Characterisation of carbon paste electrodes bulk-modified with surfactants for measurements in nonaqueous media

Voltammetric determination of flufenamic acid and adsorption studies with biochar in the absence / presence of cetyltrimethylammonium bromide

In this article, a novel method for the determination of Flufenamic acid (FFA, pharmaceutical pollutant) is presented based on voltammetric oxidation at a carbon paste electrode (CPE) in-situ modified with cetyltrimethylammonium bromide (CTAB). The experimentally proved "erosion effect" of this surfactant enhanced the sensitivity of detection in the SWASV mode allowing us to quantify the analyte down to the low nanomolar level (with a LOD of 5.5 × 10-9 mol L-1 FFA). The respective (electro)analytical procedure has been shown to be applicable in monitoring the residua of FFA in model aqueous solutions simulating polluted and then purified industrial wastewater. Furthermore, the process of removal of FFA via adsorption onto selected carbonaceous materials was studied in detail, when two conventional active carbon adsorbents were compared with biochar (BC) - a cheaper alternative. It has been found that although the latter as such does not attain the adsorption capacities of both active carbons, in-situ modification of BC with CTAB enhances its adsorption capacity up to 40% (from 125 mg g-1 to ca. 175 mg g-1), as well as fastens the adsorption process (3x); both under conditions of testing. When considering the final procedure for removal of residual pollutant from model water samples with BC and the method of choice for quantification of the corresponding change(s) of FFA before and after purification, the principal role of CTAB has been revealed and defined. Namely, the functioning of CTAB had, in fact, double benefit: (i) enhancement of adsorptive capabilities of the BC adsorbent and (ii) improved sensitivity of the voltammetric detection with in-situ modified CPE.

Carbon Paste Electrodes Surface-Modified with Surfactants: Principles of Surface Interactions at the Interface between Two Immiscible Liquid Phases

Carbon paste electrodes ex-situ modified with different surfactants were studied using cyclic voltammetry with two model redox couples, namely hexaammineruthenium (II)/(III) and hexacyanoferrate (II)/(III), in 0.1 mol L-1 acetate buffer (pH 4), 0.1 mol L-1 phosphate buffer (pH 7), and 0.1 mol L-1 ammonia buffer (pH 9) at a scan rate ranging from 50 to 500 mV s-1. Distinct effects of pH, ionic strength, and the composition of supporting media, as well as of the amount of surfactant and its accumulation at the electrode surface, could be observed and found reflected in changes of double-layer capacitance and electrode kinetics. It has been proved that, at the two-phase interface, the presence of surfactants results in elctrostatic interactions that dominate in the transfer of model substances, possibly accompanied also by the effect of erosion at the carbon paste surface. The individual findings depend on the configurations investigated, which are also illustrated on numerous schemes of the actual microstructure at the respective electrode surface. Finally, principal observations and results are highlighted and discussed with respect to the future development and possible applications of sensors based on surfactant-modified composited electrodes.

New electroanalytical method for the determination of trans-anethole in spices and sweets

A completely new method for the determination of trans-anethole (TAN) based on the anodic oxidation of this flavouring substance in pure acetonitrile using differential-pulse voltammetry has been developed. A nonaqueous carbon paste electrode bulk-modified with solid sodium dodecyl sulphate of 40 % (w/w) content was chosen as optimum. To propose TAN electrode reaction mechanism, its electrochemical behaviour was investigated at glassy carbon electrode in nonaqueous media. At optimum working conditions, the current response could be calibrated within a linear range 2-200 µmolL^-1 TAN, a coefficient of determination of 0.9971, a sensitivity of 0.1122µALµmol^-1_, and a detection limit of 0.7 µmolL^-1. A satisfactory precision (relative standard deviation of 4 %) has been achieved. The validation performed by analysis of spices and sweets provided comparable results with reference reverse-phase HPLC with spectrophotometric detection, thus confirming the practical use of the developed voltammetric method in food analysis.

Electric Field-Assisted Uptake of Hexavalent Chromium Ions with In Situ Regeneration of Carbon Monolith Adsorbents

The uptake of hexavalent chromium (Cr(VI)) ions from wastewater is of great significance for environmental remediation and resource utilization. In this study, a self-designed instrument equipped with an oxidized mesoporous carbon monolith (o-MCM) as an electro-adsorbent is developed. o-MCM with a super hydrophilic surface displayed a high specific surface area (up to 686.5 m²  g^-1 ). With the assistance of an electric field (0.5 V), the removal capacity of Cr(VI) ions is as high as 126.6 mg g^-1 , much higher than that without an electric field (49.5 mg g^-1 ). During this process, no reduction reaction of Cr(VI) to Cr(III) ions is observed. After adsorption, the reverse electrode with 10 V is used to efficiently desorb the ions on the carbon surface. Meanwhile, the in situ regeneration of carbon adsorbents can be obtained even after ten recycles. On this basis, the enrichment of Cr(VI) ions in a special solution is achieved with the assistance of an electric field. This work lays a foundation for the uptake of heavy metal ions from wastewater with the assistance of the electric field.

Vein-Like Ni-BTC@Ni3S4 with Sulfur Vacancy and Ni3+ Fabricated In Situ Etching Vulcanization Strategy for an Electrochemical Sensor of Dopamine

In this study, a novel Ni-BTC@Ni₃S₄ composite was fabricated by solvothermal reaction using an in situ etching vulcanization strategy and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and Brunauer-Emmett-Teller (BET) analyses. The existence of a sulfur vacancy and Ni³⁺ in the as-prepared vein-like Ni-BTC@Ni₃S₄ greatly promoted the electrochemical sensing activity of the materials. Herein, a simple electrochemical sensor (Ni-BTC@Ni₃S₄/CPE) has been fabricated and used for the detection of dopamine (DA). The current signal of the Ni-BTC@Ni₃S₄/CPE-modified electrode was linear with the concentration of DA in the range of 0.05-750 μM (R² = 0.9995) with a sensitivity of 560.27 μA·mM^-1·cm^-2 and a detection limit of 0.016 μM. At the same time, the sensor has good stability and anti-interference ability. This study could provide a new idea and strategy for the structural regulation of composite electrode-modified materials and sensitive sensing detection of small biological molecules.

Investigation of the photoactivation effect of TiO2 onto carbon-clay paste electrode by cyclic voltammetry analysis

In this work, a cyclic voltammetry analysis for the detection of Ascorbic Acid (AA) based on a carbon-clay paste electrode modified with titanium dioxide (CPEA/TiO₂) is presented. The electrochemical sensor was prepared using clay and carbon graphite, mixed with TiO₂ to investigate the electrode behavior towards the detection of AA. Comprehensive characterization approaches including X-ray diffraction (XRD), Selected area electron diffraction (SAED), Transmission electron microscopy (TEM), Fourier transform infra-red spectroscopy (FTIR) were carried out on different samples. The results indicated that, the electrode has been effectively modified, while the electrochemical parameters of AA on CPEA/T_iO₂/UV such as the charge transfer coefficient (α _a ), number of electrons (n) transferred and standard potential were calculated. CPEA/TiO₂/UV exhibit better photoactivity and also higher electronic conductivity under light radiation (100 W). The linear range for AA was determined between 0.150μM and 0.850 μM with the straight-line equation equivalent to I p a ( μ A ) = 2.244 [ A A ] + 1.234 (n = 8, R² = 0.993). The limit of detection was 0.732 μM (3σ) and limit of quantification was 2.440 μM. For the analytical applications, pharmaceutical tablets such as Chloroquine phosphate, Azithromycin and Hydroxychloroquine sulfate were performed. In addition, interference study in the analytical application was performed, and it was found that the electroanalytical method used can be well adopted for simultaneous electrochemical detection of AA and Azithromycin.