Biochips – fundamentals and applications - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 351

electrochemistry [116]. Fabrication of nanoarray biochips is still facing great chal-
lenges. The architecture to effi ciently and inexpensively build a nanobiochip with
functional molecules, nanowires, nanoparticles, and nanocarbontubes, etc. needs to
be further exploited. The input and output strategy that specifi es a method for inter-
facing from the nanometer-scale constructs of the systems to micrometer-scale con-
structs within or outside the system is not clear up to date. It can be possible to test
and operate the completed prototype using an integrated interconnection to the outside
world; in particular, separate external nanoprobe tips are not considered an acceptable
I/O approach. Defect and fault tolerance are essential to the nanoscale array architec-
ture because it is anticipated that a signifi cant fraction of the hierarchically assembled
nanometer-scale devices contained in the device may be defective or imprecisely posi-
tioned. It has been believed that a variety of “bottom-up” nanoassembly methods and/
or “top-down” nanofabrication methods for constructing nanodevices will be hybrid-
ized for great performance.
11.4.3 Electrochemical detection
Although optical detection techniques are perhaps the most prevalent in biology and
life sciences, electronic or electrochemical detection techniques have also been used in
biochips due to their great sensitivity, high specifi city, and low cost. These techniques
can be amenable to portability and miniaturization, when compared to optical detection
techniques [117]. Electrochemical detection include three basic types: (i) amperometry,
which measures the electric current associated with the electron transfer involved in
redox processes, (ii) potentiometry, which measures a change in potential at electrodes
due to ions or chemical - biomolecular intereactions (such as an ion-sensitive FET),
and (iii) impedimetry, which measures conductance or capacitance changes associated
with changes in the overall ionic medium between the two electrodes. There are more
reports on potentiometric and amperometric methods, particulary due to the established
fi eld of electrochemistry. Many of the sensors employing these two methods have been
commercialized.
11.4.3.1 Amperometry
The most prevalent examples of the amperometric method employ an enzyme-catalyzed
redox reaction, where the resulting redox electron current is measured at a working
electrode for high sensitivity. A prominent advantage of amperometry for the biochip
is the improvement of performance that accompanies the reduction of electrode size
into the low micrometer range. Conventional macroelectrodes (typically millimeter in
diameter) exhibit planar diffusion of electroactive species to the electrode surface for
redox reactions. As the electrode diameter is decreased into the low micrometer range,
a shift to non-planar diffusion occurs, causing an increase in the collection effi ciency
of the electroactive species at the surface. The practical result is an increase in the sig-
nal to noise ratio (S/N), which generally equals to a lower detection limit [118].