Standard Heterogeneous Rate Constant Research Articles

Electrokinetics of Nitrite to Ammonia Conversion in the Neutral Medium Over A Platinum Surface.

Polycrystalline Pt electrode was employed to selectively convert nitrite ions ( ) into useful nitrogenous compound through electrochemical reduction reaction in neutral medium. According to adsorptive stripping analysis, the reduction process produced nitric oxide (NO) on the surface of Pt electrode. The spectroscopic test and gas chromatographic studies discovered the presence of ammonia (NH3) in the electrolyzed solution, suggesting the transformation of adsorbed NO into NH3 during the reverse scan. Scan rate dependent investigation was performed to elucidate kinetic information relating to this reaction on Pt surface. From Ep vs scan rate (υ) and jp vs υ (logarithmic plot), it was found that the conversion of ion into NO is an irreversible reaction which relies on the diffusion of ions to electrode surface. The Tafel analysis unveiled that the first electron transfer sets the overall reaction rate, having formal reduction potential, E0'=-0.46 V and standard heterogeneous rate constant, k0= cm s-1. Reductive transfer coefficient (α) is another kinetics parameter, which was found to be approximate 0.77 from the difference between Ep and Ep/2 of the voltammograms obtained over scan rate range 0.005 V s-1 to 0.250 V s-1, indicating a stepwise process. According to temperature-dependent voltammograms, the nitrite reduction reaction on Pt had a calculated activation energy of about 19.8 kJ mol-1 and a pre-exponential factor of about 8.39×103 mA cm-2.

An on-site and portable electrochemical sensing platform based on spinel zinc ferrite nanoparticles for the quality control of paracetamol in pharmaceutical samples.

In this study, crystalline spinel zinc ferrite nanoparticles (ZnFe2O4 NPs) were successfully prepared and proposed as a high-performance electrode material for the construction of an electrochemical sensing platform for the detection of paracetamol (PCM). By modifying a screen-printed carbon electrode (SPE) with ZnFe2O4 NPs, the electrochemical characteristics of the ZnFe2O4/SPE and the electrochemical oxidation of PCM were investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and differential pulse voltammetry (DPV) methods. The calculated electrochemical kinetic parameters from these techniques including electrochemically active surface area (ECSA), peak-to-peak separation (ΔEp), charge transfer resistance (Rct), standard heterogeneous electron-transfer rate constants (k0), electron transfer coefficient (α), catalytic rate constant (kcat), adsorption capacity (Γ), and diffusion coefficient (D) proved that the as-synthesized ZnFe2O4 NPs have rapid electron/mass transfer characteristics, intrinsic electrocatalytic activity, and facilitate the adsorption-diffusion of PCM molecules towards the modified electrode surface. As expected, the ZnFe2O4/SPE offered excellent analytical performance towards sensing of PCM with a detection limit of 0.29 μM, a wide linear range of 0.5-400 μM, and high electrochemical sensitivity of 1.1 μA μM-1 cm-2. Moreover, the proposed ZnFe2O4-based electrochemical nanosensor also exhibited good repeatability, high anti-interference ability, and practical feasibility toward PCM sensing in a pharmaceutical tablet. Based on these observations, the designed electrochemical platform not only provides a high-performance nanosensor for the rapid and highly efficient detection of PCM but also opens a new avenue for routine quality control analysis of pharmaceutical formulations.

Bio-based hierarchical vertically aligned 2D ZnO nanostructures for ultra selective electrochemical sensing of p-Chloroaniline

Herein, we report synthesis of hierarchical 2D vertically aligned nanostructures (VANs) of ZnO onto the surface of FTO glass via bioinspired route for targeted electrocatalytic sensing of p-Chloroaniline (p-CA). The phytochemicals present in vegetal extract of Sechiun edule were employed to grow and form stable ZnO VANs onto FTO (ZnO-VANs(bio)/FTO). The morphological characterizations of ZnO-VANs(bio)/FTO showed the formation of vertical nanostructures (length ∼ 820 nm) with high density and good crystallinity. It exhibited an increased surface roughness 2.2-fold higher than seeded FTO (ZnO@seed/FTO). The ZnO-VANs(bio)/FTO@pH7 electrode showed a decrease in electron transfer resistance from 7.78 to 3.02 MΩ with 1.2-fold increase in effective surface area than FTO(bare). The oxidation of p-CA included an irreversible one-electron oxidation mechanism with formal potential (E0) of 0.978 V vs. Ag/AgCl and standard heterogeneous rate constant (k) of 0.737 s−1. ZnO-VANs(bio)/FTO@pH7 exhibited remarkable electrocatalytic sensing of p-CA with low limit of detection (0.021 µM) and excellent sensitivity (0.194 µA·µM−1·cm−2) between a linear window of 2–200 µM. The detection limit of fabricated sensor for p-CA sensing was about 2-fold lower than FTO(bare). Chlorobenzene, carbendazim, profenofos, nitrite, 4-nitrophenol, bisphenol a, and mercury could not interfere p-CA sensing, and only 4.7% reduction in peak current was observed upto 9th cycle. The ZnO-VANs(bio)/FTO@pH7 could efficiently sense p-CA in spiked river and residential water samples, comparable with HPLC (max 4.6% deviation). Therefore, ZnO VANs synthesized in a bio-based route is a potential electrocatalyst for binder-free sensor development with high stability and low detection limit for sensing pesticides and their intermediates.

Experimental and theoretical approaches to interactions between DNA and purine metabolism products

Deoxyribonucleic acid (DNA) is a significant target for small organic and inorganic drug molecules. Understanding the DNA interaction mechanism of these molecules is vital for new drug designs. In this work, interactions between xanthine (XT), theophylline (TP), and theobromine (TB) with calf-thymus double-strained DNA (dsDNA) were monitored via an experimental and theoretical approach. Experimentally, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques were used on the surface of the NiO/MWCNT/NNaM/PGE electrochemical platform in vitro. Kinetic parameters, including diffusion coefficients, surface concentrations, and standard heterogeneous rate constants, were measured in the absence and presence of DNA using scan rate studies. In the presence of DNA, kinetic parameters were observed to be reduced significantly. Thermodynamic parameters, such as DNA binding constants and standard free Gibbs energies, were calculated for each molecule using the CV and DPV techniques. Both techniques suggested a binding affinity order of XT > TB > TP. Theoretically, density functional theory was applied for geometry optimization, natural bond orbital analyses, and molecular orbital energies of XT, TP, and TB. Experimental and theoretical binding affinities confirm each other. The most energetically stable ligand-DNA complexes expressed that XT, TP, and TB interact with dsDNA via minor groove binding mode, using mostly hydrogen bonds.

Electro-sensing layer constructed of a WO3/CuO nanocomposite, for the electrochemical determination of 2-phenylphenol fungicide

The abstract highlights the development of an electroanalytical sensor for the detection of 2-phenylphenol (2-PPL) as a contaminant. The novelty of the experiment lies in the utilization of a 1-D nanostructured WO3/CuO nanocomposite integrated with a carbon paste electrode (CPE). The hydrothermal method was used to synthesize the WO3 NPs, which were then characterized using Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) techniques. Tungsten oxides (WO3) have been the subject of extensive study because of their many desirable characteristics, including their ease of preparation, tunable stoichiometry, crystal structure, particle morphology, 2.6 eV bandgap, excellent photocatalytic oxidation capacity, non-toxic nature, and widespread availability. The narrow band gap in CuO makes it an ideal sensing material. Copper oxide has applications in many different industries because it is a semiconductor metal with a narrow band gap in the spectrum of 1.2–1.9 eV and unique optical, electrical, and magnetic properties. Techniques like cyclic voltammetry (CV), and square wave voltammetry (SWV) were used. Real sample analysis was carried out in real-world samples like different types of soil, vegetables, and water. The electroanalytical sensor showed outstanding catalytic behavior by enhancing the peak current of the 2-phenylphenol with the potential shift to the less positive side compared to the unmodified carbon paste electrode in the presence of pH 7.0 phosphate buffer solution (PB). Throughout the experimental study, double distilled was used. Various electro-kinetic parameters like pH, accumulation time study, scan rate, concentration variation, standard heterogeneous rate constant, and participation of electrons, accumulation time, and transfer coefficient have been studied at WO3/CuO/CPE. The limit of detection was quantified together with the limit of quantification. Possible electrochemical oxidation mechanism of the toxic molecule was depicted. Overall, this research contributes to the field of electroanalytical sensing and offers potential applications in environmental monitoring.

Voltammetric Evidence of Proton Transport through the Sidewalls of Single-Walled Carbon Nanotubes.

Understanding ion transport in solid materials is crucial in the design of electrochemical devices. Of particular interest in recent years is the study of ion transport across 2-dimensional, atomically thin crystals. In this contribution, we describe the use of a host-guest hybrid redox material based on polyoxometalates (POMs) encapsulated within the internal cavities of single-walled carbon nanotubes (SWNTs) as a model system for exploring ion transport across atomically thin structures. The nanotube sidewall creates a barrier between the redox-active molecules and bulk electrolytes, which can be probed by addressing the redox states of the POMs electrochemically. The electrochemical properties of the {POM}@SWNT system are strongly linked to the nature of the cation in the supporting electrolyte. While acidic electrolytes facilitate rapid, exhaustive, reversible electron transfer and stability during redox cycling, alkaline-salt electrolytes significantly limit redox switching of the encapsulated species. By "plugging" the {POM}@SWNT material with C60-fullerenes, we demonstrate that the primary mode of charge balancing is proton transport through the graphenic lattice of the SWNT sidewalls. Kinetic analysis reveals little kinetic isotope effect on the standard heterogeneous electron transfer rate constant, suggesting that ion transport through the sidewalls is not rate-limiting in our system. The unique capacity of protons and deuterons to travel through graphenic layers unlocks the redox chemistry of nanoconfined redox materials, with significant implications for the use of carbon-coated materials in applications ranging from electrocatalysis to energy storage and beyond.

An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces.

The fundamental process in non-aqueous redox flow battery (NRFB) operation revolves around electron transfer (ET) between a current collector electrode and redox-active organic molecules (redoxmers) in solution. Here, we present an approach utilizing scanning electrochemical microscopy (SECM) to evaluate interfacial ET kinetics between redoxmers and various electrode materials of interest at desired locations. This spot-analysis method relies on the measurement of heterogeneous electron transfer rate constants (kf or kb ) as a function of applied potential (E-E0 '). As demonstrated by COMSOL simulations, this method enables the quantification of Butler-Volmer kinetic parameters, the standard heterogeneous rate constant, k0 , and the transfer coefficient, α. Our method enabled the identification of inherent asymmetries in the ET kinetics arising during the reduction of ferrocene-based redoxmers, compared to their oxidation which displayed faster rate constants. Similar behavior was observed on a wide variety of carbon electrodes such as multi-layer graphene, highly ordered pyrolytic graphite, glassy carbon, and chemical vapor deposition-grown graphite films. However, aqueous systems and Pt do not exhibit such kinetic effects. Our analysis suggests that differential adsorption of the redoxmers is insufficient to account for our observations. Displaying a greater versatility than conventional electroanalytical methods, we demonstrate the operation of our spot analysis at concentrations up to 100 mM of redoxmer over graphite films. Looking forward, our method can be used to assess non-idealities in a variety of redoxmer/electrode/solvent systems with quantitative evaluation of kinetics for applications in redox-flow battery research.

Fundamentals on kinetics of hydrogen redox reaction at a polycrystalline platinum disk electrode

• It is first reported that hydrogen redox reaction is a diffusion-controlled process. • Diffusion coefficient of H + is obtained as (9.7±0.9)×10 −5 cm 2 ·s −1 in 0.5 M KCl-HCl. • Diffusion coefficient of H 2 is found to be (6.3±0.9)×10 −5 cm 2 ·s −1 . • Standard heterogeneous rate constant is estimated to be ca. 6.5×10 −2 cm·s −1 . The hydrogen redox reactions are comprehensively investigated as key reactions in the electrochemistry and hydrogen production technology. However, the kinetic processes for hydrogen redox reactions are not fully elucidated. Herein, the kinetics of hydrogen redox reaction at polycrystalline platinum disk electrodes are studied in acidic solution and the diffusion-controlled reaction mechanisms are proposed. By analyzing the results obtained from cyclic voltammograms at different scan rates at pH 2.05, the diffusion coefficients of H + and H 2 are calculated to be (9.7 ± 0.9) × 10 −5 and (6.3 ± 0.9) × 10 −5 cm 2 ·s −1 respectively, which are in good agreement with those obtained at different values of pH, as well as the previous reports. The corresponding kinetic standard heterogeneous rate constant ( k 0 ) is estimated to be 6.5 × 10 −2 cm·s −1 . This work may be helpful and universal to understand electrochemical reaction kinetic questions and can be extended to other similar systems for development of highly efficient catalysts toward hydrogen redox reactions.

Cyclic voltammetric studies of electron transfer reactions associated with bioactive iron–KA complexation at different biological pH

This study explores electrochemical interactions of a pharmaceutical drug, kojic acid (5-hydroxy-2-hydroxymethyl-4 H-pyran-4-one, KA), with bioactive iron Fe(III) at different pH systems by using cyclic voltammetric technique. For this study, buffers relevant to physiological pH of blood and digestive system have been selected. Bioactive iron was found to form an electroactive complex [Fe(III)L3] in the presence of KA at all studied pH, whereas electrochemical behavior of KA was also changed by complex formation. This electrochemical investigation reveals that under the experimental conditions, [Fe(III)L3] complex undergoes a diffusion controlled, one electron redox process that is quasi-reversible in nature. It was also found that [Fe(III)L3] complex undergoes coupled chemical reaction according to EC (an electron transfer reaction followed by a chemical reaction) mechanism, and this interaction is pH dependent. Modified Nicholson and Shain, Gileadi, and Kochi methods were used to determine standard heterogeneous electron transfer rate constant ( k0) of [Fe(III)L3] complex. It was found to be in the range of 10−3 to 10−5 cm/s.

Fabrication of an Organofunctionalized Talc-like Magnesium Phyllosilicate for the Electrochemical Sensing of Lead Ions in Water Samples.

A talc-like magnesium phyllosilicate functionalized with amine groups (TalcNH2), useful as sensor material in voltammetry stripping analysis, was synthesized by a sol–gel-based processing method. The characterizations of the resulting synthetic organoclay by scanning electron microscopy (SEM), X-ray diffraction, N2 sorption isotherms (BET method), Fourier transform infrared spectroscopy (FTIR), CHN elemental analysis and UV–Vis diffuse reflectance spectroscopy (UV–Vis-DRS) demonstrated the effectiveness of the process used for grafting of amine functionality in the interlamellar clay. The results indicate the presence of organic moieties covalently bonded to the inorganic lattice of talc-like magnesium phyllosilicate silicon sheet, with interlayer distances of 1568.4 pm. In an effort to use a talc-like material as an electrode material without the addition of a dispersing agent and/or molecular glue, the TalcNH2 material was successfully dispersed in distilled water in contrast to natural talc. Then, it was used to modify a glassy carbon electrode (GCE) by drop coating. The characterization of the resulting modified electrode by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) revealed its charge selectivity ability. In addition, EIS results showed low charge transfer resistance (0.32 Ω) during the electro-oxidation of [Fe(CN)6]3−. Kinetics studies were also performed by EIS, which revealed that the standard heterogeneous electron transfer rate constant was (0.019 ± 0.001) cm.s−1, indicating a fast direct electron transfer rate of [Fe(CN)6]3− to the electrode. Using anodic adsorptive stripping differential pulse voltammetry (DPV), fast and highly sensitive determination of Pb(II) ions was achieved. The peak current of Pb2+ ions on TalcNH2/GCE was about three-fold more important than that obtained on bare GCE. The calculated detection and quantification limits were respectively 7.45 × 10−8 M (S/N = 3) and 24.84 × 10−8 M (S/N 10), for the determination of Pb2+ under optimized conditions. The method was successfully used to tap water with satisfactory results. The results highlight the efficient chelation of Pb2+ ions by the grafted NH2 groups and the potential of talc-like amino-functionalized magnesium phyllosilicate for application in electrochemical sensors.

Electron Transfer Kinetics in Ethaline/Water Mixtures: An Apparent Non‐Marcus Behavior in a Deep Eutectic Solvent

AbstractEffects of water on the charge‐transfer (CT) kinetics in deep eutectics solvents (DES) have been investigated in ethaline (1 : 2 choline chloride+ethylene glycol), taken as a prototypical example of a hydrophilic DES. Standard heterogeneous CT rate constants ks were measured for two redox couples: 1,1’‐ferrocene‐dimethanol/1,1’‐ferrocenium‐dimethanol and ferro‐/ferricyanide on a glassy carbon electrode in ethaline as function of the water amount in the DES. Contrarily to the behavior reported in classical solvents and in apparent contradiction with the Marcus Theory, ks values show very little or no variation with the amount of water in the DES or the changes of viscosities or diffusion coefficients that are observed. This unexpected phenomenon is discussed as function of the known physical‐chemistry parameters of ethaline.

Fundamentals on kinetics of electrochemical reduction of CO2 at a bismuth electrode

Electrochemical reduction of CO2 into renewable carbon–neutral fuels has been extensively studied. Current attentions mainly focus on the development of high-performance materials and molecular dynamics, fundamentals on kinetics of electrochemical reduction of CO2 are still unclear. Herein, we design a simplified electrochemical process, CO2-saturated K2SO4 solution at bismuth electrodes, to elucidate the electrochemical reaction kinetics of CO2 reduction reaction. A totally irreversible process for CO2 reduction reaction occurs and this reaction can be described as CO2 + H2O + 2e− → HCOO− + OH−. It is firstly reported that the reduction of CO2 at bismuth electrodes is of a diffusion-controlled process, and the diffusion coefficient of CO2 is (1.98 ± 0.22) × 10−5 cm2·s−1. From the well-defined linear sweep voltammograms, electron transfer coefficient is obtained as 0.18 ± 0.01. Combining with the half-wave potential (−1.503 ± 0.002 V vs. Ag/AgCl (sat. KCl)) from differential pulse voltametric results, the standard heterogeneous rate constant is estimated to be (3.4 ± 0.2) × 10−3 cm·s−1. These new findings may identify electrochemical reaction kinetic questions and be helpful to understanding the electrochemical conversion between CO2 and carbon–neutral renewable energy.

Electrochemical study on molten iron-carbon electrodes in slag and their decarburization by slag/metal reaction

An electrochemical study on molten iron-carbon electrodes in slag and their decarburization during anodic polarization was performed at 1600 °C to obtain information relevant to steelmaking. Carbon concentration was varied from 4.5 to 0.2 wt% to cover the range of interest between pig iron tapped from blast furnaces and steel tapped from oxygen converters or electric arc furnaces. Impedance spectra were measured at the rest potential and at applied direct current biases, and results were fitted to a faradaic decarburization reaction proceeding through two adsorbed intermediates. At the rest potential, carbon significantly increases faradaic activity at the metal/slag interface as evidenced by a strong increase in exchange current and interfacial capacitance with carbon activity. The apparent standard heterogeneous rate constant of the anodic decarburization half-cell was determined using the dependence of exchange current on carbon activity. During electrochemical decarburization, interfacial capacitance decreases significantly which is consistent with changes in surface concentrations of intermediates. Using a comprehensive kinetic model, surface concentrations of intermediate adsorbed species were determined during anodic polarization in the Tafel region. Results from impedance spectroscopy were compared with those obtained by gas chromatography during constant-current decarburization.

A Reliable Cyclic Voltammetry Technique for the Degradation of Salicylaldehyde: Electrode Kinetics

Salicylaldehyde (SA) is used in numerous biological, pharmaceutical, and industrial applications. Releasing effluents from these industries contaminates water. So the degradation of salicylaldehyde is necessitated. The electrochemical degradation of salicylaldehyde in buffered media was studied using the eco-friendly cyclic voltammetry (CV) technique on a platinum electrode at different scan rates. Kinetic and electrochemical parameters were evaluated for the reaction such as standard heterogeneous rate constant (k0,2.468×103 s-1 ), anodic electron transfer rate constant (kox,2.507×103 s-1), electron transfer coefficient of reaction (?,0.673), and formal potential (E0, 1.0937) under the influence of scan rate. The nature of the reaction is found to be diffusion controlled. The concentration study in the range of 1 mM to 4 mM was calibrated. The limit of detection and the limit of quantification were calculated to be 0.0031 mM and 0.0103 mM respectively.

Polydopamine nanofilms for high‐performance paper‐based electrochemical devices

Since the discovery of polydopamine (PDA), there has been a lot of progress on using this substance to functionalize many different surfaces. However, little attention has been given to prepare functionalized surfaces for the preparation of flexible electrochemical paper-based devices. After fabricating the electrodes on paper substrates, we formed PDA on the surface of the working electrode using a chemical polymerization route. PDA nanofilms on carbon were characterized by contact angle (CA) experiments, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy (topography and electrical measurements) and electrochemical techniques. We observed that PDA introduces chemical functionalities (RNH2 and RC═O) that decrease the CA of the electrode. Moreover, PDA nanofilms did not block the heterogeneous electron transfer. In fact, we observed one of the highest standard heterogeneous rate constants (ks ) for electrochemical paper-based electrodes (2.5 ± 0.1) × 10-3 cm s-1 , which is an essential parameter to obtain larger currents. In addition, our results suggest that carbonyl functionalities are ascribed for the redox activity of the nanofilms. As a proof-of-concept, the electrooxidation of nicotinamide adenine dinucleotide showed remarkable features, such as, lower oxidation potential, electrocatalytic peak currents more than 30 times higher when compared to unmodified paper-based electrodes and electrocatalytic rate constant (kobs ) of (8.2 ± 0.6) × 102 L mol-1 s-1 .

Electrochemical characteristics of molten iron electrodes in slag and electrochemical properties of their interface

Kinetic characteristics of molten iron/slag reactions and interfacial properties between molten iron/slag are of profound importance in steelmaking but quantitative electrochemical descriptions are lacking. Therefore, a detailed electrochemical investigation of pure (carbon-free) molten iron in slag was performed in the present work, using mostly impedance spectroscopy. Measurements were made between 1600–1700 °C at the rest potential and also at direct current biases to elucidate changes in interfacial properties when metal/slag reactions take place. Various fundamental physiochemical properties were measured for the first time including exchange currents, standard heterogeneous rate constants, Nernst diffusion layer thicknesses in slag, differential capacitance curves, potentials of zero charge, excess charge density curves, and electrocapillary curves. Results and discussion inform about the electric double layer formed between molten iron and slag.

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.

Electrochemical and surface enhanced infrared absorption spectroscopy studies of TEMPO self-assembled monolayers

The electron transfer kinetics of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) monolayers have been studied using both a chronocoloumetric method and surface sensitive infrared spectroelectrochemistry (attenuated total reflection surface enhanced infrared absorption spectroscopy, ATR-SEIRAS). Redox active self-assembled monolayers (SAMs) are constructed by covalently attaching 4-amino-TEMPO to preformed monolayers of different chain length alkanoic acid thiol monolayers. The density of TEMPO groups on the resulting Cn-TEMPO (n = 6, 8, 11, 12, 16) SAMs is approximately 20% of a full monolayer coverage and the resulting SAMs exhibit very little kinetic heterogeneity. Standard heterogeneous rate constants are measured as a function of SAM chain length. Tafel plots constructed for n = 12 show asymmetry between the forward (oxidation of the radical to the nitrosonium cation) and reverse (reduction to the TEMPO radical) reactions. The reorganization energy of the nitrosonium cation is estimated to be 1.0 ± 0.1 eV for the C16-TEMPO SAM. ATR-SEIRAS studies reveal that the oxidation of the TEMPO radical leads to a conformational change (chair-chair flip) in the piperdinyl ring. The nitrosonium conformer pushes the N=O+ center away from the electrode and into a polar environment. Conformational differences in the reduced and oxidized species are cited as the reason for kinetic asymmetry in the TEMPO/TEMPO+ redox system. Time-resolved ATR-SEIRAS is used to extract the standard rate constant for C12-TEMPO on gold island films deposited on an indium tin oxide (ITO) surface. Standard rate constants for SAMs formed on Au@ITO are found to be higher than those formed on gold beads due to greater kinetic heterogeneity caused by the larger roughness of the gold islands.

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 Intercalants inside Birnessite-Type Manganese Oxide Nanosheets for Sensor Applications.

Hydrazine is a common reducing agent widely used in many industrial and chemical applications; however, its high toxicity causes severe human diseases even at low concentrations. To detect traces of hydrazine released into the environment, a robust sensor with high sensitivity and accuracy is required. An electrochemical sensor is favored for hydrazine detection owing to its ability to detect a small amount of hydrazine without derivatization. Here, we have investigated the electrocatalytic activity of layered birnessite manganese oxides (MnO2) with different intercalants (Li+, Na+, and K+) as the sensor for hydrazine detection. The birnessite MnO2 with Li+ as an intercalant (Li-Bir) displays a lower oxidation peak potential, indicating a catalytic activity higher than the activities of others. The standard heterogeneous electron transfer rate constant of hydrazine oxidation at the Li-Bir electrode is 1.09- and 1.17-fold faster than those at the Na-Bir and K-Bir electrodes, respectively. In addition, the number of electron transfers increases in the following order: K-Bir (0.11 mol) < Na-Bir (0.17 mol) < Li-Bir (0.55 mol). On the basis of the density functional theory calculation, the Li-Bir sensor can strongly stabilize the hydrazine molecule with a large adsorption energy (-0.92 eV), leading to high electrocatalytic activity. Li-Bir also shows the best hydrazine detection performance with the lowest limit of detection of 129 nM at a signal-to-noise ratio of ∼3 and a linear range of 0.007-10 mM at a finely tuned rotation speed of 2000 rpm. Additionally, the Li-Bir sensor exhibits excellent sensitivity, which can be used to detect traces of hydrazine without any effect of interference at high concentrations and in real aqueous-based samples, demonstrating its practical sensing applications.