Electrochemiluminescence of Langmuir-Blodgett films of Nafion
|Thiago Kohls 1, Ligia M. Moretto 1, Paolo Ugo 1, Denis Badocco 2, Paolo Pastore 2, Neso Sojic 3|
1. Dipartimento di Chimica Fisica, Università Cà Focari di Venezia, S. Marta 2137, Venezia 30123, Italy
The success of Nafion modified electrodes in electroanalytical applications can be ascribed to their unique ion-exchange selectivity and pre-concentration capabilities, good wetting properties, chemical and biological inertness and mechanical robustness. The deposition of Nafion coatings on electrodes surfaces can be easily performed by recasting polymer solutions by microvolume evaporation or spin-coating. In some cases, however, such procedure brings to unsatisfactory reproducibility in the behaviour of the modified electrode. Significant improvements in the reliable use of ionomer-coated electrodes could indeed come from the development of deposition procedures able to achieve a better control of the deposition at a molecular level. Operating in this direction, recently, we developed original methods for preparing ultrathin films of Nafion and other ionomers by using Langmuir-Blodgett (LB) technique [1, 2]. In the present work, we exploited the potentialities of electrochemiluminescence (ECL) to study mechanism and behaviour of an analyte incorporate in a modified electrode where the film is very thin and compact. These characteristics can change dramatically properties such as the diffusion coefficient and the mechanism of charge transfer. Nafion LB coatings were deposited on transparent ITO (indium-tin oxide) electrodes and loaded by ion-exchange with Ru(bpy)32+, used as electroactive luminescent probe. Such modified electrodes are characterized electrochemically and the luminescence of electrode surface is registered by a photomultiplier tube while changing the applied electrochemical potential. The onset of luminescence occurred near 0.9–1.0V, which was consistent with the oxidation of the immobilized Ru(bpy)32+, and then the ECL intensity arose steeply. More information on the loading of the redox probe is obtained by ECL measurements; Ru(bpy)32+ emits a strong luminescence when reacts with TPrA . The experimental evidences that have to be kept in mind to justify the ECL origin in the presence of the Nafion membrane are the following: 1. both oxalate and TPrA are not electro-oxidized (both at neutral and alkaline pH); 2. TPrA is present in the film in its protonated form and is able to displace the ruthenium complex from it; 3. when the complex is in the film, the oxalate/ Ru(bpy)33+ system does not produce ECL (oxalate cannot be in the film because negatively charged); 4. when the complex is in the film together with the protonated TPrA, the system does not produce ECL. The experimental evidences suggest that ECL process occurs only when Ru(bpy)32+ is in the film and TPrA is outside. Emission should occurs at the interface and in the most external layer of the polymeric film.
The figure shows the ECL emission area vs. the TPrA concentration obtained from CV experiments (see the inset). The results obtained indicate that linearity is present only at low TPrA concentrations. Signal bends and reaches a plateau because the Ruthenium complex becomes the limiting species. Due to its good affinity for Nafion, Ru(bpy)32+ remains strongly attached to the film and can be electrochemically recycled many times. This aspect is very attractive for application to sensors development.
Presentation: Poster at SMCBS'2007 International Workshop, by Thiago Kohls
See On-line Journal of SMCBS'2007 International Workshop
Submitted: 2007-08-30 17:58 Revised: 2009-06-07 00:44
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