A novel sensitive biosensor for Ca2+ based on electropolymerized melatonin modified electrode

An electropolymerized melatonin modified glassy carbon electrode (EPMT/GCE) was prepared by electrochemically polymerizing of melatonin in a 0.04 mol l^?1 perchloric acid solution. In KCl solution, Ca²⁺ was found to perform a reversible complex reaction on the EPMT/GCE to produce a sharp complex adsorption wave. The configuration of the electropolymerized membrane was characterized by scanning electron microscope and various electrochemical techniques. Based on which, a novel sensitive Ca²⁺ electrochemical sensor was established. The peak current showed a linear relationship with Ca²⁺ concentration in range of 6.2 × 10^?7–1.0 × 10^?4 mol l^?1 with detection limit of 4.5 × 10^?7 mol l^?1. The developed electrochemical sensor has been used for determination of Ca²⁺ in body fluid.     

The effect of activation on the electrochemical behaviour of graphite felt towards the Fe3+/Fe2+ redox electrode reaction

This paper is dedicated to the study of the effect of graphite felt activation by thermal oxidation in air on its electrocatalytic activity towards Fe³⁺/Fe²⁺ redox electrode reaction. For the first time, the exchange current densities and electron transfer coefficients determined from the Tafel equation were obtained within the wide range of burn-off levels (0–50%). The maximal catalytic activity was obtained at the burn-off of 17%. The cathode having this burn-off level expressed almost three-fold enhancement in the galvanic cell performance (criterion for the performance evaluation in our case was a cell voltage at the current density of 300 mA cm⁻²) as compared to that with the non-activated graphite felt, and allowed to obtain current densities up to 670 mA cm⁻² at the cathode polarization as low as 150 mV. The correlation between electrocatalytic activity and a surface oxide chemistry of graphite felt was established. The cell performance was found to be the best when the pH at a point of zero charge and the amount of surface quinoid groups per unit area were minimal. The results obtained are of significant importance for practical applications, including the development of electrodes in redox flow batteries.     

Determination of tannic acid by adsorptive anodic stripping voltammetry at porous pseudo-carbon paste electrode

A novel and sensitive electrochemical sensor based on porous pseudo-carbon paste electrode (PPCPE) for tannic acid detection is described. PPCPE is fabricated by mixing calcium carbonate microspheres as the template, graphite powders as the filler, and pyrrole as the precursor of polymer which actually acted as the paste. After the polymerization of pyrrole catalyzed by Fe³⁺, the template calcium carbonate microspheres are removed with 0.1 M hydrochloric acid to form PPCPE. The diameters of these pores are in the range from 2 to 5 μm by SEM observations and the specific surface area of PPCPE is 59.26 m²/g by the Brunauer–Emmet–Teller (BET) method. A linear relationship between the anodic stripping peak current and the concentration of tannic acid from 0.02 to 1 μM and a limit of detection as low as 0.01 μM are obtained using PPCPE.     

An insight to the filtration mechanism of Pb(II) at the surface of a clay ceramic membrane through its preconcentration at the surface of a graphite/clay composite working electrode

The use of cyclic voltammetry (CV) and linear scan anodic stripping voltammetry (LSASV) to predict the selectivity of microfiltration ceramic membranes made from a lump of local clay towards Pb(II) ions filtration is described. The membranes were characterized by different techniques followed by CV analysis of the Fe(CN)₆^3-/Fe(CN)₆^4- redox couple and Pb(II) on bare graphite, raw clay, and clay-modified carbon paste electrode (clay-modified CPE). The effect of clay loading in the range of 1–10 % (w/w) on the electrodes is studied, where an enhanced peak current is observed for 5 % w/w clay. Moreover, a decrease in the peak current can be seen for bare graphite electrodes, suggesting that the clay mineral had played a substantial role in the sieving of heavy metal ions through the ceramic membrane. The electroactive surface area of 5% w/w raw clay towards Fe(II) ions was found to be in the order of 3.07 × 10^-2 cm² and higher than 5% w/w clay sintered to 1000 °C and bare graphite. CV analysis shows that both, 5 % w/w raw clay and 5 % w/w clay sintered to 1000 °C exhibited high peak currents towards Pb(II) ions. The mobility of the Pb(II) ions is found to increase when 5% w/w clay sintered to 1000 °C is utilized as membrane/electrode, leading to an increase in the amount of reduced Pb(II) ions on the surfaces of the clay membranes/electrodes. The study suggests successful filtration of Pb(II) ions through the proposed membrane/electrode and a much better accumulation than Fe(II) at the surface of the membrane/electrode before being subjected to filtration.     

Zinc(II) selective poly(vinyl chloride) membrane ISE using a macrocyclic compound 1,12,14-triaza-5,8-dioxo-3(4),9(10)-dibenzoylcyclopentadeca-1,12,14-triene as neutral carrier

The potentiometric response characteristics of zinc ion selective PVC-based membrane electrode employing 1,12,14-triaza-5,8-dioxo-3(4),9(10)-dibenzoyl-1,12,14-triene as an inophore was investigated. The proposed electrode exhibits a Nernstian behavior with a slope of 29.2 ± 0.4 mV per decade with a working concentration range of 1.3 × 10^?7–1.0 × 10^?1 mol L^?1 and a detection limit of 1.0 × 10^?8 mol L^?1. The membrane having the composition as TDODBCPT:O-NPOE:PVC:OA; 7:57:30:6 wt.% exhibits the best results. It has a fast response time of 7 s and can be used for at least 100 days without any considerable divergence in potential. The proposed electrode show good discrimination of Zn²⁺ ion from diverse ions. The potential response remains constant over a pH range of 3.5–9.2. The electrode found well work under laboratory conditions. The proposed sensor directly used for determination of zinc ions in human hair sample, wastewater and an indicator electrode with EDTA titration.     

High performance electrochemical sensor based on modified screen-printed electrodes with cost-effective dispersion of nanostructured carbon black

A novel sensor based on a screen-printed electrode (SPE) modified with a stable dispersion of commercially available carbon black (CB) N220 was developed. This probe showed significantly enhanced electrochemical activity relative to a bare SPE when tested with ferricyanide, epinephrine, norepinephrine, benzoquinone and NADH. When challenged in amperometric batch mode with NADH, the response was stable and revealed a linear dependence up to 2·10^?4 mol L^?1 with a detection limit of 3·10^?7 mol L^?1. The analytical performance, coupled with the low cost of the CB nanomaterial, suggests that this sensor holds promise for electrochemical applications.     

Ni(II)-selective PVC membrane sensor based on 1,2,4-triazole bis Schiff base ionophore: Synthesis,characterization and application for potentiometric titration of Ni2+ ions against EDTA

This study involves the preparation and investigation of a novel and highly selective poly(vinyl chloride)-based membrane of 2-((5-(2-hydroxy-3-methoxybenzylideneamino)-2H-1,2,4-triazol-3-ylimino)methyl)-6-methoxyphenol Schiff base ligand (HMBT), which is a neutral ionophore with sodium tetraphenyl borate (STB) in the form of an excluder and o-nitrophenyloctyl ether (o-NPOE) in the form of solvent mediators (plasticizing) as a Ni(II)-selective electrode. The observation of optimal performance was done wherein the membrane was shown to have the HMBT–PVC–NPOE-STB composition of 4:32:63:1.It worked effectively across a broad range of concentration (1.0 × 10^?8 to 1.0 × 10^?2 mol L^?1). Meanwhile, the Nernstian slope was recorded as 29.3 mV per decade of activity between pH 3.0 and 8.0. The response time of this electrode was fast at 11 s which was used for a span of 100 days with sound reproducibility. According to the selectivity coefficients for trivalent, divalent, and monovalent cations, excellent selectivity was indicated for Ni(II) ions across a large number of citations, whereas no interference was caused by anions like PO₄^3?, SO₄^2? and Cl^?. The proposed method in this study was applied successfully to determine Ni(II) content in different samples of water, obtaining suitable recoveries. Additionally, the probed sensor is utilized as indicator electrode when considering Ni²⁺ ion potentiometric titration against EDTA. In addition, the chelate’s geometry and structure of the complex formed between Ni²⁺ ions and HMBT, abbreviated as HMBT-Ni₂, was evaluated by separating the solid product. Complex structure was confirmed based on alternative analytical and spectral methods to be structured in the bimetallic form with the formula [Ni₂(HMBT)(H₂O)₂ Cl₂]. The diamagnetic nature of the complex, which was concluded from the room temperature magnetic moment measurement combined with the UV–Vis measurement, suggested the square planar geometry around the Ni centers.     

Asymmetric Schiff base as carrier in PVC membrane electrodes for manganese (II) ions

Manganese(II) complex of (E)-2-(hydroxyl-5-methoxybenzylideneamino) phenol was synthesized and used as a suitable Mn(II) – selective membrane in PVC matrix. The plasticized membrane sensor exhibits a nersian response for Mn(II) ions over a wide concentration range of 6 × 10^?6–2 × 10^?2 M with slope of 29 ± 1 mV per decade. It has a response time of <11 s and can be used for 2 months without any measurable divergence in potential. The response of the proposed sensor is independent of pH between 4 and 9.5. The proposed sensor shows a fairly good discriminating ability towards Mn(II) in comparison with some hard and soft metals. The electrode was used in the determination of Mn(II) in aqueous solutions and as an indicator electrode in potentiometer titration of manganese ions against EDTA.     

A sensitivity-controlled hydrogen peroxide sensor based on self-assembled Prussian Blue modified electrode

In this communication, a hydrogen peroxide (H₂O₂) sensor based on self-assembled Prussian Blue (PB) modified electrode was reported. Thin film of PB was deposited on the electrode by self-assembly process including multiple sequential adsorption of ferric ions and hexacyanoferrate ions. The as-prepared PB modified electrode displayed sufficient stability for practical sensing application. At an applied potential of ?0.05 V vs. Ag/AgCl (sat. KCl), PB modified electrode with 30 layers exhibited a linear dependence on H₂O₂ concentration in the range of 1 × 10^?6–4 × 10^?4 M (r = 0.9998) with a sensitivity of 625 mA M^?1 cm^?2. It was found that the sensitivity of H₂O₂ sensors could be well controlled by adjusting the number of deposition cycles for PB preparation. This work demonstrates the feasibility of self-assembled PB modified electrode in sensing application, and provides an effective approach to control the sensitivity of PB-based amperometric biosensors.     

Immobilization of biotinylated biomolecules onto electropolymerized poly(pyrrole-nitrilotriacetic acid)–Cu2+ film

A novel and simple immobilization strategy for biotinylated biological macromolecules onto electropolymerized poly(pyrrole-nitrilotriacetic acid)(NTA)–Cu²⁺ films without avidin as connecting bridge is reported. After complexation of Cu²⁺ by the polymerized NTA chelator, biotinylated biomolecules were immobilized by coordination of the biotin groups on the NTA–Cu²⁺ complex. The anchoring of biotinylated glucose oxidase was demonstrated by fluorescent characterization via FITC-labeled avidin and amperometric measurement of glucose. The resulting calibration curve led to a sensitivity and maximum current density values of 0.6 mA mol^?1 L cm^? 2 and 13.2 μA cm^? 2, respectively. Thus, biotinylated polyphenol oxidase was fixed leading to a catechol sensor with a sensitivity of 656 mA mol^?1 L cm^? 2 and maximum current density of 25.4 μA cm^? 2. This system was also applied to the efficient immobilization of biotinylated DNA, illustrated by impedimetric detection of the formation of the DNA duplex.     

Selective determination of drug resistant cancer cells on indium tin oxide electrode modified with nano titanium dioxide

A new electrochemical cell sensor, with low cost, simple fabrication, high selectivity and sensitivity was developed in this study. Titanium dioxide nanoparticles (nano-TiO₂) were assembled on the disposable indium tin oxide (ITO) electrodes for the immobilization of the drug sensitive leukemia K562/B.W. cells and drug resistant leukemia K562/ADM cells to fabricate the relative cell sensors. The different electrochemical behaviors of the probe allowed us to differentiate one type of leukemia cells from another. Furthermore, the results of electrochemical impedance spectroscopy indicated that the detection limit of the new cell sensor is 1.3 × 10³ cells ml^?1 with a linear range of 1.6 × 10⁴ to 1.0 × 10⁷ cells ml^?1. These results suggested the promising application of this nano-TiO₂ interface to construct the non-labeling potential-discriminative cell biosensors for clinical uses.     

Direct electrochemistry of glucose oxidase on a graphite nanosheet–Nafion composite film modified electrode

The direct electrochemistry of glucose oxidase (GOD) immobilized in a modified electrode based on a composite film of exfoliated graphite nanosheets (GNSs) and Nafion has been investigated for the first time. Direct electron communication between GOD and the electrode was achieved with a fast electron transfer rate (12.6 s^?1). In addition, the bioactivity of GOD was retained after immobilization in the composite film and glucose could be determined based on the decrease of the electrocatalytic response of the reduced form of GOD to dissolved oxygen. The resulting biosensor exhibited higher sensitivity (3.4 μA mM^?1). Considering much lower cost of GNSs and ready preparation from graphite, the GNSs-based modified electrode described here is superior to the carbon nanotubes (CNTs)-based modified electrodes and should have wide applications in third-generation biosensors, bioelectronics and electrocatalysis.     

Electrodeposited PbO2 thin film on Ti electrode for application in hybrid supercapacitor

PbO₂ thin films were prepared by pulse current technique on Ti substrate from Pb(NO₃)₂ plating solution. The hybrid supercapacitor was designed with PbO₂ thin film as positive electrode and activated carbon (AC) as negative electrode in the 5.3 M H₂SO₄ solution. Its electrochemical properties were determined by cyclic voltammetry (CV), charge–discharge test and electrochemical impedance spectroscopy (EIS). The results revealed that the PbO₂/AC hybrid supercapacitor exhibited large specific capacitance, high-power and stable cycle performance. In the potential range of 0.8–1.8 V, the hybrid supercapacitor can deliver a specific capacitance of 71.5 F g^?1 at a discharge current density of 200 mA g^?1(4 mA cm^?2) when the mass ratio of AC to PbO₂ was three, and after 4500 deep cycles, the specific capacitance remains at 64.4 F g^?1, or 32.2 Wh Kg^?1 in specific energy, and the capacity only fades 10% from its initial value.     

Preparation process for improving cathode electrode structure in direct methanol fuel cell

The cathode electrode structure of the direct methanol fuel cell (DMFC) was improved by a novel catalyst ink preparation method. Regulation of the solvent polarity in the cathode catalyst ink caused increases in the electrochemical active surface (EAS) for the oxygen reduction reaction (ORR) as well as decreases in the methanol crossover effect. In a two-step preparation, agglomerates consisting of catalyst and Nafion ionomers were decreased in size, and polar groups in the ionomers formed organized networks in the cathode catalyst layer. Despite Pt catalysts in the cathode being only 0.5 mg cm^? 2, the maximum power density of the improved membrane electrode assembly (MEA) was 120 mW cm^? 2, at 3 M methanol, which was much larger than that of traditional MEA (67 mW cm^? 2).     

Facile one-pot synthesis of fluorinated graphene oxide for electrochemical sensing of heavy metal ions

Doping and functionalization could significantly assist in the improvement of the electrochemical properties of graphene derivatives. Herein, we report a one-pot synthesis of fluorinated graphene oxide (FGO) from graphite. The surface morphology, functionalities and composition of the resulting FGO have been studied using various surface characterization techniques, revealing that layer-structured nanosheets with ~ 1.0 at.% F were formed. The carbon bound F exhibited semi-ionic bonding characteristic and significantly increased the capacitance of FGO compared to GO. Further, the FGO has been employed for the simultaneous detection of heavy metal ions Cd^2 +, Pb^2 +, Cu^2 + and Hg^2 + using square wave anodic stripping voltammetry; and a substantial improvement in the electrochemical sensing performance is achieved in comparison with GO.     

Electrochemical stripping analysis of cadmium on tantalum electrode

A tantalum electrode is reported as an alternative electrode for electrochemical stripping analysis for the first time. Several key operational parameters that influenced the electroanalytical signals were optimized, such as pH of the electrolyte, deposition potential and deposition time. The tantalum electrode yields well-defined and sharp stripping signals for trace cadmium analysis when combined with differential pulse anodic stripping voltammetry. Under the optimized condition the electrode shows good linear behavior in the examined concentration in the range of 20–200 μg L^?1 for cadmium, with a detection limit (3σ) of 0.57 μg L^?1 followed a 5-min deposition step under ? 1.3 V. It also shows good reproducibility with a relative standard deviation of 2.56% for ten consecutive measurements. The sensor was also employed for real sample determination and exhibited excellent performance compared with the result of inductively coupled plasma-mass spectrometry.     

The electrochemical detection of cadmium using surface-immobilized DNA

An electrochemical assay based on underpotential deposition (UPD) of metal has been utilised for the detection of Cd²⁺ using surface immobilized single-stranded DNA (ss-DNA). Since Cd²⁺ is able to undergo UPD on gold surface, this allows the opportunity to detect the amount of Cd²⁺ accumulated by immobilized ss-DNA directly via voltammetry. This is evidenced by the appearance of Cd²⁺/Cd⁰ electrochemistry at E^0′ of 92 mV (rather than E^0′ of −795 mV in bulk solution) at the ss-DNA modified gold electrode only after an exposure to Cd²⁺ solution. An association constant of 8.33 × 10⁵ M⁻¹ was determined from a Cd²⁺ calibration curve assuming a Langmuir-binding model for Cd²⁺ with surface-immobilized DNA. The high association constant is reflected in a low detectable concentration of 10 pM. The sensing layer can also be regenerated to metal-free status and can be reused up to 18 times without significant signal degradation.     

Evaluation of a platinum electrode modified with hydroxyapatite in the lead(II) determination in a square wave voltammetric procedure

The behavior of a modified carbon platinum electrode (Pt) for lead(II) determination by square wave voltammetry (SWV) was studied. The modified electrode is obtained by electrodeposition of hydroxyapatite (HAP) on the surface of a bare platinum electrode. The new electrode (HAP/Pt) revealed interesting electroanalytical detection of lead(II) based on the adsorption of this metal onto hydroxyapatite under open circuit conditions. After optimization of the experimental and voltammetric conditions, the best voltammetric responses-current intensity and voltammetric profile were obtained in 0.2 mol L^?1 KNO₃ with: 30 min accumulation time, 5 mV pulse amplitude and 1 mV s^?1 scan rate. The observed detection (DL, 3σ) and quantification (DL, 10σ) limits in pure water were 2.01 × 10^?8 and 6.7 × 10^?7 mol L^?1, respectively. The reproducibility of the proposed method was determined from five different measurements in a solution containing 2.2 × 10^?6 mol L^?1 lead(II) with a coefficients of variation of 2.08%.The electrochemical of hydroxyapatite at platinum surfaces was characterized, after calcinations 900 °C, by X-ray diffraction, infrared spectroscopy, chemical and electrochemical analysis.     

Flow injection potentiometric sensor for determination of phenylpropanolamine hydrochloride

A new polymeric membrane electrode has been constructed for the determination of phenylpropanolamine hydrochloride. The electrode was prepared by solubilizing the phenylpropanolamine-phosphomolybdate ion associate into a polyvinyl chloride matrix plasticized by dibutylphthalate as a solvent mediator. The electrode showed near-Nernstian response over the concentration range of 1 × 10^?5–1 × 10^?2 M with low detection limit of 6.3 × 10^?6 M. The electrode displays a good selectivity for phenylpropanolamine with respect to a number of common inorganic and organic species. The electrode was successfully applied to the potentiometric determination of phenylpropanolamine ion in its pure state and its pharmaceutical preparation in batch and flow injection conditions.     

Poly-xanthurenic acid as an efficient mediator for the electrocatalytic oxidation of NADH

This paper describes, for the first time, the development of a simple and highly sensitive chemical sensor based on a new electroactive material, electrogenerated in situ from xanthurenic acid on an electrode modified with MWCNT. The modified electrode shows efficient electrocatalytic oxidation activity towards NADH at an applied potential of 0.1 V vs. Ag/AgCl. The kinetic constant, k_kin, for the electrocatalytic oxidation of NADH, evaluated by chronoamperometry and voltammetry using RDE, provided values close to 10⁵ mol^?1 L s^?1.