Electrochemiluminescence Imaging of Conducting Polymer Film Structures

Abstract

Conducting polymers (CPs) is a class of attractive materials that, thanks to their unique properties, structures made on-demand and new composition mixtures, finds a wide variety of applications, ranging from material science to chemistry, from energy to environmental remediation and electroanalysis. Electrochemical synthesis of CPs is usually achieved by anodic oxidation, or electropolymerization, of suitable monomer species, which generally results in the formation of polymeric films deposited onto the surface of an electrode. Nevertheless, the use of this methodology makes rather difficult, or even impossible, to tune the morphology of the final CPs materials, limiting their applicability in material science. To overcome this limitation, electrosynthesis of CPs with different geometries such as brushes, films and fiber arrays has recently been achieved exploiting the unique wireless feature of bipolar electrochemistry1. In fact, this renewed technology enables electrochemical reactions at both extremities of a wireless electrode, or bipolar electrode (BPE), through the application of an external electric field generated in a low concentration of a supporting electrolyte.In this context, alternating current (AC)-bipolar electropolymerization of 3,4-ethylenedioxythiophene (EDOT) has recently been employed for the gradual in-plane growth of the corresponding poly(3,4-ethylenedioxythiophene) (PEDOT) films at the terminal of a gold BPE laid down on a glass surface2. This notable phenomenon could paved the way for the drawing of CPs on non-conductive substrates, opening up new perspectives for their potential application for circuit patterning in organic electronic devices. The possible mechanism of films formation seems to involve the generation of PEDOT fibers that propagate in an anisotropic morphology from the terminal of the BPE, in parallel to the external electric field under electrophoretic effect, resulting in colored film depositions visible at naked eye. However, very thin layers of PEDOT films underlying the main fibers are generally transparent, making their direct visualization rather difficult.In the present contribution, we propose the use of Electrogenerated Chemiluminescence (ECL) for the morphological imaging of PEDOT films synthetized by AC-bipolar electropolymerization. ECL is the emission of light at the electrode surface following an electrochemical stimulus and, nowadays, it is widely employed as an emerging microscopy technique3. The great success of ECL imaging technology is due to the electrochemical nature of light generation that allows very low background signal and the possibility to control the ECL emission both temporally and spatially by applying a suitable potential. Our approach employs the use of the well-known luminol/hydrogen peroxide ECL system for the visualization of PEDOT films grown on non-conductive substrates used as BPEs in bipolar systems. The different potential distribution on such PEDOT systems allows ECL emission only in confined areas of PEDOT films where the potential is sufficient to trigger the ECL reaction, allowing the visualization of different structural characteristics of the polymeric films (Figure 1). This technique would represent an innovative and promising tool to accurately control the morphology of the final polymer material, opening up new perspectives in the area of electrochemical synthesis of CPs. Figure 1: Schematic illustration of the bipolar setup used for ECL imaging of PEDOT films (a). Optical image of PEDOT film synthetized by AC-bipolar electropolymerization (b). Corresponding Luminol ECL images on PEDOT film working as BPE at 25 V (c) and 70 V (d).