Biomedical Engineering Reference
In-Depth Information
18.3.3
Fiber-Optic Biosensor
The fiber-optic biosensor is a powerful tool in biological and chemical detection (31,32),
and much of its success is due to the revolutionary developments in the telecommunica-
tion industry. There are four main characteristics that differentiate fiber-optic biosensors
from other types of biosensors: The very high bandwidth of optical fibers allows them to
carry a large amount of signal through a single fiber because optical fiber is a dielectric and
it is not subject to interference from electronic waves that might be present in the sensing
environment; and fiber-optic biosensors can function under physiological environment
with high concentrations of electrolytes and wide ranges of pH values that could interfere
with the measurement and erode metals at a rapid rate. In addition, fiber-optic biosensors
are intrinsically safe in explosive environments (no sparks), lightweight, compact, robust,
and potentially inexpensive. Indeed, fiber-optic biosensors can perform the functions of
virtually any conventional sensor—often faster and with greater sensitivity—and they can
also perform measurement tasks that cannot be achieved with conventional biosensors.
The field of optical biosensors has experienced rapid growth in the past few years (33).
This is partly due to the ever-improving optoelectronics designed for telecommunications,
and advances in material sciences, which have led to better fabrication of materials and
improved methods of signal generation and measurement (34). Fiber-optic biosensors can
be described as either direct or indirect, just like the other immunoassay platforms. The
direct systems rely solely on antigen-antibody binding to modulate the signal being meas-
ured, while indirect sensors depend on the use of labels or fluorophores to sense the bind-
ing event. The advantage of the direct type is that the assay is essentially “reagentless” in
that no additional substances are needed for detection. The disadvantages are that sensi-
tivity may be limited by nonspecific binding and the limited number of analytes that could
be detected using this format (35,36). The indirect format has the advantage of improved
sensitivity and selectivity due to the label, as well as a reduced amount of nonspecific bind-
ing effects. Normal-assay formats can still be used with indirect sensors, since the labels
will serve to amplify the optical sensor response even to small molecules bound to the
transducer. The main disadvantage of the indirect sensors is the need for labeled reagents.
Optical biosensors based on the EW use the technique of attenuated total reflection (ATR)
spectroscopy and SPR to measure real-time interaction between biomolecules. The basis of
ATR is the reflection of light inside the core of a waveguide when the angle of incidence is
less than the critical angle. Waveguides can be slab guides, planar integrated optics, or opti-
cal fibers. Light waves are propagated along fibers by the law of TIR. Even though the light
is totally internally reflected, the intensity does not abruptly fall to zero at the interface. The
intensity exponentially decays with distance, starting at the interface and extending into
the medium of lower refractive index. The EW is the electromagnetic field created in the
second medium. It is characterized by the penetration depth defined as the distance from
the interface at which it decays to 1e
-1
of its value at the interface (36). The wavelength of
light, ratio of the refractive indices, and angle of the light at the interface determine the pen-
etration depth (37). Penetration depths are typically 50-1000 nm; thus, the EW is able to
interact with many monolayers at the surface of the probe. Reactions occurring very close
to the interface perturb the evanescent field and the change in signal can be related to the
amount of binding between the target and immobilized ligand at the interface.
Fluorescent measurements can also be used to monitor the binding events occurring on
the surface of optical biosensors. When light is traveling through the optical waveguide, it
excites fluorophores within the evanescent field, and the fluorescent signal is propagated
back up the fiber and is detected by a fluorometer (Figure 18.3). By exploiting the detection
of fluorescence-emitting labels, specific antibody-antigen complexes can be monitored.
Hirschfeld and coworkers (38-40) demonstrated that EW sensing excites fluorophores that
