Redox polyelectrolyte-modified electrodes

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]. 

Molecular Description of Redox Polyelectrolyte-Modified Electrodes 89... 

While the structure of nonredox polymer and polyelectrolytes thin layers has received much attention in the past [116, 117], only recently has a molecular theory able to treat, from a molecular point of view, redox polyelectrolytes adsorbed on electrodes, been presented [118-120]. The formulation of the theory, its scope, advantages and limitations will be discussed in detail in Section 2.5.2, and therefore we will limit ourselves to show here some predictions that are relevant for the understanding of the structure of polyelectrolyte-modified electrodes. The theory was applied to study the particular system depicted in Figure 2.5, which consists of a single layer of PAH-Os adsorbed on a gold surface thiolated with negatively charged mercapto... 

Thin-film ideal or Nemstian behavior is the starting point to explain the voltammetric behavior of polyelectrolyte-modified electrodes. This condition is fulfilled when (i) the timescale of the experiment is slower than the characteristic timescale for charge transport (fjD pp, with Ithe film thickness) in the film, that is all redox within the film are in electrochemical equibbrium at any time, (ii) the activity of redox sites is equal to their concentration and (iii) all couples have the same redox potential. For these conditions, anodic and cathodic current-potential waves are mirror images (zero peak splitting) and current is proportional to the scan rate [121]. Under this regime, there exists an analytical expression for the current-potential curve ... 

When the characteristic time for charge diffusion is lower than the experiment timescale, not all the redox sites in the film can be oxidized/reduced. From experiments performed under these conditions, an apparent diffusion coefficient for charge propagation, 13app> can be obtained. In early work choroamperometry and chronocoulometry were used to measure D pp for both electrostatically [131,225] and covalently bound ]132,133] redox couples. Laviron showed that similar information can be obtained from cyclic voltammetry experiments by recording the peak potential and current as a function of the potential scan rate [134, 135]. Electrochemical impedance spectroscopy (EIS) has also been employed to probe charge transport in polymer and polyelectrolyte-modified electrodes [71, 73,131,136-138]. The methods... 

We will discuss here applications of polyelectrolyte-modified electrodes, with particular emphasis on layer-by-layer self-assembled redox polyelectrolyte multilayers. The method offers a series of advantages over traditional technologies to construct integrated electrochemical devices with technological applications in biosensors, electrochromic, electrocatalysis, corrosion prevention, nanofiltration, fuel-cell membranes, and so on. 

In both cases the top layer of these layered polyelectrolyte films contains many ion sites that can bind redox ions by ion exchange vdth the electrolyte solution. Homo polypeptides such as poly(L-lysine) and poly(L-glutamic add) have been employed to form layered polyelectrolyte films with Fe(CN)6 " electrostatically adsorbed onto ammonium sites in poly(lysine) [45]. Modified electrodes with polyelectrolytes mono-layers have also been deposited using the Langmuir-Blodgett technique [46-48]. 

During continuous redox cycling, the first cycle usually differs from the following ones. This effect is referred as break-in. In poly(vinylferrocene), PVF, films this has been related to the incorporation of solvent and ions into the film, decreasing its resistivity [132]. This effect has been observed for several polyelectrolyte and polymer-modified electrodes, for example, polyaniline [155]. 

Electrode surfaces can be modified by redox polyelectrolytes via a sol-gel process, yielding random redox hydrogels or by layer-by-layer self-assembly of different redox and nonredox polyelectrolytes by alternate electrostatic adsorption from solutions containing the polyelectrolytes to produce highly organized redox-active ultrathin multilayers. 

The LbL strategy to chemically modify electrodes with redox polyelectrolyte films has become an important tool for the fabrication of devices and electrodes with important future applications in biosensors, electrochromic devices, electrocatalysts, corrosion-resistant coatings, and so on. 

Tagliazucchi M, Grumelii D, Calvo EJ (2006) Nanostructured modified electrodes role of ions and solvent flux in redox active polyelectrolyte multilayer films. Phys Chem Chem Phys 8 5086-5095... 

Grumelii DE, Garay F, Barbero CA, Calvo EJ (2006) Dynamics of ion exchange between self-assembled redox polyelectrolyte multilayer modified electrode and liquid electrolyte. J Phys Chem B 110 15345-15352... 

Tagliazucchi M, Calvo EJ, Szleifer I (2008) Molecular theory of chemically modified electrodes by redox polyelectrolytes under equilibrium conditions comparison with experiment. J Phys Chem C 112 458 71... 

---