Conducting polymer growth in porous sol-gel thin films: Formation of nanoelectrode arrays and mediated electron transfer to sequestered macromolecules

We show the templated electrochemical growth of poly(3,4-ethylenedioxythiophene) (PEDOT) into porous sol-gel (PSG) films [50:50 tetraethyl orthosilicate/methyltriethoxysilane (TEOS/MTES)] on indium-tin oxide (ITO) substrates and PEDOT-mediated electron transfer to ferrocene-modified dendrimers encapsulated within these sol-gel matrices. We first describe conditions needed to optimize PEDOT electropolymerization within the porous sol-gel (PSG) films such that barely emergent PEDOT features, ca. 100 nm in diameter, protrude from a PSG thin film surface, functioning as an array of ultramicroelectrodes in diffusion controlled oxidation/reduction reactions of solution probe molecules. We next describe the incorporation of fourth-generation PAMAM dendrimers into these PSG films after (a) conjugation with ferrocenecarboxylic acid (Fc-COOH) to form Fc-PAMAM, (b) conjugation with both Fc-COOH and 3-thiopheneacetic acid (3TAA), or (e) conjugation with both Fc-COOH and 3,4-ethylenedioxythiophenemethanol (2,3-dihydrothieno[3,4-b]-1,4-dioxyn-2-yl or EDTM). Oxidation/reduction of the encapsulated Fc-PAMAM units could be voltammetrically detected after PEDOT growth into the sol-gel film, but with different efficiency, depending upon the composition of the encapsulated dendrimer. Background corrected cyclic voltammograms showed that ca. 0.9% of the ferrocene groups became electrochemically active when Fc-PAMAM alone was incorporated into the PSG film. Up to ca. 20% of these Fc-PAMAM units became electrochemically active, with high rates of electron transfer, when EDTM was conjugated to the Fc-PAMAM dendrimer and when the EDOT monomer was codoped into the sol-gel film. These results argue for the direct, facile electrochemical communication of encapsulated Fc,EDTM-PAMAM units to the ITO substrate via the electrochemically grown PEDOT "wires," especially when the conducting polymer and the Fc units are held in close proximity on the dendrimer. This is a new approach to control of the redox chemistry of macromolecules encapsulated in inert porous matrices.