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This work describes the fabrication and characterization of a new high surface area nanocomposite electrode containing reduced graphene oxide (rGO) and titanium nitride (TiN) for electrochemical applications. This approach involves electrochemically depositing rGO on a high surface area TiN nanorod array electrode to form a new nanocomposite electrode. The TiN nanorod array was first formed by the glancing angle deposition technique in a DC (Direct Current) sputtering system. GO flakes of ~1.5 μm in diameter, as confirmed by Dynamic Light Scattering (DLS), were electrodeposited on the nanostructured TiN electrode via the application of a fixed potential for one hour. The surface morphology of the as-prepared rGO/TiN electrode was evaluated by scanning electron microscopy (SEM) and the presence of rGO on TiN was confirmed by Raman Microscopy. The CV shows an increase in the capacitive current at rGO/TiN as compared to TiN. The rGO decorated TiN electrode was then used for analyzing the electrocatalytic oxidation of ascorbic acid and dopamine, and the reduction of nitrate by CV and linear sweep voltammetry (LSV), respectively. CV or LSV show that the electrochemical kinetics of these three analytes are significantly faster on rGO/TiN than TiN itself. Overall, the rGO/TiN electrode showed better electrochemical behavior for biomolecules like ascorbic acid and dopamine as well as another target analyte, nitrate ions, compared to TiN by itself. [ABSTRACT FROM AUTHOR]

Single-molecule microscopic methods were used to probe the uptake, mobility, and entrapment of dye molecules in cured poly(dimethylsiloxane) (PDMS) films as a function of oligomer extraction. The results are relevant to the use of PDMS in microfluidic separations, pervaporation, solid-phase microextraction, and nanofiltration. PDMS films were prepared by spin-casting dilute solutions of Sylgard 184 onto glass coverslips, yielding ~ 1.4 [micro]m thick films after curing. Residual oligomers were subsequently extracted from the films by 'spin extraction'. In this procedure, 200 [micro]L aliquots of isopropyl alcohol were repeatedly dropped onto the film surface and spun off at 2000 rpm. Samples extracted 5, 10, 20, and 40 times were investigated. Dye molecules were loaded into these films by spin-casting nanomolar dye solutions onto the films. Both neutral perylene diimide (N,N'-bis(butoxypropyl)perylene-3,4,9,10-tetracarboxylic diimide) and cationic rhodamine 6G (R6G) dyes were employed. The films were imaged by confocal fluorescence microscopy. The images obtained depict nonzero populations of fixed and mobile molecules in all films. Cross-correlation methods were used to quantitatively determine the population of fixed molecules in a given region, while a Bayesian burst analysis was used to obtain the total population of molecules. The results show that the total amount of dye loaded increases with increased oligomer extraction, while the relative populations of fixed and mobile molecules decrease and increase, respectively. Bulk R6G data also show greater dye loading with increased oligomer extraction. 10.1021/ac902250p

Silica and hybrid organic-inorganic films, ca. 100-200 nm thick, can be grown on glassy carbon electrodes through reactions initiated by electrogenerated hydroxide or hydronium ions in water under reductive and oxidative conditions, respectively. A variety of different alkoxysilanes (tetramethoxysilane and organoalkoxysilanes) and supporting electrolytes were used to evaluate whether film formation takes place on glassy carbon electrodes. The results of the study indicate that the acid-base properties of the supporting electrolyte are an important factor in determining whether film formation will take place. For cathodic electrodeposition, thin films can be formed using supporting electrolytes that are close to neutral, such as KCl, KN[O.sub.3], and NaCl[O.sub.4]. For anodic electrodeposition, thin films can be formed using supporting electrolytes that are acidic, such as, K[H.sub.2]P[O.sub.4], HN[O.sub.3], [H.sub.2]S[O.sub.4], etc. The acidity/basicity effects of the electrolytes arise in part from the strong dependence of the hydrolysis and condensation rates of the silicon alkoxide precursors on pH.

The concentration dependence of autocorrelation amplitudes obtained from orientationally fixed single molecules is explored using theory, simulation, and experimental results. Autocorrelation amplitudes obtained under such circumstances are shown to be ~2-fold larger than predicted in previous studies (Koppel, D. E. Phys. Rev. A 1974, 10, 1938-1945 and citing references), which frequently assume polarization-independent excitation and detection. A detailed derivation of the autocorrelation amplitude expected under conditions frequently employed in single-molecule experiments is given. Simulated and experimental single-molecule image data obtained from samples incorporating fixed single molecules are used to verify the correctuess of the model. These results are compared to both simulated and experimental time transient data in which the molecules exhibit predominantly fast rotational reorientation and to which previously reported models apply. The experimental results employed are obtained from dye-doped mesoporous silica thin films studied at different levels of hydration. The theory and results obtained are of importance to the determination of molecular concentrations from single-molecule image and time transient autocorrelation data, in situations where the molecules exhibit permanent or reversible adsorption at fixed orientations in or on thinfilm materials.

Single-molecule fluorescence spectroscopy is used to study dye diffusion within organically modified silicate (ORMOSIL) films. ORMOSIL films are prepared from sols containing tetraethoxysilane and isobutyltrimethoxysilane in 2:1 and 1:9 molar ratios. Nile red and a new silanized form of nile red that can be covalently attached to the silicate matrix are used as fluorescent probe molecules. The number and rate of single molecules diffusing through these films increases dramatically with increasing film organic content. Autocorrelation of the fluorescence images yields a quantitative measure of the relative populations of fixed and diffusing species. Surprisingly, both 'free' and silicate-bound nile red exhibit relatively facile translational motions. Single-molecule/single-point fluorescence correlation spectroscopy (FCS) is used to measure the dye diffusion coefficients in submicrometer-scale film regions. The most common diffusion coefficients for 'free' and silicate-bound nile red molecules in the 1:9 films are 3.9 x [10.sup.-10] and 1.6 x [10.sup.-10] [cm.sup.2]/s, respectively. The unexpectedly rapid diffusion of silicate-bound nile red is attributed to the presence of liquidlike silicate oligomers in the films. A lower bound for the molecular weight of the oligomers is estimated at 2900. Bulk solution-phase FCS experiments performed on 'free' and silicate-bound nile red species extracted into chloroform solutions provide valuable support for these conclusions. Comparison of the results derived from experimental and simulated time transients indicates film heterogeneity occurs on sub-100-nm-length scales and likely results from the presence of inorganic- and organic-rich domains.

The diffusion coefficients of redox probes entrapped in a silica matrix prepared by the sol--gel process were measured using a combination of cyclic voltammetry and chronoamperometry at an ultramicroelectrode. In this study, the porosities of the gels were varied to assess the importance of constrained environments vs intermolecular interactions on the translational mobility of guests entrapped in this solid host matrix. The average pore diameter of the gels was varied from 40 to 400 [Angstrom] by utilizing different catalysts (HCI, N[H.sub.3], NaF) or different silicon precursors (tetramethoxysilane or Ludox colloidal silica). The diffusion coefficients of cobalt(II) tris(bipyridine), ferrocenemethyltrimethylammonium ion, and dicyanobis(phenanthroline)iron(II) and their rate of change as the gel dried were found to be nearly identical for gels prepared from TMOS and catalyzed with either HCI, N[H.sub.3], or NaF. When trapped in gels prepared from Ludox, ferrocenemethanol and potassium ferricyanide diffused at rates identical to that measured in solution. In contrast, [D.sub.app] for ferrocenemethyl(trimethylammonium) dropped 1 order of magnitude over a 30-day drying period. These results attest to the importance of intermolecular interactions in governing diffusion in sol-gel-derived materials.

Sol--gel-derived silicate films were electrochemically deposited on conducting surfaces from a sol consisting of tetramethoxysilane (TMOS). In this method, a sufficiently negative potential is applied to the electrode surface to reduce oxygen to hydroxyl ions, which serves as the catalyst for the hydrolysis and condensation of TMOS. The electrodeposition process was followed by the electrochemical quartz crystal microbalance and cyclic voltammetry. The electrodeposited films were characterized for their surface morphology, porosity, and film thickness using atomic force microscopy, electrochemical probe techniques, surface area and pore size analysis, and profilometry. The electrodeposited films were found to have a completely different surface structure and to be significantly rougher relative to spin-coated films. This is likely due in part to the separation of the gelation and evaporation stages of film formation. The electrodeposited films were found to be permeable to simple redox molecules, such as ruthenium(III) hexaammine and ferrocene methanol. Film thickness can be easily varied from < 75 nm to > 15 [micro]m by varying the electrode potential from--600 mV to more than--1000 mV, respectively. The electrodeposition process was further applied for the electroencapsulation of redox molecules and organic dyes within the silicate network. Cyclic voltammograms for the gel-entrapped ferrocene methanol (FcC[H.sub.2]OH) and ruthenium(II) tris(bipyridine) (Ru[(bpy).sub.3.sup.2+]) exhibited the characteristic redox behavior of the molecules. The electroencapsulation of organic dyes in their 'native' form proved to be more difficult because these species typically contain reducible functionalities that change the structure of the dye.

The voltammetry and electrogenerated chemiluminescence (ECL) of tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)(sub 3)(super 2+)) ion-exchanged in Nafion and Nafion - silica composite materials have been investigated. The major goal of this work was to investigate and develop new materials and immobilization approaches for the fabrication of ECL-based sensors with improved reactivity and long-term stability. Nafion - silica composite materials with varying contents of Nafion (53-100 wt % relative to silica) were prepared via the two-step acid/base hydrolysis and condensation of tetramethoxysilane. The Nafion doped sols were spin cast on glassy carbon electrodes, dried, and then ion-exchanged with Ru(bpy)(sub 3)(super 2+). The shapes of the cyclic voltammetric curves and the amount of Ru(bpy)(sub 3)(super 2+) exchanged into the films strongly depends on the amount of Nafion incorporated into the hybrid sol. Nafion - silica films with a low content of Nafion ion-exchanged less Ru(bpy)(sub 3)(super 2+) and exhibited tail-shaped voltammetry at 100 mV/s. The ECL of immobilized Ru(bpy)(sub 3)(super 2+) in the presence of either tripropylamine or sodium oxalate in pH 5 acetate buffer was also strongly dependent on the amount of Nafion introduced into the composite with greater ECL observed for the Nafion-silica films relative to pure Nafion.

The electrogenerated chemiluminescence (ECL) of Ru(bpy)(sub 3)(super 2+) and tripropylamine, tributylamine, triethylamine, trimethylamine, or sodium oxalate encapsulated within sol-gel-derived silica monoliths have been investigated using an immobilized ultramicroelectrode assembly. The major purpose of this study was to investigate the role of the reductant on the magnitude and stability of the ECL in this solid host matrix. For gel-entrapped Ru(bpy)(sub 3)(super 2+)/tertiary amines, the shape and intensity of the ECL - potential curves were highly dependent on scan rate. At 10 mV/s, the ECL intensity was ca. 6-fold higher relative to that observed at 500 mV/s. When the ECL acquired at low scan rates was normalized by that obtained in solution under similar conditions, a value of 0.03 - 0.06 was obtained. In direct contrast, the ECL of the Ru(bpy)(sub 3)(super 2+)-oxalate system showed little dependence on scan rate, and the ECL was ca. 65-75% of that measured in solution. These differences can be atlributed to differences in rotational and translational mobility between the reductants (amines vs oxalate) trapped in this porous solid host. For both systems, the ECL was found to be stable upon continuous oxidation or upon drying the gels in a high-humidity environment for over 10 days.

Individual molecular-scale environments in silicate thin films are described for the first time using single molecule confocal microspectroscopic methods. The physicochemical environment in which each molecule is entrapped is probed using rhodamine dyes doped into the material at nanomolar level. According to static fluorescence spectra and time-dependent fluorescence signals for many single molecules, the silicate materials are highly inhomogeneous compared to polymer films.

High-frequency generation of electrochemiluminescence (ECL) from ion-annihilation reactions has been investigated at microelectrodes. Well-defined, stable, reproducible luminescence curves were obtained when a multicycle square wave was applied to a microelectrode in the presence of either 9,10-diphenylanthracene, 9,10-dimethylanthracene, or 9-phenylanthracene. ECL efficiencies of ca. 5%, 2%, and 2% were obtained for these compounds, respectively, using ruthenium(II) tris(bipyridine) as a standard. Due to the reduced effects of double-layer capacitance and ohmic drop, the microelectrode can be pulsed at a more rapid rate than electrodes of conventional size. The electrochemical-time constant, which was directly evaluated from the luminescence data, determined the upper frequency limit. For a 5 micrometers radius electrode in acetonitrile containing 0.1 M electrolyte, this corresponded to 20-30 kHz. The rapidity of the experiment improved the stability of the luminescence and enabled ECL to be investigated from dilute solutions as well as from solutions containing 10% added water. Through variation in both the time scale of the experiment and the precursor concentration, a lower limit on the ion-annihilation rate constant for DPA of 2 X 10 to the ninth power M to the -1 s to the -1 was determined. This is close to the diffusion-controlled value of 10 to the tenth power M to the -1 s to the -1 theoretically expected.

Tin oxide electrodes with an adsorbed cytochrome c monolayer were examined using ultraviolet-visible spectroscopy. The structural state and the distribution of the compound on the surface were readily observed without the need for complex procedures. The position and intensities of the ultraviolet spectra provided the information regarding the conformation and total surface coverage of the cytochrome c monolayer. In addition, ferricytochrome c remained intact after its adsorption on the electrode's surface even in the presence of applied potentials reaching 0.7 volts.