Electrochemical sensing chemical oxygen demand based on the catalytic activity of cobalt oxide film

Cobalt oxide sensing film was in situ prepared on glassy carbon electrode surface via constant potential oxidation. Controlling at 0.8 V in NaOH solution, the high-valence cobalt catalytically oxidized the reduced compounds, decreasing its surface amount and current signal. The current decline was used as the response signal of chemical oxygen demand (COD) because COD represents the summation of reduced compounds in water. The surface morphology and electrocatalytic activity of cobalt oxide were readily tuned by variation of deposition potential, time, medium and Co²⁺ concentration. As confirmed from the atomic force microscopy measurements, the cobalt oxide film, that prepared at 1.3 V for 40 s in pH 4.6 acetate buffer containing 10 mM Co(NO₃)₂, possesses large surface roughness and numerous three-dimensional structures. Electrochemical tests indicated that the prepared cobalt oxide exhibited high electrocatalytic activity to the reduced compounds, accompanied with strong COD signal enhancement. As a result, a novel electrochemical sensor with high sensitivity, rapid response and operational simplicity was developed for COD. The detection limit was as low as 1.1 mg L⁻¹. The analytical application was studied using a large number of lake water samples, and the accuracy was tested by standard method.     

Electrochemical behavior and antioxidant and prooxidant activity of natural phenolics

We have investigated the electrochemical oxidation of a number natural phenolics (salicylic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, protocatechuic acid, o-coumaric acid, m-coumaric acid, p-coumaric acid, caffeic acid, quercetin and rutin) using cyclic voltammetry. The antioxidant properties of these compounds were also studied. A structural analysis of the tested phenolics suggests that multiple OH substitution and conjugation are important determinants of the free radical scavenging activity and electrochemical behavior. Compounds with low oxidation potentials (Epa lower than 0.45) showed antioxidant activity, whereas compounds with high Epa values (>0.45) act as prooxidants.     

Simultaneous determination of neutral nitrogen compounds in gasoline and diesel by differential pulse voltammetry

The presence of trace neutral organonitrogen compounds as carbazole and indole in derivative petroleum fuels plays an important role in the car's engine maintenance. In addition, these substances contribute to the environmental contamination and their control is necessary because most of them are potentially carcinogenic and mutagenic. For those reasons, a reliable and sensitive method was proposed for the determination of neutral nitrogen compounds in fuel samples, such as gasoline and diesel using preconcentration with modified silica gel (Merck 70-230 mesh ASTM) followed by differential pulse voltammetry (DPV) technique on a glassy carbon electrode. The electrochemical behavior of carbazole and indole studied by cyclic voltammetry (CV) suggests that their reduction occurs via a reversible electron transfer followed by an irreversible chemical reaction. Very well resolved diffusion controlled voltammetric peaks were obtained in dimethylformamide (DMF) with tetrabutylammonium tetrafluoroborate (TBAF₄ 0.1 mol L⁻¹) for indole (−2.27 V) and carbazole (−2.67 V) versus Ag|AgCl|KCl_sat reference electrode. The proposed DPV method showed a good linear response range from 0.10 to 300 mg L⁻¹ and a limit of detection (L.O.D) of 7.48 and 2.66 μg L⁻¹ for indole and carbazole, respectively. The results showed that simultaneous determination of indole and carbazole presents in spiked gasoline samples were 15.8 ± 0.3 and 64.6 ± 0.9 mg L⁻¹ and in spiked diesel samples were 9.29 ± 1 and 142 ± 1 mg L⁻¹, respectively. The recovery was evaluated and the results shown the values of 88.9 ± 0.4 and 90.2 ± 0.8% for carbazole and indole in fuel determinations. The proposed method was also compared with UV-vis spectrophotometric measures and the results obtained for the two methods were in good agreement according to the F and t Student's tests.     

Simple and rapid voltammetric determination of morphine at electrochemically pretreated glassy carbon electrodes

A simple and rapid method for morphine detection has been described based on electrochemical pretreatment of glassy carbon electrode (GCE) which was treated by anodic oxidation at 1.75 V, following potential cycling in the potential range from 0 V to 1.0 V vs. Ag|AgCl reference electrode. The sensitivity for morphine detection was improved greatly and the detection limit was 0.2 μM. The reproducibility of the voltammetric measurements was usually less than 3% RSD for six replicate measurements. Moreover, this method could readily discriminate morphine from codeine. And an electrochemical detection of morphine in spiked urine sample was succeeded with satisfactory results.     

An electrochemical ascorbic acid sensor based on palladium nanoparticles supported on graphene oxide

In this study, an electrochemical ascorbic acid (AA) sensor was constructed based on a glassy carbon electrode modified with palladium nanoparticles supported on graphene oxide (PdNPs-GO). PdNPs with a mean diameter of 2.6 nm were homogeneously deposited on GO sheets by the redox reaction between PdCl₄²⁻ and GO. Cyclic voltammetry and amperometric methods were used to evaluate the electrocatalytic activity towards the oxidation of AA in neutral media. Compared to a bare GC or a Pd electrode, the anodic peak potential of AA (0.006 V) at PdNPs-GO modified electrode was shifted negatively, and the large anodic peak potential separation (0.172 V) of AA and dopamine (DA), which could contribute to the synergistic effect of GO and PdNPs, was investigated. A further amperometric experiment proved that the proposed sensor was capable of sensitive and selective sensing of AA even in the presence of DA and uric acid. The modified electrode exhibited a rapid response to AA within 5 s and the amperometric signal showed a good linear correlation to AA concentration in a broad range from 20 μM to 2.28 mM with a correlation coefficient of R = 0.9991. Moreover, the proposed sensor was applied to the determination of AA in vitamin C tablet samples. The satisfactory results obtained indicated that the proposed sensor was promising for the development of novel electrochemical sensing for AA determination.     

Preparation of cobalt-tetraphenylporphyrin/reduced graphene oxide nanocomposite and its application on hydrogen peroxide biosensor

A novel cobalt-tetraphenylporphyrin/reduced graphene oxide (CoTPP/RGO) nanocomposite was prepared by a π–π stacking interaction and characterized by ultraviolet–visible absorption spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS). The CoTPP/RGO nanocomposite exhibited high electrocatalytic activity both for oxidation and reduction of H₂O₂. The current response was linear to H₂O₂ concentration with the concentration range from 1.0 × 10⁻⁷ to 2.4 × 10⁻³ mol L⁻¹ (R = 0.998) at the reductive potential of −0.20 V and from 1.0 × 10⁻⁷ to 4.6 × 10⁻⁴ mol L⁻¹ (R = 0.996) at the oxidative potential of +0.50 V. The H₂O₂ biosensor showed good anti-interfering ability towards oxidative interferences at the oxidative potential of +0.50 V and good anti-interfering ability towards reductive interferences at the reductive potential of −0.20 V.     

Low potential electrosyntheses of free-standing polyfluorene films in boron trifluoride diethyl etherate

High quality free-standing polyfluorene (PFe) films were synthesized electrochemically by direct anodic oxidation of fluorene in pure boron trifluoride diethyl etherate (BFEE) on stainless steel sheet. The oxidation potential of fluorene in this medium was measured to be only 1.1 V versus SCE, which was much lower than that determined in acetonitrile + 0.1 mol L⁻¹ TBATFB. PFe films obtained from this medium showed good electrochemical behavior, good thermal stability with conductivity of 0.25 S cm⁻¹, indicating that BFEE is a better medium than acetonitrile for the electrosyntheses of PFe films. FTIR and ¹H NMR spectral investigations indicated that the polymerization of fluorene occurred mainly at 2, 7 position. As-formed PFe films can be partly dissolved in acetone, acetonitrile, tetrahydrofuran, etc.     

Multi-colored electrochromic polymer with enhanced optical contrast

A new π-conjugated monomer was synthesized which contains an electron-donating unit 3,4-ethylenedioxythiophene and electron withdrawing quinoxaline-based heterocycle to examine the effects of imine unit on the optoelectronic and redox properties of the resulting polymer. Electroactivity of monomer and electrochemical redox behavior of its polymer were investigated by cyclic voltammetry. An irreversible anodic wave at +0.85 V vs Ag wire reference electrode corresponding to the monomer oxidation was observed. Spectroelectrochemical analysis revealed that the neutral polymer has an absorbance at 820 nm. The band gap of the polymer was determined as 1.0 eV from the onset of the π-π∗ transition. The polymer shows multi-colored electrochromic behavior with five distinct states: brick red (−0.3 V), orange (+0.4 V), brown (+0.7 V), green (+0.85 V), gray (+1.2 V). The polymer revealed 34% optical contrast at 460 nm and an excellent optical contrast of 99% in the NIR region.     

Studies on electrochemical behaviors of acyclovir and its voltammetric determination with nano-structured film electrode

A multi-wall carbon nanotubes (MWNTs)-dihexadecyl hydrogen phosphate (DHP) film-coated glassy carbon electrode (GCE) was fabricated, and the electrochemical behaviors of acyclovir on the MWNTs-DHP film-coated GCE were investigated by using cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and chronocoulometry (CC). The oxidation peak current of acyclovir increased significantly and the peak potential shifted negatively at the MWNTs-DHP film-modified GCE, compared with that at a bare GCE. The results showed that this nano-structured film electrode exhibited excellent enhancement effects on the electrochemical oxidation of acyclovir. Consequently, a simple and sensitive electroanalytical method was developed for the determination of acyclovir. The oxidation peak current was proportional to the concentration of acyclovir from 8.0 × 10⁻⁸ to 1.0 × 10⁻⁵ mol/L. The detection limit was about 3.0 × 10⁻⁸ mol/L for 60 s accumulation at 0.00 V. The proposed method was demonstrated by using acyclovir tablets and the result was satisfying.     

Sodium dodecyl sulfate-modified electrochemical paper-based analytical device for determination of dopamine levels in biological samples

We report the development of an electrochemical paper-based analytical device (ePAD) for the selective determination of dopamine (DA) in model serum sample. The ePAD device consists of three layers. In the top layer, SU-8 photoresist defines a hydrophilic sample application spot on the filter paper. The middle layer was made from transparency film and contained two holes, one for sample preconcentration and the other for the surfactant to allow transfer to the third layer. A screen-printed carbon electrode formed the bottom layer and was used for electrochemical measurements. In the absence of the anionic surfactant, sodium dodecyl sulfate (SDS), the oxidation peaks of DA, ascorbic acid (AA) and uric acid (UA) overlapped. With the addition of SDS, the DA oxidation peak shifted to more negative values and was clearly distinguishable from AA and UA. The oxidation potential shift was presumably due to preferential electrostatic interactions between the cationic DA and the anionic SDS. Indeed, whilst the SDS-modified paper improved the DA current five-fold, the non-ionic Tween-20 and cationic tetradecyltrimethylammonium bromide surfactants had no effect or reduced the current, respectively. Furthermore, only the SDS-modified paper showed the selective shift in oxidation potential for DA. DA determination was carried out using square-wave voltammetry between −0.2 and 0.8 V vs. Ag/AgCl, and this ePAD was able to detect DA over a linear range of 1–100 μM with a detection limit (S/N = 3) of 0.37 μM. The ePAD seems suitable as a low cost, easy-to-use, portable device for the selective quantitation of DA in human serum samples.     

Electrocatalytic oxidation and detection of hydrazine at gold electrode modified with iron phthalocyanine complex linked to mercaptopyridine self-assembled monolayer

Electrocatalytic oxidation and detection of hydrazine in pH 7.0 conditions were studied by using gold electrode modified with self-assembled monolayer (SAM) films of iron phthalocyanine (FePc) complex axially ligated to a preformed 4-mercaptopyridine SAMs. The anodic oxidation of hydrazine in neutral pH conditions with FePc-linked-mercaptopyridine-SAM-modified gold electrode occurred at low overpotential (0.35 V versus Ag|AgCl) and the treatment of the voltammetric data showed that it was a pure diffusion-controlled reaction with the involvement of one electron in the rate-determining step. The mechanism for the interaction of hydrazine with the FePc-SAM is proposed to involve the Fe^(III)Pc/Fe^(II)Pc redox process. Using cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV), hydrazine was detected over a linear concentration range of 1.3 × 10⁻⁵ to 9.2 × 10⁻⁵ mol/L with low limits of detection (ca. 5 and 11 μM for OSWV and CV, respectively). At concentrations higher than 1.2 × 10⁻⁴ mol/L the anodic peak potential shifted to 0.40 V (versus Ag|AgCl), and this was interpreted to be due to kinetic limitations resulting from the saturation of hydrazine and its oxidation products onto the redox-active monolayer film. This type of metallophthalocyanine-SAM-based electrode is a highly promising electrochemical sensor given its ease of fabrication, good catalytic activity, stability, sensitivity and simplicity.     

A novel bimediator amperometric sensor for electrocatalytic oxidation of gallic acid and reduction of hydrogen peroxide

A novel bimediator amperometric sensor is fabricated for the first time by surface modification of graphite electrode with thionine (TH) and nickel hexacyanoferrate (NiHCF). The electrochemical behavior of the TH/NiHCF bimediator modified electrode was characterized by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The TH/NiHCF bimediator modified electrode exhibited a pair of distinct redox peaks for NiHCF and TH with formal potentials of 0.33 V and −0.27 V vs. SCE at a scan rate of 50 mV s⁻¹ in 0.1 M NaNO₃ and 0.1 M NH₄NO₃ respectively. The electrocatalytic activity of the bimediator modified electrode towards oxidation of gallic acid with NiHCF and reduction of hydrogen peroxide with TH was evaluated and it was observed that the modified electrode showed an electrocatalytic activity towards the oxidation of gallic acid in the concentration range of 4.99 × 10⁻⁶–1.20 × 10⁻³ M with a detection limit of 1.66 × 10⁻⁶ M (S/N = 3) and reduction of H₂O₂ in the concentration range of 1.67 × 10⁻⁶–1.11 × 10⁻³ M with a detection limit of 5.57 × 10⁻⁷ M (S/N = 3). The bimediator modified electrode was found to exhibit good stability and reproducibility.     

Electrochemical detection of phenolic estrogenic compounds at carbon nanotube-modified electrodes

The use of a carbon nanotube-modified glassy carbon electrode (CNT-GCE) for the LC-EC detection of phenolic compounds with estrogenic activity is reported. Cyclic voltammograms for phenolic endocrine disruptors and estrogenic hormones showed, in general, an enhancement of their electrochemical oxidation responses at CNT-GCE attributable to the electrocatalytic effect caused by CNTs. Hydrodynamic voltammograms obtained under flow injection conditions lead to the selection of +700 mV as the potential value to be applied for the amperometric detection of the phenolic estrogenic compounds, this value being remarkably less positive than those reported in the literature using other electrode materials. Successive injections of these compounds demonstrated that no electrode surface fouling occurred. A mobile phase consisting of a 50:50 (v/v) acetonitrile:0.05 mol l⁻¹ phosphate buffer of pH 7.0 was selected for the chromatographic separation of mixtures of these compounds, with detection limits ranging between 98 and 340 nmol l⁻¹. Good recoveries were obtained in the analysis of underground well water and tap water samples spiked with some phenolic estrogenic compounds at a 14 nmol l⁻¹ concentration level.     

Voltammetric behavior of naratriptan and its determination in tablets

The electrochemical behavior and the analytical application of the selective serotonin agonist naratriptan (N-methyl-3-(1-methyl-4-piperidyl)indole-5-ethanesulfonamide) are presented herein. Naratriptan exhibits an anodic response in aqueous media over a broad pH range (pH 2-12), as determined by differential pulse voltammetry and cyclic voltammetry using glassy carbon electrodes. This response is irreversible in nature, diffusion-controlled and probably caused by the oxidation of the naratriptan indole moiety. The differential pulse voltammetry technique was performed in 0.1 mol L⁻¹ Britton-Robinson buffer (pH = 3), which elicited the most reproducible results. The percentage of naratriptan recovery was 102.1 ± 1.8%, and the limits of detection and quantitation were 9.5 × 10⁻⁶ and 2.0 × 10⁻⁵ mol L⁻¹, respectively. Selectivity trials revealed that the oxidation signal of the drug was not disturbed by the presence of excipients or degradation products. Thus, we conclude that the method presented herein is useful for the quantification of naratriptan in pharmaceutical drugs and that this method requires no separations or extractions. Finally, this voltammetric method was successfully applied to determine the quantity and the content uniformity of naratriptan in drug tablets. A comparison of this technique to the standard high-performance liquid chromatography technique was conducted at the end of our study.     

Electrochemical behaviors of guanosine on carbon ionic liquid electrode and its determination

The electrochemical behaviors of guanosine on the ionic liquid of N-butylpyridinium hexafluorophosphate (BPPF₆) modified carbon paste electrode (CPE) was studied in this paper and further used for guanosine detection. Guanosine showed an adsorption irreversible oxidation process on the carbon ionic liquid electrode (CILE) with the oxidation peak potential located at 1.12 V (vs. SCE) in a pH 4.5 Britton-Robinson (B-R) buffer solution. Compared with that on the traditional carbon paste electrode, small shift of the oxidation peak potentials appeared but with a great increment of the oxidation peak current on the CILE, which was due to the presence of ionic liquid in the modified electrode adsorbed the guanosine on the surface and promoted the electrochemical response. The electrochemical parameters such as the electron transfer coefficient (α), the electron transfer number (n), and the electrode reaction standard rate constant (k_s) were calculated as 0.74, 1.9 and 1.26 × 10⁻⁴ s⁻¹, respectively. Under the optimal conditions the oxidation peak current showed a good linear relationship with the guanosine concentration in the range from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol/L by cyclic voltammetry with the detection limit of 2.61 × 10⁻⁷ mol/L (3σ). The common coexisting substances showed no interferences to the guanosine oxidation. The CILE showed good ability to distinguish the electrochemical response of guanosine and guanine in the mixture solution. The urine samples were further detected by the proposed method with satisfactory results.     

Electrosyntheses of high quality poly(5-methylindole) films in mixed electrolytes of boron trifluoride diethyl etherate and diethyl ether

High quality poly(5-methylindole) (P5MeI) films, especially with good fluorescence properties, were synthesized electrochemically by direct anodic oxidation of 5-methylindole in boron trifluoride diethyl etherate (BFEE) containing additional 50% diethyl ether (EE) (by volume). The oxidation potential onset of 5-methylindole in this medium was measured to be only 0.84 V vs. SCE, which was much lower than that determined in acetonitrile + 0.1 mol L⁻¹ TBATFB (1.08 V vs. SCE). P5MeI films obtained from this medium showed good electrochemical behavior and good thermal stability with conductivity of 10⁻² S cm⁻¹, indicating that BFEE was a better medium than acetonitrile for the electrosyntheses of P5MeI films. Dedoped P5MeI films were thoroughly soluble in strong polar solvent such as dimethyl sulfoxide (DMSO). ¹H NMR spectroscopy and FT infrared spectrum of dedoped P5MeI films strongly suggested that the monomers were linked via the positions 2 and 3. Fluorescent spectral studies indicated that P5MeI was a good violet-blue light emitter with the excitation and emission wavelength of 310 nm and 418 nm, respectively. To the best of our knowledge, this is the first case that 5-methyl group substituted polyindole films with good fluorescence properties can be electrodeposited.     

The electrochemical determination of phenolic derivates using multiple pulsed amperometry with graphite based electrodes

The electrochemical determinations of 4-chlorophenol (4-CP) and 4-nitrophenol (4-NP) by chronoamperometry (CA) and multiple pulsed amperometry (MPA) using expanded graphite-epoxy composite (EG-Epoxy) and rotating spectral graphite disc (SG) electrodes are reported. The electrochemical behaviours of both electrodes in the presence of organics informed about oxidation peak potential and the electrode fouling with organics concentration increasing. Setting up the oxidation peak potential as detection potential, only SG gave good electroanalytical performance using CA. However, by MPA applying both electrodes exhibited the capability to assess electrochemically and quantitatively the pollutants from aqueous solutions. UV spectrometric method detecting 4-CP and 4-NP at λ = 280 nm and λ = 398 nm wavelength, respectively was used for validation and parallel determinations.     

Preparation and photoelectrocatalytic activity of a nano-structured WO3 platelet film

A tungsten trioxide (WO₃) film was prepared by calcination from a precursor paste including suspended ammonium tungstate and polyethylene glycol (PEG). The ammonium tungstate suspension was yielded by an acid-base reaction of tungstic acid and an ammonium solution followed by deposition with ethanol addition. Thermogravimetric (TG) analysis showed that the TG profile of PEG is significantly influenced by deposited ammonium tungstate, suggesting that PEG is interacting strongly with deposited ammonium tungstate in the suspension paste. X-ray diffraction (XRD) data indicated that the WO₃ film is crystallized by sintering over 400 °C. The scanning electron microscopic (SEM) measurement showed that the film is composed of the nano-structured WO₃ platelets. The semiconductor properties of the film were examined by Mott-Schottky analysis to give flat band potential E_FB=0.30 V vs. saturated calomel reference electrode (SCE) and donor carrier density N_D=2.5×10²² cm⁻³, latter of which is higher than previous WO₃ films by two orders of magnitude. The higher N_D was explained by the large interfacial heterojunction area caused by the nano-platelet structure, which apparently increases capacitance per a unit electrode area. The WO₃ film sintered at 550 °C produced 3.7 mA cm⁻² of a photoanodic current at 1.2 V vs. SCE under illumination with a 500 W xenon lamp due to catalytic water oxidation. This photocurrent was 4.5-12.8 times higher than those for the other control WO₃ films prepared by similar but different procedures. The high catalytic activity could be explained by the nano-platelet structure. The photocurrent was generated on illumination of UV and visible light below 470 nm, and the maximum incident photon-to-current conversion efficiency (IPCE) was 47% at 320 nm at 1.2 V. Technically important procedures for preparation of nano-structured platelets were discussed.     

Study on electrochemical behavior of tryptophan at a glassy carbon electrode modified with multi-walled carbon nanotubes embedded cerium hexacyanoferrate

Electrochemical behavior of cerium hexacyanoferrate (CeHCF) incorporated on multi-walled carbon nanotubes (MWNTs) modified GC electrode is investigated by scanning electron microscopy (SEM) and electrochemical techniques. The CeHCF/MWNT/GC electrode showed potent electrocatalytic activity toward the electrochemical oxidation of tryptophan in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of 240 mV. The anodic peak currents increased linearly with the concentration of tryptophan in the range of 2.0 × 10⁻⁷ to 1.0 × 10⁻⁴ M with a detection limit of 2.0 × 10⁻⁸ M (at a S/N = 3). And the determination of tryptophan in pharmaceutical samples was satisfactory.     

Electrosyntheses of poly(2,3-benzofuran) films in boron trifluoride diethyl etherate containing poly(ethylene glycol) oligomers

Visible-light transparent high-quality substrate-supported poly(2,3-benzofuran) (PBF) film has been successfully electrosynthesized by direct anodic oxidation of 2,3-benzofuran on stainless steel sheet in boron trifluoride diethyl etherate (BFEE) containing 10% poly(ethylene glycol) (PEG) with molar mass of 400 (by volume). The oxidation potential of 2,3-benzofuran in this medium was measured to be only 1.0 V vs. SCE, which is lower than that determined in acetonitrile + 0.1 M Bu₄NBF₄ (1.2 V vs. SCE). The PBF films obtained in this media showed good electrochemical behaviors and good thermal stability with conductivity of 10⁻² S cm⁻¹, and the doping level of as-prepared PBF films was determined to be only 8.9%. The structure and morphology of the polymer were investigated by UV-vis, infrared spectroscopy and scanning electron microscopy (SEM), respectively. To the best of our knowledge, this is the first case for the syntheses of PBF films.