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Nucleation and growth mechanism of electropolymerization of methylene blue (MB) in a basic medium and the effect of preparation potential on poly(MB) film structure were investigated by using cyclic voltammetry, potentiostatic current-time transient, scanning tunneling microscopy (STM), atomic force microscopy (AFM), and UV-vis. absorption spectroscopy techniques. Electropolymerization of MB has been achieved by potentiodynamic (cyclic voltammetry) and potentiostatic (constant potential) techniques. The potentiostatic current-time transients fitted with a theoretical model and morphological studies indicate that nucleation and growth mechanism of poly(MB) starts with a progressive layer-by-layer nucleation and growth besides random adsorption. Nucleation and growth of poly(MB) follows a process between progressive layer-by-layer and 3-D instantaneous mechanism resulting in highly-oriented poly(MB) nanofibers with increasing poly(MB) film thickness. Cyclic voltammetry and morphological studies exhibit that poly(MB) film structure changes depending on the preparation potential. Poly(MB) films prepared at the potential values of 900 and 950 mV show a well-ordered, smooth surface but at the potential values higher than 1000 mV, rough polymer surface arises as overoxidation takes place. UV-vis. absorption spectra of poly(MB) film and MB monomer show three peaks. The peak at 410 nm for poly(MB) shows 100 nm blue shift when compared to the MB monomer and is attributed to poly(MB) formation on the electrode.

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A flexible and free-standing graphene-based hybrid paper was successfully fabricated by successive applications of vacuum filtration and electropolymerization. First, a suspension including graphene oxide (GO) and silver nanoparticles (AgNPs) was prepared, and GO/AgNPs paper was obtained by vacuum-filtration of this suspension through a membrane. This GO/AgNPs paper was transformed to rGO/AgNPs paper by using both chemical reduction with HI and thermal annealing procedures. rGO/AgNPs/poly(PyY) hybrid paper electrode was formed by electropolymerization of Pyronin Y (PyY) on rGO/AgNPs paper electrode from a PyY monomer-containing (pH 1.0) solution. Structural, chemical, and morphological characterization of this hybrid paper was carried out by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, infrared spectroscopy, UV-vis absorption spectroscopy, four-point probe conductivity measurement, and cyclic voltammetry techniques. Electrooxidation of nitrite on rGO/AgNPs/poly(PyY) hybrid paper electrode has been achieved at 860 mV with a linear range of 0.1-1000 μM, sensitivity of 13.5 μAμM(-1)cm(-2), and a detection limit of 0.012 μM. Amperometry studies have shown that the hybrid paper electrode is suitable for amperometric determination of nitrite in both standard laboratory samples and real samples. Moreover, this paper electrode selectively detects nitrite even in the presence of 100-fold common ions and exhibits an excellent operational stability and good flexibility.

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The kinetics of the electrochemical deposition and desorption of sulfur monolayers on highly oriented Au(111) and polycrystalline Au electrodes in aqueous solutions containing sodium sulfide was studied by cyclic voltammetry, chronocoulometry, and chronoamperometry. Cyclic voltammetry experiments reveal that underpotential deposition (upd) and stripping of sulfur takes place at two different potentials at polycrystalline Au substrates leading to two oxidative and two reductive peaks. However, sulfur upd consists of only one oxidative and reductive peak at single crystalline Au(111) substrates. Electrosorption valancy measurements and pH dependency showed that the upd of sulfur involved two-electron and one-step mechanism. The charge corresponding to one monolayer of sulfur was determined by cyclic voltammetry and chronocoulometry, indicating a 0.33 coverage. The chronoamperometric results at polycrystalline Au electrodes indicates that deposition takes place in a Langmuir-type mechanism, whereas the stripping of sulfur follows a two-dimensional nucleation and growth mechanism which is accompanied by Langmuir adsorption. Although the appearance of current transients, observed for the deposition of sulfur on Au(111), was similar to what seen for deposition on polycrystalline Au substrates, the stripping process was totally different from deposition, involving only a two-dimensional nucleation and growth mechanism, at pH 12. From an analysis of the desorption current transient, we describe the stripping mechanism as instantaneous. Chronoamperometry experiments suggest that sulfur is first adsorbed on to a Au(111) surface randomly, then reorganizes itself to form a well-ordered structure, and finally strips off in a two-dimensional nucleation and growth mechanism.

Highly flexible graphene/poly(methylene blue)/AgNPs composite paper was successfully prepared for amperometric biosensing of NADH. For this purpose, a dispersion including graphene oxide (GO), methylene blue (MB) and silver nanoparticles (AgNPs) was prepared and GO/MB/AgNPs paper was acquired by vacuum‐filtration of this dispersion through a suitable membrane. After peeling it off from membrane, it was transformed to rGO/MB/AgNPs paper by performing reduction with hydriodic acid. In a three‐electrode cell, which is containing 0.1 M phosphate buffer solution (pH: 9.0), rGO/MB/AgNPs paper was used as working electrode and rGO/poly(MB)/AgNPs composite paper was generated by surface‐confined electropolymerization of MB using successive cyclic voltammetry approach in a suitable potential window. Characterization of this composite paper was carried out by using scanning electron microscopy, scanning tunneling microscopy, X‐ray photoelectron spectroscopy, powder X‐ray diffraction spectroscopy, Raman spectroscopy, four‐point probe conductivity measurement and cyclic voltammetry techniques. Flexible rGO/poly(MB)/AgNPs composite paper has demonstrated high sensitivity, wide linear range and low detection limit for amperometric quantification of NADH.

We report a study of the underpotential deposition (UPD) of Te monolayers onto Au electrodes from perchloric acid solution using electrochemical quartz microgravimetry (EQCM) and chronocoulometry. We find that tellurium(IV) oxide (as TeO3 2-, TeO2 or HTeO2 +) is adsorbed onto Au at potentials positive of the UPD region. EQCM experiments, in which an Au electrode poised at +0.7 V in pure HClO4 electrolyte was monitored as a function of time before and after the addition of TeO2, showed a slow adsorption of a submonolayer of TeO2. Energy-dispersive X-ray (EDX) measurements confirm the presence of a Te-containing species on the surface. Two distinct reductive UPD features at approximately +0.400 V and −0.100 V vs Ag|AgCl were observed by cyclic voltammetry. EQCM measurements indicate that there is a small, reproducible mass decrease accompanying the first UPD wave, consistent with the loss of H2O from the surface as an adsorbed layer of HTeO2 + is reduced to a submonolayer of Te atoms. Chronocoulometry indicates a charge density of 160 μC/cm2 for the first UPD wave, which, when taken together with the mass change data, is consistent with the four-electron reduction of adsorbed HTeO2 + to Te. In addition, analysis of the current−time data indicates that this process occurs by a two-dimensional instantaneous nucleation and growth mechanism, resulting in an ordered overlayer with a fractional coverage of ca. 0.41. The second UPD peak results in the formation of a dense monolayer by reduction of HTeO2 + from solution. This process also appears to occur by a direct four-electron reduction and exhibits simple Langmuir adsorption behavior as evidenced by exponentially decaying current−time transients. The saturation coverage is approximately 0.9.