Mechanistic approach to the oxidation of phenol at a platinum/gold electrode in an acid medium

Enhancing fouling resistance of graphite sheets for electrochemical sensing of bisphenol-A

Bisphenol A (BPA) is widely used in the production of polycarbonate plastics and epoxy resins, and it is now classified as an emerging pollutant due to its extensive environmental presence. Given the need for effective BPA monitoring, this study presents a cost-effective electrochemical approach for its quantification, using pyrolytic graphite sheets (GSs) as working electrodes integrated into a 3D-printed electrochemical cell. Despite initially exhibiting an intense voltammetric peak for BPA, fouling of the GS surface resulted in a progressive decrease in the BPA signal over successive scans. A pretreatment consisting of applying +1.5 V for 30 s under continuous stirring was adopted before voltammetric measurements. This pretreatment promotes the oxygen evolution reaction, likely cleaning the electrode surface and enhancing its antifouling capability by introducing oxygenated functional groups onto the GS surface. In contrast, this pretreatment was ineffective for a glassy carbon electrode (GCE), likely due to its morphology and reduced reactive surface area, which inhibited the formation of oxygenated functional groups. Under optimized conditions, differential pulse voltammetry with the treated GS electrode yielded a linear range for BPA detection from 0.5 to 1.9 μmol L-1, achieving a detection limit of 0.12 μmol L-1. The proposed electrochemical method was applied to quantify BPA in drinking water and vacuum-pressure mouthguard sheets, providing results statistically equivalent to those obtained using high-performance liquid chromatography. Therefore, this paper presents interesting findings that we believe to be significant for the electroanalysis community, particularly in developing countries where research funding is often limited.

Base-metal oxide semiconductor electrodes for PPCP degradation: Ti-doped α-Fe2O3 for sulfosalicylic acid oxidation as an example

Sulfosalicylic acid is a typical pharmaceutical and personal care product with high toxicity, environmental persistence, and low biodegradability. Electrochemical oxidation has been demonstrated to be a promising way for hazardous organics treatment, but it is severely limited by the high cost and resource shortage of electrode materials. Base-metal oxide semiconductor anodes have the merits of low cost, diversity, and tunable energy levels for charge transfer, and thus may be alternatives to the electrodes for wastewater treatment. Herein, we found that Ti-doped α-Fe₂O₃, as an example, could be efficient for sulfosalicylic acid oxidation, reaching comparable faraday efficiency of sulfosalicylic acid to that of the boron-doped diamond electrode. Ti-doped electrodes exhibited both higher removal rates and current efficiency compared to the undoped. This could be mainly ascribed to the enhanced charge transfer rate constant. Kinetic analysis shows that the apparent reaction order, in terms of sulfosalicylic acid in bulk solution, depended on applied potential and pollutant concentration. Mechanism study shows that the oxidation of sulfosalicylic acid was mainly through indirect pathway. Moreover, the oxidation products were determined and the oxidation mechanism was proposed. This study may open a door to employ base-metal oxide semiconductor anodes for the efficient treatment of organic wastewater.

Electrochemical Sensors Based on the Electropolymerized Natural Phenolic Antioxidants and Their Analytical Application

The design and fabrication of novel electrochemical sensors with high analytical and operational characteristics are one of the sustainable trends in modern analytical chemistry. Polymeric film formation by the electropolymerization of suitable monomers is one of the methods of sensors fabrication. Among a wide range of the substances able to polymerize, the phenolic ones are of theoretical and practical interest. The attention is focused on the sensors based on the electropolymerized natural phenolic antioxidants and their analytical application. The typical electropolymerization reaction schemes are discussed. Phenol electropolymerization leads to insulating coverage formation. Therefore, a combination of electropolymerized natural phenolic antioxidants and carbon nanomaterials as modifiers is of special interest. Carbon nanomaterials provide conductivity and a high working surface area of the electrode, while the polymeric film properties affect the selectivity and sensitivity of the sensor response for the target analyte or the group of structurally related compounds. The possibility of guided changes in the electrochemical response for the improvement of target compounds' analytical characteristics has appeared. The analytical capabilities of sensors based on electropolymerized natural phenolic antioxidants and their future development in this field are discussed.

A Silver-Loaded Exfoliated Graphite Nanocomposite Anti-Fouling Electrochemical Sensor for Bisphenol A in Thermal Paper Samples

Silver nanoparticles (AgNPs) were synthesized separately and loaded onto the expanded layers of exfoliated graphite (EG) to form a silver nanoparticle-exfoliated graphite nanocomposite (AgNPs-EG). The AgNPs-EG was compressed into a pellet (0.6 cm in diameter) and used to prepare an electrochemical sensor for bisphenol A (BPA) in standard samples and in thermal paper. The synthesized materials were characterized by ultraviolet-visible spectrophotometry, X-ray diffraction spectroscopy, scanning electron microscopy, and energy-dispersive X-ray. The electrochemical behavior of BPA on the AgNPs-EG sensor was investigated by cyclic voltammetry and square wave voltammetry. Under optimized experimental conditions, the oxidation peak current was linearly proportional to bisphenol A concentrations in the range from 5.0 to100 μM, with a coefficient of determination (R2 ) of 0.9981. The obtained limit of detection of the method was 0.23 μM. The fabricated sensor was able to overcome electrode fouling with good reproducibility (RSD = 2.62%, n = 5) by mechanical polishing of the electrode on emery paper. The proposed method was successfully applied to determine bisphenol A in thermal paper samples and demonstrated good accuracy of 93.1 to 113% recovery.

Influence of expanded graphite coming from the electrochemical oxidation of phenol on cement-polymer matrix

In presented article the polymer-modified cement mortars with expanded graphite covered by the oligomer film, being a by-product of phenol electro-oxidation, were investigated. The changes in the morphology of EG and EG/oligomer as well as in the microstructure of cement-polymer composites modified with EG/oligomer were verified using the scanning electron microscopy (SEM). The evaluation of adhesion between EG/oligomer and cement-polymer mortar based on the mechanical tests, especially their flexural behavior. It was shown that the oligomer film formed on EG surface made graphite flakes more durable and resistant to bending. Moreover, the oligomer due to the interaction with polymer network in cement-polymer mortar led to the improvement of flexural toughness of composite.

Sensitive electrochemical detection of NADH and ethanol at low potential based on pyrocatechol violet electrodeposited on single walled carbon nanotubes-modified pencil graphite electrode

In this work, the electrodeposition of pyrocatechol violet (PCV) was initially investigated by the electrochemical surface plasmon resonance (ESPR) technique. Subsequently, PCV was used as redox-mediator and was electrodeposited on the surface of pencil graphite electrode (PGE) modified with single-wall carbon nanotubes (SWCNTs). Owing to the remarkable synergistic effect of SWCNTs and PCV, PGE/SWCNTs/PCV exhibited excellent electrocatalytic activity towards dihydronicotinamide adenine dinucleotide (NADH) oxidation at low potential (0.2V vs. SCE) with fast amperometric response (<10s), broad linear range (1.3-280 μM), good sensitivity (146.2 μA mM(-1)cm(-2)) and low detection limit (1.3 μM) at signal-to-noise ratio of 3. Thus, this PGE/SWCNTs/PCV could be further used to fabricate a sensitive and economic ethanol biosensor using alcohol dehydrogenase (ADH) via a glutaraldehyde/BSA cross-linking procedure.

Voltammetric determination of dopamine in the presence of ascorbic and uric acids using partial least squares regression: determination of dopamine in human urine and plasma

A new differential pulse voltammetric method for dopamine determination at a bare glassy carbon electrode has been developed. Dopamine, ascorbic acid (AA) and uric acid (UA) usually coexist in physiological samples. Because AA and UA can be oxidized at potentials close to that of DA it is difficult to determine dopamine electrochemically, although resolution can be achieved using modified electrodes. Additionally, oxidized dopamine mediates AA oxidation and the electrode surface can be easily fouled by the AA oxidation product. In this work a chemometrics strategy, partial least squares (PLS) regression, has been applied to determine dopamine in the presence of AA and UA without electrode modification. The method is based on the electrooxidation of dopamine at a glassy carbon electrode in pH 7 phosphate buffer. The dopamine calibration curve was linear over the range of 1–313 μM and the limit of detection was 0.25 μM. The relative standard error (RSE %) was 5.28%. The method has been successfully applied to the measurement of dopamine in human plasma and urine.

Preparation and characterization of Ti/SnO(2)-Sb(2)O(3)-Nb(2)O(5)/PbO(2) thin film as electrode material for the degradation of phenol

In this work, a novel electrode of titanium substrate coated with mixed metal oxides of SnO(2), Sb(2)O(3), Nb(2)O(5) and PbO(2) was successfully prepared using thermal decomposition and electrodeposition. The surface morphology and the structure of the prepared thin film were characterized by scanning electronic microscopy (SEM) and X-ray diffraction (XRD), respectively. Experimental results showed that the structure of the prepared electrode might be described as a Ti/SnO(2)-Sb(2)O(3)-Nb(2)O(5)/PbO(2) thin film and its surface was mainly comprised pyramidal-shape beta-PbO(2) crystals. The modified electrode had higher oxygen evolution potential than that of other PbO(2) modified electrodes. Electrocatalytic oxidation of phenol in aqueous solution was studied to evaluate the potential applications of this electrode in environmental science. The phenol removal efficiency in an artificial wastewater containing 0.50g/L phenol could reach 78.6% at 20 degrees C and pH 7.0 with an applied electricity density of 20mA/cm(2) and treatment time of 120min. When 21.3g/L chloride was added to this wastewater, the removal efficiency could reach to 97.2%.

The influence of TiO2 and aeration on the kinetics of electrochemical oxidation of phenol in packed bed reactor

The electrochemical oxidation of phenolic wastewater in a lab-scale reactor, packed into granular activated carbon (GAC) with Ti/SnO2 anodes and stainless steel cathodes, was interpreted in this study. GAC saturated rapidly if it was only used as sorbent, but application of suitable electric energy for the system simultaneously could recover the adsorption ability of GAC and maintain the continuous running effectively. The titanium dioxide (TiO2) as catalyst and airflow were also applied to the electrochemical reactor to examine the enhancement for phenol oxidation process. Results revealed that the electrochemical degradation of phenol could be reasonably described by first-order kinetics. In addition, it was illustrated that acid region, increased voltage, more dosage of TiO2 and higher aeration intensity were all beneficial parameters for phenol oxidation rates. By inspecting the relationship between the rate constants (k) and influencing factors, respectively, an overall kinetic model for phenol oxidation was proposed. The kinetics obtained from the experiments under corresponding electrochemical conditions could provide an accurate estimation of phenol concentration effluent and better design of the packed bed reactor.

Highly stable electrochemical oxidation of phenolic compounds at carbon ionic liquidelectrode

A carbon ionic liquid electrode (CILE) was used for the investigation of the electrochemical oxidation of phenolic compounds in acidic media using cyclic voltammetry, chronoamperometry and square wave voltammetry techniques. The results indicate that, contrary to many other electrodes, the oxidation of phenolic compounds on CILE is highly stable and does not result in electrode fouling. Cyclic voltammetry showed that phenolic compounds such as phenol, 2,4-dichlorophenol and catechol were oxidized at CILE and remained electroactive after multiple cycles and at high concentrations of phenol. The cyclic voltammetric response of the CILE is very stable with more than 99% of the initial activity remaining after 20 s of stirring of a 0.5 mM solution of phenol.

The effects of alkyl sulfates on the analysis of phenolic compounds by microchip capillary electrophoresis with pulsed amperometric detection

The effects of different surfactants (sodium 2-ethylhexyl sulfate, sodium decyl sulfate, sodium dodecyl sulfate and sodium tetradecyl sulfate) on the analysis of phenolic compounds by microchip-CE with pulsed amperometric detection were investigated. Using sodium decyl sulfate as a model surfactant, the effects of concentration and pH were examined. Under the optimized conditions, the analysis of six phenolic compounds was performed and compared with control runs performed without surfactant. When these surfactants were present in the run buffer, decreases in the migration time and increases in the run-to-run reproducibility were observed. Systematic improvements in the electrochemical response for the phenolic compounds were also obtained. According to the results presented, surfactants enhance the analyte-electrode interaction and facilitate the electron transfer process. These results should allow a more rational selection of the surfactants based on their electrophoretic and electrochemical effects.