Bond stabilities, sensor surfaces

When constructing biosensors, which are to be used continuously in vivo or in situ, maintaining sensor efficiency while increasing sensor lifetime are major issues to be addressed. Researchers have attempted various methods to prevent enzyme inactivation and maintain a high density of redox mediators at the sensor surface. Use of hydrogels, sol-gel systems, PEI and carbon paste matrices to stabilize enzymes and redox polymers was mentioned in previous sections. Another alternative is to use conductive polymers such as polypyrrole [123-127], polythiophene [78,79] or polyaniline [128] to immobilize enzymes and mediators through either covalent bonding or entrapment in the polymer matrix. Application to various enzyme biosensors has been tested. 

Although Schiff base formation can be performed with amine groups, the low stability of the bond in aqueous conditions makes hydrazide a better alternative. Hydrazides can be introduced on the sensor surface via reaction of hydrazine or carbohydrazine to carboxylic groups after activation with EDC/NHS (Fig. 11) [32]. The hydrazide-aldehyde bond forms rapidly and is relatively stable in neutral to alkaline conditions, but disintegrates slowly in acidic buffers. If necessary, the bond can be further stabilized by reduction with sodium cyanoborohydride at pH 4. 

One of the major deterrents to the successful application of electroanalytical sensors has been the lack of long-term stability of the polymer films. At least three factors effect the stability of these amperometric sensors. These factors are the mode of polymer film attachment to the electrode surface (adsorption vs. covalent bonding), solubility of the film in the contacting solution, and finally, the mode of attachment of the catalyst in the polymer film (electrostatic vs. covalent). 

Stability of Prussian blue modified screen-printed electrodes The operational stability of all the PB-modified sensors is a critical point, especially at neutral and alkaline pH. A possible explanation for reduced stability could be the presence of hydroxyl ions at the electrode surface as a product of the H202 reduction. Hydroxyl ions are known to be able to break the Fe-CN-Fe bond, hence solubilising the PB [21]. An increased stability of PB at alkaline pH was first observed by our group after adopting a chemical deposition method for the modification of graphite particles with PB for the assembling of carbon paste electrodes [48]. 

The modification of TCO surfaces can clearly be used to enhance the electrochemical performance of oxide electrodes. There are, however, issues yet to be resolved regarding the initial surface composition of the oxide, especially for ITO, which prevent realization of the full electrochemical and electronic potential of these electrodes. In some cases, the modification chemistries produce a surface, which is sufficiently robust to be used in various sensor platforms or condensed phase devices. However, it is not yet clear whether long-term stability can be achieved in those cases where the oxide is exposed to solutions that also promote the hydrolysis of the oxide unless an extremely strong covalently bonded network, or chemisorption interaction can be produced. These modification strategies will continue to evolve with the increasing need for viable interfaces between electroactive materials and the metal oxide electrode. 

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