Polymer-based thin-film electrodes

In contrast, polymer-based thin-film electrodes offer the possibility of creating a monolithic device, in which the microelectrodes and the flexible interconnect are fabricated as one structure. 

Another method of following the rate of urea hydrolysis is based on a specific-ion electrode for ammonium ions (see Section 21D). Here, the production of NH4 is monitored potentiometrically and is used to obtain the reaction rate. In yet another approach, the urease can be immobilized on the surface of a pH electrode and the rate of change of pH monitored. Many enzymes have now been immobilized onto supports such as gels, membranes, tubing walls, glass beads, polymers, and thin films. Immobilized enzymes often show enhanced stability over their soluble counterparts. In addition, they can be reused often for hundreds or thousands of analyses. 

Tetrathiafulvalene (TTE) has also been used in electrochromic devices. TTE-based polymers spin-coated onto transparent electrode surfaces form stable thin films with reproducible electrochromic properties (100). The slow response of these devices has been attributed to the rate of ion movement through the polymer matrix. 

Otero and co-workers208,212 have visually observed nuclei of oxidized polymer in thin polypyrrole films on electrodes. They attribute these to sites of counter-ion and solvent ingress. A nucleation model based on the growth of ionically conductive zones provides good agreement with experimental chronoamperometric responses. 

The concept of thin films of a molecularly imprinted sol-gel polymer with specific binding sites for a target analyte is general and can be applied also to electrochemical sensors. For example, a sensor to detect parathion in aqueous solutions is based on films cast on glass substrates and on glassy carbon electrodes (Figure 6.14).12... 

The LEC structure that involves the addition of ionic dopants and surfactants to the printable inks enables the ability to print a top electrode without restriction by the work function of the metal. Silver, nickel, or carbon particle-based pastes are generally the preferred printable electron injecting electrodes however, the shape and size of the particles combined with the softening properties of the solvent can create electrical shorts throughout the device when printed over a thin polymer layer that is only several hundred nanometers thick. For optimal performance, the commercially available pastes must be optimized for printing onto soluble thin films to make a fully screen-printed polymer EL display. 

Revzin A, Sirkar K, Pishko M. Glucose, lactate, and pyruvate biosensor arrays based on redox polymer/oxidoreductase nanocomposite thin films deposited on photo-lithographically patterned gold electrodes. Sensors and Actuators B 2002, 81, 359-368. 

Consider the polymer-on-metal interface, which might be prepared by coating a thin metal film with polymer in a polymer-based LED. The case of the counter electrode, formed by vapor-deposition, is discussed subsequently. First, assume that the substrates have clean surfaces hydrocarbon and oxide free, or naturally oxidized but still hydrocarbon free (pointed out as necessary). Typically, in connection with polymer-based LEDs, the metallic substrate could be gold, ITO (indium tin oxide) coated glass, the clean natural oxide of aluminum ( 20 A in thickness), the natural oxide which forms upon freshly etched Si( 110) wafers ( 10 A), or possibly even a polyaniline film. Dirt , which may be either a problem or an advantage, will not be taken up here. Discussions will alternate between coated polymer films and condensed model molecular solid films, as necessary to illustrate points. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

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