Quasi-reversible Electron Transfer Research Articles

Supercritical CO2 mediated construction of aluminium waste recovered γ-Al2O3 impregnated Dracaena trifasciata biomass-derived carbon composite: A robust electrocatalyst for mutagenic pollutant detection

Inspired by the waste-to-wealth concept, we have recovered the gamma phase aluminium oxide nanoparticles (γ-Al2O3 NPs) from waste aluminium (Al) foils and fabricated a composite with Dracaena trifasciata biomass-derived activated carbon matrix (DT-AC) using supercritical carbon-di-oxide (SC-CO2) pathway. The prepared samples are characterized altogether by various micro- and spectroscopic analyses. Based on the results, the recovered γ-Al2O3 NPs are well impregnated in the DT-AC surface by the action of the microbubble effect from the SC-CO2. The higher D-band and ID/IG value of 1.07 in the Al2O3/DT-AC nanocomposite indicate increased defects and the amorphous nature of the carbon materials. The effect of scan rate (ν) demonstrated greater linearity in ν1/2 vs peak current in the electrochemical detection study of the mutagenic pollutant 4-(methylamino) phenol hemi sulfate, showing a quasi-reversible electron transfer process undergoing diffusion-controlled kinetics. Furthermore, the limit of detection is determined to be 3.2 nM L–1 with an extensive linear range, spanning from 0.05 to 618.25 µM/L. The incredible sensitivity of 2.117 μA μM−1 cm−2, along with excellent selectivity, repeatability, and stability, is observed. Further, the respectable recovery percentage of 98.61 % in the environmental water sample is perceived. The observed outcomes suggest that the prepared Al2O3/DT-AC composite performs as an excellent electrocatalyst material, and the processing techniques used are thought to be sustainable in nature.

A new biosensing platform based on L-cysteine-capped Fe3O4 nanoparticles embedded in chitosan-MWCNT matrix: Electrochemical kinetic and sensing studies

We present a report on the electrochemical kinetics and sensing studies conducted on a flexible ITO-PET transducer, fabricated with a new nanocomposite of L-cysteine coated iron oxide nanoparticles (LCys@Fe3O4 NPs) and multi-wall carbon nanotubes (MWCNT), for the first time. Synthesized via the wet-chemical co-precipitation method, the LCys@Fe3O4 NPs exhibit a pure cubic phase. A nanocomposite of these LCys@Fe3O4 NPs are prepared via dispersion in chitosan (CS) solution along with MWCNT, and further drop-casted onto a low-cost, flexible ITO-PET sheet. The fabricated CS-MWCNT-LCys@Fe3O4/ITO-PET electrode shows a higher surface roughness, compared to either MWCNT or LCys@Fe3O4 modified electrode counterpart. The CS-MWCNT-LCys@Fe3O4 nanocomposite exhibits a twofold enhancement of the peak anodic voltammetric current, and a tenfold decrease in the charge transfer resistance, as compared to the LCys@Fe3O4 nanoparticles. Additionally, a mathematical model is devised to evaluate the kinetic parameters (ks, α & DFe(II)) of the redox species K4[Fe(CN)6]/K3[Fe(CN)6] on the CS-MWCNT-LCys@Fe3O4/ITO-PET surface, confirming a quasi-reversible electron transfer process. We couple this transducer with an immunosandwich format to facilitate rapid and ultrasensitive electrochemical measurement of protein biomarkers. Proof-of-concept is carried out by measuring the SARS-CoV-2 spike glycoprotein on the biofunctionalized Ab1/CS-MWCNT-LCys@Fe3O4/ITO-PET immunoelectrodes within a wide linear range (1 pg mL-1-1 μg mL-1). The immunoelectrode encompasses a limit of detection of 0.932 pg mL-1, and a sensitivity of 0.499 μA pg−1 mL cm−2. Thus, CS-MWCNT-LCys@Fe3O4 presents a new and excellent nanocomposite for biosensing application, while the Ab1/CS-MWCNT-LCys@Fe3O4/ITO-PET immunoelectrode can serve as an efficient, flexible, and cost-effective biosensing platform which can further be utilised to fabricate point-of-care biosensors.

In vitro biomedical corrosion and enzyme activity inhibition on modified Cu-Zn-Al bioalloy

Abstract The amperometric biosensor was created using a flat sheet of the Cu-Zn-Al shape memory alloy, with a shiny surface and uniform thickness below 1 mm. The high biocompatibility and a large specific surface area for enzyme loading are evaluated. In vitro biomedical corrosion testing of samples revealed successful immobilization of catalase, which has undergone quasi-reversible electron transfer from the surface and saline solution. A catalase that had been immobilized retained its basic structure and bioactivity and demonstrated a remarkable electrocatalytic response to hydrogen peroxide reduction. The reduction of hydrogen peroxide on the catalase-modified Cu-25.38Zn-3.3Al alloy was investigated using hydrodynamic amperometry in both the absence and presence of l-cysteine and K2[B3O3F4OH] inhibitors. Catalytic reduction currents have increased as a result of the gradual increase in hydrogen peroxide concentration. The study on enzyme activity inhibition has shown a lower corrosion rate of catalase-modified bioalloy than the initial sample because inhibitor ions occupy all sites of the immobilized enzyme.

Redox Active cAAC-Fluorene/Indene Systems Displaying Solvatochromism, Green Luminescence and pH Sensing: Functionalization of Fluorenyl/Indenyl Rings with Radical Carbene.

Two new series of air stable compounds of cAACX = fluorene/indene (X = Me2 , Et2 , Cy) [cAAC = cyclic (alkyl) amino carbene] have been isolated and well characterized by X-ray single crystal diffraction, photoluminescence, cyclic voltammogram (CV) and electron paramagnetic resonance (EPR) studies. Fluorescence studies reveals green light emission of cAAC bonded fluorene, whereas free fluorene generally displays a violet emission. Interestingly, the sterically crowded cAAC-fluorene analogue display solvatochromism and CF3 CO2 H sensing in solution. CV of the these compounds show a quasi-reversible electron transfer process, indicating the functionalization of fluorene/indene with radical anionic form of carbene, confirmed by CV/EPR measurements. DFT/TDDFT calculations and energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) have been carried out to study different aspects of bonding and electronic transitions. Such a class of redox active and thermally stable organic molecules may be suitable for molecule based spin memory devices in future.

Series of Fluorinated Benzimidazole-Substituted Nitronyl Nitroxides: Synthesis, Structure, Acidity, Redox Properties, and Magnetostructural Correlations.

A special series of nitronyl nitroxides was synthesized: 2-(benzimidazol-2'-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyls mono-, di-, tri-, or tetrafluorinated on the benzene ring. The structure of all paramagnets was unambiguously confirmed by single-crystal X-ray diffraction. It was found that in crystals, the radicals are assembled into chains due to intermolecular H-bonds between the benzimidazole moiety (H-bond donor) and the nitronyl nitroxide group or benzimidazole ring (H-bond acceptor). The magnetic properties of nitronyl nitroxides depend on the type of binding of radicals by H-bonds. The magnetic motif of 4-fluoro-, 5-fluoro-, 4,6-difluoro-, 4,5,6-trifluoro-, 4,5,7-trifluoro-, and 4,5,6,7-tetrafluoro-derivatives, as well as the nonfluorinated compound, consists of ferromagnetic chains (J/kB ≈ 20-40 K) formed by the McConnell type I mechanism. In the 5,6-difluoro- and 4,5-difluoro-derivatives, the distances between the paramagnetic centers are large, as a result of which the exchange interactions are weak. According to cyclic voltammetry, paramagnets are oxidized reversibly, while their reduction is a quasi-reversible electron transfer (EC mechanism); experimental redox potentials of radicals correlate well with the calculated values. Quantum chemical assessment of the acidity of benzimidazolyl-substituted nitronyl nitroxides revealed that the introduction of fluorine atoms into the benzene ring enhances the acidity of the paramagnets by more than 5 orders of magnitude.

Reversible and Quasireversible Electron Transfer under Conditions of Differential Square-Wave Voltammetry

A theoretical analysis of reversible and quasireversible electrode reactions of a dissolved redox couple under conditions of the novel technique of differential square-wave voltammetry (DSWV) is presented. The technique is recently introduced as a hybrid form between differential pulse voltammetry (DPV) and square-wave voltammetry (SWV) as the two most advanced and competitive pulse-form voltammetric techniques for a purpose of unifying their advantages and further advancing both techniques in terms of analytical performances, mechanistic analysis, and electrode kinetics. The potential modulation of DSWV consists of potential steps and pulses providing a plethora of voltammetric curves, which enable in-depth characterization of the electrode reaction with a minimal set of measurements. Based on numerical simulations, a set of criteria for characterization and differentiation between reversible and quasireversible electrode reactions are established.

Synthesis, Characterization and Bio-Potential Activities of Co(II) and Ni(II) Complexes with O and N Donor Mixed Ligands

The synthesis and characterization of Co(II) and Ni(II) mixed ligand complexes are derived from isoniazid, 9-fluorenoneandoxalate. The metal complexes were characterized on the basis of elemental analysis, IR, UV-visible, CV, PXRD, and molar conductance analytical data, viz., all the metal complexes were suggested in an octahedral geometry, respectively. The mixed ligand complexes are formed in the 1:1:2:1 (M:L1:L2:L3) ratios, as found from the elemental analyses, and originate to have the formula [M(L1)(L2)2(L3)]. Where M = Co(II), Ni(II), L1 = isoniazid, L2 = 9-fluorenone, and L3 = oxalate. The molar conductance data reveals that the complexes are non-electrolytes. The cyclic voltammogram of the Co(II) complex revealed that the quasi-reversible single electron transfer process and Ni(II) complex corresponding to a one-electron transfer process were observed during controlled potential electrolysis. IR spectra show that the ligands are coordinated to the metal ions through N and O donor sites of isoniazid-N, 9-fluorenone-O and oxalate-O. Magnetic moment values and UV-visible spectra were used to infer the coordinating of the geometrics of these complexes found to be octahedral. The PXRD patterns suggest that all the complexes are crystalline phases. The metal chelates have been screened for antimicrobial, antioxidant and anti-inflammatory activities, and our findings have been reported, explained and compared with some known antibiotics.

Multi-Analyte Sensor Based on Pencil Graphite Electrode for Riboflavin and Pyridoxine Determination

Riboflavin (VB2) and pyridoxine (VB6) are two of the eight water-soluble B vitamins that play an essential role and are correlated in normal function of the body. In the present study, cyclic voltammetry demonstrated that in Britton-Robinson buffer (pH 5.00) VB2 undergoes a quasi-reversible electron transfer reaction and the oxidation of VB6 is irreversible, both electrochemical systems being diffusion controlled at the pencil graphite electrode (PGE) surface. Also, a sensitive and selective voltammetric procedure has been developed for the simultaneous determination of the two compounds using PGE and square wave voltammetry (SWV). Two well-defined oxidation peaks with a voltammetric separation of more than 1.10 V were obtained in Britton-Robinson buffer (pH 5.00). The SWV curves exhibited linear responses with riboflavin and pyridoxine concentrations. Thus, for VB2 two linear concentration ranges of 1.00 · 10−7–5.00 · 10−5 M and 5.00 · 10−5–7.50 · 10−4 M were obtained and in the case of VB6 the linear response was between 2.50 · 10−5 M and 2.50 · 10−3 M. The detection limits for VB2 and VB6 were 7.38 · 10−8 M and 1.10 · 10−5 M, respectively. The applicability of PGE was successfully proved for simultaneous determination of VB2 and VB6 in pharmaceutical tablets with good accuracy and precision.

Analyzing electron transfer properties of ferrocene in gasoline by cyclic voltammetry and theoretical methods

Developing sensors to detect lead alternative additive like ferrocene (FC) in the gasoline is essential for automobile industries. In the current work, we have detected FC in commercial gasoline using carbon paste electrode (CPE). FC was interacted to CPE interface by grinding, immobilization, and mobilization method to understand its Electron transfer (ET) mechanism. ET properties of FC strongly depends on its interaction mode. FC displayed quasi-reversible ET behavior when they are interacted by grinding and immobilization method at the CPE interface but they showed reversible ET nature when they are interacted by mobilization method. However, mass transfer process is adsorption controlled at mobilization methods and diffusion controlled in remaining cases. As a part of work, we have also studied the mass transfer process of FC in gasoline which was also found to be diffusion controlled. Experimental results are further verified using density functional theory based calculations. Further development of this methodology will be helpful in future for developing electrochemical sensors to detect lead alternative additive in gasoline.

Electrochemical behavior of the monomeric and polymeric forms of N-phenyl-3-(phenylimino)-3H-phenothiazin-7-amine

Electrochemical behavior of new phenothiazine derivative, N-phenyl-3-(phenylimino)-3H-phenothiazin-7-amine (PhTz) and of the product of its electropolymerization has been investigated on glassy carbon electrode using electrochemical techniques. Monomeric PhTz is involved in quasi-reversible electron transfer complicated with adsorption of oxidized form. The transfer coefficient and standard heterogeneous constant were estimated in a wide pH range. In multiple cycling of the potential, the PhTz was oxidized to the polymeric film, which growth was confirmed by electrochemical impedance spectroscopy. Maximal deposition of the product was found for application of the upper potential of 1.3 V. Sulfate and nitrate anions promote electropolymerization whereas chloride ions inhibit the reaction. The reduction of polyPhTz depended on the number of the potential scans because of the increase of diffusional permeability with the number of cycles. A broad pH dependency of the potential, reversibility of the redox reactions and similarity of the behavior of monomeric and polymeric forms of PhTz make them attractive for future application in the assembly of electrochemical (bio)sensors. It was shown on the example of application of appropriately modified electrodes in acid-base and redox titration and in discrimination of native and damaged DNA adsorbed onto the polyPhTz layer based on the changes in the impedance parameters.

(Invited) Thin Layer Sonoelectrochemistry: Impacts on Slow Electron Transfer Kinetics without Visible Cavitation

Sonochemistry and sonoelectrochemistry are typically undertaken in a bulk fluid with sonication of sufficient intensity to generate visible cavitation. Sonoelectrochemistry was undertaken in a thin layer of fluid with a quartz crystal oscillator. In the thin layer configuration, no cavitation is visible and mass transport is not impacted, but impacts on electron transfer kinetics are observed. Because there is no visible cavitation, cyclic voltammetric responses typical of a quiescent electrolyte are observed. From the voltammograms for species that undergo rapid heterogeneous electron transfer such as outer sphere Ru(bpy)₃²⁺⇌Ru(bpy)₃³⁺+e, voltammetric morphology is unchanged on sonication of a thin layer of fluid. For Fe³+e⇌ Fe² that undergoes quasireversible (slower) electron transfer under cyclic voltammetric perturbation, sonication in a thin layer is found to increase the rate of heterogeneous electron transfer by roughly an order of magnitude. Sonoelectrochemical catalysis for inherently slow heterogeneous electron transfer processes is generated and observed in a thin layer of electrolyte. In the experiments reported here, the fluid layer was less than the wavelength of sonication. The aqueous electrolyte contains redox species and acid. Experiments were undertaken at a platinum wire electrode using a 41 kHz quartz crystal oscillator. For the same experimental conditions, no impact on the electron transfer reversible Ru(bpy)₃²⁺ was observed. All data were collected at a fixed frequency; impacts varied with oscillator intensity. As with many sonoelectrochemical experiments, subtle variation in experimental parameters complicate generation of reproducible results. Here, data generated by two different experimentalists yield similar outcomes. Best operating conditions differed slightly for the two experimentalists but the pattern of increased interfacial electron transfer rates for iron on sonication was found by both. Reference [1] Johna Leddy, Chester G. Duda, Jacob Lyon, and William J. Leddy III, "Thin Layer Sonoelectrochemistry and Sonoelectrochemistry Devices and Methods," published 28 May 2015 as US Patent Application 20150147594 A1.

Electrochemistry of Electroactive Species Loaded inside Polymeric Hydrogels. Effect of Viscoelastic Properties on Mass Transport and Heterogeneous Charge Transfer

The loading and mass transfer of electroactive species inside polymeric hydrogels is of interest in drug delivery, analytical chemistry and water remediation. The electrochemical properties of the species could be affected by the viscoelastic properties of the gel, local ionic force and pH, as well as interactions (e.g. hydrophobic) between the ions and the polymer chains. Usually, a thin film of the hydrogel is deposited on the electrode, loaded with electroactive species and electrochemically investigated. However, the mass transfer is controlled by finite diffusion and depends of an unknown, the thickness of the layer in the experimental conditions. On the other hand, we show here that a simple set-up, consisting of a disk electrode pressed on the soft hydrogel, allows performing electrochemistry of electroactve species (redox complexes, arsenite, nitrite) loaded inside the hydrogel matrix. The redox complexes are present as dilute solutions and the hydrogel dimensions fulfill the semi-infinite diffusion boundary conditions. Therefore, the data analysis can be performed using the theoretical framework developed for electrochemical measurements in liquid solutions. The physicochemical properties of the bulk hydrogels are evaluated by measuring the swelling kinetics, in the same conditions of the electrochemical measurements. Redox cations (e.g. tris(phenanthroline)iron(II) (TPFeII)) or anions (e.g. Fe(CN)6 -3, (FeCN) are loaded into anionic, neutral and cationic hydrogels showing no Donnan exclusion effects. The cyclic voltammetry of the loaded species show a quasireversible electron transfer mechanism (Eqre). The electrochemical parameters (diffusion coefficient and charge transfer constant) inside hydrogels are measured using chronoamperometry and digital simulation of the cyclic voltammetry. The Stokes-Einstein equation is used to calculate the effective viscosity of the hydrogel matrixes using the diffusion coefficients of redox complexes determined inside the hydrogels and in aqueous solutions (of known viscosities). The calculated viscosities correlate, with a negative slope, with the swelling rate constant of the hydrogel matrix. The heterogeneous charge transfer constants correlate with the calculated viscosities, revealing the effect of solvent dynamics on the charge transfer, according to Marcus theory for strongly adiabatic electron transfer. In that way, it is shown that the electrochemical measurements are able to monitor the local solvation properties of the hydrogel matrix. The method can be applied in electrochemical sensing of environmental contaminants. In the case of nitrite, the charge transfer is slow on glassy carbon (GC) electrodes. Therefore a redox catalyst (TPFeII) and nitrite are simultaneously absorbed in an anionic hydrogel and the redox electrocatalysis occurs inside the hydrogel matrix. On the other hand, the sensing of arsenite is made using Co oxide nanoparticles adsorbed on the GC electrode with arsenite loaded inside the hydrogel.

Manganese species in methane sulfonic acid as the solvent for zinc-manganese redox battery

Abstract Manganese(II)/Manganese(III) with a standard electrode potential of 1.51 V has been identified as a potential positive electrode for redox flow cells. The current work is to evaluate the use of methane sulfonic acid (MSA) as a solvent for Manganese(II)/Manganese(III) electric pairs as cathode materials applied to redox flow batteries (RFB). The electrochemical behavior of the MSA medium in Manganese(II)/Manganese(III) was studied by linear scanning voltammetry and cyclic voltammetry. The results show that the reaction of Manganese(II)/Manganese(III) on the graphite electrode is a quasi reversible single electron transfer step. The diffusion coefficient of Mn2+ was identified as 0.982 × 10−6 cm2 s−1 in 1 M MSA. As the concentration of MSA increases from 2 to 5 M, Mn3+ would be more stable, and the electrochemical kinetics of Manganese(II)/Manganese(III) couple would be better. When the MSA concentration reached 5 M, reduction kinetics of oxidized species Mn3+ decreased obviously. The results support the selecting of 4 M MSA. When the current density was 20 mA cm−2, the average energy efficiency and coulomb efficiency of the battery were 74.0% and 92.2% respectively. The average discharge voltage was 1.91 V and the average energy efficiency was 59%, which was higher than the total V cell. At the same time, the battery got a good charging and discharging cycle performance, which indicates that the zinc manganese oxide redox battery has a wide application prospect.

Voltammetric and adsorption study of 4-nitrophenyl-triazole-labeled 2′-deoxycytidine and 7-deazaadenosine nucleosides at boron-doped diamond electrode

Electrochemical properties of 4-nitrophenyl triazole labeled 2′-deoxycytidine (dCTRNO2) and 7-deazaadenosine (dATRNO2) were studied using alumina-polished boron doped diamond (BDD) electrode in acetate buffer pH5.0. Cyclic voltammetry revealed peak-shaped signals for reduction of the nitro group at the potentials of ca −0.6V (vs. AgƖAgClƖ3molL−1 KCl reference electrode) and for the hydroxylamine/nitroso pair exhibiting quasireversible electron transfer at potentials comparable with redox processes of other nitro aromatic compounds at BDD electrodes. All electrochemical processes are directed by adsorption. Moreover, ex situ adsorption of dATRNO2 and transfer to supporting electrolyte enables its quantitation using square wave voltammetry in small (several microliter) sample volumes after optimization of parameters for the hydroxylamine/nitroso redox couple. Competitive adsorption/desorption at the BDD surface was further studied by cyclic voltammetry with dATRNO2 in the presence of electrochemically inactive phenyl triazole 7-deazaadenosine conjugate (dATR), revealing mutual displacement of both compounds at BDD surface. Further, the redox processes of simpler aromatic nitro compounds (4-nitrophenyl acetylene and 2-nitronaphthalene) are strongly affected when the BDD surface is covered by dATR.

Electroactive Biofilms for Sensing: Reflections and Perspectives.

Microbial electrochemistry has from the onset been recognized for its sensing potential due to the microbial ability to enhance signals through metabolic cascades, its relative selectivity toward substrates, and the higher stability conferred by the microbial ability to self-replicate. The greatest challenge has been to achieve stable and efficient transduction between a microorganism and an electrode surface. Over the past decades, a new kind of microbial architecture has been observed to spontaneously develop on polarized electrodes: the electroactive biofilm (EAB). The EAB conducts electrons over long distances and performs quasi-reversible electron transfer on conventional electrode surfaces. It also possesses self-regenerative properties. In only a few years, EABs have inspired considerable research interest for use as biosensors for environmental or bioprocess monitoring. Multiple challenges still need to be overcome before implementation at larger scale of this new kind of biosensors can be realized. This perspective first introduces the specific characteristics of the EAB with respect to other electrochemical biosensors. It summarizes the sensing applications currently proposed for EABs, stresses their limitations, and suggests strategies toward potential solutions. Conceptual prospects to engineer EABs for sensing purposes are also discussed.

Facile preparation of poly(methylene blue) modified carbon paste electrode for the detection and quantification of catechin

Free radicals are formed as byproducts of metabolism, and are highly unstable due to the presence of unpaired electrons. They readily react with other important cellular components such as DNA causing them damage. Antioxidants such as (+)-catechin (CAT), neutralize free radicals in the blood stream. Hence there is a need for detection and quantification of catechin concentration in various food sources and beverages. Electro-oxidative properties of catechin were investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). A carbon paste working electrode modified by electropolymerizing methylene blue (MB) was fabricated. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) techniques were used to study the surface morphology of the electrode. Quasi-reversible electron transfer reaction occurred at +0.260V through a diffusion controlled process. In comparison to the bare carbon paste electrode (CPE), there was a significant 5.3 times increment in anodic current sensitivity at the modified electrode at physiological pH. Our findings indicate that for the electro-oxidation of CAT, CPE is a better base material for electropolymerization of MB compared to glassy carbon electrode (GCE). Nyquist plot followed the theoretical shape, indicating low interfacial charge transfer resistance of 0.095kΩ at the modified electrode. Calibration plots obtained by DPV were linear in two ranges of 1.0×10−3 to 1.0×10−6 and 1.0×10−7 to 0.1×10−8M. The limit of detection (LOD) and limit of quantification (LOQ) was 4.9nM and 14nM respectively. Application of the developed electrode was demonstrated by detecting catechin in green tea and spiked fruit juice with satisfactory recoveries. The sensor was stable, sensitive, selective and reproducible.

Electroactive polymers made by loading redox ions inside crosslinked polymeric hydrogels. Effects of hydrophobic interactions and solvent dynamics

Electroactive polymers are made by loading redox complexes inside different polymeric crosslinked hydrogels. A redox cation (tris(phenanthroline)iron(II) (TPFeII) is sorbed in anionic, neutral and cationic hydrogels while redox anions (ferricyanide) is taken up inside an anionic hydrogel. Moreover, a redox anion (nitrite) and a cation (TPFeII) are simultaneously absorbed in an anionic hydrogel. Homopolymeric hydrogels of 2-acrylamidopropanesulfonic acid (PAMPS), acrylic acid (PAA) or (3-acrylamidopropyl)trimethylammonium chloride) (PAPMTAC), crosslinked with bisacrylamide, are synthesized by radical polymerization. The same method is used to produce a 1:1 copolymeric hydrogel of acrylic acid and 2-acrylamidopropanesulfonic acid (PAA-co-AMPS). The physicochemical properties of the hydrogels are evaluated by measuring the swelling kinetics, in the same conditions of the electrochemical measurements. The cyclic voltammetric response of all electroactive polymers show quasireversible electron transfer (Eqrev mechanism) while the hydrogel loaded with TPFeII and nitrite show a catalyzed oxidation (EqrevC mechanism). The electrochemical parameters (diffusion coefficient and charge transfer constant) of TPFeII loaded inside PAMPS, PAA and PAA-co-AMPS are measured using chronoamperometry and digital simulation of the cyclic voltammetry. The Stokes-Einstein equation is used to calculate the effective viscosity of the hydrogel matrixes using the diffusion coefficients of a redox complex (TPFeII) determined inside the hydrogels and the same parameters determined in aqueous solution. The calculated viscosity correlates, with a negative slope, with the swelling rate constant of the hydrogel matrix. The heterogeneous charge transfer of the redox complex inside ionomers (PAMPS and PAA-co-PAMPS) is nearly as fast as in solution, while the charge transfer inside the neutral PAA is ca. 100 times smaller. The measured charge transfer constants correlate with the calculated viscosity, revealing the effect of solvent dynamics on the charge transfer, according to Marcus theory for strongly adiabatic electron transfer. In that way, it is shown that the electrochemical measurements are able to monitor the local solvation properties of the hydrogel matrix. These results suggest novel ways to produce electroactive polymers by loading redox active substances inside hydrogels, even when the hydrogel matrix is neutral or bear the same charge than the redox probe. The redox complexes are present as dilute solutions and the hydrogel dimensions fulfill the semi-infinite diffusion boundary conditions. Therefore, the data analysis can be performed using the theoretical framework for electrochemical measurements in liquid solvents.

Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production.

A large variety of molecular cobalt complexes are used as electrocatalysts for H2 production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient Co(III)H and Co(II)H intermediates of electrocatalytic H2 production by [Co(II)(P(tBu)2N(Ph)2)(CH3CN)3](2+) and Co(II)(dmgBF2)2(CH3CN)2. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the Co(II/I) couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a Co(III)H intermediate to Co(II)H, which is then protonated by acid to generate H2. A bridge-protonated Co(I) species was ruled out as a catalytic intermediate for Co(II)(dmgBF2)2(CH3CN)2 from voltammograms recorded at 1000 psi of H2. Density functional theory was used to calculate Co(III)-H and Co(II)-H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co-H bond strengths for the Co(III)H and Co(II)H intermediates.

Determination of tryptamine in foods using square wave adsorptive stripping voltammetry

Tryptamine, a biogenic amine, is an indole derivative with an electrophilic substituent at the C3 position of the pyrrole ring of the indole moiety. The electrochemical oxidation of tryptamine was investigated using glassy carbon electrode (GCE), and focusing on trace level determination in food products by square wave adsorptive stripping voltammetry (SWAdSV). The electrochemical responses of tryptamine were evaluated using differing voltammetric techniques over a wide pH range, a quasi-reversible electron-transfer to redox system represented by coupled peaks P1–P3, and an irreversible reaction for peak P2 were demonstrated. The proton and electron counts associated with the oxidation reactions were estimated. The nature of the mass transfer process was predominantly diffusion-limited for the oxidation process of P1, the most selective and sensitive analytical response (acetate buffer solution pH 5.3), being used for the development of SWAdSV method, under optimum conditions. The excellent response allowed the development of an electroanalytical method with a linear response range of from 4.7–54.5)×10−8molL−1, low detection limit (0.8×10−9molL−1), and quantification limit (2.7×10–9molL−1), and acceptable levels of repeatability (3.6%), and reproducibility (3.8%). Tryptamine content was determined in bananas, tomatoes, cheese (mozzarella and gorgonzola), and cold meats (chicken sausage and pepperoni sausage), yielding recoveries above 90%, with excellent analytical performance using simple and low cost instrumentation.

Sensitive and selective determination of riboflavin (vitamin B2) based on boron-doped diamond electrode

In this paper, we present electrochemical behavior and determination of riboflavin in pharmaceutical tablets by square wave voltammetry using unmodified boron-doped diamond electrode. In Britton–Robinson buffer solution at pH 2, by employing cyclic voltammetry it is demonstrated that riboflavin undergoes a diffusion-controlled quasi-reversible electron transfer reaction. The experimental parameters, such as pH of the supporting electrolyte, square wave voltammetry parameters, influence of most common interferences, were examined. Under optimized experimental conditions in Britton–Robinson buffer solution at pH 2, linear calibration curves were obtained in the wide range from 0.02 to 35 µM, with the capability of detection of 0.0037 µM. Repeatability of ten measurements was calculated at four concentrations from linear range and all values were <1.8 %. Proposed method was successfully applied for the determination of the total vitamin content in pharmaceutical preparations and urine samples. The developed approach could be beneficial for the quantification of vitamin in drugs using boron-doped diamond electrode as up-to-date electrochemical sensor and could represent inexpensive analytical alternative to separation methods.