Biomedical Engineering Reference
In-Depth Information
Fig. 7.22.
Transmission near the 2D PhC microcavity resonance for the function-
alized sensor (
a
), after exposure to the probe, glutaraldehyde (
b
), and subsequent
exposure to the target, bovine serum albumin
Figure 7.22 illustrates the normalized transmission spectra measured at
three different binding stages. The PhC microcavity consisted of a triangular
array of cylindrical air pores in a 400 nm-thick silicon (Si) slab. The lattice
constant
a
was 465 nm, and the pore diameter
r
was 0.3
a
. A defect was intro-
duced by reducing the center pore diameter to 0.18
a
, leading to a transmis-
sion resonance in the bandgap close to 1.58
m. Curve (a) shows the initial
transmission after the oxidation and silanization. Curve (b) was measured af-
ter exposure to glutaraldehyde. A resonance red shift of 1.1 nm is observed.
Curve (c) shows a red shift of 1.7 nm after BSA binding corresponding to a
total shift of 2.8 nm with respect to the initial spectrum. To verify the exper-
imental results, a plane-wave expansion calculation with 32 grid points per
supercell was performed. The simulation of the resonance red shift assumes
that the refractive index of the dehydrated proteins is 1.45. This value agrees
with an ellipsometry measurement [29] and with the literature values [39].
Figure 7.23 plots the predicted resonance red shift vs. coating thickness.
Using ellipsometry, the thickness of the dehydrated glutaraldehyde monolayer
was measured as 7
µ
1 A. We then calculated that glutaraldehyde should intro-
duce a resonance red shift of 0.98
±
0.2 nm, which is in good agreement with
the experimental data. The thickness of a dehydrated BSA layer measured
with the same method is approximately 15
±
5 A, which should introduce
±
an additional red shift of 2.7
±
1 nm or a total red shift of 3.8
±
1nm. The
experimental data are again in good agreement with the model.
