Biomedical Engineering Reference
In-Depth Information
11.6
Studying the Longevity of PSi under Environmentally and
Physiologically Relevant Conditions
Because PSi biosensors function via changes in the amount of bulk material present within
the PSi matrix, degradation and dissolution of PSi over time is a major concern. Indeed,
PSi has been touted as a delivery vehicle for pharmaceuticals precisely because of its
biocompatibility, low toxicity, and ability to dissolve over time in a physiological environ-
ment.
34
Fortunately, several methods have been developed to stabilize PSi. These include
thermal oxidation (passing the PSi device through a roughly 500-degree furnace in the
presence of ambient oxygen), oxidation via aqueous hydrogen peroxide treatment,
ozonolysis, thermal carbonization (sample annealing at high temperature in N
2
or acety-
lene atmosphere), thermal nitridation (sample annealing at upwards of 1100°C in N
2
atmosphere), and dodecene treatment. Björkvist et al
.
compared four of these treatments
in single-layer PSi samples.
35
Brunauer-Emmett-Teller (BET) methodology was employed
to examine changes in porosity (and hence surface area) following thermal oxidation,
dodecene treatment, thermal carbonization, and thermal nitridation. Thermal oxidation
and thermal nitridation were found to cause the greatest changes in porosity (correspon-
ding to an 80% loss in surface area). However, these two treatments were also found to
provide the greatest increase in device-shelf life; examining the weight increase of PSi
samples following storage at 70°C and 75% relative humidity, these were found to have
undergone the smallest change.
The tendency for PSi to degrade rapidly under physiological conditions was recognized
early by Canham and coworkers.
36,37
In their 1998 report,
38
Sailor, Ghadiri, and coworkers
examined the stability of single-layer PSi structures in 10% EtOH/90% PBS
(pH 7.4) solution as a function of surface derivatization. Measured as a time-dependent
change in effective optical thickness, “as etched” (hydride-terminated) samples were
found to degrade at a rate of 6 nm/min, while ozone oxidation reduced this to 2 nm/min
and thermal oxidation reduced it to 1 nm/min. Importantly, functionalization of the
ozone-oxidized surface with (2-pyridyldithiopropionamidobutyl)dimethylmethoxysilane
reduced the degradation rate to an almost negligible 0.05 nm/min. This suggests that care-
ful choice of probe immobilization chemistry can significantly enhance the stability of the
devices produced.
In at least one instance, salt-mediated PSi degradation has been exploited as a “feature,”
rather than viewed as a problem to be avoided. In 2004, the Ghadiri group reported that
hybridization of target DNA to immobilized nucleic acid probes in neutral buffered solu-
tion caused a blueshift in the optical-sensor response relative to probe-functionalized sam-
ples exposed to solutions lacking target DNA.
39
This observation was interpreted as a
hybridization-dependent enhancement in the rate of device dissolution.
11.7
PSi Biosensors in Hydrogels: Toward the “Smart Bandage”
A deficiency of the majority of PSi-based biosensors examined till date is that because the
PSi matrix remains attached to the backing Si wafer, diffusion of analyte solution in and out
of the matrix is substantially hindered. It has been known for some time that PSi membranes
can be obtained as free-standing entities by fracturing them from the backing wafer with a
high-current pulse postetch; however, these free-standing membranes are exceptionally
fragile. The “Smart Dust” methodology of Sailor and coworkers (described below) is a
