exponentially into media. The SP wave
propagates in the x- and y-axis along the metal/dielectric interface, for distances of
tens to hundreds of microns and decays evanescently in the z-axis. The interactions
between the biomolecular layer and metal surface lead to a change in the plasmon
resonance conduction such as angle and wavelength shift [
102
]. These interactions
can be measured the reflectivity of light as a function of either angle of incidence (at
constant wavelength) or wavelength (at constant angle of incidence).
The degree of the change in the propagation constant of an SPW depends on the
refractive index (RI) change and its distribution regarding the profile of SPW field.
Therefore, the RI change (
Δn
) by the binding event, produces a change in the propa-
gation constant (
Δβ
), which is directly proportional to the RI change:
{
}
Re
∆ ≅∆
kn
where
k
denotes the free-space wave number. If the RI change is caused by a bind-
ing event occurring within a distance from the surface
d
, the corresponding change
in the propagation constant can be expressed as follows:
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