Biology Reference
In-Depth Information
Potentiometric biosensors monitor the potential at a working elec-
trode with respect to a reference electrode, and this potential demon-
strates concentration-dependent behavior. A species-selective working
electrode gives outstanding sensitivity and selectivity, although a chal-
lenge for this type of sensor is that a highly stable and accurate reference
electrode is required. 30 Light-addressable potentiometric sensors have
also been developed ( Fig. 7.6(b) ), based on semiconductor activation
by a light-emitting diode, and have been applied to the detection of
Escherichia coli . 29
Conductometric biosensors measure the conductivity change in the
solution due to the production or consumption of ionic species e.g. by the
metabolic activity of microorganisms. Conductance measurements are very
fast and sensitive, though the selectivity is relatively poor. 30
Voltametric biosensors measure both current and potential. The peak
current position is used for identification, while the peak current den-
sity is proportional to the concentration of the corresponding species. The
advantages of this type of electrochemical biosensor are that the low noise
allows for highly sensitive measurements and that simultaneous detection
of multiple analytes is easily achieved.
7.3.3. Mass-sensitive
Quartz crystal microbalance (QCM) biosensors have been employed for
the detection of a range of analytes from proteins such as lysozyme and
BSA, 31 DNA sequences from pathogens like E. coli , 32 as well as the detec-
tion of intact pathogens, including E. coli and Cryptosporidium parvum . 33,34 In
QCM, a quartz disc is sandwiched between two electrodes, normally made
of gold. As quartz is piezoelectric, the crystal can be excited by applying
an AC voltage across the electrodes and will exhibit a resonance frequency
( Fig. 7.7 ). A mass change, Δ m , on the sensor surface results in a shift of the
resonance frequency, Δ f , from the original frequency, f , as described by the
Sauerbrey equation:
Δ f = 2 f 2 Δ m
( ρμ ) 1/2 A
where r , m , and A are the quartz crystal elastic modulus, density, and
the active area of the loaded mass, respectively. 34 QCM biosensors are
well known for their high sensitivity, 0.1 Hz (ng cm −2 ) −1 at 5 MHz, 34 and
high specificity. 35 Furthermore, this type of biosensor presents a large
 
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