**6. Sol-gel and hydrogel material in biosensors**

CPs as a convenient component, forming an appropriate environment for the immobilization of enzyme at the electrode surface. Stable immobilization of macromolecular biomolecules on conducting microsurfaces with complete retention of their biological recognition properties is a crucial problem for the commercial development of miniaturized biosensor. Most of the conventional procedure for biomolecule immobilization such as cross-linking, covalent binding and entrapment in gels or membrane suffer from a low reproducibility and a poor

Due to that CPs have considerable flexibility in the available chemical structure, which can be modified as required, CPs have attracted much interest to serve as good matrices for the immobilization of enzymes. The techniques of incorporating enzymes into electro-depositable conducting polymeric films permit the localization of biologically active molecules on electrodes of any size or geometry and are particularly appropriate for the elaboration of multianalyte micro-amperometric biosensors. Another advantage offered by CPs is that the electrochemical synthesis allows the direct deposition of the polymer on the electrode surface, while simultaneously trapping the protein molecules. In addition, the electrochemically prepared CPs can be grown with controlled thickness using lower potential and they also provide an excellent enzyme-entrapping property. The polymerization through electrochem‐ ical oxidation provides greater control over the process and enables control over the thickness of the polymer layer and even small electrode substrate to be coated. This technique offers a suitable way to make a homogeneous film that adheres strongly with the electrode surface.

Another important advantage of using CPs is that the biomolecules can be immobilized onto the nanowire structure in a single step rather than the multiple steps that are required when other non-polymeric materials are used. Nanostructured conjugated polymers and their nanocomposites represent new advanced materials that are key issues for the development of new devices and structures offering the association of the various properties required in advanced applications. As conducting polymer nanomaterials are light weight, have large surface area, adjustable transport properties, chemical specificities, low cost, easy processing and scalable productions, they are used for applications in nanoelectric devices, chemical and biological sensors [57]. There are extensive studies in the literature concerning the synthesis,

Among the CPs, PPy is one of the most extensively used conducting polymers in design of bioanalytical sensors. PPy and its derivatives play a leading role due to several interesting properties such as electroactivity, ionic exchange properties, supercapacitors for energy storage, secondary batteries, and elastic textile composites of high electrical conductivity, as well as good stability. PPy have the most versatile applicability for the construction of different types of bioanalytical sensors. The background presented illustrates that PPy is a very attractive, versatile material, suitable for preparation of various catalytic and affinity sensors and biosensors. The immobilization of biologically active molecules into PPy can be obtained during electrochemical deposition during which either some undesirable electrochemical interactions can be prevented or the electron transfer from some redox enzymes can be facilitated. The developments in nano-structured conducting polymers and polymer nano‐ composites have large impact on biomedical research. Significant advances in the fabrications

spatially controlled deposition.

80 State of the Art in Biosensors - General Aspects

characterization, and application of these CPs.

The terminology sol-gel is used to describe a broad class of processes in which a solid phase is formed through gelation of a colloidal suspension. A typical hydrogel network involve poly (vinyl alcohol) (PVA) or poly(acrylic acid) (PAA). Sol-gel is gradually attracting the attention of the electrochemical community as a versatile way for the preparation of modified electrodes and solid electrolytes. Sol-gel electrochemistry is rather young, and often researchers are still excited by the mere feasibility of realizing an application by sol-gel technologies due to its better processibility, improved diffusion rate, large ion-exchange capacity, fast proliferation of organic-inorganic hybrids, and other application specific chemical properties. The types of sol-gel materials that are useful for electrochemistry and the recent advances in the various fields of sol-gel electrochemistry have been reviewed by a few research groups [62, 63]. The ease of preparation and the wide ranging flexibility of incorporating desired functionalities by careful selection and design can be readily applied to different types of electrode materials without being restricted by electrode shapes and designs, which are suited for the immobili‐ zation of biomolecules. However, sol–gel matrices, despite being also biocompatible, are traditional fragile nature, similarly to biological membranes, and suffer from low sensitivity and reproducibility which hampered their application in biosensor. The organic–inorganic material prepared by sol–gel method can yield a highly sensitive, robust and stable biosensor.

Hydrogels have also been extensively investigated as coatings support for immobilization of enzymes. Enzymes can often denature and lose their efficiency; however this effect can be mitigated by encapsulating it inside a hydrogel, because hydrogel is a type of waterswollen and cross-linked polymer formed by the gelling process and features a highly hydrophilic structure of three dimensional networks. And the consequent swelling of the polymer matrix provides a biocompatible microenviroment for the enzyme to maintain its natural configura‐ tion, thus is an ideal matrix for the massive entrapment of cell and enzyme. Moreover, the biosensors based on the hydrogel have high sensitivities. Although it exhibits a high affinity for water, it does not dissolve, and provides sufficient permeability for both solvent and substrate molecules so that they are capable of diffusing quickly through the water-swollen polymer which has reasonably high water content. In addition, the external hydrophobic organic solvent is unable to distort the native conformation of the entrapped enzyme in the sol-gel, and the hydrogel is soft and of rubbery consistence which closely resemble living tissues. Consequently, the widely utilised application for hydrogels has been as enzyme stabilising agents [64-68].

#### **7. Conclusion and future perspectives**

Proper electrode fabrication using different materials for efficient electron transport has recently aroused much interest as a versatile tool for the constructing biosensors. Biosensors designed employing polymeric materials results in low detection limits, high sensitivities, lower applied potential, reduction of background, efficient electron transfer and easier immobilization of enzymes on electrodes. Application of organic CPs in biosensors has recently aroused much interest as potential candidates to enhance speed, sensitivity and versatility for electrochemical biosensors due to their easy preparation methods along with attractive unique properties such as high stability at room temperature, good conductivity output and facile polymerization and being compatible with biological molecules in a neutral aqueous solution. However, there are verious of challenges to be addressed in order to fulfill the applications of polymers. In addition, long laboratory synthetic pathways and costs are also involved in the production of the functional polymers. The aforementioned disadvantages of the above polymeric materials call for search for low cost biomaterials as alternative for the development of novel electrochemical sensors and biosensors. Those focuses towards design‐ ing smart polymers such as nanostructure doped polymers are the most promising for polymers to be further investigated. By combination of the unique properties of nanostructured material and various polymers, it is possible to develop novel enzyme-based bioelectronic devices with particular advantages. In particular, the integration of nanotech‐ nology, with novel polymeric materials should lead to very sensitive and fast assays.
