Chapter 1 Redox hydrogel-based electrochemical biosensors

Adam Heller University of Texas at Austin, USA.

1 Electron conducting redox polymers in biosensors 1.1 Relationship to mediator-based sensors Redox hydrogel-based sensors are a subgroup of mediator-based sensors. Unlike their older cousins, the diffusional mediator-based sensors, their electron trans- port mediating redox centers are polymer-bound. When the resulting redox polymers are crosslinked on electrodes, they become insoluble, but swell in water to form redox hydrogels (1–4). These hydrogels can be as soft as Jell-O, or as tough as leather, depending on the extent of their crosslinking (1, 5). Upon hydration and swelling, the mobility of their segments is increased. This increased segmental mobility translates to an increase in the frequency of electron-transferring collisions between the tethered redox centers, and increases the electronic conductivity of the redox hydrogels. The diffusivity of electrons in the redox hydrogels is typically in the 10~6-10~10 cm2s"a range (6-9). In addition to conducting electrons, the redox hydrogels, like other hydro- gels, also conduct ions (10-18). Because they are permeable to water-soluble species, water-soluble chemicals like nitrite and biochemicals like ascorbate and dopamine can be electrooxidized or electroreduced in their three-dimen- sional matrices (19-21). When redox enzymes are co-immobilized in the redox hydrogels, their reaction centers can also be electrooxidized or electroreduced. The enzyme-containing redox hydrogels catalyze, therefore, the electrooxidation and the electroreduction of the substrates of the enzymes (22-54). Enzymes with FAD, FMN, PQQ., heme, and copper-containing redox centers have been co-immobilized and their centers are electroactive. Examples of the electro- oxidized biochemicals include glucose, fructose, cellobiose, lactate, cholesterol, glycerol-3-phosphate, pyruvate, phenols, primary and secondary alcohols, hista- mine and other amines, D-amino acids, and glutamate. The electroreduced che- micals include O2 and H2O2.

1 ADAM HELLER

1.2 The electrocatalytic activity of redox hydrogels and the "wiring" of enzymes The electrocatalytic activity of the films formed by crosslinking depends on electron-transferring collisions between redox centers of the co-immobilized enzymes and those of the redox polymer (7-18). When these collisions are fre- quent enough to assure the efficient collection of the electrons from the enzyme reaction centers by the redox polymer, or the efficient delivery of electrons by the redox polymer to the reaction centers of the enzyme, the enzyme is said to be electrically "wired" and the redox polymer is said to "wire" the enzyme. The crosslinked redox polymers are poor electron conductors when dry: hydration is of essence for the electron transport to be fast and for the current density to be high. When the electron diffusivities reach, after hydration, 10~6-10~9 cm2s"a and the co-immobilized redox enzyme turns over at a rate of >100 s ~ \ the limiting current density of substrate-electroreduction/oxidation on a semi-infinite planar enzyme electrode is typically of 10~3-10~4 Acm~2. Because their electron-transfer mediating centers are not leached, the "wired" enzyme electrodes can be used in microelectrodes (55-59), in experiments in vivo (60-68), and in flow systems (69-79).

1.3 Dependence of the diffusivity of electrons on crosslinking and the advantage of composite electrodes Nanocomposite enzyme electrodes can be made without covalent crosslinking by sequentially adsorbing countercharge films of a redox polymer, usually a polycation and enzymes which is often a polyanion at neutral pH (80-85). In the absence of covalent crosslinking, these films disintegrate in use and their components are slowly leached. Crosslinking by forming covalent bonds prevents the disintegration. While leaching of the