Biosensors for pesticides - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 48

sensors. Herbicides can reversibly inhibit the photosynthetic electron transport (PET)
chain. The PET includes two photosystems, photosystem-I, and photosystem-II. The
main components of the reaction center photosystem-II (PSII) are D1 and D2 proteins,
where herbicidal inhibition can occur. The inhibition at the D1 protein has been moni-
tored through the mediated photocurrent by employing diaminodurene as a mediator.
With the above inhibition principle a photosynthetic cyanobacteria-based biosensor
has been developed. The detection limit for atrazine was 1 ng mL 1 [124].
2.6 MAJOR INTERFERING COMPOUNDS AND SAMPLE
PRETREATMENT
Real samples contain several interfering compounds along with the target analyte. The
most common ways to reduce matrix effects are (a) selective extraction (“cleanup”) and
(b) dilution to bring the interfering substances below a concentration [109]. According
to the literature AchE activity can be inhibited not only by insecticides but also by other
substances like heavy metals and hypochloride (e.g. sodium hypochloride) [125]. The
heavy metals such as Cu 2 , Cd 2 (pH 4.5), Fe 3 (pH 2) [126] and Hg 2 (pH 7) [127]
can produce a non-competitive AChE inhibition. In immunoassays the main interfering
compounds are anions such as azide and cations such as Ca 2 . They inhibit the enzyme
used as a label. The humic substances (humic acids) present in water or soil extracts
may bind non-specifi cally to the Ab and thereby interfere with the specifi c binding of
the analyte. Matrix effects in food samples frequently occur owing to colored extracts
or to the content of lipids, proteins or polyphenols, which may be coextracted during
sample preparation [128]. Usually, the pesticides are extracted from soil with organic
solvents such as acetone, ethyl acetate or methanol. Immunoassay is tolerant to a vari-
ety of solvents up to a certain degree [6, 129, 130]. In some cases a cleanup step is
introduced such as passing through C18 columns or immunoaffi nity columns, in which
the analyte is separated from the matrix [131-133]. Another approach is the supercriti-
cal fl uid extraction (SFE) prior to immunoanalysis [67, 114].
2.7 CONCLUSIONS
The current pesticide biosensors/immunosensors are competing with other, fairly well-
established fi eld analytical methods such as chemical sensors, immunoassays, and
chemical test kits. Although biosensors have the potential to replace chromatographic
methods in the identifi cation of the pesticides in simple and less expensive ways, accu-
rate validation methods are still very much necessary. More genetically modifi ed sen-
sitive biocatalysts and highly specifi c antibodies are necessary for the development of
stable and robust biosensors/immunosensors. This can be achieved by adopting geneti-
cally modifi ed receptor molecules. New immobilization methods/matrices should be
explored to improve the sensor stability and fast signal transfer to the transducer sur-
face. Even though a few of the methods do not require any pretreatment of sample,