Fast Electron Transfer Process Research Articles

Electrochemical detection of hydroquinone with a gold nanoparticle and graphene modified carbon ionic liquid electrode

An ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) modified carbon paste electrode was fabricated and used as the substrate electrode. Then a gold nanoparticle and graphene (GR) composite film was co-electrodeposited on the carbon ionic liquid electrode (CILE) surface by immersing CILE in the graphite oxide and tetrachloroauric acid dispersion solution with cyclic voltammetric reduction. The fabricated Au–GR/CILE exhibited good electrochemical performances with higher conductivity and lower electron transfer resistance. Electrochemical behaviors of hydroquinone (HQ) were further investigated on the modified electrode by cyclic voltammetry and differential pulse voltammetry. A pair of well-defined redox peaks appeared with the peak-to-peak separation (ΔEp) as 0.077V in 0.1mol/L pH 2.5 PBS, indicating a fast quasi-reversible electron transfer process. The result could be attributed to the presence of high conductive Au–GR nanocomposites on the electrode surface. The electrochemical parameters of HQ on the Au–GR/CILE were calculated and the experimental conditions were optimized. Under the optimal conditions, the linear relationship between the oxidation peak current of HQ and its concentration can be obtained in the range from 0.06μmol/L to 800.0μmol/L with the detection limit as 0.018μmol/L (3σ). The coexisting catechol exhibited no interference and Au–GR/CILE was applied to the detection of HQ in synthetic wastewater samples with satisfactory results.

Convertible electron transfer pathways of cytochrome c at TiO2 quantum electrode

In this work, biofunctional ultrathin TiO2 quantum wires (about 3 nm), self-assembled into flat-cable-like “stacks”, are synthesized by a simple template-free wet chemical method with high uniformity and on a large scale. On the self-assembled TiO2 nanowire (SA-Nw-TiO2) coated electrode, cytochrome c (cyt c) undergoes a fast, reversible (or quasi-reversible) and sensitive electron transfer process that is closely related to the relative ionic strength and the hydrated state of SA-Nw-TiO2 surface. At the suitably hydrated SA-Nw-TiO2 electrode in an optimum bulk ionic environment, electrons are reversibly transferred at the heme edge on the basis of an effective orientation of cyt c. It is interesting to note that in a suitable ionic microenvironment, a convertible electron transfer occurs in the following three steps: electron transfer driven partial unfolding of cyt c, electron transfer, and thermodynamics driven refolding. In addition, the partially unfolded cyt c was proven to exist only in the quasi-reversible electron transfer process, and the multi-step pathway of electron transfer can be converted into the single-step pathway at the fully hydrated SA-Nw-TiO2 electrode.

Electrochemical determination of dioxygen and hydrogen peroxide using Fe3O4@SiO2@hemin microparticles

Iron oxide microparticles were coated with 3-aminopropyltriethoxysilane and then coated with hemin via an amidation reaction. The resulting composite particles were characterized by transmission electron microscopy. FTIR spectroscopy revealed two bands (at 1,701 and 1,634 cm−1), which were assigned to the carboxy group and the amide linkage, respectively, resulting from the linkage between hemin and the amino-modified Fe3O4 particles. In addition, strong Fe-O vibrations can be observed at 563 cm−1. An electrode was modified with these microparticles and then showed a well-defined redox behavior of the immobilized hemin, with a fast heterogeneous electron transfer process (14.5 s−1). The electrode is capable of sensing both O2 and H2O2 and displays a wide linear range, high sensitivity, and fast response. The composites reported here also may serve as a support for the immobilization of proteins, which paves the way to potential applications in novel biosensors and bioelectronic devices.

Large Amplitude Fourier Transformed AC Voltammetric Investigation of the Active State Electrochemistry of a Copper/Aqueous Base Interface and Implications for Electrocatalysis

The higher harmonic components available from large-amplitude Fourier-transformed alternating current (FT-ac) voltammetry enable the surface active state of a copper electrode in basic media to be probed in much more detail than possible with previously used dc methods. In particular, the absence of capacitance background current allows low-level Faradaic current contributions of fast electron-transfer processes to be detected; these are usually completely undetectable under conditions of dc cyclic voltammetry. Under high harmonic FT-ac voltammetric conditions, copper electrodes exhibit well-defined and reversible premonolayer oxidation responses at potentials within the double layer region in basic 1.0 M NaOH media. This process is attributed to oxidation of copper adatoms (Cu*) of low bulk metal lattice coordination numbers to surface-bonded, reactive hydrated oxide species. Of further interest is the observation that cathodic polarization in 1.0 M NaOH significantly enhances the current detected in each of the fundamental to sixth FT-ac harmonic components in the Cu*/Cu hydrous oxide electron-transfer process which enables the underlying electron transfer processes in the higher harmonics to be studied under conditions where the dc capacitance response is suppressed; the results support the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis. The underlying quasi-reversible interfacial Cu*/Cu hydrous oxide process present under these conditions is shown to mediate the reduction of nitrate at a copper electrode, while the mediator for the hydrazine oxidation reaction appears to involve a different mediator or active state redox couple. Use of FT-ac voltammetry offers prospects for new insights into the nature of active sites and electrocatalysis at the electrode/solution interface of Group 11 metals in aqueous media.

Electropolymerisation of l-arginine at carbon paste electrode and its application to the detection of dopamine, ascorbic and uric acid

L-arginine was electropolymerised on a carbon paste electrode (CPE) to form the biopolymer by free radical formation in the electro oxidation process of the amino and carboxylic group containing compound by cyclic voltammetric technique. The modified electrode shows an excellent electrocatalytic activity towards the oxidation of both dopamine (DA) and ascorbic acid (AA). It was demonstrated that the deposited biopolymer has positive charges over the bare carbon electrode surface, which leads to the formation of electrical double layer made the fast electron transfer process could leads to the diffusion of dopamine, ascorbic acid and uric acid on their charge gradient by cyclic voltammetric technique. The response of the sensor was tested towards the different dopamine concentration. The catalytic peak current obtained was linearly related to DA concentrations in the ranges of 5×10(-5) to 1×10(-4)M L(-1) with correlation co-efficient of 0.9924 which reveals the adsorption controlled process. The detection limit for dopamine was 5×10(-7)M L(-1). The interference studies showed that the modified electrode exhibits excellent selectivity in the presence of large excess of ascorbic acid (AA) and response is fast stable, reliable, resistant to biofouling and can be applied for the real sample analysis in medical, pharmaceutical and biotechnological sectors. The adsorption-controlled process and kinetic parameters of the poly(L-arginine) were determined using electrochemical approaches.

Annulation of Tetrathiafulvalene to the Bay Region of Perylenediimide: Fast Electron-Transfer Processes in Polar and Nonpolar Solvents

A tetrathiafulvalene donor has been annulated to the bay region of perylenediimide through a 1H-benzo[d]pyrrolo[1,2-a]imidazol-1-one spacer affording an extended π-conjugated molecular dyad (TTF−PDI). To gain insight into its ground- and excited-state electronic properties, the reference compound Ph-PDI has been prepared via a direct Schiff-base condensation of N,N′-bis(1-octylnonyl)benzoperylene-1′,2′:3,4:9,10-hexacarboxylic-1′,2′-anhydride-3,4:9,10-bis(imide) with benzene-1,2-diamine. Both the experimental and the computational (DFT) results indicate that TTF−PDI exhibits significant intramolecular electronic interactions giving rise to an efficient photoinduced charge-separation process. Free-energy calculations verify that the light-induced process from TTF to the singlet-excited state of PDI is exothermic in both polar and nonpolar solvents. Fast adiabatic electron-transfer processes of a compactly fused, π-conjugated TTF−PDI dyad in benzonitrile, 2-methyltetrahydrofuran, anisole and toluene were observed by femtosecond transient absorption spectral measurements. The lifetimes of radical-ion pairs slightly increase with decreasing the solvent polarities, suggesting that the charge-recombination occurs in the Marcus inverted region. By utilizing the nanosecond transient absorption technique, the intermolecular electron-transfer process in a mixture of TTF-diamine/Ph-PDI has been observed via the triplet excited PDI for the first time.

Preparation and Application of Mediator‐Free H2O2 Biosensors of Graphene‐Fe3O4 Composites

AbstractGraphene‐Fe3O4 (G‐Fe3O4) composites were prepared with a solvothermal method for the immobilization of horseradish peroxidase (HRP) to construct a mediator‐free H2O2 biosensor. The morphology and structure of the G‐Fe3O4 composites were analyzed by TEM, SEM, and XRD. Chitosan (CH) was utilized to disperse the composites, and the mixture of CH, G‐Fe3O4 and HRP was spin‐coated on an indium tin oxide (ITO) electrode to form a biocomposites film. Electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CV) demonstrated the fast electron transfer process between HRP and the electrode. The resulted H2O2 biosensor exhibited a rapid response upon the addition of H2O2, a high sensitivity of 132 µA mM−1 cm−2 and a low detection limit of 6×10−7 M. The linear range of the biosensor is from 5×10−6 to 3.81×10−3 M (r=0.997). Such interesting G‐Fe3O4 composites with easy preparation and low cost can provide a novel matrix for enzyme immobilization to construct biosensors with higher performance.

Copper-poly(2-aminodiphenylamine) as a novel and low cost electrocatalyst for electrocatalytic oxidation of methanol in alkaline solution

In the present work we demonstrate the carbon paste as a new electrode substrate for the electropolymerization of 2-aminodiphenylamine and fabrication of polymer film modified electrode. Then transition metal of copper is incorporated into the polymer by electrodepositing of Cu(II) from CuCl 2 acidic solution using potentiostatic technique. The electrocatalytic oxidation of methanol was studies by cyclic voltammetry and chronoamperometry methods at the surface of obtained Cu/P(2ADPA)/MCPE. It has been found that in the course of an anodic potential sweep, the electro-oxidation of methanol follows the formation of Cu(III) and is catalyzed by this species through a mediated electron transfer mechanism. The obtained current density for this catalytic oxidation is very high which could be come from high surface area of caused by the P(2ADPA) modification. The effects of various parameters such as the copper loading, scan rate and methanol concentration on the electrocatalytic oxidation of methanol were also investigated at the surface of Cu/P(2ADPA)/MCPE. Finally, using a chronoamperometric method, the catalytic rate constant ( k) for methanol was found to be 0.2 × 10 5 cm 3 mol −1 s −1 that the high k can be ascribed for the fast electron transfer process due to electrode modification.

Effect of Large-Amplitude Alternating Current Modulation on Apparent Reversibility of Electrode Processes

We examined the effect of a large-amplitude high-frequency alternating potential modulation on direct currents associated with irreversible, quasi-reversible, and reversible electron-transfer processes occurring at microelectrodes under voltammetric conditions. All irreversible processes appear to be accelerated by the superimposed ac modulation, and under certain conditions this may even lead to an electrochemical etching of noble metal electrodes. In the case of electrode processes which are reversible on the time scale of a dc polarization, but quasi-reversible on the time scale of the ac modulation, the distortion of voltammograms caused by the ac modulation can provide useful information about the kinetics of fast electron-transfer processes. For completely reversible electrode processes the effect of the large-amplitude ac modulation is essentially trivial; the distortion of voltammetric curves causes broadening of analytical signals without providing any useful information.

Ultrafast Photophysics of Star-Like Molecules with Benzene and Triazine Core

Static and transient spectroscopic characters of newly synthesized start-like molecules, 1,3,5-tri(10-butyl-3-propenyl-10H-phenothiazine)-benzene (TP3B) and 2,4,6-tri(10-butyl-3-propenyl-10H-phenothiazine)-[1,3,5]triazine (TP3T), are studied using static, picosecond fluorescence and femtosecond transient absorption spectroscopy. The results show that when the benzene group is in the center, a large conjugation system is formed, while a fast electron-transfer process happens when the center group is triazine.

Electrochemical detection of ethidium bromide by using pure single-walled carbon nanotube sheet as the electrode

Sheets consisted of entire single-walled carbon nanotubes (SWNTs) were used as the working electrode for electrochemical measurement of ethidium bromide by covering a glassy carbon electrode (GCE) using the SWNT-sheet. This SWNT-sheet based electrode exhibited a fast electron transfer process on the electrode surface via the cyclic voltammetry with K4Fe(CN)6 as electrochemical probes. This SWNT-sheet based electrode showed also a high sensitivity toward ethidium bromide, a typically harmful, aromatic backboned chemical. The SWNT-sheet based electrode was capable of accumulating ethidium bromide to a higher concentration and therefore was capable of detecting ethidium bromide with a detection limit of 1.0 × 10−8 M.

Effects of ZnO coating on the performance of TiO2 nanostructured thin films for dye-sensitized solar cells

ZnO-coated TiO2 (ZTO) thin films were deposited on ITO substrates by a sol–gel method for application as the work electrode for dye-sensitized solar cells (DSSCs). The I–V curve and the incident photon-to-current conversion efficiency (IPCE) value of DSSCs for ZTO thin films were studied and compared with single TiO2 films. The results show that the short-circuit photocurrent (J sc) and open-circuit voltage (V oc) values increased from 3.7 mA/cm2 and 0.68 V for the DSSCs with a single TiO2 film to 4.5 mA/cm2 and 0.72 V, respectively, for the DSSCs with a ZTO thin film. It indicated that the DSSCs with a ZTO thin film contributed to provide an inherent energy barrier that suppressed charge recombination significantly. In addition, the higher IPCE value in the ZTO thin film is attributed to the better charge separation by a fast electron transfer process using two semiconductors with different conduction band edges and energy positions.

Differential Tuning of the Electron Transfer Parameters in 1,3,5-Triarylpyrazolines: A Rational Design Approach for Optimizing the Contrast Ratio of Fluorescent Probes

A large class of cation-responsive fluorescent sensors utilizes a donor-spacer-acceptor (D-A) molecular framework that can modulate the fluorescence emission intensity through a fast photoinduced intramolecular electron transfer (PET) process. The emission enhancement upon binding of the analyte defines the contrast ratio of the probe, a key property that is particularly relevant in fluorescence microscopy imaging applications. Due to their unusual electronic structure, 1,3,5-triarylpyrazoline fluorophores allow for the differential tuning of the excited-state energy DeltaE(00) and the fluorophore acceptor potential E(A/A(-)), both of which are critical parameters that define the electron transfer (ET) thermodynamics and thus the contrast ratio. By systematically varying the number and attachment positions of fluoro substituents on the fluorophore pi-system, DeltaE(00) can be adjusted over a broad range (0.4 eV) without significantly altering the acceptor potential E(A/A(-)). Experimentally measured D-A coupling and reorganization energies were used to draw a potential map for identifying the optimal ET driving force that is expected to give a maximum fluorescence enhancement for a given change in donor potential upon binding of the analyte. The rational design strategy was tested by optimizing the fluorescence response of a pH-sensitive probe, thus yielding a maximum emission enhancement factor of 400 upon acidification. Furthermore, quantum chemical calculations were used to reproduce the experimental trends of reduction potentials, excited-state energies, and ET driving forces within the framework of linear free energy relationships (LFERs). Such LFERs should be suitable to semiempirically predict ET driving forces with an average unsigned error of 0.03 eV, consequently allowing for the computational prescreening of substituent combinations to best match the donor potential of a given cation receptor. Within the scaffold of the triarylpyrazoline platform, the outlined differential tuning of the electron transfer parameters should be applicable to a broad range of cation receptors for designing PET sensors with maximized contrast ratios.

A novel hydrogen peroxide sensor based on the direct electron transfer of horseradish peroxidase immobilized on silica–hydroxyapatite hybrid film

The direct electron transfer of immobilized horseradish peroxidase (HRP) on silica–hydroxyapatite (HAp) hybrid film-modified glassy carbon electrode (GCE) and its application as H 2O 2 biosensors were investigated. On silica/HRP–HAp/GCE, HRP displayed a fast electron transfer process accompanied with one proton participate in. This sensor exhibited an excellent electrocatalytic response to the reduction of H 2O 2 without the aid of an electron mediator. The proposed biosensor showed good reproducibility and high sensitivity to H 2O 2 with the detection limit of 0.35 μM. In the range of 1.0–100 μM, the catalytic reduction current of H 2O 2 was proportional to H 2O 2 concentration. The apparent Michaelis–Menten constant ( k m app ) of the biosensor was calculated to be 21.8 μM, exhibiting a high enzymatic activity and affinity for H 2O 2.

Study of the Underlying Electrochemistry of Polycrystalline Gold Electrodes in Aqueous Solution and Electrocatalysis by Large Amplitude Fourier Transformed Alternating Current Voltammetry

Polycrystalline gold electrodes of the kind that are routinely used in analysis and catalysis in aqueous media are often regarded as exhibiting relatively simple double-layer charging/discharging and monolayer oxide formation/removal in the positive potential region. Application of the large amplitude Fourier transformed alternating current (FT-ac) voltammetric technique that allows the faradaic current contribution of fast electron-transfer processes to be emphasized in the higher harmonic components has revealed the presence of well-defined faradaic (premonolayer oxidation) processes at positive potentials in the double-layer region in acidic and basic media which are enhanced by electrochemical activation. These underlying quasi-reversible interfacial electron-transfer processes may mediate the course of electrocatalytic oxidation reactions of hydrazine, ethylene glycol, and glucose on gold electrodes in aqueous media. The observed responses support key assumptions associated with the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis.

Excited State Interactions in Calix[4]arene−Perylene Bisimide Dye Conjugates: Global and Target Analysis of Supramolecular Building Blocks

The photophysical properties of two supramolecular building blocks oc and oc2 consisting of a perylene bisimide chromophore substituted with either one or two calix[4]arene units in the N-imide position as well as those of the reference compound oref without calix[4]arene substituents were investigated. A complete picture of the processes taking place after photoexcitation in toluene, CH2Cl2, and benzonitrile was obtained by means of UV/vis absorption, steady state and time-resolved emission, and femtosecond transient absorption spectroscopy. It has been found that the fluorescence emission of compounds oc (Φfl = 0.03 in CH2Cl2) and oc2 (Φfl < 0.01 in CH2Cl2) is almost completely quenched compared with the highly emissive reference compound oref (Φfl = 0.99 in CH2Cl2) because of fast electron-transfer processes from the calix[4]arene moieties to the perylene bisimide chromophore. This predominantly takes place with rate constants of kCS = 3 × 1010 s-1 (τCS = 32 ps) for compound oc and kCS = 9 × 1010 s-1 (τCS = 11 ps) for compound oc2 (in CH2Cl2) leading to a short-lived charge-separated state consisting of the reduced perylene bisimide unit and the oxidized calix[4]arene moiety.

Energy transfer pathways in pyridylporphyrin Re(I) adducts

Dimeric and pentameric adducts between a meso-4′ pyridylporphyrin core and either one or four rhenium(I) bipyridine tricarbonyl units, fac-[Re(CO) 3(bipy)(4′MPyP)][CF 3SO 3] and fac-[{Re(CO) 3(bipy)} 4(4′TPyP)][CF 3SO 3] 4(4′MPyP = 4′-monopyridylporphyrin, 4′TPyP = 4′-tetrapyridylporphyrin), respectively, were synthesized and their photophysical behaviors were investigated by emission and absorption time resolved experiments. The adducts exhibit distinctive supramolecular features, different from those of the molecular components. Upon excitation of the core, the typical porphyrin fluorescence is quenched. This effect is attributed to enhanced intersystem crossing in the porphyrin unit, owing to the heavy atom effect provided by the attached rhenium unit(s). Following excitation of rhenium fragments the typical rhenium MLCT emission is not observed while partial sensitization of the porphyrin fluorescence occurs, indicating that fast intercomponent energy and/or electron transfer processes take place in competition with the intersystem crossing in the rhenium unit.

Dynamic Chemical Devices: Photoinduced Electron Transfer and Its Ion‐Triggered Switching in Nanomechanical Butterfly‐Type Bis(porphyrin)terpyridines

A series of butterfly-type molecular constructs has been prepared in good yield by using a double Stille coupling synthetic protocol. They are composed of a terpyridine (terpy) scaffold and two wings composed of appended porphyrins that are capable of switching from an extended W geometry to a compact U geometry upon cation coordination of the terpy unit. The porphyrin moieties exist in the constructs either as free bases or they can be sequentially metallated, thus giving rise to wings of different "colours". Stationary and time-resolved emission studies of the HZn, ZnAu and Zn2Au constructs show that the electronic properties are strongly dependent on the geometry. In the extended W conformation an energy-transfer process is seen from the free base to the Zn-metallated porphyrin. In the U conformation in Zn2Au the donor luminescence resulting from the singlet excited state of the Zn wing is strongly, quenched not only due to the heavy atom effect but also due to a fast electron-transfer process to the ground state of the Au wing. Furthermore, the binding of (alpha,omega)-diamine substrates to the Zn(II)-porphyrin sites can also influence the conformation of the system. For the Zn2Zn construct, single-crystal diffraction experiments with synchrotron radiation allowed the structure to be solved by direct methods and fully refined; it shows the expected U conformation. The central Zn atom is six-coordinate, whereby the zinc atom is coordinated by the eta3-terpy ligand as well by monodentate and semi-chelating acetate anions. The structure is made rigid by hydrogen bonds involving the aqua ligands on the outer Zn centres and acetate oxygen atoms. The present system thus represents a double-trigger-modulated optomechanical switching device with selective substrate binding for either metal atoms or tailored ligands. Both energy- and electron-transfer processes can be controlled opening a means of improving the on/off ratio in future constructs.

Ultrafast Energy-Electron Transfer Cascade in a Multichromophoric Light-Harvesting Molecular Square

A molecular square with dimensions of about 4 nm, incorporating sixteen pyrene chromophores attached to four ditopic bay-functionalized perylene bisimide chromophores, has been synthesized by coordination to four Pt(II) phosphine corner units and fully characterized via NMR spectroscopy and ESI-FTICR mass spectrometry. Steady-state and time-resolved emission as well as femtosecond transient absorption studies reveal the presence of a highly efficient (>90%) and fast photoinduced energy transfer (k(en) approximately equal to 5.0 x 10(9) s(-1)) from the pyrene to the perylene bisimide chromophores and a very fast and efficient electron transfer (>94%, k(et) approximately equal to 5 x 10(11) up to 43 x 10(11) s(-1)). Spectrotemporal parametrization indicates upper excited-state electron-transfer processes, various energy and electron-transfer pathways, and chromophoric heterogeneity. Temperature-dependent time-resolved emission spectroscopy has shown that the acceptor emission lifetime increases with decreasing temperature from which an electron-transfer barrier is obtained. The extremely fast electron-transfer processes (substantially faster and more efficient than in the free ligand) that are normally only observed in solid materials, together with the closely packed structure of 20 chromophoric units, indicate that we can consider the molecular square as a monodisperse nanoaggregate: a molecularly defined ensemble of chromophores that partly behaves like a solid material.

Electron-Transfer Kinetics of Nitroxide Radicals as an Electrode-Active Material

Abstract The electron-transfer rate constants of nitroxide derivatives, 4-(N-t-butyl-N-oxylamino)-t-butylbenzene, 4-(N-t-butyl-N-oxylamino)methoxybenzene, and 2,2,6,6-tetramethylpiperidinyl-N-oxyl, were investigated by electrochemical methods, which demonstrated that these radicals can potentially be used as a high power-rate electrode-active material due to their fast electron-transfer process.