Enzyme biosensors MWCNTs

An amperometric uric acid biosensor based on functionalized MWCNTs with Sn02 nanoparticles has been developed by Zhang et al. This MWCNTs-Sn02 electrode acts as an efficient promoter, and the system exhibits a linear dependence for the uric acid concentration over the range from 1.0 x 10"7 to 5.0 x 10 4 mol L 1. This biosensor exhibits high sensitivity of the MWCNTs-Sn02 modified enzyme electrode. This electrode has been used to monitor trace levels of uric acid in dialysate samples in rat striatum [104],... 

As discussed in sec 3, CNTs have been extensively used to develop pesticide sensors with higher sensitivity and longer stability. In this section we discuss about the design and the development of CNT based pesticide sensors. Joshi et al. reported the detection of OP compounds at a disposable biosensor with AChE-functionalized acid purified multi-wall carbon nanotubes (MCNTs) modified SPE [10]. The degree of inhibition of AChE by OP compounds was determined by measuring the electro oxidation current of the thiocholine generated by the AChE catalyzed hydrolysis of ATCh. The large surface area and electro-catalytic activity of MWCNTs lowered the over potential for thiocholine oxidation to + 0.2 Y. Further, mediators were not used in this case and enzyme immobilization was done by physical adsorption. 

As mentioned in the previous section, the response, the stability and the enzyme activity found greatly enhanced at the MWCNT platform. Other than CNTs, AuNPs also possess some unique properties and recent years it has been widely employed in the biosensors to immobilize biomolecules. Thus in this section we discuss about the application of AuNP matrix for the immobilization of AChE for pesticide sensor development. With the use of AuNPs, the efficiency and the stability of the pesticide sensor gets greatly amplified. Moreover, the nanoparticles matrix offers much friendly environment for the immobilized enzyme and thus the catalytic activity of the enzyme got greatly amplified. Interestingly, Shulga et al. applied AChE immobilized colloidal AuNPs sensor for the nM determination of carbofuran, a CA pesticide [16], The enzyme-modified electrode sensor was also utilized for the sensitive electrochemical detection of thiocholine from the enzyme catalyzed hydrolysis of acetylthiocholine chloride (ATCl). The fabrication and the enzyme catalyzed reaction at the AuNPs coated electrode surface is shown in Fig. 6. 

Another technique to increase the dispersability and the biocompatibility of the carbon nanotubes is the insertion of terminal groups on the surface of CNTs with a procedure called functionalization [104, 105, 118, 123, 136]. The creation of free carboxylic acid and amino acid moieties on the surface are the most commonly used functionalization for enzyme immobilization and stabilization [104, 105, 118,123, 136]. Apart from these two functional groups, other groups are also used, for various purposes. For example, Jeykumari and Narayanan functionalized MWCNTs with toluidine blue, in order to prevent the leakage of the redox mediator from their bienzymic biosensor [114]. 

Drop-casting of the enzyme solution on the porous PANl-CNT composite film followed by (hying has been a common way to prepare biosensors. Direct drop-casting of acetylcholine esterase onto a electropotymerized PANl-c-MWCNT composite... 

Interestingly, the sepioUte-MWCNT hybrid buckypapers combine the adsorption properties of the fibrous siUcate with the electrical conductivity characteristic of the CNTs, aUowing functionalization of the system through reaction with the silanol groups at the external surface of sepioUte. In this way, a new amperometric biosensor based on immobilization of horseradish peroxidase (HRP) enzyme oti sepioUte—MWCNT materials has been developed [259]. 

Monoamine oxidase amperometric biosensor based on SPE were also modified with MWCNT by using the drop casting technique for the determination of antidepressants in model solutions and dosage forms. The authors used BSA protein which provided a matrix for the immobilization of the enzyme and protection of the enzyme activity when glutaraldehyde is used as a linker. Serafin et aZ. developed a label free dual immunosensor for the determination of human growth and prolactin hormones. The electrochemical immunosensor was based on CNT modify carbon SPE platform with the presence of poly(ethylene-dioxythiophene) (PEDOT) and gold nanoparticles. Again, the hybrid nano-material composite facilitated a proper immobilization of the antibody on the electrode matrix. 

The electropolymerization of MN4-MC catalysts on carbon supports is another possibility concerning the fabrication of modified electrodes. Qiu et al. [54] reported the development of a glucose biosensor based on poly-Ni-Pc/MWCNTs. The enzyme glucose oxidase (GOx) was also immobilized on the surface of the functionalized electrode. The sensor gready improved the emission of luminol electrochemiluminescence (ECL) in the presence of H2O2, one of the products of glucose oxidation by GOx. 

Figure 3.7 Scheme of cholesterol oxidase (ChOx) enzyme-based cholesterol biosensor using MWCNT/ GCE. 

In addition to the previously described dehydrogenase-based CNT electrodes, electrochemical biosensors that employ other types of enzyme-modified CNTs have also been reported. Kowalewska and Kulesza applied CNTs with adsorbed redox mediator tetrathiafulvalene (TTF) for electrochemical detection of glucose." TTF-modified CNTs were found to facilitate electron transfer between GOx and the electrode surface for glucose detection. Jia et al. reported a similar strategy for the detection of lactate using MWCNTs modified with TTF and lactate oxidase. Since TTF does not cause skin irritation and the CNT/TTF platform also enables low-potential sensing of lactate, CNT/ TTF/lactate oxidase-based electrochemical biosensors conld be used to detect lactate in perspiration directly on human skin. This was accomplished by preparing temporary tattoos from CNT/ TTF/lactate oxidase-conductive carbon ink that was transferred onto a human subject s skin. ... 

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