Electropolymerized films

In contrast to the area of redox protein electrochemistry, redox enzyme electrochemistry has received much greater attention, driven in many cases by the desire to construct practical, self-contained enzyme electrodes for commercial applications. Redox enzyme electrochemistry is also easier to study in many ways because the substrate or product is often detected electrochemically rather than the enzyme itself. Various types of electroactive polymers have been used with redox enzymes, including redox polymers, redox-active hydrogels, and electropolymer-ized films of conducting and nonconducting, polymers. We discuss each type of polymer in turn, starting with electropolymerized films. 

Electropolymerization is an attractive approach to enzyme immobilization, a fact already recognized in the earliest studies in this area. The use of electropolymerized films to immobilize enzymes has recently been reviewed, so we provide an abridged discussion here. 

Electrochemical polymerization is simple to carry out, and it can be controlled by manipulating the electrode potential. In principle this allows the polymer film 

A significant proportion of the published work has been concerned with the study of glucose oxidase (GOx EC 1.1.3.4) immobilized in poly(pyrrole). This is in part due to interest in amperometric enzyme electrodes for glucose measurement and in part to the fact that GOx is a very robust enzyme and poly(pyrrole) is very easy to polymerize from aqueous solutions thus conditions required to immobilize the enzyme are not demanding. In addition to the GOx/poly(pyrrole) system, a range of other enzymes and other electropolymerized films have been investigated. 

Many redox enzymes can be denatured if the pH of the solution is taken too far from neutral or if nonaqueous solvents are used. It is therefore desirable, although not always essential, to use neutral aqueous solutions in the electropolymerization process, thus restricting the choice of monomers that can be used. One way to overcome this problem is to incorporate, or immobilize, the enzyme after deposition of the polymer film. This allows a much wider choice of deposition conditions. In addition to adsorbing the enzyme onto preformed poly(pyrrole) films, GOx has also been covalently immobilized onto a wide range of preformed 3- and N-derivatized pyrrole carboxylates or onto N-amino-substituted pyrroles. This technique may increase current densities at a given glucose concentration by as much as twentyfold when compared to the best results obtained elsewhere. The authors make no claims for direct electron transfer between enzyme 

---

Other usefiil gas sensors include the potentiometric ammonia (64) or hydrogen cyanide probes (65), and amperometric carbon monoxide (66) and nitrogen dioxide (67) devices. The hydrogen cyanide probe is an example of a modem device that relies on changes in the conductivity of electropolymerized film (polyanihne) in the presence of a given gas. 

Chain length is another factor closely related to the structural characterization of conducting polymers. The importance of this parameter lies in its considerable influence on the electric as well as the electrochemical properties of conducting polymers. However, the molecular weight techniques normally used in polymer chemistry cannot be employed on account of the extreme insolubility of the materials. A comparison between spectroscopic findings (XPS, UPS, EES) for PPy and model calculations has led some researchers to conclude that 10 is the minimum number of monomeric units in a PPy chain, with the maximum within one order of magnitude n9- 27,i28) mechanical qualities of the electropolymerized films,... 

Fig. 2. Curve A Eleotropolymerization of ImH H2(o-NH2)TPP in 0.1M Et NClO /CH CN by sweeping potential at 200mV/s on Pt electrode. Numbers represent scan number. Curve B Cyclic voltammogram of an electropolymerized film of poly-[H2(o-NH2)TPP] on a Pt electrode, in 0.1M Et NClO /CH CN at 200 mV/s. Integration of the charge under the wave shows that coverage is 3.5X10 9 mol/cm of the porphyrin sites. Curve C Rotated disk electrode voltammetry of the Os(lII,Il) reaction for 0.2 mM...

Dloxygen reduction electrocatalysis by metal macrocycles adsorbed on or bound to electrodes has been an Important area of Investigation (23 ) and has achieved a substantial molecular sophistication in terms of structured design of the macrocyclic catalysts (2A). Since there have been few other electrochemical studies of polymeric porphyrin films, we elected to inspect the dloxygen electrocatalytic efficacy of films of electropolymerized cobalt tetraphenylporphyrins. All the films exhibited some activity, to differing extents, with films of the cobalt tetra(o-aminophenylporphyrin) being the most active (2-4). Curiously, this compound, both as a monomer In solution and as an electropolymerized film, also exhibited two electrochemical waves... 

Metallopolymer films have also been prepared by oxidative polymerization of complexes of the type [M(phen)2(4,4 -bipy)2]2+ (M = Fe, Ru, or Os phen= 1,10-phenanthroline, 4,4 -bipy = 4,4 -bipyridine).23 Such films are both oxidatively and reductively electrochromic reversible film-based reduction at potentials below —IV lead to dark purple films,23 the color and potential region being consistent with the viologen dication/radical cation electrochromic response. A purple state at high negative potentials has also been observed for polymeric films prepared from [Ru(L13)3]2+.24 Electropolymerized films prepared from the complexes [Ru(L16)-(bipy)2][PF6]22 and [Ru(L17)3][PF6]226,27 exhibit reversible orange/transparent electrochromic behavior associated with the Run/Ruin interconversion. 

A. Ciszewski and G. Milczarek, Electrocatalysis of NADH oxidation with an electropolymerized film of l,4-bis(3,4-dihydroxyphenyl)-2,3-dimethylbutane. Anal. Chem. 72, 3203-3209 (2000). 

The surface EXAFS and near edge structure of electropolymerized films of [M(v-bpy)3]+2 (v-bpy is 4-vinyl-4 -methyl,-2,2 -bipyrdine and M = Ru,... 

Although much less so than pyrrole polymers, indole polymers are beginning to be synthesized and studied as new materials. Electropolymerized films of indole-5-carboxylic acid are well-suited for the fabrication of micro pH sensors and they have been used to measure ascorbate and NADH levels. The three novel pyrroloindoles shown have been electrochemically polymerized, and the polymeric pyrrolocarbazole has similar physical properties to polyaniline. 

Manipulation of the Donnan potential in random polymer-modified electrodes can also be achieved. In the case of cast redox polyelectrolyte-modified electrodes one can control ion permselectivity by mixing the redox polymer with an oppositely charged polyelectrolyte in an appropriate ratio before film casting [123]. The same strategy can be followed in electropolymerized films by mixing the electroactive monomer with one of opposite charge [124]. 

Formate production has also been reported for electropolymerized films of [Co(4-vinylterpyridine)2] " on glassy carbon electrodes in dimethylformamide solutions [63]. Interestingly, the product of this same catalytic system in aqueous solutions is formaldehyde [81]. Other heterogeneous systems that produce formate include Cd, Sn, Pb, In, and Zn electrodes in aqueous media [12] (see also Vol VII 5.2.3). It is likely that the pathway to formate formation on metal electrodes follows the sequence of M—H bond formation followed by CO2 insertion to form a M—0C(0)H species followed by desorption from the electrode surface. 

Polymer films can also be electropolymerized directly onto the electrode surface. For example, Abruna et al. have shown that vinylpyridine and vinyl-bipyridine complexes of various metal ions can be electropolymerized to yield polymer films on the electrode surface that contain the electroactive metal complex (see Table 13.2) [27]. The electronically conductive polymers (Table 13.2) can also be electrosynthesized from the corresponding monomer. Again, a polymer film that coats the electrode surface is obtained [25]. Electropolymerized films have also been obtained from styrenic, phenolic, and vinyl monomers. 

Immunosensors for clinical and environmental applications based on electropolymerized films analysis of cholera toxin and hepatitis C virus antibodies in water and serum... 

Geise RJ, Adams JM, Barone NJ, Yacynych AM. Electropolymerized films to prevent interferences and electrode fouling in biosensors. Biosensors Bioelectronics 1991, 6, 151-160. 

It should be noted that, although both the electropolymerized films and the PVC electrodes that use cobalt porphyrins as ionophores respond primarily to thiocyanate, their selectivity properties are quite different. Specifically, the electropolymerized [Co(0-NH2)TPP] films demonstrate a much larger discrimination for thiocyanate... 

Electropolymerized Films for the Construction of Ultramicrobiosensors and Electron-Mediated Amperometric Biosensors... 

Biosensors based on carbon fiber ultramicroelectrodes have been used to determine pyruvate (2) and glucose (3). Glucose sensors using platinum ultramicroelectrodes have also been reported (4), including the entrapment of glucose oxidase in an electropolymerized film of polyaniline (5,6). Ikariyama and co-workers have used platinum ultramicroelectrodes modified with platinum black to construct very sensitive glucose sensors (7-13). 

Electropolymerized films have been used to prevent interferences and fouling in biosensors constructed from reticulated vitreous carbon and platinum disk electrodes (14,15). A biosensor constructed using electropolymerized films can have significantly improved diffusional properties due to the thinness of the film. By engineering the components and properties of a biosensor on a microscopic scale, rather than using "bulk-technology" and physically assembling discrete macroscopic components, as is the conventional practice, an all-chemical method of construction can be achieved. All-chemical methods of construction would... 

The effects of fouling were studied by obtaining glucose calibration curves for two ultramicrobiosensors, one without electropolymerized film, the other with poly(1,3-DAB). Both sensors were then placed in a solution containing 3% w/v bovine serum albumin, at temperature of 4°C, for 6 h. After 6 h, both sensors were again calibrated. 

For the electropolymerization of poly (1,3-DAB), four UMBs were placed in parallel to the working electrode lead, to insure that the electropolymerized film was formed under the same conditions and same length of time for all four UMBs, and to save time. Since the electropolymerization is a potentiostatically controlled process, all four... 

---