The first electrochemical sensor for determination of mangiferin based on an ionic liquid–graphene nanosheets paste electrode

A novel carbon paste electrode modified with graphene nanosheets and an ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate) was fabricated and used for the electrochemical study of mangiferin for the first time. This modified electrode offers a considerable improvement in voltammetric sensitivity toward mangiferin, compared to the bare electrode. Square wave voltammetry (SWV) exhibits a linear dynamic range from 5.0?×?10^?8 to 2.0?×?10^?4 M and a detection limit of 20.0 nM for mangiferin. Finally, the proposed method was successfully applied to the determination of mangiferin in real samples such as serum and urine.     

Voltammetric sensor for simultaneous determination of ascorbic acid,acetaminophen, and tryptophan in pharmaceutical products

A novel carbon paste electrode modified with carbon nanotubes and 5-amino-2′-ethyl -biphenyl-2-ol was fabricated. The electrochemical study of the modified electrode, as well as its efficiency for electrocatalytic oxidation of ascorbic acid (AA), is described. The electrode was employed to study the electrocatalytic oxidation of AA, using cyclic voltammetry, chronoamperometry, and square-wave voltammetry (SWV) as diagnostic techniques. It has been found that the oxidation of AA at the surface of modified electrode occurs at a potential of about 250 mV less positive than that of an unmodified carbon paste electrode. SWV exhibits a linear dynamic range from 2.0?×?10^?7 to 5.0?×?10^?4 M and a detection limit of 1.0?×?10^?7 M for AA. In addition, this modified electrode was used for simultaneous determination of AA, acetaminophen (AC), and tryptophan (TRP). Finally, the modified electrode was used for determination of AA, AC, and TRP in pharmaceutical products.     

Nanomolar detection limit for determination of norepinephrine in the presence of acetaminophen and tryptophan using carbon nanotube-based electrochemical sensor

A carbon paste electrode modified by carbon nanotubes and a synthesized hydroquinone derivative (abbreviated as DHB) was fabricated. It was used as an electrochemical sensor for simultaneous determination of norepinephrine (NE), acetaminophen (AC), and tryptophan (Trp). Oxidation potential of NE decreased about 220 mV at the modified electrode in comparison with unmodified electrode because of electrocatalysis of oxidation of NE via E? mechanism at the modified electrode. Differential pulse voltammetry was used for obtaining the calibration plot of NE and two linear range of 0.2–20.0 μM and 20.0–1,500.0 μM and an interesting detection limit (3σ) of 40.0 nM were obtained for NE. Also, simultaneous determination of NE, AC, and Trp was described by the proposed sensor and linear range of 20.0–800.0 μM was found for AC and Trp. Finally, the electrochemical sensor was used for the determination of NE, AC, and Trp in mixture.     

Nanostructured base electrochemical sensor for voltammetric determination of homocysteine using a modified single-walled carbon nanotubes paste electrode

A carbon paste electrode modified with benzoylferrocene (BF) and carbon nanotubes (CNTs) have been applied to the electrocatalytic oxidation of homocysteine which reduced the overpotential by about 165 mV with an obvious increase in the current response. The transfer coefficient (α) for the electrocatalytic oxidation of homocysteine and diffusion coefficient of this substance under the experimental conditions were also investigated. In a phosphate buffer solution (PBS) of pH 7.0, the oxidation current increased linearly with two concentration intervals of homocysteine; one is 0.1 to 10.0 μM, and the other is 10.0 to 80.0 μM. The detection limit (3σ) obtained by square wave voltammetry (SWV) was 50.0 nM. The proposed method was successfully applied to the determination of homocysteine in real samples.     

Electrochemical and catalytic investigations of epinephrine,acetaminophen and folic acid at the surface of titanium dioxide nanoparticle-modified carbon paste electrode

In the present paper, the use of a carbon paste electrode modified with 1-(4-(1, 3-dithiolan-2-yl)-6, 7-dihydroxy-2-methyl-6, 7-dihydrobenzofuran-3-yl)ethanone (DDE) and TiO₂ nanoparticles prepared by a simple and rapid method was described. The modified electrode showed excellent properties for electrocatalytic oxidization of epinephrine (EP), acetaminophen (AC) and folic acid (FA). The apparent charge transfer rate constant, k _s?=?1.14 s^?1, and transfer coefficient, α?=?0.54, for electron transfer between the modifier and carbon paste electrode were calculated. It has been found that under optimum condition (pH?=?7.0) in cyclic voltammetry, the oxidation of EP occurs at a potential about 280 mV less positive than that of an unmodified carbon paste electrode. The values of transfer coefficients (α?=?0.46), catalytic rate constant (k?=?1.2?×?10⁴ M^?1 s^?1) and diffusion coefficient (D?=?2.70?×?10^?5 cm² s^?1) were calculated for EP. Differential pulse voltammetry (DPV) exhibited two linear dynamic ranges of 0.5 to 50.0 μM and 50.0 to 1,000 μM for EP. This modified electrode is quite effective not only for the detection of EP, AC and FA but also for the simultaneous determination of these species in a mixture. The limit of detection for EP, AC and FA is 0.10, 1.80 and 2.36 μM, respectively.     

Developing a sensor for the simultaneous determination of adrenaline,uric acid,and tryptophan

A chemically modified electrode is constructed based on a coumestan derivative and multiwall carbon nanotubes modified carbon paste electrode (CMWCNT-CPE). The surface charge transfer rate constant, k _s, and the charge transfer coefficient, α, for the electron transfer between coumestan and MWCNT-CPE were estimated. CMWCNT-CPE presents a highly catalytic activity for adrenaline (AD) electrooxidation. The results show that the peak potential of AD at the CMWCNT-CPE surface shifted by about 145 mV toward negative values compared with that at the MWCNT-CPE surface. Differential pulse voltammetry exhibited three linear ranges and a detection limit of 0.2 μM for AD. For a mixture containing AD, uric acid (UA), and tryptophan (Trp), three signals corresponding to the analytes could well separate them from each other. Moreover, CMWCNT-CPE was used to determine AD in an adrenaline injection solution and UA in a human urine sample with satisfactory results. To confirm the proposed method, the AD injection solution and the urine sample were spiked with different certain amounts of AD, UA, and Trp.     

Electrochemical detection of aqueous nitrite based on poly(aniline-<Emphasis Type="Italic">co</Emphasis>-<Emphasis Type="Italic">o</Emphasis>-aminophenol)-modified glassy carbon electrode

Nitrite is a common contaminant in drinking water and groundwater with high environmental and health risks. Electrochemical sensing method is a selective and easy technique to detect nitrite in water. In this study, we report a research about a poly(aniline-co-o-aminophenol)-modified glassy carbon electrode (PAOA/GCE) for aqueous nitrite detection. With stable redox activity and conductivity in a wide pH range compared with polyaniline, PAOA is suitable to be used as electrode material in a neutral medium. The PAOA/GCE was prepared by cyclic voltammogram method by electrochemical copolymerization of o-aminophenol and aniline. SEM and FT-IR results proved the formation of PAOA, and the electrode exhibited higher responses toward nitrite oxidation compared with polyaniline-modified GCE and bare GCE. We also studied the impact of scan rate, pH, and temperature on nitrite detection. The PAOA/GCE could be used in a wide pH range from 2 to 8 and used to detect nitrite in the linear range from 5.0 × 10^?6 to 2.0 × 10^?3 M with the detection limit of 2 × 10^?6 M. Its excellent reproducibility, stability, and anti-interference ability make it a promising electrode in detecting aqueous nitrite in drinking water and groundwater.     

Highly sensitive electrochemical detection of adenine on a graphene-modified carbon ionic liquid electrode

A new electrochemical method for the sensitive detection of adenine was established on a chitosan (CTS)- and graphene (GR)-modified carbon ionic liquid electrode (CILE). CILE was prepared by mixing 1-butylpyridinium hexafluorophosphate (BPPF₆) and paraffin with graphite powder. Due to the synergistic effects of GR, CILE, and the interaction of GR with IL on the electrode surface, the electrochemical performance of CTS/GR/CILE were greatly enhanced. Electrochemical behaviors of adenine on the modified electrode was investigated with a single well-defined oxidation peak appeared. The electrochemical reaction of adenine was an adsorption-controlled irreversible process, and the electrochemical parameters were further calculated. Under the optimal conditions, the oxidation peak current was proportional to adenine concentration in the range from 1.0 nmol L^?1 to 70.0 μmol L^?1 with a detection limit of 0.286 nmol L^?1 (3σ) by differential pulse voltammetry. The established method showed the advantages such as good selectivity, stability, and repeatability.     

Sensitive voltammetric determination of cysteamine using promazine hydrochloride as a mediator and modified multi-wall carbon nanotubes carbon paste electrodes

This paper presents a sensitive electrochemical method for the determination of cysteamine (CA) using promazine hydrochloride-modified multi-wall carbon nanotubes carbon paste electrode (PrH/MWCNTs CPE). Because of the good electrochemical activity of MWCNTs and the acceptable performance of promazine hydrochloride (PrH) as an electrocatalytic mediator, the modified electrode significantly enhanced the sensitivity for the detection of CA in comparison to the bare carbon paste electrode (CPE). All chemical parameters such as pH of solution, concentration of PrH and kinetic parameters of the system were investigated. Linear sweep voltammetric (LSV) method was used to follow the electrocatalytic effect of CA on the current–potential response of PrH. Under optimum conditions, the obtained net peak current ?I _p(I _sample???I _blank) was linear with CA concentrations in two dynamic ranges of 2.0–346.5 μmol l^?1 (?I _p?=?(0.0195?±?0.0043)C _CA?+?(0.7648?±?0.0397) (r ²?=?0.9948)) and 346.5–1,912.5 μmol l^?1 (?I _p?=?(0.0100?±?0.0026)C _CA?+?(3.8981?±?0.0828) (r ²?=?0.9911)) with a detection limit of 0.8 μmol l^?1. Finally, the PrH/MWCNTs CPE was successfully applied for the determination of CA in urine and drug samples with satisfactory results.     

Enhanced electrocatalytic oxidation of hydroxylamine on Pt/polypyrrole composite modified glassy carbon electrode

A sensitive hydroxylamine sensor is developed by electrodeposition of Pt nanoparticles on pre-synthesized polypyrrole nanoparticles modified glassy carbon electrode. The modified electrode presents distinctly electrocatalytic activity toward hydroxylamine oxidation. The kinetic parameters such as the overall numbers of electrons involved in hydroxylamine oxidation, the electron transfer coefficient, standard heterogeneous rate constant, and diffusion coefficient are evaluated. The current response increases linearly with increasing hydroxylamine concentrations and exhibits two wide linear ranges of 5.0?×?10^?7–1.1?×?10^?3 and 1.1?×?10^?3–18.8?×?10^?3 M with a detection limit of 0.08 μM (s/n?=?3). The proposed electrode presents excellent operational and storage ability for determining hydroxylamine. Moreover, the sensor shows good sensitivity, selectivity, and reproducibility properties.     

Sensitive voltammetric determination of diclofenac using room-temperature ionic liquid-modified carbon nanotubes paste electrode

An ionic liquid-modified carbon nanotubes paste electrode (IL/CNTPE) has been fabricated using hydrophilic ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate) as a binder. This electrode showed enhanced electrochemical response and strong analytical activity towards the direct electrochemical oxidation of diclofenac (DCF). The electron transfer coefficient, α, and charge transfer resistance (R _ct) of DCF at the modified electrode were calculated. Under optimal conditions at pH 7.0, the anodic peak currents increased linearly with the concentration of DCF in the range of 0.5–300 μmol L^?1 with a detection limit of 0.2 μmol L^?1 (3σ). The interferences of foreign substances were investigated. Differential pulse voltammetry was used to check the applicability of the proposed sensor to the determination of DCF in real samples with satisfactory results.     

Voltammetric determination of hydroxylamine in water samples using a 1-benzyl-4-ferrocenyl-1H-[1,2,3]-triazole/carbon nanotube-modified glassy carbon electrode

A modified glassy carbon electrode has been constructed using a 1-benzyl-4-ferrocenyl-1H-[1,2,3]-triazole along with multiwalled carbon nanotubes. The electrochemical behaviour of modified electrode has been investigated by cyclic voltammetry. Electrocatalytic activity of the modified electrode was investigated for the oxidation of hydroxylamine in 0.1 M phosphate-buffered solution of pH 8. The modified electrode showed electrocatalytic response to the oxidation of hydroxylamine at the potential of 330 mV. The linear range and detection limit for the detection of hydroxylamine in the optimum condition were found to be 4.0?×?10^?7 to 6.75?×?10^?4 M and 28.0?±?1.0 nM, respectively. Finally, the method was employed for the determination of hydroxylamine in water samples.     

A fast and sensitive nanosensor based on MgO nanoparticle room-temperature ionic liquid carbon paste electrode for determination of methyldopa in pharmaceutical and patient human urine samples

In this study, we describe an ionic liquid–MgO nanoparticle modified carbon paste electrode (MgO/NPs/IL/CPE) was used as a simple, fast, and sensitive tool for the investigation of the electrochemical oxidation of methyldopa (MDOP) using voltammetric methods. The MgO/NPs was characterized with different methods such as TEM, SEM, and XRD. The oxidation peak potential of the MDOP at a surface of MgO/NPs/IL/CPE appeared at 450 mV that was about 100 mV lower than the oxidation peak potential at the surface of the traditional carbon paste electrode (CPE) under similar conditions. The electro-oxidation of MDOP occurred in a pH-dependent 2e^? and 2H⁺ process, and the electrode reaction followed a diffusion-controlled pathway. Under optimal conditions at pH 7.0, the anodic peak currents increased linearly with the concentration of MDOP in the range of 0.08–380 μmol L^?1 with a detection limit of 0.03 μmol L^?1 (3σ). The proposed sensor was successfully applied to the determination of MDOP in real samples such as drug and urine.     

Benzoylferrocene-modified carbon nanotubes paste electrode as a voltammetric sensor for determination of hydrochlorothiazide in pharmaceutical and biological samples

The electrooxidation of hydrochlorothiazide (HCT) at the surface of a benzoylferrocene modified multi-walled carbon nanotube paste electrode was studied using electrochemical approaches. Under the optimized conditions (pH 7.0), the square wave voltammetric peak current of HCT increased linearly with HCT concentration in the ranges of 6.0?×?10^?7 to 3.0?×?10^?4 M. The detection limit was 9.0?×?10^?8 M HCT. The diffusion coefficient (D?=?1.75?×?10^?5 cm²/s) and electron transfer coefficient (α?=?0.45) for HCT oxidation were also determined. The proposed sensor was successfully applied for the determination of HCT in human urine and tablet samples.     

Highly sensitive electrochemical detection of dopamine and uric acid on a novel carbon nanotube-modified ionic liquid-nanozeolite paste electrode

A novel biosensor has been constructed by incorporating modified nanosized natural zeolite and 3-hydroxypropanaminium acetate (HPAA) as a novel room temperature ionic liquid, supported on multiwalled carbon nanotube (MWCNTs) and employed for the simultaneous determination of dopamine (DA) and uric acid (UA). A detailed investigation by transmission electron microscopy and electrochemistry is performed in order to elucidate the preparation process and properties of the composites. The voltammetric studies using the modified carbon paste electrode show two well-resolved anodic peaks for DA and UA with a potential difference of 160 mV, revealing the possibility of the simultaneous electrochemical detection of these compounds. The modified carbon paste electrode shows good conductivity, stability, and extraction effect due to the synergic action of HPAA, MWCNTs, and iron ion-doped natrolite zeolite. Under optimized conditions, the peak currents are linear from 8.12?×?10^?7 to 3.01?×?10^?4?mol?L^?1 and from 9.31?×?10^?7 to 3.36?×?10^?4?mol?L^?1 with detection limits of 1.16?×?10^?7 and 1.33?×?10^?7?mol?L^?1 for DA and UA using the differential pulse voltammetric method, respectively. Finally, the modified carbon paste electrode proved to have good sensitivity and stability and is successfully applied for the simultaneous determination of DA and UA in human blood serum and urine samples.     

Electrochemical detection of trace cadmium in soil using a Nafion/stannum film-modified molecular wire carbon paste electrodes

In this paper, a novel Nafion polymer-coated stannum film-modified carbon paste electrode was developed for the analysis of trace cadmium by square wave anodic stripping voltammetry. The electronic conductive material—molecular wire (diphenylacetylene)—was employed as the binder instead of traditional mineral oil for fabricating this electrode. It was found that the prepared electrode possessed excellent electrochemical performance and increased electron transfer rate due to the introduction of molecular wire as a binder, and exhibited a better sensitivity and stability as well as high resistance to surfactants due to the synergistic effect of Nafion and stannum film. Under the optimal conditions, the stripping peak currents showed a good linear relationship with the Cd(II) concentration in the range from 1.0 to 80.0 μg L^?1 with a detection limit of 0.13 μg L^?1 (S/N?=?3). The developed electrode was further applied to the determination of Cd(II) in soil extracts with satisfactory results.     

Application of an ionic liquid-functionalized Mg2Al layered double hydroxide for the electrochemical myoglobin biosensor

In this paper, an ionic liquid 1-carboxyl-methyl-3-methylimidazolium tetrafluoroborate (CMMIMBF₄)-functionalized Mg₂Al layered double hydroxide (LDH) was synthesized and further used for the immobilization of myoglobin (Mb) on the surface of a carbon ionic liquid electrode to get a new electrochemical biosensor. Ultraviolet–visible and Fourier transform–infrared spectroscopies confirmed that Mb in the CMMIMBF₄-LDH remained its native secondary structure, which was attributed to the biocompatibility of the materials used. On the cyclic voltammograms, a pair of well-defined redox peaks appeared, indicating that direct electron transfer of Mb was realized in the modified electrode. The formal peak potential was calculated as ?0.209 V (vs. SCE), which was the typical characteristics of the Mb heme Fe(III)/Fe(II) redox couples. The fabricated Mb sensor exhibited good electrocatalytic activity to the reduction of trichloroacetic acid in the range from 1.0 to 17.0 mmol L^?1 with the detection limit as 0.344 mmol L^?1 (3σ), and the apparent Michaelis–Menten constant was calculated as 13.5 mmol L^?1. Thus, the ionic liquid-functionalized LDH exhibited the potential application in the electrochemical sensor for redox proteins.     

Simultaneous detection of 14NO and 15NO using Faraday modulation spectroscopy

We describe a technique of simultaneous detection of ¹⁴NO and ¹⁵NO by means of Faraday Modulation Spectroscopy (FAMOS) based on a cw distributed feedback quantum cascade laser (QCL) operating near 5.4 μm. FAMOS is a spectroscopic method for selective, sensitive, and time-resolved detection of free radical molecules such as NO, in the mid-infrared spectral region. The selected spectral lines are the Q (1.5) for ¹⁵NO located at 1842.76 cm^?1 and the P (9.5) for ¹⁴NO located at 1842.93 cm^?1. The detection limit (1σ) of 6 ppb $/\sqrt{\mathrm{Hz}}$ for ¹⁵NO and 62 ppb $/\sqrt{\mathrm{Hz}}$ for ¹⁴NO has been achieved. The simultaneous detection was performed using a fast laser frequency switching between the two isotopologues with a time resolution of 2 s. The isotope ratio (δ ¹⁵N) has been determined with a precision (1σ) of 0.52‰ at 800-s averaging time for 100 ppm NO-gas with a time resolution of 2 s. δ ¹⁵N is determined after NO release from nitrite by chemical reduction with potassium iodine.     

A sensitive amperometric detection of dopamine agonist drug pramipexole at functionalized multi-walled carbon nanotubes (f-MWCNTs) modified electrode

The electrochemical detection of dopaminergic agonist drug pramipexole dihydrochloride monohydrate (PPX) has been investigated by cyclic voltammetric (CV) and amperometric i–t techniques at functionalized multi-walled carbon nanotubes-modified glassy carbon electrode. For the first time, a sensitive and rapid electrochemical method was developed for the determination of PPX. The surface morphological characteristics of the proposed electrode have been studied by using transmission electron microscopy (TEM); further, electrochemical impedance spectroscopy (EIS) and Fourier transform infrared spectroscopy (FTIR) have been employed. PPX shows an irreversible anodic peak, which may be ascribed to the oxidation of the –NH groups of PPX. The proposed method was showing good sensitivity of 0.993 μA μM^?1 cm^?2 with a linear range of 5 to 340 μM by amperometric i–t and CV technique shows a linear range of 12.5 to 313 μM with a sensitivity of 1.92 μA μM^?1 cm^?2. The recovery of PPX from blood serum samples was found 100.6 and 98.9 %, respectively. Furthermore, the proposed method has been demonstrated for the determination of PPX in commercially available pharmaceutical samples and good agreement of results obtained.     

The Reaction of Synechocystis Catalase–Peroxidase (KatG) with Isoniazid Investigated by Multifrequency (9–285 GHz) EPR Spectroscopy

Three distinct electron paramagnetic resonance (EPR) spectra of radical intermediates formed as reactive intermediates in the catalytic cycle of Synechocystis catalase–peroxidase were identified. Multifrequency EPR spectroscopy, combined with site-directed mutagenesis and selective deuterium labeling of Trp and Tyr residues, allowed us to unequivocally assign such intermediates to an [Fe(IV) = O Por ^·+] species, the first committed intermediate in monofunctional peroxidases and two protein-based radicals, identified as Trp106 ^· and a Tyr ^· , formed subsequently to the [Fe(IV) = O Por ^·+] species by intramolecular electron transfer. Our recent characterization of the Mycobacterium tuberculosis catalase–peroxidase showed that the Trp ^· sites differ among these enzymes, and that the [Fe(IV) = O Trp ^· ] species was the reactive intermediate with the prodrug isoniazid. Accordingly, the question to address was whether the dissimilarity in the sites for the formation of the Trp ^· intermediates and in the geometry of the distal side was reflected by differences in the peroxidase-like reaction of Synechocystis and Mycobacterium tuberculosis catalase–peroxidases with the prodrug isoniazid. Our findings show that in the Synechocystis enzyme, the isoniazid substrate can get closer to the heme distal side and can react readily with the [Fe(IV) = O Por ^·+] species, at variance to the situation in the M. tuberculosis catalase–peroxidase. These results indicate that, as in the case of monofunctional peroxidases, the difference in the sites for the formation of the Trp ^· as alternative reactive intermediates to the [Fe(IV) = O Por ^·+] species is correlated to differences in substrate binding sites.