The Impact of Pesticides - Academy Publish

these limitations can be overcome by adsorbing enzymes onto CNT-modifiedelectrodes decorated with metallic nanoparticles, such as Pt-NP. By subsequentlydepositing a Nafion film onto the electrode, it is possible to reduce leaching of theenzyme and to improve the stability of the biosensor.Covalent Attachment: Covalent immobilization approach yields the directanchoring of the enzymes to the carbon framework. One of the most used types ofenzyme immobilization is the chemical immobilization by means of cross-linkingwith glutaraldeyde. Dan Du et al proposed the immobilization of AChE by usingglutaraldehyde as cross-linker to MWNTs-chitosan (MC) composite, leading to astable AChE biosensor for rapid determination of triazophos quantitatively (Du et al,2007b).Layer-by-layer self assembly: The Layer-by-layer (LBL) electrostatic self-assemblydeposition method, initially reported by Decher, is one of the most convenienttechniques for fabricating molecularly controlled ultrathin multilayer films (Decherand Hong, 1991). This technique based on the successive deposition of very thinlayers of cationic and anionic species from a solution. The film fabrication isperformed under mild conditions, which is particularly important for preservingactivity of biomolecules. Guodong Liu et al reported the immobilization of AChEon the negatively charged CNT surface by alternatively assembling a PDDA layerand an AChE layer (Liu and Lin, 2006).Electropolymerization: Electropolymerization is an attractive and well-controlledmethod for immobilizing enzymes onto electrodes. In this methodology, the enzymeis mixed with a monomer which is electropolymerized at a GCE or a metal electrode,whereupon the enzyme becomes embedded into the polymer matrix. Theincorporation of the enzyme into the matrix is often promoted through electrostaticinteractions. Numerous enzymes have been incorporated into electropolymerizedfilms (Bartlett and Cooper, 1993). In many cases conductive polypyrrole (PPy) hasbeen used as a polymer matrix. This choice relates to the fact that pyrrole can beelectropolymerized at low oxidation potentials in aqueous solutions at neutral pH,which is compatible with a wide range of biological molecules. Recently, a simplemethod to immobilize AChE on PPy and polyaniline (PAn) copolymer doped withmulti-walled carbon nanotubes (MWCNTs) was proposed (Du, et al, 2010b). Thesynthesized polyaniline/polypyrrole/MWCNTs copolymer presented a porous andhomogeneous morphology which provided an ideal size to entrap enzyme molecules.The surface hydrophilicity was improved greatly after forming a complex structureinstead of a separate layer. It provided an excellent environmental and chemicalstability around the enzyme molecule to stabilize its biological activity to a largeextent, resulting in a stable AChE biosensor for screening of organophosphatesexposure.As the oxidation of enzymatic product thiocholine occurs at a relatively highpotential on conventional electrodes, mediators such as cobalt (II) phthalocyanine(Skladal, 1991; Hart and Hartley, 1994), prussian blue ( Ricci et al, 2004; Sun andAcademyPublish.org - The Impact of Pesticides401