Stability sensors

NO sensors can be characterized in terms of sensitivity, detection limit, selectivity, response time, stability, linear range, lifetime, reproducibility, and biocompatibility. Sensor stability is important especially when measuring low NO concentrations. For example, when measuring low NO concentrations it must be the case that the noise... 

Some earlier developments and applications of various implantable pH sensors or measurement systems have been reported [128, 129, 130, 131]. However, reliable pH sensors for long-term implantations are still not available, and widespread clinical usage of implantable pH sensors has not been reached. Similar to other implantable sensors, the development of implantable pH microelectrodes, either fully implanted in the body or needle type sensors applied through the skin (percutaneous), has faced serious obstacles including sensor stability deterioration, corrosion, and adverse body reactions [48, 132, 133], Among them, encapsulation to prevent corrosion represents a major challenge for the implantable sensor devices [51]. Failure of encapsulation can cause corrosion damage on internal components, substrate materials, and electrical contacts [48], The dissolution of very thin pH sensitive layers will also limit the stability and lifetime of implantable micro pH sensors. 

Sensors or analyzers exist for some of the priority analytes, such as 09, pH, and N03 . The challenge in these cases is to improve sensor stability, response rates, or lifetime. However, for most of the priority analytes, there is no existing sensor or analyzer system that will operate for long time periods without operator intervention. The development of sensors for most of these analytes, such as chlorofluorocarbons or dissolved iron, must circumvent the difficulties posed by low analyte concentrations or interference from other dissolved material. Development of specific sensing chemistry is the ultimate means of circumventing these problems. 

Substrate Enzyme Sensor Stability Response Range [M]... 

The good agreement between sensor and laboratory results proves the effectiveness of the pre-calibration procedure and sensor stability. The results suggest that these devices can be used for short term glucose-lactate monitoring in for example the intensive care unit, the operation theater, in sports, medicine, rehabilitation and in diabetology. 

Binyamin, Chen and Heller reported that wired enzyme electrodes constituted of glassy carbon electrodes coated with poly(4-vinylpyridine) complexed with [Os(bpy)2Cl] and quarternized with 2-bromoethylamine or poly[(iV-vinylimidazole) complexed with [Os(4,4 -dimethyl-2,2 -bypyridine)2Cl] or poly(vinylpyridine) complexed with [Os(4,4 -dimethoxy-2,2 -bypyridine)2Cl] quaternized with methyl groups lost their electrocatalytic activity more rapidly in serum or saline phosphate buffer (pH 7.2) in the presence of urate and transitional metal ions such as Zn and Fe " " than in plain saline phosphate buffer (pH 7.2). It was reported that as much as two-thirds of the current is lost in 2 h in some anodes. However, when a composite membrane of cellulose acetate, Nafion, and the polyaziridine-cross-linked co-polymer of poly(4-vinyl pyridine) quaternized with bromoacetic acid was applied, the glucose sensor stability in serum was improved and maintained for at least 3 days [27,50]. 

Gorton et al. reported carbon paste electrodes based on Toluidine Blue O (TBO)-methacrylate co-polymers or ethylenediamine polymer derivative and NAD" " with yeast alcohol dehydrogenase for the analysis of ethanol [152,153] and with D-lactate dehydrogenase for the analysis of D-lactic acid [154]. Use of electrodes prepared with dye-modified polymeric electron transfer systems and NAD+/NADH to detect vitamin K and pyruvic acid has also been reported by Okamoto et al. [153]. Although these sensors showed acceptable performances, insensitivity to ambient oxygen concentration, sensor stability and lifetime still need to be improved to obtain optimal dehydrogenase based enzyme biosensors. 

Novel materials are thus needed to improve the mechanical and chemical stability of the sensor for practical applications in various conditions and, on the other hand, to improve the immobilization scheme in order to ensure sensor stability and the spatial control of biomolectdes. The most important materials for chemical and biochemical sensors include organic polymers, sol-gel systems, semiconductors and other various conducting composites. This chapter reviews the state-of-the-art biosensing materials and addresses the limitations of existing ones. 

In most amperometric cytochrome b2 electrodes the reaction is followed by anodic oxidation of ferrocyanide at a potential of +0.25 V or above. The first of such sensors was assembled by Williams et al. (1970), who immobilized the enzyme (from baker s yeast) physically at the tip of a platinum electrode within a nylon net of 0.15 mm thickness. The large layer thickness resulted in a response time of 3-10 min. Owing to the low specific enzyme activity used, the sensor was kinetically controlled. Therefore the linear measuring range extended only up to 0.1 Km-A similar sensor has been applied by Durliat et al. (1979) to continuous lactate analysis. The enzyme was contained in a reaction chamber of 1 pi volume in front of the electrode. This principle has also been employed in the first commercial lactate analyzer using an enzyme electrode (Roche LA 640, see Section 5.2.3.3X With a sensor stability of 30 days and a C V below 5%, 20-30 samples/h can be processed with this device. 

Immobilisation of the antibody to the crystal surface is of critical importance to the overall performance of the sensor. Stability, orientation and reproducibility of the immobilised layer will affect the sensitivity, lifetime, reusability and potential applications of the sensor. 

A first step toward the decrease of mediator leakage and hence concomitant increase in the sensor stability has been seen in the development of carbon-paste electrodes [104]. A graphite powder is thoroughly mixed with mineral oil, and the paste obtained is pressed into a tube and contacted from the back by the insertion of a copper wire. To... 

In comparison to enzyme-based biosensors, microbial biosensors show lower analyte selectivity, slightly slower response times, but often much better stability. Microbial biosensor determination of analytes such as amino acids, alcohol, and lactate show sensor stability over several days, whilst enzyme-based sensors may have operational lives of only 2-24 h. 

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