Modification, characterization and electrochemical properties of redox active and biologically relevant thin films on electrode surfaces
Date
2017
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Abstract
The function of materials often relies heavily on properties that derive from the interface. There are often specific developments of surface modifications within certain fields of study to alter these properties, but because thin films span such a vast array of applications, there is need for a concentrated effort on surface functionalization development. To this, we have developed cross coupling methods to construct 10 unique thin films on electrode materials, 1 known thin film for the first time on a carbon paper support and 2 new thin films that relied on well-developed surface chemistry primarily for energy storing devices and sensing applications. ☐ In all 13 unique thin films were synthesized on carbon based and indium tin oxide (ITO) conducting surfaces. The 13 thin films can be segregated by the type of chemistry used to synthesize the thin film (chapter 2), the conductive support (chapter 3), and finally thin films for applied use (chapter 4 and 5). ☐ Carbon paper electrodes (CPE) were capped with alkynes based on the electrochemical reduction of aryl diazonium salts. Briefly, triisopropylsiyl protected ethynyl benzene diazonium (4-TIPS-ethynyl-ph-N2+) was electrodeposited on the substrate and subsequently treated with tetrabutylammonium fluoride (TBAF) to remove the TIPS group exposing a terminal alkyne monolayer. The grafted substrates were characterized by X-ray photoelectron spectroscopy (XPS) for the presence and absence of the silicon (from the TIPS group in 4-TIPS-ethynyl-ph-N2+) before and after deprotection. ☐ Sonogashira cross coupling and Glaser cross coupling chemistries were developed as a method to build molecular wires starting from the alkyne terminated CPE’s. Ferrocene appended reactants were used to determine the success of cross coupled products by observing the presence of iron by XPS and the ferrocene/ferrocenium redox couple by cyclic voltammetry. Glaser cross coupled electrodes varied from Sonogashira cross coupled electrodes by an additional alkynyl unit, which extends the π-conjugation within the film. We also performed copper catalyzed azide alkyne cycloaddition (CuAAC) for the construction of a third type of wire. This series of molecular wires on CPE’s are compared with respect to the impact on the ability to modulate charge transfer. We found the rate of electron transport is effected by molecular wire architecture with the surface Glaser cross coupled product producing remarkably fast kinetics. ☐ Next, three additional carbon electrode platforms (isomolded graphite (IG), glassy carbon (GC) and diamond (DE)) were modified using identical conditions developed for the surface Sonogashira cross coupling for CPE’s. The native carbon substrates were extensively studied to obtain a clear picture of the chemical environment of the surface before attaching the alkyne. Surface oxides were determined by XPS. The distribution of these oxides within the material were examined by XPS depth profiling and electron dispersive X-ray spectroscopy. These techniques show that GC and DE have the highest amount of surface oxides, but they are confined to the top several nanometers of the material. IG on the other hand contains oxides that penetrate deep within the bulk of the film. Carbon paper contains negligible detectable oxides. Once the surfaces were cross coupled via Sonogashira cross coupling chemistry with 4-iodophenylferrocene, the electrode dynamics were measured using the Laviron formalism. Comparisons were made between the electron kinetics through the wire and the parent electrode’s surface resistivity. The parent electrode’s resistivity’s were measured using a 4-point kelvin probe and their conductivities were measured using cyclic voltammetry in an electrolyte solution. It was found that the electron kinetics through the wire does not out compete with the native materials resistivity’s. ☐ We next exchanged the redox active halogen reactant (4-iodophenylferrocene) with a maleimide appended halogen (4-iodophenylmaleimide) for surface Sonogashira cross coupling. The maleimide functionalized carbon paper was characterized by XPS for the presence of nitrogen from the maleimide group located at 400.7 eV. There was significant iodination of the carbon paper surface after Sonogashira reaction with 4-iodophenylmaleimide with atomic percentages in excess of 1 %. This contrasts with the Sonogashira cross coupling with 4-iodophenylferrocene in which iodide on the surface was intermittently found, and always less than < 0.1%. We attempted to remove the iodide from the surface electrochemically, resulting in successful removal of the iodide but a compromised film. Hypothesizing that the iodide was the unwanted side product of the possible competing addition of a deprotonated maleimide backbone with the immobilized alkyne, (leaving the iodide apex on the wire) we replaced the maleimide with a methylated version. Sonogashira cross coupling with the methylated maleimide resulted in removal of the iodide contamination and preservation of the electronics of the molecular wire suggesting that the competing pathway was a source of the iodide fouling. We performed the Michael addition with the ferrocene appended cysteine on 4-iodophenylmaleimide and methylated 4-iodophenylmaleimde cross coupled product. We found that the methylated maleimide group slows the addition of the cysteine, but we were able to achieve comparable surface coverage of ferrocene at 24 hr reaction time at room temperature. ☐ We also performed a Michael addition with a thiol appended Ru (tris)bipyridine derivative. In the presence of a sacrificial reductant we observed a catalytic enhancement of current as well as photonic emission. Immobilization of the Ru (tris)bipyridine luminophore successfully produced an ECL device which can be used for sensing applications. ☐ Finally, ITO electrodes were terminated with COOH groups, passivating the electrode. EDC/NHS coupling was used to couple and immobilize amine terminated DNA. After DNA was conjugated to the surface, the electrode regained conductance. The immobilized DNA was characterized by XPS, electrochemical impedance spectroscopy (EIS) and time-of-flight secondary mass spectrometry (TOF-SIMS); the last technique demonstrated a covalent bond between the ITO and the appended DNA. EIS was utilized to show to the reversibility of protein binding. This was done by showing an increased resistance to charge transfer after ITO-dsDNA was incubated with target protein. A decrease in the resistance to charge transfer was observed after the incubated IT-dsDNA electrodes were further soaked in a concentrated solution of ds-DNA. This result is evidence of the reversible binding of the protein to and from the ITO-dsDNA.
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Keywords
Pure sciences, Biologically, Electrochemical, Electrode, Films, Redox, Thin