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for detecting Prunus necrotic ringspot virus extracts using electrochemical
impedance spectroscopy.
138
The immunosensor uses glassy carbon electrodes
and polyclonal antibody. It was capable of discriminating between samples
consisting of extracts from healthy plants and leaf extracts from infected plants
diluted 10,000 times with extract from healthy plants. Important efforts have
been carried out for detecting, concentrating, and separating the virus from a
sample using dielectrophoresis. Morgan studied cowpea mosaic virus
139
and
tobacco mosaic virus.
140
In 2007, Balasubramanian presented a microfluidic
system based on electrophoretic transport and electrostatic trapping to study
MS-2 virus and Echovirus 11 found in potable water.
141
Two gold electrodes
were used for dielectrophoretic trapping. MS-2 virus was captured with 88%-
99% efficiency, and Echovirus showed capture efficiency above 70%. Grom
reported the combination of electrohydrodynamic flow and dielectrophoretic
forces to trap Hepatitis A virus.
142
They showed that the microsystem can be
useful to accumulate viral particles from relatively large sample volumes.
Several works have been reported in the literature based on monitoring
of bacteria metabolism. For instance, oxygen consumption can be mea-
sured by amperometric technique.
143
Impedance biosensor chips were
developed for detection of
Escherichia coli
O157:H7 based on the surface
immobilization of affinity-purified antibodies onto indium tin oxide elec-
trode chips.
144
The biosensor can detect the target bacteria with a detection
limit of 6 × 10
3
cells mL
−1
. An interesting strategy to detect bacteria through
their metabolic process was that based on the electrochemical detection of
specific marker enzymes (i.e. β-D-galactosidase). Perez described a method
to detect viable
E. coli
in water samples using an amperometric sensor for
the detection of 4-aminophenol (4-AP) after hydrolysis of the substrate
4-aminophenyl-β-D-galactopyranoside (4-APGal) by the bacterial enzyme
β-D-galactosidase.
145
Immunological detection with antibodies has been
employed successfully for the detection of specific bacteria and their toxins.
Brewster developed an assay for detection of
E. coli
O157:H7 measuring
the current produced by the oxidation of an electroactive enzyme product.
The sensor had a detection limit of 5 × 10
3
cfu mL
−1
with a 25-min analysis
time.
146
Salmonella typhimurium
was sandwiched between antibody-coated
magnetic beads and an enzyme-conjugated antibody.
147
The beads were
transported onto the surface of graphite electrodes with the aid of a mag-
net. Conversion of enzyme substrate to an electroactive product was mea-
sured giving a minimum detectable level of 8×10
3
cells mL
−1
. The same
group described the development of an immunoligand assay in conjunction
with a LAPS for the rapid detection of
E. coli
O157:H7 cells in buffer.
148
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