In situ Raman spectroelectrochemistry of azobenzene monolayers on glassy carbon

In situ Raman spectra of chemisorbed azobenzene (AB) monolayers on glassy carbon (GC) electrodes were observed under potentiostatic conditions in acetonitrile (ACN) with tetrabutyl-ammonium tetrafluoroborate (TBA-BF₄). The Raman intensities of these spectra were high below −1000 mV, and this is attributed to the change in absorbance of AB on GC. In this paper, we describe chemisorbed AB molecules on GC electrode surfaces under potentiostatic conditions.     

In situ optical spectroelectrochemistry of single-walled carbon nanotube thin films

A detailed study is presented on the optical absorption of thin films of single-walled carbon nanotubes (SWNT) under electrochemical conditions. The procedure for the preparation of free-standing semitransparent films of SWNT is used for the fabrication of a working electrode for transmission optical spectroelectrochemistry. The analysis of the potential dependent spectroscopic response of the SWNT film benefits from the widest possible electrochemical window, in which the charging of SWNT can safely be investigated. This electrochemical window is not limited by parasitic electrochemistry and/or galvanic breakdown reactions occurring at supporting electrode materials such as indium–tin oxide conducting glass or semitransparent Pt film, which were employed in earlier studies. Electrochemical doping of SWNT is observable at the optical absorptions, which are assigned to allowed electronic transitions between van Hove singularities in the density of states of SWNT. Furthermore, the spectral response of counterions, balancing the charging of the nanotube skeleton, is traceable at certain conditions. The latter effect is monitored here through the overtones of C–H stretching vibrations from tetrabutylammonium cations.     

In situ electrodeposition of Pt nanoclusters on glassy carbon surface modified by monolayer choline film and their electrochemical applications

Pt nanoclusters attached to the monolayer choline (Ch) modified glassy carbon surface were successfully synthesized by use of in situ cyclic voltammetry (CV) method. Field emission scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS) were used to characterize the properties of this modified electrode. It was demonstrated that Ch was immobilized onto the carbon surface forming a covalently planted Ch monolayer, which could induce the formation of Pt nanoclusters. The preliminary study found that the homogeneous nanostructured Pt/Ch film exhibited remarkable electrocatalytic activity towards the oxidation of methanol and nitrite.     

Direct electron transfer and voltammetric determination of roxithromycin at a single-wall carbon nanotube coated glassy carbon electrode

The electrochemical behavior of roxithromycin (RM) at a single-wall carbon nanotube (SWNT) coated glassy carbon (GC) electrode was studied. It was found that RM could produce an irreversible anodic peak at the electrode. When the pH of supporting electrolyte (i.e. phosphate buffer solution) was 7 the peak potential was 0.86V (vs. SCE). The electrochemical reaction contained electron and proton transfer, and the electron-transfer coefficient (α) was ca. 0.87. The anodic peak depended on the adsorption of RM, the maximum adsorption amount was about 3.99×10(-10)molcm(-2). The adsorbed RM could be removed by cycling between 0.1 and 1.1V in a blank solution for about two minutes, and the electrode thus could be regenerated. Under the optional conditions, the anodic peak current was linear to RM concentration over the range of 5.0×10(-6) to 1.0×10(-4)M. The limit of detection was 5.0×10(-7)M (S/N=3) for 180s accumulation at -0.8V. The modified electrode had good stability and repeatability, and it was successfully applied to the determination of RM in medicine samples.     

Local electron transfer rate measurements on modified and unmodified glassy carbon electrodes

A natural and artificial distribution of electron transfer activity on glassy carbon electrodes can be observed and quantified by the use of scanning electrochemical microscopy (SECM). A large (sevenfold) spread in rate constant is found for randomly sampled sites on polished, untreated glassy carbon surfaces. Direct-mode oxidation with the SECM tip was used to produce small regions of oxidized carbon on a polished surface. A large increase in electron transfer rate for the Fe(II/III) ion is observed on the locally oxidized carbon surface in comparison to the unoxidized region. Rate constant measurements made along a line profiles the transition from unoxidized to oxidized surfaces. SECM images of defect sites show reaction–rate variations. Rate constants measured at several locations of the defective surface allows discrimination between the kinetic and topographic components of the SECM image. Dedicated to the 80th birthday of Keith B. Oldham     

In situ EPR and UV-vis spectroelectrochemistry of hole-transporting organic substrates

A newly developed in situ electron paramagnetic resonance (EPR)/ultraviolet-visible (UV-vis) spectroelectrochemical cell equipped with a laminated indium-tin oxide (ITO) working electrode was used in the investigation of various organic substrates which are potential hole-transporting materials. The experiment demonstrated the possibility of using such a technique for examining redox behavior of conducting polymers (polypyrrole, PPy), oligomers (thiophene dimmer and quarterthiophene) and bis-anilines (N,N,N',N'-tetraphenylbenzidine, TPB). All investigated structures formed stable paramagnetic intermediates in the first oxidation step characterised with UV-vis spectra in the region 400-600 nm. In the second oxidation step EPR-silent di-cationic structures are formed with broad vis bands in the region 600-1000 nm. The measurement of the reference UV-vis spectra direct in the EPR cavity was possible using a specially-constructed non-contacted ITO plate in the spectroelectrochemical cell in the case of polypyrrole.     

Direct electron transfer and bioelectrocatalysis of hemoglobin on nano-structural attapulgite clay-modified glassy carbon electrode

Direct electrochemistry of hemoglobin (Hb) on natural nano-structural attapulgite clay film-modified glassy carbon (GC) electrode was investigated. The interaction between Hb and attapulgite was examined using UV-vis, FTIR spectroscopy, and electrochemical methods. The immobilized Hb displayed a couple of well-defined and quasi-reversible redox peaks with the formal potential (E(0')) of about -0.366 V (versus SCE) in 0.1 M phosphate buffer solution of pH 7.0. The current was linearly dependent on the scan rate, indicating that the direct electrochemistry of Hb in that case was a surface-controlled electrode process. The formal potential changed linearly from pH 5.0 to 9.0 with a slope value of -48.2 mV/pH, which suggested that a proton transfer was accompanied with each electron transfer in the electrochemical reaction. The immobilized Hb exhibited excellent electrocatalytic activity for the reduction of hydrogen peroxide without the aid of an electron mediator. The electrocatalytic response showed a linear dependence on the H(2)O(2) concentration ranging from 5.4 x 10(-6) to 4.0 x 10(-4) M with the detection of 2.4 x 10(-6) M at a signal-to-noise ratio of 3. The apparent Michaelis-Menten constant K(M)(app) for the H(2)O(2) sensor was estimated to be 490 microM, showing a high affinity.     

Trans-membrane electron transfer in red blood cells immobilized in a chitosan film on a glassy carbon electrode

We have studied the trans-membrane electron transfer in human red blood cells (RBCs) immobilized in a chitosan film on a glassy carbon electrode (GCE). Electron transfer results from the presence of hemoglobin (Hb) in the RBCs. The electron transfer rate (k _s) of Hb in RBCs is 0.42 s^?1, and <1.13 s^?1 for Hb directly immobilized in the chitosan film. Only Hb molecules in RBCs that are closest to the plasma membrane and the surface of the electrode can undergo electron transfer to the electrode. The immobilized RBCs displayed sensitive electrocatalytic response to oxygen and hydrogen peroxide. It is believed that this cellular biosensor is of potential significance in studies on the physiological status of RBCs based on observing their electron transfer on the modified electrode.

Cyclic voltammetric and computational study of a 4-bromophenyl monolayer on a glassy carbon electrode

A glassy carbon (GC) surface modified with monolayer of 4-bromophenyl was examined as voltammetric electrode for some redox systems. The modified electrode exhibited very slow electron transfer in comparison to the unmodified surface by factors which varied with the redox systems. However, after scanning the modified electrode in 0.1 M tetrabutylammonium tetrafluoroborate (TBABF₄) in acetonitrile from 0.4 to −1.1 V vs. Ag/AgCl for 20–25 cycles, the modified electrode showed much faster electron transfer kinetics, e.g., the results for Fe(CN)₆ ^3−/4− were approaching those observed with unmodified surfaces. The effect is attributed to an apparently irreversible structural change in the 4-bromophenyl monolayer, which increases the rate of electron tunneling. The transition to the conducting state is associated with electron injection into the monolayer and causes a significant decrease in the calculated HOMO-LUMO gap for the monolayer molecule. Once the monolayer is switched to the conducting state, it supports rapid electron exchange with the redox system, but not with dopamine, which requires adsorption to the electrode surface. A conductive surface modified electrode may have useful properties for electroanalytical applications and possibly in electrocatalysis. Correspondence: Abbas A. Rostami, Department of Chemistry, Faculty Basic of Science, University of Mazandaran, Babolsar, Iran.     

In situ Raman spectroscopy of bias-induced structural changes in nitroazobenzene molecular electronic junctions

Carbon/nitroazobenzene (NAB)/titanium/gold molecular electronic junctions with active thicknesses of 7-8 nm were constructed having partially transparent Ti/Au top contacts, which permitted in situ monitoring of molecular structure with Raman spectroscopy for applied biases between +3 and -3 V. Deposition of the Ti/Au top contacts resulted in spectral changes similar to those accompanying NAB reduction in a conventional spectroelectrochemical experiment. Upon application of +3 V (carbon relative to Ti), the spectrum indicated reoxidation of the NAB reduction product, and this redox cycle could be repeated at least three times. When a voltage excursion to -2 or -3 V occurred, the spectra indicated irreversible loss of the nitro group, and a dramatic but reversible decrease in Raman intensity over the entire shift range examined. Negative applied voltage causes formation of reduced NAB and a high oxidation state of titanium, while positive voltage forms oxidized NAB and injects electrons into the titanium oxide layer. The spectral changes were correlated with current/voltage curves in order to probe the mechanism of rectification and conductance switching reported previously. Overall, the combination of spectroscopic and voltammetric results implies a conduction mechanism involving both NAB and titanium oxide, possibly mediated by the injection of carriers into the semiconducting titanium oxide, or by reduction of an insulating titanium oxide to a more conductive form.     

Solvent-controlled photoinduced electron transfer between porphyrin and carbon nanotubes

beta-Cyclodextrin (beta-CD)-modified multiwalled carbon nanotubes (MWCNTs) were successfully prepared by reaction of surface-bound carboxylic chloride groups of MWCNTs with aminoethyleneamino-deoxy-beta-CD (ENCD) and comprehensively characterized by FTIR, Raman, and X-ray photoelectron spectroscopy and thermogravimetric and differential thermal analysis. The beta-CD-modified MWCNTs (ENCD-MWCNTs) are highly water-soluble, with a solubility of ca. 9.0 mg x mL(-1). Furthermore, the photoinduced electron transfer (PET) process between tetrakis(4-carboxyphenyl)porphyrin (TCPP) and ENCD-MWCNTs was investigated by means of fluorescence, fluorescence decay, transient absorption spectroscopy, and cyclic voltammetry. Obvious quenching processes were observed upon addition of both MWCNT-COOH and ENCD-MWCNTs to the aqueous solutions of TCPP, indicating that the PET process between TCPP and carbon nanotubes takes place upon irradiation. When 1-adamantane acetic acid was added to the aqueous solutions of TCPP/MWCNT-COOH and TCPP/ENCD-MWCNTs, respectively, the former fluorescence remains, while the latter fluorescence recovers. On the contrary, the fluorescence intensity of TCPP in the DMF solution was hardly decreased upon addition of ENCD-MWCNTs, whereas its fluorescence was quenched in the presence of MWCNT-COOH. The observations indicate that the CD cavities play a vital role on the control of the PET process.     

In situ infrared spectroelectrochemical studies on adsorption and oxidation of nucleic acids at glassy carbon electrode

The adsorption and oxidation of yeast RNA and herring sperm DNA (HS DNA) at glass carbon (GC) electrode are studied by differential pulse voltammetry (DPV) and in situ FTIR spectroelectrochemistry. Two oxidation peaks of yeast RNA are obtained by DPV, whose peak potentials shift negatively with increasing pH. The peak currents decrease gradually in successive scans and no corresponding reduction peaks occur, thus indicating that the oxidation process of yeast RNA is completely irreversible. The IR bands in the 1200-1800 cm(-1) range, attributed to the stretching and ring vibrations of nucleic acid bases, show the main spectral changes when the potential is shifted positively, which gives evidence that the oxidation process takes place in the base residues. The oxidation process of HS DNA is similar to that of yeast RNA. The results both from DPV and in situ FTIR spectroelectrochemistry confirm that the guanine and adenine residues can be oxidized at the electrode surface, which is consistent with the oxidation mechanism of nucleic acids proposed previously.     

In situ spectroscopy and spectroelectrochemistry of uranium in high-temperature alkali chloride molten salts

Soluble uranium chloride species, in the oxidation states of III+, IV+, V+, and VI+, have been chemically generated in high-temperature alkali chloride melts. These reactions were monitored by in situ electronic absorption spectroscopy. In situ X-ray absorption spectroscopy of uranium(VI) in a molten LiCl-KCl eutectic was used to determine the immediate coordination environment about the uranium. The dominant species in the melt was [UO 2Cl 4] (2-). Further analysis of the extended X-ray absorption fine structure data and Raman spectroscopy of the melts quenched back to room temperature indicated the possibility of ordering beyond the first coordination sphere of [UO 2Cl 4] (2-). The electrolytic generation of uranium(III) in a molten LiCl-KCl eutectic was also investigated. Anodic dissolution of uranium metal was found to be more efficient at producing uranium(III) in high-temperature melts than the cathodic reduction of uranium(IV). These high-temperature electrolytic processes were studied by in situ electronic absorption spectroelectrochemistry, and we have also developed in situ X-ray absorption spectroelectrochemistry techniques to probe both the uranium oxidation state and the uranium coordination environment in these melts.     

Direct electron transfer enhancement of covalently bound tyrosinase to glassy carbon via Woodward's reagent K

This work describes the reaction mechanism for chemical modification of tyrosinase by Woodward's Reagent K and its covalent attachment to a glassy carbon electrode. The spectrophotometric studies revealed that the modification does not cause a significant structural change to tyrosinase. The direct electrochemistry of modified enzyme was achieved after immobilization on an oxidatively activated glassy carbon electrode. The enzyme film exhibited a pair of well-defined quasi-revesible voltammetric peaks corresponding to the Cu (II)/Cu (I) redox couple located in the active site of tyrosinase. The formal potential of immobilized enzyme was measured to be 90mV (vs. Ag/AgCl) in phosphate buffer solution at pH 7.0. The charge-transfer coefficient and apparent heterogeneous electron transfer rate constant were estimated to be 0.5 and 0.9±0.06s(-1), respectively. Finally, the electrochemical behavior of the immobilized enzyme in the presence of caffeic acid and L-3,4-dihydroxyphenylalanine as substrates was investigated. The amperometric study of biosensor toward L-3,4-dihydroxyphenylalanine resulted a linear response in the concentration range from 1.66×10(-6) to 8.5×10(-5)M with detection limit of 9.0×10(-5)M and sensitivity of 135mAμM(-1)cm(-2).     

Effects of monolayer structures on long-range electron transfer in helical peptide monolayer

Self-assembled monolayers of alpha-helical peptides were prepared on gold, and the effects of the monolayer structures (kind of constituent amino acid, molecular orientation, and molecular packing) on long-range electron transfer through the helical peptides were studied. The helical peptides were 16mer peptides having a thiophenyl linker at the N-terminal for immobilization on gold and a redox active ferrocene moiety at the C-terminal as an electron-transfer probe. The peptides were immobilized on gold by a gold-sulfur linkage and the electron transfer from the ferrocene moiety to gold was studied by electrochemical methods. When two types of the peptides, one with the repeating unit of Leu-Aib (Aib represents 2-aminoisobutyric acid) and the other with that of Ala-Aib, were compared, the electron transfer was found one order slower in the Leu-Aib peptide monolayer than that in the Ala-Aib peptide monolayer. The self-assembled monolayers of the Ala-Aib peptide with mixing of three different lengths of the peptides, 8mer, 12mer, and 16mer without a ferrocene moiety, were also prepared. The monolayer regularity in terms of molecular orientation and packing was higher roughly in the order of the monolayers mixed with 16mer > 12mer > no additive > 8mer, but the electron transfer became faster in the opposite order. The logarithms of the standard rate constants showed a nearly linear relationship with the direct distances between the ferrocene moiety and gold (beta = 0.32 A (-1)). Some data deviated from this linear relationship, but the deviations could be explained from the difference in the molecular packing, which was evaluated from the monolayer capacitance. It is thus concluded that an electron is transferred along a few molecules along the surface normal so that the vertical orientation or the increase of the interchain backbone separation slows down the electron transfer. Further, it is demonstrated that a tightly packed monolayer, where vibrational mode is restricted, suppresses the electron transfer. Three models are proposed to account for the observed molecular dynamics effects on the basis of either electron-transfer mechanism of electron tunneling or sequential hopping.     

In situ time-resolved FTIR spectroelectrochemistry: study of the reduction of TCNQ

The efficiency and versatility of time-resolved FTIR spectroscopy has been used to follow concentration profiles of species produced during a cyclic voltammetric scan. It has been tested in situ and in resolved time, by probing the reduction of tetracyanoquinodimethane (TCNQ) on its first and second electrochemical wave. Besides the establishment of the method, the individual concentrations of TCNQ, of the monoanion and of the dianion were monitored at distinct infrared frequencies and the time derivatives of the concentration profiles were compared to the voltammograms.     

The charging of the carbon nanotube/oligothiophene interphase as studied by in situ electron spin resonance/UV-Vis-NIR spectroelectrochemistry

A detailed in situ Electron Spin Resonance (ESR)/UV-Vis-NIR spectroelectrochemical study of the oligothiophene/single walled carbon nanotube (SWCNT) interphase is presented to provide an insight into the interaction of nanotubes with oligothiophenes. Used as electrode materials these composites are followed in situ with respect to the paramagnetic and diamagnetic states formed upon electrochemical charging. The variation of the oligomer chain length and the type, position and number of substituents at the oligomer is used to understand the structural influence on the formation of the charged states in the material upon electrochemical reaction. For β,β'-dihexylsexithiophene (β,β'-DHST)-SWCNT the enlarged current in the composite and a decreased radical cation concentration can be explained by the formation of π-dimers. By interaction with SWCNTs the π-dimerization of oligothiophenes and the formation of multi π-stack structures occur. For α,ω-dicyano-β,β'-dibutylquaterthiophene (DCNDBQT)-SWCNT a new paramagnetic structure of the oligomer is formed as an intermediate which undergoes follow-up reactions. Using different substituted oligothiophenes their interaction with nanotubes can be understood with respect to the structure of the oligomer.     

In situ NMR spectroelectrochemistry for the structure elucidation of unstable intermediate metabolites

In situ NMR spectroelectrochemistry is presented in this study as a useful hybrid technique for the chemical structure elucidation of unstable intermediate species. An experimental setting was designed to follow the reaction in real time during the experimental electrochemical process. The analysis of ¹H NMR spectra recorded in situ permitted us (1) to elucidate the reaction pathway of the electrochemical oxidation of phenacetin and (2) to reveal the quinone imine as a reactive intermediate species without using any trapping reaction. Phenacetin has been considered as hepatotoxic at high therapeutic amounts, which is why it was chosen as a model to prove the applicability of the analytical method. The use of 1D and 2D NMR experiments led to the elucidation of the major species produced from the oxidation process. We demonstrated that in situ NMR spectroelectrochemistry constitutes a fast way for monitoring unstable quinone imines and elucidating their chemical structures.

Effect of electrode surface microstructure on electron transfer induced conformation changes in cytochrome c monitored by in situ UV and CD spectroelectrochemistry

Electrochemical redox processes of bovine heart cytochrome c were investigated by in situ UV-vis and CD spectroelectrochemistry at bare glassy carbon electrode (GCE) and single-wall carbon nanotubes (SWNTs) modified glassy carbon electrode (SWNTs/GCE) using a long optical path thin layer cell. The spectra obtained at GCE and SWNTs/GCE reflect electrode surface microstructure-dependent changes in protein conformation during redox transition. Potential-dependent conformational distribution curves of cytochrome c obtained by analysis of in situ circular dichroism (CD) spectra using singular value decomposition least square (SVDLS) method show that SWNTs can retain conformation of cytochrome c. Some parameters of the electrochemical reduction process, i.e. the product of electron transfer coefficient and number of electrons (alpha n = 0.3), apparent formal potential (E0' = 0.04 V), were obtained by double logarithmic analysis and standard heterogeneous electron transfer rate constant k0 was obtained by electrochemistry and double logarithmic analysis, respectively.     

Direct electron transfer of hemoglobin founded on electron tunneling of CTAB monolayer

Direct electron transfer and stable adsorption of hemoglobin (Hb) on a carbon paste (CP) electrode were achieved with the aid of a single-chain cationic surfactant, namely, cetyltrimethylammonium bromide (CTAB). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) indicated that CTAB could form a complete monolayer with a high density of positive charges on the surface of the CP electrode, which strongly adsorbed negatively charged protein molecules via electrostatic interactions. The surfactant molecules anchored the protein molecules to align in suitable orientations and acted as electron-tunneling pathways between the protein molecules and the CP electrode. The bioelectrocatalytic activity of the immobilized Hb was confirmed by RAIR and UV-vis spectroscopies, and rapid electrochemical responses to the reduction of oxygen (O2), hydrogen peroxide (H2O2), and nitrite (NO2-) were also obtained.