Biomedical Engineering Reference
In-Depth Information
applications when easy infiltration of the target throughout the entire mul-
tilayer structure is required. For a given porosity contrast, the
Q
-factor can
also be increased by increasing the number of periods of the Bragg mirror.
In practice, the number of periods cannot be increased arbitrarily for two
reasons. First, uniform infiltration of the molecules becomes more di
cult in
thicker devices. Second, maintaining a constant HF concentration at the tip
of very deep pores is di
cult [27], which may lead to an undesired porosity
or refractive index gradient.
PSi-based PhC sensors are not perturbed by large, unwanted bioparticles.
When the PSi sensor is exposed to a complex biological mixture, only the
molecules that are smaller than the pores can be infiltrated into the sensor.
Furthermore, an increase of refractive index on the top of the microcavity
due to the presence of large, unwanted objects only causes changes to the
side lobes in the reflectivity spectrum, not to the resonance dip [28]. Thus,
PSi microcavities are more reliable than planar sensing platforms, where the
nonspecific binding of large size objects present in a “dirty” environment may
produce a false-positive signal.
The pore size also affects the sensitivity of PSi biosensors because the
targets do not completely fill the pores but instead are attached to the pore
walls. For a PSi layer of a given porosity, the internal surface area decreases
as the pore size increases. The effective refractive index change of a layer with
larger pores is thus smaller as the percentage of the pore volume occupied
by the biological species is smaller. The spectra for microcavities with fixed
porosities (e.g., 80% for the high porosity layer and 70% for the low porosity
layer) but different pore diameters (ranging from 20 to 180 nm) have been
calculated. For a given coating thickness (with
n
layer
= 1.42, a typical value
for biomolecules), the resonance red shift decreases as the pore size increases,
as shown in Fig. 7.12 [8]. For a 0.3 nm thick coating layer, a microcavity with
40 nm pores produces a red shift of 10 nm, while a microcavity with 100 nm
pores produces a red shift of only 3 nm. Thus, to optimize the sensitivity, the
pore size should be as small as possible while still allowing for easy infiltration
of the biological material. Note that the amount of red shift can be used to
precisely measure the amount of material captured inside the pores [29].
7.2.4 Fabrication of One-Dimensional PhC Biosensors
The porosity can be controlled by the etching current density. Once a layer
has been etched, further etching using a different current density does not
affect it, as explained in Sect. 2.1. Stacks of layers with different refractive
indices can thus be formed by switching the current density during etch-
ing [30]. Figure 7.13 shows a cross-sectional SEM image of a multilayer
structure etched, using a periodic current density pulse train. The current den-
sity determines the porosity and the pulse duration determines the thickness.
Figure 7.14 shows top view and cross-sectional SEM images, and reflectance
spectra for two different types of microcavities with different pore sizes. The
