A Sensitive Electrochemical Approach for Melamine Detection Using a Disposable Screen Printed Carbon Electrode

We report here a simple and easy electrochemical approach for sensitive detection of non‐electroactive melamine using a disposable screen printed carbon electrode (SPCE) with uric acid as the recognition element. It is based on the competitive adsorptive behavior of melamine at the preanodized SPCE causing suppression in the oxidation current of uric acid. A linear range up to 126 ppb with a detection limit of 1.6 ppb (S/N=3) is achieved at the preanodized SPCE by differential pulse voltammetry. The electrochemical method is successfully applied to detect the melamine content in tainted milk powder and dog food.     

Sensitive amperometric detection of omeprazole and pantoperazole at electrodeposited nickel oxide nanoparticles modified glassy carbon electrode

Glassy carbon electrode modified with electrodeposited nickel oxide nanoparticles (NiOxNPs) was used as electrocatalyst for oxidation of omeprazole and pentoperazole in alkaline solution. The modified electrode exhibited efficient electrocatalytic activity for the oxidation of omeprazole and pentoperazole with relatively high sensitivity, excellent stability, and long lifetime. Hydrodynamic amperometric method is used for determination of selected analytes. Under optimized condition, the linear concentration range, detection limit, and sensitivity of modified electrode toward omeprazole detection are 4.5–120 μM, 0.4 μM (at signal to noise 3), and 40.1 nA μM⁻¹ cm⁻², respectively. For pantoperazole, hydrodynamic amperometric determination yielded calibration curve with linear range of 2.5–180 μM, detection limit of 0.2 μM, and sensitivity of 39.2 nA μM⁻¹ cm⁻², respectively. The proposed method was successfully applied to pentoperazole and omeprazole determination in drug samples.     

Electrochemical behavior of isoproterenol in the presence of uric acid and folic acid at a carbon paste electrode modified with 2,7-bis(ferrocenyl ethyl)fluoren-9-one and carbon nanotubes

This paper reports the selective determination of isoproterenol (IP) in the presence of uric acid (UA) and folic acid (FA) using 2,7-bis(ferrocenyl ethyl)fluoren-9-one modified carbon nanotube paste electrode (2,7-BFCNPE) in 0.1 M phosphate buffer solution (PBS) (pH 7.0). The bare carbon paste electrode does not separate the voltammetric signals of IP, UA, and FA. However, 2,7-BFCNPE not only resolved the voltammetric signals of IP, UA, and FA with potential differences of 150, 325, and 475 mV between IP–UA, UA–FA, and IP–FA, respectively, but also dramatically enhanced the oxidation peak currents of them when compared to bare carbon paste electrode. In PBS of pH 7.0, the oxidation current increased linearly with two concentration intervals of IP, one is 0.08 to 17.5 μM and the other is 17.50 to 700.0 μM. The detection limit (3σ) obtained by DPV was 26.0 ± 2 nM. The practical application of the modified electrode was demonstrated by determining IP in IP injection, urine, and human blood serum.     

Electrochemical oxidation of salicylic acid at well-aligned multiwalled carbon nanotube electrode and its detection

In this paper, an electrochemical sensor for sensitive and convenient determination of salicylic acid (SA) was constructed using well-aligned multiwalled carbon nanotubes as electrode material. Compared to the glassy carbon electrode, the electro-oxidation of SA significantly enhanced at the multiwalled carbon nanotube (MWCNT) electrode. The MWCNT electrode shows a sensitivity of 59.25 μA mM⁻¹, a low detection limit of 0.8 × 10⁻⁶ M and a good response linear range with SA concentration from 2.0 × 10⁻⁶ to 3.0 × 10⁻³ M. In addition, acetylsalicylic acid was determined indirectly after hydrolysis to SA and acetic acid, which simplified the detection process. The mechanism of electrochemical oxidation of SA at the MWCNT electrode is also discussed.     

Electrochemical Determination of Pyrogallol at Conducting Poly(3,4‐ethylenedioxythiophene) Film‐Modified Screen‐Printed Carbon Electrodes

The electrochemical oxidation of pyrogallol at electrogenerated poly(3,4‐ethylenedioxythiophene) (PEDOT) film‐modified screen‐printed carbon electrodes (SPCE) was investigated. The voltammetric peak for the oxidation of pyrogallol in a pH 7 buffer solution at the modified electrode occurred at 0.13 V, much lower than the bare SPCE and preanodized SPCE. The experimental parameters, including electropolymerization conditions, solution pH values and applied potentials were optimized to improve the voltammetric responses. A linear calibration plot, based on flow‐injection amperometry, was obtained for 1–1000 µM pyrogallol, and a slope of 0.030 µA/µM was obtained. The detection limit (S/N=3) was 0.63 µM.     

Electrochemical sensor using glassy carbon electrode modified with acylpyrazolone-multiwalled carbon nanotube composite film for determination of xanthine

In this work, an electrochemical sensor 1-phenyl-3-methyl-4-(2-furoyl)-5-pyrazolone/multiwalled carbon nanotubes/glassy carbon electrode (GCE) was prepared for the determination of xanthine (XN) in the presence of an excess of uric acid. Cyclic voltammetry and differential pulse voltammetry were used to characterize the electrode. The oxidation of XN occurred in a well-defined peak having E _p 0.73 V in phosphate buffer solution of pH 6.0. Compared with the bare GCE, the electrochemical sensor greatly enhanced the oxidation signal of XN with negative shift in peak potential about 110 mV. Based on this, a sensitive, rapid, and convenient electrochemical method for the determination of XN has been proposed. Under the optimized conditions, the oxidation peak current of XN was found to be proportional to its concentration in the range of 0.3~50 μM with a detection limit of 0.08 μM. The analytical utility of the proposed method was demonstrated by the direct assay of XN in urine samples and was found to be promising at our preliminary experiments.     

Electrocatalytic oxidation of glucose on nanoporous gold membranes

With characteristic of structural integrity and high surface area, nanoporous gold (NPG) prepared by dealloying method is proposed to be a highly sensitive catalyst for glucose electrooxidation. It can be found that a-NPG which obtained by electrochemical corrosion method has the highest sensitivity for glucose electrooxidation among the three studied samples. Under alkaline conditions, the catalytic current density of a-NPG is over 1.5 times and 17 times higher than that of f-NPG (prepared by free corrosion) and poly-Au electrode, respectively. Using a-NPG sample for glucose detection, the obtained minimum sensible concentration are 413 nM in alkaline media and 1 μM in neutral solutions. The a-NPG electrode also shows stable recovery and reproducibility characteristics. These results indicate that NPG may work as an efficient electrode material for electrochemical sensors and a promising catalyst for alkaline glucose fuel cells.     

Electrochemical oxidation and determination of ceftriaxone on a glassy carbon and carbon-nanotube-modified glassy carbon electrodes

The electrochemical behavior of ceftriaxone was investigated on a carbon-nanotube-modified glassy carbon (GC-CNT) electrode in a phosphate buffer solution, pH = 7.40, and the results were compared with those obtained using the unmodified one [glassy carbon (GC) electrode]. During oxidation of ceftriaxone, an irreversible anodic peak appeared, using both modified and unmodified electrodes. Cyclic voltammetric studies indicated that the oxidation process is irreversible and diffusion-controlled. The number of electrons exchanged in the electrooxidation process was obtained, and the data indicated that ceftriaxone is oxidized via a one-electron step. The results revealed that carbon nanotube promotes the rate of oxidation by increasing the peak current. In addition, ceftriaxone was oxidized at lower potentials, which thermodynamically is more favorable. These results were confirmed by impedance measurements. The electron-transfer coefficients and heterogeneous electron-transfer rate constants for ceftriaxone were reported using both the GC and GC-CNT electrodes. Furthermore, the diffusion coefficient of ceftriaxone was found to be 2.74 × 10⁻⁶ cm² s⁻¹. Binding of ceftriaxone to human serum albumin forms a kind of electroreactive species. The percentage of interaction of ceftriaxone with protein was also addressed. A sensitive, simple, and time-saving differential-pulse voltammetric procedure was developed for the analysis of ceftriaxone, using the GC-CNT electrode. Ceftriaxone can be determined with a detection limit of 4.03 × 10⁻⁶ M with the proposed method.     

A selective determination of norepinephrine on the glassy carbon electrode modified with poly(ethylenedioxypyrrole dicarboxylic acid) nanofibers

A glassy carbon electrode modified with poly(3,4-ethylenedioxypyrrole-2,5-dicarboxylic acid) nanofibers (PEDOPA-NFs) was prepared for the determination of norepinephrine (NE) in phosphate buffer saline. The modified electrode demonstrated an improved sensitivity and selectivity toward the electrochemical detection of NE and could detect separately ascorbic acid (AA), uric acid (UA), and NE in their mixture. The separations of the oxidation peak potentials of NE–AA and NE–UA were 160 and 150 mV, respectively. Meanwhile, the modified electrode showed higher sensitivity and selectivity toward NE than dopamine and epinephrine. Using differential pulse voltammetry, the oxidation peak current of NE was found to be linearly dependent on its concentration within the range of 0.3–10 μM, and the detection limit of the NE oxidation current was 0.05 μM at a signal-to-noise ratio of 3. The PEDOPA-NFs promoted the electron transfer reaction of NE, while the PEDOPA-NFs, acting as a negatively charged linker, combined with the positively charged NE to induce NE accumulation in the NFs at pH under 7.4. However, the PEDOPA-NFs restrained the electrochemical response of the negatively charged AA and UA due to the electrostatic repulsion. The result indicates that the modified electrode can be used to determine NE without interference from AA and UA and selectively in the mixture of catecholamines.     

A new cobalt complex of 1-decyl-1<Emphasis Type="Italic">H</Emphasis>-benzo[<Emphasis Type="Italic">d</Emphasis>]imidazole: synthesis,characterization and electrocatalysis

The present work reports on the synthesis, characterization and performance of a new cobalt(II) complex, [Co(C₁₀H₂₁-bim)₂(SCN)₂] (bim = benzimidazole) as electrocatalyst for trichloroacetic acid and bromate reduction. Its structure was characterized by X-ray crystallography, IR spectroscopy and elemental analysis. The cobalt atom adopts a distorted tetrahedral geometry by coordinating to four nitrogen atoms from two thiocyanates and two 1-decyl-1H-benzo[d]imidazole ligands. The electrochemical behavior and electrocatalysis of the title complex bulk-modified carbon paste electrode (Co-CPE) have been studied by cyclic voltammetry. The Co-CPE shows good electrocatalytic activities toward the reduction of trichloroacetic acid and bromate. The detection limit and the sensitivity are 0.02 μM, 34.63 μA μM⁻¹ for trichloroacetic acid detection, and 0.03 μM, 78.92 μA μM⁻¹ for bromate detection, respectively. This modified electrode shows good reproducibility, high stability, low detection limit, technical simplicity and possibility of rapid preparation, which is important for practical application.     

Determination aminopyrine in pharmaceutical formulations based on APTS-Fe3O4 nanoparticles modified glassy carbon electrode

In this work, 3-aminopropyltriethoxysilane modified Fe₃O₄ nanoparticles (ATPS-Fe₃O₄) were used to modify glassy carbon electrode for aminopyrine determination. ATPS-Fe₃O₄ showed obviously catalytic activity and adsorptivity towards aminopyrine oxidation proven by the increased oxidation peak current and the decreased oxidation peak potential. The best analytical response was obtained by immobilizing 8 μL 3 mg/mL APTS-Fe₃O₄ dispersion with an accumulation time of 200 s at −0.2 V in 0.1 M phosphate buffer solution (pH 9.0). The oxidation peak current of aminopyrine showed linear relationship with its concentration in the range from 0.5 to 100 and 100 to 1600 μM. The detection limit was 0.1 μM (S/N = 3). The proposed method showed satisfactory repeatability and anti-interference ability. The fabricated electrode was successfully applied to determine aminopyrine in pharmaceutical formulations.     

Pd/Pt core–shell nanowire arrays as highly effective electrocatalysts for methanol electrooxidation in direct methanol fuel cells

Highly ordered Pd/Pt–core–shell nanowire arrays (Pd/Pt NWAs) have been prepared by anodized aluminum oxide (AAO) template-electrodeposition and magnetron sputtering methods. Pd/Pt NWA electrode shows a very high electrochemical active surface area and high electrocatalytic activity for the methanol electrooxidation in acid medium for direct methanol fuel cells (DMFCs). The mass specific anodic peak current density is 756.7 mA mg⁻¹ Pt for the methanol oxidation on the Pd/Pt NWA electrode, an increase by a factor of four as compared to conventional E-TEK PtRu/C electrocatalysts. The mechanism of the significant enhancement of the Pd/Pt core/shell NWA nanostructure in the efficiency and electrocatalytic activity of Pt for the methanol electrooxidation in acid medium is discussed.     

Oxidation and determination of Gabapentin on nanotubes of nickel oxide-modified carbon paste electrode

The electrooxidation of Gabapentin was studied on nanotubes of nickel oxide-modified carbon paste electrode for the first time. Cyclic voltammetry was employed to investigate the electrooxidation process. A simple, sensitive, and efficient amperometric method was developed for the analysis of the drug, and the corresponding analytical parameters were reported. For Gabapentin, a detection limit of 0.3 μM was obtained in a linear range of 2.4–50 μM. The proposed amperometric method was also applied to the analysis of commercial capsules, and the results were in good agreement with the declared values. Also, the applicability of the method to the direct assay of the drug in human serum and urine was described.     

Simultaneous voltammetric detection of dopamine and uric acid in the presence of high concentration of ascorbic acid using multi-walled carbon nanotubes with methylene blue composite film-modified electrode

An electrochemically functional nanocomposite through the adsorption of methylene blue onto the multi-walled nanotubes (MB-MWNTs) was prepared, and a sensitive voltammetric sensor was fabricated. The modified electrode showed excellent electrocatalytic activity toward dopamine (DA) and uric acid (UA) in 0.1 M phosphate solution medium (pH 3.0). Compared to the bare electrode, the MB-MWNTs film-modified electrode not only remarkably enhanced the anodic peak currents of DA and UA, i.e., shifted the anodic peak potential of DA negatively, but also avoided the overlapping of the anodic peaks of DA and UA. The interference of ascorbic acid (AA) was eliminated. Under the optimized conditions, the peak separation between AA and DA and between DA and UA was 219 and 174 mV, respectively. In the presence of 1.0 mM AA and 10.0 μM UA, the anodic peak current was linear to the concentration of DA in the range of 0.4–10.0 μM with a detection limit of 0.2 μM DA. The anodic peak current of UA was linear to the concentration in the range of 2.0–20.0 and 20.0–200.0 μM with a lowest detection limit of 1.0 μM in the presence of 1.0 mM AA and 1.0 μM DA.     

Electrochemical behavior of electrodeposited rutin film on a multi-wall carbon nanotubes modified glassy carbon electrode. Improvement of the electrochemical reversibility and its application as a hydrazine sensor

By immobilizing rutin at the surface of a glassy carbon electrode (GCE) modified with multi-wall carbon nanotubes (MWCNT), a new modified electrode has been fabricated and its electrochemical behavior was investigated by cyclic voltammetry. Cyclic voltammograms of the resulting modified electrode show stable and a well defined redox couple with surface confined characteristics. The results show that the reversibility of rutin is significantly improved at a MWCNT modified GCE in comparison with GCE alone. The charge transfer coefficient, α, was calculated to be 0.4, and charge transfer rate constant, k_s, was 46.7 s⁻¹ in pH 8, indicating great facilitation of the electron transfer between rutin and MWCNT deposited on the electrode surface. The rutin MWCNT (RMWCNT) modified GCE showed excellent mediation of hydrazine oxidation: a decrease in the overvoltage of hydrazine electrooxidation was observed as well as a dramatic increase in the peak current compared to that seen at a rutin modified GCE (RMGCE), activated GCE or bare GCE. Hydrazine was determined amperometrically at the surface of RMWCNT modified GCE in pH 8. Under the optimized conditions the calibration curve is linear in the concentration range 2.0–190.0 μM hydrazine. The detection limit and sensitivity are 0.61 μM and 0.0656 μA μM⁻¹, respectively. Finally the kinetic parameters of the electron transfer coefficient, α, the heterogeneous rate constant of dependent to different potentials, k′(E), and the standard heterogeneous rate constant, k⁰, for oxidation of hydrazine at the RMWCNT surface were determined using various electrochemical methods. The advantages of this modified electrode for hydrazine determination are high sensitivity, excellent catalytic activity, short response time, wide linear range, and high exchange current density.     

Simultaneous voltammetric determination of ascorbic acid and uric acid using a Nafion/multi-wall carbon nanotubes composite film-modified electrode

A Nafion/multi-wall carbon nanotubes (MWNT) composite film-modified electrode was fabricated. The modified electrode showed excellent electrocatalytic activity toward ascorbic acid (AA) and uric acid (UA) in 0.1-mol L⁻¹ NaCl medium (pH 6.5). Compared to the bare electrode that only displayed a broad and overlapped oxidation peak, the Nafion/MWNT film-modified electrode not only remarkably enhanced the anodic peak currents of AA and UA but also avoided the overlapping of the anodic peaks of AA and UA with a 320-mV separation of both peaks. Under the optimized conditions, the peak currents of AA and UA were proportional to their concentration at the ranges of 8.0 × 10⁻⁵ to 6.0 × 10⁻³ mol L⁻¹ and 6.0 × 10⁻⁷ to 8.0 × 10⁻⁵ mol L⁻¹, respectively. The proposed method was used for the detection of AA and UA in real samples with satisfactory results.     

Electrochemical characterization of 2, 2′-[1, 2-ethanediylbis (nitriloethylidyne)]-bis-hydroquinone-carbon nanotube paste electrode and its application to simultaneous voltammetric determination of ascorbic acid and uric acid

The preparation and electrochemical characterization of a carbon nanotube paste electrode modified with 2,2′-[1,2-ethanediylbis (nitriloethylidyne)]-bis-hydroquinone, referred to as EBNBH, was investigated. The EBNBH carbon nanotube paste electrode (EBNBHCNPE) displayed one pair of reversible peaks at E _pa = 0.18 V and E _pc = 0.115 V vs Ag/AgCl. Half wave potential (E _1/2) and ΔE _p were 0.148 and 0.065 V vs Ag/AgCl, respectively. The electrocatalytic oxidation of ascorbic acid (AA) has been studied on EBNBHCNPE, using cyclic voltammetry, differential pulse voltammetry and chronoamperometry techniques. It has been shown that the oxidation of AA occurs at a potential where oxidation is not observed at the unmodified carbon paste electrode. The heterogeneous rate constant for oxidation of AA at the EBNBHCNPE was also determined and found to be about 1.07 × 10⁻³ cm s⁻¹. The diffusion coefficient of AA was also estimated as 5.66 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometry. Also, this modified electrode presented the property of electrocatalysing the oxidation of AA and uric acid (UA) at 0.18 and 0.35 V vs Ag/AgCl, respectively. The separations of anodic peak potentials of AA and UA reached 0.17 V. Using differential pulse voltammetry, the calibration curves for AA and UA were obtained over the range of 0.1–800 μM and 20–700 μM, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of AA and UA in biological samples.     

Biomolecule-free,selective detection of clenbuterol based on disposable screen-printed carbon electrode

We report here the development of a selective clenbuterol sensor made of disposable screen-printed carbon electrode (SPCE) without the need of adding any biorecognition element. Good analytical performance was achieved through the proper function of both the oxygen functionalities and edge plane sites on the “preanodized” SPCE (SPCE*). It is the amino group of clenbuterol to effectively form hydrogen bond with the SPCE* to induce the adsorption of clenbuterol. The edge plane sites enhance the electron transfer process and further help the dimer formation of clenbuterol to generate electroactivity for analysis. Square wave voltammetry was applied to increase the detection sensitivity with a linear response in the range of 7–1000 ppb and a detection limit of 0.51 ppb (S/N = 3). In the real sample analysis, results observed were satisfactory with meat, human blood, and human urine. High reproducibility in sensor fabrication further favors the disposable purpose of applications.     

Flow injection amperometric sensing of uric acid and ascorbic acid using the self-assembly of heterocyclic thiol on Au electrode

Au electrode modified with the self-assembled monolayer of a heterocyclic thiol, mercaptotriazole (MTz), is used for the electroanalysis of uric acid (UA) and ascorbic acid (AA). MTz forms a less compact self-assembly on Au electrode. The self-assembly of MTz on Au electrode favors the oxidation of UA and AA at less positive potential. Significant decrease (∼400 mV) in the overpotential and enhancement in the peak current for the oxidation of interfering AA with respect to the unmodified electrode is observed. The negative shift in the oxidation peak potential of AA favors electrochemical sensing of UA without any interference. Two well-separated voltammetric peaks for AA and UA are observed in their coexistence. The large separation between the two voltammetric peaks allows the simultaneous or selective sensing of the analytes without compromising the sensitivity. Linear response is obtained for a wide concentration range. This electrode could sense as low as 1 μM of UA in the presence of 10-fold excess of interfering AA. No change in the sensitivity (0.012 μA/μM) of the electrode toward UA in the presence and absence of AA is observed. Reproducible and stable amperometric flow injection response was obtained upon repetitive injection.     

Electrochemical solid phase micro-extraction and determination of salicylic acid from blood samples by cyclic voltammetry and differential pulse voltammetry

The electrochemical solid phase micro-extraction of salicylic acid (SA) at graphite-epoxy-composed solid electrode surface was studied by cyclic voltammetry. SA was oxidized electrochemically in pH 12.0 aqueous solution at 0.70 V (vs. saturated calomel electrode) for 7 s. The oxidized product shows two surface-controlled reversible redox couples with two proton transferred in the pH range of 1.0∼6.0 and one proton transferred in the pH range of 10.0∼13.0 and is extracted on the electrode surface with a kinetic Boltzman function of i _p = 3.473–4.499/[1 + e^{(t − 7.332)/6.123}] (χ ² = 0.00285 μA). The anodic peak current of the extracted specie in differential pulse voltammograms is proportional to the concentration of SA with regression equation of i _p = −5.913 + 0.4843 c (R = 0.995, SD = 1.6 μA) in the range of 5.00∼200 μM. The detection limit is 5.00 μM with RSD of 1.59% at 60 μM. The method is sensitive and convenient and was applied to the detection of SA in mouse blood samples with satisfactory results.