An Electrochemical Assay for Monitoring Differentiation of the Osteoblastic Cell Line (MBA‐15) on the Sensor Chip

An electrochemical assay for the indication of the activity of the cell bound differentiation marker alkaline phosphatase (ALP) is proposed using voltammetry on an in‐vitro cell culture. The basis of the assay is cultivation of cells on gold microelectrodes in wells of a microplate, catalytic hydrolysis of p‐aminophenyl phosphate by ALP and indication of p‐aminophenol oxidation by square wave voltammetry (SWV) with the sensors onto which the cells attached. The morphology of the bone marrow stromal cell line (MBA‐15) on the electrode surface was investigated and it exhibited in vitro osteogenic characteristics. Since ALP is expressed on the cell surface in early differentiation stage of osteoblastic cells, its activity was followed after different culture times over a period of 144 h by recording repetitive voltammograms at different time points upon addition of the substrate p‐aminophenyl phosphate. The ALP activity was estimated from the signal increase related to formation rate of p‐aminophenol and the number of cells. The highest value was measured at 120 h, when the cells reached confluence. The results of the electrochemical activity assay are consistent with the colorimetric acquired value from p‐nitrophenol formation rate.     

Electrocatalytic Oxidation of Some Carbohydrates by Nickel/Poly(o‐Aminophenol) Modified Carbon Paste Electrode

This study investigates the electrocatalytic oxidation of glucose and some other carbohydrates on nickel/poly(o‐aminophenol) modified carbon paste electrode as an enzyme free electrode in alkaline solution. Poly(o‐aminophenol) was prepared by electropolymerization using a carbon paste electrode bulk modified with o‐aminophenol and continuous cyclic voltammetry in HClO₄ solution. Then Ni(II) ions were incorporated to electrode by immersion of the polymeric modified electrode having amine group in 1 M Ni(II) ion solution. Cyclic voltammetric and chronoamperometric experiments were used for the electrochemical study of this modified electrode; a good redox behavior of Ni(OH)₂/NiOOH couple at the surface of electrode can be observed, the capability of this modified electrode for catalytic oxidation of glucose and other carbohydrates was demonstrated. The amount of α and surface coverage (Γ*) of the redox species and catalytic chemical reaction rate constant (k) for each carbohydrate were calculated. Also, the electrocatalytic oxidation peak currents of all tested carbohydrates exhibit a good linear dependence on concentration and their quantification can be done easily.     

Electrochemical Glutathione Sensor Based on Electrochemically Deposited Poly‐m‐aminophenol

Preparation and characterization of electrodes suitable for determination of glutathione is reported in this study. For this poly‐m‐aminophenol (PmAP), poly‐o‐aminophenol, and poly‐p‐aminophenol were electrochemically deposited from aqueous solution on the surface of glassy carbon (GC) electrode by potential cycling in the range of +0.2–+1.0 V. The modified GC electrodes were characterized by cyclic voltammetry, electrochemical impedance spectroscopy, contact angle measurement and ellipsometry. It was found that poly‐m‐aminophenol modified GC electrode (PmAP/GC‐electrode) is most suitable for electroanalytical determination of glutathione. An electroanalytical system for the determination of glutathione based on the PmAP/GC‐electrode was developed. The analytical system was characterized by low limit of detection, good stability, high sensitivity and wide linear detection range.     

Electrocatalytic Determination of 6‐Tioguanine at a p‐Aminophenol Modified Carbon Paste Electrode

A p‐aminophenol modified carbon paste electrode (p‐APMCPE) was constructed for determination of an anticancer drug 6‐thioguanine (6‐TG). The cyclic voltammogram showed that the electrocatalytic oxidation of 6‐TG at the surface of p‐APMCPE occurs at a potential about 840 mV less positive than at an unmodified electrode. Square‐wave voltammetry results presented that the electrocatalytic oxidation peak currents of 6‐TG in pH 9.0 had two linear dynamic ranges in the range of 0.2 to 8.0 and 8.0 to 350.0 μM 6‐TG with a detection limit of 0.08 μM. The kinetic parameters such as electron transfer coefficient (α) and rate constant were determined for the chemical reaction between 6‐TG and p‐aminophenol. Finally, this method was evaluated for the determination of 6‐TG in 6‐thioguanine tablets and urine samples.     

Comparison of Electrochemical and Surface Plasmon Resonance Immunosensor Responses on Single Thin Film

This paper reports results obtained when comparing an electrochemical enzyme immunosensor and a surface plasmon resonance (SPR) based immunosensor on the same gold surface installed in an electrochemical SPR flow cell. Simultaneous electrochemical and SPR measurements were performed on a gold surface modified with multilayers of poly‐L ‐lysine and poly‐styrenesulfonate assembled with the layer‐by‐layer method. First, we obtained the SPR response induced by the formation of an immunocomplex from the shift in the SPR angle by injecting an anti tumor necrosis factor‐α antibody solution labeled with alkaline phosphatase into the flow cell containing the multilayer modified with tumor necrosis factor‐α. Then we compared this SPR result with that obtained for the electrochemical oxidation current of p‐aminophenol catalyzed by alkaline phosphatase from p‐aminophenolphosphate on the same gold film. We compared the two immunosensor responses obtained using the different measurement principles and found that there was a high correlation efficient of 0.973 between them. This was because we were able to immobilize the immunoreagents with good stability and without losing the transport of the enzyme product in the multilayer whose thickness we easily controlled with nanometer scale accuracy. We also report that the detection limit of our electrochemical immunosensor after optimization was around 100 pg/mL (0.4 pM), which is one of the lowest values yet reported for an electrochemical immunosensor.     

Electroanalytical Oxidation of p‐Coumaric Acid

Abstract

The mechanism of the electrochemical oxidation of p‐coumaric acid on a glassy carbon electrode was investigated using cyclic, differential pulse, and square wave voltammetry at different pHs. The oxidation of p‐coumaric acid is irreversible over the whole pH range. After successive scans, the p‐coumaric acid oxidation product deposits on the electrode surface, forming a polymeric film that undergoes reversible oxidation at a lower potential than p‐coumaric acid. This polymeric film increases in thickness with the number of scans, covering the electrode surface, and impeding the diffusion of the p‐coumaric acid and its oxidation on the electrode. The oxidation of p‐coumaric acid is pH dependent up until values close to the pK_a. For pHs higher than pK_a, the p‐coumaric acid oxidation process is pH independent. An electroanalytical determination procedure of p‐coumaric in pH 8.7 0.2 M ammonium buffer was developed, and a detection limit, LOD=83 nM, and the limit of quantification, LOQ=250 nM, were obtained.     

Synthesis of Highly Stable,Water‐Dispersible Copper Nanoparticles as Catalysts for Nitrobenzene Reduction

We report an aqueous‐phase synthetic route to copper nanoparticles (CuNPs) using a copper–surfactant complex and tests of their catalytic efficiency for a simple nitrophenol reduction reaction under atmospheric conditions. Highly stable, water‐dispersed CuNPs were obtained with the aid of polyacrylic acid (PAA), but not with other dispersants like surfactants or polymethacrylic acid (PMAA). The diameter of the CuNPs could be controlled in the range of approximately 30–85 nm by modifying the ratio of the metal precursor to PAA. The catalytic reduction of p‐nitrophenol to p‐aminophenol takes place at the surface of CuNPs at room temperature and was accurately monitored by UV/Vis spectroscopy. The catalytic efficiency was found to be remarkably high for these PAA‐capped CuNPs, given the fact that at the same time PAA is efficiently preventing their oxidation as well. The activity was found to increase as the size of the CuNPs decreased. It can therefore be concluded that the synthesized CuNPs are catalytically highly efficient in spite of the presence of a protective PAA coating, which provides them with a long shelf life and thereby enhances the application potential of these CuNPs.     

Electrodeposition of Cobalt Oxide Nanostructure on the Glassy Carbon Electrode for Electrocatalytic Determination of para‐Nitrophenol

Cobalt oxide nanostructure (CoO_xNS) deposited on the glassy carbon (GC) electrode surface is proposed as a novel electrocatalytic system for the reduction of para‐Nitrophenol. Cyclic voltammetry, electrochemical impedance spectroscopy, atomic force microscopy and scanning electron microscopy were used for characterization of deposited CoO_xNS. CoO_xNS deposited by cycling at positive potentials (0 to +1.3 V) show less charge‐transfer resistance (Rct) and more catalytic activity for the electroreduction of p‐nitrophenol compared to those CoO_xNS obtained by scanning the applied potential throughout a negative V range. The GC/CoO_xNS electrode showed good electrocatalytic activity toward the reduction of p‐nitrophenol at different pH values.     

Indirect detection of substituted phenols and cannabis based on the electrochemical adaptation of the Gibbs reaction

The voltammetric behaviour of 2,6-dichloro-p-aminophenol (PAP) in aqueous solution at an edge plane pyrolytic graphite electrode was explored and its sensitivity to additions of substituted phenols examined. Proof of concept is shown for the electrochemical adaptation of the Gibbs reaction, where reaction of the oxidised form of PAP with substituted phenols provides an indirect methodology for the analytical detection of these compounds. This indirect protocol provides an attractive alterative to the direct electrochemical oxidation of phenolic compounds, since the latter is plagued by electrode passivation, leading to low sensitivity. It is observed that phenol, 4-phenoxyphenol, methylphenol (para and meta), nitrophenol and most importantly, tetrahydrocannabinol, can be detected voltammetrically. Such a protocol is particularly attractive for roadside testing for cannabis in drug drivers.     

Direct and Simultaneous Determination of Phenol, Hydroquinone and Nitrophenol at Boron-Doped Diamond Film Electrode

The electrochemical characteristics of multi-component phenolic pollutants, such as phenol （Ph）, hydroquinone （HQ） and 4-nitrophenol （4-NP）, were investigated on boron-doped diamond （BDD） film electrode by differential pulse voltammetry （DPV） technique. A simple and feasible platform was accordingly established for the direct and simultaneous determination of these three phenolic pollutants. Results showed that, Ph, HQ and 4-NP gave obvious oxidation peaks on BDD electrode at the potential of 1.24, 0.76 and 1.52 V, respectively. Each of them displayed good linear relationship between their oxidation peak currents and their corresponding concentrations in a rather wide range coexisting with one or two of the other phenolic pollutants. The detection limits of Ph, HQ and 4-NP were estimated to be as low as 1.82×10^-6, 1.67×10^-6 and 1.44×10^-6 mol·L^-1, respectively. Therefore, a promising direct and simultaneous electrochemical determination method of multi-component phenolic pollutants in wastewater samples was constructed successfully on BDD electrode with advantages being rapid, simple, convenient, sensitive, in situ and inexpensive.     

Arthrobacter sp. JS443‐Based Whole Cell Amperometric Biosensor for p‐Nitrophenol

An amperometric microbial biosensor for highly sensitive and selective determination of p‐nitrophenol (PNP) is reported. The biosensor consisted of PNP‐degrader Arthrobacter sp. JS443 immobilized by entrapment in Nafion polymer deposited on the top of the carbon paste electrode transducer. The biosensor was based on the measurement of the oxidation current of the intermediates 4‐nitrocatechol and 1,2,4‐benzenetriol formed by the highly selective oxidation of PNP by Arthrobacter sp. The sensor signal and response time were optimized with applied potential of +0.4 V (vs. Ag/AgCl reference electrode) and 0.03 mg of cells and operating in pH 7.5, 50 mM citrate‐phosphate buffer at room temperature. When operated at optimized conditions, the Arthrobacter sp.‐based biosensor measured as low as 5 nM (0.7 ppb) of PNP. The biosensor demonstrated excellent selectivity with no interference from phenolic compounds such as 2‐nitrophenol, phenol and 3‐chlorophenol but was interfered by 3‐nitrophenol and 3‐methyl‐4‐nitrophenol. It had good precision and intra‐ and inter‐day reproducibility, accuracy and was stable up to 3 days when stored in buffer at 4 °C. When applied for measurement in water from Lake Elsinore, CA, the results obtained were in excellent agreement with the amounts determined spectrophotometrically.     

Sensitive Voltammetric Response of p‐Nitroaniline on Single‐Wall Carbon Nanotube‐Ionic Liquid Gel Modified Glassy Carbon Electrodes

An ionic liquid (i.e., 1‐butyl‐3‐methylimidazolium hexafluorophosphate, BMIMPF₆)‐single‐walled carbon nanotube (SWNT) gel modified glassy carbon electrode (BMIMPF₆‐SWNT/GCE) is fabricated. At it the voltammetric behavior and determination of p‐nitroaniline (PNA) is explored. PNA can exhibit a sensitive cathodic peak at ?0.70 V (vs. SCE) in pH 7.0 phosphate buffer solution on the electrode, resulting from the irreversible reduction of PNA. Under the optimized conditions, the peak current is linear to PNA concentration over the range of 1.0×10^?8–7.0×10^?6 M, and the detection limit is 8.0×10^?9 M. The electrode can be regenerated by successive potential scan in a blank solution for about 5 times and exhibits good reproducibility. Meanwhile, the feasibility to determine other nitroaromatic compounds (NACs) with the modified electrode is also tested. It is found that the NACs studied (i.e., p‐nitroaniline, p‐nitrophenol, o‐nitrophenol, m‐nitrophenol, p‐nitrobenzoic acid, and nitrobenzene) can all cause sensitive cathodic peaks under the conditions, but their peak potentials and peak currents are different to some extent. Their peak currents and concentrations show linear relationships in concentration ranges with about 3 orders of magnitude. The detection limits are 8.0×10^?9 M for p‐nitroaniline, 2.0×10^?9 M for p‐nitrophenol, 5.0×10^?9 M for o‐nitrophenol, 5.0×10^?9 M for m‐nitrophenol, 2.0×10^?8 M for p‐nitrobenzoic acid and 8.0×10^?9 M for nitrobenzene respectively. The BMIMPF₆‐SWNT/GCE is applied to the determination of NACs in lake water.     

Electrochemical Behavior of o‐Nitrophenol at Hexagonal Mesoporous Silica Modified Carbon Paste Electrodes

A hexagonal mesoporous silica (HMS) modified carbon paste electrode (CPE) was fabricated and characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods (ferrocene as a probe). The electrochemical behavior of nitrophenol (o‐NP) at the HMS modified electrode (HMSCPE) was investigated. Compared with CPE, a well‐defined reduction peak and a remarkably peak current response was observed. It is indicated that mesoporous HMS exhibited remarkable enhancement effects on the electrochemical reduction of o‐NP. The electrochemical reduction mechanism was also discussed. Consequently, a simple and sensitive electrochemical method was proposed for the determination of o‐NP, which was used to determine o‐NP in waste water samples.     

Graphene Functionalized Graphite Electrode with Diphenylacetylene for Sensitive Electrochemical Determination of Adenosine‐5′‐triphosphate

In this paper a molecular wire modified carbon paste electrode (MW‐CPE) was firstly prepared by mixing graphite powder with diphenylacetylene (DPA). Then a graphene (GR) and chitosan (CTS) composite film was further modified on the surface of MW‐CPE to receive the graphene functionalized electrode (CTS‐GR/MW‐CPE), which was used for the sensitive electrochemical detection of adenosine‐5′‐triphosphate (ATP). The CTS‐GR/MW‐CPE exhibited excellent electrochemical performance and the electrochemical behavior of ATP on the CTS‐GR/MW‐CPE was carefully studied by cyclic voltammetry with an irreversible oxidation peak appearing at 1.369 V (vs. SCE). The electrochemical parameters such as charge transfer coefficient (α) and electrode reaction standard rate constant (k_s) were calculated with the results of 0.53 and 5.28×10^?6 s^?1, respectively. By using differential pulse voltammetry (DPV) as detection technique, the oxidation peak current showed good linear relationship with ATP concentration in the range from 1.0 nM to 700.0 µM with a detection limit of 0.342 nM (3σ). The common coexisting substances, such as uric acid, ascorbic acid and guanosine‐5′‐triphosphate (GTP), showed no interferences and the modified electrode was successfully applied to injection sample detection.     

Synthesis of gold nanoparticles decorated on sulfonated three‐dimensional graphene nanocomposite and application as a highly efficient and recyclable heterogeneous catalyst for Ullmann homocoupling of aryl iodides and reduction of p‐nitrophenol

Gold nanoparticles were decorated onto sulfonated three‐dimensional graphene (3DG‐SO₃H) through spontaneous chemical reduction of HAuCl₄ by 3DG‐SO₃H. This nanocomposite exhibited excellent catalytic activity for the synthesis of symmetric biaryls via the Ullmann homocoupling of aryl iodides in an aqueous medium. Additionally, this nanocomposite was used as a catalyst for the reduction of p‐nitrophenol to p‐aminophenol. The catalyst could be used more than six times successively without significant deactivation.     

Reduced Graphene Oxide/Carbon Nanotube/Gold Nanoparticles Nanocomposite Functionalized Screen‐Printed Electrode for Sensitive Electrochemical Detection of Endocrine Disruptor Bisphenol A

We fabricated a highly sensitive electrochemical sensor for the determination of bisphenol A (BPA) in aqueous solution by using reduced graphene oxide (RGO), carbon nanotubes (CNT), and gold nanoparticles (AuNPs)‐modified screen‐printed electrode (SPE). GO/CNT nanocomposite was directly reduced to RGO/CNT on SPE at room temperature. AuNPs were then electrochemically deposited in situ on RGO/CNT‐modified SPE. Under optimized conditions, differential pulse voltammetry (DPV) produced linear current responses for BPA concentrations of 1.45 to 20 and 20 to 1,490 nM, with a calculated detection limit of an ultralow 800 pM. The sensor response was unaffected by the presence of interferents such as phenol, p‐nitrophenol, pyrocatechol, 2,4‐dinitrophenol, and hydroquinone.     

Nanostructured Platinum‐iridium Alloy Microelectrode for Ammonia Determination

Nanostructured platinum‐iridium alloy microelectrode with high surface area was successfully prepared by applying successive potential cycles to a conventional PtIr microdisc in ionic liquid electrolyte containing ZnCl₂ at elevated temperature. Scanning‐electron microscope studies show that a very thin nanostructured film was created on the electrode upon 20 potential cycles between −2.0 and 0.75 V versus a Ag pseudo‐reference electrode. The film nanostructures are characteristic of regular hill‐like nano‐spacings separated by valley‐like nano‐cracks, and a roughness factor of approximately 40. The nanostructured electrode is highly active towards electrochemical oxidation of ammonia, and generates a linear relation between voltammetric peak currents (or chronoamperometric currents), and logarithm of ammonia concentration in a range of approximately 1 ppm to 10000 ppm. It has been proposed that the Temkin adsorption of ammonia from the bulk solution onto the electrode surfaces was involved in its electrochemical oxidation and could be responsible for the linear current‐logarithmic concentration relation.     

Fabrication of Acid Violet 34/Nickel Hydroxide Ultrathin Film and Its Electrocatalytic Performance for Glucose

This article reports the fabrication of Acid Violet 34 (AV34)/nickel hydroxide nanosheets ultrathin film on the glassy carbon electrode (GCE) via the electrostatic layer‐by‐layer (LBL) technique, and its electrocatalytic oxidation for glucose was demonstrated. UV‐vis absorption and electrochemical impedance spectra indicate the uniform deposition of the LBL film, with a continuous and smooth film surface observed by SEM and AFM. The electrochemical performance of the ultrathin film was studied by cyclic voltammetry and chronoamperometry. The (AV34/Ni(OH)₂)₅ ultrathin film modified electrode displays a fast direct electron transfer attributed to the Ni²⁺/Ni³⁺ redox couple as well as remarkable electrocatalytic activity towards the oxidation of glucose. The linear response was obtained in the range 0.5–13.5 mM (R=0.9994) with a low detection limit (14 µM), high sensitivity (25.9 µA mM^?1 cm^?2), rapid response (less than 1 s) and excellent anti‐interference properties to the species including ascorbic acid (AA), uric acid (UA), acetamidophenol (AP) and structurally related sugars. Therefore, the AV34/Ni(OH)₂ ultrathin film can be potentially used as a feasible electrochemical sensor for the determination of glucose.     

Electrocatalytic Oxidation of Methanol at Lower Potentials on Glassy Carbon Electrode Modified by Platinum and Platinum Alloys Incorporated in Poly(o‐Aminophenol) Film

The electrocatalytic oxidation of methanol at a glassy carbon electrode modified by a thin film of poly(o‐aminophenol) (PoAP) containing Pt, Pt‐Ru and Pt‐Sn microparticles has been investigated using cyclic voltammetry as analytical technique and 0.10 M perchloric acid as supporting electrolyte. It has been shown that the presence of PoAP film increases considerably the efficiency of deposited Pt microparticles toward the oxidation of methanol. The catalytic activity of Pt particles is further enhanced when Ru or specially Sn is co‐deposited in the polymer film. The effects of various parameters such as the thickness of polymer film, concentration of methanol, medium temperature as well as the long term stability of modified electrodes have also been investigated.