Electrochemical determination diethylstilbestrol by a single-walled carbon nanotube/platinum nanoparticle composite film electrode

Platinum nanoparticles (Pt_nano) were prepared and used in combination with single-wall carbon nanotube (SWNT) for fabricating electrochemical sensors with remarkably improved sensitivity toward diethylstilbestrol (DES). The glassy carbon (GC) electrode modified with SWNT/Pt_nano composite film exhibited excellent electrochemical behaviors toward the redox of DES. Compared with the bare GC electrode and SWNTs film modified GC electrode, the redox peak currents at the SWNTs/Pt_nano composite film modified GC electrode was enhanced greatly. The experimental parameters, which influence the peak current of DES, were optimized. Under optimal conditions, a linear response of DES was obtained in the range from 1.0 × 10⁻⁷ to 2.0 × 10⁻⁵ mol L⁻¹ (R = 0.997) and with a limit of detect (LOD) of 1.5 × 10⁻⁸ mol L⁻¹. The proposed procedure was successfully applied to determine the active ingredient in the DES tablet with satisfactory results.     

Fabrication of carbon paste electrode containing [PFeW11O39] polyoxoanion supported on modified amorphous silica gel and its electrocatalytic activity for H2O2 reduction

[PFeW₁₁O₃₉]⁴⁻ (PFeW₁₁) supported on the surface of 3-aminopropyl(triethoxy)silane modified silica gel was synthesized and used as a bulk modifier to fabricate a renewable three-dimensional chemically modified electrode. The electrochemical behavior of the modified electrode was investigated. Cyclic voltammetry studies showed that the PFeW₁₁ on the electrode surface sustained the same electrochemical properties as that of the PFeW₁₁ in solution. The preparation of chemically modified electrode is simple and quiet reproducible using inexpensive material. The modified electrode had high electrocatalytic activity toward H₂O₂ reduction and it was successfully applied as an electrochemical detector to monitor H₂O₂ in flow injection analysis (FIA). The electrocatalytic peak current was found to be linear with the H₂O₂ concentration in the range 10-200 μmol L⁻¹ with a correlation coefficient of 0.998 and a detection limit (3σ) of 7.4 μmol L⁻¹ H₂O₂. The electrode has the remarkable advantage of surface renewal owing to bulk modification, as well as simple preparation, good mechanical and chemical stability and reproducibility.     

Green synthesized Au–Ag bimetallic nanoparticles modified electrodes for the amperometric detection of hydrogen peroxide

Simple and eco-friendly electro deposition method was employed for the fabrication of Au–Ag bimetallic nanoparticles modified glassy carbon electrode. Nano Au–Ag film modified glassy carbon electrode surface morphology has been examined using atomic force microscopy. Electrodeposited Au–Ag bimetallic nanoparticles were found in the average size range of 15–50 nm. The electrochemical investigations of nano Au–Ag/1-butyl-3-methylimidazolium tetrafluoroborate-nafion film have been carried out using cyclic voltammetry and electrochemical impedance spectroscopy. The nano Au–Ag/1-butyl-3-methylimidazolium tetrafluoroborate-nafion film modified glassy carbon electrode holds the good electrochemical behavior and stability in pH 7.0 phosphate buffer solutions. The nano Au–Ag/1-butyl-3-methylimidazolium tetrafluoroborate-nafion modified glassy carbon electrode was successfully employed for the detection of H₂O₂ in the linear range of 1–250 μM in lab samples, and 1 × 10⁻³–2 × 10⁻² M in real samples, respectively.     

Development of nonenzymatic hydrogen peroxide sensor based on catalytic properties of copper nanoparticles/Rutin/MWCNTs/IL/Chit

A new electrochemical sensor based on copper nanoparticles for detection of hydrogen peroxide has been developed. Copper nanoparticles/Rutin/Multiwall Carbon Nanotubes/Ionic liquid/Chitosan modified glassy carbon electrode (CuNPs/Rutin/MWCNTs/IL/Chit/GCE) prepared by consecutive coating of MWCNTs/IL/Chit nanocomposite and rutin on the GCE, followed by the electrodeposition of copper. Surface physical characteristics of modified electrode were studied by scanning electron microscopy (SEM). The electrochemical performance of the sensor for detection of H₂O₂ was investigated by cyclic voltammetry and chronoamperometry techniques. The modified electrode exhibits an enhanced electrocatalytic property, low working potential, high sensitivity, excellent selectivity, good stability, and fast amperometric sensing towards reduction of hydrogen peroxide. The response to H₂O₂ is linear in the range between 0.35 μM to 2500 μM, and the detection limit is 0.11 μM.     

Synthesis of 3D cubic of Eu3+/Cu2O with clover-like faces nanostructures and their application as an electrochemical sensor for determination of antiretroviral drug nevirapine

In this experiment, an original three-dimensional (3D) cubic of europium (Eu) ³⁺/cuprous oxide (Cu₂O) with clover-like face-centered nanostructures (Eu³⁺/Cu₂O CLFNs) was successfully synthesized to determine nevirapine (C₁₅H₁₄N₄O), using electrochemical methods. The surface morphology of the Ns was correspondingly identified through different techniques, including energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), and scanning electron microscopy (SEM). In this sense, the synergistic influence of the Eu³⁺/Cu₂O CLFNs enhanced the electrocatalytic capability of the electrode via a modified glassy carbon (GC) and raised the active site. Employing various approaches, the modified electrode was then analyzed through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Moreover, kinetic parameters and electrochemical ones were estimated by means of voltammetric methods. For the determination of nevirapine applying the Eu³⁺/Cu₂O CLFNs/GC electrode (GCE), differential pulse voltammetry (DPV) was further utilized. Under optimized conditions, the range of linear responses was between 0.01 and 750.0 μM with the limit of detection (LOD) of 3.6 nM, whereas the recommended sensor sensitivity was equal to 0.1244 μA/μM. The response time of approximately 3.5 min and the reproducibility (inter- and intra-electrode reproducibility of 2.26% and 1.51%, respectively) were subsequently achieved. It was concluded that sensors have been thus far exploited to determine nevirapine in real samples with favorable outcomes, indicating that electrocatalysis modifier can be assumed as one of appropriate catalysts.     

Direct electrochemistry of chemically modified catalase immobilized on an oxidatively activated glassy carbon electrode

Catalase (Ct) was modified using Woodward’s reagent K (WRK) as a specific modifier of carboxyl residues. The modified Ct was immobilized on an oxidatively activated glassy carbon electrode surface to investigate its direct electrochemistry. Using cyclic voltammetry an irreversible reduction peak was obtained at approximately −0.362 V vs. Ag/AgCl in buffer solution, pH 7, and at a scan rate of 0.1 V s⁻¹. The electrochemical parameters, including charge-transfer coefficient (0.27), apparent heterogeneous electron transfer rate constant (13.51 ± 0.42 s⁻¹) and formal potential of the Ct film (−0.275 V) were determined. The prepared enzyme electrode exhibited a response to H₂O₂.     

A modified composite film electrode of polyoxometalate/carbon nanotubes and its electrocatalytic reduction

Keggin-type polyoxometalate (H₄SiMo₁₂O₄₀) and carbon nanotubes (CNTs) coated by poly(allylamine hydrochloride) (PAH) were alternately deposited on glassy carbon (GC) electrodes by an electrochemical growth method in acidic aqueous solution. The preparation of the film electrode was simple and convenient. Thus-prepared multilayer films and the electrochemical behavior of the composite film modified electrode were characterized by UV–vis spectroscopy and cyclic voltammetry. It was shown that the multilayer films are uniform and stable. The resulting multilayer film modified electrode behaves as an electrochemical sensor because of its low overpotential for the catalytic reduction of S₂O₈ ²⁻ and NO₂ ⁻ in acidic aqueous solution.     

Studies of electrochemical reduction of dioxygen with RRDE

The electrochemical behaviors of dioxygen (O₂) were studied by using rotate ring-disk electrode (RRDE) and other electrochemical methods at bare glassy carbon electrode (GCE) and single-walled carbon nanotubes (SWNTs)-dihexadecyl hydrogen phosphate (DHP) film modified GCE. The results showed that the electrochemical reduction of dioxygen was considered to proceed by a two-step two-electron reduction pathway at both bare GCE and SWNTs-DHP film modified GCE in 0.1 mol/L air-saturated sodium hydroxide (NaOH). Maybe because each reaction rate for two cases was different the cyclic voltammograms measurements exhibited different behaviors. The detection of ring current confirmed the presence of middle product hydrogen peroxide (H₂O₂). Furthermore, larger current and more positive reduction potential indicated that SWNTs showed a catalytic effect towards the electrochemical reduction of dioxygen.     

Copper phosphate compounds with visible-to-near-infrared-active photo-fenton-like photocatalytic properties

Copper pyro-, ortho-, and tetra-phosphate compounds (Cu₂P₂O₇, Cu₃P₂O₈, and Cu₄P₂O₉) with the general composition, Cu_xP₂O_5+x (x = 2, 3, 4), were prepared through the conventional solid-state synthesis route, and their optical properties and visible-infrared light-induced photocatalytic activities were investigated for the first time. The copper phosphate compounds showed comparable particle sizes, and their primary optical bandgaps were estimated to lie in the range of 2.8-3.6 eV. Their photocatalytic activity was evaluated by the kinetics of decomposition of rhodamine B (RhB) solution under visible-light irradiation (>420 nm). Among the three kinds of copper phosphate, Cu₄P₂O₉ exhibited the highest photocatalytic activity due to its smallest bandgap (of the three compounds considered) and appropriate band positions. More interestingly, the three copper phosphate compounds, particularly Cu₃P₂O₈, show strong near-infrared (NIR) light absorption, thereby enabling the NIR-induced photocatalytic decomposition of RhB in the presence of hydrogen peroxide. Finally, a possible photo-Fenton photocatalytic reaction mechanism was also proposed based on the band structure/position.     

The pH-dependence of oxygen reduction on multi-walled carbon nanotube modified glassy carbon electrodes

The pH-dependence of oxygen electroreduction has been investigated on multi-walled carbon nanotube (MWCNT) modified glassy carbon (GC) electrodes. Various surfactants were used in the electrode modification: dihexadecyl hydrogen phosphate, cetyltrimethylammonium bromide, sodium dodecyl sulfate and Triton X-100. Electrochemical experiments were carried out in 0.5 M H₂SO₄ solution, acetate buffer (pH 5), phosphate buffers (pH 6, 7 and 8), borate buffer (pH 10), 0.01 M KOH, 0.1 M KOH and in 1 M KOH solution, using the rotating disk electrode (RDE) method. The oxygen reduction behaviour of MWCNT-modified GC electrodes at different pHs was compared. The RDE results revealed that the half-wave potential (E_1/2) of oxygen reduction was higher in solutions of high pH. At lower pHs (pH < 10) the value of E_1/2 did not essentially depend on the solution pH. A comparison with previous studies on bare GC showed that the pH-dependence of the half-wave potential of oxygen reduction on MWCNT-modified GC electrodes follows a similar trend to that observed for bare GC.     

Electrochemical fabrication of Rh–Pd particles and electrocatalytic applications

Electrochemical deposition method was employed for the fabrication of rhodium–palladium (Rh–Pd) particles on the glassy carbon electrode (GCE) and indium tin oxide (ITO) electrode surface. Surface morphological analysis of Rh–Pd film has been carried out using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Here, the electrodeposited Rh–Pd particles were found in the average size range of 30–200 nm. The electrochemical activities of the Rh–Pd film have been investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopic (EIS) analysis. The Rh–Pd particles-modified GCE successfully detects the hydrogen peroxide (H₂O₂) (in pH 7.0 phosphate buffer solution (PBS)) in the linear range in the lab (10–460 μM) and real samples (10–340 μM). The Rh–Pd particles-modified GCE possesses the good sensitivity and selectivity for the detection of H₂O₂ in lab and real samples.     

Direct electrochemistry of horseradish peroxidase on graphene-modified electrode for electrocatalytic reduction towards H2O2

Graphene was synthesized by a chemical method to reduce graphite oxide and well characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and Fourier transform infrared (FTIR) spectra. Horseradish peroxidase (HRP) immobilized on a graphene film glassy carbon electrode was found to undergo direct electron transfer and exhibited a fast electron transfer rate constant of 4.63 s⁻¹. The HRP-immobilized electrode was investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The CV results showed that the modified electrode gave rise to well-defined peaks in phosphate buffer, corresponding to the electrochemical redox reaction between HRP–Fe(III) and HRP–Fe(II). The obtained electrode also displayed an electrocatalytic reduction behavior towards H₂O₂. The new H₂O₂ sensor shows a linear range of 0.33–14.0 μM (R² = 0.9987) with a calculated detection limit of 0.11 μM (S/N = 3). Furthermore, the biosensor exhibits both good operational storage and storage stability.     

Polymethylthiophene/Nafion-modified glassy carbon electrode for selective detection of dopamine in the presence of ascorbic acid

The possible use of an electrode modified with electroactive conductive poly(3-methylthiophene) (PMeT)/Nafion as a chemical sensor was investigated for the voltammetric analysis of Dopamine (DA). The electrochemical behavior of dopamine was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. By using a PMeT-modified glassy carbon (GC/PMeT) electrode, DA and Ascorbic Acid (AA) signals could be separated but the AA at high concentrations still caused significant interference by overlapping the DA peak. In comparison to the GC/PMeT electrode, the glassy carbon (GC/Nafion/PMeT) electrode modified with hybrid film Nafion/PMeT was found to permit a superior separation by shifting the oxidation of AA peak toward the less positive potential. The DPV curves for a mixture of DA and AA at an GC/Nafion/PMeT electrode in a 0.1 M H₂SO₄ solution showed peaks of DA and AA, at 0.45 and 0.21 V, respectively, indicating that the difference in the oxidation potential was 240 mV. In the 0.1 M H₂SO₄ solution, the oxidation peak current on the differential pulse voltammograms for the GC/PMeT electrode increased linearly with the concentration of DA in the range 1 × 10⁻⁶ to 1 × 10⁻³ M, and the oxidation peak current on the differential pulse voltammograms for the GC/Nafion/PMeT electrode in the range 5 × 10⁻⁷ to 2 × 10⁻⁴ M. The DA detection sensitivity of the GC/Nafion/PMeT electrode (26.7 μA μM⁻¹ cm⁻²) was 22 times higher than that of the GC/PMeT electrode (1.21 μA μM⁻¹ cm⁻²).     

A novel non-enzymatic hydrogen peroxide sensor based on single walled carbon nanotubes–manganese complex modified glassy carbon electrode

A simple procedure was developed to prepare a glassy carbon (GC) electrode modified with single wall carbon nanotubes (SWCNTs) and phenazine derivative of Mn-complex. With immersing the GC/CNTs modified electrode into Mn-complex solution for a short period of time 20–100 s, a stable thin layer of the complex was immobilized onto electrode surface. Modified electrode showed a well defined redox couples at wide pH range (1–12). The surface coverages and heterogeneous electron transfer rate constants (k_s) of immobilized Mn-complex were approximately 1.58 × 10⁻¹⁰ mole cm⁻² and 48.84 s⁻¹. The modified electrode showed excellent electrocatalytic activity toward H₂O₂ reduction. Detection limit, sensitivity, linear concentration range and k_cat for H₂O₂ were, 0.2 μM and 692 nA μM⁻¹ cm⁻², 1 μM to 1.5 mM and 7.96(±0.2) × 10³ M⁻¹ s⁻¹, respectively. Compared to other modified electrodes, this electrode has many advantageous such as remarkable catalytic activity, good reproducibility, simple preparation procedure and long term stability.     

Electrochemical oxidation of hydrogen peroxide at a bromine adatom-modified gold electrode in alkaline media

Bromine (Br)-adatom (Br_(ads)) was in situ fabricated onto polycrystalline gold (Au (poly)) electrode in Br⁻-containing alkaline media. The surface coverage of Br_(ads) (Γ_Br) varied only in the submonolayer coverage within the investigated potential window under potentiodynamic condition because of the coadsorption of hydroxyl ion (OH⁻) in alkaline media. The in situ fabricated Br_(ads)-submonolayer-coated Au (poly) electrode was successfully used for the electrochemical oxidation of hydrogen peroxide (H₂O₂). About five times higher oxidation current was achieved at the modified electrode as compared with the bare electrode. The enhancement of the electrode activity towards the electrochemical oxidation of H₂O₂ was explained based on the enhanced electrostatic attraction between the anionic HO₂⁻ molecules and Br_(ads)-adlayer-induced positively polarized Au (poly) electrode surface.     

Hydrogen peroxide sensor based on riboflavin immobilized at the nickel oxide nanoparticle-modified glassy carbon electrode

A simple and sensitive electrochemical sensor based on nickel oxide nanoparticles/riboflavin-modified glassy carbon (NiONPs/RF/GC) electrode was constructed and utilized to determine H₂O₂. By immersing the NiONPs/GC-modified electrode into riboflavin (RF) solution for a short period of time (5–300 s), a thin film of the proposed molecule was immobilized onto the electrode surface. The modified electrode showed stable and a well-defined redox couples at a wide pH range (2–10), with surface-confined characteristics. Experimental results revealed that RF was adsorbed on the surface of NiONPs, and in comparison with usual methods for the immobilization of RF, such as electropolymerization, the electrochemical reversibility and stability of this modified electrode has been improved. The surface coverage and heterogeneous electron transfer rate constants (k _s) of RF immobilized on a NiO _x –GC electrode were approximately 4.83 × 10^?11 mol cm^?2, 54 s^?1, respectively. The sensor exhibits a powerful electrocatalytic activity for the reduction of H₂O₂. The detection limit, sensitivity and catalytic rate constant (k _cat) of the modified electrode toward H₂O₂ were 85 nM, 24 nA μM^?1 and 7.3 (±0.2) × 10³ M^?1 s^?1, respectively, at linear concentration rang up to 3.0 mM. The reproducibility of the sensor was investigated in 10 μM H₂O₂ by amperometry, the value obtained being 2.5 % (n = 10). Furthermore, the fabricated H₂O₂ chemical sensor exhibited an excellent stability, remarkable catalytic activity and reproducibility.     

Electrogeneration of hydrogen peroxide in gas diffusion electrodes modified with tert-butyl-anthraquinone on carbon black support

Hydrogen peroxide (H₂O₂) is a versatile oxidizing agent that is synthesized commercially by the reduction of oxygen in organic medium. Electrochemical technology employing a modified gas diffusion electrode (MGDE) offers a viable alternative for the industrial-scale synthesis of the oxidant. Addition of 1% (w/w) of tert-butyl-anthraquinone (TBAQ) to carbon black deposited in the form of a microporous layer onto the disk of a rotating ring-disk electrode produced an increase in the ring current, which is directly related to H₂O₂ formation, and presented an efficiency of H₂O₂ generation of 89.6% compared with 76.6% for carbon black alone. No significant changes were detected in the number of electrons transferred in the presence of the catalyst suggesting an electrochemical/chemical mechanism for H₂O₂ formation. Analogous improvements in the generation of H₂O₂ were obtained with MGDEs comprising TBAQ on carbon black. The highest concentrations of H₂O₂ (301 mg L⁻¹) were produced at the fastest rate (5.9 mg L⁻¹ min⁻¹) with the lowest energy consumption (6.0 kWh kg⁻¹) when a potential of −1.0 V vs SCE was applied to a MGDE containing 1.0% of TBAQ on carbon black. It is concluded that the application of MGDEs comprising TBAQ on carbon black support offers considerable advantages in the electrogeneration of H₂O₂.     

Electrochemical behavior of glassy carbon electrodes modified by multi-walled carbon nanotube/surfactant films in a buffer solution and an ionic liquid

The electrochemical behavior of glassy carbon (GC) electrodes coated with multi-walled carbon nanotube (MWCNT)/surfactant films was studied in an ionic liquid and a phosphate buffer solution (pH = 6.86), using cyclic voltammetry. The dispersion of MWCNTs in different media was investigated by scanning and transmission electron microscopy. Cast films of MWCNT/zwitterionic dodecyldimethylamine oxide on a GC electrode show a typical redox couple in phosphate buffer solution, which is better than that of MWCNT/anionic sodium dodecyl sulfate and cationic alkyltrimethylammonium bromide. However in the ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), the GC electrode modified by MWCNT/cationic surfactant films shows a well-defined irreversible reduction of MWCNTs. The cyclic voltammograms clearly show that the surfactant hydrophilic group plays an important role in the electrochemical behavior of the MWCNTs. The electrolytes also have an important effect. In an ionic liquid, the strong binding of the ionic liquid cations with the MWCNTs may change the structure of the modified films and lead to changes of electrochemical behavior.     

Electrochemical behavior and its electrocatalytic properties of chemically modified electrode with Keggin-type [SiNi(H2O)W11O39]

A monolayer of Keggin-type heteropolyanion [SiNi(H₂O)W₁₁O₃₉]⁶⁻ was fabricated by electrodepositing [SiNi(H₂O)W₁₁O₃₉]⁶⁻ on cysteamine modified gold electrode. The monolayer of [SiNi(H₂O)W₁₁O₃₉]⁶⁻ modified gold electrode was characterized by atomic force microscopy (AFM) and electrochemical method. AFM results showed the [SiNi(H₂O)W₁₁O₃₉]⁶⁻ uniformly deposited on the electrode surface and formed a porous monolayer. Cyclic voltammetry exhibited one oxidation peak and two reduction peaks in 1.0 M H₂SO₄ in the potential range of −0.2 to 0.7 V. The constructed electrode could exist in a large pH (0-7.6) range and showed good catalytic activity towards the reduction of bromate anion (BrO₃⁻) and nitrite (NO₂⁻), and oxidation of ascorbic acid (AA) in acidic solution. The well catalytic active of the electrode was ascribed to the porous structure of the [SiNi(H₂O)W₁₁O₃₉]6⁻ monolayer.     

Iron-enriched natural zeolite modified carbon paste electrode for H2O2 detection

This work demonstrates that iron-enriched natural zeolitic volcanic tuff (Paglisa deposit, Cluj county, Transilvania, Romania) resulting from a previous use as adsorbent in wastewater treatment can be recycled into effective electrode modifier applied to the electrocatalytic detection of hydrogen peroxide. After physico-chemical characterization of tuff samples using various techniques such as chemical analysis, X-ray diffraction, scanning electron microscopy, infrared spectroscopy, BET analysis and X-ray photoelectron spectroscopy, the electrochemical response of the iron-enriched zeolites was studied on the basis of solid carbon paste electrodes modified with these samples. The results indicate that iron centers in the zeolite are electroactive and that they act as electrocatalysts in the voltammetric and amperometric detection of H₂O₂. Best performance was achieved in phosphate buffer at pH 7, showing a sensitivity of 0.57 mA M⁻¹ cm⁻², a detection limit down to 60 μM, and a linear domain up to 100 mM H₂O₂.