Optical Waveguide-Based Biosensors Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 81

Figure 5.1 A schematic illustration of the principle and experimental
setup for the FO-PPR biosensor to make measurements. The
setup consists of A, function generator; B, light emitting diode
(530 nm); C, FO-PPR sensing chip; D, photodiode; E, lock-in
ampliier; F, computer. Biomolecular binding at the surface
of the functionalized gold nanoparticle layer results in an
increase in absorbance and hence a decrease in light intensity
exiting the optical iber.
Optical iber evanescent wave absorption sensors are based
on attenuated total relection (ATR) spectroscopy. This technique
relies on the penetration of the evanescent wave (EW) in the
absorbing medium outside the iber core and interacts with the
analyte, whose distance from the iber surface is generally smaller
than the wavelength of the traveling light. 36 Since light propagates
in optical iber by virtue of multiple TIR, the summation of each
local evanescent ield results in a continuous evanescent ield at the
waveguide surface immediately adjacent to the region occupied by a
propagating mode. As a result, the overall sensitivity of the structure
depends on the modes of light propagation in the iber, 37 the length
of the iber core exposed to the chromophore, 38 and the chemical
interaction between the analyte and the immobilized binding sites. 39
Hence, the low absorbance of a NMNP layer can be enhanced by a
iber optic EW sensing scheme. 23
Fiber optic EW absorbance measurements typically are
performed by comparing the light intensity of a sensor immersed
in an analyte solution ( I S ) to the intensity of the sensor in a blank