Biomedical Engineering Reference
In-Depth Information
had difficulty entering the pores and/or became entrained in smaller channels within the
PSi matrix in an inactive conformation. Since increasing the pore diameter was an essen-
tial prerequisite of moving forward, we examined the effect of dilute solutions of KOH
post-etch on the microcavities.
The use of KOH solutions to enlarge pore diameters in mesoporous Si was initially
explored by Tinsley-Bown, et al.
25
In that case, a solution of 83 mM KOH in 17% water or
83% ethanol was used to effectively double the diameter of mesopores etched in
n
-type
silicon from roughly 50-100 nm for 12 min. For
P
-type silicon microcavities, we found that
postetch treatment with a solution of 0.5-1.5 mM ethanolic KOH was effective in expand-
ing the pore diameter by as much as 15% (from roughly 15 nm to 19 nm).
26
Treatment with
more concentrated KOH solutions caused complete destruction of the multilayer micro-
cavity structure in a matter of seconds. Pore opening was found to be independent of the
starting porosity of the material, as measured by blueshift in the optical reflection
spectrum.
Two additional important observations were derived from this study. First, KOH treat-
ment almost instantaneously destroyed the photoluminescence of the microcavities. We
presume that this is the result of very rapid dissolution of the manocrystalline silicon par-
ticles that are believed to be the source of PSi's room-temperature photoluminescence.
Thus, such samples must be measured in reflectivity mode. Second, as one might antici-
pate, KOH treatment caused degradation of the microcavity Q (“Quality” factor, essen-
tially a measure of the narrowness of the microcavity line width) as a function of length of
KOH exposure (Figure 11.6). However, microcavities treated for
30 min— a time suffi-
cient to enlarge pores— were still of sufficient quality for general sensor use.
11.5
Using Enzyme Assays as a Secondary Monitor of Sensor Performance
Since the properties of PSi are a result of its three-dimensional matrix structure, gaining
a detailed understanding of the relationship between the structure of the PSi device and
its sensor performance is considerably more difficult than for a planar device (i.e., a sur-
face plasmon resonance (SPR) chip or fluorescence microarray) because the active sur-
face cannot be observed directly. In the biosensing examples discussed so far, one
observes a change in the optical spectrum of the device on attachment of a probe mole-
cule or binding of an analyte, and based on control experiments with “nonbinding” ana-
lytes one attempts to relate this too; but it is not certain that there is a way to correlate
these observations with another measure of the presence and amount of the active,
immobilized recognition molecule. In early experiments, we employed radiolabeled
oligonucleotides to correlate sensor response with the presence of immobilized probe
oligonucleotides.
27
One could potentially employ fluorescence-tagged molecules in a
similar manner. More recently, in a series of articles published in 2004 and 2005, we used
the enzymatic activity of glutathione-S-transferase (GST) covalently immobilized in PSi
as a measure of immobilization capacity of the matrix, and to examine in detail how the
optical response of the sensor material correlates with the amount and activity of the
immobilized molecules.
PSi has many characteristics analogous to other substrates used for immobilized
enzyme bioreactor systems, thus combining this concept with biosensing seemed a natu-
ral course of action. GST is an ideal model enzyme for testing such systems, since it is rel-
atively small (25 kDa per monomer, and active as the dimer
28
), robust, and inexpensive.
The dimeric, active form of the enzyme is essentially spherical (Figure 11.7), permitting the
