Electron Transfer Rate Constant Research Articles

Cyclic voltammetry, square wave voltammetry or electrochemical impedance spectroscopy? Interrogating electrochemical approaches for the determination of electron transfer rates of immobilized redox proteins

In this article, we cross-examine three well-established electrochemical approaches, namely cyclic voltammetry (CV), cyclic square-wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) to dissect the electron transfer (ET) rate of electrostatically immobilized cytochrome c on Ag electrodes. A detailed analysis supported by simulations of redox transition provided three distinct values for the heterogeneous electron transfer (HET) rate constant of cyt c interfaced on COOH-terminated C10-long alkanethiol, i.e., kHET= 47.8 (±2,91) s−1 in CV, kHET= 64.8 (±1,27) s−1 in SWV, and kHET= 26.5 s−1 in EIS. We discuss the obtained discrepancies obtained from electrochemical methods and compare them with the data from spectro-electrochemical experiments. A comprehensive selection list is created from which the most applicable approach can be chosen for studying proteins of interest. CV is most applicable to study the interfaced proteins exhibiting kHET of ca. 0.5 - 70 s−1, SWV is suitable for a broader range of kHET of 5 – 120 s−1 and EIS for kHET of 0.5 to 5 s−1 if alkanethiols are used as immobilization strategy.

A disposable and low-cost laser-scribed graphene electrochemical sensor for simultaneous detection of hydroquinone, paracetamol and methylparaben

• A new disposable LSG sensor was fabricated using Polyetherimide substrate. • Sensor presented high reproducibility, sensitivity, and selectivity. • A LSG sensor was applied to simultaneous detection of emerging pollutants. • A porous graphene-like material with high electrochemical performance was developed. A novel and inexpensive laser-scribed graphene (LSG) sensor using polyetherimide (PEI) substrate was fabricated using CO 2 laser. The material was characterized by scanning electron microscopy (SEM), Raman spectroscopy, and electrochemical techniques, demonstrating a high porous carbonaceous material, which resulted in an electroactive area with 3.4-fold enhancement compared to geometric area and a high heterogeneous electron transfer rate constant (k°) of 3.20x10 -2 cm/s for ferricyanide redox probe. As a proof of applicability, the LSG-PEI sensor was applied for simultaneous detection of hydroquinone (HQ), paracetamol (PA), and methylparaben (MP) emerging pollutants (EP) in tap water and natural (lake) water samples, providing recoveries between 93.1% and 118% with limits of detection of 9.42x10 -8 , 3.23x10 -7 , and 2.95x10 -7 mol/L for HQ, PA, and MP, respectively.

An energy decomposition and extrapolation scheme for evaluating electron transfer rate constants: a case study on electron self-exchange reactions of transition metal complexes.

A simple approach to the analysis of electron transfer (ET) reactions based on energy decomposition and extrapolation schemes is proposed. The present energy decomposition and extrapolation-based electron localization (EDEEL) method represents the diabatic energies for the initial and final states using the adiabatic energies of the donor and acceptor species and their complex. A scheme for the efficient estimation of ET rate constants is also proposed. EDEEL is semi-quantitative by directly evaluating the seam-of-crossing region of two diabatic potentials. In a numerical test, EDEEL successfully provided ET rate constants for electron self-exchange reactions of thirteen transition metal complexes with reasonable accuracy. In addition, its energy decomposition and extrapolation schemes provide all the energy values required for activation-strain model (ASM) analysis. The ASM analysis using EDEEL provided rational interpretations of the variation of the ET rate constants as a function of the transition metal complexes. These results suggest that EDEEL is useful for efficiently evaluating ET rate constants and obtaining a rational understanding of their magnitudes.

A highly efficient and portable laser-scribed graphene-based electrochemical system for forensic-oriented determination of acepromazine.

Acepromazine (ACP) is a phenothiazine derivative drug commonly used as a tranquilizer veterinary medication due to its sedative properties. Benefiting from sedative properties, ACP has emerged as a drug of abuse and has been associated with drug-facilitated sexual assaults. Herein, we report, for the first time, the electrochemical behavior of ACP using a miniaturized and environmentally friendly laser-scribed graphene-based (LSG) sensor fabricated on a polyetherimide (PEI) substrate. The LSG device presented high porosity, as demonstrated by scanning electron microscopy (SEM). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements of the PEI-LSG electrode confirmed the enhanced electroactive area (3.1-fold increase) caused by the rough surface and revealed a low charge transfer resistance of the electrode material, with a heterogeneous electron transfer rate constant (k0) of 8.66 × 10-3 cm s-1 for potassium ferricyanide redox probe. A simple and accurate method was applied to quantify ACP by using square wave voltammetry (SWV) under optimized experimental conditions, which exhibited high sensitivity (0.686 ± 0.008 A L mol-1 cm-2) and a low limit of detection (LOD) of 7.43 × 10-8 mol L-1, with a linear concentration ranging from 0.5 to 100 μmol L-1 ACP. Aiming for on-site analysis, the PEI-LSG sensor was integrated with a miniaturized potentiostat controlled by using a smartphone and applied as proof of applicability to ACP detection in commercial beverage and synthetic urine samples. These studies demonstrated adequate recoveries, ranging from 95.1% to 115.8%. The analytical parameters highlight the robustness and reliability of the proposed method for analyses of ACP directly at a potential crime scene.

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.

Structurally and electrochemically tunable pyrylium platforms: A new class of redox anolyte for non-aqueous organic redox flow battery operating at a high-current density

Non-aqueous all-organic redox flow batteries (NORFB) are a proficient candidate for large-scale energy storage applications. The higher operating potential window of non-aqueous solvents, diversity of redox-active organic molecules (ROMs), and independent scaling of power and energy, etc., captivate NORFB to be a potential choice for grid energy storage applications. Besides, the invention of new redox-active motifs significantly expands the development of NORFBs; thus, it is highly demanded. But, the low operational current density and slower redox kinetics are the bottlenecks for the further development of NORFB. Herein, we introduced a new class of anolyte material based on pyrylium salt (2,4,6-triphenylpyrylium tetrafluoroborate (TPT)) and demonstrated for the first time its applicability as a potential anolyte in NORFB. The substitutions at the 2, 4, and 6th positions of pyrylium salt prominently tune the structural and electrochemical behavior, thus providing a versatile platform to explore them in flow battery chemistry. TPT exhibited a one-electron reduction process with a high electron transfer rate constant of 9.94 × 10−3 cm s−1 and diffusion coefficient of 5.83 × 10−6 cm2 s−1, which is higher for the non-aqueous medium. By coupling with N-decylphenothiazine (DPTZ) catholyte, a mixed electrolyte flow cell was demonstrated with a capacity of 1.08 Ah/L and coulombic efficiency of 97 % at a high current density of 40 mA cm−2.

Theory of electron transmission probability at metal/semiconductor Interface system

We focus in this review on the theoretical description of the probability of electron transmission at metal/semiconductor at the interface depending on the quantum theory and density functional theory .Electronic devices technology largely rely on metal/ semiconductor system interfaces, whose most important coefficient parameters for the probability of electron transmission such that :reorientation free energy , potential barrier height ( ),work function ,affinity of semiconductor ,coupling coefficient matrix element, concentration of electron ,volume of unit cell for semiconductor ,petration factor , and temperature T(K) . The probability of electron transport rate constant in Au/ Ge ,and Au/Si interface system has been calculated and the transmission through interface is examined .A Mat lab program has been used to calculate the rate constant of electron transfer and all parameters of transmission by solvingthe suitable formulas .Our results data show that the probability of the rate constant for electron transfer increases with the increasing of the coupling coefficient and temperature, and decreasing of the reorganization energy

Well-Dispersive Polypyrrole and MoSe2 Embedded in Multiwalled Carbon Nanotube@Reduced Graphene Oxide Nanoribbon Electrocatalysts as the Efficient Counter Electrodes in Rigid and Plastic Dye-Sensitized Solar Cells

In this study, a core–shell structured multiwall carbon nanotube@reduced graphene oxide nanoribbon (MWCNT@rGNR) was prepared by the facile unzipping of MWCNTs with mild conditions to synthesize the composites of the MWCNT@rGNR and polypyrrole (PPy) or MoSe2, which were used as the electrocatalysts for the counter electrodes (CEs) in dye-sensitized solar cells (DSSCs). The transmission electron microscopy image of the MWCNT@rGNR showed a graphene sheet structure found on both sides of the MWCNTs, indicating a central core of the MWCNTs and its shell of the rGNR. The obtained MWCNT@rGNR functionalized with PPy or MoSe2 was envisaged to have the advantages of both component, that are the unique electronic structure of the MWCNT@rGNR and the synergistic effects of conductive PPy or electrocatalytically active MoSe2, respectively. The DSSCs with PPy/MWCNT@rGNR0.3 and MoSe2/MWCNT@rGNR0.3 showed comparable efficiency (photoelectric conversion efficiency, PCE = 7.48 and 8.34%, respectively), short-circuit current (Isc = 17.97 and 17.11 mA cm–2, respectively), and open-circuit voltage (Voc = 0.80 and 0.84 V, respectively) without any loss of the fill factor (FF = 0.52 and 0.58, respectively) as compared to the cell fabricated using a standard Pt electrode. The PCE of a standard Pt CE was 7.70% with an Isc of 16.09 mA cm–2, Voc of 0.76 V, and FF of 0.63. According to the results of the electrochemical measurements, composites with PPy or MoSe2 showed better electrocatalytic activity, higher redox-active surface area, and extremely large heterogeneous electron transfer rate constant. The plastic devices assembled with the PPy/MWCNT@rGNR0.3 and MoSe2/MWCNT@rGNR0.3 coated on a flexible plastic substrate (indium tin oxide-coated polyethylene naphthalate; ITO/PEN)exhibited impressive PCEs of 4.61 and 5.25%, respectively, comparable to those coated on the rigid fluorine-doped tin oxide glass substrate. This work presented a facile low-temperature method to fabricate high-performance MWCNT@rGNR composite-based CEs to make them applicable to large-scale plastic DSSCs.

Effect of the Donor/Acceptor Size on the Rate of Photo-Induced Electron Transfer

The photo-induced electron transfer has been under intensive investigation for a few decades already, and a good understanding of the reaction was developed based on thorough study of the molecular donor–acceptor (DA) system. The recent shift to hybrid DA systems opens the question of transferring the knowledge to analyze and design these new materials. One of the apparent differences is the size increase of the donor or acceptor entities. The electronic wave functions of larger entities occupy a larger volume, but since these are still one-electron wave functions, their amplitudes are lower. A simple analysis proposed here demonstrates that this leads to roughly inverse third power dependence of the electron transfer rate constant on the donor or acceptor size, kET∝R−3. This dependence can be observed upon switching from molecular to quantum dot donor in DA systems with a fullerene acceptor.

Fabrication of a polyvinyl alcohol-bentonite composite coated on a carbon felt anode for improving yeast microbial fuel cell performance

Bentonite-Polyvinyl Alcohol (PVA/Bentonite) is constructed by mixing bentonite with PVA. It is then coated on carbon felt (CF) and proposed as a replacement for conventional carbon-based materials in the anode of yeast Microbial Fuel Cell (MFC) with biofilm development. The selected anodic benchmarks are three-dimensional CF that is unmodified, inexpensive, flexible, spongy, and electrically conductive. Adding a [PVA/Bentonite] composite to a CF anode increased the power output of yeast MFC by 6–100%, while the voltage increased by 37–62%. Comparatively, the CF/[PVA0.2/Bentonite] layer resulted in superior catalytic activities, such as a high electron transfer rate constant of 0.580 s−1, a low Rct value of 1.3 Ω, a low Cs of 20.27 F/g, a low Cdl of 162 F, and a W value of 66.59 μΩ/s0.5. The MPD of the yeast MFC using the layer was 25.70 ± 5.5 mW/m2. The more PVA and bentonite in the CF, more challenging it is for biofilm to develop on the anode surface in yeast MFCs. At certain concentration, however, this [PVA/Bentonite] mixture may tend to prefer protons and electrons generated by yeast. This research reveals that the close interaction of bentonite and CF through PVA might enhance the performance of yeast MFC.

Polypyrrole-MXene supported gold nanoparticles for the trace-level detection of nitrofurantoin

High usage of nitrofurantoin (NFT) has resulted in unacceptable levels in water and foods with negative impacts on animals, humans, and surrounding ecosystems. The accurate and ultra-low sensing of NFT in waterways presents a significant societal challenge. To address this issue, gold nanoparticles (AuNPs) were integrated into polypyrrole (PPy) on Titanium carbide MXene (Ti3C2Tx) using a simple sonochemical route involving the in-situ, oxidant-free polymerization of pyrrole monomer in the presence of HAuCl4. The AuNP-PPy-MXene composite produced was used to modify a glassy carbon electrode (GCE) to construct an ultrasensitive electrochemical sensor for the direct and on-site sensing of NFT in complex samples. Electrochemical impedance spectroscopy (EIS) investigations showed Au-PPy-MXene-GCEs had a heterogeneous electron transfer rate constant (k°) of 1.03 × 10-2 cm s−1 and a lower charge-transfer resistance (Rct = 73 Ω) than Au-MXene, Au-PPy, and PPy-MXene GCEs. In addition, Au-PPy-MXene-GCE exhibited better electrocatalytic reduction performance at a higher cathodic signal intensity and a lower reduction overpotential of −0.38 V (vs Ag/AgCl) for NFT than Au-MXene, Au-PPy, and PPy-MXene GCEs. The fabricated sensor had a wide linear range from 6 to 172 nM (measured at −0.38 V vs Ag/AgCl), an ultra-low detection limit (LOD) of 0.26 nM (at S/N = 3), and an excellent sensitivity of 6.4121 μA nM−1 cm−2 for the electroreduction of NFT. Au-PPy-MXene-GCE also provided specific and accurate amperometric analysis for NFT in the presence of cationic, anionic, and other potentially interfering pesticides. The practicality of the sensor was tested to quantify NFT at trace levels in complex matrices (honey, pond water, and hospital wastewater samples), and achieved recoveries ranging between 97.6 % and 113.2 % and a relative standard deviation of<3.6 %.

Acceleration of Nonradiative Charge Recombination Reactions at Larger Distances in Kinked Donor-Bridge-Acceptor Molecules.

Photoinduced electron transfer in donor-bridge-acceptor (D-B-A) molecular systems can occur via tunneling over long distances (rDA) of well over 10 Å. We commonly observe decreasing rates of electron transfer with increasing distances, a result of a decrease in the electronic coupling of the donor and acceptor moiety. In the study of D-B-A molecules with Ru(bpy)32+ as a bridge/core, Kuss-Petermann and Wenger observed the opposite trend (J. Am. Chem. Soc.2016, 138, 1349); a maximum rate constant of electron transfer was observed at an intermediate electron transfer distance. Within the high-temperature limit of the classical Marcus equation, their observation was qualitatively explained by a sharp distance dependence of outer sphere (or solvent) reorganization energy, as predicted by Sutin and co-workers (J. Am. Chem. Soc.1984, 106, 6858), and almost distance-independent electronic couplings. Here, we report another example of such an underexplored behavior with three kinked D-B-A systems of rDA ∼ 10-19 Å, showing increasing rates of nonradiative charge recombination with increasing rDA. The three D-B-A systems are based on boron dipyrromethene and triphenylamine as electron acceptor and donor groups, respectively, with aryl bridges where the donor and acceptor moieties are connected at meso-positions. These D-B-A molecules exhibit radiative electron transfer reactions (or charge-transfer emission), which enables us to experimentally determine the solvent reorganization energy and the electronic couplings. The analysis of charge-transfer emission that explicitly considers electron-vibration coupling, in conjunction with the temperature-dependent analysis and computational method, revealed that the solvent reorganization energy indeed increases with distance, and at the same time, the electronic coupling decreases with distance expectedly. Therefore, under the right conditions for solvent reorganization energy and electronic coupling values, our results show that we can observe the acceleration of electron transfer reactions with increasing distance, even when we have the expected distance dependence of electronic coupling. This work indicates that the acceleration of electron transfer with increasing distance may be achieved with a fine-tuning of molecular design.

Field-Effect Tuning of Electrochemistry at 2D Semiconductor Electrodes: Charge Transfer Kinetics Can be Modulated with a Back Gate Voltage

Semiconductor electrodes are widely used in electrocatalytic reactions. The ability to improve and modulate the heterogeneous charge transfer kinetics on semiconducting electrodes is a major challenge for the electrochemical application of these materials. Instead of changing the electronic occupation of the semiconductor by chemical doping, we adopt an electronic method to modulate band edge alignment and solution electrochemistry at 2D ZnO working electrodes.In our approach, a field-effect transistor with ultrathin ZnO semiconductor (3-5 nm) is fabricated, the heart of which is a metal-insulator-semiconductor (MIS) stack. A high-dielectric-constant material (HfO2) is used as the insulator to realize low-voltage tunability of the device. The ZnO layer of the device functions as the working electrode (WE) for electrochemical reactions. By applying a back gate voltage to this MIS field-effect transistor, charge carriers are electrostatically induced and the ZnO band edges shift at the front face of the ZnO in contact with electrolyte. These effects can be exploited to manipulate the charge transfer kinetics on the ZnO electrode at a given electrode potential. We demonstrate this by focusing on outer-sphere redox chemistry. In order to eliminate mass transfer effects, microfluidic flow cells are coupled to the ZnO devices. Steady state kinetic analysis is carried out to extract the electron transfer rate constant of the redox process as a function of both electrochemical and back gate potentials. The heterogeneous electron transfer rate can be tuned by over an order of magnitude using the back gate voltage. The work demonstrates that a back gate provides another degree of freedom in determining electrochemical reaction kinetics at ultrathin semiconductor working electrodes. The method also opens possibilities for control over a wider range of semiconductor electrochemistry, such as electrocatalytic reactions. Figure 1

Bentonite Nanoclay Optoelectrochemical Property Improvement through Bimetallic Silver and Gold Nanoparticles

This study assesses the physical and electrochemical changes of bimetallic Ag-Au nanoparticle-functionalized bentonite nanoclay. Nanoclay was studied to deduce a better sensing material/film. A chemical co-reduction method was used to synthesize bimetallic Ag-Au c nanoparticles, which were used to prepare a Ag-Au/PGV bentonite composite. Bimetallic Ag-AuNPs and their nanoclay composite were optically characterized using the scanning electron microscope, ultraviolet visible spectroscopy, X-ray diffraction, and Fourier transform infrared, whilst cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to ascertain their electrochemical activity and properties. The results of surface morphological inspection showed an average size of 10 nm, in agreement with XRD. The bimetallic Ag-AuNPs UV/Vis characteristic wavelengths of 414 nm and 516 nm confirmed the presence of Ag and Au metals, respectively. XRD exhibited diffraction planes related to 2θ values of Ag and Au metals, whilst FTIR indicated mainly COO- functional groups from the citrate capping of bimetallic Ag-Au NPs. CV and DPV showed that bentonite nanoclay is largely insulated by silicates but exhibited a small electroactivity of Fe. The electroactivity of Ag-Au/PGV bentonite exhibited peak potentials due to Ag/Ag+ and Au/Au3+ redox couples at 0.19 V/−0.20 V and 1.37 V/0.42, respectively. The Ag-Au/PGV bentonite nanocomposite exhibited the highest surface concentration of 3.25 × 10−2 cm2, a diffusion coefficient of 2.36 × −11 cm2/s, and an electron transfer rate constant (Ks) of 1.99 × 10−4 cm2. The outcome of these results indicated that the Ag-Au/PGV bentonite nanocomposite was more electroactive than PGV. Therefore, this study accentuates Ag-Au/PGV bentonite nanocomposite as a novel and promising platform for electrochemical sensing with higher sensitivity and efficiency than other sensing materials.

A machine learning-based multimodal electrochemical analytical device based on eMoSx-LIG for multiplexed detection of tyrosine and uric acid in sweat and saliva

Multiplexed detection of biomolecules is of great value in various fields, from disease diagnosis to food safety and environmental monitoring. However, accurate and multiplexed analyte detection is challenging to achieve in mixtures using a single device/material. In this paper, we demonstrate a machine learning (ML)-powered multimodal analytical device based on a single sensing material made of electrodeposited molybdenum polysulfide (eMoSx) on laser induced graphene (LIG) for multiplexed detection of tyrosine (TYR) and uric acid (UA) in sweat and saliva. Electrodeposition of MoSx shows an increased electrochemically active surface area (ECSA) and heterogeneous electron transfer rate constant, k0. Features are extracted from the electrochemical data in order to train ML models to predict the analyte concentration in the sample (both singly spiked and mixed samples). Different ML architectures are explored to optimize the sensing performance. The optimized ML-based multimodal analytical system offers a limit of detection (LOD) that is two orders of magnitude better than conventional approaches which rely on single peak analysis. A flexible and wearable sensor patch is also fabricated and validated on-body, achieving detection of UA and TYR in sweat over a wide concentration range. While the performance of the developed approach is demonstrated for detecting TYR and UA using eMoSx-LIG sensors, it is a general analytical methodology and can be extended to a variety of electrochemical sensors to enable accurate, reliable, and multiplexed sensing.

Epitaxial Self-Assembly of Interfaces of 2D Metal-Organic Frameworks for Electroanalytical Detection of Neurotransmitters.

This paper identifies the electrochemical properties of individual facets of anisotropic layered conductive metal–organic frameworks (MOFs) based on M3(2,3,6,7,10,11-hexahydroxytriphenylene)2 (M3(HHTP)2) (M = Co, Ni). The electroanalytical advantages of each facet are then applied toward the electrochemical detection of neurochemicals. By employing epitaxially controlled deposition of M3(HHTP)2 MOFs on electrodes, the contribution of the basal plane ({001} facets) and edge sites ({100} facets) of these MOFs can be individually determined using electrochemical characterization techniques. Despite having a lower observed heterogeneous electron transfer rate constant, the {001} facets of the M3(HHTP)2 systems prove more selective and sensitive for the detection of dopamine than the {100} facets of the same MOF, with the limit of detection (LOD) of 9.9 ± 2 nM in phosphate-buffered saline and 214 ± 48 nM in a simulated cerebrospinal fluid. Langmuir isotherm studies accompanied by all-atom MD simulations suggested that the observed improvement in performance and selectivity is related to the adsorption characteristics of analytes on the basal plane versus edge sites of the MOF interfaces. This work establishes that the distinct crystallographic facets of 2D MOFs can be used to control the fundamental interactions between analyte and electrode, leading to tunable electrochemical properties by controlling their preferential orientation through self-assembly.

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 dynamics and electrocatalytic oxygen evolution activities of the Co3O4 nanoparticles attached to indium tin oxide by self-assembled monolayers.

The Co3O4 nanoparticle-modified indium tin oxide-coated glass slide (ITO) electrodes are successfully prepared using dicarboxylic acid as the self-assembled monolayer through a surface esterification reaction. The ITO-SAM-Co3O4 (SAM = dicarboxylic acid) are active to electrochemically catalyze oxygen evolution reaction (OER) in acid. The most active assembly, with Co loading at 3.31 × 10−8 mol cm−2, exhibits 374 mV onset overpotential and 497 mV overpotential to reach 1 mA cm−2 OER current in 0.1 M HClO4. The electron transfer rate constant (k) is acquired using Laviron’s approach, and the results show that k is not affected by the carbon chain lengths of the SAM (up to 18 -CH2 groups) and that an increase in the average diameter of Co3O4 nanoparticles enhances the k. In addition, shorter carbon chains and smaller Co3O4 nanoparticles can increase the turn-over frequency (TOF) of Co sites toward OER. The Co3O4 nanoparticles tethered to the ITO surface show both a higher number of electrochemically active Co sites and a higher TOF of OER than the Co3O4 nanoparticles bound to ITO using Nafion.

Electroactive Biofilms of Activated Sludge Microorganisms on a Nanostructured Surface as the Basis for a Highly Sensitive Biochemical Oxygen Demand Biosensor.

The possibility of the developing a biochemical oxygen demand (BOD) biosensor based on electroactive biofilms of activated sludge grown on the surface of a graphite-paste electrode modified with carbon nanotubes was studied. A complex of microscopic methods controlled biofilm formation: optical microscopy with phase contrast, scanning electron microscopy, and laser confocal microscopy. The features of charge transfer in the obtained electroactive biofilms were studied using the methods of cyclic voltammetry and electrochemical impedance spectroscopy. The rate constant of the interaction of microorganisms with the extracellular electron carrier (0.79 ± 0.03 dm3(g s)−1) and the heterogeneous rate constant of electron transfer (0.34 ± 0.02 cm s−1) were determined using the cyclic voltammetry method. These results revealed that the modification of the carbon nanotubes’ (CNT) electrode surface makes it possible to create electroactive biofilms. An analysis of the metrological and analytical characteristics of the created biosensors showed that the lower limit of the biosensor based on an electroactive biofilm of activated sludge is 0.41 mgO2/dm3, which makes it possible to analyze almost any water sample. Analysis of 12 surface water samples showed a high correlation (R2 = 0.99) with the results of the standard method for determining biochemical oxygen demand.