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 behavior of catechol, resorcinol and hydroquinone at graphene–chitosan composite film modified glassy carbon electrode and their simultaneous determination in water samples

Graphene–chitosan composite film modified glassy carbon electrode was prepared and characterized. The fabricated electrode showed excellent electrochemical catalytic activities towards the oxidation of catechol (CT), resorcinol (RS) and hydroquinone (HQ). The oxidation overpotentials of CT, RS and HQ decreased significantly and the corresponding oxidation currents increased remarkably compared with those obtained at the bare GCE and chitosan modified GCE. Some kinetic parameters, such as the electron transfer number (n), proton transfer number (m), charge transfer coefficient (α) and the apparent heterogeneous electron transfer rate constant (k_s), were calculated. Differential pulse voltammetry was used for the simultaneous determination of CT, RS and HQ in their ternary mixture. The peak-to-peak potential separations between CT and RS, RS and HQ, and HQ and CT were 0.388, 0.484 and 0.096 V, respectively. The calibration curves for CT, RS and HQ were obtained in the range of 1 × 10⁻⁶ to 4 × 10⁻⁴, 1 × 10⁻⁶ to 5.5 × 10⁻⁴ and 1 × 10⁻⁶ to 3 × 10⁻⁴ mol L⁻¹, respectively. The detection limits were 7.5 × 10⁻⁷ mol L⁻¹ (S/N = 3).     

Electrochemical behavior and simultaneous determination of dihydroxybenzene isomers at a functionalized SBA-15 mesoporous silica modified carbon paste electrode

The simultaneous voltammetric determination of dihydroxybenzene isomers was investigated using cyclic and differential pulse voltammetries at the amino-functionalized SBA-15 mesoporous silica-modified carbon paste electrode (NH₂-SBA15/CPE) in phosphate buffer solution (pH 6.0). The NH₂-SBA15/CPE showed a larger peak current and higher selectivity for the dihydroxybenzene isomers in comparison with the bare carbon paste electrode (CPE) and SBA-15 mesoporous silica-modified carbon paste electrode (SBA15/CPE). The oxidation peak potential difference between hydroquinone (HQ) and catechol (CC) was 115 mV and was 396 mV between catechol and resorcinol (RC). This indicated that catechol, resorcinol and hydroquinone could be identified entirely at the NH₂-SBA15/CPE. Under the optimized conditions, the amperometric currents were linear over ranges from the following: 0.8–160 μmol L⁻¹ for hydroquinone, 1.0–140 μmol L⁻¹ for catechol and 2.0–160 μmol L⁻¹ for resorcinol. The detection limits were 0.3, 0.5 and 0.8 μmol L⁻¹, respectively. The proposed electrode can be applied to the simultaneous determination of dihydroxybenzene isomers in mixtures without previous chemical or physical separations.     

Comparative investigation on electrochemical behavior of hydroquinone at carbon ionic liquid electrode, ionic liquid modified carbon paste electrode and carbon paste electrode

Ionic liquid, 1-heptyl-3-methylimidazolium hexafluorophosphate (HMIMPF₆), has been used to fabricate two new electrodes, carbon ionic liquid electrode (CILE) and ionic liquid modified carbon paste electrode (IL/CPE), using graphite powder mixed with HMIMPF₆ or the mixture of HMIMPF₆/paraffin liquid as the binder, respectively. The electrochemical behaviors of hydroquinone at the CILE, the IL/CPE and the CPE were investigated in phosphate buffer solution. At all these electrodes, hydroquinone showed a pair of redox peaks. The order of the current response and the standard rate constant of hydroquinone at these electrodes were as follows: CILE > IL/CPE > CPE, while the peak-to-peak potential separation was in an opposite sequence: CILE < IL/CPE < CPE. The results show the superiority of CILE to IL/CPE and CPE, and IL/CPE to CPE in terms of promoting electron transfer, improving reversibility and enhancing sensitivity. The CILE was chosen as working electrode to determine hydroquinone by differential pulse voltammetry, which can be used for sensitive, simple and rapid determination of hydroquinone in medicated skin cosmetic cream.     

Glassy carbon electrode modified with a bilayer of multi-walled carbon nanotube and polypyrrole doped with new coccine: Application to the sensitive electrochemical determination of Sumatriptan

A promising electrochemical sensor was developed based on a layer by layer process by electro-polymerization of pyrrole in the presence of new coccine (NC) as dopant anion on the surface of the multi-walled carbon nanotubes (MWCNTs) pre-coated glassy carbon electrode (GCE). The modified electrode was used as a new and sensitive electrochemical sensor for voltammetric determination of sumatriptan (SUM). The electrochemical behavior of SUM was investigated on the surface of the modified electrode using linear sweep voltammetry (LSV). The results showed a remarkable increase (∼12 times) in the anodic peak current of SUM in comparison to the bare GCE. The effect of experimental variables such as, drop size of the casted MWCNTs suspension, pH of the supporting electrolyte, accumulation conditions and the number of cycles in the electro-polymerization process on the electrode response was investigated. Under the optimum conditions, the modified electrode showed a wide linear dynamic range of 0.02–10.0 μmol L⁻¹ with a detection limit of 6 nmol L⁻¹ for the voltammetric determination of SUM. The prepared electrode showed high sensitivity, stability and good reproducibility in response to SUM. This sensor was successfully applied for the accurate determination of trace amounts of SUM in pharmaceutical and clinical preparations.     

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.     

不同浓度四氢呋喃水溶液制备高合金化Pt-Ru/CMK-3催化剂

提出了在四氢呋喃(THF)和H2O混合溶液中用一般的化学还原法在室温下制备高合金化Pt-Ru/CMK-3催化剂的新方法。与在纯水中制得的Pt-Ru催化剂相比,其Pt-Ru粒子的合金化程度高、平均粒径较小且相对结晶度低,因此该催化剂对甲醇氧化的电催化活性远高于在纯水中制得的Pt-Ru催化剂。高合金化程度的原因是H2PtCl6和RuCl3在THF和H2O混合溶液中的起始还原电位相近。进一步发现在THF和H2O混合溶液中,THF和H2O体积比的改变并不影响制得的Pt-Ru/CMK-3催化剂中Pt-Ru粒子的合金化程度,但对Pt-Ru粒子的粒径以及相对结晶度有较大影响。     

Simultaneous determination of hydroquinone and catechol using poly(p‐aminobenzoic acid) modified glassy carbon electrode

The electrochemical redox reaction of hydroquinone (HQ) and catechol (CC)was investigated with poly‐(p‐aminobenzoic acid) modified glassy‐carbon electrode (poly‐p‐ABA/GCE) via cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The poly‐p‐ABA/GCE has shown an excellent electrocatalytic activity for HQ and CC in 0.1 mol L^?1 phosphate buffer solution (PBS). The oxidation and reduction separation (ΔE) has been decreased from 353 to 32 mV for HQ and from 228 to 33 mV vs. SCE for CC at the bare GCE and poly‐p‐ABA/GCE respectively. DPV curves show that the oxidation potential of HQ and CC has a separation about 105 mV at the poly‐p‐ABA/GCE. Moreover, the oxidation current of HQ and CC has been enhanced two and four times respectively at the modified electrode. Using DPV method, a highly selective and simultaneous determination of HQ and CC has been explored at the poly‐p‐ABA/GCE. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Modification of glassy carbon electrode with multi-walled carbon nanotubes and iron(III)-porphyrin film: Application to chlorate, bromate and iodate detection

In this study, multi-wall carbon nanotubes (MWCTs) is evaluated as a transducer, stabilizer and immobilization matrix for the construction of amperometric sensor based on iron-porphyrin. 5,10,15,20-Tetraphenyl-21H,23H-porphine iron(III) chloride (Fe(III)P) adsorbed on MWCNTs immobilized on the surface of glassy carbon electrode. Cyclic voltammograms of the Fe(III)P-incorporated-MWCNTs indicate a pair of well-defined and nearly reversible redox couple with surface confined characteristics at wide pH range (2-12). The surface coverage (Γ) and charge transfer rate constant (k_s) of Fe(III)P immobilized on MWCNTs were 7.68 × 10⁻⁹ mol cm⁻² and 1.8 s⁻¹, respectively, indicating high loading ability of MWCNTs for Fe(III)P and great facilitation of the electron transfer between Fe(III)P and carbon nanotubes immobilized on the electrode surface. Modified electrodes exhibit excellent electrocatalytic activity toward reduction of ClO₃⁻, IO₃⁻ and BrO₃⁻ in acidic solutions. The catalytic rate constants for catalytic reduction of bromate, chlorate and iodate were 6.8 × 10³, 7.4 × 10³ and 4.8 × 10² M⁻¹ s⁻¹, respectively. The hydrodynamic amperometry of rotating-modified electrode at constant potential versus reference electrode was used for detection of bromate, chlorate and iodate. The detection limit, linear calibration range and sensitivity for chlorate, bromate and iodate detections were 0.5 μM, 2 μM to 1 mM, 8.4 nA/μM, 0.6 μM, 2 μM to 0.15 mM, 11 nA/μM, and 2.5 μM, 10 μM to 4 mM and 1.5 nA/μM, respectively. Excellent electrochemical reversibility of the redox couple, good reproducibility, high stability, low detection limit, long life time, fast amperometric response time, wide linear concentration range, technical simplicity and possibility of rapid preparation are great advantages of this sensor. The obtained results show promising practical application of the Fe(III)P-MWCNTs-modified electrode as an amperometric sensor for chlorate, iodate and bromate detections.     

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.     

邻对苯二酚在石墨烯-聚噻吩复合膜修饰电极上的电化学行为及测定

研制了一种新型的石墨烯/聚噻吩复合膜修饰玻碳电极(GR/PTh/GCE),在0.100 mol/L的柠檬酸-磷酸氢二钠缓冲溶液中(pH 7.0),利用循环伏安法(CV)研究了该修饰电极对邻苯二酚(CC)和对苯二酚(HQ)的电催化作用,同时利用差分脉冲溶出伏安法(DPSV)测定邻苯二酚和对苯二酚的含量,线性范围为1.00×10-6～4.00×10-4mol/L,检出限为1.50×10-7 mol/L,并应用于模拟废水中邻苯二酚和对苯二酚的测定,回收率分别为98.0％～103.0％、96.0％～104.O％,效果良好.     

Electrochemistry and electrocatalysis of hemoglobin on multi-walled carbon nanotubes modified carbon ionic liquid electrode with hydrophilic EMIMBF4 as modifier

The direct electrochemistry of hemoglobin (Hb) on multi-walled carbon nanotubes (MWCNTs) modified carbon ionic liquid electrode (CILE) was achieved in this paper. By using a hydrophilic ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) as the modifier, a new CILE was fabricated and further modified with MWCNTs to get the MWCNTs/CILE. Hb molecules were immobilized on the surface of MWCNTs/CILE with polyvinyl alcohol (PVA) film by a step-by-step method and the modified electrode was denoted as PVA/Hb/MWCNTs/CILE. UV-vis and FT-IR spectra indicated that Hb remained its native structure in the composite film. Cyclic voltammogram of PVA/Hb/MWCNTs/CILE showed a pair of well-defined and quasi-reversible redox peaks with the formal potential (E^0′) of −0.370 V (vs. SCE) in 0.1 mol/L pH 7.0 phosphate buffer solution (PBS), which was the characteristic of the Hb heme Fe^III/Fe^II redox couples. The redox peak currents increased linearly with the scan rate, indicating the direct electron transfer was a surface-controlled process. The electrochemical parameters of Hb in the film were calculated with the results of the electron transfer coefficient (α) and the apparent heterogeneous electron transfer rate constant (k_s) as 0.49 and 1.054 s⁻¹, respectively. The immobilized Hb in the PVA/MWCNTs composite film modified CILE showed excellent electrocatalytic activity to the reduction of trichloroacetic acid (TCA) and hydrogen peroxide. So the proposed electrode showed the potential application in the third generation reagentless biosensor.     

Fabrication of Fc-SWNTs modified glassy carbon electrode for selective and sensitive determination of dopamine in the presence of AA and UA

The electrochemistry of dopamine (DA) was investigated by cyclic voltammetry (CV) and differential pulse voltammograms (DPV) at a glassy carbon electrode modified by the hybridization adducts of Fc-SWNTs. The electro-oxidation of DA could be catalyzed by Fc/Fc⁺ couple as a mediator and had a higher electrochemical response due to the unique carbon surface of carbon nanotubes. The anodic peaks of DA, ascorbic acid (AA) and uric acid (UA) in their mixture can be well separated by the prepared electrode. Under optimum conditions linear calibration graphs were obtained over the DA concentration range 5.0 × 10⁻⁶ to 3.0 × 10⁻⁵ M with a correlation coefficient of 0.9998 and a detection limit of 5.0 × 10⁻⁸ M based on the equation C_m = 3s_b1/m. The modified electrode has been successfully applied for the assay of DA in human blood serum. This work provides a simple and easy approach to selectively detect DA in the presence of AA and UA.     

Electrocatalytic oxidation of ethanol at copper bromide modified copper electrode in comparison to bare and copper chloride modified copper electrodes

Copper bromide modified copper electrode was prepared and used to electrocatalytic oxidation of ethanol. Scanning electron microscopy and energy dispersive x-ray experiments suggested the formation of thin layer of copper bromide on the copper surface. The j₀ for copper bromide modified copper and copper chloride modified copper electrodes are 9.8 and 5.7 folds respectively higher than for that of bare copper electrode. For copper bromide modified copper electrode, the charge transfer coefficient (α) and the number of electrons involved in the rate determining step (n_α) were calculated as 0.44 and 1 respectively.     

Electrocatalytic reduction of dioxygen by cobalt porphyrin-modified glassy carbon electrode with single-walled carbon nanotubes and nafion in aqueous solutions

Cobalt porphyrin (CoP)-modified glassy carbon electrode (GCE) with single-walled carbon nanotubes (SWNTs) and Nafion demonstrated a higher electrocatalytic activity for the reduction of dioxygen in 0.1 M H₂SO₄ solution. Cyclic and hydrodynamic voltammetry at the CoP-SWNTs/GCE-modified electrodes in O₂-saturated aqueous solutions was used to study the electrocatalytic pathway. Compared with the CoP/GCE-modified electrodes, the reduction potential of dioxygen at the CoP-SWNTs/GCE-modified electrodes was shifted to the positive direction and the limiting current was greatly increased. Especially, the Co(TMPP)-SWNTs/GCE-modified electrode was catalyzed effectively by the 4e⁻ reduction of dioxygen to water, because hydrodynamic voltammetry revealed the transference of approximately four electrons for dioxygen reduction and the minimal generation of hydrogen peroxide in the process of dioxygen reduction.     

Sensitive determination of Cd and Pb by differential pulse stripping voltammetry with in situ bismuth-modified zeolite doped carbon paste electrodes

Here we investigated the analytical performances of the bismuth-modified zeolite doped carbon paste electrode (BiF-ZDCPE) for trace Cd and Pb analysis. The characteristics of bismuth-modified electrodes were improved greatly via addition of synthetic zeolite into carbon paste. To obtain high reproducibility and sensitivity, optimum experimental conditions for bismuth deposition were studied. Voltammetric responses of the BiF-ZDCPEs prepared with different ratios of zeolite, carbon powder, and silicone, were examined under same conditions. The in situ plated (zeolite/graphite powder/silicone, 10/190/80 w/w) BiF-ZDCPEs exhibited the most sensitive response to Cd and Pb in 0.10 M acetate buffer (pH 4.5). The detection limits of the modified electrode were 0.08 μg L⁻¹ for Cd(II) and 0.10 μg L⁻¹ for Pb(II) based on three times the standard deviation of the baseline with a preconcentration time of 120 s under optimal conditions, respectively. The modified electrode showed well linear response to both Cd(II) and Pb(II) over the concentration range from 1.0 to 20.0 μg L⁻¹. The BiF-ZDCPEs were successfully applied to the determination of Cd(II) and Pb(II) in real samples, and the results were in agreement with those of atomic absorption spectroscopy (AAS).     

Formation of a robust and stable film comprising ionic liquid and polyoxometalate on glassy carbon electrode modified with multiwalled carbon nanotubes: Toward sensitive and fast detection of hydrogen peroxide and iodate

A robust and stable film comprising n-octylpyridinum hexafluorophosphate ([C₈Py][PF₆]) and 1:12 phosphomolybdic acid (PMo₁₂) was prepared on glassy carbon electrodes modified with multiwall carbon nanotubes (GCE/MWCNTs) by dip-coating. The cyclic voltammograms of the GCE/MWCNTs/[C₈Py][PF₆]-PMo₁₂ showed three well-defined pairs of redox peaks due to the PMo₁₂ system. The surface coverage for the immobilized PMo₁₂ and the average values of the electron transfer rate constant for three pairs of redox peaks were evaluated. The GCE/MWCNTs/[C₈Py][PF₆]-PMo₁₂ showed great electrocatalytic activity towards the reduction of H₂O₂ and iodate. The kinetic parameters of the catalytic reduction of hydrogen peroxide and iodate at the electrode surface and analytical features of the sensor for amperometric determination of hydrogen peroxide and iodate were evaluated.     

Studies on the electrochemical behavior of 3-nitrobenzaldehyde thiosemicarbazone at glass carbon electrode modified with nano-γ-Al2O3

Nano-γ-Al₂O₃ is dispersed onto the glass carbon electrode (GCE) by polishing. This nanostructured modified GCE exhibits a great enhancement to the redox responses of 3-nitrobenzaldehyde thiosemicarbazone (3-NBT). In comparison with bare GCE, 3-NBT gives a more sensitive voltammetric response because of the nanoparticle’s unique properties. The lowest detectable concentration (3σ) of 3-NBT is estimated to be 1.18 × 10⁻⁶ M (accumulation for 4 min). The linear relationship between peak current and concentration of 3-NBT holds in the range 1.0 × 10⁻⁵ M to 1.0 × 10⁻⁴ M (r = 0.9981). The electrochemical properties of 3-NBT on this modified electrode have been investigated with various electrochemical methods. The results indicate that the transference of one electron and one proton involves electrode radical reaction processes I and II, respectively. The coverage value (Γ) of 1.62 × 10⁻⁹ mol cm⁻² was calculated and the electrochemical parameters, diffusion coefficient D (2.54 × 10⁻³ cm² s⁻¹, 2.03 × 10⁻³ cm² s⁻¹) and reaction rate constant k_s (5.9573 s⁻¹, 7.15 × 10⁻² cm s⁻¹) were obtained for quasi-reversible system I and irreversible system II, respectively.     

Cyclic voltammetry of lipophilic compounds in oil: Direct determination of lipid peroxide with a carbon past electrode

In preliminary experiments, we demonstrated that a cyclic voltammogram of a lipophilic compound can be conveniently obtained with the past electrode that was made from graphite powder and an oil in which the compound is dissolved. This finding was applied to the direct determination of lipid peroxides. Oxygenation of lipids at 120°C was followed by the cathodic current created by the reduction of the peroxides. The validity of the method was investigated by comparison with conventional methods (peroxide value, thiobarbituric acid, and weight gain). The amount of peroxides in an oil and its cathodic current show a linear relation in the initial stages of peroxidation (ca. 12 h). This electrochemical method was applied successfully to the evaluation of the antioxidant activity of butylated hydroxytoluene in linseed oil.     

Promethazine determination in plasma samples by using carbon paste electrode modified with molecularly imprinted polymer (MIP): Coupling of extraction, preconcentration and electrochemical determination

A promethazine (PMZ) molecularly imprinted polymer (MIP) and a non-imprinted polymer (NIP) were synthesized by two different formulations of methacrylic acid-ethylene glycol dimethacrylate (MAA-EGDMA) and vinyl benzene-divinyl benzene (VB-DVB). Then, the MIPs were used to modify the carbon paste electrode (CP). The response difference between MIP-CP and NIP-CP electrodes, containing VB-DVB polymer, was higher than that for MIP-CP and NIP-CP modified with polymer of MAA-EGDMA, indicating the lower nonselective surface adsorption property of the VB-DVB based MIP. The MIP, incorporated in the carbon paste electrode, functioned as selectively recognition element and pre-concentrator agent for PMZ determination. The prepared electrode was used for PMZ measurement by the three steps procedure including analyte extraction in the electrode, electrode washing and electrochemical measurement of PMZ. It was shown that the electrode washing, after PMZ extraction, led to enhanced selectivity. Square wave voltammetry (SWV) for PMZ determination by proposed electrode was proved to be better than that of differential pulse voltammetry. Some parameters, effective on the electrode response, were optimized and then a calibration curve was plotted. Two dynamic linear range of 7 × 10⁻⁹ to 4 × 10⁻⁷ and 4 × 10⁻⁷ to 7 × 10⁻⁶ mol L⁻¹ were obtained. The detection limit of the method was calculated equal to 3.2 × 10⁻⁹ mol L⁻¹. This method was used successful for PMZ determination in blood serum sample.