Biomedical Engineering Reference
In-Depth Information
variable ionic background of clinical samples and the relatively small conductivity
changes observed in high ionic strength solutions [138]. The capacitance measure-
ment is limited to detect relatively large biomolecules, and requires more precision of
capacitance measurements with commercial instruments [139]. Further, it is unlikely
that individual electrodes in a biochip have identical electronic and physical proper-
ties, particularly in the case of fi lm coated electrodes. This would cause signifi cant var-
iation. It is not practical to use simple subtraction of impedance changes before and
after biomolecular interaction in biochip experiments. Actually, the simple subtraction
method leads to a messy, scattering patterned plot of impedances vs target concentra-
tions of the target molecule, due to variations from individual electrodes [95]. Li
et al.
reported impedance labelless detection of DNA and proteins [85, 94, 95] on polypyr-
role deposited electrodes. In order to eliminate or reduce the variations from different
single electrodes in multi-concentration analysis, a concept of normalized dimension-
less impedance unit change was introduced to analyze the measured impedance data
[95]. In this method, for example, the resistances measured at an Ab impregnated elec-
trode before and after the target antigen incubation are assumed as
R
1
and
R
2
, respec-
tively. The normalized resistance unit change,
∆
R
N
, is
RR
R
2
1
∆
R
N
1
(6)
The physical meaning of
R
N
is the dimensionless unit resistance change. This nor-
malization is different from the normalized change of intensity used in optical array
biochips, which uses the responses at biomarked spots to be divided by the responses
at the negative spots, and also is different from the normalized resistance change
(
∆
g
o
;
g
o
is the conductance of the sensor
without any interest analyte and
g
is the conductance of the same sensor in the pres-
ence of the analyte. The normalized dimensionless unit resistance change introduced
here is based on results from a single electrode. By using
∆
g
/
g
o
) used in literature [94] where
∆
g
g
∆
R
N
to process impedance
data, the S/N ratio was signifi cantly improved.
11.5 LAB-ON-CHIPS
The principle of the lab-on-chip (LOC) is to integrate all the necessary devices on a sin-
gle small chip to perform complicated biological and chemical processes that are usu-
ally done with larger volumes in well-equipped laboratories. In the past ten years, the
research and application in this area has been rapidly growing [140]. After an initial
focus on electrokinetic separation techniques on the chip, the scope of the technology
has widened to include topics like microfl uidics, DNA analysis, cell analysis, micro-
reactors, and mass spectrometer interfacing. Microfabrication and the drive to analyze
thousands or hundreds of samples quickly and effi ciently have led to the development of
this new form of analytical technology. As well as the analytical chemistry community,
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