Carbon Microdisk Research Articles

Voltammetric method for chemical oxygen demand in a dairy wastewater using homemade minimal instrumentation

This paper presents a voltammetric method for Chemical Oxygen Demand (COD) quantification in a dairy wastewater using homemade minimal electrochemical instrumentation. The working electrode was a carbon microdisc, the auxiliary electrode was a carbon rod, and a silver electrode was used as reference. A low-cost non-commercial potentiostat was employed. The linear working range was 1-20 g/L. Repeatability was evaluated on 10 sample quantifications, the coefficient of variation was 3.6%, lower than that obtained for the reference spectrophotometric method (8.2%). The voltammetric method did not require treatment of the sample. The trueness was evaluated by comparing the voltammetric results with the spectrophotometric method. Mean COD of 10 quantifications using low-cost voltammetry was 10.2 g/L, whereas for the spectrophotometric method it was 11.0 g/L. A Paired Sample t-Test did not show statistically significant differences between both. Detection limit was 0.6 g/L, and quantification limit was 1.8 g/L. Both are acceptable for the sample in which there is a high organic content.

Exposure Studies of Different Kinds of Volatile Organic Compounds in Vapour Phase with a Narrow-gap Cell Containing CuxS Modified Carbon Microdisc Electrode

In this article, the feasibility of the CuxS modified carbon microdisc electrode was examined by exposure to four different volatile organic compounds (2-propanol, acetic acid, ethyl acetate and n-butylamine) directly in their vapour phase using cyclic voltammetry and amperometry. The performance of the modified microdisc was compared with the bare carbon microdisc (30 μm in diameter) which was involved in a narrow-gap cell. By using both methods high current increase was observed for 2-propanol with the modified electrode and its sensitivity was sufficiently higher than with the bare electrode. The modified electrode showed lower current signals in case of acetic acid and n-butylamine. The latter formed a condensation layer at the interelectrode gap. Neither the bare nor the modified electrode was sensitive to ethyl acetate.

Understanding electroanalytical measurements in authentic human saliva leading to the detection of salivary uric acid

The electroanalytical responses of several redox-active species are investigated in authentic human whole saliva. First, we show that ferrocenemethanol (FcCH2OH) and ferrocyanide ([Fe(CN)6]4−) display well-defined voltammetry in undiluted saliva at a carbon microdisc electrode without the addition of supporting electrolyte. Second, we demonstrate that dissolved oxygen is detectable in saliva. The ferrocenemethanol oxidation is shown not to be altered by the medium apart from the steady-state currents which change according to the viscosity of the solvent. In contrast, the voltammetry of ferrocyanide oxidation and oxygen reduction in authentic saliva are significantly different from those in synthetic saliva or aqueous electrolyte. It is demonstrated that the electrode is partially blocked by organic molecules or other electrochemically inert species when placed in authentic saliva samples. The distortion in voltammetry thus reflects the different surface sensitivity of the two processes. Ohmic drop is proved to be minimal at microelectrodes even without added electrolyte. Microelectrodes are thus suitable for use in electrochemical analysis of saliva samples. Finally, we demonstrate that uric acid which is a potential biomarker for gout, oral cavity cancer and other several diseases can be directly detected in authentic saliva at a carbon microelectrode with the interference of ascorbic acid and dopamine of no more than 10%.

Improving Limits of Detection. Microdisc versus Microcylinder Electrodes

AbstractThe specific apparent capacitance (Farads per unit geometric area) of commercial carbon microdisc electrodes and in‐house fabricated carbon microcylinders is studied through the use of cyclic voltammetry. Home‐made microcylinder electrodes exhibit lower specific apparent capacitances and as a result facilitate lower limits of detection. We further demonstrate practically how the electroanalytical limit of the detection of solution phase trace oxygen is significantly improved using such microcylinder electrodes. Finally we show that after the degassing of a solution by nitrogen purging even in the most commonly encountered ‘best’ case scenarios approximately tens of μM of oxygen remain present in the electrochemical cell, unobservable by the commercial microdisc electrodes, but detectable with microcylinder electrodes, testifying to their analytical superiority.

Electrophoretic separation of chlorophenols on silica nanospheres-coated poly(dimethylsiloxane) microchip using a Nafion/graphene-modified carbon electrode for detection.

In the present work, the micellar electrophoretic separation of five chlorophenols (CPs) on a functionalized poly(dimethylsiloxane) (PDMS) microchip with amperometric detection was performed. In order to achieve high resolution by controlling the electroosmotic flow (EOF) as well as signal detection by suppressing analytes adsorption, the microchannel was functionalized by poly(diallyldimethylammonium chloride) (PDDA)/SiO2 nanospheres (NSs)/ poly(sodium-p-styrenesulfonate) (PSS), via an approach of layer-by-layer assembly. Five chlorophenols (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,3-dichlorophenol and 2,4,6-trichlorophenol) were efficiently separated in this microchannel (3.7 cm of length) within 120 s. A resolution of at least 2.4 was obtained with a 10 mM phosphate buffer solution (PBS) (pH 9.48) containing 20 mM sodium dodecyl sulfate (SDS) and 50% (v/v) acetonitrile as a carrier under optimized conditions. A graphene-modified carbon microdisk electrode was used for high-sensitivity detection. Its characteristics were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). For those CPs, linear ranges of 0.08-5, 0.06-5, 0.04-5, 0.04-5 and 0.30-20 μM and detection limits of 0.021, 0.026, 0.022, 0.019 and 0.054 μM were obtained, respectively. The method was successfully applied for the analysis of some wastewater samples with satisfactory recovery.

The effect of acetate concentration, solution pH and conductivity on the anodic stripping voltammetry of lead and cadmium ions at in situ bismuth-plated carbon microelectrodes

The use of carbon microdisc electrode substrates allowed Pb(II) and Cd(II) anodic stripping voltammetry at in situ plated bismuth films to be studied in an extended acetate buffer/electrolyte concentration range that includes very low or zero acetate levels. The change of the Pb and Cd Square Wave Anodic Stripping Voltammetry (SWASV) peak height with acetate concentration, pH and conductivity has been studied systematically. It was found that the stripping peak signal is considerably enhanced in the absence of added acetate or in acidic solutions. This behaviour has been attributed to the extent of metal ion complexation at different levels of free acetate ions that are present at different buffer concentrations or pH values. Recording both the forward and the reverse voltammograms of each SWASV curve, the signal variation could be attributed to changes in the metal ion deposition rate (as its complexation state is varied) both during the preconcentration step and the reverse pulse of the SWASV potential sequence.

Specific Oxygen-Containing Functional Groups on the Carbon Surface Underlie an Enhanced Sensitivity to Dopamine at Electrochemically Pretreated Carbon Fiber Microelectrodes

The in vivo use of carbon-fiber microelectrodes for neurochemical investigation has proven to be selective and sensitive when coupled with background-subtracted fast-scan cyclic voltammetry (FSCV). Various electrochemical pretreatments have been established to enhance the sensitivity of these sensors; however, the fundamental chemical mechanisms underlying these enhancement strategies remain poorly understood. We have investigated an electrochemical pretreatment in which an extended triangular waveform from -0.5 to 1.8 V is applied to the electrode prior to the voltammetric detection of dopamine using a more standard waveform ranging from -0.4 to 1.3 V. This pretreatment enhances the electron-transfer kinetics and significantly improves sensitivity. To gain insight into the chemical mechanism, the electrodes were studied using common analytical techniques. Contact atomic force microscopy (AFM) was used to demonstrate that the surface roughness was not altered on the nanoscale by electrochemical pretreatment. Raman spectroscopy was utilized to investigate oxide functionalities on the carbon surface and confirmed that carbonyl and hydroxyl functional groups were increased by electrochemical conditioning. Spectra collected after the selective chemical modification of these groups implicate the hydroxyl functionality, rather than the carbonyl, as the major contributor to the enhanced electrochemical signal. Finally, we have demonstrated that this electrochemical pretreatment can be used to create carbon microdisc electrodes with sensitivities comparable to those associated with larger, conventionally treated cylindrical carbon fiber microelectrodes.

A New Chromium(III) Microelectrode Based on Self‐Assembled Diethylenetriamminepentaacetic Acid‐Poly(fuchsin basic) Modified Electrode

AbstractA new selective, sensitive, and rapid response microelectrode for microamount chromium(III) determination was developed. For the electrode preparation, fuchsin basic was electropolymerized onto a carbon microdisk electrode, and then diethylenetriamminepentaacetic acid (DTPA) was self‐assembled on the electrode surface by the reaction between DTPA and poly(fuchsin basic). The determination conditions were optimized. The electrode showed a linear response to Cr3+ ions in the concentration range of 1.0×10−6−1.0×10−4 M and exhibits a super‐Nernstian slope of 32.5±0.4 mV per decade. The detection limit is 3.6×10−7 M. The response time of the electrode is less than 10 s, and it can be used for at least 2 months with limited considerable divergences in its potentials. The proposed electrode was applied for monitoring the chromium ion level in wastewater of chromate industries.

Anodic stripping voltammetry at in situ bismuth-plated carbon and gold microdisc electrodes in variable electrolyte content unstirred solutions

Carbon and gold microdisc electrodes (30 and 10 μm, respectively) have been tested as substrates for in situ bismuth film plating from unstirred solutions of variable acetate buffer content and were subsequently used in the anodic stripping voltammetry determination of Pb(II) and Cd(II) ions. The effects of Bi(III) concentration, analyte accumulation time, stirring as well as supporting electrolyte content have been studied. Under optimal conditions good voltammetric responses were obtained by means of square wave anodic stripping voltammetry in unstirred analyte solutions of 5 × 10 −8 to 10 −6 M, even in the absence of added buffer. In an indicative application, Pb(II) ion levels were determined in tap water using bismuth-plated carbon microdisc electrodes.

Anodic Stripping Voltammetry at Bismuth‐Coated and Uncoated Carbon Microdisk Electrodes: Application to Trace Metals Analysis in Food Samples

This article reports on the performance of a bismuth‐coated carbon microdisk electrode (BiFμE) for the determination of trace heavy metals by anodic stripping voltammetry (ASV). The BiFμE was prepared by electrodeposition of a metallic bismuth film onto the microdisk, by applying an in‐situ electroplating procedure. To test the performance of the BiFμE, ASV measurements were performed on synthetic solutions containing Cd2+, Pb2+, and Cu2+ as target ions. The results indicated that cadmium and lead gave well‐defined ASV peaks with no interference, and their quantitative determination could be carried out straightforwardly. In particular, linear calibration curves over the range 5.0 × 10−8–1.0 × 10−6 M for both ions, and detection limits of 7.8 and 2.9 nM, for cadmium and lead, respectively, after applying a 60 sec preconcentration step, were obtained. The reproducibility was also satisfactory, the relative standard deviation (RSD) being within 2.5% for both ions. Copper, instead, gave an ASV response that, in most experimental conditions, overlapped with that of bismuth. This circumstance made the determination of copper at the BiFμE difficult. Since the latter element could be detected reliably at the uncoated carbon microdisk electrode (CμE), both BiFμE and CμE were employed, respectively, for the determination of lead and copper ions in drinking water, wine, and tomato sauce.

Electroporation Introduction of Diclofenac Sodium into Human Erythrocytes and Its Determination

A method of electroporation for introducing diclofenac sodium (a drug) into human erythrocytes was developed. The conditions (the pulse strength, pulse duration and pulse number) for the introduction of diclofenac sodium into erythrocytes by electroporation were investigated. The diclofenac sodium in single human erythrocytes introduced by the electroporation was analyzed qualitatively and quantitatively by capillary zone electrophoresis (CZE) with end-column ampermetric detection at a carbon microdisk array electrode.

Electroporation Introduction of Diclofenac Sodium into Human Erythrocytes and Its Determination

A method of electroporation for introducing diclofenac sodium (a drug) into human erythrocytes was developed. The conditions (the pulse strength, pulse duration and pulse number) for the introduction of diclofenac sodium into erythrocytes by electroporation were investigated. The diclofenac sodium in single human erythrocytes introduced by the electroporation was analyzed qualitatively and quantitatively by capillary zone electrophoresis (CZE) with end-column ampermetric detection at a carbon microdisk array electrode.

Quantitation of caffeine by capillary zone electrophoresis with end-column amperometric detection at a carbon microdisk array electrode.

Capillary zone electrophoresis is employed for the determination of caffeine using end-column amperometric detection with a carbon fiber microdisk array electrode at a constant potential of 1.45 V versus a saturated calomel electrode. The optimum conditions of separation and detection are 0.1 52mM NaH2PO4-0.648mM Na2HPO4 for the buffer solution, 20 kV for the separation voltage, 5 kV for the injection voltage, and 10s for the injection time. The limit of detection is 2.9 x 10(-4)mM or 1.2 fmol (signal-to-noise ratio = 2). The relative standard deviation is 0.68% for the migration time and 2.3% for the electrophoretic peak current. The method is applied to determining caffeine in human serum and a cola drink.

A steady-state voltammetric investigation on the oxidation of ferrocene in ethanol–water mixtures

The oxidation process of ferrocene was studied in ethanol–water mixtures, under steady-state conditions, with platinum and carbon microdisk electrodes. The electrode process was found to be essentially reversible in mixtures up to 60 vol.% water. In media with a larger water content, a weak adsorption of the reactant was involved. Half-wave potentials determined with both microelectrodes, quoted against an aqueous Ag|AgCl (KCl saturated) reference electrode, shifted towards less positive values as the water content was increased. In contrast, the half-wave potential referenced against the cobaltocenium|cobaltocene system was calculated to be −1.318 (±0.005) V, independent of the medium composition. The limiting current, at a given concentration of ferrocene, strongly depended on the medium composition and mainly reflected the change of viscosity of the media. The overall results allowed us to conclude that ferrocene can be used as a voltammetric standard in the mixtures examined.

A Study on the Suitability of Carbon Disk Microelectrodes for Trace Analysis of Lead and Copper by ASV

The performance of carbon microdisk electrodes for the simultaneous trace analysis of lead and copper by anodic stripping voltammetry is investigated. The evaluation of the two metal ions in a single step, i.e., by employing a plating potential at which both cations are reduced simultaneously, gives unpredictable results due to the interaction between lead and copper deposited onto the electrode surface. In particular, the calibration plots of lead are not linear, while the copper stripping peak is quite broad and irreproducible. A two-steps deposition procedure, based on a preelectrolysis of copper performed at a potential at which lead ion is not reduced, followed by a simultaneous reduction of the two cations at large enough negative potentials, provides, instead, linear calibration graphs for lead. Copper can be determined without interference by plating the metal at a negative potential where lead ion is not reduced. The reliability of the two-steps procedure for the determination of lead and copper is verified in rain and wine samples.

Application of a large assembly of microelectrodes to the study of cytochrome‐c and other redox proteins

AbstractA novel microelectrode assembly containing more than 1000 carbon microdisks has been used to study the interfacial electron transfer kinetics of cytochrome c and several other redox proteins. It is demonstrated that near‐steady‐state current–voltage curves can be obtained without solution stirring, and without significant mains interference or capacitive charging currents. In addition, it is demonstrated that concentrations of redox proteins as low as 1 μM can be detected in solution. Finally, it is shown that the surfaces of carbon microdisk assemblies can be successfully coated with thin films of poly(ester sulfonic acid), which allows reactants to be studied in a membrane‐like environment.

Use of Nafion ® coated carbon disk microelectrodes in solution without and with different concentrations of supporting electrolyte

Carbon microdisk electrodes modified with Nafion ® have been characterised in aqueous solutions without and with different concentrations of supporting electrolyte, by using Ru(NH 3) 6Cl 6 as the probe molecule. The experimental results indicate that, with the microelectrode, good quality responses can be obtained under any conditions of ionic strength of the solutions containing the electroactive species. Comparative measurements, carried out with conventional modified carbon disk electrodes under low ionic strength conditions, display voltammograms strongly affected by ohmic drop and capacitive currents. Non-steady state chronoamperometric measurements were used to evaluate the apparent diffusion coefficient D app and concentration c f of Ru(NH 3) 6Cl 3 in the polymer film. A concentration dependence of D app has been observed under all the conditions investigated here. The possibility of exploiting the modified microelectrodes for quantitative measurements in solution at low ionic strength has also been ascertained.

Au(III) in Borate Medium: Electrochemical Behavior and Application to Electroplating

A well‐defined steady‐state current for the reduction of gold on a glassy carbon microdisk electrode is obtained from borate medium. Coulometric experiments show that this current is due to the reduction of gold(III) to gold(0). This current is proportional to the concentration of gold(III) and suitable for analytical purposes. Gold(III) is more stable in borate electrolytes than in solutions of an indifferent electrolyte at the same ionic strength and pH. However, UV‐vis spectrometry and pH‐metric titrations suggest that borate does not form stoichiometric complexes with gold(III). This medium yields an excellent quality of deposit on test masks with feature size in the range of 5 to 250 μm and highest aspect ratio 1:1.

A potentiostatic study of silver centres and their catalytic effect on hydrogen peroxide using carbon microelectrodes

Potentiostatic measurements are reported for the growth of silver centres in the absence and in the presence of mixed reduction/oxidation of hydrogen peroxide on carbon microdisc electrodes. The application of growth laws and polarization effects are discussed and it is shown that the silver growth process is kinetically controlled at very short times, becoming diffusion controlled as time increases. From the polarization curves, the rate constants of silver deposition were determined. For the system silver/hydrogen peroxide the results show that the reduction/oxidation of H 2O 2 at silver substrates is controlled by the occurrence of a non-electrochemical reaction.

Determination of Some Polynuclear Aromatic Hydrocarbons by Liquid Chromatography with Electrochemical Detection at a Carbon Microdisk Electrode

Amperometric detection with a 5- to 12-μm-diameter carbon microdisk electrode shows considerable promise for the determination of polynuclear aromatic hydrocarbons following their separation by reverse-phase high-performance liquid chromatography. Trial mixtures containing anthracene, l,2-benzanthracene, and benzo[ a]pyrene were chromatographically resolved by elution with acetonitrile containing little or no purposely added electrolyte. The detector current response was stable, reproducible, and linear versus concentration even though the cell resistance was very high. The apparent detection limit for these analytes was 10-15 pg. Addition of pyrene to the analyte mixtures resulted in peak convolution but electrode fouling, which often is a problem during electro-oxidation of pyrene, did not occur at the carbon microsensor. Resolution of these four analytes was achieved by eluting with 80% acetonitrile-20% ethylene glycol.