Edge Plane Pyrolytic Graphite Research Articles

Label-free voltammetric screening of human blood serum

The current study presents a comprehensive voltammetric investigation into the direct analysis of untreated human blood serum in a phosphate buffer at an unmodified, graphite electrode by means of voltammetry. By employing advanced square-wave voltammetry at an edge plane pyrolytic graphite electrode (EPPGE), the basic principles were established for developing a sensitive, fast, simple, and label-free method for the simultaneous screening of uric acid, bilirubin, and albumin analytes that are present in human blood serum and are quite essential for rapid medical diagnostics. The electrochemical protocol utilizes the specific structural patterns of the EPPGE, the inherent redox and adsorption properties of the analysed analytes, and the sensitivity and rapidity of the employed advanced voltammetric technique. The methodology has been successfully applied for quantification of the considered analytes in a series of samples of human blood serum and was compared with the standard methods used in a clinical biochemical laboratory. This novel method represents a significant advancement towards the development of point-of-care devices aimed at swiftly and simultaneously quantifying uric acid, bilirubin, and albumin levels in human serum.

A novel and intelligent chemometric-electrochemical-enzymatic biosensing procedure and mimicking a clinical condition environment to trick the red blood cells for counting them under physiological conditions: A new connection among chemometry, electrochemistry and hematology

Here, a novel electrochemical biosensing procedure has been developed for determination of the number of red blood cells (RBCs) under physiochemical conditions based on chemometric modeling of hydrodynamic differential pulse voltammetric (HDPV), and amperometric data as responses of a modified edge plane pyrolytic graphite electrode (EPPGE). In order to obtain a good sensitivity from the EPPGE, its surface was modified with a thin layer of multiwalled carbon nanotubes-ionic liquid (MWCNTs-IL). Catalase (CAT) was immobilized onto the surface of MWCNTs-IL/EPPGE with help of nafion. The response of the biosensor was based on electrochemical reduction of oxygen of the blood samples which was enhanced by a trick based on addition of hydrogen peroxide (H2O2) to blood samples which can be reduced by the CAT to produce extra oxygen. Prior to experiments, the solution in electrochemical cell was bubbled with pure N2 to purge the oxygen in the solution, but in order to increase the selectivity of the biosensor towards detection of the oxygen obtained from the red blood cells, voltammetric responses of the biosensor were modeled by multivariate chemometric calibration methods with the help of radial basis function-partial least squares (RBF-PLS), least squares-support vector machines (LS-SVM), recursive weighted partial least squares (rPLS), ant colony optimization-mathematical pre-processing selection by genetic algorithm-sample selection through a distance-based procedure-partial least squares-1 (ACO-GA-SS-PLS1), and radial basis function-artificial neural networks (RBF-ANN) to select the best method for determination of the number of the RBCs. The results confirmed the amperometric methods modeled by RBF-ANN showed the best performance for supporting the biosensor in determination of the number of the RBCs with a performance which had an excellent compatibility with the results of a hemocytometer. The results of this study as the newest application of chemometric-electrochemical methods can make a strong connection among electrochemists, chemometricians and hematologists to expand their collaborations on determination of blood factors.

A Study of V(IV/V) and V(II/III) Reactions on Carbon Based Electrodes

The vanadium redox flow battery (VRFB) is one of the most promising energy storage technologies for large scale commercialization. Carbon based electrodes, such as carbon felts or carbon paper, are used for V(II/III) and V(IV/V) reactions. To improve the vanadium redox reaction kinetics, the carbon electrodes are usually activated by a variety of methods such as thermal treatment, heteroatom doping, catalyst incorporation etc. [1] It is generally considered that surface functional group such as C–OH, C=O play an important role in vanadium redox reaction [1-6]. Even though there are reports showing higher surface functional group content e.g. total C–O content leads to improved vanadium redox reaction kinetics [2-6], qualitative correlation of the surface property of the carbon materials and the vanadium redox reaction is not reported. There is a need for fundamental understanding of the surface properties of the carbon electrodes and their relation with the reaction kinetics, including the key parameters that determine them.In this work, using cyclic voltammetry (CV), we studied the surface properties of a variety of carbon based electrodes (glassy carbon (GC), edge plane pyrolytic graphite, base plane pyrolitic graphite, graphite (from Pine Research, called graphite-Pine thereafter), graphite rod, and H2SO4 soaked graphite rod), and the V(II/III) and V(IV/V) reaction kinetics on these electrodes. The goal of soaking the graphite rod electrode in H2SO4 is to activate the electrode and make it comparable to the carbon felt electrode [1]. XPS was also used to analyze the functional group. The relationship between the surface properties and the reaction kinetic parameters was analyzed.Fig. 1 compares the cyclic voltammograms (CV) of the edge plane, basal plane graphite and GC electrodes in 2 M H2SO4. Two redox couples are observed, ascribed to C=O and COOH group redox peak respectively [7]. On basal plane electrode, these two redox peaks are less significant, and an extra reduction peak is observed at 0.24 V due to the C–OH reduction. On the GC electrode, the C=O redox peak is still discernable but the COOH redox peak is almost undetectable. A capacitance region appears between 0.6 – 0.75 V vs Ag/AgCl for all these electrodes. Similar features are observed with the graphite-Pine, graphite rod and H2SO4 soaked graphite rod electrodes with significantly enhanced redox peaks and larger capacitance. The H2SO4 soaked graphite rod electrode presents the highest surface functional group density and capacitance. XPS also shows that the H2SO4 soaked graphite rod electrode has the highest total C–O content.CV of V(IV/V) and V(II/III) were measured using these electrodes at different scan rates. Fig. 2 shows the CV of V(IV/V) reaction on these electrodes. The three graphite rod electrodes show better reversible feature than the edge plane, basal plane and GC electrodes. Diffusion coefficient, transfer coefficient and reaction rate constant were obtained from the CVs. The diffusion coefficient follows the sequence of soaked graphite rod > graphite rod > Graphite-Pine > Edge plane > GC > basal plane electrode. The transfer coefficient and reaction rate constant are also different for these electrodes. Similarly, the diffusion coefficient, transfer coefficient and reaction rate constant are obtained for the V(II/III) reaction. The diffusion coefficient and reaction rate constant for the V(II/III) reaction are smaller than that for the V(IV/V) reaction on the corresponding electrodes. For both V(IV/V) and V(II/III), a linear relationship was found for diffusion coefficient versus the logarithm of capacitance and C=O group functional group. Higher value of log(capacitance) and log(C=O density) leads to larger diffusion coefficient. However, there is no clear relationship between transfer coefficient, reaction rate constant vs capacitance or C=O density. It seems that the capacitance and C=O functional group density are the importance parameters determining the vanadium redox reaction kinetics.References He, Y. Lv, T. Zhang, Y. Zhu, L. Dai, S. Yao., W. Zhu, L. Wang, Chem. Eng. J., 427 (2022) 131680Zhang, J. Xia, Z. Li, H. Zhou, L. Liu, Z. Wu, X. Qiu, Electrochim. Acta 89 (2013) 429– 435Liu, L. Yang, Q. Xu, C. Yan, RSC Adv., 2014, 4, 55666Choi, H. Noh, S. Kim, R. Kim, R. Kim, J. Lee, J. Heo, H.-T. Kim, J. Energy Storage, 21 (2019) 321 – 327Eifert, R. Banerjee, Z. Jusys, R. Zeis, J. Electrochem. Soc., 165 (2018) A2577 – A2586Leuna, D. Priyadarshani, A. K. Tripathi, M. Neergat, J. Electroanal. Chem., 878 (2020) 114590K. Singh, M. Pahlevaninezhad, N. Yasri, E. P. L. Roberts, ChemSusChem 2021, 14, 2100–2111 Figure 1

A Theoretical and Experimental Square-Wave Voltammetric Study of Ascorbic Acid in Light of Multi-Step Electron Transfer Mechanism

A detailed theoretical and experimental study of the complex redox mechanism of ascorbic acid in aqueous phosphate buffer solution (pH = 7.3) by square-wave voltammetry is presented. Experimental square-wave voltammograms at edge plane pyrolytic graphite electrode consist of a typical irreversible peak for a direct two-step electrode oxidation of the ascorbic acid. The complex mechanism of oxidation of the ascorbic acid was represented by a theoretical model for E1C’E2C mechanism. Morphology of theoretical square-wave voltammograms greatly depended on different specific parameters of the model. This feature was used to make the best fit between experimental and theoretical voltammograms for 2 different step potentials, which resulted in revelation of all specific parameters (standard electrochemical rate constant, diffusion coefficient, electron transfer coefficient and rate constants for both chemical reactions) of the model. Therefore, we showed that using only square-wave voltammetry, both experimentally and theoretically, it is possible to make a detailed study of the complex oxidation of ascorbic acid and further enlighten its redox mechanism.

Square-wave voltammetry of human blood serum

A study on voltammetric analysis of blood serum diluted in a phosphate buffer is presented using advanced square-wave voltammetry at an edge plane pyrolytic graphite electrode. The results demonstrate that even in a complex medium like human blood serum, electrochemical characterization can be achieved through the use of advanced voltammetric techniques in conjunction with an appropriate commercially available electrode, such as the edge plane pyrolytic graphite electrode, which boosts superior electrocatalytic properties. Without undergoing any chemical treatment of the serum sample, the square-wave voltammetry technique reveals, for the first time, the electrode reactions of uric acid, bilirubin, and albumin in a single experiment, as represented by well-defined, separated, and intense voltammetric signals. All electrode processes are surface-confined, indicating that the edge plane sites of the electrode serve as an ideal platform for the competitive adsorption of electroactive species, despite the extensive chemical complexity of the serum samples. The speed and differential nature of square-wave voltammetry are crucial for obtaining an outstanding resolution of the voltammetric peaks, maintaining the quasi-reversible nature of the underlying electrode processes, while reducing the impact of follow-up chemical reactions that are coupled to the initial electron transfer for all three detected species, and minimizing fouling of the electrode surface.

Electrochemical Sensing of Chlorpyrifos, a Carcinogen Responsible for Breast Cancer, in Milk and Plasma of Lactating Mothers

AbstractIn view of the increase in breast cancer cases at the global level, electrochemical sensing of the carcinogenic pesticide, chlorpyrifos (CPF) in breast milk is proposed. The determination is based on the nucleophilic substitution reaction of pralidoxime (PAM) with CPF. The proposed method offers a linear concentration range of 0.002 to 0.08 μmol/L. The limit of detection and limit of quantification was found to be 0.05×10−9 and 0.167×10−9M, respectively. The offered “unmodified edge plane pyrolytic graphite sensor” proved to be a better substrate than the earlier reported modified sensors. The limit of detection for the proposed method was found to be nearly fifty times lower than reported at modified electrodes. The interference study proved the adequate selectivity of the offered sensor. The sensor has good stability and reproducibility along with high sensitivity. The offered sensor is very useful for cancer hospitals, pesticide industries, and the study of environmental toxicity‐related issues.

Comparison of two pyrolytic graphite representatives in the construction of hybrid electrochemical DNA biosensors for monitoring DNA damage

• Fabrication of hybrid electrochemical DNA biosensors via electrostatic adsorption. • In-depth optimization process of the most crucial biosensor parameters. • Large potential window allowing dsDNA cathodic/anodic processes at single electrode. • First direct monitoring of DNA damage via cathodic signals at a carbon electrode. Electrochemical DNA biosensors represent a variety of versatile analytical tools successfully used in many important applications. Their utilization in the area of monitoring DNA damage is considered as the prominent field that can provide us with an improved general knowledge regarding DNA damaging events occurring in vivo by an unorthodox overall insight employing the various electrochemical methods. Herein, we present development, optimization process, and subsequent analytical testing of the novel hybrid electrochemical DNA biosensors based on the two representatives of the pyrolytic graphite – “basal-plane” pyrolytic graphite (BPPG) and “edge-plane” pyrolytic graphite (EPPG) – and low-molecular-weight double-stranded DNA (dsDNA). Closer resolution of the nature of the oxidation/reduction signals of the dsDNA, as well as the most important optimization of parameters securing satisfying operational stability of the proposed hybrid biosensors, is investigated. Subsequent analytical testing of the novel hybrid biosensors and the evaluation of their advantages/disadvantages in terms of monitoring DNA damage caused by an UV light irradiation employing various voltammetric methods (square wave (SWV), linear sweep (LSV), or cyclic (CV) voltammetry) and electrochemical impedance spectroscopy (EIS) is also presented.

Poly(Alizarin Red S) on pyrolytic graphite electrodes as a new multi-electronic system for sensing oxandrolone in urine

This study presents a new polymeric and multielectronic system, the poly-Alizarin Red S (PARS), obtained from the electropolymerization of Alizarin Red S (ARS) dye on an edge-plane pyrolytic graphite electrode (EPPGE) surface. During EPPGE/PARS electrochemical characterization, we identified seven stable and reversible redox peaks in acidic medium (0.10 mol L−1, pH 1.62 KH2PO4), which indicated its mechanisms underlying electropolymerization and electrochemical behavior. To the best of our knowledge, this is the first study to use an EPPGE/PARS electrode to detect oxandrolone (OXA) in artificial urine, where PARS acts as a synthetic receptor for OXA. The interactions of OXA with EPPGE/PARS as well as the properties of PARS were investigated using density functional theory (DFT). Atomic force microscopy (AFM) was used to characterize EPPGE/PARS, and it was found that the PARS polymer formed a semi-globular phase on the EPPGE surface. The limit of detection for OXA found by the sensor was close to 0.50 nmol L−1, with a recovery rate of approximately 100% in artificial urine. In addition to the application proposed in this study, EPPGE/PARS is a low-cost product that could be applied in several devices and processes, such as supercapacitors and electrocatalysis.

Edge plane pyrolytic graphite as a sensing surface for the determination of fluvoxamine in urine samples of obsessive-compulsive disorder patients

There is an increasing demand for fast and sensitive determination of antidepressants in human body fluids because of the present scenario of rising depression cases at the global level. A simple and sensitive voltammetric method using edge plane pyrolytic graphite electrode (EPPGE) as a novel sensor is presented for the determination of antidepressant fluvoxamine in urine and blood plasma samples of obsessive-compulsive disorder (OCD) patients. EPPGE is delineated the first time for this determination. EPPGE exhibited strong electrocatalytic activity and enhanced reduction signal towards the sensing of fluvoxamine. Fluvoxamine gave a well-defined reduction peak at ~ – 670 mV using EPPGE. The fluvoxamine reduction peak current was linear to its concentration in the range 5.00 × 10−9 – 0.1 × 10−6 mol L−1 and the limit of detection was found to be 3.5 × 10−9 mol L−1. The pre-eminence of EPPGE over mercury electrodes has been proved in terms of sensitivity and imperative analytical parameters. The pH study reveals the involvement of an equal number of electrons and protons in the reduction reaction mechanism. The frequency study indicated the adsorption controlled irreversible reaction mechanism. The stability and reproducibility of the offered sensor were also found most favorable. The interference study confirmed the optimum selectivity of the proposed sensor. The edge plane pyrolytic graphite sensing platform is recommended as a potential contender for the accurate and fast determination of fluvoxamine in depression medications as well as biological specimens of OCD patients.

Cobalt ferrite nanoparticles as a source of intrinsic metals for simultaneous electrosynthesis of Prussian blue and cobalt hexacyanoferrate

A novel method is proposed for the simultaneous electrochemical formation of Prussian blue (PB) and cobalt hexacyanoferrate (CoHCF) on the surface of edge-plane pyrolytic graphite electrodes (EPPGE) modified by CoFe2O4/Chit/PTCA nanoparticle (NP) (where CoFe2O4 = cobalt ferrite, Chit = chitosan, and PTCA = Perylene-3,4,9,10-tetracarboxylic acid). The PB and CoHCF films are formed from the NP intrinsic metals (Fe3+ and Co2+) in the presence of ferrocyanide ([FeII(CN)6]4−) dissolved in a Britton-Robinson (BR) buffer at pH 1.60. The extent of the film formation is determined by the amount of the PTCA immobilised on the CoFe2O4/Chit NP surface and is influenced by the components of the BR buffer medium. Larger amounts of immobilised PTCA enhance electrosynthesis and generate modified electrodes in situ with excellent electrochemical properties. Optimized electrodes increased electroactive area approximately 420% compared to bare EPPGE. We believe that the combination of NP intrinsic metals, PTCA immobilisation, use of [FeII(CN)6]4− and BR buffer components provides a new electrochemical structure for simultaneous production of PB and CoHCF films. The quality of the electrodes generated in situ by the present method is evidenced by their increased sensitivity and enhanced active surface area relative to bare EPPGE, which indicates its potential for future application in electrochemical sensors.

Electrochemical Properties of pH-Dependent Flavocytochrome C 3 from Shewanella Putrefaciens Adsorbed Onto Catechol-Modified Carbon Electrode

A century after the demonstration that electrons from microbial metabolism can be harvested with electrodes, electron transfer processes in biofilms of microorganisms performing respiration on extracellular solids are now being studied in order to increase the performance of microbial electrochemical technologies such as microbial fuel cells. Extracellular electron transfer occurring at the microorganism/electrode interface stimulated a wide range of fundamental and applied studies because of important implications in biotechnology, bioenergetics and bioremediation. Most studies focus on two Gram-negative mesophilic bacteria able to transfer electrons from their respiratory metabolism to extracellular solids, namely Shewanella oneidensis MR-1 and Geobacter sulfurreducens. In these model electroactive microorganisms, multiheme c-type cytochromes transfer electrons from cytoplasmic and inner-membrane oxidizing enzymes towards cell surface redox proteins that are responsible for the reduction of solid phase electron acceptors. It is also known that electron transfer is usually coupled to proton transfer which causes acidification in the anodic biofilms and in the anolyte with deleterious consequences for the biofilm metabolism and stability. The fundamental understanding of these processes that involve large proteins (>70 kDa) often with complex redox properties (with ≥10 heme centers in some multihemes cytochromes) is still in its infancy and precludes the optimization of microbial electrochemical technologies. In Gram-negative electroactive bacteria such as Shewanella, several redox proteins have pH-dependent potentials, such as flavocytochrome c 3 (M r = 63.8 kDa) which is one of the most abundant proteins in the periplasm and the only functional fumarate reductase in this microorganism.This work focuses on the design and the development of an efficient and versatile electrochemical platform able to probe charge (electron/proton) transfer properties of proteins from Gram-negative electroactive bacteria on modified carbon electrodes.[1] The electrodes are first modified with pH-responsive electrophores such as quinone units and then proteins are immobilized for electrochemical studies. If relevant, the protein electroactivity can be probed directly at the modified electrode while proton transport, if any, performed by the protein can be detected thanks to the grafted pH sensor.The electrochemical detection of a pH-dependent redox protein from electroactive bacteria at an electrode modified to act additionally as an efficient pH sensor based on a redox readout is demonstrated. The pH sensing electrode was previously designed and showed to allow the immobilization and study of pH-independent and redox active cytochrome c.[1] Here we extend this work to flavocytochrome c 3, a tetraheme FAD-containing periplasmic flavoenzyme isolated from the bacterium Shewanella putrefaciens, taken as a model pH-dependent redox protein from electroactive bacteria [2]. The modification of the electrode surface with the pH sensing modifier (catechol) stems from our previous experience in the tailoring of bio-interfaces of carbon electrodes with covalent electrografting. Flavocytochrome c 3 adsorption onto the modified electrode surface is successfully achieved by cyclic voltammetry in a flavocytochrome c 3 solution containing polymyxin as co-adsorbate. The electrochemical activity of the immobilized flavocytochrome c 3 is detected without alteration of its native structure and by keeping intact its electrochemical properties and its catalytic fumarate reductase activity.[2] The redox activity of the protein arises from its FAD and four hemes cofactors. The experiments evidence that the hemes’ redox potentials of flavocytochrome c 3 from Shewanella putrefaciens, for which no crystal structure is yet available, depend on pH which is at variance with data from the other strains Shewanella frigidimarina or Shewanella oneidensis.[1] Lebègue, E.; Louro, R. O.; Barrière, F. Electrochemical Detection of pH-Responsive Grafted Catechol and Immobilized Cytochrome c onto Lipid Deposit-Modified Glassy Carbon Surface. ACS Omega 2018, 3 (8), 9035–9042. https://doi.org/10.1021/acsomega.8b01425.[2] Lebègue, E.; Costa, N. L.; Fonseca, B. M.; Louro, R. O.; Barrière, F. Electrochemical Properties of pH-Dependent Flavocytochrome C3 from Shewanella Putrefaciens Adsorbed onto Unmodified and Catechol-Modified Edge Plane Pyrolytic Graphite Electrode. Journal of Electroanalytical Chemistry 2019, 847, 113232. https://doi.org/10.1016/j.jelechem.2019.113232. Figure 1

Activation of edge plane pyrolytic graphite in screen printed carbon electrodes on OHP sheet, Whatman paper and textile substrates

Screen printed carbon electrodes (SPCEs) are widely used in the field of electrochemical sensors, especially in the areas of food, water, environmental quality, and healthcare applications. In this context, microband SPCEs were fabricated on Whatman paper, overhead projection (OHP) sheet and cotton textile substrates for the development of cost-effective electrochemical biosensors. The fabricated electrodes were subjected to three consecutive processes such as thermal, electrochemical and oxygen plasma treatment. Surface morphologies of SPCEs after each treatment stage were observed using scanning electron microscope for process standardization. Electrochemical performances of the fabricated SPCEs were analyzed using cyclic voltammetry. SPCEs on OHP substrate exhibited a single electron transfer rate at ΔEp of 57 mV and ipa/ipc = 1 (n = 3) with significant repeatability and reproducibility compared to SPCEs on paper and textile substrates. The role of activation of edge plane pyrolytic graphite of SPCEs in achieving the reversible reaction with single electron transfer characteristics has been analysed and reported. The effectiveness of the activated SPCEs on the OHP substrate was tested by detecting 1 mM of uric acid and ascorbic acid with KCl as an electrolyte.

Comparison of Different Unmodified and Nano-Material Modified Sensors for the Ultrasensitive Determination of Serotonin

A comparison of response of different unmodified and polymer nanocomposite modified sensors has been monitored for the sensitive determination of serotonin (5-hydroxytryptamine; 5-HT). For the modification of sensors, polymer nanocomposite of poly-bromocresol green (p-BG) with single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs) and graphene (GRA) were prepared by electro polymerization at the surface of edge plane pyrolytic graphite (EPPG). Surface morphology of the different unmodified and modified sensor surfaces was studied by using Electrochemical Impedance Spectroscopy, Field Emission Scanning Electron Microscopy, cyclic voltammetry and square wave voltammetry (SWV). The calibration curves for 5-HT were recorded using SWV and linear relations between peak current and concentration were found in the wide concentration range. For unmodified electrode surfaces, limit of detection were found as 1.339, 0.850, 0.737, 0.405 and 0.104 μM for gold, palladium, glassy carbon, basal plane pyrolytic graphite and EPPG surfaces, respectively. The limit of quantification was found as 4.462, 2.835, 2.456¸ 1.349 and 0.348 μM respectively. Among unmodified sensors, EPPG sensor exhibited lowest detection limit. The modified sensors p-BG/SWCNTs/EPPG, p-BG/MWCNTs/EPPG and p-BG/GRA/EPPG electrocatalyzed the oxidation of 5-HT and a well-defined oxidation peak with enhanced peak current was observed. The detection limit was observed as 5.9, 4.3 and 2.4 nM with quantification limit of 19.66, 14.33 and 8 nM, respectively. The lowest detection limit was found for p-BG/GRA/EPPG sensor in comparison to all unmodified and other modified surfaces. An excellent electrocatalytic activity of p-BG/GRA/EPPG surface towards the sensitive determination of 5-HT was noticed, which exhibited much lower L.O.D in comparison to the ones reported in recent years.

Direct electrochemistry of bacterial surface displayed cytokinin oxidase and its application in the sensitive electrochemical detection of cytokinins

Cytokinin oxidase from Nipponbare (OsCKX4) was successfully displayed on the surface of E. coli cells by an ice nucleation protein from Pseudomonas borealis DL7 as an anchoring motif and a maltodextrin-binding protein(MBP) from E. coli as a solubility enhancer. The OsCKX4-displayed bacteria can be directly immobilized onto an electrode to selectively detect cytokinins, thus eliminating the need for enzyme extraction and purification. Direct electrochemistry of the cofactor FADH2 in OsCKX4 has been achieved on an edge-plane pyrolytic graphite electrode (PGE) with a formal potential (E0’) of −0.45 V at pH 7.0 in phosphate buffer. With the addition of isopentenyladenine, the reduction peak current for FADH2 decreased, and the oxidative peak current increased gradually. Therefore, a bacteria-OsCKX4-modified PGE has been developed for the detection of isopentenyladenine with a linear range of 1.0–11.0 μM and a lower limit of detection of 0.7 μM (S/N = 3). Slight interference was observed in the presence of other phytohormones, including brassinosteroid, abscisic acid, methylene jasminate and gibberellin. The proposed bacterial biosensor is stable, specific and simple and has great potential for applications that require the detection of cytokinins.

Electroanalysis of the Anthelmintic Drug Bithionol at Edge Plane Pyrolytic Graphite Electrode

AbstractAn electrochemical study of the anthelmintic drug bithionol using edge plane pyrolytic graphite electrode (EPPGE) is presented for the first time by applying different electrochemical techniques, such as cyclic voltammetry (CV), square‐wave voltammetry (SWV), square‐wave adsorptive stripping voltammetry (SWAdSV), and alternating current (AC) impedance spectroscopy. Mechanistic aspects of the electrode reaction were studied implying a quasireversible electrode reaction from an adsorbed state of the reactant, coupled with a follow‐up chemical reaction to a final electroinactive product. The overall mechanism appears totally irreversible under conditions of CV at moderate scan rate, while being quasireversible under conditions of the fast SWV. Furthermore, an optimisation of the analytical procedure for quantitative determination of bithionol was conducted by applying SWV in an adsorptive stripping mode. The calibration curve was constructed in the concentration range of 0.1–1.0 μmol L−1 (R2=0.9984) with a sensitivity of 3.6 μA L μmol−1 and LOD of 26.7 nmol L−1. The simple and sensitive SWAdSV procedure was proved to be suitable for the analysis of spiked urine samples.

Electrochemical properties of pH-dependent flavocytochrome c3 from Shewanella putrefaciens adsorbed onto unmodified and catechol-modified edge plane pyrolytic graphite electrode

The electroactivity of adsorbed flavocytochrome c3, a tetraheme FAD-containing flavoenzyme isolated from the bacterium Shewanella putrefaciens, is investigated by cyclic voltammetry at an edge plane pyrolytic graphite electrode before and after modification with grafted catechol serving as an efficient pH sensor based on a redox readout. Flavocytochrome c3 adsorption onto the unmodified or modified electrode surface is successfully achieved by cyclic voltammetry (100 consecutive cycles) in a flavocytochrome c3 solution containing polymyxin as co-adsorbate. The immobilized flavocytochrome c3 retains its electrochemical activity and its catalytic fumarate reductase activity. The redox activity of the protein arises from its FAD and four hemes cofactors. The experiments evidence that the hemes' redox potential of flavocytochrome c3 from Shewanella putrefaciens, for which no crystal structure is yet available, depend on pH which is at variance with data from the other strains Shewanella frigidimarina or Shewanella oneidensis.

Oxidation potentials of guanine, guanosine and guanosine-5′-monophosphate: Theory and experiment

Guanine, having lower one-electron oxidation potential than other nucleobases, is of relevance to oxidative degradation of nucleic acids in mutagenesis, carcinogenesis, and aging. Here we compare oxidation potentials of guanine (G), guanosine (Guo), deoxyguanosine (dGuo), guanosine -5′- monophosphate (GMP) and 2′- deoxyguanosine -5′- monophosphate (dGMP) obtained by theoretical and experimental methods. Structures of G species were optimized and the identities of minima were verified by vibration frequency calculations. Redox equilibria were modelled in terms of corresponding thermochemical cycles. The changes in free energy were calculated at DFT level using the two different functionals: (i) general purpose B3LYP functional, and (ii) more specific ωB97X-D functional (both with 6-31 + G(d) basis set). Experimental oxidation potentials of all G analogues were measured voltammetrically on a polymer pencil graphite electrode (pPeGE) providing the best results from all carbon electrodes used (glassy carbon electrode, basal and edge plane pyrolytic graphite electrodes). The oxidation process is strongly dependent on the pH value and with increasing pH a linear shift of G oxidation peaks (Epa) towards negative potentials is observed. The theoretically and experimentally obtained oxidation potentials were compared for the pH 5. Anodic peak potentials increase in the order G « dGMP ≤ GMP < dGuo ≤ Guo and correlate with the calculated thermodynamic redox potentials as well as with NBO charges in purine moiety. The oxidation of deoxy analogues was predicted theoretically to occur at lower potentials than that of corresponding parent compounds and this fact was experimentally verified. The assumption that due to negatively charged phosphate group of GMP or dGMP their oxidation potentials could be observed at lower positive potential has not been confirmed and the significant difference (more than 200 mV) between the oxidation potentials of G nucleobase and its nucleosides and nucleotides is discussed. Moreover, conformity of theoretical and experimental data for radicals (cation, neutral) indicates that while the deprotonation process of G differs from its analogues, the oxidation process of all species takes place on imidazole ring.

Electrochemical behaviors of alizarin and Cr(VI)–Alizarin complex adsorbed on edge-plane pyrolytic graphite electrode

Metal complexes have been subject of study worldwide. These compounds are useful for a multitude of applications in industry and medicine, or in chemical and electrochemical analysis. For the first time, the electrochemical behaviors of alizarin (Alz) and a Cr(VI)–Alz complex adsorbed on an edge-plane pyrolytic graphite electrode (EPPGE) surface were studied. The complex was investigated at pH 5.10–6.52; the best voltammetric response was obtained at pH 6.52. Alz and the metallic complex both exhibited adsorptive behavior and high electrochemical stability and were strongly influenced by the electrolyte pH. EPPGE modification with Alz, together with square-wave-adsorptive stripping voltammetry, enabled quantification of Cr(VI) ions in an aqueous medium (Britton–Robinson buffer); the detection limit was 0.107 nmol L−1. A linear response was achieved in the concentration range between 0.10 and 2.30 nmol L−1. Therefore, this system is expected to be a useful electrochemical sensor for Cr(VI) at ultra-trace levels.

Development and first application of the edge plane pyrolytic graphite electrode modified with graphene nanoplatelets for highly sensitive voltammetric determination of oxolinic acid

In this paper, a graphene nanoplatelets (GNPs) were used as a modifier of an edge plane pyrolytic graphite electrode (EPPGE) surface, and the graphene nanoplatelets modified edge plane pyrolytic graphite electrode (GNPs–EPPGE) was prepared by simple drop casting procedure. The as-prepared GNPs–EPPGE was characterized by a cyclic voltammetry (CV), a scanning electron microscopy (SEM), and an atomic force microscopy (AFM). The electrochemical behavior of OxA on the EPPGE and the GNPs–EPPGE was performed using the cyclic voltammetry and an electrochemical impedance spectroscopy (EIS). Further, a sensitive and selective square–wave voltammetric (SWV) procedure using the GNPs–EPPGE was developed for the determination of a quinolone antibiotic oxolinic acid (OxA). An oxidation peak of OxA was observed in Britton–Robinson buffer solution with the highest response at pH 5.0 at potential c.a. +1.4 V vs. Ag/AgCl reference electrode. Compared to the unmodified EPPGE, the EPPGE modified GNPs exhibited a marked enhancement in the current response of OxA. Calibration curve on the GNPs–EPPGE was constructed in the concentration range of 3.0 × 10−7–9.1 × 10−6 mol L−1 with a correlation coefficient of 0.9997, a sensitivity of 1.39 μA L μmol−1, a limit of detection (LOD) of 5.2 × 10−8 mol L−1, and a limit of quantification (LOQ) of 1.7 × 10−7 mol L−1. The determination of OxA in the spiked human urine samples was carried out using the GNPs–EPPGE with the satisfactory recovery of 99.6%. The selectivity of the developed method for the determination of OxA using the GNPs–EPPGE was tested.

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.