Electropolymerization features

Interestingly, such a study very recently led to the serendipitous discovery that, when operating in the presence of very low levels of nucleophiles, may be oxidatively electropolymerized, thus forming stable conducting polymeric films able to transport both holes and electrons, a feature that is not possessed by many organic systems.27... 

For consistent and reproducible operation it is important to control chemosensor features like thickness of the MIP coatings. Generally, MIPs were prepared and immobilized on PZ resonators by using two distinct procedures, namely in situ assembling on the resonator surface [103] or physical entrapment of the preprepared MIP particles in an inert polymer matrix attached to this surface [21]. Typically, thickness of the polymer is 20 nm to 5 pm. In situ immobilization of MIPs (see below in Sects. 3.2.1-3.2.4) can be accomplished by surface grafting, sandwich casting, electropolymerization, physical entrapment or chemical coupling [102],... 

A molecularly imprinted polypyrrole film coating a quartz resonator of a QCM transducer was used for determination of sodium dodecyl sulphate (SDS) [147], Preparation of this film involved galvanostatic polymerization of pyrrole, in the presence of SDS, on the platinum-film-sputtered electrode of a quartz resonator. Typically, a 1-mA current was passed for 1 min through the solution, which was 0.1 mM in pyrrole, 1 mM in SDS and 0.1 M in the TRIS buffer (pH = 9.0). A carbon rod and the Pt-film electrode was used as the cathode and anode, respectively. The SDS template was then removed by rinsing the MlP-film coated Pt electrode with water. The chemosensor response was measured in a differential flow mode, at a flow rate of 1.2 mL min-1, with the TRIS buffer (pH = 9.0) as the reference solution. This response was affected by electropolymerization parameters, such as solution pH, electropolymerization time and monomer concentration. Apparently, electropolymerization of pyrrole at pH = 9.0 resulted in an MIP film featuring high sensitivity of 283.78 Hz per log(conc.) and a very wide linear concentration range of 10 pM to 0.1 mM SDS. 

In oxidative electropolymerization, monomers such as pyrrole, thiophene, alkylth-iophenes or aniline are dissolved in an appropriate solvent containing an electrolyte that can act as a source for the anions needed to neutralize the cations formed during the oxidation process [33,34]. The nature of this electrolyte is of great importance to the structural features obtained for the electropolymerized layer since the dopants become an intrinsic part of the polymer layer structure. A general outline of a mechanism describing the electropolymerization process is shown in Figure 4.21. 

The size of the working electrode is also important. This will influence the electropolymerization process in that the conductivity decrease during deposition can be minimized also, depletion effects are more pronounced with large electrodes. With smaller (< 20 pm diameter) electrodes, electropolymerization can be carried out in low-conductivity media also, the rate of transport to and from the electroactive center is markedly enhanced. In some cases, this latter feature can be a problem in that the enhanced rate of transport away from the microelectrode causes increased difficulty in obtaining a polymer deposit. 

Electropolymerization of phenols proceeds similarly to that for pyrrole poly(phenol) itself is probably a mixture of para- and meta-linked units. Films are generally continuous and free from such defects as pinholes. They have been used for corrosion protection, as permselective films, " and as pH sensors. Like poly(pyrrole) films, electropolymerized phenol films have a number of features that make them attractive for immobilizing an enzyme. First these films can be grown under electrochemical control from aqueous buffered solution at neutral pH. Second a wide variety of phenol derivatives are available that allow some control over the films physical characteristics. Third these films are permselective, which could be useful in preventing interfering species from reaching the... 

A polyrotaxane sensitive to transition metal ion 98 was prepared by electropolymerization [205]. The precursor was obtained by complexation of a bipyridine sandwiched between two 2,2 -biEDOT moieties with Cu or Zn in the presence of a Sauvage s phenanthroUne macrocycle incorporating a polyether chain. A reversible ionochromic process was observed. On removal of metal ions by ethylenediamine treatment, the color of the polymer film turns from blue-green to red while the reverse effect was observed upon reintroduction of the metal. Another salient feature is the fact that the matching of the polyrotaxane and couple redox potentials results in a contribution to conductivity. 

FIGURE 18.1 Classical feature for cyclic voltammetries (CVs) recorded upon electropolymerization process. The arrows indicate the current variation on each successive scan. 

In a more recent work, Paik et al. described transport properties and stability of sulphonated porphyrin derivatives (3) in films of conducting polymers PPy and PEDOT films using EQCM . As expected, it was evidenced that such anions were not expelled from these polymer films by extensive potential cycles. The authors also analyzed the effects of potential cycling on PEDOT-H2-3 films obtained by two different modes of electropolymerization (i) at a constant current and (ii) with repeated cyclic potential sweeps. Figure 8.2 shows the results of potential cycles in 0.1 M NaCl solution for the two types of films (noted A and B, respectively). In both cases, the general feature of mass increases of the negative... 

To prepare polymer film electrodes there are different techniques like dip coating, spin coating, electropolymerization from monomers, and molecularly imprinted polymers (MIPs) developed very recently. They are often used for both potentiometric and amperometric ion sensors. Some feature of different types of ion sensors based on polymer CMEs are shown in Table 1. Conducting polymer-based CMEs... 

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