Optical Waveguide-Based Biosensors Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 94

On the other hand, planar waveguides represent practically the
same concept as optical ibers but in a microfabricated planar
design. However, planar waveguides offer advantages such as ease
of miniaturization in form of an integrated optical sensor and
compatibility with microelectronic technology.
Richardson and coworkers 81 used indium tin oxide (ITO) as
planar waveguide (PW) and immobilized GNPs on the surface of ITO.
The sensing region had a length of 2 cm. 82 A study of the refractive
index sensitivity of this sensing coniguration showed that the PW-
PPR sensor had a sensitivity of 9.2 AU/RIU. Such a high sensitivity is
due to the extremely high number of relections along the waveguide
since the thickness of an ITO layer is typically around 50-100 nm,
while the FO-PPR sensor as described above typically has a core
diameter of 400 nm and sensing length of 2 cm. The optical path
length of the PW sensor can be estimated by using a ray optics model
to describe light propagation in the guiding layer. 83 The work also
demonstrated that the sensor can detect adsorption of ibrinogen on
GNP surface and is more sensitive than the PPR sensor employing
a single pass transmission measurement. 19 Since ITO is conductive,
this sensing coniguration also allows spectroelectrochemistry to be
performed.
Although a thin ilm PW can be used to increase the effective
pathlength of an ATR measurement and enhance the sensitivity,
light coupling into the waveguide is not an easy task. Typically,
prism or grating coupling is used. Prism coupling is only practical
for benchtop applications since the prisms must be placed on the
surface of the thin ilm and it is dificult to isolate the prisms from
the liquid sample. Moreover, coupling eficiency is very sensitive
to the input beam alignment and prism contact with the surface.
On the other hand, the coupling eficiency with grating is low and
sophisticated microfabrication processes are often required to
construct the gratings on the sensing chips. An alternative is to use
direct coupling but precise alignment of the incident light beam to
the end face of the PW is required, especially for thin ilm PWs.
In an attempt to study the sensitivity of a PW-PPR sensor, a
simple optical setup and a thin glass slide (0.1 mm) as shown in
Fig. 5.5 were used. GNPs were immobilized on one side of the slide
and a PMMA chip with a microluidic channel (0.4 mm × 3 mm × 30
mm) was placed on top of the glass slide in a manner that the GNPs
could be exposed in the microluidic channel. When I S / I R is plotted