Quartz crystal microbalance electropolymerization

Endres et al. [82] have demonstrated the suitability of an air- and water-stable ionic liquid for the electropolymerization of benzene. This synthesis is normally restricted to media such as concentrated sulfuric acid, liquid SO2 or liquid HF as the solution must be completely anhydrous. The ionic liquid used, l-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, can be dried to below 3 ppm water, and this ionic liquid is also exceptionally stable, particularly in the anodic regime. Using this ionic liquid, poly(para-phenylene) was successfully deposited onto platinum as a coherent, electroactive film. Electrochemical quartz crystal microbalance techniques were also used to study the deposition and redox behavior of the polymer from this ionic liquid (Section 7.4.1) [83]. 

Through the combination of SPR with a - poten-tiostat, SPR can be measured in-situ during an electrochemical experiment (electrochemical surface plasmon resonace, ESPR). Respective setups are nowadays commercially available. Voltammetric methods, coupled to SPR, are advantageously utilized for investigations of - conducting polymers, thin film formation under influence of electric fields or potential variation, as well as - electropolymerization, or for development of -> biosensors and - modified electrodes. Further in-situ techniques, successfully used with SPR, include electrochemical - impedance measurements and -+ electrochemical quartz crystal microbalance. 

PPy films modified by platinum catalyst particles were also considered for electrocatalytic reactions (oxygen reduction and methanol oxidation) by Hepel et al. [41], The incorporation of a PtCl anion was performed during the electropolymerization of pyrrole and monitored by the electrochemical quartz crystal microbalance (EQCM) technique, allowing us to evaluate the amount of platinum obtained after reduction of the PPy/PtCl film. 

Fig.1 Electrochemical quartz crystal microbalance schematic of the three-electrode setup used for carrying out the electropolymerization/adsorption studies presented later in this chapter. This figure was reprinted with permission from [71]...

J. Petitjean, J. Tanguy, J.C. Lacroix, K.I. Chane-Ching, S. Aeiyach, M. Delamar, and P.C. Lacaze, Interpretation of the ultra-fast electropolymerization of pyrrole in aqueous media on zinc in a one-step process The specific role of the sahcylate salt investigated by X-ray photoelectron spectroscopy (XPS) and by electrochemical quartz crystal microbalance (EQCM), J. Electroanal. Chem., 581, 111-121 (2005). 

As discussed, the electropolymerized PEDOT-PSS (o- = 80 S cm , 1 s/t ratio = 0.68) has a completely different composition than the chemically polymerized PEDOT-PSS (Electrochemical quartz crystal microbalance (EQCM) analyses have shown that the composition of the electropolymerized PEDOT-PSS is not dependent on the anion concentration [135]. This indicates that the mechanism of synthesis of the polymer strongly influences its composition. During electropolymerization, the first formed (and doped) oligomers are very close to the metal electrode as charge transfer occurs at a tunnel distance. Consequently, those doped oligomers interact electrostatically with the closest sulfonate anions of a PSS chain at the vicinity of the electrode. This mechanism leads to a high concentration of PEDOT in the PEDOT-PSS film, independent on the anion concentration. 

Electrolytes and solvents. The electropolymerization reaction may be sensitive to the nucleophilic nature of the solvent and electrolyte. For this reason, many of the films are prepared in aprotic solvents, such as acetonitrile, which are poor nucleophiles. Electro-oxidative polymerization in the presence of small anions simultaneously incorporates the anions which render the polymer film conductive. Upon reduction, the anions are released from the film. Cycling the film through oxidation and reduction leads to insertion and release in the respective parts of the cycle. Simultaneous incorporation or removal of the solvent and/or cations may also occur, as shown by measurements on the quartz crystal microbalance [51-52]. Polymerization in the presence of large anions such as poly(vinylsulphonate) and poly(4-styrene sulphonate) (PSS") also incorporates the anion during growth [53-56]. Subsequent cycling, however, does not release the anions which are trapped because of their... 

Polymerization of pyrrole has also been carried out chemically by mixing the monomer with a homogeneous oxidant (Fe ) in solution. Mermillod et al [61] found that poly(pyrrole) synthesized in water by action of Fe ( 104)3 produces particles in solution as well as films on the reactor walls. The material was electrochemically identical with electropolymerized product. Gregory et al [62] chemically deposited very uniform films of poly(pyrrole) on textile fibres and woven glass cloth. Gottesfeld et al [63] found that chemically deposited films were uniform and could serve as a conducting substrate for metallization structures for microelectronic circuitry. Hillman et al [64] studied the electrodeposition of poly(vinylfer-rocene) with the quartz crystal microbalance. 

Chemical sensors are generally used in a liquid or a gas medium. In either case their interaction with the sample, and the generation of the primary signal, can take place at the surface (by adsorption), in the bulk (by absorption), or by both mechanisms. This seemingly trivial fact has some important implications with respect to the use of the selective layer in the sensor. For example, an electropolymerized layer can be used in a quartz crystal microbalance in a simple and predictable way with the response originating in the bulk. The use of the same material in a chemi-resistor may depend mostly on the modulation of the surface resistance by adsorption. 

Terthiophene (2.179, n = 2) and EDOT (2.181) derivatives, in which 18-crown-6 moieties were attached directly to the 3,4-position of the central thiophene ring, were prepared by Zotti s group (Chart 1.37) [275]. These oligomers showed maximum absorption at 365 and 366 nm, respectively, which were significantly blue shifted (AA, = 20-30 nm) after the addition of 0.1 MNa" " or K+ ions. The polymer films prepared by electropolymerization of the oligomers were little influenced by the addition of alkali metal ions. Furthermore, electrochemical quartz crystal microbalance analysis of the alkali metal coordination ability of the polymer films in acetonitrile solution revealed a lower degree of coordination, which was attributed to the loss of degrees of freedom in the crown-ether moiety. 

Figure 2. Frequency(F) and admittance (Y) changes observed under double potential steps (+0.98 Vand 4), 3 V). a) no insertion in thin polymer layer, b) large insertion in moderately thick polymer, c) no insertion measured. (Reprintedfrom J. Electroanal Chem., 476, Lagjrost, Tanguy, Aeiyach, Lacroix, Jouini, Chane-Ching, and Lacaze, Polymer chain encapsulation followed by a quartz crystal microbalance during electropolymerization of bithiqphene-J3-cyclodextrin hot-guest compounds in aqueous solution, pp. 1-14, Copyright 1999 with permission from Elsevier Science).

Schweiss R, Liibben JF, Johannsmarm D, Knoll W (2005) Electropolymerization of ethylene dioxythiophene (EDOT) in micellar aqueous solutions studied by electrochemical quartz crystal microbalance and surface plasmon resonance. Electrochim Acta 50(14) 2849-2856... 

Baute N, Martinot L, Jerome R (1999) Investigation of the cathodic electropolymerization of aCTylethyl acrylate and methyl methacrylate by coupled quartz crystal microbalance analysis and cyclic voltammetry. J Electroanal Chtan 472 83-90... 

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