Electrocatalytic determination of dopamine in the presence of uric acid using an indenedione derivative and multiwall carbon nanotubes spiked in carbon paste electrode

In the present study, a modified carbon paste electrode (CPE) containing multi-wall carbon nanotubes and an indenedione derivative(IMWCNT?CPE) was constructed and was successfully used for dopamine(DA) electrocatalytic oxidation and simultaneous determination of DA and uric acid (UA). Cyclic voltammograms of the IMWCNT?CPE show a pair of well-defined and reversible redox. The obtained results indicate that the peak potential of DA oxidation at IMWCNT?CPE shifted by about 65 and 185 mV toward the negative values compared with that at a MWCNT and indenedione modified CPE, respectively. The electron transfer coefficient, α, and the heterogeneous electron transfer rate constant, k′, for the oxidation of DA at IMWCNT?CPE were calculated 0.4 ± 0.01 and (1.13 ± 0.03) × 10^? 3 cm s^? 1, respectively. Furthermore, differential pulse voltammetry (DPV) exhibits two linear dynamic ranges of 1.9–79.4 μM, and 79.4–714.3 μM and a detection limit of 0.52 μM for DA determination. Then IMWCNT?CPE was applied to the simultaneous determination of DA and UA with DPV. Finally, the activity of the modified electrode was also investigated for determination of DA and UA in real samples, such as injection solution of DA and urine, with satisfactory results.     

Electrocatalytical oxidation and sensitive determination of acetaminophen on glassy carbon electrode modified with graphene–chitosan composite

The electrochemical behaviors of acetaminophen (ACOP) on a graphene–chitosan (GR–CS) nanocomposite modified glassy carbon electrode (GCE) were investigated by cyclic voltammetry (CV), chronocoulometry (CC) and differential pulse voltammetry (DPV). Electrochemical characterization showed that the GR–CS nanocomposite had excellent electrocatalytic activity and surface area effect. As compared with bare GCE, the redox signal of ACOP on GR–CS/GCE was greatly enhanced. The values of electron transfer rate constant (k_s), diffusion coefficient (D) and the surface adsorption amount (Γ^?) of ACOP on GR–CS/GCE were determined to be 0.25 s^? 1, 3.61 × 10^? 5 cm² s^? 1 and 1.09 × 10^? 9 mol cm^? 2, respectively. Additionally, a 2e^?/2H⁺ electrochemical reaction mechanism of ACOP was deduced based on the acidity experiment. Under the optimized conditions, the ACOP could be quantified in the range from 1.0 × 10^? 6 to 1.0 × 10^? 4 M with a low detection limit of 3.0 × 10^? 7 M based on 3S/N. The interference and recovery experiments further showed that the proposed method is acceptable for the determination of ACOP in real pharmaceutical preparations.     

A comparative study on fabrication of Mn2 + selective polymeric membrane electrode and coated graphite electrode

Poly(vinyl chloride)-based membranes of two ligands 2,4-bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L₁) and N2,N4-di(cyanoethyl)-2,4-bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L₂) were fabricated and explored as Mn^2 + ion selective electrodes. The performance of the polymeric membranes electrodes of ionophores with different plasticizers (dibutylphthalate, benzoic acid, o-nitrophenyloctyl ether, 1-chloronapthalene and tri-n-butylphosphate) and anion excluders (sodium tetraphenylborate and potassium tetrakis p-(chloro phenyl)borate) was looked in to and the better results were obtained with the membrane having composition L₂: NaTPB: DBP: PVC as 6: 3: 56: 35 (w/w; mg). The coated graphite electrode (CGE) with same composition was also fabricated and investigated as Mn^2 + selective electrode. It was found that CGE showed better response characteristics than PME. The potentiometric response of CGE was independent of pH in the range 3.0–9.0 exhibiting the Nernstian slope 29.5 ± 0.3 mV decade^? 1 of activity and working concentration range 4.1 × 10^? 7–1.0 × 10^? 1 mol L^? 1 with a limit of detection 6.7 × 10^? 8 mol L^? 1. The electrode showed a fast response time of 12 s with a shelf life of 105 days. The proposed CGE could be successfully used for the determination of Mn^2 + ions in different water, soil, vegetables and medicinal plants also used as an indicator electrode in potentiometric titration with EDTA.     

Iodide selective membrane electrodes based on a Molybdenum–Salen as a neutral carrier

A new polymeric membrane electrode (PME) and a coated platinum disk electrode (CPtE) based on Schiff base complex of Mo(VI) as a suitable carrier for I^? ion were described. The influence of membrane composition, pH and possible interfering anions were investigated on the response properties of the electrodes. The electrodes exhibited a Nernstian slope of 63.0 ± 0.5 (CPtE) and 60.3 ± 0.4 (PME) mV decade^? 1 in I^? ion over a wide concentration range from 7.9 × 10^? 7 to 1.0 × 10^? 1 M for CPtE and 9.1 × 10^? 6 to 1.0 × 10^? 1 M I^? for PME. The potentiometric response of the electrodes was independent of the pH of the test solution in the pH range 2.0–8.5 with a fast response time (< 10 s). The process of transfer of iodide across the membrane interface was investigated by use of the AC impedance technique. The proposed sensors were successfully applied to direct determination of iodide in samples containing interfering anions, waste water and as indicator electrodes in precipitation titrations.     

Transmission electron microscopy study of carbon nanophases produced by ion beam implantation

The formation of carbon nanocrystals, produced by ion implantation of carbon ions into fused SiO₂ substrates, followed by 1 h thermal annealing at 1000 °C, in an Ar + 5% H atmosphere has been studied. Combined high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) have been employed for structural characterization of carbon nanophases embedded in the quartz substrate. The dependence of grain size and sample morphology of the carbon nanophases on implantation dose was studied. The carbon nanocrystals formed by the implantation for a dose of 1 × 10¹⁶ C/cm² at 320 keV have been identified as a mixture of c-diamond nanophase and a modified diamond nanophase known as n-diamond. For a higher implantation dose, 5 × 10¹⁶ C/cm², besides n-diamond, another solid carbon nanophase was observed, with a structure known as i-carbon. Following the highest implantation dose 1 × 10¹⁷ C/cm² the sample contained the i-carbon nanophase only. A least-square refinement of SAED patterns was employed for the calculation of unit-cell parameters of identified carbon nanophases.     

A simple and efficient electrochemical sensor for folic acid determination in human blood plasma based on gold nanoparticles–modified carbon paste electrode

Folic acid (FA) is a water soluble vitamin that exists in many natural species. The lack of FA causes some deficiencies in human body, so finding a simple and sensitive method for determining the FA is important. A new chemically modified electrode was fabricated for determination of FA in human blood plasma using gold nanoparticles (AuNPs) and carbon paste electrode (CPE). Gold nanoparticles–modified carbon paste electrode (AuNPs/CPE) was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The experimental parameters such as pH, scan rate (ν) and amount of modifier were studied by cyclic voltammetry and the optimized values were chosen. The electrochemical parameters such as diffusion coefficient of FA (D_FA), electrode surface area (A) and electron transfer coefficient (α) were calculated. Square wave voltammetry as an accurate technique was used for quantitative calculations. A good linear relation was observed between anodic peak current (i_pa) and FA concentration (C_FA) in the range of 6 × 10^? 8 to 8 × 10^? 5 mol L^? 1, and the detection limit (LOD) achieved 2.7 × 10^? 8 mol L^? 1, that is comparable with recently studies. This paper demonstrated a novel, simple, selective and rapid sensor for determining the FA in the biological samples.     

Biosensor based on laccase immobilized on plasma polymerized allylamine/carbon electrode

In this work, a simple and rapid method was used to functionalize carbon electrode in order to efficiently immobilize laccase for biosensor application. A stable allylamine coating was deposited using a low pressure inductively excited RF tubular plasma reactor under mild plasma conditions (low plasma power (10 W), few minutes) to generate high density amine groups (N/C ratio up to 0.18) on rough carbon surface electrodes. The longer was the allylamine plasma deposition time; the better was the surface coverage. Laccase from Trametes versicolor was physisorbed and covalently bound to these allylamine modified carbon surfaces. The laccase activities and current outputs measured in the presence of 2,2′-azinobis-(3-ethylbenzothiazole-6-sulfonic acid) (ABTS) showed that the best efficiency was obtained for electrode plasma coated during 30 min. They showed also that for all the tested electrodes, the activities and current outputs of the covalently immobilized laccases were twice higher than the physically adsorbed ones. The sensitivity of these biocompatible bioelectrodes was evaluated by measuring their catalytic efficiency for oxygen reduction in the presence of ABTS as non-phenolic redox substrate and 2,6-dimethoxyphenol (DMP) as phenolic one. Sensitivities of around 4.8 μA mg^? 1 L and 2.7 μA mg^? 1 L were attained for ABTS and DMP respectively. An excellent stability of this laccase biosensor was observed for over 6 months.     

Carbon layers formed on steel and Ti alloys after ion implantation of C+ at very high doses

Ion implantation is a useful technique to tailor surface properties of steel and Ti alloys. In particular, very high dose C⁺ implantation (in the range of 10¹⁸ ions cm⁻²) offers the possibility of forming carbon layers without a sharp interface with the substrate material. In this study, ion implantation of carbon doses up to 8×10¹⁸ ions cm⁻² has been performed on 440C martensitic stainless steel and Ti6Al4V substrates under similar conditions and tribological and surface analysis results have been compared. Surface hardening occurred for all ion implantation conditions up to doses of 10¹⁸ ions cm⁻²1, 2, 3. Higher doses resulted in a different behaviour for both materials. The stainless steel showed a softening while a twofold hardness increase was maintained in the Ti alloy. Nevertheless, at the higher implanted dose a decrease in hardness was also observed in the Ti alloy. Small area XPS analyses were performed to evaluate the chemical states after ion implantation and establish a relationship with the observed surface hardening. Depth profile XPS analyses showed that for a dose of 4×10¹⁸ ions cm⁻² a carbon layer (with concentration over 85% at. C) was formed in the near surface region for both materials.     

Determination of Pb2+ ions by a modified carbon paste electrode based on multi-walled carbon nanotubes (MWCNTs) and nanosilica

A novel carbon paste ion selective electrode for determination of trace amount of lead was prepared. Multi-walled carbon nanotubes (MWCNTs) and nanosilica were used for improvement of a lead carbon paste sensor response. MWCNTs have a good conductivity which helps the transduction of the signal in carbon paste electrode. The electrode composition of 20 wt% paraffin oil, 57% graphite powder, 15% ionophore (thiram), 5% MWCNTs, and 3% nanosilica showed the stable potential response to Pb²⁺ ions with the Nernstian slope of 29.8 (±0.2) mV decade^?1 over a wide linear concentration range of 10^?7–10^?2 mol L^?1. The electrode has fast response time, and long term stability (more than 2 months). The proposed electrode was used to determine the concentration of lead ions in waste water and black tea samples.     

Determination of benzalkonium chloride preservative in pharmaceutical formulation of eye and ear drops using new potentiometric sensors

A novel approach for determination of low concentrations of the preservative, benzalkonium chloride (BCCl), in pharmaceutical formulation constitutes is presented. New chemically modified carbon paste electrodes (CMCPEs) are developed. The first is based on an ion-association of BCCl with phosphomolybdic acid (PMA) as the ion-exchanger (BC–PM) dissolved in the mixed plasticizers dibutyl phthalate (DBP) and dioctyl sebacate (DOS) encoded sensor A. In the other electrode, encoded sensor B, the plasticizers DBP and dioctyl phthalate (DOP) are more suitable solvent mediators for the paste. These electrodes exhibit a Nernstian slope of 58.2 ± 0.6 and 62.3 ± 0.7 mV/decade in concentration range 1.3 × 10^? 7–1.7 × 10^? 4 M and 2.5 × 10^? 7–1.7 × 10^? 4 M with the limit of detection of 1.0 × 10^? 7 M and 1.6 × 10^? 7 for sensors A and B, respectively. The sensors have short and stable response time 5–8 s, good reproducibility and can be used in pH range of 5.7–8.6. The present electrodes show good discrimination of BCCl from several inorganic, organic ions and some common drug excipients. These characteristics of the electrodes make them useful in successful determination of BCCl in its pharmaceutical preparations (eye and ear drops) and aqueous solutions. The results obtained were satisfactory with excellent percentage recovery comparable and sometimes better than those obtained by other routine methods for the assay.     

Investigation of traps in AlGaN/GaN HEMTs by current transient spectroscopy

Deep levels in AlGaN/GaN HEMTs on Si substrate are known to be responsible for trapping processes like: threshold voltage shift, leakage current, degradation in saturation current and hysteresis effect. The related deep levels are directly characterized by Conductance Deep Level Transient Spectroscopy (CDLTS) method. Hereby we have detected four carrier traps with activation energy of 0.83, 0.50, 0.20 and 0.07 eV and capture cross-section respectively of σ = 3.14 × 10^− 14 cm², σ = 2.57 × 10^− 15 cm², σ = 3.03 × 10^− 17 cm² and σ = 2.65 × 10^− 15 cm². All these traps are located between the substrate and the two-dimensional electron gas (2DEG) channel.     

Potentiometric determination of ketoprofen and piroxicam at a new PVC electrode based on ion associates of Rhodamine 6G

The characteristics, performance and application of ion-selective electrodes for ketoprofen and piroxicam ions based on Rhodamine 6G as electrode-active substances are described. These electrodes respond with sensitivities of (58.0 ± 1.0) and (57.0 ± 2.0) mV/decade over the range 1.0 × 10^? 4–1.0 × 10^? 1 and 1.0 × 10^? 4–5.0 × 10^? 2 mol/l at pH 5–9 and 6–10 and a detection limit of 6.3 × 10^? 5 and 3.2 × 10^? 5 mol/l for ketoprofen and piroxicam, respectively. The electrodes are easily constructed at a relatively low cost, have a fast response time and can be used for a period of 5 months without any considerable divergence in potential. The proposed sensor displayed good selectivity for ketoprofen and piroxicam in the presence of several substances and inorganic anions. It was used for the direct assay of ketoprofen and piroxicam in commercial pharmaceutical preparations.     

EPR and optical absorption studies of Cr3+ doped lithium potassium sulphate single crystals

X-band electron paramagnetic resonance (EPR) studies of Cr³⁺ doped lithium potassium sulphate single crystals have been done at room temperature. The Cr³⁺ crystal field and spin Hamiltonian parameters have been evaluated by employing resonance line positions observed in the EPR spectra for different orientations of external magnetic field. The evaluated g, D and E values are: g_x = 2.0763 ± 0.0002, g_y = 1.9878 ± 0.0002, g_z = 1.8685 ± 0.0002 and D = 549 ± 2 × 10^?4 cm^?1, E = 183 ± 2 × 10^?4 cm^?1. Using EPR data the site symmetry of Cr³⁺ ion in the crystal is discussed. Cr³⁺ ion enters the lattice substitutionally replacing K⁺ site. The optical absorption study of the single crystal is also done in 195–925 nm wavelength range at room temperature. By correlating optical and EPR data the nature of bonding in the crystal is discussed. The calculated values of Racah parameters (B and C), crystal field parameter (D_q) and nephelauxetic parameters (h and k) are obtained as: B = 697, C = 3247, D_q = 2050 cm^?1, h = 1.146 and k = 0.21.     

Designing and synthesis of bis(2,4-dihydroxybenzylidene)-1,6-diaminohexane and its efficient application as neutral carrier for preparation of new copper selective electrode

A new copper carbon paste electrode (CPE) based on incorporation bis(2, 4-dihydroxybenzyliden)-1,6-diaminohexane (DHBDAH) in graphite powder matrix has been described. The influence of variables including an amount of graphite, sodium tetraphenylborate (NaTPB), DHBDAH and nujol on the Cu²⁺ carbon paste electrode response was studied and optimized. The optimum carbon paste composition was set as follows, graphite powder: NaTPB: Nujol: DHBDAH with amount of 150:2.3:30:4 mg, respectively. At the optimum conditions, the potential response is linear over the concentration range of 5.0 × 10^? 8 to 1.0 × 10^? 1 mol L^? 1 with a Nernstian slope of 29.5 ± 1.1 mV per decade of Cu²⁺ ion concentration. The good performance of electrode such as low detection limit of (LOD) (4 × 10^? 8 mol L^? 1), wide applicable pH range (2.5–5.5), fast response time (?10 s) and adequate shelf life (69 days) indicate the utility of the proposed electrode for evaluation of Cu²⁺ ion content in various analysis. Due to moderate potentiometric selectivity coefficients of proposed electrode obtained by fixed interference method (FIM) and separate solution method (SSM), the proposed electrode successfully can be applied for the determination of Cu²⁺ ions content in some real samples.     

Platinum deposition from hydrogen-ion beam irradiated solid precursor

We report a particular method of Pt/glassy carbon (GC) surface formation, based on a 15 keV H^+/? ion beam irradiation of thin H₂PtCl₆ × nH₂O layer placed over the GC surface. Hydrogen-ion beam irradiation provided an excellent adherence of Pt deposit, unlike to any other Pt-deposition method. Furthermore, the morphology and electrochemical activity of GC/Pt catalyst obtained at the fluence of 5 × 10¹⁷ cm^? 2 was found to be sensitive to the sign of charge of hydrogen ions. The electrochemical activity of such obtained Pt/GC surface toward oxygen reduction and ethanol oxidation was compared with the activity of the Pt deposits obtained by other more common reduction procedures.     

A comparison of Ga:ZnO and Ga:ZnO/Ag/Ga:ZnO source/drain electrodes for In–Ga–Zn–O thin film transistors

We compared the characteristics of single Ga:ZnO (GZO) and GZO/Ag/GZO multilayer electrodes for source/drain (S/D) contacts in amorphous In–Ga–Zn–O (a-IGZO)-based thin film transistors (TFTs). Due to the existence of a Ag metallic layer between the GZO layers, the GZO/Ag/GZO multilayer electrode exhibited low sheet resistance (3.95 ohm/sq.) and resistivity (3.32 × 10^?5 ohm-cm). The saturation mobility (10.2 cm² V^?1 s^?1) of the a-IGZO TFT with GZO/Ag/GZO S/D electrodes is much higher than that attained for the a-IGZO TFT with single GZO S/D electrodes (0.7 cm² V^?1 s^?1) due to the lower resistivity of the GZO/Ag/GZO multilayer S/D electrode. Furthermore, it is expected that the high transparency of the GZO/Ag/GZO multilayer will allow for the possible realization of fully transparent a-IGZO TFTs.     

Direct electron transfer and biosensing of glucose oxidase immobilized at multiwalled carbon nanotube-alumina-coated silica modified electrode

Investigations are reported regarding the direct electrochemical performance of glucose oxidase (GOD) immobilized on a film of multiwalled carbon nanotube-alumina-coated silica (MWCNT-ACS). The surface morphology of the GOD/MWCNT-ACS nanobiocomposite is characterized by scanning electron microscopy. In cyclic voltammetric response, the immobilized GOD displays a pair of well-defined redox peaks, with a formal potential (E°′) of ? 0.466 V versus Ag/AgCl in a 0.1 M phosphate buffer solution (pH 7.5) at a scan rate of 0.05 V s^? 1; also the electrochemical response indicates a surface-controlled electrode process. The dependence of formal potential on solution pH indicates that the direct electron transfer reaction of GOD is a reversible two-electron coupled with a two-proton electrochemical reaction process. The glucose biosensor based on the GOD/MWCNT-ACS nanobiocomposite shows a sensitivity of 0.127 A M^? 1 cm^? 2 and an apparent Michaelis–Menten constant of 0.5 mM. Furthermore, the prepared biosensor exhibits excellent anti-interference ability to the commonly co-existed uric acid and ascorbic acid.     

Gadolinium(III) ion selective sensor using a new synthesized Schiff's base as a sensing material

According to a solution study which showed a selective complexation between N,N′-bis(methylsalicylidene)-2-aminobenzylamine (MSAB) and gadolinium ions, MSAB was used as a sensing element in construction of a gadolinium(III) ion selective electrode. Acetophenon (AP) was used as solvent mediator and sodium tetraphenyl borate (NaTPB) as an anion excluder. The electrode showed a good selectivity towards Gd(III) ions over a wide variety of cations tested. The constructed sensor displayed a Nernstian behavior (19.7 ± 0.3 mV/decade) in the concentration range of 1.0 × 10^? 6 to 1.0 × 10^? 2 mol L^? 1 with detection limit of 5.0 × 10^? 7 mol L^? 1 and a short response time (< 10 s). The working pH range of the electrode was 3.5–10.1 and lifetime of the sensor was at least 10 weeks. Analysis of certified reference materials confirmed the accuracy of the proposed sensor. The electrode was successfully applied as an indicator electrode in gadolinium titration with EDTA.     

Effect of electron irradiation on characteristics of Mo Schottky barriers on epitaxial silicon

The results of studies on influence of 6 MeV electron irradiation on avalanche breakdown voltage (U_b) and on forward voltage (U_F) at different values of direct current (I_F) for the Mo Schottky diodes on epitaxial silicon of n-type conductivity are presented. It was found out that the avalanche breakdown voltage of the diodes is very sensitive to electron irradiation. A decrease in U_b was observed after electron irradiation with a fluence as low as 1 × 10¹¹ cm^?2. An increase in electron irradiation fluence from 1 × 10¹¹ cm^?2 to 5 × 10¹⁴ cm^?2 resulted in 30% decrease in U_b, however, further increase in electron irradiation fluence from 5 × 10¹⁴ cm^?2 to 3 × 10¹⁶ cm^?2 led to some increase in the avalanche breakdown voltage. Monotonic increases in U_F values at different I_F with the increase in electron irradiation fluence were observed starting from a fluence of 5 × 10¹⁴ cm^?2. Radiation-induced changes in U_b were unstable at room temperature and a significant recovery of U_b occurred after maintaining the irradiated diodes at room temperature for 30 days. Annealing at 120 °C for 20 min resulted in the almost complete recovery of U_b. Radiation-induced changes in U_F values were stable up to 300 °C. Mechanisms of the observed radiation-induced changes in the U_b and U_F values and defects responsible for the changes are discussed.     

Fabrication of an iron(III) PVC-membrane sensor based on bis-benzilthiocarbohydrazide as a selective sensing material

The construction, performance characteristics, and application of a novel iron(III) membrane sensor based on a new bis-benzilthiocarbohydrazide (BBTC) are reported in this paper. The sensor is prepared by incorporating of BBTC, nitrobenzene (NB), and sodium tetraphenyl borate (NaTPB) into a plasticized poly(vinyl chloride) membrane. The electrode reveals a Nernstian behavior over a wide iron ion concentration range (1.0 × 10^? 2–1.0 × 10^? 7 mol L^? 1), and relatively low detection limit (8.6 × 10^? 8 mol L^? 1). The potentiometric response is independent on the pH of the solution in the range of 1.6–4.3. The electrode shows a very short response time (< 10 s). The proposed electrode can be used for at least nine weeks without any considerable divergence in potentials. It exhibits very good selectivity relative to a wide variety of alkali, alkaline earth, transition and heavy metal ions. In fact, the selectivity of the proposed sensor shows great improvements compared to the previously reported electrodes for the iron ion. Also, the sensors accuracy was investigated in two ways: (i) with the potentiometric titration of a Fe(III) solution with EDTA and (ii) with the Fe(III) monitoring in river and wastewater samples.