Electrochemical Behavior of Uric Acid and Epinephrine at a Meso-2,3-Dimercaptosuccinic Acid Self-Assembled Gold Electrode

A self-assembled electrode with a meso-2,3-dimercaptosuccinic acid (DMSA) monolayer has been characterized by electrochemical quartz crystal microbalance and complex impedance analysis, surface enhanced Raman spectroscopy and cyclic voltammetry. The self-assembled electrode was used for the simultaneous electrochemical detection of epinephrine (EP) and uric acid (UA) in phosphate buffer of pH 7.7. The simultaneous oxidation of EP and UA was performed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), and the signals for each method were well separated with a potential difference of over 330 mV and without interference by each other. The detection limit of EP is 5.4 × 10⁻⁸ mol L⁻¹ by CV and 5.3 × 10⁻⁸ mol L⁻¹ by DPV and that of UA is 8.4 × 10⁻⁸ mol L⁻¹ by CV and 4.2 × 10⁻⁸ mol L⁻¹ by DPV. The DMSA self-assembled electrode can be applied to the simultaneous determination of EP and UA.     

Electrochemical behavior and determination of epinephrine at a mercaptoacetic acid self-assembled gold electrode

The electrochemical behavior of epinephrine (EP) at a mercaptoacetic acid (MAA) self-assembled monolayer modified gold electrode was studied. The MAA/Au electrode is demonstrated to promote the electrochemical response of epinephrine by cyclic voltammetry. The possible reaction mechanism is also discussed. The diffusion coefficient D of EP is 6.85 × 10⁻⁶ cm² s⁻¹. In 0.1 mol L⁻¹ phosphate buffer (pH 7.20), a sensitive oxidation peak was observed at 0.177 V, and the peak current is proportional to the concentration of EP in the range of 1.0 × 10⁻⁵–2.0 × 10⁻⁴ mol L⁻¹ and 1.0 × 10⁻⁷–1.0 × 10⁻⁶ mol L⁻¹. The detection limit is 5 × 10⁻⁸ mol L⁻¹. The modified electrode is highly stable and can be applied to the determination of EP in practical injection samples. The method is simple, quick, sensitive and accurate.     

Preconcentration and Voltammetric Determination of the Antihypertensive Doxazosin on a C8 Modified Carbon Paste Electrode

 An electrochemical study of the doxazosin oxidative process at carbon paste electrodes using different voltammetric techniques has been carried out. The process is irreversible and controlled by adsorption, giving rise to an oxidation wave around 1.0 V in citric acid-citrate buffer (pH 3.0). A mechanism based on the oxidation of the amine group is postulated. Two methods based on adsorptive stripping (AdS) of doxazosin at the C₈-modified carbon paste electrode (C₈-MCPE), before its voltammetric determination, are studied, using differential pulse voltammetry (DPV) and square wave voltammetry (SWV) as redissolution techniques. By means of AdS-DPV and C₈-MCPE, doxazosin can be determined over the 1.0 × 10⁻⁹ to 3.0 × 10⁻⁸ mol L⁻¹ range with a variation coefficient of 2.2% (2.0 × 10⁻⁸ mol L⁻¹) and a limit of detection of 7.4 ×10⁻¹⁰ mol L⁻¹. If AdS-SWV is used, a linear range from 1.0 × 10⁻⁹ to 4.0 × 10⁻⁸ mol L⁻¹ is obtained, the variation coefficient being 2.8% (2.0 × 10⁻⁸ mol L⁻¹, and the limit of detection reached 7.7 × 10⁻¹⁰ mol L⁻¹. The AdS-DPV procedure was applied to the determination of doxazosin in urine and formulations. Received March 13, 1999. Revision December 23, 1999.     

Ultrasensitive Assay of Rhein in Medicine Based on its Enhanced Luminol-K3Fe(CN)6 Chemiluminescence Reaction Using the Flow Injection Technique

Determination of the effective components in traditional Chinese medicine is one of the key steps for its identification. In this paper a novel and sensitive chemiluminescence (CL) method for the determination of rhein coupled with flow-injection analysis (FIA) is developed. It is based on the strong sensitizing effect on the weak CL reaction between luminol and ferricyanide in alkaline solution. Under optimal experimental conditions, the relative CL intensity is proportional to the concentration of rhein in the range of 7.0 × 10⁻¹²–7.0 × 10⁻¹⁰ mol L⁻¹ and 1.0 × 10⁻⁹–4.0 × 10⁻⁵ mol L⁻¹, the detection limit is 1.478 × 10⁻¹³ mol L⁻¹, and the relative standard deviation (RSD) for 9 parallel measurements of 1.408 × 10⁻⁷ mol L⁻¹ rhein is 3.4%. The method was successfully applied to the determination of rhein in pharmaceutical preparations. The possible mechanism of CL is also briefly discussed.     

A novel method for the determination of Pb2+ based on the quenching of the fluorescence of CdTe quantum dots

A novel method for the determination of Pb²⁺ has been developed based on quenching of the fluorescence of thiol-capped CdTe quantum dots (QDs) by Pb²⁺ in aqueous solutions. Under optimum conditions, the relative fluorescence intensity was linearly proportional to the concentration of Pb²⁺ between 2.0 × 10⁻⁶ and 1.0 × 10⁻⁴ mol L⁻¹ with a detection limit of 2.7 × 10⁻⁷ mol L⁻¹. The relative standard deviation (RSD) was 4.6% for a 4.0 × 10⁻⁵ mol L⁻¹ Pb²⁺ solution (N = 5). As an application, the proposed method was successfully applied to the analysis of Pb²⁺ in food samples, and the results were satisfactory, i.e. consistent with those of flame atomic absorption spectrometry (FAAS). Correspondence: Heyou Han, College of Science, Institute of Chemical Biology, State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, P.R. China     

Covalent Modification of Glassy Carbon Electrode with L-Cysteine for the Determination of Acetaminophen

A novel L-cysteine film modified electrode has been fabricated by means of an electrochemical oxidation procedure, and it was successfully applied to the electrochemical determination of acetaminophen. This method utilizes the electrooxidation of amines to their analogous cation radicals to form a chemically stable covalent linkage between the nitrogen atom of the amine and edge plane sites at the glassy carbon electrode surface. The electrochemical behaviour of acetaminophen at the film electrode was investigated in 0.1 mol L⁻¹ phosphate buffer (pH 6.20). It was found that the redox peak current of acetaminophen was enhanced greatly on the film electrode. Linearity between the oxidation peak current and the acetaminophen concentration was obtained in the range of 1.0 × 10⁻⁴–2.0 × 10⁻⁷ mol L⁻¹ with a detection limit of 5.0 × 10⁻⁸ mol L⁻¹. For seven parallel detections of 1.0 × 10⁻⁵ mol L⁻¹ acetaminophen, the relative standard deviation (RSD) was 1.46%, suggesting that the film electrode has excellent reproducibility. Application to the determination of acetaminophen in drug tablets and human urine demonstrated that the film electrode has good stability and high sensitivity.     

Electroreduction and Quantification of Furazolidone and Furaltadone in Different Media

 Adsorptive cathodic stripping voltammetry was used for the determination of furazolidone (FZ) and furaltadone (FD) in borax and phosphate buffers, respectively, using HMDE as working electrode. The influence of different factors upon the peak current response such as accumulation potential, scan rate, preconcentration time, pH and other variables was studied. Furazolidone and furaltadone showed an adsorption character on HMDE in presence of borax and phosphate buffers, respectively. A single cathodic peak at −0.36 V in borax (pH = 9.5) was observed for FZ, while FD gave a cathodic peak at −0.32 V in phosphate buffer (pH = 8.5). The calibration graph showed a linear behavior over the range 3×10⁻⁹–9×10⁻⁸ mol dm⁻³ for furazolidone. In the case of FD, concentrations from 3×10⁻⁹ to 2×10⁻⁷ mol dm⁻³ gave a linear relationship with the peak current. A detection limit of 2×10⁻⁹ mol dm⁻³ and 1×10⁻⁹ mol dm⁻³ was obtained for furazolidone and furaltadone, respectively. This method was applied to determine these drugs in pharmaceutical formulations, urine and serum samples. Received December 15, 1998. Revision February 4, 2000.     

Application of a Single-Wall Carbon Nano-Tube Film Electrode to the Determination of Trace Amounts of Folic Acid

Single-wall carbon nano-tubes were used to modify the surface of a glassy carbon electrode (GC) and applied in the determination of folic acid with voltammetry. The experiments demonstrated that the presence of a carbon nano-tube film on the electrode greatly increased the reduction peak current of folic acid. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were used in a comparative investigation of the electrochemical reduction of folic acid with the film electrode. Effects of pH on the peak current and the peak potential were studied in the pH range of 4.0–8.0 with Britton-Robinson buffer solution. The reduction peak current was found to be linearly related to folic acid concentration over the range of 1 × 10⁻⁸ to 1 × 10⁻⁴ mol L⁻¹ with a detection limit of 1 × 10⁻⁹ mol L⁻¹ after 5 min accumulation. The film electrode provides an efficient way for eliminating interferences from some inorganic and organic species in the solution. The high sensitivity, selectivity and stability of the film electrode demonstrate its practical application from a simple and rapid determination of folic acid in tablets.     

Electrochemical determination of acetaminophen using a glassy carbon electrode coated with a single-wall carbon nanotube-dicetyl phosphate film

Single-wall carbon nanotubes (SWNT) were dispersed into water in the presence of dicetyl phosphate (DCP), and then a SWNT-DCP film-coated glassy carbon electrode (GCE) was constructed. The electrochemical behavior of acetaminophen at bare GCE and SWNT-DCP modified GCE were compared, suggesting that the SWNT-DCP-modified GCE significantly enhances the oxidation peak current of acetaminophen. A sensitive and simple electrochemical method with a good linear relationship in the range of 1.0 × 10⁻⁷–2.0 × 10⁻⁵ mol L⁻¹, was developed for the determination of acetaminophen. The detection limit is 4.0 × 10⁻⁸ mol L⁻¹ for 3-min accumulation. This method was successfully demonstrated with tablets.     

Simple sensor for simultaneous determination of dihydroxybenzene isomers

A simple sensor based on bare carbon ionic liquid electrode was fabricated for simultaneous determination of dihydroxybenzene isomers in 0.1 mol L⁻¹ phosphate buffer solution (pH 6.0). The oxidation peak potential of hydroquinone was about 0.136 V, catechol was about 0.240 V, and resorcinol 0.632 V by differential pulse voltammetric measurements, which indicated that the dihydroxybenzene isomers could be separated absolutely. The sensor showed wide linear behaviors in the range of 5.0 × 10⁻⁷–2.0 × 10⁻⁴ mol L⁻¹ for hydroquinone and catechol, 3.5 × 10⁻⁶–1.535 × 10⁻⁴ mol L⁻¹ for resorcinol, respectively. And the detection limits of the three dihydroxybenzene isomers were 5.0 × 10⁻⁸, 2.0 × 10⁻⁷, 5.0 × 10⁻⁷ mol L⁻¹, respectively (S/N = 3). The proposed method could be applied to the determination of dihydroxybenzene isomers in artificial wastewater and the recovery was from 93.9% to 104.6%.     

Electrochemical determination of uric acid using a mesoporous SiO2-modified electrode

A mesoporous SiO₂-modified carbon paste electrode for the determination of uric acid is described. Owing to the regular and specific mesoporous channels, numerous active sites and a large surface area, the mesoporous SiO₂-modified electrode greatly increases the oxidation peak current of uric acid. Based on this, a highly sensitive, rapid and convenient electrochemical method was developed for the determination of uric acid after optimizing the experimental parameters (supporting electrolyte, content of mesoporous SiO₂, accumulation potential and time). The linear range is from 2.5 × 10⁻⁷ to 2.0 × 10⁻⁵ mol L⁻¹, and the limit of detection is estimated to be 8.0 × 10⁻⁸ mol L⁻¹. The relative standard deviation for 10 mesoporous SiO₂-modified electrodes is 5.8%. The method was used to determine uric acid in human serum samples. Correspondence: Kangbing Wu, Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, P.R. China     

Voltammetric Sensor for the Determination of Parathion Using an Electropolymerized Poly(Carmine) Film Electrode

A voltammetric sensor for the determination of parathion has been developed based on the use of a poly(carmine) film electrode. The reduction of parathion at the poly(carmine) modified glassy carbon electrode (GCE) is studied by cyclic voltammetry (CV) and linear scan voltammetry (LSV). Parathion yields a well-defined reduction peak at a potential of −0.595 V on the poly(carmine) modified GCE in pH 6.0 phosphate buffer solution (PBS). Compared with that on a bare GCE, the reduction peak current of parathion is significantly enhanced. All the experimental parameters are optimized for the determination of parathion. The reduction peak current is linear with the parathion concentration in the range of 5.0 × 10⁻⁸ to 1.0 × 10⁻⁵ mol L⁻¹, and the detection limit is 1.0 × 10⁻⁸ mol L⁻¹.     

Voltammetric determination of 2-chlorophenol using a glassy carbon electrode coated with multi-wall carbon nanotube-dicetyl phosphate film

Multi-wall carbon nanotubes (MWNT) were dispersed into water in the presence of dicetyl phosphate (DCP), and MWNT-DCP composite film coated glassy carbon electrodes (GCE) were constructed. The electrochemical properties of 2-chlorophenol at a bare GCE and MWNT-DCP modified GCE were compared. It was found that MWNT-DCP modified GCEs significantly enhance the oxidation peak current of 2-chlorophenol and lowers its oxidation overpotential, suggesting great potential in the sensitive determination of 2-chlorophenol. Finally, a sensitive and simple voltammetric method was developed for the determination of 2-chlorophenol. The oxidation peak current increases linearly with the concentration in the range of 1.0 × 10⁻⁷–2.0 × 10⁻⁵ mol L⁻¹, and the detection limit is 4.0 × 10⁻⁸ mol L⁻¹ for 2 min accumulation. The method was successfully used to determine 2-chlorophenol in waste water samples.     

Adsorption voltammetry of the zirconium-alizarin red S complex at a multi-walled carbon nanotubes modified carbon paste electrode

We used a carbon paste electrode modified with multi-walled carbon nanotubes as a working electrode and studied the electrochemical behavior of zirconium-alizarin red S complex on it. It was found that the modified electrode exhibited a significant catalytic effect toward the reduction of free alizarin red S and the complex. The second derivative linear scan voltammograms of the complex were recorded by a polarographic analyser from 0 to −1000 mV (vs. SCE), and it was found that the complex can be adsorbed on the surface of the modified electrode, yielding a peak at about −470 mV, corresponding to the reduction of alizarin red S in the complex. The linear range was found to be 2.0 × 10⁻¹¹–8.0 × 10⁻⁷ mol L⁻¹, and the detection limit was 1.0 × 10⁻¹¹ mol L⁻¹ (S/N = 3) for 3 min accumulation. The procedure was successfully applied to the determination of trace amounts of zirconium in the ore samples. Correspondence: Pei-Hong Deng, Department of Chemistry and Material Science, Hengyang Normal University, Hengyang Hunan 421008, P.R. China     

Electrochemical behavior of lead(II) at poly(phenol red) modified glassy carbon electrode, and its trace determination by differential pulse anodic stripping voltammetry

Poly(phenol red) (denoted as PPR) films were electrochemically synthesized on the surface of a glassy carbon electrode (GCE) by cyclic voltammetry to obtain a chemically modified electrode (denoted as PPR-GCE). The growth mechanism of PPR films was studied by attenuated total reflection spectroscopy. This PPR-GCE was used to develop a novel and reliable method for the determination of trace Pb²⁺ by anodic stripping differential pulse voltammetry. At optimum conditions, the anodic peak exhibits a good linear concentration dependence in the range from 5.0 × 10⁻⁹ to 5.0 × 10⁻⁷ mol L⁻¹ (r = 0.9989). The detection limit is 2.0 × 10⁻⁹ mol L⁻¹ (S/N = 3). The method was employed to determine trace levels of Pb²⁺ in industrial waste water samples. Correspondence: Gongjun Yang, Ming Shen, College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P.R. China     

Determination of Amitraz Pesticide by Adsorptive Stripping Voltammetry on the Hanging Mercury Drop Electrode

 A sensitive method for the determination of amitraz pesticide at nanomolar level by adsorptive stripping voltammetry at a hanging mercury drop electrode is described. The cyclic voltammograms demonstrate the adsorption of this compound on the mercury electrode. A systematic study of the various experimental parameters, that affect the stripping response, was carried out by differential pulse voltammetry. Using an accumulation potential of −0.50 V, and 30 s accumulation time, the limit of detection was found to be 2.3 × 10⁻⁹ mol L⁻¹ and the relative standard deviations (n = 5) was 2.2% at concentration level of 5.0 × 10⁻⁸ mol L⁻¹ of amitraz. The influence of diverse ions and some other pesticides was investigated. Finally, the method was applied to the determination of amitraz in spiked soil and water. The relative standard deviation is 4.5% for 5 determinations of amitraz in water and 3.2% for 5 determinations in soil. Received December 6, 2000. Revision March 1, 2001.     

Electrochemical behaviors of GMP based on solid-phase extractionon at Cu-Mg-Al hydrotalcite-like compound (HTLC) modified glass carbon electrode

Guanosine-5′-monophosphate (GMP) was investigated the electrochemical behaviors based on solid-phase extractionon (SPE) at Cu-Mg-Al hydrotalcite-like compound (HTLC) modified glass carbon electrode. Cu-Mg-Al hydrotalcite-like compound (HTLC) was proved as a new sorbent for SPE of GMP, which showed an irreversible adsorption oxidation process on the HTLC/GCE with the oxidation peak potential located at 1.15 V (vs. SCE) in a pH 5.0 acetate buffer solution. Influencing factors of the electrochemical behavior of GMP on the HLTC/GCE were optimized and kinetic parameters were calculated. Under the optimal conditions, with differential pulse voltammetry (DPV), a linear relationship was obtained between the oxidation peak current and the GMP concentration in the range from 1.0 × 10^− 6 to 8.0 × 10⁻⁴ mol L⁻¹ with the detection limit as 5.0 × 10⁻⁷ mol L⁻¹ (signal-to-noise ratio of 3). The modified electrode surface has very good reproducibility and stability.     

Simultaneous voltammetric determination of dihydroxybenzene isomers using a poly(acid chrome blue K)/carbon nanotube composite electrode

A novel modified electrode was fabricated by electropolymerization of acid chrome blue K at a multi-walled carbon nanotubes modified glassy carbon electrode. The electrode developed was used for simultaneous determination of the isomers of dihydroxybenzene in environmental samples using first order linear sweep derivative voltammetry with background subtraction. A linear relationship between peak current and concentration of hydroquinone, catechol and resorcinol was obtained in the range of 1 × 10⁻⁶–1 × 10⁻⁴ mol L⁻¹, and the detection limits were estimated to be 1 × 10⁻⁷, 1 × 10⁻⁷ and 9 × 10⁻⁸ mol L⁻¹, respectively. The constructed electrode showed excellent reproducibility and stability. Real water samples were analyzed and satisfactory results were obtained. This method provides a new way of constructing electrodes for environmental and biological analysis.     

Application of rhodamine B hydrazide as a new fluorogenic indicator in the highly sensitive determination of hydrogen peroxide and glucose based on the catalytic effect of iron(III)-tetrasulfonato-phthalocyanine

A sensitive, selective and rapid spectrofluorimetric method is proposed for the determination of hydrogen peroxide using rhodamine B hydrazide as a fluorogenic substrate catalyzed by iron(III)-tetrasulfonatophthalocyanine. It is based on the oxidation of rhodamine B hydrazide, a colorless, non-fluorescent spirolactam hydrazide, by hydrogen peroxide which generates the highly fluorescent product rhodamine B. Under optimum conditions, the responses for hydrogen peroxide were linear from 2.0 × 10⁻⁸ to 2.0 × 10⁻⁶ mol L⁻¹, with a detection limit of 3.7 × 10⁻⁹ mol L⁻¹ in a 3.5 min reaction period. It can easily be incorporated into the determination of biochemical substances that produce hydrogen peroxide under catalytic oxidation in the presence of their oxidase. The possibility has been tested for the determination of glucose in human sera as an example.     

Covalent modification of a glassy carbon electrode with penicillamine for simultaneous determination of hydroquinone and catechol

A simple and highly selective electrochemical method has been developed for the simultaneous determination of hydroquinone (HQ) and catechol (CC) at a glassy carbon electrode covalently modified with penicillamine (Pen). The electrode is used for the simultaneous electrochemical determination of HQ and CC and shows an excellent electrocatalytical effect on the oxidation of HQ and CC upon cyclic voltammetry in acetate buffer solution of pH 5.0. In differential pulse voltammetric measurements, the modified electrode was able to separate the oxidation peak potentials of HQ and CC present in binary mixtures by about 103 mV although the bare electrode gave a single broad response. The determination limit of HQ in the presence of 0.1 mmol L⁻¹ CC was 1.0 × 10⁻⁶ mol L⁻¹, and the determination limit of CC in the presence of 0.1 mmol L⁻¹ HQ was 6.0 × 10⁻⁷ mol L⁻¹. The method was applied to the simultaneous determination of HQ and CC in a water sample. It is simple and highly selective.