Biomedical Engineering Reference
In-Depth Information
FIGURE 10.42
Principle of a fiber optic immunoassay biosensor.
component of an immunosensor is the biological recognition element, which typically con-
sists of antibodies or antibody fragments. Immunological techniques offer outstanding
selectivity
4
and sensitivity through the process of antibody-antigen interaction. This is the
primary recognition mechanism by which the immune system detects and fights foreign
matter and has therefore allowed the measurement of many important compounds at
micromolar and even picomolar concentrations in complex biological samples.
Evanescent-type biosensors can be used in immunological diagnostics to detect anti-
body-antigen binding. Figure 10.42 shows a conceptual diagram of an immunoassay bio-
sensor. The immobilized antibody on the surface of the unclad portion of the fiber
captures the antigen from the sample solution, which is normally introduced into a small
flow through a chamber where the fiber tip is located. The sample solution is then removed
and a labeled antibody is added into the flow chamber. A fluorescent signal is excited and
measured when the labeled antibody binds to the antigen that is already immobilized by
the antibody.
10.6.8 Surface Plasmon Resonance Sensors
When monochromatic polarized light (e.g., from a laser source) impinges on a transpar-
ent medium having a conducting metallized surface (e.g., Ag or Au), there is a charge den-
sity oscillation at the interface. When light at an appropriate wavelength interacts with the
dielectric-metal interface at a defined angle, called the resonance angle, there is a match of
resonance between the energy of the photons and the electrons at the metal interface. As a
result, the photon energy is transferred to the surface of the metal as packets of electrons,
called plasmons, and the light reflection from the metal layer will be attenuated. This
results in a phenomenon known as surface plasmon resonance (SPR) and is shown schema-
tically in Figure 10.43. The resonance is observed as a sharp dip in the reflected light inten-
sity when the incident angle is varied. The resonance angle depends on the incident
wavelength, the type of metal, the polarization state of the incident light, and the nature
of the medium in contact with the surface. Any change in the refractive index of the
medium will produce a shift in the resonance angle and thus provide a highly sensitive
means of monitoring surface interactions.
4
The sensor's ability to detect a specific substance in a mixture containing other substances.
