Derivatized electrode

The historical development of chemically electrodes is briefly outlined. Following recent trends, the manufacturing of modified electrodes is reviewed with emphasis on the more recent methods of electrochemical polymerization and on new ion exchanging materials. Surface derivatized electrodes are not treated in detail. The catalysis of electrochemical reactions is treated from the view of theory and of practical application. Promising experimental results are given in detail. Finally, recent advances of chemically modified electrodes in sensor techniques and in the construction of molecular electronics are given. 

Electrooxidation of the pyrrole unit results in the formation of a pyrrole polymer that coats the electrode surface as it is formed. The amount of polymer deposited can be controlled by the number of CV cycles into the pyrrole oxidation wave. With 30, thick polymer layers give broad CV waves in the quinone voltage region, but thinner layers produce a well-resolved wave for the quinone 0/—1 reduction, which is reasonably stable when the electrodes are placed into fresh electrolyte solution with no 30. As in solution, addition of different urea derivatives causes this wave to shift positive. The relative magnitude of the shifts mirror that seen in solution. Furthermore, the 2 moves back to the original potential when the derivatized electrode is put back into a blank solution containing no urea. 

Values of ket were determined as previously reported for H20 or EtOH solvent (16). The derivatized electrode is first characterized by cyclic voltammetry in solvent/electrolyte solution without added I". The value of ket is then determined from the time dependence of the surface-concentration of (FeCp2+)surf. in the presence of variable I" concentration and as a function of solvent. The (FeCp2+)Surf. is generated in a linear potential sweep from -0.6 to +0.5 V vs. SCE while the... 

Two points of interest regarding derivatized electrodes emerge from the data for mediated I- oxidation. First, it is noteworthy that ferricenium in H20 will not oxidize I- the... 

Dendrimers Containing Fe4 Clusters at the Periphery Recognize ATP2 Better than the Model H2PO and also Form Derivatized Electrodes... 

Gold nanopartide-alkanethiolate-ferrocenyl Stars and Dendrimers are Excellent Oxo-Anion Sensors that can Provide Derivatized Electrodes... 

For all the derivatized electrodes the photocurrent onset is at a wavelength greater than 600 nm, where the photon energy is 2.0 eV in contrast to the 3.1 eV band gap of the semiconductor. For untreated Ti02 electrodes the photocurrent action spectrum follows the absorption edge of the semiconductor, with little response in the visible. The onset potential in all cases is the same as with the untreated electrode (Figure 6). 

We briefly mention here the use of the ferrocene/ferrocenium redox couple to mediate electron transfer on the oxidation (anodic) side, especially in derivatized electrode. This broad area has been reviewed [349]. For instance, polymers and dendrimers containing ferrocene units have been used to derivatize electrodes and mediate electron transfer between a substrate and the anode. Recently, ferrocene dendrimers up to a theoretical number of 243 ferrocene units were synthesized, reversibly oxidized, and shown to make stable derivatized electrodes. Thus, these polyferrocene dendrimers behave as molecular batteries (Scheme 42). These modified electrodes are characterized by the identical potential for the anodic and cathodic peak in cyclic voltammetry and by a linear relationship between the sweep rate and the intensity [134, 135]. Electrodes modified with ferrocene dendrimers were shown to be efficient mediators [357-359]. For the sake of convenience, the redox process of a smaller ferrocene dendrimer is represented below. 

To assess the suitability of the nanocrystals as optically active centers for their incorporation into optoelectronic devices, a monolayer of particles was deposited onto mercaptopropionic acid derivatized ITO substrates. Their photoelectrochemical response was assessed under conditions of illumination using LED whose peak intensity (A.pk = 470 nm) is greater than the calculated bandgap. As can be seen from the inset of Fig. 2, upon illumination of the SnS-derivatized electrode the current is observed to quickly increase and remain relatively constant during the illumination time, here 20 s, and upon switching off of the LED the current returns to its preillumination value. This photocurrent response profile is reproducible over many cycles (in a number of trials for periods in excess of an hour). An average photocurrent (current under illumination minus background current) for a number of similarly prepared electrodes has yielded values of between 6 and 8 nA cm . ... 

An example is shown in Fig. 6.31 for a polymer with a redox couple M /M that is attached to the silicon surface through chemical bonding. On illumination is converted to M. The resulting oxidized surface is then capable of oxidizing a second species B to B. The redox species B is not efficient in capturing holes directly from silicon due to its distance from the valence band. When the Si surface is covered with a polymer, the location of electron transfer is within the polymeric film rather than at the solid interface. " The photocurrent response of derivatized electrode is similar to that of bare silicon surface but the stability is significantly improved. ... 

FIGURE 6.31. Contrast of situations for photooxidation of B to B at a naked semiconductor and a deriva-tized electrode. The mediation system M /M is the Pe(Cp)2/Fe(Cp)2 couple attached via hydrolytic reaction of Si-Cl bonds with surface OH groups. At the naked semiconductor, B must capture the photogenerated h which comes to Evb. while at the derivatized electrode M oxidizes B and the photogenerated h" need only be transferred to M to generate the M. Ideally, the M /M system would be more negative than Eg in order to suppress decomposition of the semiconductor Eg is the decomposition potential of semiconductor. (Reprinted with permission from Wrighton et al " 1978, American Vacuum Society.)... 

The electrochemical behavior of these three derivatized electrodes, and the information this provides about the nature of their oxide films are discussed. 

In general, however, the two derivatized electrodes are quite similar. Eq values are comparable, coverage is persistent, and the redox process is reversible (equal anodic and cathodic integrated charge) in both cases (6). 

Iteratively fitting all the voltammetry and chronoamperometry data for SPS-derivatized electrodes yields a set of values for i°ps, aSps, hnc and q that simultaneously provide a good description of all the experimental results, for example, Figures 2.18, 2.19 and 2.21. 

With the parameters, iPEGj oCpeg, isps, asps. fcmc(q) and q(q) derived from experiments using either freshly abraded or S PS-derivatized electrodes in S PS-free PEG-C1 electrolyte, the hysteretic q-i data for the full SPS-PEG-C1 electrolyte were used to fit... 

Lewis N. S. and Wrighton M. S. (1981), Electrochemical rednction of horse heart ferricytochrome c at chemically derivatized electrodes . Science 211, 944-947. 

Figure 17.3.7 XPS responses for derivatized glassy carbon electrodes, (a) Curves A, following treatment with y-aminopropyltriethoxysilane. Curves B, unreacted surfaces, (b) Nitrogen Is spectra for surfaces treated with DNPH A, derivatized electrode cycled between 0 and — 1.2 V vs. SCE ...

F. Nuesch et al., Derivatized Electrodes in the Constmction of Organic Light Emitting Diodes, Adv. Mater. 1997, 9, 222-225. 

Figure 2. Top. Schematic of a p3MeT-based solid-state microelectrochemical device. Center. Cyclic voltammetry at the p3MeT derivatized electrodes. At left, the device is characterized in the solution electrolyte CH3CN/O.I M L1CF3S03 before the application of MEEP. At right, the same device is characterized under MEEP/LiCF3S03 (5 1). Bottom. Comparison of the steady-state vs. Vq of the p3MeT device in fluid solution electrolyte and under MEEP/LiCF3S03. Electrodes 3 and 4 are source and drain respectively (see Figure 1).

---