Biomedical Engineering Reference
In-Depth Information
are glass, fused silica (high purity synthetic amorphous silicon dioxide), or
mono-crystalline silicon as used in large quantities for the semiconductor in-
dustry. Since the interface between the coupling prism and the biochip does
not need to be patterned - it must only be optically flat - glass or fused silica
are the preferred substrate materials for the prism coupling approach. This is
particularly true for surface plasmon resonance (SPR) biosensors.
On the other hand, grating coupling requires the fabrication of coupling
gratings on the substrate. Although glass is still the material of choice for this
application, the use of plastic or polymeric materials such as PMMA (poly-
methylmethacrylate), PDMS (polydimethylsiloxane) or Ormoceres
TM
is being
investigated, since surface gratings can easily be replicated in these materials
with nanometer precision. Before these materials are employed routinely in
practice, however, several problems regarding their surface roughness, the me-
chanical stability, and the physical/chemical inertness of the surfaces need to
be resolved satisfactorily. An additional problem that needs to be addressed
is the thermal expansion coe
cient of polymeric materials, which is typically
ten times as large as the one of glass or silicon [17].
Fluorescence and luminescence sensing, in particular, using the special
types of microarrays discussed in Sect. 12.2, is most often realized on glass
or silicon substrates. Since the materials must neither be transparent nor
of particular optical quality, the choice of cost-effective substrates is larger
than that for the other optical biosensors. Because of the ease with which the
microfluidic functionality can be integrated into plastic or polymeric materials
(e.g., by injection molding), these materials are also of interest for fluorescence
biosensors [20].
12.7 Realization Example of an Optical
Biosensor/Biochip: WIOS
As a practical realization example of an optical biochip and its associated
readout system, the wavelength-interrogated optical sensor (“WIOS”) princi-
ple described in [18] is explained in more detail. This label-free optical biosen-
sor is based on a dielectric waveguide with grating couplers for coupling light
into and out of the waveguide.
The disposable biochip consists of a 12
.
5
12
.
5mm
2
AF45 glass substrate
of 0.7 mm thickness covered with a 150 nm thick film of Ta
2
O
5
(
n
=2
.
13)
with a grating structure that was previously etched into the glass using dry
etching. This grating has a period of
Λ
= 360 nm and a thickness of 13.2 nm.
To minimize interference effects between the input and the output grating,
the Ta
2
O
5
film thickness was fabricated 150 nm thicker at the output grating,
resulting in a smaller out-coupling angle. Since the complete input/output
grating structure measures only 0
.
8
×
1
.
0mm
2
, several of them can be produced
on the same biochip. The input grating of the sensing pads is selectively
sensitized with suitable receptor molecules that are photo-bonded to the input
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