Biosensors for pesticides - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 37

100

M methyl parathion 200

(a) (b)

80

60

40
20
0 0

50 100 150 200

I 0

10 min Concentration/ M

I t

2 m M ACH

FIGURE 2.5 (a) Amperometric response of CNT modifi ed screen-printed biosensor for methyl parathion
in 0.1 M PBS containing 0.1 M NaCl (pH 7.4) at applied potential of 0.50 V and (b) the relative inhibition of
CNT AChE/CHO biosensor as a function of methyl parathion concentration (adapted from [40]).

et al. [54] used a recombinant, OPH-expressing Eschericia coli ( E. coli ) immobilized
on a pH electrode, to develop a microbial biosensor for the direct detection of OP.
Mulchandani et al . [55] purifi ed OPH from a recombinant E. coli and immobilized it
on a pH electrode to develop a potentiometric biosensor by catalyzing the hydrolysis
of organophosphorus pesticides (parathion, paraoxon, methyl parathion) to release pro-
tons, the concentration of which was proportional to the amount of hydrolyzed sub-
strate. Mulchandani et al. [56] reported another amperometric OP biosensor using thick
fi lm strip electrode and genetically engineered E. coli OPH. The detection limit of this
sensor was 7 10 8 M.

2.3.4 Flow injection biosensors
Flow injection analysis (FIA) is a highly convenient method for fast detection and
automatization. Several reports exist on the FIA of pesticides using enzyme-based
biosensors with a wide linear range of detection. Kindervator et al. [57] proposed an
exchangeable enzyme reactor by immobilizing AChE and COx on magnetic parti-
cles using glutaraldehyde as a cross-linker. The main advantages of this method were
that the magnetic particles could be easily held inside the reactor under the magnetic
fi eld and the completely inactivated enzyme could be replaced. An FIA biosensor for
paraoxon by immobilizing AChE in PVA-SbQ polymer and integrating with a continu-
ous fl ow system exhibited a detection limit of 1.0 nM (0.3 ng mL 1 ) (inhibition % 10).

The analytical time for one sample including inhibition reactivation using 1.0 mM
2-pyridinealdoxime methiodide was 1 h [44]. Recently Kandimalla and Ju [11] reported
an FIA system for the detection of the organophosphorus insecticide sufl otep by modi-
fying the GCE with cross-linked chitosan-multiwall carbon nanotube (MWNT) com-
posite and covalently linking AChE to the chitosan via glutaraldehyde. This membrane
was crack free and showed a homogeneous porous structure and distribution of MWNTs
(Fig. 2.6). Both biocompatibility of chitosan and the inherent conductive properties of