Biosensors for pesticides - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 36

Acid phosphatase

Glucose 6 phosphate H 2 O

Inorganic phosphate

Glucose

GOD

Gluconolactone H 2 O 2

Glucose O 2 H 2 O

FIGURE 2.4

Acid phosphatase and GOD bioenzymatic reaction for pesticide detection.

“O” ring. At 20 min inhibition time the detection limits for malathion, parathion methyl
and paraoxon were 3, 0.5 and 5

g l 1 , respectively. Although these bienzymatic sys-
tems look simple, it is diffi cult to provide optimal conditions for both enzymes. In
general the optimum pH, temperature and buffer molarity for different enzymes are
different. The experimental conditions are at the levels below the optimum capacity
of both enzymes [14]. This disadvantage can be minimized by use of a single enzyme
system, which is readily inhibited by the pesticide.
Usually the inhibition results in the decay of the enzyme activity so that the number
of consecutive measurements with the same biosensor is limited. To overcome the above
limitations and determine the pesticides faster and at a cheaper cost disposable sen-
sors have been developed [7, 39, 40, 50]. Screen-printed electrodes have been widely
used in the design of disposable sensors using AChE as a biocatalyst [7, 50] and doped
TCNQ in carbon paste to decrease the working potential and to avoid the interfering
infl uence of electroactive impurities by reducing the applied potential. Using thick fi lm
technology, a biosensor strip was reported by integrating photolithographic conduct-
ing copper tracks, graphite-epoxy composite applied by screen printing and enzyme
immobilized by cross-linking with the bifunctional reagent glutaraldehyde [51]. The
detection limit of the disposable strip was on the order of 10 9 to 10 11 M for paraoxon
and carbofuran. Wang et al. [52] reported a tyrosinase-based screen-printed biosensor
for the determination of carbamate pesticides with fast response time and without the
preincubation period. Recently a disposable carbon nanotube modifi ed screen-printed
biosensor has been reported using the bienzymatic system (AChE/CHO) [40]. After
inhibition by methyl parathion, the bienzymatic amperometric response shows a wide
dynamic linear range (up to 200

µ

M (Fig. 2.5). These
characteristics are attributed to the catalytic activity of carbon nanotubes to promote
the redox reaction of the H 2 O 2 produced from the AChE/CHO enzymatic reaction with
their substrates and a large surface of the carbon nanotube materials.

µ M) and a detection limit of 0.05 µ

2.3.3 Catalysis-based biosensors
Although the inhibition-based biosensors are sensitive, they are poor in selectivity and
are rather slow and tedious since the analysis involves multiple steps of reaction such
as measuring initial enzyme activity, incubation with inhibitor, measurement of residual
activity, and regeneration and washing. Biosensors based on direct pesticide hydrolysis
are more straightforward. The OPH hydrolyzes ester in a number of organophospho-
rus pesticides (OPPs) and insecticides (e.g. paraoxon, parathion, coumaphos, diazinon)
and chemical warfare agents (e.g. sarin) [53]. For example, OP parathion hydrolyzes by
the OPH to form p -nitrophenol, which can be measured by anodic oxidation. Rainina