Biosensor stability

Fig. 24.4. Study of the biosensor stability with biological sample. Arrows indicate where (i.e. biological sample) a solution obtained by dialysing a human serum with the microdialysis probe was flowed in the biosensor cell. At the beginning a perfusion solution and control solution (glucose 5 mmol l-1) were used instead of the serum to test the biosensor response. Control solution of glucose was also used during and at the end of the experiment to evaluate the stability of the biosensor. Continuous flow mode 10 pi min-1). Applied potential —50 mV vs. int. ref. Reprinted from Ref. [59] with permission from Elsevier.

The factors that are generally important to know are the operational stability (also termed the useful lifetime) and the storage stability. Comparison of literature data on these properties is sometimes difficult because the experimental conditions employed to establish the biosensor stability vary widely. It appears most practical to use the sensor intermittently for analysis and... 

Materials used in electrochemical biosensors can greatly benefit from nanotechnology. Nanomaterials are especially used to immobilize the recognition elements and improve the transport of the electrical signal from the biorecognition element to the electrode surface (the transducer). A weak immobilization protocol affects the biosensor stability during use and storage and prejudices the overall functionality of the biosensor [85-89]. 

A reference electrode, such as the standard calomel electrode (SCE), is placed next to the enzyme electrode. The reference electrode may be combined with the working electrode, as is the case in the pH-sensitive glass electrode. The electrodes are connected to a millivoltmeter for potentiometric measurements, or to a potendostat for amperometric measurements. The system is connected to a recorder which monitors the biosensor stability and the progression of its response curve towards a steady state. This recorder may be replaced with a data acquisition unit which also gives the slopes of the response curves and relates them directly to the analyte concentration thereby increasing the rate of measuremenL... 

In 1994, thiols were firstly used as stabilizers of gold nanoparticles [6a]. Thiols form monolayer on gold surface [18] and highly stable nanoparticles could be obtained. Purification of nanoparticles can be carried out, which makes chemical method of metal nanoparticles a real process for nanomaterial preparation. Various thiol derivatives have been used to functionalize metal nanoparticles [6b, 19]. Cationic and anionic thiol compounds were used to obtain hydrosols of metal nanoparticles. Quaternary ammonium-thiol compounds make the nanoparticle surface highly positively charged [20]. In such cases, cationic nanoparticles were densely adsorbed onto oppositely charged surfaces. DNA or other biomolecule-attached gold nanoparticles have been proposed for biosensors [21]. 

Enzyme electrodes belong to the family of biosensors. These also include systems with tissue sections or immobilized microorganism suspensions playing an analogous role as immobilized enzyme layers in enzyme electrodes. While the stability of enzyme electrode systems is the most difficult problem connected with their practical application, this is still more true with the bacteria and tissue electrodes. 

AET activities are based upon its expertise in the field of protein biotechnology and are oriented to the area of protein stabilization technology and to the development and production of stabilized biosensors. The AET biosensor activities are enhanced by support and synergy with its sister companies Gwent Electronic Materials Ltd. and Gwent Sensors Ltd. 

AET has also demonstrated very high operational thermal stability for immobilized biocatalysts, which is applicable to a multitude of industrial areas including the biocat-alytic and biosensor industries. 

The work in the biosensor industry permitted the testing and proved of stability and reproducibility of enzymes, within the conditions employed in that area. Enzymes with demonstrated stability include lactate oxidase, malate dehydrogenase, alcohol oxidase, and glutamate oxidase. 

With regard to biosensor applications, a wide variety of electrochemically active species (ferrocene, ruthenium complexes, or carbon and metal (Pt, Pd, Au...) [185,186] were also introduced into the sol-gel matrices or adsorbed to improve the electron transfer from the biomolecules to the conductive support [187,188]. For instance, glucose oxidase has been trapped in organically modified sol-gel chitosan composite with adsorbed ferrocene to construct a low-cost biosensor exhibiting high sensitivity and good stability [189]. 

A flow injection optical fibre biosensor for choline was also developed55. Choline oxidase (ChOX) was immobilized by physical entrapment in a photo-cross-linkable poly(vinyl alcohol) polymer (PVA-SbQ) after adsorption on weak anion-exchanger beads (DEAE-Sepharose). In this way, the sensing layer was directly created at the surface of the working glassy carbon electrode. The optimization of the reaction conditions and of the physicochemical parameters influencing the FIA biosensor response allows the measurement of choline concentration with a detection limit of 10 pmol. The DEAE-based system also exhibited a good operational stability since 160 repeated measurements of 3 nmol of choline could be performed with a variation coefficient of 4.5%. 

Enzyme-based optical sensor applications will be further described in this book. They are still the most widespread optical biosensors but work is needed to overcome limitations such as shelf life, long term stability, in situ measurements, miniaturization, and the marketing of competitive devices. 

Biosensors are the analytical systems, which contain sensitive biological elements and detectors. Plant cells as a possible biosensors have natural structure that determinates their high activity and stability. Criteria in the screening of the plant cells as biosensors for allelopathy should be as under (i) Reaction is fast based on the time of response, (ii) Reaction is sensitive to small doses of analysed compounds or their mixtures and (iii) Methods of detection viz., biochemical, histochemical, biophysical (in particular, spectral changes in absorbance or fluorescence) are easy in laboratory and in the field conditions. The search of biosensors in active plant species is suitable to determine the mechanisms of action of biologically active substances or external factors of the environment (Roshchina and Roshchina, 2003 Roshchina, 2004 2005 c)). 

M.D. Gouda, M.S. Thakur, and N.G. Karanth, Stability studies on immobilized glucose oxidase using an amperometric biosensor. Biotech. Techniq. 11, 653-655 (1997). 

S. Sasso, R. Pierce, R. Walla, and A. Yacynych, Electropolymerized 1,2-diaminobenzene as a means to prevent interferences and fouling and to stabilize immobilized enzyme in electrochemical biosensors. Anal. Chem. 62, 1111-1117 (1990). 

B. Haghighi, S. Varma, F.M. Alizadeh, Y. Yigzaw, and L. Gorton, Prussian blue modified glassy carbon electrodes - study on operational stability and its application as a sucrose biosensor. Talanta 64, 3-12 (2004). 

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