Heterogeneous Electron Transfer Rate Research Articles

Mapping Electron Transfer at MoS2 Using Scanning Electrochemical Microscopy.

Understanding the role of macroscopic and atomic defects in the interfacial electron transfer properties of layered transition metal dichalcogenides is important in optimizing their performance in energy conversion and electronic devices. Means of determining the heterogeneous electron transfer rate constant, k, have relied on the deliberate exposure of specific electrode regions or additional surface characterization to correlate proposed active sites to voltammetric features.Few studies have investigated the electrochemical activity of surface features of layered dichalcogenides under the same experimental conditions. Herein, MoS2 flakes with well-defined features were mapped using scanning electrochemical microscopy (SECM). At visually flat areas of MoS2, k of hexacyanoferrate(III) ([Fe(CN)6]3-) and hexacyanoferrate(II) ([Fe(CN)6]4-) was typically smaller and spanned a larger range than that of hexaammineruthenium(III) ([Ru(NH3)6]3+), congruent with the current literature. However, in contrast to previous studies, the reduction of [Fe(CN)6]3- and the oxidation of [Fe(CN)6]4- exhibited similar rate constants, attributed to the dominance of charge transfer through surface states. The comparison of SECM with optical and atomic force microscopy images revealed that while most of the flake was electroactive, edge sites associated with freshly exposed areas that include macrosteps consisting of several monolayers as well as recessed areas exhibited the highest reactivity, consistent with the reported results.

Electrochemical, spectroscopic and theoretical monitoring of anthracyclines' interactions with DNA and ascorbic acid by adopting two routes: Cancer cell line studies.

Pharmacodynamic interactions of three anthracycline antibiotics namely doxorubicin (DXH), epirubicin (EpiDXH) and daunorubicin (DNR) with DNA in the absence and presence of ascorbic acid (AA) as natural additive were monitored under physiological conditions (pH = 7.4, 4.7 and T = 309.5K). Route–1 (Anthracycline–AA–DNA) and Route–2 (Anthracycline–DNA–AA) were adopted to see the interactional behavior by cyclic voltammetry (CV) and UV-visible spectroscopy. In comparison to Route–2; voltammetric and spectral responses as well as binding constant (Kb) and Gibb’s free energy change (ΔG) values revealed strongest and more favorable interaction of anthracycline–AA complex with DNA via Route–1. Kb, s (binding site sizes) and ΔG evaluated from experimental (CV, UV-Vis) and theoretical (molecular docking) findings showed enhanced binding strength of tertiary complexes as compared to binary drug–DNA complexes. The results were found comparatively better at pH 7.4. Consistency was observed in binding parameters evaluated from experimental and theoretical techniques. Diffusion coefficients (Do) and heterogeneous electron transfer rate constant (ks,h) confirmed the formation of complexes via slow diffusion kinetics. Percent cell inhibition (%Cinh) of anthracyclines for non-small cell cancer cell lines (NSCCLs) H-1299 and H-157 were evaluated higher in the presence of AA which further complimented experimental and theoretical results.

Synthesis of 3D-coral like polyaniline nanostructures using reactive oxide templates and their high performance for ultrasensitive detection of blood cancer

We report the synthesis of 3D coral like polyaniline nanostructures (PANI-NC), using in situ sacrificial oxidative template route. The coral-like nanostructures were developed as electrode materials for biosensing applications by depositing it electrophoretically onto indium tin oxide (ITO) coated glass substrates. These fabricated electrodes were characterized using various microscopic, and spectroscopic techniques which confirms the deposition of 3D PANI-NC on ITO electrode (PANI-NC/ITO). Electrochemical studies of the PANI-NC/ITO showed improved electron transfer behavior with a diffusion coefficient of 3.097 × 10−8 cm2/s and heterogeneous electron-transfer rate constant of 4.52 × 10−4 cm/s. Further, these electrodes were utilized for the immobilization of biotinylated DNA sequence (22 base pairs), identified from BCR–ABL fusion gene using avidin-biotin coupling. The electrochemical impedance spectroscopy analysis showed that the nucleic acid sensor exhibits a linear dynamic range from 10-6 M to 10-17 M of target DNA with a low detection limit (7 × 10−18 M). The high differentiation ability and specificity of the nucleic acid biosensor with various target DNA sequences (complementary, noncomplementary, one base mismatch and two base mismatch) and clinical samples of CML positive patients demonstrate its potential for clinical diagnostics.

Graphene-based “hybrid” aerogels with carbon nanotubes: Mesoporous network–functionality promoted defect density and electrochemical activity correlations

Electrochemical activity of graphene and graphene-based “hybrid” nanomaterials is crucial for energy and water sustainability applications, which requires fine tuning over combined geometric and electronic structures. We demonstrate that precise control of defects, porosity, and topological interconnectedness, invoked in hydrothermally synthesized graphene aerogel integrated with multi-walled carbon nanotubes, promotes finely tuned morphology, structure, defect number density, hierarchical mesoporosity, and conductivity and enhances the electrochemical heterogeneous electron transfer rate (kET). We prepared a range of graphene-based “hybrid” scaffolds (or monolithic aerogels) and their nitrogenated equivalents with varying graphene–carbon nanotube compositions using two synthetic schemes known as approaches 1 and 2. This study allows us to correlate quantitatively between number defect density (via Raman spectroscopy; RS) and heterogeneous electron transfer rate (via scanning electrochemical microscopy). RS provided microscale structural characterization revealing localized lattice vibrations. The first- and second-order Raman bands were analyzed in terms of band position, intensity ratio, and integrated intensity determining structural disorder, in-plane cluster size, inter-defect distance, and number defect density. The role of oxygenated (carbonyl; C═O, carboxyl; —COOH) and nitrogenated (pryridinic-N and graphitic/pyrrolic-N) functionalities and bonding configurations besides mesoporosity is emphasized for understanding the role of surface chemistry in regionally improved physicochemical (electroactivity and catalytic) properties. The defect-induced increase in finite electronic density of states (DOS) near Fermi level calculated using density functional theory under hydration helped in establishing moderate defect density for enhanced heterogeneous electron transfer rate as a critical onset such that the carbon system is electroactive while maintaining integral sp2 C structural network. Moreover, the defect sites allow sufficient overlap between DOS for graphene-based aerogels and redox probe wavefunctions, which emphasizes the experimental correlation establishments.

Stress-activated pyrolytic carbon nanofibers for electrochemical platforms

Carbon's electrochemistry depends on its type and microstructure, and how these affect the electrode's electronic density of states. We demonstrate how pyrolysis of electro-mechanically stressed Polyacrylonitrile (PAN) nanofibers, infused with carbon nanotubes, will result in a unique graphitic electrode, which possesses enhanced and multifaceted electrochemical behavior. As corroborated by materials characterization, the microstructure of the stress-activated pyrolytic carbon (SAPC) characteristically contains a high proportion of disorders in the forms of edge planes and embedded heterogeneous nitrogen atoms. These disorders introduce a range of energy states near the Fermi level, yielding enhanced kinetics in the as-synthesized SAPC electrodes. A comprehensive electrochemical study of the SAPC electrode in surface sensitive ([Fe(CN)6]3-/4-), surface insensitive ([IrCl6]2-/3-), and adsorption sensitive (dopamine) redox probes demonstrates 5–14-fold increases in its heterogeneous electron transfer rate compared to regular PAN-based carbon electrodes. The fast kinetics of SAPC electrodes in adsorption sensitive analytes translates into its capability for simultaneous detection of dopamine, uric acid, and ascorbic acid. The results point to a new class of pyrolytic carbon electrodes with an attractive electrocatalytic capacity, geared toward electrochemical sensing platforms.

Hierarchically Ordered Two-Dimensional Coordination Polymers Assembled from Redox-Active Dimolybdenum Clusters.

Coordination polymers (CPs) supporting tunable through-framework conduction and responsive properties are of significant interest for enabling a new generation of active devices. However, such architectures are rare. We report a redox-active CP composed of two-dimensional (2D) lattices of coordinatively bonded Mo2(INA)4 clusters (INA = isonicotinate). The 2D lattices are commensurately stacked and their ordering topology can be synthetically tuned. The material has a hierarchical pore structure (pore sizes distributed between 7 and 33 Å) and exhibits unique CO2 adsorption (nominally Type VI) for an isotherm collected at 195 K. Furthermore, cyclic voltammetry and electrokinetic analyses identify a quasi-reversible feature at E1/2 = -1.275 V versus ferrocene/ferrocenium that can be ascribed to the [Mo2(INA)4]0/-1 redox couple, with an associated standard heterogeneous electron transfer rate constant ks = 1.49 s-1. The tunable structure, porosity, and redox activity of our material may render it a promising platform for CPs with responsive properties.

Selective Determination of Total Capsaicinoids in Plant Material Using Poly(Gallic Acid)‐modified Electrode

AbstractNovel voltammetric approach for the selective determination of total capsaicinoids has been developed using glassy carbon electrode modified with multi‐walled carbon nanotubes and poly(gallic acid) (PGA/MWNT/GCE). The modified electrode provides significant improvements in the capsaicinoids voltammetric characteristics in comparison to GCE and MWNT/GCE. The electrooxidation of capsaicinoids is irreversible adsorption‐controlled process with the anodic transfer coefficient of 0.49–0.53 and heterogeneous electron transfer rate constant of 1300–2400 s−1. The analytical ranges of 0.010–1.0 and 1.0–50 μM for capsaicin, 0.025–0.75 and 0.75–75 μM for dihydrocapsaicin and 0.025–5.0 and 5.0–75 μM for nonivamide with the detection limits of 2.9, 5.9 and 6.1 nM, respectively, have been obtained using differential pulse voltammetry (DPV). The selectivity of the capsaicinoids quantification in the presence of ascorbic acid, α‐tocopherol and carotenoids is shown. The method has been tested on the samples of red hot pepper spices and Capsicum annuum L. tinctures. The results correspond to the chromatographic data.

A cross examination of electron transfer rate constants for carbon screen-printed electrodes using Electrochemical Impedance Spectroscopy and cyclic voltammetry

Heterogeneous electron transfer rate constants of a series of chemical systems are estimated using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS), and critically compared to one another. Using aqueous, quasi-reversible redox systems, and carbon screen-printed electrodes, this work has been able to quantify rate constants using both techniques and have proved that the two methods sometimes result in measured rate constants that differ by as much as one order of magnitude. The method has been converted to estimate k0 values for irreversible electrochemical systems such as ascorbic acid and norepinephrine, yielding reasonable values for the electron transfer of their respective oxidation reactions. Such electrochemically irreversible cases are compared to data obtained via digital simulations. The work is limited to finite concentration ranges of electroactive species undergoing simple electron processes (‘E’ type reactions). The manuscript provides the field with a simple and effective way estimating electron transfer rate constants for irreversible electrochemical systems without using digital software packages, something which is not possible using either Nicholson or Laviron methods.

Electrochemical behavior of horseradish peroxidase on WS2 nanosheet‐modified electrode and electrocatalytic investigation

In this paper, a WS2 nanosheet was modified on the surface of a carbon ionic liquid electrode (CILE), and horseradish peroxidase (HRP) was further fixed on the electrode with a Nafion film. Direct electrochemistry and bioelectrocatalysis of HRP incorporated on the modified electrode were investigated in detail. On Nafion/HRP/WS2/CILE, a pair of well‐defined quasi‐reversible redox peaks appeared on the cyclic voltammogram, indicating that the presence of the WS2 nanosheet on the electrode surface could provide a specific interface with large surface area for HRP and its direct electron transfer rate was greatly enhanced. The formal potential (E0) obtained was –0.179 V, which was the typical feature of heme Fe(III)/Fe(II) in HRP. The electron transfer coefficient (α) and the heterogeneous electron transfer rate constant (ks) of HRP were calculated as 0.44 and 1.01 s–1, respectively. This HRP‐modified electrode showed excellent electrocatalytic activity for the reduction of trichloroacetic acid and NaNO2 with a wide linear range and low detection limit. Real samples were detected by this proposed method, indicating the successful fabrication of a new third‐generation electrochemical enzyme sensor utilizing the WS2 nanosheet.

Thermodynamic and kinetic properties of the outer membrane cytochrome OmcF, a key protein for extracellular electron transfer in Geobacter sulfurreducens

Gene knock-out studies on Geobacter sulfurreducens have shown that the monoheme c-type cytochrome OmcF is essential for the extracellular electron transfer pathways involved in the reduction of iron and uranium oxy-hydroxides, as well as, on electricity production in microbial fuel cells. A detailed electrochemical characterization of OmcF was performed for the first time, allowing attaining kinetics and thermodynamic data. The heterogeneous electron transfer rate constant was determined at pH 7 (0.16 ± 0.01 cm s−1) indicating that the protein displays high electron transfer efficiency compared to other monoheme cytochromes. The pH dependence of the redox potential indicates that the protein has an important redox-Bohr effect in the physiological pH range for G. sulfurreducens growth. The analysis of the structures of OmcF allowed us to assign the redox-Bohr centre to the side chain of His47 residue and its pKa values in the reduced and oxidized states were determined (pKox = 6.73; pKred = 7.55). The enthalpy, entropy and Gibbs free energy associated with the redox transaction were calculated, pointing the reduced form of the cytochrome as the most favourable. The data obtained indicate that G. sulfurreducens cells evolved to warrant a down-hill electron transfer from the periplasm to the outer-membrane associated cytochrome OmcF.

Silver Electrodeposition on Nanocrystalline Graphite: Galvanostatic Control As a Tool in the Study Electrodes with High Resistance

Carbon based materials, such as glassy carbon or highly oriented pyrolytic carbon (HOPG), have been used extensively as working electrodes for electrodeposition. Due to their chemical inertness and electrochemical stability they are commonly used as substrates for electrodeposition studies. HOPG in particular has attracted great deal of attention due to its close relation to carbon nanotubes and graphene [1]. Miranda-Hernández et al. [2] investigated silver electrodeposition on HOPG, mechanically polished glassy carbon and fractured glassy carbon. Their results showed that the surface conditions, including morphology of the carbon substrate, have an effect on the overpotential required to start a 3D silver electrocrystallization. Additionally, their results led to the conclusion that the active sites do not depend on physical features on the electrode's surface and thus their nature must be electronic. In carbon materials, the shape and magnitude of the density of electronic states (DOS) distribution vary according to their structure [3]. Since the rate of heterogeneous electron transfer depends on the electrode’s DOS [4], it is expected that different carbon materials would exhibit different electrodeposition behaviors. Furthermore, simple electrodeposition systems could be used to characterize the properties of novel carbon materials. Recently, Cholula-Díaz et al. [5] used aerosol assisted chemical vapor deposition to synthesize nanocrystalline graphite (NCG) thin films. The low nanoscale roughness of this material may lead to some insight into the electrodeposition process. Additionally, their complex electronic behavior may lead to the observation of new phenomena. These samples were used as working electrodes for silver electrodeposition. A single compartment electrochemical cell was used, with Hg|HgSO4 and a Pt wire as reference and counter electrodes, respectively. The composition of the platting bath was: 10 mM AgNO3, 1 M KNO3 and 1.6 M NH4OH [2]. Cyclic voltammetry experiments resulted in tilted voltammograms, which are typical of a high ohmic resistance. Since the platting bath had a high ionic concentration, this behavior is adscribed to the resistance of the NCG sample. The cell voltage U is equal to the sum of the double-layer potential φDL and the ohmnic drop. U= φDL + iR Hence, under potentistatic control, current changes result in variations of the ohmic term, to which the system responds by adjusting the double-layer potential to comply with the applied voltage value. This feedback loop between the double-layer potential and the current makes it difficult to establish a clear relationship between the experimental conditions and the characteristics of the deposited material. In contrast, galvanostatic control offers a far simpler scenario; since the current is fixed in the experiment, it is easier to determine the double-layer potential. Figure 1a shows the time evolution of the double-layer potential during controlled current experiments. The time scale on which the potential stabilizes is larger for the one observed for similar experiments on glassy carbon. This shows the importance of galvanostatic control as means to study electrodeposition samples with high resistivity. Figure 1b shows an SEM image in which submicrometric silver particles can be identified. We explore the dependence between the current pulses applied and the resulting particle size distributions. References R. Unwin, Faraday Discuss., 172, 521 (2014)Miranda-Hernández, I. González, and N. Batina, J. Phys. Chem. B, 105, 4214 (2001).Richard L. McCreery, Chem. Rev., 108, 2646 (2008)J. Royea, T. W. Hamann, B. S. Brunschwig, N. S. J. Lewis, Phys. Chem. B, 110, 19433 (2006).L. Cholula-Díaz, J. Barzola-Quiquia, H. Krautscheid, U. Teschner, P. Esquinazi, Carbon, 67, 10 (2014). Figure 1. a) Time dependence of nanocrystalline graphite electrode potential after applying cathodic current pulses from −50 to −325 µA in 10 mM AgNO3 + 1 M KNO3 + 1.6 M NH4OH electrolyte. b) SEM image (BS detector) of the surface of the nanocrystalline graphite showing particles of silver deposit. Figure 1

Evidence of a Magnetic Effect on the Electron Transfer of the Hydrogen Evolution Reaction (HER)

In 1972, Trasatti carefully compiled the rates of hydrogen evolution reaction (HER) on 31 metal electrodes near pH 0 [1]. The HER is a surface reaction that can be represented as Hsoln⁺ ⇌ Hads⁺ Hads⁺ + e ⇌ Hads ∙ 2 Hads ∙ ⇌ H2,ads H2,ads ⇌ H2,soln Trasatti also defined the electrochemical work function Φ, which is the energy needed to remove an electron from the electrode surface to a redox species immediately at the electrode surface. The rates of the HER reaction are embedded in the standard exchange current density j0 (A/cm2) that measures the standard heterogeneous rate of electron transfer, k0. Trasatti demonstrated that plot of log j0 versus Φ yields two parallel lines as shown, where regression yields log j₀ = (6.4₄ ± 0.2₄) Φ (eV) - (35.₄±1.₁) with R² = 0.98 for the higher rate line and log j₀ = (6.5 ± 0.5₆) Φ (eV) - (38.₅ ± 2.₄) with R² = 0.93 for the lower rate line. The slopes are the same but the intercepts differ by 3. For a given Φ, metals on the upper line have HER rates that are 1000 x higher than metals on the lower line. In this presentation, the magnetic properties of the electrodes are shown to describe the binary segregation of the data into two discrete lines. The magnetic properties of the electrodes are set by paired and unpaired electron spins. The results are evidence of a magnetic effect on electron transfer.

Synthesis and Characterization of Novel Polypyrrole/Cerium Oxide Nanocomposite Sensor for the Voltammetric Quantification of Flupirtine Maleate

The highly conducting and novel Polypyrrole/Cerium Oxide (Ppyl/CeO2) nanocomposite material was synthesized and characterized by various spectral (FTIR, XRD, SEM) and electrochemical methods (voltammetry) along with electron impedance spectroscopy (EIS). The spectral data as shown in figure 1 clearly describes the successful synthesis of the Ppyl/CeO2 nanocomposite material. The Ppyl/CeO2 nanocomposite was used to modify the glassy carbon electrode (GCE) for the fabrication of Ppyl/CeO2/GCE nanocomposite sensor. The electrochemical characterization of the Ppyl/CeO2/GCE along with its other modified forms viz CeO2/GCE, Ppyl/GCE including bare GCE confirms the successful fabrication of the nanocomposite sensor. The surface area, heterogeneous electron transfer (HET) rate constant (k0 eff), and charge transfer resistance of the fabricated sensor were also determined by cyclic voltammetry (CV) and EIS respectively. The fabricated sensor was further optimized to carry out the voltammetric quantification of the Flupirtine Maleate (FPM). The sensor under optimized experimental conditions exhibits a linear relationship over a concentration range of 100 ng mL−1 to 500 ng mL−1 with the correlation coefficient (r2) of 0.9940 for the oxidation of FPM. The limit of detection (LOD) and limit of quantification (LOQ) calculated from calibration curve was found to be 26.76 ng mL−1 and 89.20 ng mL-1 respectively. The results thus clearly demonstrated that the newly fabricated voltammetric nanocomposite sensor possesses an ultra sensitivity for the oxidation of FPM. Figure 1

Energy-Efficient Hydrogen Evolution by Fe-S Electrocatalysts: Mechanistic Investigations.

The intrinsic catalytic property of a Fe-S complex toward H2 evolution was investigated in a wide range of acids. The title complex exhibited catalytic events at -1.16 and -1.57 V (vs Fc+/Fc) in the presence of trifluoromethanesulfonic acid (HOTf) and trifluoroacetic acid (TFA), respectively. The processes corresponded to the single reduction of the Fe-hydride-S-proton and Fe-hydride species, respectively. When anilinium acid was used, the catalysis occurred at -1.16 V, identical with the working potential of the HOTf catalysis, although the employment of anilinium acid was only capable of achieving the Fe-hydride state on the basis of the spectral and calculated results. The thermodynamics and kinetics of individual steps of the catalysis were analyzed by density functional theory (DFT) calculations and electroanalytical simulations. The stepwise CCE or CE (C, chemical; E, electrochemical) mechanism was operative from the HOTf or TFA source, respectively. In contrast, the involvement of anilinium acid most likely initiated a proton-coupled electron transfer (PCET) pathway that avoided the disfavored intermediate after the initial protonation. Via the PCET pathway, the heterogeneous electron transfer rate was increased and the overpotential was decreased by 0.4 V in comparison with the stepwise pathways. The results showed that the PCET-involved catalysis exhibited substantial kinetic and thermodynamic advantages in comparison to the stepwise pathway; thus, an efficient catalytic system for proton reduction was established.

Evaluation of new cholinium-amino acids based room temperature ionic liquids (RTILs) as immobilization matrix for electrochemical biosensor development: Proof-of-concept with Trametes Versicolor laccase

In this work, we present new cholinium-amino acids room temperature ionic liquids (ChAARTILs) that can be used as an efficient immobilization matrix for electrochemical biosensor development. The ideal immobilization strategy should be able to ensure the highest enzyme loading and a tight enzymatic immobilization, preserving its native structure and biological activity. In this regard, ChAARTILs present different side chains on the amino acids giving rise to van der Waals, π-π stacking and hydrogen bonding interactions. All these interactions can affect the nanomaterial organization onto the electrode surface. To this aim, we have evaluated the main electrochemical parameters, namely electroactive area (AEA) and the heterogeneous electron transfer rate constant (k0), showing how both cations and anions of room temperature ionic liquids (RTILs) can independently affect multi-walled carbon nanotubes (MWCNTs) organization. In particular, [Ch][Phe] showed the best performance in terms of AEA (3.432 cm2) and k0 (4.71·10−3 cm s−1) with a homogeneous distribution of MWCNTs bundles onto the electrodes and a faster electron transfer rate.Finally, the modified electrode (MWCNTs-[Ch][Phe]) has been tested with a model enzyme, namely Trametes versicolor laccase (Tvl), in order to evaluate the possibility to use ChAARTILs as immobilization matrix, preventing enzymatic denaturation phenomena which would affect the biosensor performance in terms of sensitivity, linear range, and stability.

Synthesis of MCM-41 nanoparticles from stem of common reed ash silica and their application as substrate in electrooxidation of methanol

In this work, stem of common reed ash (SCRA) is introduced as a new source of silica in the preparation of mesoporous materials. Mesoporous silicate MCM-41 nanoparticles were synthesized hydrothermally using sodium silicate prepared from SCRA as a silica source. The characterization of MCM-41was carried out by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), N $$_{2}$$ adsorption/desorption (BET) and transmission electron microscopy (TEM). SEM shows that MCM-41 nanoparticles are sphere-like with size in the range of 30–50 nm with some degree of agglomeration. TEM image of the synthesized sample shows the open framework structure of MCM-41. A type IV isotherm can be observed from adsorption/desorption curves, which is the characteristic of mesoporous materials. The prepared MCM-41 nanoparticles were used as substrate to facilitate the oxidation of methanol through the modification with an electroactive species. The modification was achieved by impregnation of MCM-41 pores with $$\hbox {Ni}^{2+}$$ ions (Ni-doped MCM-41). A modified carbon paste electrode (CPE) was prepared by mixing Ni-doped MCM-41 with carbon paste (NiMCM-41CPE). Cyclic voltammetry of NiMCM-41CPE shows an increment in current density of methanol oxidation in comparison with CPE in alkaline solution. Moreover, a decrease in the overpotential of methanol oxidation occurred on the surface of modified electrode. The effects of some parameters such as scan rate and methanol concentration are also investigated on the behaviour of NiMCM-41CPE. Also, the heterogeneous electron transfer rate for the catalytic reaction (k) of methanol is calculated.

Improved electrocatalytic metabolite production and drug biosensing by human liver microsomes immobilized on amine-functionalized magnetic nanoparticles

A biocatalytic system constructed by electrostatically immobilizing drug metabolizing human liver microsomes (HLM, negatively charged due to phospholipids) onto positively charged amine-functionalized magnetic nanoparticles (MNPamine, 100 nm hydrodynamic diameter) for biosensing of cytochrome P450 (CYP)-specific drug candidates and electrocatalyzing drug conversion into metabolites is reported. The MNP-adsorbed HLM biomaterial (denoted as MNPamine/HLM) was magnetically separated from the bulk suspension of HLM, washed twice in phosphate buffer (pH 7.0), and adsorbed onto polished edge-plane pyrolytic graphite (EPG) electrodes. Direct electrochemical properties and oxygen reduction currents were obtained by cyclic voltammetry (CV). Direct electron transfer kinetics from the microsomal redox proteins present in the adsorbed film of MNPamine/HLM on EPG surface was accomplished using square wave voltammetry due to its smaller background charging currents and improved faradaic peak intensity than CV. The MNPamine/HLM bioelectrodes exhibited a formal potential of −0.46 V vs Ag/AgCl reference (1 M KCl), which was in agreement with the FAD/FMN cofactor containing CYP-reductase (CPR) as the electron receiver from the electrode. This conclusion was drawn based on the formal potentials of only CPR (∼-0.47 V vs Ag/AgCl) containing bactosomal films adsorbed on EPG electrodes (bactosomes are bacterial membrane expressed enzymes). Whereas, bactosomal CYP 2C9 film showed a much more positive formal potential (−0.34 V vs Ag/AgCl). A heterogeneous electron transfer rate constant of 19 ± 5 s−1 was exhibited by the MNPamine/HLM film. Oxygen reduction currents and electrocatalytic diclofenac hydroxylation characteristics of the CYP enzymes present in the HLM film indicated the electron mediation by the CYP-reductase from the electrode to CYP-heme centers to initiate the catalytic cycle. Amperometric i-t curves with diclofenac drug concentration yielded the apparent Michaelis-Menten affinity constants (KMapp) of MNPamine/HLM and only HLM films as 302 ± 33 and 365 ± 36 μM, respectively. This is the first electrocatalysis report of bioelectrodes featuring HLM-bound MNPamine for applications in drug activity assays, enhanced metabolite production, and sensitive biosensing of CYP-specific drugs and environmental pollutants. Eliminations of tedious isolation and purification of membrane-containing CYPs, and the need for expensive NADPH cofactor (as electrode donates the reducing equivalents in an inexpensive and sustainable manner) are added advantages. Our future work aims to gain quantitative metabolite estimations upon scalability of such bioelectrode designs useful for pharmacological and toxicological evaluations of new drugs in development.

MoSe2 Dispersed in Stabilizing Surfactant Media: Effect of the Surfactant Type and Concentration on Electron Transfer and Catalytic Properties.

Layered transition metal dichalcogenides (TMDs) have gained attention from the scientific community because of their extended range of applications. Molybdenum diselenide (MoSe2) has been proven to be an efficient catalyst for the hydrogen evolution reaction (HER), having implications in the research of new catalysts for clean energy production. One way to produce large quantities of these materials involves the use of surfactants for liquid exfoliation. Herein, we investigate the effects of cationic, anionic, and nonionic surfactants within a concentration range on the heterogeneous electron transfer rates, electrocatalytic efficiency toward the HER of MoSe2, and on the stability of the dispersions. We found that surfactants can have a detrimental effect on the electrocatalytic properties of the material when used above a concentration threshold. In some cases, high surfactant levels also had a negative effect on the stability of the material. This report serves to gain an understanding on how the way TMDs are prepared, processed, and stabilized can have dramatic effects on their efficiency toward HER, one of their most popular applications, and how choosing the appropriate surfactant type and concentration is crucial to gain in stability without compromising the intrinsic properties of the material.

Cyclic voltammetry study of the electrochemical behavior of vanadyl sulfate in absence and presence of antibiotic

The cyclic voltammetry technique was used to study the electrochemical behavior of vanadyl sulfate in absence and presence of antibiotic (cefazolin) in 0.1 M KCl under different pH values at 300.15 K. The redox behavior of vanadyl sulfate has been studied by using glassy carbon electrode, Ag/AgCl as a reference electrode and Pt wire as a counter electrode under potential from +1500 mV to −1000 mV. One anodic peak and one cathodic peak are observed in cyclic voltammograms. Peak current ratio and peak potential separation (ΔE) was calculated, the higher values of them gave an indication about systems under study are quasi-reversible. The effect of pH, scan rate and concentration of electroactive species on the interaction between vanadyl sulfate and antibiotic were studied. Charge transfer coefficients (α), the heterogeneous electron transfer rate constants (ks) and the diffusion coefficients (D) involved in the redox reaction were evaluated.

A simple ultrasensitive electrochemical sensor for simultaneous determination of gallic acid and uric acid in human urine and fruit juices based on zirconia-choline chloride-gold nanoparticles-modified carbon paste electrode

The determination of gallic acid (GA) and uric acid (UA) is essential due to their biological properties. Numerous methods have been reported for the analysis of GA and UA in various real samples. However, the development of a simple, rapid and practical sensor still remains a great challenge. Here, a carbon paste electrode (CPE) was modified by nanocomposite containing zirconia nanoparticles (ZrO2NPs), Choline chloride (ChCl) and gold nanoparticles (AuNPs) to construct ZrO2-ChCl-AuNPs/CPE as electrochemical sensor for the simultaneous electro-oxidation of GA and UA. Characterization was performed by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. The modified electrode was investigated by different methods including electrochemical impedance spectroscopy and cyclic voltammetry. Kinetic parameters such as charge transfer coefficient, standard heterogeneous electron transfer rate constant and other parameters were calculated via voltammetry techniques. Differential pulse voltammetry was used for simultaneous determination of GA and UA applying the ZrO2-ChCl-AuNPs/CPE electrode. At the optimum conditions, this sensor showed a linear response in the ranges 0.22– 55 and 0.12–55 µM for GA and UA, respectively. In addition, low detection limits of 25 and 15 nM were obtained for GA and UA, respectively. Furthermore, ZrO2-ChCl-AuNPs/CPE was successfully applied for the independent determination of GA in green tea and fruit juice as well as the simultaneous determination of GA and UA in human urine samples.