Hematoxylin multi-wall carbon nanotubes modified glassy carbon electrode for electrocatalytic oxidation of hydrazine

A new hydrazine sensor has been fabricated by immobilizing hematoxylin at the surface of a glassy carbon electrode (GCE) modified with multi-wall carbon nanotube (MWCNT). The adsorbed thin films of hematoxylin on the MWCNT modified GCE show one pair of peaks with surface confined characteristics. The hematoxylin MWCNT (HMWCNT) modified GCE shows highly catalytic activity toward hydrazine electro-oxidation. The results show that the peak potential of hydrazine at HMWCNT modified GCE surface shifted by about 167 and 255 mV toward negative values compared with that at an MWCNT and activated modified GCE surface, respectively. In addition, at HMWCNT modified electrode surface remarkably improvement the sensitivity of determination of hydrazine. The kinetic parameters, such as the electron transfer coefficient, α, and the standard heterogeneous rate constant, k⁰, for oxidation of hydrazine at the HMWCNT modified GCE were determined and also is shown that the heterogeneous rate constant, k′, is strongly potential dependent. The overall number of electron involved in the catalytic oxidation of hydrazine and the number of electrons involved in the rate-determining steps are 2 and 1, respectively. The amperometric detection of hydrazine is carried out at 220 mV in 0.1 M phosphate buffer solution (pH 7) with linear response range 2.0-122.8 μM hydrazine, detection limit of 0.68 μM and sensitivity of 0.0208 μA μM⁻¹. Finally the amperometric response for hydrazine determination is reproducible, fast and extremely stable, with no loss in sensitivity over a continual 400 s operation.     

Gold nanoparticle/carbon nanotube hybrids as an enhanced material for sensitive amperometric determination of tryptophan

In this work, a highly sensitive electrochemical sensor for the determination of tryptophan (Trp) was fabricate by electrodeposition of gold nanoparticles (AuNPs) onto carbon nanotube (CNT) films pre-cast on a glassy carbon electrode (GCE), forming an AuNP-CNT composite-modified GCE (AuNP-CNT/GCE). Scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used for the surface analysis of the electrode. The results indicate that the hybrid nanomaterials induced a substantial decrease in the overpotential of the Trp oxidation reaction and exhibited a remarkable synergistic effect on the electrocatalytic activity toward the oxidation of Trp. In phosphate buffer solution (pH 7.4), the modified electrode showed excellent analytical performance for the amperometric determination of Trp. The peak currents possess a linear relationship with the concentration of Trp in the range of 30 nM to 2.5 μM, and the detection limit is 10 nM (S/N = 3). In addition, the modified electrode was used to determine Trp concentration in pharmaceutical samples with satisfactory results.     

Electrochemical behavior and voltammetric determination of 4-aminophenol based on graphene-chitosan composite film modified glassy carbon electrode

The graphene-chitosan composite film modified glassy carbon electrode (GCE) was fabricated and used to determine 4-aminophenol (4-AP). In 0.1 M pH 6.3 phosphate buffer solution, the redox peak currents of 4-AP increased significantly and the peak-to-peak separation decreased greatly at graphene-chitosan composite film modified GCE compared with bare GCE and chitosan modified GCE, indicating that graphene possessed electrocatalytic activity towards 4-AP. The experimental conditions were optimized and the kinetic parameters were investigated. The oxidation mechanism was discussed. Under the optimal experimental conditions, the oxidation peak current was proportional to 4-AP concentration in the range from 0.2 to 550 μM with the correlation coefficient of 0.9930. The detection limit was 0.057 μM (S/N = 3). Using the proposed method, 4-AP was successfully determined in water samples and paracetamol tablets with standard addition method, suggesting that this method can be applied to determine 4-AP in environments and pharmaceuticals.     

Towards the conception of an amperometric sensor of l-tyrosine based on Hemin/PAMAM/MWCNT modified glassy carbon electrode

A novel amperometric sensor was fabricated based on the immobilization of hemin onto the poly (amidoamine)/multi-walled carbon nanotube (PAMAM/MWCNT) nanocomposite film modified glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and ultraviolet visible (UV-vis) adsorption spectroscopy were used to investigate the possible state and electrochemical activity of the immobilized hemin. In the Hemin/PAMAM/MWCNT nanocomposite film, MWCNT layer possessed excellent inherent conductivity to enhance the electron transfer rate, while the layer of PAMAM greatly enlarged the surface average concentration of hemin (Γ) on the modified electrode. Therefore, the nanocomposite film showed enhanced electrocatalytical activity towards the oxidation of l-tyrosine. The kinetic parameters of the modified electrode were investigated. In pH 7.0 phosphate buffer solution (PBS), the sensor exhibits a wide linear range from 0.1 μM to 28.8 μM l-tyrosine with a detection limit of 0.01 μM and a high sensitivity of 0.31 μA μM⁻¹ cm⁻². In addition, the response time of the l-tyrosine sensor is less than 5 s. The excellent performance of the sensor is largely attributed to the electro-generated high reactive oxoiron (IV) porphyrin (O = Fe^IV-P) which effectively catalyzed the oxidation of l-tyrosine. A mechanism was herein proposed for the catalytic oxidation of l-tyrosine by oxoiron (IV) porphyrin complexes.     

Study of nimesulide and its determination using multiwalled carbon nanotubes modified glassy carbon electrodes

A new method for the determination of nimesulide was established based on the multiwalled carbon nanotubes (MWCNTs) modified glassy carbon electrode (MWCNTs/GCE). In 0.2 M PBS (pH 6.6) buffer solution, the MWCNTs/GCE showed a remarkable catalytic and enhancement effect on reduction of the nimesulide. The reduction peak potential of nimesulide shifted positively from −0.665 V at bare GCE to −0.553 V at MWCNTs/GCE, and the sensitivity increased ca. 7 times. A linear dynamic range of 3.2 × 10⁻⁷-6.5 × 10⁻⁵ M (R = 0.9992) with a detection limit of 1.6 × 10⁻⁷ M was obtained. The electrochemical behaviors of nimesulide were studied and electron-transfer coefficient (α = 0.45), proton number (X = 1) and electron-transfer number (n = 2) have been determined. This method has been used to determine the content of nimesulide in medical tablets. The recovery was determined to be 93.2-106.2% by means of standard addition method. Compared with UV-vis spectrometry, the method was not remarkable difference.     

Electrocatalytic oxidation and simultaneous determination of uric acid and ascorbic acid on the gold nanoparticles-modified glassy carbon electrode

A new gold nanoparticles-modified electrode (GNP/LC/GCE) was fabricated by self-assembling gold nanoparticles to the surface of the l-cysteine-modified glassy carbon electrode. The modified electrode showed an excellent character for electrocatalytic oxidization of uric acid (UA) and ascorbic acid (AA) with a 0.306 V separation of both peaks, while the bare GC electrode only gave an overlapped and broad oxidation peak. The anodic currents of UA and AA on the modified electrode were 6- and 2.5-fold to that of the bare GCE, respectively. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of UA and AA has been explored at the modified electrode. DPV peak currents of UA and AA increased linearly with their concentration at the range of 6.0 × 10⁻⁷ to 8.5 × 10⁻⁴ mol L⁻¹ and 8.0 × 10⁻⁶ to 5.5 × 10⁻³ mol L⁻¹, respectively. The proposed method was applied for the detection of UA and AA in human urine with satisfactory result.     

Electrochemical determination of bisphenol A at Mg-Al-CO3 layered double hydroxide modified glassy carbon electrode

The electrochemical behavior of bisphenol A (BPA) was investigated on Mg-Al layered double hydroxide (LDH) modified glassy carbon electrode (GCE) by cyclic voltammetry (CV), differential pulse voltammetry (DPV), linear sweep voltammetry (LSV) and chronocoulometry (CC). The cyclic voltammogram of BPA on the modified electrode exhibited a well defined anodic peak at 0.454 V in 0.1 M pH 8.0 phosphate buffer solution (PBS). The experimental parameters were optimized and the kinetic parameters were investigated. The probable oxidation mechanism was proposed. Under the optimized conditions, the oxidation peak current was proportional to BPA concentration in the range from 1 × 10⁻⁸ to 1.05 × 10⁻⁶ M with the correlation coefficient of 0.9959. The detection limit was 5.0 × 10⁻⁹ M (S/N = 3). The fabricated electrode showed good reproducibility, stability and anti-interference. The proposed method was successfully applied to determine BPA in plastic products and the results were satisfactory.     

Electrocatalytic hydrazine oxidation on quinizarine modified glassy carbon electrode

The electrocatalytic oxidation of hydrazine has been studied on glassy carbon modified by electrodeposition of quinizarine, using cyclic voltammetry and chronoamperometry techniques. It has been shown that the oxidation of hydrazine to nitrogen occurs at a potential where oxidation is not observed at the bare glassy carbon electrode. The apparent charge transfer rate constant and transfer coefficient for electron transfer between the electrode surface and immobilized quinizarine were calculated as 4.44 s⁻¹ and 0.66, respectively. The heterogenous rate constant for oxidation of hydrazine at the quinizarine modified electrode surface was also determined and found to be about 4.83 × 10³ M⁻¹ s⁻¹. The diffusion coefficient of hydrazine was also estimated as 1.1 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometry.     

Monolayer covalent modification of 5-hydroxytryptophan on glassy carbon electrodes for simultaneous determination of uric acid and ascorbic acid

5-Hydroxytryptophan (5-HTP) was covalently grafted on the surface of glassy carbon electrodes (GCEs) using cyclic voltammetric method in a phosphate buffer solution. The prepared electrode, denoded as 5-HTP/GCE, was characterized by X-ray photoelectron spectroscopy, cyclic voltammetry and differential pulse voltammetry (DPV). Tryptophan grafted GCE (TRP/GCE) and 5-hydroxytryptamine grafted GCE (5-HTP/GCE) were also prepared by the same method for comparison. It was found that the electrocatalytic activities toward the oxidation of uric acid (UA) and ascorbic acid (AA) was in the order of 5-HT/GCE > 5-HTP/GCE > TRP/GCE for UA oxidation and 5-HT/GCE = 5-HTP/GCE > TRP/GCE for AA oxidation. However, the CV current sensitivity was estimated as 4:2:1 for 5-HTP/GCE:5-HT/GCE:TRP/GCE. The DPV peaks for UA and AA oxidation appeared at 0.07 V and 0.34 V versus SCE, respectively, allowing simultaneous determination in mixtures. A linearly response in the range of: 5.0 × 10⁻⁷ to 1.1 × 10⁻⁵ M with the detection limit (s/n = 3) of 2.8 × 10⁻⁷ M for UA determination, and a linear response in the range of: 5.0 × 10⁻⁶ to 1.0 × 10⁻⁴ M with the detection limit of 4.2 × 10⁻⁶ M for AA determination were obtained. This electrode was used for UA and AA determinations in human urine samples satisfactorily.     

Electrochemical synthesis of belt-like polyaniline network on p-phenylenediamine functionalized glassy carbon electrode and its use for the direct electrochemistry of horse heart cytochrome c

Three-dimension (3D) belt-like polyaniline (PAN) network has been prepared via electrochemical polymerization of aniline on p-phenylenediamine (PDA) functionalized glassy carbon electrode (GCE) using a three-step electrochemical deposition procedure. PDA was covalently binded on GCE via the formation of carbon-nitrogen bond between amine cation radical and the aromatic moiety of GCE surface using electrochemical oxidation procedure. X-ray photo-electron spectroscopy (XPS) and cyclic voltammetry have been performed to characterize the attachment of PDA on GCE. The images of scanning electron microscope (SEM) show that the 3D belt-like PAN network is uniform. The width and thickness of the PAN belt varies in the range of 1.5-5.5 μm and 0.1-0.8 μm, respectively. The distance between the belt-contacts ranges from 2.5 to 15 μm. The 3D belt-like PAN network modified GCE (PAN-PDA/GCE) exhibits an improved electro-activity of PAN at an extended pH up to 7.0. The PAN-PDA/GCE not only immobilizes but also leads to a direct electrochemical behavior of cytochrome c (Cyt c). The immobilized Cyt c maintains its activity, showing a surface-controlled electrode process with the electron-transfer rate constant (k_s) of 14.8 s⁻¹ and electron-transfer coefficient (α) of 0.48, and could be used for the electrocatalytic reduction of hydrogen peroxide (H₂O₂).     

Electrochemical determination of bromide at a multiwall carbon nanotubes-chitosan modified electrode

A multiwall carbon nanotubes (MWNTs)-chitosan modified glassy carbon electrode (GCE) exhibits attractive ability for highly sensitive cathodic stripping voltammetric measurements of bromide (Br⁻). In pH 1.8 H₂SO₄ solution, a substantial increase in the stripping peak current of Br⁻ (compared to bare GCE and chitosan modified GCE) is observed using MWNTs-chitosan modified electrode. Operational parameters were optimized and the electrochemical behaviors of Br⁻ were studied by different electrochemical methods. The kinetics parameters were measured, the number of electron transfer (n) was 1 and the transfer coefficient (α) is 0.17. A wide linear calibration range (3.6 × 10⁻⁷-1.4 × 10⁻⁵ g mL⁻¹) was achieved, with a detection limit of 9.6 × 10⁻⁸ g mL⁻¹. The mechanism of electrode reaction was fully discussed.     

A hydroxylamine electrochemical sensor based on electrodeposition of porous ZnO nanofilms onto carbon nanotubes films modified electrode

A novel route (electrodeposition) for the fabrication of porous ZnO nanofilms attached multi-walled carbon nanotubes (MWCNTs) modified glassy carbon electrodes (GCEs) was proposed. The morphological characterization of ZnO/MWCNT films was examined by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). The performances of the ZnO/MWCNTs/GCE were characterized with cyclic voltammetry (CV), Nyquist plot (EIS) and typical amperometric response (i-t). The potential utility of electrodes constructed was demonstrated by applying them to the analytical determination of hydroxylamine concentration. An optimized limit of detection of 0.12 μM was obtained at a signal-to-noise ratio of 3 and with a fast response time (within 3 s). Additionally, the ZnO/MWCNTs/GCE exhibited a wide linear range from 0.4 to 1.9 × 10⁴ μM and higher sensitivity. The ease of fabrication, high stability, and low cost of the modified electrode are the promising features of the proposed sensor.     

Glassy carbon electrode modified with Nafion-Au colloids for clenbuterol electroanalysis

Stable Nafion-Au colloids were immobilized on a glassy carbon electrode (GCE) for detection of β-agonist clenbuterol by electroanalysis. The Au colloids were prepared by a one-step electrodeposition onto GCE, with obvious electrocatalytic activity present. The negatively charged Nafion film was an efficient barrier to negatively charged interfering compounds, resulting in accumulation of positively charged clenbuterol at the Nafion film. The electrochemical characters of the electrode during various modified steps in a redox probe system of K₄[Fe(CN)₆]/K₃[Fe(CN)₆] were confirmed by cyclic voltammetry (CV) and AC-impedance. In Britton-Robinson (B-R) buffer solution (pH = 2.0) and the potential range of −0.2 to 1.2 V, the Nafion-Au colloid modified electrode, compared to a bare GCE, exhibits obvious electrocatalytic activity towards the redox of clenbuterol by greatly enhancing the peak current with a linear calibration curve from 8.0 × 10⁻⁷ to 1.0 × 10⁻⁵ mol/L and a detection limit of (1.0 × 10⁻⁷ mol/L) (R = 0.996). The modified electrode shows high sensitivity, selectivity and reproducibility. The recovery for detecting clenbuterol (∼10⁻⁶ mol/L) in human serum is up to 98.19%.     

Electrooxidation of alcohols at a nickel oxide/multi-walled carbon nanotube-modified glassy carbon electrode

Electrocatalytic oxidation of methanol and some other primary alcohols on a glassy carbon electrode modified with multi-walled carbon nanotubes and nano-sized nickel oxide (GCE/MWNT/NiO) was investigated by cyclic voltammetry and chronoamperometry in alkaline medium. The results were compared with those obtained on a nickel oxide-modified glassy carbon electrode (GCE/NiO). Both the electrodes were conditioned by potential cycling in the range of 0.1–0.6 V versus Ag/AgCl in a 0.10 M NaOH solution. The effects of various parameters such as scan rate, alcohol concentration, thickness of NiO film, and real surface area of the modified electrodes were also investigated and compared. It was found that the GCE/MWNT/NiO-modified electrode possesses an improved electrochemical behavior over the GC/NiO-modified electrode for methanol oxidation.     

Poly-phenothiazine derivative-modified glassy carbon electrode for NADH electrocatalytic oxidation

Electropolymerization of a new phenothiazine derivative (bis-phenothiazin-3-yl methane; BPhM) on glassy carbon (GC) electrode generates a conducting film of poly-BPhM, in stable contact with the electrode surface. The heterogeneous electron-transfer process corresponding to the modified electrode is characterized by a high rate constant (50.4 s⁻¹, pH 7). The GC/poly-BPhM electrode shows excellent electrocatalytic activity toward NADH oxidation. The rate constant for catalytic NADH oxidation, estimated from rotating disk electrode (RDE) measurements and extrapolated to zero concentration of NADH, was found to be 9.4 × 10⁴ M⁻¹ s⁻¹ (pH 7). The amperometric detection of NADH, at +200 mV vs. SCE, is described by the following electroanalytical parameters: a sensitivity of 1.82 mA M⁻¹, a detection limit of 2 μM and a linear domain up to 0.1 mM NADH.     

Highly sensitive and simultaneous determination of hydroquinone and catechol at poly(thionine) modified glassy carbon electrode

A simple and highly sensitive electrochemical method for the simultaneous and quantitative detection of hydroquinone (HQ) and catechol (CT) was developed, based on a poly(thionine)-modified glassy carbon electrode (GCE). The modified electrode showed excellent electrocatalytic activity and reversibility towards the oxidation of both HQ and CT in 0.1 M phosphate buffer solution (PBS, pH 7.0). The peak-to-peak separations (ΔE_p) between oxidation and reduction waves in CV were decreased significantly from 262 and 204 mV at the bare GCE, to 63 and 56 mV, respectively for HQ and CT at the poly(thionine) modified GCE. Furthermore, the redox responses from the mixture of HQ and CT were easily resolved in both CV and DPV due to a difference in the catalytic activity of the modified GCE to each component. The peak potential separation of ca. 0.1 V was large enough for the simultaneous determination of HQ and CT electrochemically. The oxidation peak currents of HQ and CT were linear over the range from 1 to 120 μM in the presence of 100 and 200 μM of HQ and CT, respectively. The modified electrode showed very high sensitivity of 1.8 and 1.2 μA μM⁻¹ cm⁻² for HQ and CT, respectively. The detection limits (S/N = 3) for HQ and CT were 30 and 25 nM, respectively. The developed sensor was successfully examined for real sample analysis with tap water and revealed stable and reliable recovery data.     

Electrochemical study on cobalt film modified glassy carbon electrode and its application

A novel electroactive material for ascorbic acid (AA) determination was successfully prepared by plating/potential cycling method. The cobalt film was first deposited on the surface of glassy carbon electrode (GCE) in CoSO₄ solution by potential cycling, and then a cobalt film on the surface of GCE was activated by potential cycling in 0.1 mol L⁻¹ NaOH. The electrochemical performance of the resulted film (Co/GCE) and factors affecting its electrochemical activity were investigated by cyclic voltammetry and amperometry. This film electrode exhibited good electrocatalytic activity to the oxidation of AA. This biosensor had a fast response of AA less than 3 s and excellent linear relationships were obtained in the concentration range of 3 × 10⁻⁷ to 1 × 10⁻⁴ mol L⁻¹ with a detection limit of 2 × 10⁻⁷ mol L⁻¹ (S/N = 3) under the optimum conditions. Moreover, the selectivity, stability and reproducibility of this biosensor were evaluated with satisfactory results.     

Voltammetric studies of a novel bicopper complex modified glassy carbon electrode for the simultaneous determination of dopamine and ascorbic acid

A novel modified glassy carbon electrode (GCE) with a binuclear copper complex was fabricated using a cyclic voltammetric method in phosphate buffer solution. This modified electrode shows very efficient electrocatalytic activity for anodic oxidation of both dopamine (DA) and ascorbic acid (AA) via substantial decrease in anodic overpotentials for both compounds. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using this modified electrode show two well-resolved anodic waves for the oxidation of DA and AA in mixed solution, which makes it possible for simultaneous determination of both compounds. Linear analytical curves were obtained in the ranges 2.0–120.0 μM and 5.0–160.0 μM for DA and AA concentrations by using DPV methods, respectively. The detection limits were 1.4 × 10⁻⁶ M of DA and 2.8 × 10⁻⁶ M of AA. This electrode was used for AA and DA determinations in medicine and foodstuff samples with satisfactory results.     

Simultaneous voltammetric determination of dopamine and adenosine using a single walled carbon nanotube – Modified glassy carbon electrode

The simultaneous voltammetric determination of adenosine (ADS) and dopamine (DA) using a single wall carbon nanotube (SWCNT) modified glassy carbon electrode (GCE) is reported. This has physiological importance in controlling Parkinson’s disease. In phosphate buffer medium of pH 7.2, the concentration vs. peak current plots were linear in the range 1–100 μM for ADS and DA. A comparison of the voltammetric response of DA at acid-treated (purified and super-purified) and untreated SWCNT modified GCE indicates that the oxidation peak current of DA decreases considerably at the treated SWCNT modified GCE. This indicates that metallic impurities in nanotubes play a large role in enhancing the electrochemical current. The detection limit (3σ) and sensitivity observed for ADS and DA were 34.7 μM, 7 μM and 9.5 nA μM⁻¹, 77.9 nA μM⁻¹, respectively.     

Enhancement of electrochemiluminesence of lucigenin by ascorbic acid at single-wall carbon nanotube film-modified glassy carbon electrode

The electrochemiluminescent behavior of lucigenin on a single-wall carbon nanotube/DMF film-modified glassy carbon electrode was studied in this paper. Comparing with the bare glassy carbon electrode, the electrochemiluminescent of lucigenin at modified electrode is more stable and without tedious procedure for clean-up the surface of modified electrode. It has been found that ascorbic acid could enhance the electrochemiluminescent intensity of lucigenin greatly at this modified electrode. Based on which, a new sensitive and simple electrochemiluminescent method for determination of ascorbic acid could be developed. The condition for the determination of ascorbic acid was optimized. Under the optimized condition, the enhanced electrochemiluminescent intensity versus ascorbic acid concentration was linear in the range of 1.0 × 10⁻⁸ to 4.0 × 10⁻⁶ mol/L with a detection limit of 2.0 × 10⁻¹⁰ mol/L, and the relative standard derivation for 1.0 × 10⁻⁷ mol/L ascorbic acid was 3.8% (n = 8). The possible mechanism was also discussed.