Biomedical Engineering Reference
In-Depth Information
according to a photonic model indicated that one can view such PSi microcavities as a one-
dimensional band-gap structure. 14,15 The narrow FWHM displayed by these microcavity
devices suggested that they might provide a greater sensitivity to changes in the local
refractive index over other device configurations (single-layer or multilayer stack), mak-
ing them ideal for biosensing.
Our initial efforts focused on the detection of nucleic acid targets using microcavities
derivatized with synthetic oligonucleotides. 16,17 After etching each microcavity, exposure to
high-temperature oxidation was used to both stabilize the underlying structure and provide
a reactive surface for probe immobilization. Treatment with (glycidoxypropyl)trimethoxysi-
lane followed by an amino-terminated DNA oligonucleotide provided the sensor in its final
form. As shown in Figure 11.4, incubation of this sensor with a solution of the complemen-
tary oligonucleotide, followed by a rinsing and drying procedure, causes a strong shift in the
visible photoluminescence spectrum. Importantly, no analogous shift is observed in the pres-
ence of noncomplementary DNA. As a first attempt at detecting a “real” biological target, we
also incubated the sensor with a solution of lambda bacteriophage. Lambda bacteriophage is
a 48,502-bp double-stranded DNA virus that, as its name implies, infects bacteria. 18 It was
gratifying to observe a strong redshift in the luminescence following incubation with this
infectious agent. Preliminary experiments at the time suggested that the sensor was capable
of detecting DNA at a very low concentration. However, it should be noted that the sensitiv-
ity of the detection will depend on the size of the targeted DNA, since larger targets will
produce a larger redshift regardless of the size of the probe sequence. Significant work
remains to be done in this area as well with regard to the effect of salt on both the stability of
PSi / DNA
PSi / DNA / cDNA
FIGURE 11.4
Top: Photoluminescence spectrum for 50
M of DNA attached to a porous silicon
microcavity structure. Middle: The same
chip following exposure to a 1-
M solution
of complementary DNA. A 7-nm redshift
is observed, consistent with DNA binding.
The difference spectrum is shown at the
bottom.
Differential PL signal
600
700
750
800
850
900
650
Wavelength (nm)
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