Biosensors for pesticides - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 44

2.4.2.3 Electrochemical immunosensors
Although optical and piezoelectric immunosensor systems are capable of achieving
the required detection limits for pesticides and herbicides, these are at the expense of
experimental complexity and instrumental portability. Most potentiometric biosen-
sors for detection of environmental pollutants use enzyme to catalyze the consump-
tion or production of protons. However, the reports on potentiometric immunosensors
for pesticide determination are less. Dzantiev et al . [90] reported a detection of 50 ng
mL 1 of the herbicide 2,4,5-trichlorophenoxyacetic acid. This method was based on
the competitive binding of free pesticide and pesticide-peroxidase conjugate with anti-
bodies immobilized on a graphite electrode. Another potentiometric immunosensor for
simazine has been reported by using HRP as tracer [91]. The antibodies immobilized
on gold planar electrodes were stable and could be easily regenerated with 0.04 M
HCl. Among the electrochemical methods a widely used method was amperometric
detection. Amperometric biosensors typically rely on an enzyme system. The enzyme
can convert catalytically electrochemically non-active analytes into products that can
be oxidized or reduced at a working electrode. The current produced is directly related
to the concentration of the electroactive species, which in turn is proportional to the
non-electroactive enzyme substrate. Recently simazine antibodies and recombinant
HRP with histidine tag were coimmobilized on gold electrode surface to construct
an amperometric immunosensor, which was used in a fl ow immunoassay [79]. The
detection was performed by passing analyte and GOD conjugated analyte through the
immobilized antibody surface in the presence of glucose and dissolved oxygen at a
potential of 50 mV vs Ag/AgCl. The detection limit was 0.1 ng mL 1 .

Gascón et al. [92] reported a fl ow injection immunoassay (FIIA) method for the
direct and accurate determination of atrazine, without purifi cation or preconcentration.
This FIIA method showed an IC 50 of 2 nM (0.47 ng mL 1 ) and a detection limit of
0.35 nM (0.075 ng mL 1 ). López et al. [93] reported an immunosensor by modifying a
GCE with redox polymer PVPOs(bpy) 2 Cl 2 , where PVP was poly(4-vinylpyridine), and
the specifi c antibody. Atrazine could be detected up to 1 ng mL 1 using this immuno-
sensor. A label-free electrochemical immunosensor has been reported by immobilizing
the antibody on gold nanoparticles [94]. Gold nanoparticle labeled antibody was fi xed
on GCE surface by Nafi on. After the formation of paraoxon immunocomplex, analyte
could be directly quantifi ed at an applied potential of 0.03 mV.

Screen-printed electrodes (SPEs) have also been used in immunosensor design [95,
96]. Kaláb and Skládal [97] reported a disposable screen-printed immunosensor for the
detection of 2,4-D. 2,4-D was covalently immobilized on the surface of SPE. Through
the indirect competitive immunoassay the detection limit for 2,4-D was close to 0.1 ng
mL 1 . Keay and McNeil [98] reported a separation-free and disposable electrochemi-
cal screen-printed atrazine immunosensor. The SPE was prepared with carbon ink
incorporating HRP, and an atrazine antibody immobilized Biodyne C membrane was
placed over the electrode surface. The assay was carried out based on the competition
between available Ab binding sites and atrazine and atrazine-GOD conjugate. In the
presence of glucose, H 2 O 2 formed by the conjugated GOD was reduced by enzyme