Enzyme-based biosensors, improvements

In recent years the electrochemistry of the enzyme membrane has been a subject of great interest due to its significance in both theories and practical applications to biosensors (i-5). Since the enzyme electrode was first proposed and prepared by Clark et al. (6) and Updike et al. (7), enzyme-based biosensors have become a widely interested research field. Research efforts have been directed toward improved designs of the electrode and the necessary membrane materials required for the proper operation of sensors. Different methods have been developed for immobilizing the enzyme on the electrode surface, such as covalent and adsorptive couplings (8-12) of the enzymes to the electrode surface, entrapment of the enzymes in the carbon paste mixture (13 etc. The entrapment of the enzyme into a conducting polymer has become an attractive method (14-22) because of the conducting nature of the polymer matrix and of the easy preparation procedure of the enzyme electrode. The entrapment of enzymes in the polypyrrole film provides a simple way of enzyme immobilization for the construction of a biosensor. It is known that the PPy-... 

Additives such as polyethylene glycol, cationic antibiotics, polymers, small uncharged molecules, and negatively charged proteins have been used extensively in order to avoid the denaturing of enzymes or to improve the sensitivity and operational stability of biosensors. DNA has been proposed as an additive to improve the response and stability of biosensors based on CP. The biomolecules studied, such as tyrosinase [93], peroxidase [94], cytochrome C [95], have been shown to improve its performance by using adsorbed DNA within CP as an additive. 

Cellular biosensors have been widely described [11-55]. In many cases, the cells have been used in a manner analogous to that of enzyme based devices simply because they contain substantial quantities of particular enzymes. There are, of course, advantages to this approach since the enzymes do not have to be isolated and so may be cheaper but also more active and more stable than the purified components. However, the reproducibility, speed of response and selectivity of the cell based devices will, in general, be less favorable than their enzyme based counterparts. This is because of the relatively large physical size of the cells, the presence of membranes that hinder diffusion and the presence of enzymes other than the one(s) of particular interest. Nevertheless, a range of approaches has been adopted to improve the selectivity and other characteristics of whole cell biosensor devices. These were reviewed by Racek [11] and include ... 

Applications. A biotinylated GOX-based biosensor was developed based on a new electropolymerized material consisting of a pol3rp3uidyl complex of ruthenium(II) functionalized with a pyrrole group [90]. Because histidine, lysine and arginine functions also coordinate Os /Os , biosensors based on co-electrodeposited GOX, HRP, soybean peroxidase (SBP) and laccase with redox Os /Os polymer have been developed [89]. A metal chelate formed by nickel and nitrilotriacetic acid was used to modify a screen-printed electrode surface. The functionalized support allowed stable attachment of acetylcholinesterase and the resulting biosensor was used for sensitive detection of organophosphorus insecticides [91]. This method is attractive because it ensures a controlled and oriented enzyme immobilization, considerably improving the sensitivity and the detection limit. 

Lakard et al. have described HRP modified PANI nanoparticles-based biosensor for sensing [159]. This biosensor format exhibits improved enzyme deposition and improved signal-to-noise ratio. There is a strong relationship between nano-dimension and biosensing performance. 

The potential for the coexistence of several enzymes in tissue materials can also be a major drawback as it can affect the selectivity of the device when used in complex sample media. Some of the strategies that have been employed for improving the selectivity of tissue-based biosensors include the use of activators to promote the primary reaction, inhibitors to suppress the undesirable reactions, or preincubation of the desired substrate. Under favorable conditions, the multienzyme activity of the tissue can be exploited for the detection of multicomponents in real samples. For example, with amperometric detection, such multicomponent detection may require the application of different potentials to achieve improved selectivity. 

Similarly, nanocomposites of graphene and PANl were used by Ruecha et al. [54] to develop a paper based biosensor for the enzyme based detection of cholesterol and the electrochemical detection of the by-product H2O2 of the enzymatic reaction. Interestingly they incorporated polyvinylpyrrolidone in the modified electrode in order to improve the stabilization of the graphene concentration. Those biosensors based on composites of different materials often show a combination of the parameters of the materials and therefore offer a higher thermal stability, better condnctivity and improved resistance to salts. 

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