Mechanism of Implantable Biosensor

The above two types of biosensors work on the same principle, i.e., enzymatic conversion ofglucose to pyruvate and hydrogen peroxide by GOD. The by-product produced, hydrogen peroxide, is further reduced and the potential difference is measured with the help of a signal processor. The signal processor amplifies the signal and the digital reading is observed on the display.

Nonenzymatic Glucose Biosensors

For the determination of glucose concentration in biological and chemical samples a biosensor has been developed. However, researchers are working toward the development of novel nonenzymatic biosensor. The most commonly faced problem is the insufficient stability of the biosensor that originated from the enzyme, which is very difficult to overcome. GOD is very stable relative to other enzymes. However, this enzyme may be degraded due to thermal and chemical insults. Moreover, the enzyme may be degraded during fabrication and storage. Moreover, GOD is easily affected by the severe interferences from other oxidizable species in the samples such as ascorbic and uric acids. The electrodes need to be poised at >7 V vs Ag/ AgCl for the electrochemical detection of hydrogen peroxide. Because of all these limitations, researchers have focused on the development of enzymeless glucose biosensors. Enzymeless biosensors have the advantages of stability, simplicity, being oxidation free, and most importantly reproducibility.

Enzymeless biosensors have been developed with disposable pencil graphite electrode. Glucose detection based on overoxidized polypyrrole nanofiber electrode modified with cobalt(II)pthalocyanine tetrasulfonate has been developed [79]. This newly developed enzymeless biosensor displays electrocatalytic activity in alkaline solution for the oxidation of glucose. The performance for glucose determination was significantly improved with a wide linear range, 0.25—20 mM, long-term stability, low percentage of interference, and highly reproducible response. The detection limit is 0.1 mM.

A higher ordered nickel nanowire array electrode for the determination of electrocatalytic oxidation of glucose in alkaline medium has been developed [80]. The linear range for quantification of glucose is 5.0 x 10 to 7.0 x 10_ M with a high sensitivity of 1043 pA/mM. The lower detection limit is 1 x 10 7 M. The biosensor is free of interference from oxidizable species, exhibiting good reproducibility and long-term stability. This novel Ni-nanowire assay provides an improved method for the development of enzymeless biosensor.

Mechanism of Nonenzymatic Glucose Biosensors

The mechanism for the nonenzymatic glucose biosensors is similar to that of the enzymatic glucose biosensors. The difference lies in the absence of enzyme for glucose detection. The mechanism of nonenzymatic glucose biosensor is described as the chemical reaction that occurs on the biosensor surface as follows:

Nanosystems for Diagnostic Imaging, Biodetectors, and Biosensors 239

The Ni(OH)O produced will oxidize the glucose to glucolactone liberating two protons. This reaction is catalyzed by Ni(IH)/(II) redox couple according to the following reactions:

Ni(III) rapidly oxidizes glucose at the anode to produce Ni(II). As a result the concentrations ofNi(II) and Ni(III) species are changed. This causes an increase in the anodic peak current with simultaneous diminishing cathode peak current. These potential changes can be taken into account to generate electric signal for the detection of glucose in test samples.

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