Simultaneous voltammetric detection of dopamine and uric acid in the presence of high concentration of ascorbic acid using multi-walled carbon nanotubes with methylene blue composite film-modified electrode

An electrochemically functional nanocomposite through the adsorption of methylene blue onto the multi-walled nanotubes (MB-MWNTs) was prepared, and a sensitive voltammetric sensor was fabricated. The modified electrode showed excellent electrocatalytic activity toward dopamine (DA) and uric acid (UA) in 0.1 M phosphate solution medium (pH 3.0). Compared to the bare electrode, the MB-MWNTs film-modified electrode not only remarkably enhanced the anodic peak currents of DA and UA, i.e., shifted the anodic peak potential of DA negatively, but also avoided the overlapping of the anodic peaks of DA and UA. The interference of ascorbic acid (AA) was eliminated. Under the optimized conditions, the peak separation between AA and DA and between DA and UA was 219 and 174 mV, respectively. In the presence of 1.0 mM AA and 10.0 μM UA, the anodic peak current was linear to the concentration of DA in the range of 0.4–10.0 μM with a detection limit of 0.2 μM DA. The anodic peak current of UA was linear to the concentration in the range of 2.0–20.0 and 20.0–200.0 μM with a lowest detection limit of 1.0 μM in the presence of 1.0 mM AA and 1.0 μM DA.     

An amperometric sensor for hydrazine based on nano-copper oxide modified electrode

A sensitive hydrazine sensor has been fabricated using copper oxide nanoparticles modified glassy carbon electrode (GCE) to form nano-copper oxide/GCE. The nano-copper oxide was electrodeposited on the surface of GCE in CuCl₂ solution at −0.4 V and was characterized by Scanning electron microscopy and X-ray diffraction. The prepared modified electrode showed a good electrocatalytic activity toward oxidation of hydrazine. The electrochemical behavior of hydrazine on nano-copper oxide/GCE was explored. The oxidative current increased linearly with improving concentration of hydrazine on nano-copper oxide/GCE from 0.1 to 600 μM and detection limit for hydrazine was evaluated to be 0.03 μM at a signal-to-noise ratio of 3. The oxidation mechanism of hydrazine on the nano-copper oxide/GCE was also discussed. The fabricated sensor could be used to determine hydrazine in real water.     

Catalase biosensor for the determination of hydrogen peroxide,fluoride and cyanide

The enzyme catalase, which catalyses the decomposition of hydrogen peroxide to oxygen and water, was immobilized in a membrane by entrapping it in polyacryl amide and contacted to a Clark-type oxygen electrode. With the resulting catalase biosensor it was possible to detect the substrate hydrogen peroxide and the inhibitors fluoride and cyanide in phosphate buffer.The sensor was integrated into a flow system. In the concentration range from 5–200 mg/l a linear dependence of the peak height on the hydrogen peroxide concentration was obtained. The average decrease in activity during 30 days of storage at 6 °C was 17%. Fluoride and cyanide could be determined by measuring the inhibition of the enzymatic reaction in the same flow system. The analysis was executed in three steps; namely determination of the original activity by pumping substrate solution, inhibition of the enzyme by pumping inhibitor solution, and determination of the activity after the inhibition.The decrease in activity correlated with the inhibitor concentration of the sample, but a linear dependence was not found. The inhibition of fluoride and cyanide was both reversible, the enzyme membrane could be reactivated completely by pumping substrate solution. The detection limit was 1 mg/l for fluoride and 1.5 mg/l for cyanide.     

New enzyme sensors for morphine and codeine based on morphine dehydrogenase and laccase

Two new enzymatic methods have been developed to quantify morphine and codeine simultaneously in a flow injection system (FIA). The first enzyme sensor for morphine or codeine is based on immobilizing morphine dehydrogenase (MDH) and salicylate hydroxylase (SHL) on top of a Clark-type oxygen electrode. Morphine or codeine oxidation by MDH leads to a consumption of oxygen by SHL via the production of NADPH. This decreases the oxygen current of the Clark electrode. Concentrations of codeine and morphine are detected between 2 and 1000 μM and between 5 and 1000 μM, respectively. The second enzyme sensor for morphine is based on laccase (LACC) and PQQ-dependent glucose dehydrogenase (GDH) immobilized at a Clark oxygen electrode. Morphine is oxidized by laccase under consumption of oxygen and regenerated by glucose dehydrogenase. Since laccase cannot oxidize codeine, this sensor is selective for morphine. Morphine is detected between 32 nM and 100 μM. Both sensors can be operated simultaneously in one flow system (FIA) giving two signals without the requirement for a separation step. This rapid and technically simple method allows discrimination between morphine and codeine in less than 1 min after injection. The sampling rate for quantitative measurements is 20 h^–1. The method has been applied to the quantitative analysis of codeine or morphine in drugs. Received: 10 August 1998 / Revised: 29 January 1999 / Accepted: 5 February 1999     

Multi-wall carbon nanotubes and TiO2 as a sensor for electrocatalytic determination of epinephrinein the presence of p-chloranil as a mediator

In this work, we describe an electrochemical method using p-chloranil as a mediator and multi-wall carbon nanotube and TiO₂ as sensors for sensitive determination of epinephrine (EP) in aqueous solution at pH = 10.0. It has been found that under optimum condition (pH 10.0) in cyclic voltammetry, the oxidation of EP occurred at a potential about 171 mV less positive than that unmodified carbon nanotube paste electrode. The diffusion coefficient (D) and the kinetic parameters, such as electron transfer coefficient, (α) and heterogeneous rate constant (k _h) for EP were also determined using electrochemical approaches. The electrocatalytic currents increase linearly with the EP concentration over the range 0.6–135 μM. The detection limits for EP will be equal to 0.25 μM. The relative standard deviation percentage values for 10.0 and 15.0 μM EP were 1.7% and 1.9%, respectively. Finally, this modified electrode was also examined as a selective, simple, and precise new electrochemical sensor for the determination of EP in real sample such as urine and epinephrine injection solution.     

Electrochemical sensor using glassy carbon electrode modified with acylpyrazolone-multiwalled carbon nanotube composite film for determination of xanthine

In this work, an electrochemical sensor 1-phenyl-3-methyl-4-(2-furoyl)-5-pyrazolone/multiwalled carbon nanotubes/glassy carbon electrode (GCE) was prepared for the determination of xanthine (XN) in the presence of an excess of uric acid. Cyclic voltammetry and differential pulse voltammetry were used to characterize the electrode. The oxidation of XN occurred in a well-defined peak having E _p 0.73 V in phosphate buffer solution of pH 6.0. Compared with the bare GCE, the electrochemical sensor greatly enhanced the oxidation signal of XN with negative shift in peak potential about 110 mV. Based on this, a sensitive, rapid, and convenient electrochemical method for the determination of XN has been proposed. Under the optimized conditions, the oxidation peak current of XN was found to be proportional to its concentration in the range of 0.3~50 μM with a detection limit of 0.08 μM. The analytical utility of the proposed method was demonstrated by the direct assay of XN in urine samples and was found to be promising at our preliminary experiments.     

Electrochemical behavior and differential pulse voltammetric detection of thiobencarb on 2-(4-((4-ethoxyphenyl)diazenyl)phenylamino)ethanol-modified carbon paste electrode

Cyclic voltammetry, chronoamperometry, and rotating disk electrode voltammetry were used to investigate the electrochemical behavior of thiobencarb (TB) at carbon paste electrode modified with an azo dye, 2-(4-((4-ethoxyphenyl)diazenyl)phenylamino)ethanol (EDPE), EDPE/modified carbon paste electrode (MCPE). The modified electrode showed high electrocatalytic activity toward thiobencarb. The current was enhanced significantly relative to the situation prevailing when a bare glassy carbon electrode was used. The kinetics parameters of this process were calculated, the apparent electron transfer rate constant k _s and α (charge transfer coefficient between electrode and EDPE) were 14.6 s⁻¹ and 0.48, respectively. The experimental parameters were optimized, and the mechanism of the catalytic process was discussed. The best defined cathodic peak was obtained with 0.1 M acetate buffer (pH 3.0). The response of the sensor was very quick, and response time was approximately 5 s. The differential pulse voltammetry response of the MCPE was linear against the concentration of TB in the range of 0.96 to 106 μg L⁻¹. The limit of detection was found to be 0.025 μg L⁻¹. The precision was examined by carrying out eight replicate measurements at a concentration of 25 μg L⁻¹ TB; the relative standard deviation was 2.9%.     

Immobilization of nickel-dipicolinic acid onto a glassy carbon electrode modified with bimetallic Au-Pt inorganic-organic hybrid nanocomposite: Application to micromolar detection of fructose

This work describes the electrochemical behavior of nickel-dipicolinic acid (Ni-DPA) film immobilized on the surface of bimetallic Au-Pt inorganic-organic hybrid nanocomposite glassy carbon electrode and its electrocatalytic activity toward the oxidation of fructose. The electrode possesses a three-dimensional (3D) porous network nano architecture, in which the bimetallic Au-Pt serving as metal nano-particle based microelectrode ensembles are distributed in the matrix of interlaced 3,3′,5,5′-tetramethylbenzidine (TMB) organic nanofibers (NFs). The surface structure and composition of the sensor was characterized by scanning electron microscopy (SEM). Electrocatalytic oxidation of fructose on the surface of modified electrode was investigated with cyclic voltammetry and chronoamperometry methods and the results show that the Ni-DPA film displays excellent electrochemical catalytic activities towards fructose oxidation. The hydrodynamic amperometry at rotating modified electrode at constant potential versus reference electrode was used for detection of fructose. Under optimized conditions the calibration plots are linear in the concentration range 0.5 to 70 μM and detection limit was found to be 0.1 μM.     

Electrochemistry and electrocatalytic activity of catechin film on a glassy carbon electrode toward the oxidation of hydrazine

A very stable electroactive film of catechin was electrochemically deposited on the surface of activated glassy carbon electrode. The electrochemical behavior of catechin modified glassy carbon electrode (CMGCE) was extensively studied using cyclic voltammetry. The properties of the electrodeposited films, during preparation under different conditions, and the stability of the deposited film were examined. The charge transfer coefficient (α) and charge transfer rate constant (k _s) for catechin deposited film were calculated. It was found that the modified electrode exhibited excellent electrocatalytic activity toward hydrazine oxidation and it also showed a very large decrease in the overpotential for the oxidation of hydrazine. The CMGCE was employed to study electrocatalytic oxidation of hydrazine using cyclic voltammetry, rotating disk voltammetry, chronoamperometry, amperometry and square-wave voltammetry as diagnostic techniques. The catalytic rate constant of the modified electrode for the oxidation of hydrazine was determined by cyclic voltammetry, chronoamperometry and rotating disk voltammetry and was found to be around 10⁻³ cm s⁻¹ . In the used different voltammetric methods, the plot of the electrocatalytic current versus hydrazine concentration is constituted of two linear segments with different ranges of hydrazine concentration. Furthermore, amperometry in stirred solution exhibits a detection limit of 0.165 μM and the precision of 4.7% for replicate measurements of 40.0 μM solution of hydrazine.     

A novel nitrite sensor based on poly-1-naphthylamine doped by a ferrocenesulfonic-acid-modified electrode

A novel sensor for detecting nitrite based on poly-1-naphthylamine doped by a ferrocenesulfonic acid (PNAFc) modified electrode has been proposed. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) have confirmed that ferrocene-sulfonic acid (Fc) can be doped in poly-1-naphthylamine (PNA) and enhances its electrochemical activity. In a nitrite solution, the PNAFc electrode shows an excellent electrocatalytic activity for the oxidation of nitrite. Based on differential pulse voltammetry (DPV), the response was evaluated with respect to pH, scan rate, temperature, and other variables. The optimum analytical conditions for the determination of nitrite were established. Under optimum conditions, the linear range for nitrite determination was from 0.5 to 100 μM with a detection limit of 0.5 μM. The stability and anti-interference ability of the PNAFc electrode were also evaluated. The results indicate that this sensor is feasible for the determination of nitrite in real water samples. The text was submitted by the authors in English.     

Cell biosensor for detection of phenol in aqueous solutions

A microbial sensor for concentration measurement of phenol in aqueous solutions has been developed. Phenol-utilizing cellsPseudomonas putida GFS-8 immobilized in poly(vinyl)alcohol cryogel were used as a biological transducer. Relationships between phenol concentration in the activating medium and endogenic cell respiration have been established. Cell respiration and phenol concentration in the assay solution positively correlated at a phenol concentration range of 0.1–2.0 mg/L and were linearly dependent in the range of 0.1–1.0 mg/L. A Clark membrane electrode was the physiochemical transducer. The assay may be completed within 5 min. The cells oxidize phenol, pyrocatechol, mesityl oxide, aniline, and do not react with a number of xenobiotics, sugars, and alcohol. With the exception of aniline, most components found in waste waters from phenol production affect neither the assay process nor the ability of these cells to use phenol as exogenic respiratory substrate. The immobilized cells retained their ability to utilize phenol as an exogenic respiratory substrate for up to 1 mo.     

Microwave-enhanced electrochemical processes in micellar surfactant media

High intensity microwave radiation effects are demonstrated for electron transfer processes at 25 or 50-μm diameter platinum electrodes immersed in micellar sodium dodecylsulfate (SDS) solutions. First, a solution containing 2 mM Fe(CN)₆³⁻ and 2 mM Fe(CN)₆⁴⁻ in aqueous 0.1 M NaCl with and without SDS is employed to calibrate the electrode temperature and mass transport conditions. Addition of 0.1 M SDS has only a small effect on the microwave enhanced voltammetry for the Fe(CN)₆^3-/4− system. Next, two highly water-insoluble redox systems are studied. A solution of 1 mM tert-butylferrocene in aqueous 0.1 M NaCl containing 0.1 M SDS is shown to give no current response in the absence of microwaves. In the presence of focused microwaves at a platinum disc electrode, a strong current for the one electron oxidation of tert-butylferrocene is detected presumably due to localized disruption of the micellar solution. Concentrations of tert-butylferrocene down to the micromolar level are detected. α-Tocopherol, a lipophilic vitamin and antioxidant, is soluble in aqueous 0.1 M SDS/0.1 M NaCl. In the presence of microwave radiation, a strong and concentration dependent anodic current response consistent with the two-electron oxidation of α-tocopherol is observed. A heptode array of seven individual 50 μm diameter platinum microelectrodes placed in ca. 720 μm distance of each other is shown to allow microwave enhanced currents to be increased sevenfold with each electrode exhibiting the same microwave effect.     

Dopamine sensor based on molecularly imprinted electrosynthesized polymers

Molecularly imprinted polymers (MIPs) have been applied as molecular recognition elements to chemical sensors. In this paper, we combined the use of MIPs and electropolymerization to produce a sensor which was capable of detecting dopamine (DA). The MIP electrode was obtained by electrocopolymerization of o-phenylenediamine and resorcinol in the presence of the template molecular DA. The MIP electrode exhibited a much higher current response compared with the non-imprinted electrode. The response of the imprinted sensor to DA was linearly proportional to its concentration over the range 5.0 × 10⁻⁷-4.0 × 10⁻⁵ M. The detection limit of DA is 0.13 μM (S/N = 3). Moreover, the proposed method could discriminate between DA and its analogs, such as ascorbic acid and uric acid. This method was successfully applied to the determination of DA in dopamine hydrochloride injection and healthy human blood serum. These results revealed that such a sensor fulfilled the selectivity, sensitivity, sped, and simplicity requirements for DA detection and provided possibilities of clinical application in physiological fields.     

A new sensor for electrochemical determination of captopril using chlorpromazine as a mediator at a glassy carbon electrode

A new simple, selective and rapid cyclic voltammetric method is reported for the accurate and precise determination of captopril using chlorpromazine as a suitable electrocatalyst. It has been shown by cyclic voltammetry, single step chronoamperometry and electrochemical impedance spectroscopy that chlorpromazine can catalyze the oxidation of captopril in aqueous buffer solution and produces a sharp oxidation peak current at about 0.625 V vs. saturated calomel reference electrode. The catalytic oxidation peak current of captopril is linearly dependent on its concentration and enables the determination of captopril over the concentration range of 8–1000 μM at pH 5.0, with a detection limit of 4.8 μM. The relative standard deviation for the determination of 400 μM captopril is 0.66% (n = 9). The influence of potential interfering substances on the determination of captopril was studied. The method was satisfactorily applied to the determination of captopril in real samples such as drug and urine.     

Sensitive amperometric detection of omeprazole and pantoperazole at electrodeposited nickel oxide nanoparticles modified glassy carbon electrode

Glassy carbon electrode modified with electrodeposited nickel oxide nanoparticles (NiOxNPs) was used as electrocatalyst for oxidation of omeprazole and pentoperazole in alkaline solution. The modified electrode exhibited efficient electrocatalytic activity for the oxidation of omeprazole and pentoperazole with relatively high sensitivity, excellent stability, and long lifetime. Hydrodynamic amperometric method is used for determination of selected analytes. Under optimized condition, the linear concentration range, detection limit, and sensitivity of modified electrode toward omeprazole detection are 4.5–120 μM, 0.4 μM (at signal to noise 3), and 40.1 nA μM⁻¹ cm⁻², respectively. For pantoperazole, hydrodynamic amperometric determination yielded calibration curve with linear range of 2.5–180 μM, detection limit of 0.2 μM, and sensitivity of 39.2 nA μM⁻¹ cm⁻², respectively. The proposed method was successfully applied to pentoperazole and omeprazole determination in drug samples.     

Development of an amperometric sulfite biosensor based on a gold nanoparticles/chitosan/multiwalled carbon nanotubes/polyaniline-modified gold electrode

A sulfite oxidase (SOx) purified from leaves of Syzygium cumini (Jamun) was immobilized covalently onto a gold nanoparticles (AuNPs)/chitosan (CHIT)/carboxylated multiwalled carbon nanotubes (cMWCNTs)/polyaniline (PANI) composite that was electrodeposited onto the surface of a gold (Au) electrode. A novel and highly sensitive sulfite biosensor was developed that used this enzyme electrode (SOx/AuNPs/CHIT/cMWCNT/PANI/Au) as the working electrode, Ag/AgCl as the standard electrode, and Pt wire as the auxiliary electrode. The modified electrode was characterized by Fourier transform infrared (FTIR) spectroscopy, cyclic voltammetry (CV), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) before and after the immobilization of the SOx. The sensor produced its optimum response within 3 s when operated at 50 mVs⁻¹ in 0.1 M phosphate buffer, pH 7.0, and at 35 °C. The linear range and detection limit of the sensor were 0.75–400 μM and 0.5 μM (S/N = 3), respectively. The biosensor was employed to determine sulfite levels in fruit juices and alcoholic beverages. The enzyme electrode was used 300 times over a period of three months when stored at 4 °C.     

Single-drop method for determination of cyanide in solution with a piezoelectric quartz crystal

The change in frequency of a horizontal quartz crystal in contact with a single drop of solution is measured. When the gold electrode of the crystal is dissolved by reaction with cyanide in alkaline solution, the further change of frequency is linearly related to cyanide concentration in the range 10^-3–10^-4 M at pH 10.4. Only silver(I) and mercury(II) interfere if EDTA is added.     

Electrochemical solid phase micro-extraction and determination of salicylic acid from blood samples by cyclic voltammetry and differential pulse voltammetry

The electrochemical solid phase micro-extraction of salicylic acid (SA) at graphite-epoxy-composed solid electrode surface was studied by cyclic voltammetry. SA was oxidized electrochemically in pH 12.0 aqueous solution at 0.70 V (vs. saturated calomel electrode) for 7 s. The oxidized product shows two surface-controlled reversible redox couples with two proton transferred in the pH range of 1.0∼6.0 and one proton transferred in the pH range of 10.0∼13.0 and is extracted on the electrode surface with a kinetic Boltzman function of i _p = 3.473–4.499/[1 + e^{(t − 7.332)/6.123}] (χ ² = 0.00285 μA). The anodic peak current of the extracted specie in differential pulse voltammograms is proportional to the concentration of SA with regression equation of i _p = −5.913 + 0.4843 c (R = 0.995, SD = 1.6 μA) in the range of 5.00∼200 μM. The detection limit is 5.00 μM with RSD of 1.59% at 60 μM. The method is sensitive and convenient and was applied to the detection of SA in mouse blood samples with satisfactory results.     

Carbon ceramic electrode incorporated with zeolite ZSM-5 for determination of Piroxicam

A new chemically modified electrode is constructed based on carbon ceramic electrode incorporated with zeolite ZSM-5. Voltammetric behavior of piroxicam at the carbon ceramic zeolite modified electrode (CCZME) was investigated. The modified electrode exhibited catalytic activity toward the electrooxidation of piroxicam. Experimental parameters such as solution pH, scan rate, concentration of piroxicam and zeolite amount were studied. It has been shown that using the CCZME, piroxicam can be determined by differential pulse voltammetry (DPV) and hydrodynamic amperometry (HA). Under the optimized conditions the calibration plots are linear in the concentration ranges of 0.20–25.00 and 0.20–50.10 μM with limit of detections of 0.65 and 0.29 μM for DPV and HA, respectively. The modified electrode with DPV and HA methods was successfully applied for analysis of piroxicam in pharmaceutical formulations. The results were favorably compared to those obtained by the spiked method. The results of the analysis suggest that the proposed method has promise for the routine determination of piroxicam in the products examined.     

Self-assembled sulfonated β-Cyclodextrin layer on gold electrode for the selective electroanalysis of dopamine

Gold electrode with self-assembled D,L-cysteine grafted β-cyclodextrin sulfonic acid (Cys-β-CD∼SO₃) layer was fabricated and used to investigate the electrochemical behavior of dopamine. The experimental results indicated that the self-assembled Cys-β-CD∼SO₃ layer modified gold electrode has selective electrochemical response to dopamine with high sensitivity and excellent tolerance of ascorbic acid, which is the most common accompanying component in biological samples. Dopamine could be accurately determined in the concentration range of 1–200 μM in the presence of ascorbic acid of 5 mM. The relative standard deviation of 1.9% (n = 5) was achieved at a dopamine concentration of 5 × 10⁻⁵ M. The proposed sensor was successfully applied to the determination of dopamine in human blood serum samples.