Walter J. Doherty

Correlating molecular orientation distributions and electrochemical kinetics in subpopulations of an immobilized protein film.

Understanding the relationship between the structure and electrochemical activity of a protein film immobilized on an electrode surface is a prerequisite to the rational design of protein-based bioelectronic devices. In a monolayer of horse heart cytochrome c (cyt c) adsorbed to an indium-tin oxide (ITO) electrode, only about half of the film is electroactive, which makes it difficult to correlate the broad orientation distribution (measured spectroscopically on the entire film) with the electron transfer rate constant (measured electrochemically on the electroactive portion of the film) [Runge, A. F.; Mendes, S. B.; Saavedra, S. S. J. Phys. Chem. B 2006, 110, 6732−6739]. To address this problem, a novel form of electroreflectance spectroscopy, potential-modulated, attenuated total reflectance (PM-ATR), is used to monitor changes in the absorbance of a cyt c while a modulated potential is simultaneously applied to the ITO-coated, planar waveguide electrode. From measurements as a function of the light pol...

Broadband Spectroelectrochemical Attenuated Total Reflectance Instrument for Molecular Adlayer Studies

The study of thin film molecular architectures is an increasingly important endeavor with respect to the development and characterization of materials ranging from liquid crystalline displays to receptor-based biosensors. Here we describe an apparatus capable of simultaneously acquiring broadband spectroscopic and electrochemical information on molecularly thin films deposited on a transparent electrode surface through a multiple internal reflection geometry. To demonstrate the capabilities of this system, the spectroelectrochemical behavior of a single, neutral copper phthalocyanine bilayer was evaluated.

Voltammetric and waveguide spectroelectrochemical characterization of ultrathin poly(aniline)/poly(acrylic acid) films self-assembled on indium-tin oxide

We report on the spectroelectrochemical characterization of conducting polymer (CP) films, composed of alternating layers of poly(aniline) (PANI) and poly(acrylic acid) (PAA), deposited on ITO-coated, planar glass substrates using layer-by-layer self-assembly. Absorbance changes associated with voltammetrically induced redox changes in ultrathin films composed of only two bilayers (ITO/PANI/PAA/PANI/PAA) were monitored in real time using a unique multiple reflection, broadband attenuated total reflection (ATR) spectrometer. CP films in contact with pH 7 buffer undergo a single oxidation/reduction process, with ca. 12.5% of the aniline centers in the film being oxidized and reduced. The ATR spectra indicate that during an anodic sweep, the leucoemeraldine form of PANI in these films is oxidized to generate both the emeraldine and pernigraniline forms simultaneously. A comparison of the behavior observed during anodic and cathodic sweeps suggests that the rate of oxidation is limited by structural changes in the polymer film originating in electrostatic repulsion between positively charged PANI chains.

Molecular Ordering in Monolayers of an Alkyl-Substituted Perylene-Bisimide Dye by Attenuated Total Reflectance Ultraviolet–Visible Spectroscopy

Surface-relative orientational parameters were determined for monolayer films of N, N′-ditridecylperylenetetracarboxylic dianhydridediimide (C13-PTCDI) in terms of the relative electronic transition dipole strengths, providing a three-dimensional view of the absorption dipole distribution. In order to obtain a macroscopically ordered film, C13-PTCDI was deposited by (1) horizontal Langmuir–Blodgett (LB) transfer onto methyl- and phenyl-silanized glass, and (2) vapor deposition onto oriented films of poly(tetrafluoroethylene) (PTFE) on glass. Films of LB-deposited C13-PTCDI were found to be completely isotropic prior to annealing. After annealing, these films remained isotropic in the plane of the substrate while the out-of-plane anisotropy was significantly enhanced. In contrast, films of C13-PTCDI vapor deposited onto oriented poly(tetrafluoroethylene) (PTFE)-modified substrates yielded films with a high degree of both in- and out-of-plane anisotropy. Atomic force microscopy (AFM) images of both the LB- and vapor-deposited films show substantial differences in film morphology and long-range order. These results demonstrate that molecular orientation in C13-PTCDI films can be controlled by varying substrate surface chemistry and post-deposition processing.

Potential modulated attenuated total reflectance spectroscopy of prussian blue films on ITO

Potential modulated attenuated total reflectance (PM-ATR) spectroscopy has been employed to study charge transfer processes in Prussian blue (PB) films deposited on indium tin oxide (ITO) electrodes. PM-ATR is a planar waveguide-based spectroelectrochemical technique in which the optical response of an electroactive film is measured as a function of applied potential and modulation frequency. The multiple internal reflection geometry of PM-ATR provides a significant sensitivity advantage over the single external reflectance geometry that has been employed in most prior electroreflectance studies. The apparent electron transfer rate of PB on ITO obtained using PM-ATR was compared to that obtained with conventional cyclic voltammetry; the respective rates, 0.33 ± 0.15 S -1 (n = 3) and 0.71 ± 0.37 S -1 (n = 10), are in good agreement.