Biomedical Engineering Reference
In-Depth Information
the interference-based signal. The analytical arm incorporates an evanescent
field which interacts with, in this application, a biochemical moiety. Much as
with SPR as described above, this process results in a change in refractive index
and phase alteration which is sensed as an intensity change at the photodetector
on light recombination. This technology was employed in earlier years for the
detection of protein chemistry, where analytical detection limits of around 50
pM were found.
127
Since this work, several attempts have been made to
improve the detection capability of the sensor such as enhancement of wave
guide physics and increase of sensing surface area.
128
Although promising it
does not appear that this device is being employed widely in general biosensing
applications, possibly due the specialist nature of the fabrication that is
required. An additional interference based structure is Young's device.
129
The
overall principle is similar to the sensor mechanism employed in the
Mach-Zehnder device, except that the detection mode is different. In this case the
output from the sample and reference channels is combined to form interference
fringes on a CCD screen. The method involves Fourier transform of the spatial
intensity measure at the detector screen. As with the Mach-Zehnder sensor a
number of improvements have been made with respect to design and applications
of protein chemistry at the sensor surface have been demonstrated.
Finally, two other devices are the Hartman interferometer
130
(and back-
scattering interferometer.
131
In the former sensor, electromagnetic radiation is
coupled into a waveguide using gratings, which allows the interference
phenomenon to occur between a reference strip and a sample strip. As with the
other two devices outlined above protein chemistry was detected as imposed on
the waveguide strip. One interesting aspect of this work was that the reference
strip was employed to 'remove' signals connected to non-specific binding
effects. In backscattering interferometry, laser radiation is focused onto the
biomolecule sample which is attached to a quite small sensing area. The
reflected light intensity is detected via interference patterns at the detector
surface. As with all the cases mentioned above the system has been used to
detect the surface chemistry of proteins.
d
n
4
t
3
n
g
|
1
d
n
3
.
1.4.3.4 The Quantum Dot
Quantum dots (QDs) are semiconductor nanocrystals that consist of some
1000-10 000 atoms. Since their discovery in the 1980s, they have become an
important component of the field what we now call 'nanotechnology'
132
rather
than the more traditional, colloid science. QDs are composed of semiconductor
materials with properties, because of their dimensions, that lie between a bulk
substrate and individual molecules. The particles have been employed in a
variety of technologies such as medical imaging, light emitting diodes,
computing and photovoltaic structures for solar energy collection.
133
The
potential of QDs in medical science with respect to both imaging and therapy is
particularly exciting.
134-136
QDs are fabricated from semiconductor materials such as Si, CdSe, ZnSe,
ZnS and PbS. Often the dot has a core of one material with a shell of a different
