Electrochemical Behavior of Deoxycholic Acid on Multiwalled Carbon Nanotubes Modified Electrode

Electrochemical reduction of Deoxycholic acid (DCA), as important biological molecule has been studied in 0.05 M KH₂PO₄ aqueous methanol solution (1 : 1, v/v) at a multiwalled carbon nanotubes modified electrode. Based on cyclic voltammetry, Tafel plot, the possible mechanism of the electrochemical reduction of DCA was investigated, as well as DCA mechanical calculations. The transfer coefficient, α, the exchange current density, i₀, the diffusion coefficient, D0 at the MWCNT modified GCE were determined as 0.505, 5.655×10^?7 A/cm² and 1.579×10^?5 cm²/s, respectively. The results of chronoamperometric suggested that the reductive product of DCA was the corresponding hydroxymethyl compound in an over‐all four‐electron process.     

A Polypyrrole‐Imprinted Electrochemical Sensor Based on Nano‐SnO2/Multiwalled Carbon Nanotubes Film Modified Carbon Electrode for the Determination of Oleanolic Acid

A sensitive molecularly imprinted electrochemical sensor with specific recognition ability for oleanolic acid was synthesized by modification of multiwalled carbon nanotubes (MWNTs) decorated with tin oxide nanoparticles (nano‐SnO₂/MWNTs) and polypyrrole‐imprinted polymer on a carbon electrode. The morphology and electrochemical performance of the imprinted sensor were investigated by using scanning electron microscope (SEM), X‐ray diffraction (XRD), cyclic voltammetry (CV), linear sweep voltammetry (LSV) and amperometric i–t curve. The results showed that the imprinted sensor displayed excellent selectivity toward oleanolic acid. A linear relationship between the response currents and oleanolic acid concentrations ranging from 5.0×10^?8 g/L to 2.0×10^?5 g/L was obtained for the imprinted sensor. The limit of detection (LOD) of the imprinted sensor toward oleanolic acid was calculated as 8.6×10^?9 g/L at a signal to noise ratio (S/N) of 3. This imprinted sensor was successfully applied to the determination of oleanolic acid in Acitinidia deliciosa root samples.     

Electrochemistry and Electrocatalysis of Hemoglobin on 1‐Pyrenebutanoic Acid Succinimidyl Ester/Multiwalled Carbon Nanotube and Au Nanoparticle Modified Electrode

A biocompatible nanocomposite film was fabricated for hemoglobin (Hb) molecules immobilization. This film consists of multiwalled carbon nanotubes (MWNTs), 1‐pyrenebutanoic acid, succinimidyl ester (PASE), hemoglobin (Hb) and Au nanoparticles (AuNPs). Herein, PASE molecules physically adsorbed onto MWNTs, and its groups then covalently bond with Hb. AuNPs were then linked with Hb/PASE/MWNTs via electrostatic adsorption force. UV‐visible adsorption spectra, Fourier transform infrared spectra, scanning electron microscope and electrochemical impedance spectroscopy have characterized the film. Cyclic voltammetry (CV) scans showed that in the film Hb has well‐defined redox reaction, with the formal potential (E°) at about ?0.27 V (vs. Ag/AgCl). Herein, differential pulse voltammetry (DPV) was employed to electrochemically detect the Hb in the film with a detection limit of 9.3×10^?8 M. The proposed method was also succeeded for Hb detection in clinical blood samples. Furthermore, the Hb in the film showed good electrocatalytic activities to the reduction of H₂O₂, TCA, NaNO₂ and O₂.     

Modification of Glassy Carbon Electrode With Single‐Walled Carbon Nanotubes and α‐Silicomolybdate: Application to Sb(III) Detection

A simple procedure was developed to prepare a glassy carbon (GC) electrode modified with single‐walled carbon nanotubes (SWCNTs) and polyoxometalate. With immersing SWCNTs modified GC electrode in silicon polyoxomolybdate (α‐SiMo₁₂O₄₀^4?) solution (direct deposition) for a short period of time (2–10 s) oxoanion adsorbed strongly and irreversibly on SWCNTs. Cyclic voltammograms of the α‐SiMo₁₂O₄₀^4? incorporated‐SWCNTs indicates three well‐defined and reversible redox couples with surface confined characteristic at wide pH range (1–7). The surface coverage (Γ) of α‐SiMo₁₂O₄₀^4? immobilized on SWCNTs was 2.14 (±0.11)×10^?9 mol cm^?2 indicating high loading ability of SWCNTs for polyoxometalate. The charge transfer rate constant (k_s) of three redox couples of adsorbed α‐SiMo₁₂O₄₀^4? were 9.20 (±0.20), 8.02 (±0.20), and 3.70 (±0.10) s^?1, respectively, indicate great facilitation of the electron transfer between α‐SiMo₁₂O₄₀^4? and CNTs. In this research the attractive mechanical and electrical characteristics of CNTs and unique properties and reactivity of polyoxometalates were combined. The modified electrode in buffer solution containing Sb(III) shows a new redox system at 0.38 V in pH 1. The voltammetric peak current increased with increasing Sb(III) concentration. The differential pulse voltammetry (DPV) technique was used for detection micromolar concentration of antimony. Furthermore, the interference effects various electroactive compounds on voltammetric response of Sb(III) were negligible. Finally the ability of the modified electrode for antimony detection in real samples was evaluated.     

Highly Sensitive and Selective Amperometric Detection of Periodate at Glassy Carbon Electrode Modified with a Cyclometalated Iridium(III) Complex and Single‐Wall Carbon Nanotubes

Single‐wall carbon nanotubes (SWCNTs) were used as an immobilization matrix to incorporate [Ir(ppy)₂(phen‐dione)](PF₆) complex onto a glassy carbon electrode for the study of electrocatalytic reduction of periodate ion. Detailed preliminary electrochemical data for the Ir(III)‐complex in acetonitrile solution and for the modified GCE/SWCNTs/[Ir(ppy)₂(phen‐dione)](PF₆)/CGE are presented. The modified electrode was applied to selective amperometric detection of periodate through its electrocatalytic reduction to iodide at 0.200 V and pH 2.0. The use of amperometry resulted in two calibration plots over the concentration ranges of 1‐20 μM and 20‐450 μM, with a detection limit of 0.6 μM and sensitivity of 198 nA μM^?1.     

Electrocatalytic Oxidation and Determination of Nitrite at Multi‐walled Carbon Nanotubes Modified Carbon Ceramic Electrode

In the present work, the electrochemical oxidation of nitrite on carbon ceramic electrode (CCE) modified with multi‐walled carbon nanotubes (MWCNTs) was investigated. The modified electrode exhibited catalytic activity toward the electrooxidation of nitrite. Experimental parameters such as solution pH, scan rate, concentration of nitrite and nanotubes amount were studied. It was shown nitrite can be determined by differential pulse voltammetry (DPV) and hydrodynamic amperometry (HA) using the modified electrode. Under the optimized conditions the calibration plots are linear in the concentration ranges of 15‐220 and 50‐3000 μM with limit of detections of 4.74 and 35.8 μM for DPV and HA, respectively. The modified electrode was successfully applied for analysis of nitrite in spinach sample. The results were favorbly compared to those obtained by UV‐Visible spectrophotometric method. The results of the analysis suggest that the proposed method has promise for the routine determination of nitrite in the examined products.     

A NADH Sensor Based on 1,2‐Naphththoquinone Electropolymerized on Multi‐walled Carbon Nanotubes Modified Glassy Carbon Electrode

A new carbon nanotubes modified electrode (poly‐Nq‐MWCNTs/GCE) was fabricated by electropolymerization of 1,2‐naphththoquinone to the surface of multi‐walled carbon nanotubes modified electrode by casting method. The morphology of the nanocomposite was characterized by scanning electron microscopy. Cyclic voltammetry and chronoamperometry were applied to investigate the electrochemical properties of the poly‐Nq‐MWCNTs nanocomposite modified electrode. The result of electrochemical experiments showed that such modified electrode had a favorable catalytic ability to oxidation of β‐nicotinamide adenine dinucleotide (NADH). The resulted sensor was sensitiveness to NADH and achieved 95β of the steady‐state current within 5s. Furthermore, the anodic peak current was linear to the concentration of NADH for the range from 1.0 μM to 0.14 mM. The linear equation was: I(μA) = 0.3987 + 0.1035c (μmol/L), the correlation coefficient r = 0.9962, the detect limit is down to 1 × 10^?7 M (S/N = 3) and the sensitivity is 0.1035 μA/mmol. The well catalytic activity of the sensor was ascribed to the synergistic effect role played by MWCNTs and poly‐Nq. Moreover, the based sensor possesses good stability and reproducibility.     

维生素C在多壁碳纳米管/壳聚糖复合膜修饰玻碳电极上的电化学行为及测定

研究了维生素C在多壁碳纳米管/壳聚糖复合膜修饰玻碳电极上的电化学行为及测定。实验结果表明,在0.2 mol/L PBS(pH6.0)缓冲溶液中,修饰电极对抗坏血酸的氧化具有明显的催化和增敏效应,其氧化峰电位由 0.5 V负移至 0.1 V(vs.AgCl/Ag)。对修饰剂碳纳米管的用量、支持电解质、富集电位和富集时间等进行了优化。采用半微分伏安法进行定量测定,其线性范围为4.0×10-6~2.0×10-3mol/L,r=-0.998 3,检出限为1.0μmol/L。对抗坏血酸在修饰电极上的电化学行为进行了探讨,其电极反应为具有吸附特性和不可逆的电极过程,测得参加反应的质子数为2,电极反应的电子转移系数为0.59。测定了维生素C药片中抗坏血酸的含量,回收率在93%~105%。     

Electrochemical Behavior and Determination of L‐Tyrosine at Single‐walled Carbon Nanotubes Modified Glassy Carbon Electrode

Based on single‐walled carbon nanotubes (SWCNTs) modified glassy carbon electrode (GCE/SWCNTs), a novel method was presented for the determination of L ‐tyrosine. The GCE/SWCNTs exhibited remarkable catalytic and enhanced effects on the oxidation of L ‐tyrosine. In 0.10 mol/L citric acid‐sodium citrate buffer solution, the oxidation potential of L ‐tyrosine shifted negatively from +1.23 V at bare GCE to +0.76 V at GCE/SWCNTs. Under the optimized experimental conditions, the linear range of the modified electrode to the concentration of L ‐tyrosine was 5.0×10^?6–2.0×10^?5 mol/L (R₁=0.9952) and 2.7×10^?5–2.6×10^?4 mol/L (R₂=0.9998) with a detection limit of 9.3×10^?8 mol/L. The kinetic parameters such as α (charge transfer coefficient) and D (diffusion coefficient) were evaluated to be 0.66, 9.82×10^?5 cm² s^?1, respectively. And the electrochemical mechanism of L ‐tyrosine was also discussed.     

Application of a Carbon‐Paste Electrode Modified with 2,7‐Bis(ferrocenyl ethyl)fluoren‐9‐one and Carbon Nanotubes for Voltammetric Determination of Levodopa in the Presence of Uric Acid and Folic Acid

A carbon‐paste electrode modified with 2,7‐bis(ferrocenyl ethyl)fluoren‐9‐one and carbon nanotubes was used for the sensitive voltammetric determination of levodopa (LD). The electrochemical response characteristics of the modified electrode toward LD, uric acid (UA) and folic acid (FA) were investigated. The results showed an efficient catalytic activity of the electrode for the electrooxidation of LD, which leads to lowering its overpotential by more than 320 mV. The modified electrode exhibits an efficient electron mediating behavior together with well‐separated oxidation peaks for LD, UA and FA. Also, the modified electrode was used for determination of LD in some real samples.     

Application of Glassy Carbon Electrode Modified with a Bilayer of Multiwalled Carbon Nanotube and Polypyrrole Doped with Nitrazine Yellow for Voltammetric Determination of Naltrexone

A new sensitive electrochemical sensor was fabricated based on a layer by layer process. In this process the glassy carbon electrode (GCE) is first coated by a thin film of multiwalled carbon nanotubes (MWCNTs). In the next step, the electropolymerization of pyrrole in the presence of Nitrazine Yellow (NY) as a dopant anion is performed on the surface of the MWCNTs precoated electrode. The electrochemical response characteristics of the modified electrode toward naltrexone (NTX) were studied by means of linear sweep voltammetry (LSV). A remarkable increase (～19 times) was observed in the anodic peak current of NTX on the surface of the modified electrode relative to the bare GCE. The effects of experimental parameters on the electrode response such as, drop size of the cast MWCNTs suspension, pH of the supporting electrolyte, accumulation conditions and the number of cycles in the electropolymerization process were investigated. Under the optimum conditions, the modified electrode showed a wide linear response to the concentration of NTX in the range of 4.0×10^?8–1×10^?5 mol L^?1 with a detection limit of 12 nmol L^?1. The prepared sensor exhibited high sensitivity, stability and good reproducibility for the determination of NTX. This sensor was successfully applied for the accurate determination of trace amounts of NTX in pharmaceutical and clinical preparations.     

碳纳米管表面绿原酸印迹固相萃取材料的制备及应用

在多壁碳纳米管表面接枝的双键键合,以绿原酸为模板,甲基丙烯酸为功能单体,乙二醇二甲基丙烯酸酯为交联剂,采用沉淀聚合技术,在碳纳米管表面成功制备绿原酸印迹材料.采用红外光谱、扫描电镜和热重分析研究此印迹材料的性能.结果表明,在碳纳米管表面接枝一层稳定、均匀、30～40 nm厚的印迹材料.采用高效液相色谱研究此印迹材料的吸附动力学及吸附容量,实验结果表明,此印迹材料对绿原酸的结合存在两个结合位点,最大吸附容量Qmax分别为21.5和32.7 μmol/g.以此印迹材料作为固相萃取剂,优化萃取条件,成功应用于金银花提取液中绿原酸的富集分离研究,富集因子达25.     

ABTS‐Multiwalled Carbon Nanotubes Nanocomposite/Bi Film Electrode for Sensitive Determination of Cd and Pb by Differential Pulse Stripping Voltammetry

A 2,2′‐azinobis (3‐ethylbenzothiazoline‐6‐sulfonate) diammonium salt (ABTS)‐multiwalled carbon nanotubes (MWCNTs) nanocomposite/Bi film modified glassy carbon (GC) electrode was constructed for the differential pulse stripping voltammetric determination of trace Pb²⁺ and Cd²⁺. This electrode was more sensitive than ABTS‐free Bi/GC and Bi/MWCNTs/GC electrodes. Linear responses were obtained in the range from 0.5 to 35 μg L^?1 for Cd²⁺ and 0.2 to 50 μg L^?1 Pb(II), with detection limits of 0.2 μg L^?1 for Cd²⁺ and 0.1 μg L^?1 for Pb²⁺, respectively. This sensor was applied to the simultaneous detection of Cd²⁺ and Pb²⁺ in water samples with satisfactory recovery.     

Fabrication of New Magnetic Nanoparticles (Fe3O4) Grafted Multiwall Carbon Nanotubes and Heterocyclic Compound Modified Electrode for Electrochemical Sensor

Magnetic Fe₃O₄ nanoparticles functionalized multiwalled carbon nanotubes (nano‐Fe₃O₄ MWNTs) were prepared for electrochemical sensors. 2‐amino‐5‐mercapto‐1,3,4‐thiadiazole was used as a connecter to form a network that connected nano‐Fe₃O₄ MWNTs to the Au electrode surface. Modified process of the electrode was studied with SEM, TEM and cyclic voltammetry. Cyclic voltammetry and amperometric i‐t curve were used to investigate characteristics of the obtained electrode. The sensor has been successfully used on the direct detection of catechol and showed excellent performances. The linear regression equation was I_pa(μA)=0.07763+0.16739 C (μmol/L); R=0.9993 and the detection limit was 5.38×10^?8 mol/L. The modified electrode showed good reproducibility and stability.     

Gold Electrode Modified with Self‐Assembled Monolayer of Cysteamine‐Functionalized MWCNT and Its Application in Simultaneous Determination of Dopamine and Uric Acid

The voltammetric behavior of dopamine (DA) and uric acid (UA) on a gold electrode modified with self‐assembled monolayer (SAM) of cysteamine (CA) conjugated with functionalized multiwalled carbon nanotubes (MWCNTs) was investigated. The film modifier of functionalized SAM was characterized by means of scanning electron microscopy (SEM) and also, electrochemical impedance spectroscopy (EIS) using para‐hydroquinone (PHQ) as a redox probe. For the binary mixture of DA and UA, the voltammetric signals of these two compounds can be well separated from each other, allowing simultaneous determination of DA and UA. The effect of various experimental parameters on the voltammetric responses of DA and UA was investigated. The detection limit in differential pulse voltammetric determinations was obtained as 0.02 µM and 0.1 µM for DA and UA, respectively. The prepared modified electrode indicated a stable behavior and the presence of surface COOH groups of the functionalized MWCNT avoided the passivation of the electrode surface during the electrode processes. The proposed method was successfully applied for the determination of DA and UA in urine samples with satisfactory results. The response of the gold electrode modified with MWCNT‐functionalized SAM method toward DA, UA, and ascorbic acid (AA) oxidation was compared with the response of the modified electrode prepared by the direct casting of MWCNT.     

新型聚N，N-二甲基苯胺/多壁碳纳米管修饰电极的制备、表征与应用

采用多壁碳纳米管(MWNT)改性聚N,N-二甲基苯胺(PDMA)膜,制备了新型复合膜修饰玻碳电极,并用SEM、电化学方法对修饰电极进行表征。 结果表明,无论MWNT是以掺杂还是先滴涂MWNT再聚合DMA多层修饰方式,均会改变PDMA膜的形貌和电化学性能。 复合膜修饰电极比单一PDMA膜修饰电极大幅度提高了比表面积和电活化面积,同时使PDMA和MWNT更好地协同发挥其优良的电化学特性。 实验结果表明,层层修饰制备的聚N,N-二甲基苯胺/多壁碳纳米管复合膜修饰电极对香草醛的电化学响应远大于基体电极和其它方法制备的修饰电极,电催化作用显著提高,其过电位降低了148 mV,氧化峰电流约增加了6倍;其电极反应是吸附控制的不可逆氧化过程,转移电子数n为2,质子数m为1,传递系数α为0.4062,吸附量为Γ=3.527×10-9 mol/cm2;检出下限为8.0×10-7 mol/L,样品平均回收率为99.87%。     

Facile and Simultaneous Stripping Determination of Zinc,Cadmium and Lead on Disposable Multiwalled Carbon Nanotubes Modified Screen‐Printed Electrode

Screen‐printed electrodes (SPEs) are cheap and disposable. But their application for heavy metal detection is limited due to the low sensitivity and poor selectivity. Here we report the ultrasensitive and simultaneous determination of Zn²⁺, Cd²⁺ and Pb²⁺ on a multiwalled carbon nanotubes and Nafion composite modified SPE with in situ plated bismuth film (MWCNTs/NA/Bi/SPE). The linear curves range from 0.5–100 µg L^?1 for Zn²⁺ and 0.5–80 µg L^?1 for Cd²⁺. Uniquely, the linear curve for Pb²⁺ ranges from 0.05–100 µg L^?1 with a detection limit of 0.01 µg L^?1. The practical application was verified in real samples with satisfactory results.     

N‐(3,4‐Dihydroxyphenethyl)‐3,5‐dinitrobenzamide‐Modified Multiwall Carbon Nanotubes Paste Electrode as a Novel Sensor for Simultaneous Determination of Penicillamine,Uric acid,and Tryptophan

N‐(3,4‐dihydroxyphenethyl)‐3,5‐dinitrobenzamide modified multiwall carbon nanotubes paste electrode was used as a voltammetric sensor for oxidation of penicillamine (PA), uric acid (UA) and tryptophan (TP). In a mixture of PA, UA and TP, those voltammograms were well separated from each other with potential differences of 300, 610, and 310 mV, respectively. The peak currents were linearly dependent on PA, UA and TP concentrations in the range of 0.05–300, 5–420, and 1.0–400 µmol L^?1, with detection limits of 0.021, 2.0, and 0.82 µmol L^?1, respectively. The modified electrode was used for the determination of those compounds in real samples.     

Simultaneous Voltammetric Determination of Uric Acid and Ascorbic Acid Using a Carbon‐Paste Electrode Modified with Multi‐Walled Carbon Nanotubes/Nafion and Cobalt(II)nitrosalophen

A composition of multiwalled carbon nanotube (MWCNT), Nafion and cobalt(II)‐5‐nitrosalophen (CoNSal) is applied for the modification of carbon‐paste electrode (CPE). The pretreated MWCNT is well dispersed in the alcoholic solution of Nafion under the ultrasonic agitation, and the resulted suspension is used as modifier (with 10% w/w) in the matrix of the paste electrode. The prepared electrode further modified by addition of 3 wt% of CoNSal. The resulted modified electrode is used as a sensitive voltammetric sensor for simultaneous determination of uric acid (UA) and ascorbic acid (AA). The electrode showed efficient electrocatalytic activity in lowering the anodic overpotentials and enhancement of the anodic currents. This electrode is able to completely resolve the voltammetric response of UA and AA. The effects of potential sweep rate and pH of the buffer solution on the response of the electrode, toward UA and AA, and the peak resolution is thoroughly investigated by cyclic and differential pulse voltammetry (CV and DPV). The best peak resolution for these compounds using the modified electrode is obtained in solutions with pH 4. The ΔE_p for UA and AA in these methods is about 315 mV, which is considerably better than previous reports for these compounds. A linear dynamic range of 1×10^?7 to 1×10^?4 M with a detection limit of 6×10^?8 M is resulted for UA in buffered solutions with pH 4.0. The voltammetric response characteristics for AA are obtained as, the linear range of 5×10^?7 to 1×10^?4 M with the detection limit of 1×10^?7 M. The voltammetric detection system was very stable and the reproducibility of the electrode response, based on the six measurements during one month, was less than 3.5% for the slope of the calibration curves of UA and AA. The prepared modified electrode is successfully applied for the determination of AA and UA in mixture samples and reasonable accuracies are resulted.     

A Simple Layer‐by‐Layer Assembly Strategy for a Reagentless Biosensor Based on a Nanocomposite of Methylene Blue‐Multiwalled Carbon Nanotubes

A simple layer‐by‐layer (LBL) assembly strategy was established for constructing a novel reagentless biosensor based on a nanocomposite of methylene blue multiwalled carbon nanotubes (MB‐MWNTs). A nanocomposite of MB‐MWNTs was obtained by direct premixing and possessed good dispersion in barbital‐HCl buffer. Through electrostatic interactions, the nanocomposite of MB‐MWNTs could alternately be assembled with horseradish peroxidase (HRP) on the Au electrode modified with precursor films. UV/Vis spectra and scanning electron microscopy (SEM) were applied to reveal the formation of the nanocomposite of MB‐MWNTs. The LBL assembly process was also verified by electrochemical impedance spectroscopy (EIS). The MB is a well‐established mediator and efficiently facilitated the electron shuttle between the HRP and the electrode, as demonstrated by the cyclic voltammetry (CV) measurements. The as‐prepared reagentless biosensor exhibited a fast response for the determination of hydrogen peroxide (H₂O₂) and reached 95% of the steady‐state current within 3 s. It was found that the linear response range of the reagentless biosensor for H₂O₂ was from 4.0 μM to 3.78 mM with a detection limit of 1.0 μM and a sensitivity of 22.5 μA mM⁻¹. The biosensor exhibited a high reproducibility and stability.