Biomedical Engineering Reference
In-Depth Information
measure binding events, and are relatively immune from interferents in the
bulk solution. Optrode-type sensors are adaptable to a much wider variety
of measurements and produce a higher power excitation light in the sensing
area, but the assays must be formatted to accommodate excitation light in the
bulk solution and signal collection from a very small surface, i.e. the fiber tip.
A history of the earliest applications of fiber optics for biosensor applications
can be found in [13].
11.4.1 Fiber Optics for Biosensor Applications
Total internal reflectance (TIR) is observed at the interface between two di-
electric media with different indices of refraction as described by Snell's law:
n
1
n
2
sin
θ
1
= sin
θ
2
,
(11.1)
where
n
1
and
n
2
are the refractive indices of the fiber optic core and the
surrounding medium, respectively. The angle of light incident through the
core of the optical fiber is represented by
θ
1
and the angle of either the light
refracting into the surrounding medium or the internal reflection back into
the core is represented by
θ
2
. Total internal reflection requires that
n
1
>n
2
and occurs when the angle of incidence is greater than the critical angle,
θ
c
,
defined as:
θ
c
= sin
−
1
n
2
n
1
.
(11.2)
This parameter must be considered when designing any biosensor on the ba-
sis of optical fibers. However, although Snell's law describes the macroscopic
optical properties of waveguides, it does not account for the electromagnetic
component of the reflected light, known as the evanescent wave. The evanes-
cent wave is an electric field that extends from the fiber surface into the lower
index medium and decays exponentially with distance from the surface, gener-
ally over a distance of hundred to several hundred nanometers. For multimode
waveguides, the penetration depth, d
p
, the distance at which the strength of
the evanescent wave is 1/
e
of its value at the surface, is approximated by:
λ
(4
π
[
n
1
sin
2
θ − n
2
]
1
/
2
)
d
p
=
(11.3)
where
n
1
and
n
2
are refractive indices of the optical fiber and surrounding
medium, respectively, and
θ
is the angle of incidence [14].
The importance of the evanescent wave is its ability to couple light out
of the fiber into the surrounding medium, thereby providing excitation for
fluorophores bound to or in proximity to the fiber core surface. This confined
range of excitation is one of the major factors responsible for the relative im-
munity of evanescent wave-based systems to the effects of matrix components
or interferents beyond the reaction surface (Fig. 11.3). Love and Button [15]
