Electrode modification

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these are (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 84] heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 eV [75] and therefore are used as anode materials, typically on top of ITO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74] to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

Modification of the top electrode may also be achieved. This was done by adding a small amount of surfactant, such as an ether phosphate or an ether sulfate, to the spin-coat solution of the luminescent polymer [89]. The lipophobic ether chains segregate at the surface of the (predominantly) hydrocarbon polymer, becoming available for complexation with the aluminum cathode which is deposited on top. Thus, the dipole in the surfactant points away from the electrode and lowers its work function to improve the injection of electrons. 

---

SCHEME 4 (a) Carbon fiber microelectrode, and (b) the process of electrode modification. (Reprinted from [158], with permission from Elsevier.)... 

Among the variety of materials used for electrode modification the electroactive organic and inorganic polymers seem to be the most prominant ones. In this chapter the electroactive polycrystals of transition metals, hexacyanoferrates, will be discussed for the development of chemical and biological sensors. 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

Considerable effort had to be invested, for example, before contamination levels during transfer of a sample from an electrochemical cell to a vacuum chamber could be adequately assessed and controlled 121. With this provision, electrode modifications by electrochemical processes can be studied in much greater detail than is possible in situ and many interesting results have been obtained by such experiments. Intrinsic limitations of this transfer method arise, however, with loosely bound... 

However, because of the mostly very slow electron transfer rate between the redox active protein and the anode, mediators have to be introduced to shuttle the electrons between the enzyme and the electrode effectively (indirect electrochemical procedure). As published in many papers, the direct electron transfer between the protein and an electrode can be accelerated by the application of promoters which are adsorbed at the electrode surface [27], However, this type of electrode modification, which is quite useful for analytical studies of the enzymes or for sensor applications is in most cases not stable and effective enough for long-term synthetic application. Therefore, soluble redox mediators such as ferrocene derivatives, quinoid compounds or other transition metal complexes are more appropriate for this purpose. 

However, the oxidation of NAD(P)H to NAD(P)+ at practical rates on most electrode materials proceeds only at high overpotentials [182] and often fouling of the electrode surface has been observed. This situation induces the search for suitable mediators or electrode modification processes to accelerate the highly irreversible oxidation of NAD(P)H. The stability of the reaction products in the presence of each other is also a necessary condition for usefulness. 

As in solution phase electrochemistry, selection of solvent and supporting electrolytes, electrode material, and method of electrode modification, electrochemical technique, parameters and data treatment, is required. In general, long-time voltam-metric experiments will be preferred because solid state electrochemical processes involve diffusion and surface reactions whose typical rates are lower than those involved in solution phase electrochemistry. 

Chemisorption requires direct contact between the chemisorbed molecule and the electrode surface as a result, the highest coverage achievable is usually a monomolecular layer. This may be contrasted with several of the methods to be discussed later that allow the electrode surface to be covered with thick films (i.e., multimolecular layers) of the desired molecule. In addition to this coverage limitation, chemisorption is rarely completely irreversible. In most cases, the chemisorbed molecules slowly leach into the contacting solution phase during electrochemical or other investigations of the chemisorbed layer. For these reasons, electrode modification via chemisorption was quickly supplanted by other methods, most notably polymer-coating methods. 

A large amount of research has been done on chemical modification of electrodes. The authoritative treatment of this subject can be found in Bard and Faulkner (2001). Because it is a very active area of electrochemistry, this subject is being periodically reviewed. From the sensing point of view, the motivation for electrode modification has been to introduce additional flexibility in the design of, and additional control over, the electrochemical processes taking place at the electrodes. We have seen one example of such a modification already (Section 7.3 Soukharev et al., 2004). 

Electrode modification by the attachment of various types of biocomponents holds considerable promise as a novel approach for electrochemical (potentiometric, conductometric, and amperometric) biosensors. Potentiometric sensors based on coupled biochemical processes have already demonstrated considerable analytical success [26,27]. More recently, amperometric biosensors have received increasing attention [27,28] partially as a result of advances made in the chemical modification of electrode surfaces. Systems based on... 

The reported semi-permeable membrane consists of an electropolymerized di-amino-benzene in phosphate buffer (pH 7). Polymerization is achieved by cycling the potential between 200 mV and + 800 mV for a certain period. In principle such an electrode modification hinders fouling in an excellent way [77]. 

---