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.     

Selective determination of uric acid in the presence of ascorbic acid using a penicillamine self-assembled gold electrode

The electrochemical behaviors of uric acid (UA) at the penicillamine (Pen) self-assembled monolayers modified gold electrode (Pen/Au) have been studied. The Pen/Au electrode is demonstrated to promote the electrochemical response of UA by cyclic voltammetry (CV). The diffusion coefficient D of UA is 6.97 × 10⁻⁶ cm² s⁻¹. In differential pulse voltammetric (DPV) measurements, the Pen/Au electrode can separate the UA and ascorbic acid (AA) oxidation potentials by about 120 mV and can be used for the selective determination of UA in the presence of AA. The detection limit was 1 × 10⁻⁶ mol L⁻¹. The modified electrode shows excellent sensitivity, good selectivity and antifouling properties.     

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.     

Simultaneous Determination of Dopamine and Ascorbic Acid at a Triazole Self-Assembled Monolayer-Modified Gold Electrode

A 3-amino-5-mercapto-1,2,4-triazole (TA) self-assembled monolayer-modified gold electrode (TA SAM/Au) is characterized by X-ray photoelectron spectroscopy, A.C. impedance, cyclic voltammetry, chronoamperometry and chronocoulometry. The TA SAM/Au exhibited good promotion of the electrochemical oxidation of dopamine. Some electrochemical parameters of dopamine such as electron transfer number, exchange current density, standard heterogeneous rate constant, diffusion coefficient, etc., were measured by different electrochemical methods. The peak currents of dopamine were linearly dependent on its concentration in the range of 1.5 × 10⁻⁶–1.0 × 10⁻⁴ mol L⁻¹, with a detection limit of 5.0 × 10⁻⁷ mol L⁻¹. The oxidative peak potentials of dopamine and ascorbic acid were well separated at about 190 ± 10 mV in pH 2.0 BR buffers at TA SAM/Au, the oxidation peak current increases approximately linearly with increasing concentration of both dopamine and ascorbic acid in the concentration range of 9.98 × 10⁻⁶–4.54 × 10⁻⁴ mol L⁻¹. It can be used for simultaneous determination of dopamine and ascorbic acid.     

Simultaneous voltammetric determination of ascorbic acid and uric acid using a Nafion/multi-wall carbon nanotubes composite film-modified electrode

A Nafion/multi-wall carbon nanotubes (MWNT) composite film-modified electrode was fabricated. The modified electrode showed excellent electrocatalytic activity toward ascorbic acid (AA) and uric acid (UA) in 0.1-mol L⁻¹ NaCl medium (pH 6.5). Compared to the bare electrode that only displayed a broad and overlapped oxidation peak, the Nafion/MWNT film-modified electrode not only remarkably enhanced the anodic peak currents of AA and UA but also avoided the overlapping of the anodic peaks of AA and UA with a 320-mV separation of both peaks. Under the optimized conditions, the peak currents of AA and UA were proportional to their concentration at the ranges of 8.0 × 10⁻⁵ to 6.0 × 10⁻³ mol L⁻¹ and 6.0 × 10⁻⁷ to 8.0 × 10⁻⁵ mol L⁻¹, respectively. The proposed method was used for the detection of AA and UA in real samples with satisfactory results.     

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.     

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.     

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.     

Sensitive voltammetric sensor of dihydromyricetin based on Nafion/SWNT-modified glassy carbon electrode

A novel voltammetric sensor, based on single-walled carbon nanotubes (SWNT) dispersed in Nafion and modified glassy carbon electrode (GCE), was fabricated and used to determine the trace amounts of dihydromyricetin (DMY). The electrochemical behavior of DMY at this sensor was investigated in 0.1 mol L⁻¹ sulfuric acid solutions + 0.1 mol L⁻¹ NaCl by cyclic voltammetry and squarewave voltammetry. Compared with bare GCE, the electrode presented an excellent response of DMY through an adsorption-controlled quasi-reversible process. Under the optimum conditions, the response peak currents were linear relationship with the DMY concentrations in the range of 1.0 × 10⁻⁷–1.0 × 10⁻⁵ mol L⁻¹ with a detection limit of 9 × 10⁻⁸ mol L⁻¹. Based on this voltammetric sensor, a simple and sensitive electroanalytical method for DMY was proposed and applied to quantitative determination of DMY in Ampelopsis grossedentata samples. In addition, the oxidation mechanism was proposed and discussed, which could be a reference for the pharmacological action of DMY in clinical study.     

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 determination of uric acid and ascorbic acid at a ferrocenium–thioglycollate modified electrode

Self-assembled monolayers (SAMS) of chemisorbed thioglycollate on a gold electrode surface have been used as a base interface for the electrostatic adsorption of ferrocenium ion. Electrochemical impedance spectra (EIS) and cyclic voltammetry (CV) were used to evaluate the electrochemical properties of the supramolecular film. The bare gold electrode failed to distinguish the oxidation peaks of ascorbic acid (AA) and uric acid (UA) in phosphate buffer solution (PBS, pH 7.0), while the ferricinium–thioglycollate modified electrode could separate them efficiently. In differiential pulse voltammetric measurements, the prepared gold electrode could separate AA and UA signals, allowing the simultaneous determination of AA and UA. Under optimal conditions and within the linear range of 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ M, the detection limits of AA and UA achieved were 2.0 × 10⁻⁷ and 1.0 × 10⁻⁷ M, respectively. The applicability of the prepared electrode was demonstrated by measuring AA and UA in human urine without any pretreatment. Figure Fabrication process for the modified electrode     

Voltammetric Analysis of the Novel Atypical Antipsychotic Drug Quetiapine in Human Serum and Urine

The electrooxidative behaviour and determination of quetiapine (QTP), a dibenzothiazepine derivative and antipsychotic agent, on a glassy carbon disc electrode was investigated using cyclic (CV), linear sweep (LSV), differential pulse (DPV) and Osteryoung square wave voltammetry (OSWV). Fully validated DP and SW voltammetric procedures are described for the determination of QTP. QTP in pH 3.5 acetate buffer solution presents a well-defined anodic response, studied by the proposed methods. This main response was due to the irreversible, diffusion-controlled, one-electron and one-proton oxidation of the aliphatic nitrogen of the piperazine ring. Under optimal conditions, a detection limit of 4.0 × 10⁻⁸ mol L⁻¹ for DPV and 1.33 × 10⁻⁷ mol L⁻¹ for OSWV, and a linear calibration graph in the range from 4.0 × 10⁻⁶ to 2.0 × 10⁻⁴ mol L⁻¹ were obtained for both methods. The procedure was successfully applied to the determination of the drug in tablets, human serum and human urine with good recoveries. The detection limits were 6.20 × 10⁻⁷ mol L⁻¹ and 5.92 × 10⁻⁷ mol L⁻¹ in human serum and 1.44 × 10⁻⁷ mol L⁻¹ and 1.31 × 10⁻⁶ mol L⁻¹ in human urine, for the DPV and OSWV method, respectively.     

Electrogenerated Chemiluminescence (ECL) Determination of Ephedrine with a Self-Assembly Multilayer Ni(II)-Polyluminol Modified Electrode

By combining the layer-by-layer (LBL) self-assembly technique with the electrochemical polymerization method, multilayer Ni(II)-polyluminol films were modified on the surface of a vaseline-impregnated graphite electrode. It was found that, compared with an electrode modified by direct electrochemical polymerization, this modified electrode offered a suitable ECL reaction micro-environment created by the special multilayer films, which was beneficial to the ephedrine hydrochloride enhancing effect for luminol ECL intensity. The ECL enhancing effect of ephedrine hydrochloride on the electro-oxidation luminol was improved on this modified electrode. Based on this finding, a new sensitive ECL method was developed for ephedrine hydrochloride determination under the optimal conditions. At the same time, a new idea is proposed for improving the analytical performance of the luminol ECL system by modifying the ECL reaction micro-environment with the layer-by-layer self- assembly method. Under the optimum experimental conditions, the ephedrine hydrochloride concentration in the range of 2.0 × 10⁻⁸–7.0 × 10⁻⁶ mol L⁻¹ was proportional to the enhanced ECL signal, and it offered an 8.0 × 10⁻⁹ mol L⁻¹ detection limit for ephedrine hydrochloride.     

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.     

Development of electrochemical DNA biosensor for Trichoderma harzianum based on ionic liquid/ZnO nanoparticles/chitosan/gold electrode

Electrochemical DNA biosensor was successfully developed by depositing the ionic liquid (e.g., 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][Otf])), ZnO nanoparticles, and chitosan (CHIT) nanocomposite membrane on a modified gold electrode (AuE). The electrochemical properties of the [EMIM][Otf]/ZnO/CHIT/AuE for detection of DNA hybridization were studied. Under optimal conditions using cyclic voltammetry, the target DNA sequences could be detected in the concentration range of 1.0 × 10⁻¹⁸ to 1.82 × 10⁻⁴ mol L⁻¹, and with the detection limit of 1.0 × 10⁻¹⁹ mol L⁻¹. This DNA biosensor detection approaches provide a quick, sensitive, and convenient method to be used in the identification of Trichoderma harzianum.     

Electrochemical methods for simultaneous determination of trace amounts of dopamine and uric acid using a carbon paste electrode incorporated with multi-wall carbon nanotubes and modified with α-cyclodextrine

In this work, we investigate the electrochemical activity of dopamine (DA) and uric acid (UA) using both a bare and a modified carbon paste electrode as the working electrode, with a platinum wire as the counter electrode and a silver/silver chloride (Ag/AgCl) as the reference electrode. The modified carbon paste electrode consists of multi-walled carbon nanotubes (>95%) treated with α-cyclodextrine, resulting in an electrode that exhibits a significant catalytic effect toward the electro-chemical oxidation of DA in a 0.2-M Britton–Robinson buffer solution (pH 5.0). The peak current increases linearly with the DA concentration within the molar concentration ranges of 2.0 × 10⁻⁶ to 5.0 × 10⁻⁵ M and 5.0 × 10⁻⁵ to 1.9 × 10⁻⁴ M. The detection limit (signal to noise >3) for DA was found to be 1.34 × 10⁻⁷ M, respectively. In this work, voltammetric methods such as cyclic voltammetry, chronoamperometry, chronocuolometry, differential pulse and square wave voltammetry, and linear sweep and hydrodynamic voltammetry were used. Cyclic voltammetry was used to investigate the redox properties of the modified electrode at various scan rates. The diffusion coefficient (D, cm² s⁻¹ = 3.05 × 10⁻⁵) and the kinetic parameters such as the electron transfer coefficient (α = 0.51) and the rate constant (k, cm³ mol⁻¹ s⁻¹ = 1.8 × 10³) for DA were determined using electrochemical approaches. By using differential pulse voltammetry for simultaneous measurements, we obtained two peaks for DA and UA in the same solution, with the peak separation approximately 136 mV. The average recovery was measured at 102.45% for DA injection.     

Selective and sensitive molecularly imprinted sol–gel film-based electrochemical sensor combining mercaptoacetic acid-modified PbS nanoparticles with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@Au–multi-walled carbon nanotubes–chitosan

A sensitive molecularly imprinted electrochemical sensor was developed for selective detection of streptomycin by combination of mercaptoacetic acid-modified PbS nanoparticles with Au-coated Fe₃O₄ magnetic nanoparticles dispersed multi-walled carbon nanotubes doped chitosan film. The imprinted sensor was fabricated onto the Au electrode via stepwise modification of nanocomposites and an electrodeposited thin film of molecularly imprinted polymers via sol–gel technology. The morphologies and electrochemical behaviors of the imprinted sensor were characterized by scanning electron microscope, cyclic voltammetry, and differential pulse voltammetry, respectively. The prepared sensor showed very high recognition ability and selectivity for streptomycin. Under optimal conditions, the imprinted sensor displayed good electrocatalytic activity to the redox of streptomycin. And the differential voltammetric anodic peak current was linear to the logarithm of streptomycin concentration in the range from 1.0 × 10⁻⁶ to 1.0 × 10⁻³ mol L⁻¹, and the detection limit obtained was 1.5 × 10⁻⁹ mol L⁻¹. This proposed imprinted sensor was used successfully for streptomycin determination in different injection solution samples.     

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.     

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.     

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.