Electroinactive Species Research Articles

The effect of weak proton donors on the steady-state behavior of hydrogen evolution in mildly acidic media

The influence of anionic and cationic electro-inactive species on the steady-state current of the Hydrogen Evolution Reaction (HER) is often not considered. Here, the effects of these widely-used electrolyte species on the steady-state behavior of HER are investigated on a Au RDE at a bulk pH of 4. A current enhancement is observed from the coupling of proton reduction with homogeneous acid-base equilibria, with HSO4− having the largest influence despite the electrolyte condition being far beyond its typical buffering range, while the cations, specifically K+ and Cs+, serve as weak buffers with limited buffer capacity. Further quantitative analysis illustrates that from the steady-state current of HER the thermodynamic parameters of the homogeneous reactions can be evaluated, taking advantage of the linear dependence of the Koutecký-Levich Slope on the concentration of the electro-inactive species. Based on this evaluation, the pKa‘s of HSO4−, K+ and Cs+ were determined to be 2.06, 2.52 and 2.48, respectively. These results confirm the previously postulated proton-donating capability of certain (alkali) cations and show that these effects influence the HER current in mildly acidic media.

Recent Progress in Organic Electrochemical Transistor-Structured Biosensors.

The continued advancement of organic electronic technology will establish organic electrochemical transistors as pivotal instruments in the field of biological detection. Here, we present a comprehensive review of the state-of-the-art technology and advancements in the use of organic electrochemical transistors as biosensors. This review provides an in-depth analysis of the diverse modification materials, methods, and mechanisms utilized in organic electrochemical transistor-structured biosensors (OETBs) for the selective detection of a wide range of target analyte encompassing electroactive species, electro-inactive species, and cancer cells. Recent advances in OETBs for use in sensing systems and wearable and implantable applications are also briefly introduced. Finally, challenges and opportunities in the field are discussed.

Construction of MIP/Bi2S3 nanoparticles/rGO nanoprobe for simultaneous electrochemical determination of amoxicillin and clavulanic acid

Herein, a dual-template electrochemical sensor was developed for simultaneous determination of electro-active drug e.g. amoxicillin (AMOX) and electro-inactive drug e.g. clavulanate potassium (CLAV). The sensor consists of bismuth selenide nanoparticles (Bi2Se3 NPs) modified reduced graphene oxide (rGO), over which a thin film of molecular imprinted polymer was formed (MIP/Bi2Se3 NPs@ rGO). AMOX was directly oxidized on the surface of the as-fabricated sensor while CLAV decreased the current peak of a redox probe. The proposed MIP-based electrochemical sensor exhibited wide-linear ranges of 1.42–1100 μM and 0.2–1100 μM for AMOX and CLAV, respectively, with respective limit of detection (S/N = 3) values of 0.43 μM AND 0.06 μM. Additionally, the proposed sensor was efficiently applied for the analysis of pharmaceutical tablets and human serum samples with recoveries % in the range of 98.0–103.1% and RSDs % less than 3.6%. The results of the determination agreed well with HPLC/UV method. The synergism between MIP and Bi2Se3 NPs@ rGO resulted in many advantages including high selectivity, low detection limits, good stability, and high reliability. The proposed sensor exhibited a new venue for simultaneous determination of electro-active and electro-inactive species during therapeutic drug monitoring (TDM).

Hydrogel-Coated Microelectrode Resists Protein Passivation of In Vivo Amperometric Sensors.

Passivation of electrodes caused by nonspecific adsorption of protein can dramatically reduce sensing sensitivity and accuracy, which is a great challenge for in vivo neurochemical monitoring. However, most antipassivation strategies are not suitable to carbon fiber microelectrodes (CFMEs) for in vivo measurement, and these methods also do not work on electrochemical biosensors that fix biometric elements. In this study, we demonstrate that chitosan hydrogel-coated microelectrodes can avoid the current passivation caused by protein adsorption on the surface of carbon fiber because the chitosan hydrogel prepared by local pH gradient caused by hydrogen evolution reaction has three-dimensional networks containing large amounts of water. The highly hydrophilic three-dimensional structure of hydrogel not only forms a biocompatible interface to confine enzymes but also keeps the fast mass transfer of analytes, such as dopamine, ascorbic acid, and glucose. The consistency of the precalibration and postcalibration of the prepared sensor enables in vivo amperometric detection of both electroactive species based on their redox property and electroinactive species based on the enzyme. This study provides a simple and versatile strategy to constitute an amperometric sensor interface to resist passivation of protein adsorption in a complex biological environment such as the brain.

Unraveling the formation of optimum point in NiCo-based electrocatalysts for urea oxidation reaction

The urea oxidation reaction (UOR) has been recognized as a potential replacement for the hydrogen oxidation reaction in anion exchange membrane fuel cells, as well as the oxygen evolution reaction in water electrolyzers. For the successful application of this technology, advanced electrocatalysts for UOR must be developed to overcome the high overpotential. This can be achieved by introducing Co into Ni-based catalysts. Normally, an optimum ratio of Ni:Co would show the highest activity; however, a higher content of Co adversely affects the catalyst performance. In this study, we provide an explanation for the optimum point in NiCo-based catalysts. In addition to the positive effect of reducing the UOR onset potential, electrochemical analyses suggested that an electro-inactive species for UOR was formed by doping with Co, and this species exerted its influence more strongly as the content of Co increased. Thus, the performance was maximized at an optimum ratio of Ni:Co, at which the advantages and disadvantages of Co-doping were balanced. The results of our study indicated that Co-doping at 10 at% showed the highest UOR catalytic activity as proven by the electrochemical measurements.

(Invited) Potential Applications of Nano-Electrochemistry in Point-of-Care Sensing Devices

Using low noise electronics and micro-electrodes we can now electrochemically detect and count single, nanometer-scale objects in solution.1,2 This talk will cover recent discoveries in the field of single entity electrochemistry showing that electro-inactive species, including proteins, DNA, and even cells, can be detected individually using unmodified electrodes.3–5 Despite these advances, however, limitations persist. The location upon which the individual analyte adsorbs onto the electrode surface influences the magnitude of the electrochemical signal it produces.6,7 Consequently, the electronic output does not reflect the true physical properties of the biological entity, thus inhibiting the analytical resolution desired for applications.In response, we have shown that coupling the electrochemical reaction to a homogeneous, rate-limiting chemical reaction ameliorates this problem, improving the precision of single-entity electrochemical blocking measurements. Using both finite element simulations and experimental statistical analysis we provide guidance for the sizing of redox-inactive materials with high analytical precision under a range of conditions, including variations in electrode size and analyte concentration. The presented nanoscale electrocatalytic approach could offer a novel means of achieving cost-effective, rapid, real-time detection of nanometer-scale “insulators” in complex media, an advance that could be of significant relevance to point-of-care medical diagnostics.

Enzymatic electrochemical biosensors for in situ neurochemical measurement

Abstract In situ real-time electroanalysis of neurochemical concentrations offers rich information for exploring the spatiotemporal heterogeneity of molecular events in the nervous system. Biosensors with enzyme-modified sensing interfaces are well suited for measuring neurologically important but electroinactive species, greatly broadening the range of electrochemically detectable analytes. Minimally invasive micro/nanoelectrodes bearing enzymatically functional layers are used to fulfill the spatiotemporal requirements for in situ neurochemical studies at various neuronal levels as discussed in this minireview. Current achievements on nanoelectrode biosensors for intracellular analysis and microelectrode biosensors for single-cell and in vivo analysis are highlighted to enlighten emerging trends in this realm, and views of prospective enzymatic biosensors for neurochemical analysis are provided in the end.

In Vivo and Online Optical Ascorbic Acid Monitoring in Living Rat Brains Using a Brightfield Microscope-Based Online Detection Platform

Online electrochemical systems (OECSs) possess a distinct continuously monitoring ability toward electro-active species and have been intensively used for single/multiple neurochemical measurements. However, detecting electro-inactive species or some substances being oxidized/reduced at close potentials is difficult in OECS, and it is highly demanded to broaden such useful online methodology to more new principles, which, however, is still lacking. The system reported in this Letter demonstrates a solution to the aforementioned problem. We report an online optical detection platform (OODP) based on visible light absorption for the continuous monitoring of neurochemicals in rat brains with ascorbic acid (AA) as an example. The OODP is constructed with a homemade online capillary-based microfluidic line as the optical detector built under a brightfield microscope. Cobalt oxyhydroxide (CoOOH) nanoflakes directly oxidizing 3,3′,5,5′-tetramethylbenzidine (TMB) into a blue-colored product (oxTMB) which can be f...

Bioelectrochemistry for in vivo analysis: Interface engineering toward implantable electrochemical biosensors

Summary With a long history in research, bioelectrochemistry has emerged as a hot spot in the development of implantable biosensors with desired selectivity, sensitivity and spatiotemporal resolution for accurate analysis of electroinactive species in vivo . To catalyze electrode reactions (oxidation or reduction) of target analytes, enzymes are employed as the efficient bioelectrocatalysts immobilized on the electrodes. In bioelectrochemistry, the biggest challenge is the interfacial electron transfer between the redox active centers of enzymes and electrode surfaces. Besides, enzymatic electrodes are prone to inhibition and inactivation by environmental factors (co-substrates, inhibitors, pH, etc.). Rational design of the electrode surface/interface is necessitated to fulfill the requirements of in vivo applications. This mini review covers the recent and representative progress on interface engineering to enable bioelectrochemistry for in vivo analysis with an emphasis on heterogeneous electron transfer, oxygen dependence and microenvironment tolerance of bioelectrodes and self-powered electrochemical biosensors as well.

Scanning Electrochemical Microscopy Determination of Heterogeneous Electron Transfer and Homogeneous Association and Comproportionation Rate Constants of Anthrarufin

AbstractWe report the use of scanning electrochemical microscopy (SECM) in determining the heterogeneous electron transfer and homogeneous association and comproportionation kinetics of anthrarufin (HQ) in acetonitrile at a Pt tip UME (radius 12.5 µm). HQ undergoes two consecutive one‐electron reductions with comproportionation occurring between HQ2−, the product of the second reduction, and bulk HQ to produce HQ− and association occurring between HQ− and bulk HQ to produce an electroinactive species. Standard rate constants, k10=5.1±0.4 cm/s, for the first reduction, and a comparatively smaller rate constant, k20=0.37±0.03 cm/s, for the second reduction, were determined by tip voltammetry over a large Pt substrate. An association rate constant, kassoc=2.9×102 M−1 s−1, was determined using positive feedback approach curves over co‐axially aligned tip and substrate UMEs of the same radius. A comproportionation rate constant, kcomp=7.1×101 M−1 s−1, was determined in the presence of the association reaction by tip pulse chronoamperometry at the same co‐axially aligned tip and substrate UMEs. Diffusion coefficients of the HQ species and standard potentials for the reductions were also determined. Experimental results were compared with 2D transient axisymmetric simulations.

Cyclic Voltammetry Studies of H-Bond Complex of a p-Phenylenediamine-Based Urea with 1,8-Naphthyridine As the Proton-Coupled Electron Transfer Guest with Platinum and Glassy Carbon As Working Electrodes

Reactions in which both electron and proton are transferred, called proton-coupled electron transfer (PCET) reactions, are essential to many of the fundamental chemical processes of life. While PCET reactions have been well studied, it is only relatively recently that the role of H-bonding intermediates has been examined. The role that H-bonding can play in PCET was demonstrated by studies in the Smith group with U(H)H, a p-phenylenediamine-based urea which undergoes a reversible oxidation in methylene chloride.1 The overall oxidation of U(H)H is a 1 e- transfer with a surprising 2 e-/ 1 H+ mechanism where removal of the first electron produces a radical cation with an acidic NH proton that is then removed by the dimethylamino group from another U(H)H. This leads to removal of a second electron and creates the final products: the protonated electroinactive species, HU(H)H+, and the doubly oxidized form, U(H)+. Since this corresponds to 2 e- per 2 ureas, the process appears to be a net 1 e-. The interesting observation is that on the return scan two distinct pathways are observed. At the more negative potential, one pathway proceeds via the reduction of the U(H)+. This path dominates at low concentrations and fast scan rates. We hypothesize that the other path, which occurs at less negative potential, involves reduction of a H-bond complex formed between HU(H)H+ and U(H)+, leading to an overall electrochemically reversible process. In this study, cyclic voltammetry of U(H)H in the presence of 1,8-naphthyridine has been examined. The addition of naphthyridine results in an increase in current height and the appearance of a second oxidation wave at a more positive potentials . The increase in current is due to competition for the acidic proton between naphthyridine and the dimethylamino group of another U(H)H. NMR titration analysis show that naphthyridine H-bonds to the reduced U(H)H meaning naphthyridine blocks the H-bond sites from another U(H)H. Interestingly, the shape and behavior of the more positive oxidation wave alters depending on the electrode material. We hypothesize that the change in the CV wave is due to electrode fouling on the platinum (Pt) and poorly polished glassy carbon (GC) electrodes. With a carefully polished GC electrode, which minimizes the effect of electrode fouling, the observed second oxidation wave shifts slightly to more negative potentials and appears to merge with the first oxidation wave as the concentration of naphthyridine increases. This behavior reflects a mechanism that is similar to that of U(H)H by itself except the dimethylamino group of the other urea is replaced with naphthyridine. Therefore, the appearance of the second oxidation wave is due to oxidation of the H-bonding complex between the radical cation urea, U(H)H+, and naphthyridine, which is accompanied by proton transfer to naphthyridine. 1. Clare, L. A.; Pham A. T.; Magdaleno, F.; Acosta, J.; Woods, J. E.; Cooksy, A. L.; Smith, D. K. J. Am. Chem. Soc., 2013, 135 (50), 18930–18941.

Highly selective and sensitive analysis of γ-aminobutyric acid using a new molecularly imprinted polymer modified at the surface of abrasively immobilized multi-walled carbon nanotubes on pencil graphite electrode

This work describes the development of an electrochemical sensor based on a new molecularly imprinted polymer for the detection of γ-amino butyric acid at ultra-trace level. This is comprised of a thin imprinted film exploiting an abrasive immobilization of functionalized multi-walled carbon nanotubes on the tip of a pencil graphite electrode. For the reason that γ-aminobutyric acid is an electro-inactive species, its o-pthalaldehyde/sulphite derivative was used as a template for imprinting polymer made from a typical functional monomer (5-fluorouracil-N-acetylacrylamide) and cross-linking agent (ethylene glycol dimethacrylate) in the porogen, dimethyl sulphoxide. The electrodics of the template was explored using various techniques, viz., cyclic voltammetry, differential pulse anodic stripping voltammetry, and chronocoulometry. The template detection, in terms of γ-aminobutyric acid, at stringent limits of clinical settings was feasible by differential pulse anodic stripping voltammetry technique in the linear concentration range, 0.75–205.19ngmL−1 (correlation coefficients 0.999, detection limits 0.28ngmL−1), without any cross-reactivity and false positives in aqueous, human serum, and cerebrospinal fluid.

Analytical expression of non steady-state concentration for the CE mechanism at a planar electrode

The analytical solutions of the non-steady-state concentrations of species at a planar microelectrode are discussed. The analytical expression of the kinetics of CE mechanism under first or pseudo-first order conditions with equal diffusion coefficients at planar electrode under non-steady-state conditions are obtained by using Homotopy perturbation method. These simple new approximate expressions are valid for all values of time and possible values of rate constants. Analytical equations are given to describe the current when the homogeneous equilibrium position lies heavily in favour of the electroinactive species. Working surfaces are presented for the variation of limiting current with a homogeneous kinetic parameter and equilibrium constant. In this work we employ the Homotopy perturbation method to solve the boundary value problem. Furthermore, in this work the numerical simulation of the problem is also reported using Scilab program. The analytical results are found to be in excellent agreement with the numerical results.

Analytical expression of non steady-state concentration profiles at planar electrode for the CE mechanism

The analytical solutions of the non-steady-state concentrations of species at a planar microelectrode are presented. These simple new approximate expressions of concentrations are valid for all values of time and possible values of rate constants. Analytical equations are given to describe the current when the homogeneous equilibrium position lies heavily in favour of the electroinactive species. Working surfaces are presented for the variation of limiting current with a homogeneous kinetic parameter and equilibrium constant. Moreover, in this work we employ the Homotopy perturbation method to solve the boundary value problem.

A non-oxidative electrochemical approach to online measurements of dopamine release through laccase-catalyzed oxidation and intramolecular cyclization of dopamine

A new electrochemical approach to selective online measurements of dopamine (DA) release in the cerebral microdialysate is demonstrated with a non-oxidative mechanism based on the distinct reaction properties of DA and the excellent biocatalytic activity of laccase. To make the successful transition of the distinct sequential reaction properties of DA from a conceptual determination protocol to a practical online analytical system, laccase enzyme is immobilized onto magnetite nanoparticles and the nanoparticles are confined into a fused-silica capillary through an external magnetic field to fabricate a magnetic microreactor. The microreactor is placed in the upstream of the thin-layer electrochemical flow cell to efficiently catalyze the oxidation of DA into its quinonoid form and thereby initialize the sequential reactions including deprotonation, intramolecular cyclization, disproportionation and/or oxidation to finally give 5,6-dihydroxyindoline quinone. The electrochemical reduction of the produced 5,6-dihydroxyindoline quinone at bare glassy carbon electrode is used as the readout for the DA measurement. The laccase-immobilized microreactor is also found to catalyze the oxidation of ascorbic acid (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC) into electroinactive species and, as such, to eliminate the great interference from both species. Moreover, the successful transition of the mechanism for DA detection from the conventional oxidative electrochemical approach to the non-oxidative one substantially enables the measurements virtually interference-free from physiological levels of uric acid, 5-hydroxytryptamine, norepinephrine, and epinephrine. The current response is linear with DA concentration within a concentration range from 1 to 20 μM with a sensitivity of 3.97 nA/μM. The detection limit, based on a signal-to-noise ratio of 3, is calculated to be 0.3 μM. The high selectivity and the good linearity as well as the high stability of the online method make it very potential for continuous monitoring of cerebral DA release in physiological and pathological processes.

A theory of square-wave voltammetry of surface-active, electroinactive compounds

A potential-dependent adsorption of electroinactive species on the surface of working electrode is investigated theoretically for square-wave voltammetry (SWV). It is demonstrated that the tensammetric peak originates from the difference in surface coverage during the forward and backward series of pulses of SWV signal. Linear dependence of SWV net peak current on SW frequency, as well as of SWV net peak potential on dimensionless bulk concentration of reactant and Frumkin coefficient is predicted by the model.

Numerical simulation of the limiting current for the CE mechanism at a microdisc electrode

The finite difference method in conjunction with a conformal mapping technique is used to simulate the diffusion limiting current for three different CE mechanisms at a microdisc electrode. Working surfaces are presented for the variation of the limiting current with a normalised homogeneous kinetic parameter and equilibrium constant. Empirical equations are given to describe the current when the homogeneous equilibrium position lies heavily in favour of the electroinactive species.

Laccase-catalyzed oxidation and intramolecular cyclization of dopamine: A new method for selective determination of dopamine with laccase/carbon nanotube-based electrochemical biosensors

This study demonstrates a new electrochemical method for the selective determination of dopamine (DA) with the coexistence of ascorbic acid (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC) with laccase/multi-walled carbon nanotube (MWNT)-based biosensors prepared by cross-linking laccase into MWNT layer confined onto glassy carbon electrodes. The method described here is essentially based on the chemical reaction properties of DA including oxidation, intramolecular cyclization and disproportionation reactions to finally give 5,6-dihydroxyindoline quinone and on the uses of the two-electron and two-proton reduction of the formed 5,6-dihydroxyindoline quinone to constitute a method for the selective determination of DA at a negative potential that is totally separated from those for the redox processes of AA and DOPAC. Instead of the ECE reactions of DA with the first oxidation of DA being driven electrochemically, laccase is used here as the biocatalyst to drive the first oxidation of DA into its quinone form and thus initialize the sequential reactions of DA finally into 5,6-dihydroxyindoline quinone. In addition, laccase also catalyzes the oxidation of AA and DOPAC into electroinactive species with the concomitant reduction of O 2. As a consequence, a combinational exploitation of the chemical properties inherent in DA and the multifunctional catalytic properties of laccase as well as the excellent electrochemical properties of carbon nanotubes substantially enables the prepared laccase/MWNT-based biosensors to be well competent for the selective determination of DA with the coexistence of physiological levels of AA and DOPAC. This demonstration offers a new method for the selective determination of DA, which could be potentially employed for the determination of DA in biological systems.

Charge-transfer reactions at liquid/liquid interfaces and their applications in bioassays

Charge (electron and ion)–transfer reactions at a liquid/ liquid (L/L) interface are one of the most fundamental physicochemical processes. The liquid/liquid interface is actually made up of two immiscible electrolyte solutions. For instance, one organic solvent with a supporting electrolyte usually contacts with an immiscible aqueous solution. The study of such soft molecular interfaces by various electrochemical techniques is called electrochemistry at liquid/liquid interfaces. Although Nernst et al. carried out experiments at a water/phenol interface in 1902, this field has only been developed fully in the last 30 years because it has relevance to mimicking biomembranes, studying drug release, chemical and biological sensing, and understanding the mechanism of phase-transfer catalysis etc. [1–5]. Unlike a solid/electrolyte interface in conventional electrochemistry, in which redox species are the main task and electron-transfer reactions and sometimes adsorption/desorption processes are involved, one of the characteristics of electrochemistry at liquid/liquid interfaces is the diversity of charge-transfer reactions (CTRs), which can be studied and applied [5]. These CTRs can be classified into three main categories: (1) ion-transfer (IT) reactions; (2) facilitated ion-transfer (FIT) reactions; and (3) electron-transfer (ET) reactions. Apparently, apart from electroactive species, many electro-inactive species can also be investigated electrochemically at a liquid/liquid interface, and this actually opens many potential analytical applications. Charge– transfer reactions at liquid/liquid interfaces

Electrochemical flow injection analysis study of ion partitioning at high surface area carbon fiber electrodes.

Charge-selective electrochemistry was previously shown to occur at high surface area carbon fibers that were produced by fracturing the outer periphery with anodic current or positive potential. The cyclic voltammetric behavior of electroactive species observed at these fibers exhibited a distinct pH dependence related to the protonation/deprotonation of oxygen-containing functional groups at the surface of the carbon fiber. In this paper, electrochemical flow injection analysis (EC-FIA) is used to probe ion partitioning in to and out of the interior microstructure of the treated carbon fiber, for both electroactive and electroinactive species. It was found that the extent of partitioning was the result of both ion charge and hydrated ionic radius, in addition to the level of fracture. It was further observed that the direction of movement for an injected ionic species could be controlled relative to the ion concentration, the pH of the carrier solution, or both. EC-FIA allowed the simultaneous observation of current due to ion movement and that due to electron transfer to a redox-active species. The results presented are consistent with a model in which fixed negatively charged sites in the interior of fractured fibers govern ion partitioning with positively charged ions in the carrier solution, with counterions located in the interior "free" volume.