Nanotechnology-Enabled Sensors

450 Chapter 7: Organic Nanotechnology Enabled Sensors
DNA can also help to amplify or quench the optical signals when it is
attached to a porous surface. Such a phenomenon can be used in reflectometric
interference based sensors. A classic example of such a sensor was
described in Chap. 6, when a nanostructured thin film, such as a porous
silicon surface, displays well-resolved Fabry-Perot fringes in its reflectometric
interference spectrum.
The same structure can also be efficiently used for biosensing applications.
Lin et al showed that the binding of an analyte to its corresponding
recognition partner which is immobilized on a porous silicon substrate results
in a change in the refractive index of the layer medium. 142 This
change in the refractive index can be detected as a corresponding shift in
the interference pattern. They employed an electrochemically etched silicon
layer to produce porous surfaces with cavities as wide as 200 nm in
diameter. Subsequently, they attached DNA oligonucleotides onto this porous
silicon film.
In the presence of cDNA sequences, pronounced wavelength shifts in
the interference pattern of the porous silicon films were observed. However,
in the presence of non-cDNA sequences, but under similar conditions,
no significant shift in the wavelength of the interference fringe
pattern was seen. The lowest DNA concentration measured with this sensor
was 9 fg/mm 2 (Fig. 7.63) which is far smaller than those of the sensing
platforms described in Chap. 3.
Fig. 7.63 Change in the effective optical thickness vs. DNA concentrations for a
porous silicon reflectometric interference spectrum biosensor. Reprinted with
permission from the Science Magazine publications. 142