Electron Transfer Rate Constant Research Articles

Influence of pH on radical reactions between kynurenic acid and amino acids tryptophan and tyrosine. Part II. Amino acids within the protein globule of lysozyme

An acidosis, a decrease of pH within a living tissue, may alter yields of radical reactions if participating radicals undergo partial or complete protonation. One of photosensitizers found in the human eye lens, kynurenic acid (KNA−), possesses pKa 5.5 for its radical form that is close to physiological pH 6.89 for a healthy lens. In this work we studied the influence of pH on mechanisms and products of photoinduced radical reactions between KNA− and amino acids tryptophan (Trp) and tyrosine (Tyr) within a globule of model protein, Hen White Egg Lysozyme (HEWL). Our results show that the rate constant of back electron transfer from kynurenyl to HEWL• radicals with the restoration of initial reagents – the major decay pathway for these radicals – does not change in the pH 3–7. The quantum yield of HEWL degradation is also pH independent, however a shift of pH from 7 to 5 completely changes the outcome of photoinduced damage to HEWL from intermolecular cross-linking to oxygenation. HPLC-MS analysis has shown that four of six Trp and all Tyr residues of HEWL are modified in different extents at all pH, but the lowering of pH from 7 to 5 significantly changes the direction of main photodamage from Trp62 to Trp108 located at the entrance and bottom of enzymatic center, respectively. A decrease of intermolecular cross-links via Trp62 is followed by an increase in quantities of intramolecular cross-links Tyr20-Tyr23 and Tyr23-Tyr53. The obtained results point out the competence of cross-linking and oxygenation reactions for Trp and Tyr radicals within a protein globule and significant increase of oxygenation to the total damage of protein in the case of cross-linking deceleration by coulombic repulsion of positively charged protein globules.

Electron Transfer Inside a Decaferrocenylated Rotaxane Analyzed by Fast Scan Cyclic Voltammetry and Impedance Spectroscopy

AbstractThe electrochemical behavior of a giant rotaxane bearing 10 electroactive ferrocene centers has been analyzed by both fast scan cyclic voltammetry and impedance measurements. The measurements support the hypothesis that all redox centers are not equivalent and display different electron transfer rate constants, which would arise from several possible conformations of the adsorbed molecule. Effectively using such systems in molecular devices thus necessitates to minimize or at least control those dispersions.

Photoinduced electron transfer reactions in mixed micelles of a star block copolymer and surface active ionic liquids: Role of the anion

This work investigates the effect of different counter anions of ionic liquid (IL) co-surfactants, viz. [NTf2]−, [DCN]−, [PF6]−, [BF4]−, [Br]−, associated with the common cation (1-decyl-3-methylimidazolium, [DMIm]+), on the microenvironments of mixed micelles formed with a star block copolymer, T1304, through spectrofluorometric studies. The role of IL anions has been explored for modulating photoinduced electron transfer (PET) reactions between a series of coumarin dyes as the electron acceptors and N,N-dimethylaniline (DMAN) as the electron donor in different T1304-IL mixed micelles. It is revealed that IL co-surfactants increase the polarity in the corona region of micelles to different extents, depending on the nature of the IL anion. The PET reactions in all micellar systems follow the Marcus Inversion (MI) behavior, as predicted by Marcus electron transfer theory, but the rate constant of electron transfer and the onset of the MI along the reaction exergonicity scale, (−ΔG°)onset, varies with the IL anion. It is observed that rather than being correlated with their chaotropic/kosmotropic nature, the effect of the IL in T1304 micelles can be correlated with the hydrophobicity/hydrophilicity, and the steric bulkiness of the IL anions.

Effect of plasma-assisted electrochemical treatment of the boron-doped synthetic diamond compact electrodes on the oxygen electroreduction kinetics

The effect of plasma electrochemical activation of boron-doped diamond compact electrodes on the oxygen electroreduction reaction kinetics is studied. The conducting compact electrodes are prepared via graphite-to-diamond conversion in C–B growth system at high pressure and high temperature. The diamond compact electrode surface was modified in Na2SO4 aqueous solution by exposition to cathodic–anodic electrolytic plasma produced by the application of voltage pulses with amplitude up to 300 V. The cathodic–anodic plasma alone proved to produce but negligible catalytic effect with respect to the oxygen reduction reaction. However, on the cathodic reduction of thus treated electrode, it acquired significant electrocatalytical activity manifested itself in the predominant O2-to-H2O reduction by a four-electron pathway. At the same time, the cathodic treatment of the plasma-modified electrodes has no real impact on the rate constant of electron transfer in the “fast” [Ru(NH3)6]2+/3+ and [Fe(CN)6]4–/3– redox systems. The observed electrocatalytical effect with respect to the oxygen reduction reaction is assumed to be caused by the quinone groups formation at the boron-doped diamond electrode surface under the combined action of cathodic–anodic plasma and cathodic polarization, these groups being active centers for the four-electron oxygen reduction.

Characteristics of the voltammetric behavior of the hydroxide ion oxidation at disordered mesoporous titanium dioxide electrocatalyst

The alkaline water electrochemical splitting reactions need economical, very energetic, and durable catalysts. Here, a disordered mesoporous and highly defected titanium dioxide (dom-TiO2) electrocatalyst for the oxidation of hydroxide ion was prepared via ligand-assisted evaporation-induced self-assembly. The (dom-TiO2) electrocatalyst showed significant electrocatalytic performance for the oxidation of hydroxide ion compared to that of non-porous TiO2 (bare-TiO2) and highly-ordered hexagonal mesoporous (hm-TiO2) electrodes. The chemical and electrochemical parameters of the diffusion (D), concentration in the bulk (Cb), the number of transferred electrons (n), rate constant of heterogeneous electron transfer (ks), redox potential (E°), and homogeneous chemical rate constant (kc) for the oxidation of hydroxide ion reaction at the porous TiO2 electrodes were determined via the convolution–deconvolution voltammetry and competed against that of non-porous (bare-TiO2) and hm-TiO2 and catalysts. In addition to the effect of dom-TiO2 film thickness and the type of supporting electrolytes on the electrochemical parameters of the electrocatalytic oxidation of OH– ions have been estimated. The convolutive–deconvoluted results show that the dom-TiO2 electrode catalyst exhibits a superior reaction rate constant among the studied electrodes that depend on the film thickness and type of supporting electrolyte.

Surface Chemistry Matters: How Ligands Influence Excited State Interactions between CsPbBr3 and Methyl Viologen

The photocatalytic properties of cesium lead bromide (CsPbBr3) perovskite nanocrystals make them attractive for designing light harvesting assemblies. Often ignored, the surface chemistry can dictate the excited state interactions of these semiconductor nanocrystals with charge-shuttling redox molecules. We have now explored the impact of CsPbBr3 nanocrystal surface modification on the excited state interactions with methyl viologen (MV2+) for three different ligand environments: prototypical oleic acid/oleylamine (OA/OAm) ligands, PbSO4-oleate capping, and didodecyldimethylammonium bromide (DDAB) ligands. Native OA/OAm ligands and PbSO4-oleate capping exhibit the strongest complexation with MV2+, whereas the bulky DDAB ligand environment shows an order of magnitude weaker complexation. The electron transfer rate constants as measured from transient absorption spectroscopy vary in the range of 1.2–3.6 × 1011 s–1 for different ligand environments. For DDAB-CsPbBr3 NCs, the efficiency of electron transfer (Φet) is 73%. Despite a protective capping layer, PbSO4-oleate capped CsPbBr3 maintains a redox-active surface which is viable for photocatalytic applications. These results highlight the impact of surface chemistry on excited state interactions of CsPbBr3 NCs and photocatalytic applications. Figure 1

(Invited) Distance Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine – C60 dyads has been investigated. Herein we will presented the synthesis and photophysical properties of two C60-SiPc-C60 dyads, 1 and 2, varying in their donor-acceptor distance. In the case of C60-SiPc-C60 1 where the SiPc and C60 are separated by a phenyl spacer, faster electron transfer was observed with k cs equal to 2.7 x 109 s-1 in benzonitrile. However, in the case of C60-SiPc-C60 2, where SiPc and C60 are separated by a biphenyl spacer, a slower electron transfer rate constant, k cs equal to 9.1 x108 s-1 was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ~ 4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ~3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of 3SiPc* as the end product and suggested that the final lifetime of charge separated state to be in the 3-20 ns range. Energy level diagram established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc .+ -C60 .- charge separated states (1.57 eV) to populate the low-lying 3SiPc* (1.26 eV) prior returning to the ground state. [1].AcknowledgmentsThis work was supported by the Ministerio de Ciencia e Innovación of Spain and FEDER funds (Grants CTQ2017-87102-R and RED2018- 102471-T to ASS) and National Science Foundation (Grant No. 1401188 and 2000988 to FD).References L. Martín-Gomis, S. Seetharaman, D. Herrero, P. A. Karr, F. Fernández-Lázaro, F. D’Souza, and Á. Sastre-Santos, Chem Phys. Chem., 2020, 21, 2254-2262. Figure 1

Electrochemiluminescence at 3D Printed Titanium Electrodes.

The fabrication and electrochemical properties of a 3D printed titanium electrode array are described. The array comprises 25 round cylinders (0.015 cm radius, 0.3 cm high) that are evenly separated on a 0.48 × 0.48 cm square porous base (total geometric area of 1.32 cm2). The electrochemically active surface area consists of fused titanium particles and exhibits a large roughness factor ≈17. In acidic, oxygenated solution, the available potential window is from ~-0.3 to +1.2 V. The voltammetric response of ferrocyanide is quasi-reversible arising from slow heterogeneous electron transfer due to the presence of a native/oxidatively formed oxide. Unlike other metal electrodes, both [Ru(bpy)3]1+ and [Ru(bpy)3]3+ can be created in aqueous solutions which enables electrochemiluminescence to be generated by an annihilation mechanism. Depositing a thin gold layer significantly increases the standard heterogeneous electron transfer rate constant, ko, by a factor of ~80 to a value of 8.0 ± 0.4 × 10−3 cm s−1 and the voltammetry of ferrocyanide becomes reversible. The titanium and gold coated arrays generate electrochemiluminescence using tri-propyl amine as a co-reactant. However, the intensity of the gold-coated array is between 30 (high scan rate) and 100-fold (slow scan rates) higher at the gold coated arrays. Moreover, while the voltammetry of the luminophore is dominated by semi-infinite linear diffusion, the ECL response is significantly influenced by radial diffusion to the individual microcylinders of the array.

Electron Transfer in Films of Atomically Precise Gold Nanoclusters

In applications of monolayer-protected gold clusters, assessing the dynamic behavior of the capping monolayer is crucial, as this determines the cluster’s effective size and electron-transfer (ET) properties. Here, we describe a systematic study on the effect of the monolayer thickness on the ET between molecular Au25(SR)180 nanoclusters in films. The length of the ligands protecting the Au core was varied by using a series of linear-chain thiolate (SCnH2n+1) protected Au25 clusters, where n = 3, 4, 5, 6, 7, 8, and 10. The effect of branching was assessed by using the 2-methyl-1-butanethiolate-protected Au25 cluster. Conductivity measurements were carried out on dry films obtained by drop-casting Au25(SR)180 solutions onto interdigitated gold electrodes. Changing the alkyl chain from C3 to C10 induces a smooth decrease in the film conductivity by 3.5 orders of magnitude. By using electrochemically determined Stokes radii, the conductivity results were transformed into the corresponding ET rate constants (kETs) between neighboring clusters. kET exponentially depends on the distance, and the data show that the average Au center-to-center distance in the film is not larger than the sum of two Stokes radii. This indicates that linear alkyl chains hold a detectable degree of fluidity in films, although not as much as in solution, as shown by the trend of the corresponding heterogeneous standard rate-constant values. For both physical states, these conclusions are precisely confirmed by the much slower ET rate determined for the Au25 cluster protected by the hindered thiolate. ET in films was also studied as a function of temperature, and the results were analyzed in the framework of the ET theory. We found that the outer and inner reorganization energies increase with n. For these ETs, the electronic coupling resonance between the reactant and product states, HRP, ranges from 12 (n = 3) to 0.7 cm–1 (n = 10) and therefore ETs are nonadiabatic. The data also show that 3/4 of the effect of distance on kET is related to HRP, while the rest is associated with the intrinsic barrier and thus the distance dependence of the outer and inner reorganization energies.

Novel SeS2-loaded Co MOF with Au@PANI comprised electroanalytical molecularly imprinted polymer-based disposable sensor for patulin mycotoxin

An SeS2-loaded Co MOF and Au@PANI nanocomposite comprising the base matrix of the electrode was developed with electropolymerized molecularly imprinted polymer (MIP) consisting of p-aminobenzoic acid (PABA) and patulin (PT) to detect PT molecules based on the PT imprinted cavities. SeS2@Co MOF and Au@PANI were synthesized using hydrothermal synthesis and interfacial polymerization strategies, respectively. A suitable functional monomer to fabricate the MIP platform was selected using the density functional theory (DFT/M06-2X method). Higher electrochemical active surface area (0.985 cm2 which is 6.99 times higher than the bare SPE) and a lower charge transfer resistance (Rct = 27.8 Ω) at the MIP/Au@PANI/SeS2@Co MOF electrode was achieved based on the higher number of adsorptive sites and enhanced conductivity (electron transfer rate constant (ks = 3.24 × 10−3 s−1) of the sensing platform. The fabricated MIP sensor performance was studied in 10 mM PBS (pH = 6.4), where an improved detection limit (0.66 pM) for PT and a broad logarithmic linear dynamic range (0.001–100 nM) were both observed. The sensor possessed higher selectivity (Imprinting factor = 15.4 for PT), excellent reusability (%RSD of 10 cycles = 2.49%), high storage stability (6.7% lost after 35 days), and robust reproducibility (%RSD = 3.22%) The as-prepared MIP-based PT sensor was applied to detect PT in a real-time apple juice sample (10% diluted with PBS) with a recovery % ranging from 94.5 to 106.4%. The proposed sensor possesses great advantages in terms of cost-effectiveness, providing a simple detection strategy for long-term storage stability, and reversible cycle measurements.

Cyclic voltammetry studies of bioanode microbial fuel fells from batch culture of Geobacter sulfurreducens

The present study aims to investigate the performance of batch culture of Geobacter sulfurreducens (G. sulfurreducens) for electrical current generation via cyclic voltammetry (CV) method. The CV study was performed with an applied voltage in the range of -0.1 to 0.1 V against the standard calomel electrode (SCE) during the cell growth and attachment of G. sulfurreducens on graphite felt and initial acetate concentration of 20 mM. The kinetics of electrode reaction was investigated by conducting CV experiments at different scanning rates of 5, 10, 20, 50 and 100 mVs -1. The diffusion coefficients (D) and heterogeneous electron transfer rate constant (k o) of both anodic and cathodic process were 1.04×10 -5 cm 2·s -1, 1.73×10 -6 cm 2.s -1, 0.0004 cm.s -1 and 0.0011 cm.s -1, respectively. The obtained results showed that the anode exhibits high bioeletrocatalytic activity due to the attachment of G. sulfurreducens on the anode surface.

Kinetics analysis of the electron transfer from nano-TiO2 to O2 through on-line absorptions and theoretical modeling

On-line optical absorptions were monitored under steady light illuminations to study the electron relaxations happening through the transfer from nano-TiO2 to O2, which are found to be slow and dispersive. A quasi-equilibrium (QE) theory and Monte Carlo simulations are developed to model the electron transfer, and they give good fittings to the early stage electron relaxations (over 70%). It is shown that the electron QE population at traps is kept during the whole electron relaxations. The slow kinetics is attributed to both the low probability (ptr) for an electron transferring to an O2 from a trap and the multi-trapping transport. The dispersive feature is ascribed to the dynamic decrease in the quasi-Fermi level (EF). The electron transfer rate constants just after the termination of light illuminations are taken out from the QE model fittings to analyze the relaxation kinetics. It is found that O2 amounts mainly affect the electron transfer by changing ptr; light intensities and temperatures mainly affect the electron transfer by changing the multi-trapping transport. The difference between the conduction band edge and the EF is the thermal barrier of the electron transfer from TiO2 to O2. The apparent activation energy (Eapp) of the electron transfer, determined from the absorption decays measured at different temperatures, is smaller than the real thermal barrier because of the decrease of EF with temperatures. The disagreement between the simulations and the later stage relaxations is not caused by the none-QE electron distribution at deep traps, and additional deep traps with a different distribution should also contribute to the electron relaxations.

Charge transfer in self-assembled monolayers of molecular conductors containing tripodal anchor and terpyridine-metal redox switching element

This work presents charge transfer characteristics of molecular switches based on [M(terpyridine)2]2+/3+ (M = Ru, Os) electroactive element functionalized by a tripodal thiol-based anchoring platform allowing the formation of self-assembled monolayers (SAMs). It is demonstrated that the Os-tripod compound forms more compact film with less pronounced repulsive interactions among individual [M(terpyridine)2]2+/3+ units compared to Ru-tripod compound. The electron transfer rate constants were determined from cyclic voltammograms and electrochemical impedance spectra by three independent methods. The value of 1.4 × 103 s–1 was obtained for Os-tripod SAM and 1.6 × 103 s–1 for that of Ru-tripod SAM. These values are almost one order of magnitude higher than those reported in the literature for osmium and ruthenium dipodal thiol-based anchors and more than two orders of magnitude higher than those reported for single monopodal aliphatic anchoring groups.

The effects of the chemical environment of menaquinones in lipid monolayers on mercury electrodes on the thermodynamics and kinetics of their electrochemistry

The effects of the chemical environment of menaquinones (all-trans MK-4 and all-trans MK-7) incorporated in lipid monolayers on mercury electrodes have been studied with respect to the thermodynamics and kinetics of their electrochemistry. The chemical environment relates to the composition of lipid films as well as the adjacent aqueous phase. It could be shown that the addition of all-trans MK-4 to TMCL does not change the phase transition temperatures of TMCL. In case of DMPC monolayers, the presence of cholesterol has no effect on the thermodynamics (formal redox potentials) of all-trans MK-7, but the kinetics are affected. Addition of an inert electrolyte (sodium perchlorate; change of ionic strength) to the aqueous phase shifts the redox potentials of all-trans MK-7 only slightly. The formal redox potentials of all-trans MK-4 were determined in TMCL and nCL monolayers and found to be higher in nCL monolayers than in TMCL monolayers. The apparent electron transfer rate constants, transfer coefficients and activation energies of all-trans MK-4 in cardiolipins have been also determined. Most surprisingly, the apparent electron transfer rate constants of all-trans MK-4 exhibit an opposite pH dependence for TMCL and nCL films: the rate constants increase in TMCL films with increasing pH, but in nCL films they increase with decreasing pH. This study is a contribution to understand environmental effects on the redox properties of membrane bond redox systems.Graphical abstract

Synthesis of homogeneously distributed gold nanoparticles built-in metal free organic framework: Electrochemical detection of riboflavin in pharmaceutical and human fluids samples

This study describes the synthesis of gold nanoparticles (AuNPs) incorporated melamine based covalent organic framework (MCOF) and their modification on glassy carbon electrode (GCE) for the sensitive and selective detection of riboflavin (RF). The MCOF was synthesized by refluxing melamine and terephthalaldehyde followed by AuNPs were incorporated into MCOF. The synthesized nanocomposite (MCOF-AuNPs) was confirmed by different spectral, microscopic and electrochemical methods. The TEM images show spherical AuNPs with an average diameter of 8 nm on the sheet-like structure of MCOF. The heterogeneous electron transfer rate constant (ket) was calculated for 1 mM K3/K4[Fe(CN)6] containing 0.1 M KCl at bare GC, MCOF and MCOF-AuNPs/GCEs was found to be 1.14, 1.72 and 4.31 × 10-4 cm2 s−1, respectively. The higher ket value achieved at MCOF-AuNPs/GCE indicates that the electron transfer rate reaction was faster at this electrode than others. The MCOF-AuNPs modified electrode was then utilized for the reduction of RF in 0.2 M phosphate-buffered saline (PBS) (pH 7.0). The MCOF-AuNPs/GCE showed 3.0 and 1.7-fold higher reduction current with 40 and 60 mV less positive potential shift against RF than bare GCE and MCOF/GCE. Under the optimized sensing parameters, the linear range concentration was achieved from 5 nM to 0.2 mM with the LOD of 2.0 nM. Finally, the present sensor was applied for the quantitative determination of RF in human biological fluids samples (human blood, urine, and saliva). Besides, the practical application was also demonstrated by determining RF in vitamin B-complex tablets and satisfactory results were obtained.

Ferrocene-Containing DNA Monolayers: Influence of Electrostatics on the Electron Transfer Dynamics.

A 153-mer target DNA was amplified using ethynyl ferrocene dATP and a tailed forward primer resulting in a duplex with a single-stranded DNA tail for hybridization to a surface-tethered probe. A thiolated probe containing the sequence complementary to the tail as well as a 15 polythimine vertical spacer with a (CH2)6 spacer was immobilized on the surface of a gold electrode and hybridized to the ferrocene-modified complementary strand. Potential step chronoamperometry and cyclic voltammetry were used to probe the potential of zero charge, PZC, and the rate of heterogeneous electron transfer between the electrode and the immobilized ferrocene moieties. Chronoamperometry gives three, well-resolved exponential current–time decays corresponding to ferrocene centers located within 13 Å (4 bases) along the duplex. Significantly, the apparent standard heterogeneous electron transfer rate constant, kappo, observed depends on the initial potential, i.e., the rate of electron transfer at zero driving force is not the same for oxidation and reduction of the ferrocene labels. Moreover, the presence of ions, such as Sr2+, that strongly ion pair with the negatively charged DNA backbone modulates the electron transfer rate significantly. Specifically, kappo = 246 ± 23.5 and 14 ± 1.2 s–1 for reduction and oxidation, respectively, where the Sr2+ concentration is 10 mM, but the corresponding values in 1 M Sr2+ are 8 ± 0.8 and 150 ± 12 s–1. While other factors may be involved, these results are consistent with a model in which a low Sr2+ concentration and an initial potential that is negative of the PZC lead to electrostatic repulsion of the negatively charged DNA backbone and the negatively charged electrode. This leads to the DNA adopting an extended configuration (concertina open), resulting in a slow rate of heterogeneous electron transfer. In contrast, for ferrocene reduction, the initial potential is positive of PZC and the negatively charged DNA is electrostatically attracted to the electrode (concertina closed), giving a shorter electron transfer distance and a higher rate of heterogeneous electron transfer. When the Sr2+ concentration is high, the charge on the DNA backbone is compensated by the electrolyte and the charge on the electrode dominates the electron transfer dynamics and the opposite potential dependence is observed. These results open up the possibility of electromechanical switching using DNA superstructures.

Effect of electron donor and acceptor on the photovoltaic properties of organic dyes for efficient dye-sensitized solar cells

In this work, the molecular engineering approach was applied to the organic dyes of AE, AQ, TE, and TQ having the donor-acceptor (D-A) configuration to investigate the solar cell efficiency. In these structures, dimethylaniline and triphenylamine were used as the electron donors and 1,1,4,4-tetracyanobuta-1,3-dienes (originating from TCNE) and 2-(4-(4,4-dicyanobut-3-en-2-ylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (originating from TCNQ) as the electron acceptors. The incorporation of tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) due to the strong electron-withdrawing ability enhance the performance of the dye-sensitized solar cell (DSSC). The results reveal that a change in the electron donor and acceptor moieties affects the photovoltaic processes. Also, the dyes having 2-(4-(4,4-dicyanobut-3-en-2-ylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile electron acceptor (AQ and TQ) have better charge transfer indices such as longer charge transfer distance (DCT) and lower overlap of electron-hole distribution (S) in comparison with the dyes having 1,1,4,4-tetracyanobuta-1,3-dienes as an electron acceptor (AE and TE). Moreover, the properties of the absorption spectra indicate that TCNQ-adduct dyes (AQ and TQ) illustrate a red-shift compared with TCNE-adduct dyes (AE and TE) due to the higher ability of the electron-withdrawing. Finally, AQ and TQ dyes are the preferred candidates to improve the efficiency of the studied DSSCs because of higher charge transfer, electron transfer rate constant, and lower chemical hardness.

Electrochemical characterization of graphene-type materials obtained by electrochemical exfoliation of graphite

The oxygenated functional groups of graphene oxide (GO) allow for its dispersion in water, biocompatibility and influence the heterogeneous electron transfer rate of redox processes. Therefore, it’s important to determine the experimental conditions leading to GO materials with the desired electrochemical properties for a given application. Herein, few layers electrochemically exfoliated graphene oxide (EGO) flakes were prepared by electrochemical exfoliation of graphite in 0.1 M H2SO4 and the effect of the electric field (applied voltage divided by the distance between the electrodes) was investigated. Raman and XPS analysis evidence two different regimes during the synthesis: slow kinetics of exfoliation at low voltage vs high concentration of OH• radicals at high voltage. The EGO sheets were successfully assembled on a glassy carbon electrode through an aminophenyl-film linker and characterized by cyclic voltammetry in 1 mM [Fe(CN)6]3-/4- to determine electrochemical surface area (ESA) and standard rate constant of electron transfer (k0). The two parameters scale with each other and are sensitive to the type of EGO, confirming the suitability of the platform used in this work to characterize the EGO materials. The highest ESA (0.08 cm2) and the high k0 (0.13 cm−1) were measured with EGO materials obtained at 8–10 V and 6 cm.

Disposable and low-cost electrochemical sensor based on the colorless nail polish and graphite composite material for tartrazine detection

A new method to manufacture electrochemical devices based on the graphite and colorless nail polish (N-grap) film was developed for tartrazine (Tz) detection. Scanning Electron Microscopy (SEM) demonstrates that the composite material presents a high porous carbon structure. Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were employed to electrochemically characterize the electrode material, which corroborates the porous structure of the N-graph due to the enhanced electroactive area (5.4-fold increase) and presented a heterogeneous electron transfer rate constant (k0) of 5.82 × 10−3 cm s−1 for potassium ferricyanide. The electrochemical determination of the Tz was carried out using square-wave voltammetry (SWV), under the optimized experimental conditions, which showed high sensitivity (0.793 A L mol−1) and a lower limit of detection (LOD) of 2.10 × 10−8 mol L−1 with a linear concentration ranging from 2.0 to 50.0 μmol L−1. The developed sensor was applied for the analysis of Tz in sports drink samples and the result obtained by N-grap device was statistically compared with a spectrophotometric method demonstrating good accordance and the accuracy of the proposed method. Based on these results, we believe that this new fabrication method to produce disposable and low-cost electrochemical devices can be an alternative method for in-field analysis of dye in commercial sport drink samples and other relevant applications.

A Mononuclear Non-Heme Manganese(III)–Aqua Complex in Oxygen Atom Transfer Reactions via Electron Transfer

Metal-oxygen complexes, such as metal-oxo [M(O2-)], -hydroxo [M(OH-)], -peroxo [M(O22-)], -hydroperoxo [M(OOH-)], and -superoxo [M(O2•-)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenylphosphine (Ph3P). However, OAT of metal-aqua complexes, [M(OH2)]n+, has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(III)-aqua complex, [(dpaq)MnIII(OH2)]2+ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), to PhSMe and Ph3P derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(III)-hydroxo complex, [(dpaq)MnIII(OH)]+ (2), to the substrates. Mechanistic studies reveal that OAT reaction of 1 occurs via electron transfer from 4-methoxythioanisole to 1 to produce the 4-methoxythioanisole radical cation and [(dpaq)MnII(OH2)]+, followed by nucleophilic attack of H2O in [(dpaq)MnII(OH2)]+ to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)2C6H3S•(OH)Me, which disproportionates or undergoes electron transfer to 1 to yield methyl 4-methoxyphenyl sulfoxide. Formation of the thioanisole radical cation derivatives is detected by the stopped-flow transient absorption measurements in OAT from 1 to 2,4-dimethoxythioanisole and 3,4-dimethoxythioanisole, being compared with that in the photoinduced electron transfer oxidation of PhSMe derivatives, which are detected by laser-induced transient absorption measurements. Similarly, OAT from 1 to Ph3P occurs via electron transfer from Ph3P to 1, and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and Ph3P derivatives, to 1 are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.