Paraoxon electrochemical

FIGURE 2.1 Reaction scheme for OPH hydrolysis of methyl parathion and paraoxon (a) followed by the electrochemical oxidation of p-phenol (b). R and R are ethoxy and methoxy and X is O and S in paraoxon and methyl parathion, respectively. 

MWNTs favored the detection of insecticide from 1.5 to 80 nM with a detection limit of InM at an inhibition of 10% (Fig. 2.7). Bucur et al. [58] employed two kinds of AChE, wild type Drosophila melanogaster and a mutant E69W, for the pesticide detection using flow injection analysis. Mutant AChE showed lower detection limit (1 X 10-7 M) than the wild type (1 X 10 6 M) for omethoate. An amperometric FIA biosensor was reported by immobilizing OPH on aminopropyl control pore glass beads [27], The amperometric response of the biosensor was linear up to 120 and 140 pM for paraoxon and methyl-parathion, respectively, with a detection limit of 20 nM (for both the pesticides). Neufeld et al. [59] reported a sensitive, rapid, small, and inexpensive amperometric microflow injection electrochemical biosensor for the identification and quantification of dimethyl 2,2 -dichlorovinyl phosphate (DDVP) on the spot. The electrochemical cell was made up of a screen-printed electrode covered with an enzymatic membrane and combined with a flow cell and computer-controlled potentiostat. Potassium hexacyanoferrate (III) was used as mediator to generate very sharp, rapid, and reproducible electric signals. Other reports on pesticide biosensors could be found in review [17],... 

Other enzymes have also been immobilized on CNTs for the construction of electrochemical biosensors. Deo et al. [115] have described an amperometric biosensor for organophosphorus (OP) pesticides based on a CNT-modified transducer and OP hydrolase, which is used to measure as low as 0.15 pM paraoxon and 0.8 pM parathion with... 

Figure 5 The Principle of electrochemical immunosensing ofparaoxon-AChE (A) Zr02 nanoparticle modified SPE (B) selective capturingparaoxon-AChE adducts (C) immunoreaction between bound paraoxon-AChE adducts and QD-labeled anti-AChE antibody (D) dissolution of nanoparticle with acidfollowing an electrochemical stripping analysis.

The CNT-based electrochemical sensor was used for the measurement of ChE enzyme activity in saliva samples. Paraoxon and the 10-fold diluted naive rat saliva sample were chosen for this study. The inhibition of ChE enzyme activity (with different concentrations of paraoxon) over an increasing incubation time is shown in Figure 9. It can be seen from this Figureure that the enzyme lost almost 97% activity after 0.5 h incubation with 7.0 nM paraoxon, and 80% after 0.5 h incubation with 0.7 nM paraoxon. Thereafter, the enzyme activity corresponding to 0.7 nM paraoxon continues to decrease through 2.0 h post-treatment, while the activity that corresponds to 7.0 nM paraoxon was nearly completely inhibited within 0.5 h This demonstrates that the... 

Liu and collaborators designed new electrochemical immunosensors based on chemical assembly of vertically aligned SWCNT on carbon substrates for direct detection of the pesticide endosulfan and the insecticide paraoxon " in spiked environmental water samples. In both electrochemical immunosensors, the applied SWCNT forest was fixed onto a GC electrode exploring the formation of amide bonds between the carbo)ylic functional groups at the SWCNT ends and the amine groups on the GC surface, which was previously modified with mixed layers of 4-aminophenyl and phenyl. This architecture was named as GC-Ph-NH2/SWCNT. 

The complex multicomponent starting sol-gel solution explains why there are still so few successful electrochemical manifestations of this approach. A notable exception is the successful imprinting of iron complexes and the pesticide paraoxon which was demonstrated by the groups of Mandler and Marx (Shustak, 2003). 

The method is already widely used for obtaining sol-gel electrochemical sensors. Target molecules used include molecules such as the catecholamines, dopamine [251,295], and epinephrine [302], a beta blocker, propranolol [303], and parathion and paraoxon pesticides [304]. Marx and Lion [304] reported that the selectivity for binding of parathion compared to paraoxon increased from 1.7 to 30 by the sol-gel imprinting. The insert of Figure 4.14 shows that paraoxon is merely a thio-parathion that is, the P=S bond in parathion is replaced by a P=0 bond in paraoxon, but this meager difference was sufficient to drive very large molecular selectivity. 

Improved activity upon immobilization in mesoporous silicates was also observed by electrochemical studies. It was demonstrated that the specific activity of organophosphorus hydrolase entrapped in mesoporous amine functionalized ordered-silicate powder could reach more than twice as high as that of the free enzyme, and in a subsequent article it was reported that enzymatic sensor for paraoxon based on the immobilization of enzyme-grafted mesoporous silicate particulates in Nation film was reported [334]. 

Hu SQ, Xie JW, Xu QH, Rong KT, Shen GL, Yu RQ (2003) A label-free electrochemical immunosensor based on gold nanoparticles for detection of paraoxon. Talanta 61 769-777... 

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