Biodetection Using Silicon Photonic Crystal Microcavities - Biophotonics - page 116

Fig. 7.18. A PSi microcavity biosensor functionalized for the capture of one type
of IgG can discriminate against another type of IgG with a contrast ratio 10 ×

The detection of rabbit IgG (150 kDa) was investigated through multiple lay-
ers of biomolecular interactions in a macroporous silicon microcavity sensor.
The silanized sensor was first derivatized with biotin, which can selectively
capture streptavidin. Exposure of biotinylated goat anti-rabbit IgG to the
sensor results in its attachment to the surface through the binding between
biotin and streptavidin. The sensor used goat anti-rabbit IgG as the probe
molecule to selectively capture rabbit IgG. A red shift of the spectrum can
be detected when each layer of molecules is added to the sensor. As shown in
Fig. 7.18 when the sensor was exposed to 50 ml of a solution containing rabbit
IgG at a 1mgml − 1 concentration, a 6 nm red shift was detected [25]. When
the sensor was exposed to 50 ml goat IgG (1 mg ml − 1 ), which does not bind
to the goat anti-rabbit IgG, the red shift was negligible ( < 0.5 nm).

7.4 Two-Dimensional PhC Biosensors

7.4.1 Sample Preparation and Measurement

Silicon-on-insulator (SOI) wafers were used to fabricate 2D PhC microcavity
biosensors [36]. The Si slab thickness was approximately 400 nm. Rigorous 3D
simulations using the FDTD method were performed prior to the fabrication.
The incident light was coupled along the Γ -M direction because the in-plane
leakage of the resonance mode is mainly in the Γ -M direction and hence the
coupling e ciency is higher along the Γ -K direction. The ridge waveguide
was tapered down from 2 to 0.6 µ m with an external surface cross-section
of 2 400 nm, which predominantly select fundamental TE mode as the

propagation mode.

m ×

µ