Biochips – fundamentals and applications - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 354

changes cause changes in enzyme activity, the pH is maintained at a constant value by
the addition of acid or base. The rate of titrant addition is then proportional to the rate
of the enzymatic reaction. Precise measurements using the pH-Stat require low buffer
concentrations in the enzymatic assay mixture.
Besides, potentiometric sensors with ion-selective ionophores in modifi ed poly(vinyl
chloride) (PVC) have been used to detect analytes from human serum [128]. Cellular
respiration and acidifi cation due to the activity of the cells has been measured with
CMOS ISFETS [129]. Some potentiometric methods employ gas-sensing electrodes
for NH 3 (for deaminase reactions) and CO 2 (for decarboxylase reactions). Ion-selective
electrodes have also been used to quantitate penicillin, since the penicillinase reaction
may be mediated with I or CN .
In recent years, potentiometric sensors have been downscaled to nanometer dimen-
sion through the use of silicon nanowires [130] and carbon nanotubes as fi eld-effect
sensors [131], to take advantage of enhanced sensitivity due to higher surface area to
volume ratio. The integration of these nanoscale sensors in lab-on-chips is somewhat
diffi cult but recent advances in top-down fabrication techniques have been used to dem-
onstrate such nanoscale structures [132]. Potentiometric sensors at the microscale have
also been used to perform label-free detection of hybridization of DNA [133]. These
sensors were incorporated within cantilevers so that they can be used within microfl u-
idic channels. The DNA hybridization was detected by measuring the fi eld effect in sili-
con by the intrinsic molecular charge on the DNA, using a buffer of poly-L-lysine later.
The potentiometric methods often suffer from low sensitivity and sluggish response.
11.4.3.3 Impedimetry
Impedimetry measures the changes in the electrical impedance between two electrodes.
The impedance changes at the electrode interface or in the bulk region can be used to
identify biomolecular interreactions between DNAs, proteins, antigen-antibody bind-
ings or excretion of cellular metabolic products. Impedimetric techniques are attractive
due to their simplicity and ease of use since an electrochemical label into the target
molecule is not needed, and have been used to detect a wide variety of entities such as
agents of biothreat [134], biochemicals [135], toxins, and nucleic acids [94]. In addi-
tion, impedimetric sensors provide information on the ionic strength in electrolytes and
can provide selectivity if coupled with enzyme membranes. These sensors have been
used to detect different analytes, for example urea, glucose, etc. [136]. Measurement of
impedance was also used to measure the metabolic activity of microorganisms within
microfl uidic biochips. As bacterial cells are grown within microfl uidic channels and
wells, the impedance changes in the medium can be detected using electrodes placed
appropriately within the channels [137]. Electrical measurements of DNA hybridiza-
tion using impedance techniques have been demonstrated where the binding of oli-
gonucleotides functionalized with gold nanoparticles leads to conductivity changes
associated with binding events [115].
In impedimetry, conductivity could simply provide a measure of the ionic concen-
tration and mobility in a solution. However, the measurements are diffi cult due to the