S. Scott Saavedra

Adsorptive immobilization of cytochrome c on indium/tin oxide (ITO): electrochemical evidence for electron transfer-induced conformational changes

Abstract The adsorptive immobilization and electrochemistry of horse and yeast cytochrome c on indium/tin oxide (ITO) electrodes is reported. Near-monolayer coverage was achieved in pH 7 phosphate buffers of ionic strength equal to 10 and 50 mM, respectively, for the horse and yeast species. The layers exhibit very well-behaved voltammetry and are stable on the timescale of hours to days. Cyclic voltammetry revealed quasireversible behavior that is a product of both electron transfer (ET) kinetics and ET-induced conformational changes. A square scheme mechanism linking the redox states and the conformational states is proposed. Using a simple ET kinetic model that adequately describes the voltammetry at higher scan rates, a standard ET rate constant of 18 s −1 was determined for adsorbed horse cytochrome c . With decreasing scan rate, we observed a limiting peak separation of approximately 10 mV, an example of unusual quasireversibility (UQR) that we attribute to the effect of conformational changes. Finally we note that the intrinsic cytochrome c ET rate on ITO is some 6 orders of magnitude less than for gold.

Synthesis and Colloidal Polymerization of Ferromagnetic Au−Co Nanoparticles into Au−Co3O4 Nanowires

The preparation of cobalt oxide nanowires with gold nanoparticle (AuNP) inclusions (Au-Co(3)O(4) nanowires) via colloidal polymerization of dipolar core-shell NPs is reported. Polystyrene-coated ferromagnetic NPs composed of a dipolar metallic cobalt shell and a gold NP core (PS-AuCoNPs) were synthesized by thermolysis of octacarbonyldicobalt [Co(2)(CO)(8)] in the presence of AuNP seeds and polymeric ligands. The colloidal polymerization process of these dipolar PS-AuCoNPs comprises dipolar nanoparticle assembly and solution oxidation of preorganized NPs to form interconnected cobalt oxide nanowires via the nanoscale Kirkendall effect, with AuNP inclusions in every repeating unit of the one-dimensional mesostructure. Calcination of the polymer-coated nanowires afforded polycrystalline Au-Co(3)O(4) nanowires that were determined to be electroactive. Nanocomposite materials were characterized by transmission electron microscopy, field-emission scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, and cyclic voltammetry. We demonstrate that the optical and electrochemical properties of Au-Co(3)O(4) nanowires are significantly enhanced in comparison with hollow Co(3)O(4) nanowires prepared via colloidal polymerization.

Evanescent sensing in doped sol-gel glass films

Abstract Bromophenol blue, a pH indicator, was entrapped in porous glass films prepared by the sol-gel method. The colorimetric response of the films to pH was assessed visibly and by attenuated total reflectance spectrometry. The entrapped indicator was nonleachable and responded to solution pH changes in a reversible manner. The immobilization of indicator molecules in porous sol-gel films appears to be a particulary promising approach to optical waveguide sensor development.

The Electroactive Integrated Optical Waveguide: Ultrasensitive Spectroelectrochemistry of Submonolayer Adsorbates

Highly sensitive spectroelectrochemistry of adsorbed films on ITO is demonstrated with the electroactive integrated optical waveguide (EA-IOW). The EA-IOW, a single-mode planar waveguide coated with an ITO layer, is ∼10(4)-fold more sensitive to changes in absorbance occurring during electrochemical events versus a single-pass transmission spectroelectrochemical experiment, as demonstrated by reduction of surface-adsorbed methylene blue. Furthermore, the EA-IOW is selective to near-surface events, as it is relatively insensitive to absorbance by solutions of dissolved chromophores at <1 mM. The EA-IOW is also used to monitor the formation of Prussian Blue during the reduction of ferricyanide, an event that is not easily followed using current-detected cyclic voltammetry, due to interfering faradaic and non-faradaic electrochemical events. The optical background of the EA-IOW is potential-dependent and is explained by ion diffusion into the ITO and by voltage-dependent changes in optical constants for the material. Finally, the high sensitivity of the EA-IOW (relative to other evanescent-field-based spectroelectrochemical techniques) is discussed in terms of its design.

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...

Planar integrated optical methods for examining thin films and their surface adlayers

Thin film integrated optical waveguides (IOWs) have gained acceptance as a method for characterizing ultrathin dielectrical films and adlayers bound to the film surface. Here, we present the expressions that govern IOW methods as well as describe the common experimental configurations used in attenuated total reflection, fluorescence and Raman applications. The applications of these techniques to the study of adsorbed or surface-bound proteins to polymer and glass waveguides are reviewed.

Sol-gel-based, planar waveguide sensor for water vapor

A water vapor sensor based on a combination of sol-gel processing and planar optical waveguide technologies has been developed. The indicator erythrosin B was entrapped in a thin sol-gel film (thickness ∼100 nm) prepared from methyltriethoxysilane, dimethyldiethoxysilane, and tetraethoxysilane. This dye exhibits an increase in absorbance in the presence of liquid or gaseous water. The sol-gel layer containing the dye was deposited onto a sol-gel-derived, single-mode planar waveguide. Outcoupled light intensity measurements (at 514.5 nm) over a range of water vapor concentrations (in a nitrogen gas stream) yielded a response over a wide range of relative humidity (<1-∼70%) at room temperature. Response and reversal times were less than 1 min, which may make this sensor attractive for real-time monitoring applications.

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.

Sol−gel-based, planar waveguide sensor for gaseous iodine

A novel sensor for gaseous iodine has been developed using a combination of sol-gel processing and planar integrated optical waveguiding technologies. The sensing principle is based on the detection of a charge transfer complex formed between iodine and phenyl groups that have been incorporated into a porous, methylated glass film. The glass film was prepared from siloxane precursors by the sol-gel method. Sensors were fabricated by coating the film over a single-mode planar waveguide. Light was coupled into and out of the laminate structure using integral grating couplers, and formation of the charge transfer complex was monitored as attenuated total reflection of the guided wave. The sensor exhibits a linear response to I(2) in the range of 100 ppb to 15 ppm with response and recovery times less than 15 s. The response is selective to 4 ppm iodine in the presence of 10 ppm chlorine and is stable for at least 3 months.

Phosphonic Acid Functionalized Asymmetric Phthalocyanines: Synthesis, Modification of Indium Tin Oxide, and Charge Transfer

Metalated and free-base A(3)B-type asymmetric phthalocyanines (Pcs) bearing, in the asymmetric quadrant, a flexible alkyl linker of varying chain lengths terminating in a phosphonic acid (PA) group have been synthesized. Two parallel series of asymmetric Pc derivatives bearing aryloxy and arylthio substituents are reported, and their synthesis and characterization through NMR, combustion analysis, and MALDI-MS are described. We also demonstrate the modification of indium tin oxide (ITO) substrates using the PA functionalized asymmetric Pc derivatives and monitoring their electrochemistry. The PA functionalized asymmetric Pcs were anchored to the ITO surface through chemisorption and their electrochemical properties characterized using cyclic voltammetry to investigate the effects of PA structure on the thermodynamics and kinetics of charge transfer. Ionization energies of the modified ITO surfaces were measured using ultraviolet photoemission spectroscopy.