Polymer film coated electrode

A number of different methods can be used to prepare polymer film-coated electrodes. The simplest is to dip the surface to be coated into a solution of the polymer, remove the electrode from the solution, and allow the solvent to evaporate. While this method is simple, it is difficult to control the amount of material that ends up on the electrode surface. Alternatively, a measured volume of solution can be applied to the surface to be coated. This allows for accurate control of the amount of polymer applied. The polymer film may also be spin-coated onto the electrode surface. Spin-coating is used extensively in the semiconductor industry and yields very uniform film thicknesses. 

Assume that a disk-shaped electrode (gold, platinum, carbon, etc.) has been coated with a Film of poly (vinyl ferrocene) (Table 13.2). This can be accomplished by dissolving the polymer in chloroform, applying a drop of the solution to the electrode surface, and allowing the solvent to evaporate. The electrochemistry of the resulting polymer film-coated electrode can be investigated using the same electrochemical cell and equipment as described in the previous example. 

The key feature of this polymer film-coated electrode is that it will always be thicker than the monolayer Film considered in the previous example. Indeed, the film thickness can be controlled at will by varying the volume of solution applied and/or the concentration of polymer in the solution. The Film thickness could be anywhere from tens of angstroms to hundreds of microns, or even thicker. This multilayer Film situation is illustrated schematically in Figure 13.3A. Note that like the previous case, there are Fc groups sitting essentially... 

So, we have two possibilities for the case of the ion-exchange polymer film-coated electrode reduction could occur by physical diffusion of the Fe3+ through the film, or reduction could occur via electron hopping through the film. How can we know which process is operative Electrochemists have devoted a considerable amount of research effort to answering this question. The answer clearly depends on the nature of the polymer, the extent of swelling of the... 

Raman spectroscopy can offer vibrational information that is complementary to that obtained by IR. Furthermore, since the Raman spectrum reveals the backbone structure of a molecular entity [55], it is particularly useful in the examination of polymer film-coated electrodes. There are also some distinct advantages over in situ IR. For example, both the mid and far infrared spectral regions can be accessed with the same instrumental setup (in IR spectroscopy, these two regions typically require separate optics) [55]. Second, solvents such as water and acetonitrile are weak Raman scatterers thus the solvent medium does not optically obscure the electrode surface as it does in an in situ IR experiment. 

Magnetic electrode has been made of polycrystalline Ni plate. Copper has been used as non-magnetic metal and second electrode. Polymer film has been made by means of the spin-coating technique. The special semiconductor... 

Fig. 2 Conductance versus frequency as a function of pH for the polypyrrole-coated electrode. Polymer film thickness = 72 nm. The pH of the solution was adjusted by the addition of HCl to 10 M KCl in deionised water.

Fig. 5 Conductance at 5 Hz versus pH for polypyrrole-coated electrode. Polymer film thickness = 72 nm. Points are experimental (from Figure 2). The inset gives the fitting function used to obtain the full line.

An SECM feedback response to an electroactive polymer film can be controlled either by ET kinetics at the film-solution interface or by film conductivity. The contribution of lateral film conductivity to the effective ET rate measured by SECM was addressed in the recent study of polyaryl multilayers attached with ferrocenes [65] or ferrocene-terminated dendrimers [69] on unbiased carbon electrodes. In the latter study, the dependence of apparent ET rate constant, feei. on the generation of dendrimers (Table 6.2) was ascribed to the different efficiencies of electron transport inside and between dendrimers (Figure 6.22). Interestingly, the theory of the aforementioned triple potential step method predicts its potential to separately determine heterogeneous ET rate and lateral conductivity for a thin polymer film coated on an insulating surface [70]. 

Polymer film coated electrodes, mainly platinum or carbon electrodes, have also been prepared for potentiometric measurements. The first such electrodes have shown a response to pH variations, their selective activity being explained by the ability of protons, by virtue of their small size, to move through the polymer matrix to the platinum or carbon surface . ... 

In a different line of research, we have proposed electrochemical manipulation of a single cell using either a bare or a conducting polymer film-coated microelectrode. In a previous report, we have shown with a three electrode system that erythrocytes burst on the surface of electrodes at an applied potential much lower than ca. 1.5 V vs. Ag/AgCl . The cause of erythrocyte breakdown remains unsolved. Electrical as well as physicochemical effects, caused by potential application, may induce erythrocyte lysis due to their susceptibility to breakdown by changes in pH, osmotic pressure, and so on. 

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. 

The changes in the optical absorption spectra of conducting polymers can be monitored using optoelectrochemical techniques. The optical spectmm of a thin polymer film, mounted on a transparent electrode, such as indium tin oxide (ITO) coated glass, is recorded. The cell is fitted with a counter and reference electrode so that the potential at the polymer-coated electrode can be controlled electrochemically. The absorption spectmm is recorded as a function of electrode potential, and the evolution of the polymer s band stmcture can be observed as it changes from insulating to conducting (11). 

By 1988, a number of devices such as a MOSFET transistor had been developed by the use of poly(acetylene) (Burroughes et al. 1988), but further advances in the following decade led to field-effect transistors and, most notably, to the exploitation of electroluminescence in polymer devices, mentioned in Friend s 1994 survey but much more fully described in a later, particularly clear paper (Friend et al. 1999). The polymeric light-emitting diodes (LEDs) described here consist in essence of a polymer film between two electrodes, one of them transparent, with careful control of the interfaces between polymer and electrodes (which are coated with appropriate films). PPV is the polymer of choice. 

It is now 20 years since the first report on the electrochemistry of an electrode coated with a conducting polymer film.1 The thousands of subsequent papers have revealed a complex mosaic of behaviors arising from the multiple redox potentials and the large changes in conductivity and ion-exchange properties that accompany their electrochemistry. 

Figure 16. General transmission-line model for a conducting polymer-coated electrode. CF is the faradaic pseudo-capacitance of the polymer film, while Rt and Rt are its electronic and ionic resistance, respectively. R, is the uncompensated solution resistance.

Theoretical aspects of mediation and electrocatalysis by polymer-coated electrodes have most recently been reviewed by Lyons.12 In order for electrochemistry of the solution species (substrate) to occur, it must either diffuse through the polymer film to the underlying electrode, or there must be some mechanism for electron transport across the film (Fig. 20). Depending on the relative rates of these processes, the mediated reaction can occur at the polymer/electrode interface (a), at the poly-mer/solution interface (b), or in a zone within the polymer film (c). The equations governing the reaction depend on its location,12 which is therefore an important issue. Studies of mediation also provide information on the rate and mechanism of electron transport in the film, and on its permeability. 

Rotating-disk voltammetry is the most appropriate and most commonly employed method for studying mediation. In most systems that have been studied, there has been little penetration of the substrate in solution into the polymer film. This can be demonstrated most easily if the polymer film is nonconductive at the formal potential of the substrate. Then the absence of a redox wave close to this potential for an electrode coated with a very thin film provides excellent evidence that the substrate does not penetrate the film significantly.143 For cases where the film is conductive at the formal potential of the substrate, more subtle argu-... 

The rqjroducibility of polymer film formation is greatly improved by the spin coating technique where the polymer solution is applied by a microsyringe onto the center of a rapidly rotated disk electrode Rather thick films can be produced by repeated application of small volumes of stock solution. A thorough discussion and detailed experimental description of a reliable spin coating procedure was given recently... 

The alcohol tolerance of O2 reduction by bilirubin oxidase means that membraneless designs should be possible provided that the enzymes and mediators (if required) are immoblized at the electrodes. Minteer and co-workers have made use of NAD -dependent alcohol dehydrogenase enzymes trapped within a tetraaUcylammonium ion-exchanged Nafion film incorporating NAD+/NADH for oxidation of methanol or ethanol [Akers et al., 2005 Topcagic and Minteer, 2006]. The polymer is coated onto an electrode modified with polymethylene green, which acts as an electrocatalyst... 

The above mechanistic aspect of electron transport in electroactive polymer films has been an active and chemically rich research topic (13-18) in polymer coated electrodes. We have called (19) the process "redox conduction", since it is a non-ohmic form of electrical conductivity that is intrinsically different from that in metals or semiconductors. Some of the special characteristics of redox conductivity are non-linear current-voltage relations and a narrow band of conductivity centered around electrode potentials that yield the necessary mixture of oxidized and reduced states of the redox sites in the polymer (mixed valent form). Electron hopping in redox conductivity is obviously also peculiar to polymers whose sites comprise spatially localized electronic states. 

Polymers films formed from tr i sbipvr idineruthenium complexes and coated on electrode surfaces have been found to have interesting electrochromic and conductivity properties. 

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