Biomedical Engineering Reference
In-Depth Information
in a combined fashion. By far the majority of these involve a form of elec-
trochemistry as one component.
Magnetism has been employed in conjunction with electrochemical metho-
dology through the long-established magnetoresistive effect. This technique is
concerned with the experimental measurement of the resistance displayed by a
material under the influence of a magnetic field. As with the Ru complex
technique outlined above, a biosensor approach in this case involves the use of
a magnetic tagging agent. An example is the detection of nucleic acid hybri-
dization on the surface of a magnetoresistive material.
143
Single strand DNA
on the device surface attaches to biotinylated complementary strand which, in
turn, interacts with magnetically labeled streptavidin. The change in magnetic
fringing filed is detected with the sensor. Analogous strategies have been used in
immunochemical detection via magnetically labeled particles.
144
Surface plasmon resonance, described above, has been employed successfully
together with electrochemistry.
145
The necessary gold layer for supporting
plasmons can also be utilized as a conventional electrode. This combination is
particularly useful for studying the optical properties of an electropoly-
merization process under an electric field.
146
In an analogous approach, elec-
trochemical impedance spectroscopy has proven to be extremely useful for
monitoring SPR responses for correlation with various electrochemical signals
obtained from DNA hybridization.
147
Other optical combination metho-
dologies include wave guide and ellipsometric physics. The former is concerned
with evanescent field optical effects combined with electrochemistry of a
material surface.
148
Ellipsometry, a well-known method for measuring the
thickness of layers and films in electronic fabrication processes, in the combi-
nation with electrochemistry has been employed for detecting aspects of protein
adsorption on surfaces.
149
Finally, we mention the use of method combinations with scanning probe
technology in the detection of biochemical events. (More detail regarding these
techniques for the direct study neural cells is considered in Chapter 5.) The well-
known atomic force microscope (AFM) concerns the 'dragging' of a tip across
a surface with measurement of the force involved with a cantilever device. It has
been used extensively in conjunction with ultra-microelectrode technology
where forms of amperometric or potentiometric electrochemistry complement
the structural information produced by the AFM tip.
150
This technique is more
often than not referred to as scanning electrochemical microscopy and the main
application focus has been on surface characterization rather than biochemical
binding events.
The scanning Kelvin nanoprobe (SKN) can be considered as an adjunct-type
instrument since it measures, simultaneously, the topography and electrical
contact potential difference on a substrate. In essence, a vibrating, guarded tip
is scanned over a surface and the resulting Kelvin physics is used to detect
biochemical binding events such as oligonucleotide duplex formation and
immunochemical interactions through alteration of surface dipole charge
changes.
151
The instrument has been used successfully to examine cells
including neurons on surfaces (see Chapter 6).
d
n
4
t
3
n
g
|
1
d
n
3
.
