Amperometric enzyme electrodes for the determination of volatile alcohols in the headspace above fruit and vegetable juices

The surface of a glassy carbon electrode (GCE) was modified by electropolymerization of acridine red followed by drop-coating of graphene. The morphology was characterized by scanning electron microscopy. Uric acid (UA) is effectively accumulated on the surface of the modified electrode and generates a sensitive anodic peak in solutions of pH 6.5. Differential pulse voltammetry was used to evaluate the electrochemical response of the modified GCE to UA. Compared to the bare GCE, the GCE modified with acridine red, and to the graphene modified electrode, the new GCE displays high electrochemical activity in giving an oxidation peak current that is proportional to the concentration of UA in the range from 0.8 to 150?μM, with a detection limit of 0.3?μM (at an S/N of 3). The modified electrode displays excellent selectivity, sensitivity, and a wide linear range. It has been applied to the determination of UA in real samples with satisfactory results.

A glassy carbon electrode modified with antimony and poly(p-aminobenzene sulfonic acid) for sensing lead(II) by square wave anodic stripping voltammetry

We have developed a sensor for the square wave anodic stripping voltammetric determination of Pb(II). A glassy carbon electrode was modified with a thin film of an antimony/poly(p-aminobenzene sulfonic acid) composite in air-saturated aqueous solution of pH 2.0. Compared to a conventional antimony film electrode, the new one yields a larger stripping signal for Pb(II). The conditions of polymerization, the concentration of Sb(III), the pH value of the sample solution, the deposition potential and time, frequency, potential amplitude, and step increment potential were optimized. Under the optimum conditions, a linear response was observed for Pb(II) in the range of 0.5 to 150.0 μg?L^?1. The detection limit for Pb(II) is 0.1 μg?L^?1.

The electrochemistry of uric acid at a gold electrode modified with L-cysteine, and its application to sensing uric urine

The electrochemistry of uric acid at a gold electrode modified with a self-assembled film of L-cysteine was studied by cyclic voltammetry and differential pulse voltammetry. Compared to the bare gold electrode, uric acid showed better electrochemical response in that the anodic peak current is stronger and the peak potential is negatively shifted by about 100 mV. The effects of experimental conditions on the oxidation of uric acid were tested and a calibration plot was established. The differential pulse response to uric acid is linear in the concentration range from 1.0?×?10^?6 to ~?1.0?×?10^?4 mol?L^?1 (r?=?0.9995) and from 1.0?×?10^?4 to ~?5.0?×?10^?4 mol?L^?1 (r?=?0.9990), the detection limit being 1.0?×?10^?7 mol?L^?1 (at S/N?=?3). The high sensitivity and good selectivity of the electrode was demonstrated by its practical application to the determination of uric acid in urine samples.

Sensitive voltammetric determination of rutin in pharmaceuticals, human serum, and traditional Chinese medicines using a glassy carbon electrode coated with graphene nanosheets, chitosan, and a poly(amido amine) dendrimer

The electrochemical behavior of rutin was investigated in pH 6.0 buffer solution using a glassy carbon electrode coated with graphene nanosheets, chitosan and a poly (amidoamine) dendrimer in pH?6.0 buffer solution. The results indicate that the modified electrode displays electrochemical redox activity towards rutin, and that the oxidation peak current of rutin increases significantly compared to that at other electrodes. The amount of immobilized graphene and dendrimer, pH value, scan rate, accumulation time and accumulation potential were optimized. The kinetic parameters, charge transfer coefficient, transfer electron number, proton transfer number, standard rate constant, were calculated. Under the optimized conditions, the oxidation peak current is proportional to the concentration of rutin in the range between 0.001 and 2.0???mol L^?1 (R?=?0.9991). The detection limit is 0.6?nmol L^?1 (at S/N?=?3). The electrode exhibits satisfactory selectivity and reproducibility and was applied to the determination of rutin in pharmaceutical preparations, spiked human serum, and traditional Chinese medicine, with recoveries between 97.2 and 104.67%.

Carbon paste electrode modified with a binuclear manganese complex as a sensitive voltammetric sensor for tryptophan

We report on a carbon paste electrode that was modified with a binuclear manganese(II) complex by the drop-coating method. A study on the mechanism of the electro-oxidation of tryptophan (Trp) at this electrode indicated that it enables Trp to be determined with good sensitivity and selectivity. Second-order derivative linear sweep voltammetry at pH 4.1 revealed that a sensitive anodic peak appears at 812?mV (vs. SCE) whose current is proportional to the concentration of Trp in the concentration range from 0.1 to 1.0???mol?L^?1 and 1.0 to 80???mol?L^?1, with a detection limit (S/N?=?3) of 0.08???mol?L^?1 (60?s of accumulation). The method was applied to the determination of Trp in amino acid injection solutions with satisfactory results.

A thin poly(acridine orange) film containing reduced graphene oxide for voltammetric simultaneous sensing of ascorbic acid and uric acid

We have fabricated, in a single step, carbon ceramic electrodes modified with a poly(acridine orange) film containing reduced graphene oxide. They display electrocatalytic activity to ascorbic acid (AA) and uric acid (UA) at pH 4.5. The anodic peak potentials of AA and UA are separated by 276 mV so that they can be well resolved in cyclic voltammetry. UA and AA were simultaneously determined in a mixture at working potentials of 170 and 400 mV, respectively. Under optimized conditions, the calibration curves for AA and UA cover the 0.8–5,000 μM and 0.6–900 μM concentration range, respectively, while detection limits are 0.3 μM and 0.2 μM. The electrode was applied to determine AA and UA in urine samples.

A glassy carbon electrode modified with MoS<Subscript>2</Subscript> nanosheets and poly(3,4-ethylenedioxythiophene) for simultaneous electrochemical detection of ascorbic acid,dopamine and uric acid

We describe a chemical exfoliation method for the preparation of MoS₂ nanosheets. The nanosheets were incorporated into poly(3,4-ethylenedioxythiophene) (PEDOT) by electrodeposition on a glassy carbon electrode (GCE) to form a nanocomposite. The modified GCE is shown to enable simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Due to the synergistic effect of MoS₂ and PEDOT, this electrode displays better properties in terms of electrocatalytic oxidation of AA, DA and UA than pure PEDOT, which is illustrated by cyclic voltammetry and differential pulse voltammetry (DPV). Under optimum conditions and at pH 7.4, the respective sensitivities and best working potentials are as follows: AA: 1.20 A?mM^?1?m^?2, 30 mV; DA: 36.40 A?mM^?1?m^?2, 210 mV; UA: 105.17 A?mM^?1?m^?2, 350 mV. The calculated detection limits for AA, DA and UA are 5.83 μM, 0.52 μM and 0.95 μM, respectively. The modified electrode was applied to the detection of the three species in human urine samples and gave satisfactory results.

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Graphical abstract MoS₂ nanosheets were prepared by a facile chemical exfoliation method. MoS₂ and poly(3,4-ethylenedioxythiophene) nanocomposite modified glassy carbon electrodes were fabricated, which are shown to enable simultaneous determination of ascorbic acid, dopamine and uric acid with high sensitivity and selectivity.

     

Simultaneous determination of dopamine and uric acid in the presence of ascorbic acid using a gold electrode modified with carboxylated graphene and silver nanocube functionalized polydopamine nanospheres

A voltammetric sensor is presented for the simultaneous determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA). It is based on a gold electrode (GE) modified with carboxyl-functionalized graphene (CFG) and silver nanocube functionalized DA nanospheres (AgNC@PDA-NS). The AgNC@PDA-NS nanocomposite was characterized by scanning electron microscopy and UV-Vis spectroscopy. The electrochemical behavior of the modified electrode was evaluated by electrochemical impedance spectroscopy, cyclic voltammetry and differential pulse voltammetry. The modified electrode displays good electrocatalytic activity towards DA (typically at 0.14 V vs. Ag/AgCl) and UA (typically at 0.29 V vs. Ag/AgCl) even in the presence of ascorbic acid. Response to DA is linear in the concentration range of 2.5 to 130 μM with a detection limit of 0.25 μM. Response to UA is linear in the concentration range of 10 to 130 μM with a detection limit of 1.9 μM. In addition, the sensitivity for DA and UA is 0.538 and 0.156 μA μM^?1 cm^?2, respectively. The modified electrode also displays good stability, selectivity and reproducibility.

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Graphical abstract The gold electrode modified with polydopamine nanospheres functionalized with silver nanocube and carboxylated graphene is used for simultaneous determination of DA and UA in the presence of AA, with wide linear range and low detection limit.

     

Determination of thiourea using a carbon paste electrode decorated with copper oxide nanoparticles

We report on a novel sensor for the electrochemical determination of thiourea (TU). It is based on an active carbon paste electrode modified with copper oxide nanoparticles. The modified electrode and the electrochemical properties of thiourea on its surface were investigated using cyclic voltammetry and differential pulse voltammetry. Under optimized conditions, the detection limit is 20 μg?L^?1 of TU. The method was applied to the determination of thiourea in fruit juice, orange peel and industrial waste water.

Electrochemical determination of paraquat using a DNA-modified carbon ionic liquid electrode

Deoxyribonucleic acid (DNA) was electrochemically deposited on a carbon ionic liquid electrode to give a biosensor with excellent redox activity towards paraquat as shown by cyclic voltammetry and differential pulse voltammetry. Experimental conditions were optimized with respect to sensing paraquat by varying the electrochemical parameters, solution pH, and accumulation time of DNA. Under the optimized conditions, a linear relation exists between the reduction peak current and the concentration of paraquat in the range from 5?×?10^?8 mol L^?1 to 7?×?10^?5 mol L^?1, with a detection limit of 3.6?×?10^?9 mol L^?1. The utility of the method is illustrated by successful analysis of paraquat in spiked real water samples.

Voltammetric detection of guanosine and adenosine using a carbon paste electrode modified with 1-ethyl-3-methylimidazolium ethylsulfate

A novel kind of carbon paste electrode (CPE) was prepared by mixing graphite powder, liquid paraffin and the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate. The resulting electrode was used for the simultaneous determination of guanosine and adenosine by differential pulse voltammetry. Compared to a conventional CPE, the oxidation peak currents are largely increased, and the oxidation peak potentials are negatively shifted. The electrochemical responses to guanosine and adenosine were investigated. Under optimized conditions, the calibration curves are linear in the concentration range from 1.0?×?10^-6?mol?L^-1 to 1.6?×?10^-4?mol?L^-1 for guanosine, and from 1.0?×?10^-6?mol?L^-1 to 2.7?×?10^-4?mol?L^-1 for adenosine at pH 3.5. Substances potentially interfering in the biological matrix do no interfere. The method was successfully applied to detect adenosine and guanosine in human urine without sample treatments.

Square wave anodic stripping voltammetric determination of lead(II) using a glassy carbon electrode modified with a lead ionophore and multiwalled carbon nanotubes

We report on a glassy carbon electrode (GCE) modified with a lead ionophore and multiwalled carbon nanotubes. It can be applied to square wave anodic stripping voltammetric determination of Pb(II) ion after preconcentration of Pb(II) at ?1.0?V (vs. SCE) for 300?s in pH?4.5 acetate buffer containing 400?μg?L^?1 of Bi(III). The ionophore-MWCNTs film on the GCE possesses strong and highly selective affinity for Pb(II) as confirmed by quartz crystal microbalance experiments. Under the optimum conditions, a linear response was observed for Pb(II) ion in the range from 0.3 to 50?μg?L^?1. The limit of detection (at S/N?=?3) is 0.1?μg?L^?1. The method was applied to the determination of Pb(II) in water samples with acceptable recovery.

Electrochemical synthesis of a graphene sheet and gold nanoparticle-based nanocomposite, and its application to amperometric sensing of dopamine

We describe a simple, green and controllable approach for electrochemical synthesis of a nanocomposite made up from electrochemically reduced graphene oxide (ERGO) and gold nanoparticles. This material possesses the specific features of both gold nanoparticles and graphene. Its morphology was characterized by scanning electron microscopy which reveals a homogeneous distribution of gold nanoparticles on the graphene sheets. Cyclic voltammetry was used to evaluate the electrochemical properties of this nanocomposite towards dopamine by modification of it on surface of glassy carbon electrode (GCE). Compared to the bare GCE, the electrode modified with gold nanoparticles, and the electrode modified with ERGO, the one modified with the nanocomposite displays better electrocatalytic activity. Its oxidation peak current is linearly proportional to the concentration of dopamine (DA) in the range from 0.1 to 10?μM, with a detection limit of 0.04?μM (at S/N?=?3). The modified electrode also displays good storage stability, reproducibility, and selectivity.

Glassy carbon electrode modified with organic�Cinorganic pillared montmorillonites for voltammetric detection of mercury

A glassy carbon electrode modified with organic?Cinorganic pillared montmorillonite was used for voltammetric detection of mercury(II) in water. High sensitivity is obtained due to the use of the montmorillonites which displays outstanding capability in terms of adsorbing mercury ion due to its high specific surface and the presence of multiple binding sites. The experimental parameters and the effect of a chelating agent were optimized to further enhance sensitivity and selectivity. Linear calibration curves were obtained over the Hg(II) concentration range from 10 to 800???g?L^?1 for 5?min accumulation, with a detection limit of 1???g?L^?1. Simultaneous determination of Hg(II) and Cu(II) was also studied, and no interference was observed.

Poly(2-amino-5-(4-pyridinyl)-1, 3, 4-thiadiazole) film modified electrode for the simultaneous determinations of dopamine, uric acid and nitrite

Poly(2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole) (PAPT) modified glassy carbon electrode (GCE) was fabricated and used for the simultaneous determinations of dopamine (DA), uric acid (UA) and nitrite (NO₂ ^?) in 0.1 mol?L^?1 phosphate buffer solution (PBS, pH 5.0) by using cyclic voltammetry and differential pulse voltammetry (DPV) techniques. The results showed that the PAPT modified GCE (PAPT/GCE) not only exhibited electrocatalytic activities towards the oxidation of DA, UA and NO₂ ^? but also could resolve the overlapped voltammetric signals of DA, UA and NO₂ ^? at bare GCE into three strong and well-defined oxidation peaks with enhanced current responses. The peak potential separations are 130 mV for DA–UA and 380 mV for UA–NO₂ ^? using DPV, which are large enough for the simultaneous determinations of DA, UA and NO₂ ^?. Under the optimal conditions, the anodic peak currents were correspondent linearly to the concentrations of DA, UA and NO₂ ^? in the ranges of 0.95–380 μmol?L^?1, 2.0–1,000 μmol?L^?1 and 2.0–1,200 μmol?L^?1 for DA, UA and NO₂ ^?, respectively. The correlation coefficients were 0.9989, 0.9970 and 0.9968, and the detection limits were 0.2, 0.35 and 0.6 μmol?L^?1 for DA, UA and NO₂ ^?, respectively. In 0.1 mol?L^?1 PBS pH 5.0, the PAPT film exhibited good electrochemical activity, showing a surface-controlled electrode process with the apparent heterogeneous electron transfer rate constant (k _s) of 25.9 s^?1 and the charge–transfer coefficient (α) of 0.49, and thus displayed the features of an electrocatalyst. Due to its high sensitivity, good selectivity and stability, the modified electrode had been successfully applied to the determination of analytes in serum and urine samples.     

Dopamine sensor based on a glassy carbon electrode modified with a reduced graphene oxide and palladium nanoparticles composite

We report on a sensitive electrochemical sensor for dopamine (DA) based on a glassy carbon electrode that was modified with a nanocomposite containing electrochemically reduced graphene oxide (RGO) and palladium nanoparticles (Pd-NPs). The composite was characterized by scanning electron microscopy, energy dispersive spectroscopy, and electrochemical impendence spectroscopy. The electrode can oxidize DA at lower potential (234 mV vs Ag/AgCl) than electrodes modified with RGO or Pd-NPs only. The response of the sensor to DA is linear in the 1–150 μM concentration range, and the detection limit is 0.233 μM. The sensor was applied to the determination of DA in commercial DA injection solutions.

A graphene-based electrochemical sensor for sensitive and selective determination of hydroquinone

Graphene was prepared by electrochemical reduction of exfoliated graphite oxide at cathodic potentials, and used to fabricate a graphene-modified glassy carbon electrode (GCE) which was applied in a sensor for highly sensitive and selective voltammetric determination of hydroquinone (HQ). Compared to a bare (conventional) GCE, the redox peak current for HQ in pH 5.7 acetate buffer solution is significantly increased, indicating that graphene possesses electrocatalytic activity towards HQ. In addition, the peak-to-peak separation is significantly improved. The modified electrode enables sensing of HQ without interference by catechol or resorcinol. Under optimal conditions, the sensor exhibits excellent performance for detecting HQ with a detection limit of 0.8?μM, a reproducibility of 2.5% (expressed as the RSD), and a recoveries from 98.4 to 101.2%.

Glucose biosensor based on glucose oxidase immobilized on a nanofilm composed of mesoporous hydroxyapatite, titanium dioxide, and modified with multi-walled carbon nanotubes

We report on a highly sensitive glucose biosensor that was fabricated from a composite made from mesoporous hydroxyapatite and mesoporous titanium dioxide which then were ultrasonically mixed with multi-walled carbon nanotubes to form a rough nanocomposite film. This film served as a platform to immobilize glucose oxidase onto a glassy carbon electrode. The morphological and electrochemical properties of the film were examined by scanning electron microscopy and electrochemical impedance spectroscopy. Cyclic voltammetry and chronoamperometry were used to characterize the electrochemical performances of the biosensor which exhibited excellent electrocatalytic activity to the oxidation of glucose. At an operating potential of 0.3?V and pH 6.8, the sensor displays a sensitivity of 57.0?μA?mM^?1?cm^?2, a response time of <5?s, a linear dynamic range from 0.01 to 15.2?mM, a correlation coefficient of 0.9985, and a detection limit of 2?μM at an SNR of 3. No interferences are found for uric acid, ascorbic acid, dopamine and most carbohydrates. The sensor is stable and was successfully applied to the determination of glucose in real samples.

Dispersive solid phase micro-extraction of dopamine from human serum using a nano-structured Ni-Al layered double hydroxide, and its direct determination by spectrofluorometry

We report on a simple method for dispersive solid-phase micro-extraction of dopamine (DA) from human serum. It is based on a layered double hydroxide (LDH) of nickel-aluminum, which acts as the extractant. DA is extracted at pH 8 using the LDH sol solution as a dispersed solid-phase extractor. The extracted DA is quantified by spectrofluorometry at 285?nm excitation and 315?nm emission wavelengths. A comparison of the fluorescence of DA in bulk solution and that of DA intercalated in the LDH revealed a nearly 5-fold improvement in intensity. Factors such as pH, concentration of LDH in the sol solution, and temperature were optimized. Under these conditions, the limit of detection is 0.015?μg?L^?1. The inter- and intra-day relative standard deviations for six replicate determinations of 1?μg?L^?1 DA were 1.7 and 1.1?%, respectively. The method was successfully applied to the determination of DA in human serum samples.

A glassy carbon electrode modified with the nickel(II)-bis(1,10-phenanthroline) complex and multi-walled carbon nanotubes, and its use as a sensor for ascorbic acid

A glassy carbon electrode (GCE) was modified with the nickel(II)-bis(1,10-phenanthroline) complex and with multi-walled carbon nanotubes (MWCNTs). The nickel complex was electrodeposited on the MWCNTs by cyclic voltammetry. The modified GCE displays excellent electrocatalytic activity to the oxidation of ascorbic acid (AA). The effects of fraction of MWCNTs, film thickness and pH values were optimized. Response to AA is linear in the 10 to 630 μM concentration range, and the detection limit is 4 μM (at a signal-to-noise ratio of 3:1). The modified electrode was applied to determine AA in vitamin C tablets and in spiked fruit juice.