Enzyme biosensors features

Enzymes used in pesticide biosensors and their features... 

In view of the conductive and electrocatalytic features of carbon nanotubes (CNTs), AChE and choline oxidases (COx) have been covalently coimmobilized on multiwall carbon nanotubes (MWNTs) for the preparation of an organophosphorus pesticide (OP) biosensor [40, 41], Another OP biosensor has also been constructed by adsorption of AChE on MWNTs modified thick film [8], More recently AChE has been covalently linked with MWNTs doped glutaraldehyde cross-linked chitosan composite film [11], in which biopolymer chitosan provides biocompatible nature to the enzyme and MWNTs improve the conductive nature of chitosan. Even though these enzyme immobilization techniques have been reported in the last three decades, no method can be commonly used for all the enzymes by retaining their complete activity. 

Cell-based biosensors have comparable response characteristics to enzyme electrodes, but offer several advantageous features. ... 

Biosensors may exhibit some of these characteristics. They could, in theory, be inserted in situ, in order to provide a near real-time continuous measurement of a given analjde and allow the selective determination of a single analyte in a complex solution, this detection being achieved by the use of specific enzymes or antibodies. However, from a practical point of view, biosensors suffer from poor stability, often associated with the harsh conditions of a bioreactor. In fact, they are also susceptible to negative effects from physicochemical features of the sample, such as the pH, temperature, and from components in the sample such as inhibitors or other active compounds. Recahbration is not likely to be feasible in in situ conditions. Moreover, the sterilization necessary for the in situ implant could destroy or merely reduce the activity of the enzyme. 

Layer-by-layer (LbL) assembly is a unique technique for the fabrication of composite films with precise thickness control at the nanometer scale [111, 112], The method is based on the alternate adsorption of oppositely charged species from their solutions. The attractive feature of this approach is its ability to assemble complex structures from modular components, and integrate them into self-assembling constructions for a wide range of applications. The LbL method has been successfully exploited in the construction of dendrimer biosensors [113,114], The LbL films provide a favorable environment for the intimate contact between the dendrimer and biomolecule (enzymes or proteins), promoting a direct electron transfer between them and the underlying electrodes. 

As shown the thickness of these nanosheets is about 20 nm. There are numerous nanoscaled cavities on the surface of ZnO microspheres. The size of the cavity is about several hundred nanometers, which is accessible for the enzymes to sequester in the cavities or bind on the surface. Furthermore the cavities may provide a protective microenvironment for the enzymes to retain their enzymatic stability and activity by limiting the conformational change and unfolding of the entrapped enzyme. The FTIR spectra of hemoglobine (Hb) and Hb-ZnO- nafion composite film is shown in Fig.4B. The similarities of two spectra suggested that Hb retained the essential features of its native secondary structure in ZnO nafion composite film, and revealed the excellent biocompability of ZnO nafion composite film. Cyclic voltammetry response of the biosensor at different scan rates was shown in Fig.5. 

Flow injection analysis offers many attractive features to biosensor analysis. The reproducibility and the speed are two dominant characteristics when combining FIA with proper sampling and sample handling. It can be used for both enzyme-based assays and immunochemical binding assays. 

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