Biomedical Engineering Reference
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phenomenon in which incident light excites a charge-density wave at the interface
between a highly conductive metal and a dielectric material. The conditions for
excitation are determined by the permittivity of the metal and the dielectric
material. For measuring small changes in the refractive index of a thing region
adjacent to the metal surface, the transduction principle of SPR is basically used as
an analytical tool. For sensitive detection of chemical species such as odor, vapor,
and liquid, the optical excitation of surface plasmon on a thin metallic film has
been recognized as a promising technique. To monitor the excitation of SPR by
measuring the light reflected from the sensor interface, several methods have been
employed which include analysis of angle modulation, wavelength modulation,
intensity modulation, and phase modulation. Optical SPR sensors are sensitive to
change in the refractive index of a sample surface. The change in refractive index
of a sample surface sensitivity is measured by optical SPR sensors. Gases such as
ammonia, toluene, xylene, ethylacetate, 4-methyl-2-pentanone, and propionic acid
can be detected by measuring the SPR using angle modulation. The SPR is
measured with a prism and a thin highly conductive gold metal layer deposited on
the prism base. To excite the SPR, the LED emitting 660 nm light is used as light
source.
The reflection spectrum (reflected light intensity versus angle of incidence with
respect to the normal of metal/dielectric interface) of SPR is measured by coupling
transverse magnetically polarized monochromatic light into the prism and mea-
suring the reflected light intensity of the ray exciting the prism versus the inci-
dence angle. A very thin film of methyl-methacrylate, polyester-resin, or
propylene-ether as a sensing membrane is deposited on gold metal thin film using
spin-coating method to utilize this system as a gas sensor. Using CCD camera
attached to a personal computer the reflected light is measured. The angle at which
the minimum reflection intensity occurs is the resonance angle at where coupling
of energy occurs between the incident light and the surface plasmon waves. Four
channel images of reflected light are observed by using the CCD camera. The SPR
sensor with synthetic polymer thin film on the gold metal film as a sensing
membrane exhibits high sensitivity for toxic gases such as ammonia, toluene,
xylene, ethyl acetate, 4-methyl-2-pentanone, and prop ionic acid.
6.9.3 Fluorescent
Another sensing device that is designed as an array of optically based chemo-
sensors providing input to a pattern recognition system on the E-nose technology
has been developed. This type of chemosensor consists of optical fibers deposited
with fluorescent indicator Nile Red dye in polymer matrices of varying polarity,
hydrophobicity, pore size, elasticity, and swelling tendency to create unique
sensing regions that interact differently with vapor molecules [ 15 ].
Fiber-optic sensors most often consist of an analyte sensing element deposited
at the end of an optical fiber. Individual optical fibers with a diameter as small as
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