Electrochemically polymerized films

The use of electrochemically polymerized films to immobilize enzymes at electrode surfaces was reviewed by Bartlett and Cooper up to 1992 [24]. While a large variety of studies have been performed mainly using poly(pyrrole) as the conducting polymer, many problems still have to be solved, particularly with respect to the question of the speed of... 

Cosnier, S. (1999) Biomolecule immobilizahon on electtode surfaces by entrapment or attachment to electrochemically polymerized films. A review. Biosensors ic Bioelectronics, 14 (5), 443 456. 

Fig. 1.7 SEM image ofthe surface of an electrochemically polymerized film of polypyrrole p-toluene sulfonate.

Another interesting conjugated polymer is polythiophene, which has good environment stability compared to polyacetylenes (Fig. 49.12) (Table 49.7). Polythiophene has a value of 3 x 10 at a wavelength of 1.06 /Am. The resonantly enhanced value is about 2 orders of magnitude larger than the nonresonant value. Table 49.7 shows values of a polythiophene derivatives. Polythiophenes are colored materials [186-193]. The behavior of polythiophene has been studied in several forms. Electrochemically polymerized films of polythiophene results a value of 4 X 10 esu as measured by DFWM at 602 nm [194]. A monolayer prepared by the LB technique (thickness 22 A) yields a value of 10 esu at 602 nm as measured with femtosecond DWFM technique [195]. 

P. N. Bartlett, D. H. Dawson, J. Farrington, Electrochemically polymerized films of 5-carboxyindole - preparation and properties, of the Chemical Society - Faraday Transactions 1992, 88, 2685. 

The plasma-polymerized (cf. Sect. 5.1.6) films (e.g. PT and PMT films) have a higher content of hydrogen than that in electrochemically polymerized films, and the electrical conductivities of the doped films are lower than those of electrochemically polymerized films [601]. The surface morphology of the PT films deposited away from the high radio frequency flux-density region is similar to that of films prepared by electrochemical methods [600]. Electropolymerized PT films have more compact morphology compared to that of chemically synthesized PT films [146]. 

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Conducting polymer composites have also been formed by co-electrodeposition of matrix polymer during electrochemical polymerization. Because both components of the composite are deposited simultaneously, a homogenous film is obtained. This technique has been utilized for both neutral thermoplastics such as poly(vinyl chloride) (159), as well as for a large variety of polyelectrolytes (64—68, 159—165). When the matrix polymer is a polyelectrolyte, it serves as the dopant species for the conducting polymer, so there is an intimate mixing of the polymer chains and the system can be appropriately termed a molecular composite. 

The electrochemical oxidation of monomers such as pyrrole,2-5 thiophene,6-9 aniline,10-13 etc., or their derivatives, initiates a polymerization process at the electrode/electrolyte interface that promotes the formation of a polymeric film that adheres to the electrode. A similar homogeneous polymerization process can be initiated by chemical oxidation or chemical polymerization.14-21 Some monomers can be polymerized as well by electrochemical or chemical reduction. 

Ideal electrochemical polymerization was considered to give ideal linear and conjugated polymeric chains. The real situation is that films electrogenerated from the basic monomers are insoluble and infusible. Only polyaniline films are partially soluble in some solvents. 

Not much effort has been made, except for the Tafel studies, to establish the empirical kinetics and models of interfacial reactions to obtain thick polymeric films (>100 nm) of industrial interest from different monomers. However, this is much more than the few kinetic studies performed until now to understand the mechanism of chemically initiated polymerization. Electrochemical models still have an advantage in obtaining priority in the industrial production of tailored materials. 

Figure 37. Lateral section of a polymeric film during the nucleation and growth of the conducting zones after a potential step. (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, A new model for electrochemical oxidation of polypyrrole under conformational relaxation control. /. Electroanal. Chem. 394, 211, 1995, Figs. 2-5. Copyright 1995. Reprinted with permission from Elsevier Science.)...

The action of a muscle is a consequence of electrochemically stimulated conformational relaxation processes that occur along every electroactive chain inside a polymeric film. A free-volume model dependent on the... 

The reproducibility of the electrodeposition of conducting polymer films has been a very difficult issue. It has long been realized that each laboratory produces a different material and that results from different laboratories are not directly comparable.82 We have experienced reproducibility problems with almost all of the electrochemically polymerized materials used in our work. 

Fawcett, and the structure of the mercury-ethanol interface, 59 Feldberg and Rubinstein, theory of electrochemical polymerization, 560 Film... 

---