Electrochemical and spectroscopic characterization of a tungsten electrode as a sensitive amperometric sensor of small inorganic ions

Electrochemical Biosensor Based on Horseradish Peroxidase and Black Phosphorene Quantum Dot Modified Electrode

Black phosphorene quantum dots (BPQDs) were prepared by ultrasonic-assisted liquid-phase exfoliation and centrifugation with morphologies proved by TEM results. Furthermore, an electrochemical enzyme sensor was prepared by co-modification of BPQDs with horseradish peroxidase (HRP) on the surface of a carbon ionic liquid electrode (CILE) for the first time. The direct electrochemical behavior of HRP was studied with a pair of well-shaped voltammetric peaks that appeared, indicating that the existence of BPQDs was beneficial to accelerate the electron transfer rate between HRP and the electrode surface. This was due to the excellent properties of BPQDs, such as small particle size, high interfacial reaction activity, fast conductivity, and good biocompatibility. The presence of BPQDs on the electrode surface provided a fast channel for direct electron transfer of HRP. Therefore, the constructed electrochemical HRP biosensor was firstly used to investigate the electrocatalytic behavior of trichloroacetic acid (TCA) and potassium bromate (KBrO3), and the wide linear detection ranges of TCA and KBrO3 were 4.0-600.0 mmol/L and 2.0-57.0 mmol/L, respectively. The modified electrode was applied to the actual samples detection with satisfactory results.

Effective and selective electroreduction of aqueous nitrate catalyzed by copper particles on multi-walled carbon nanotubes

Drinking-water contamination with nitrate ions is inevitable and wide spreading, which demands feasible removal. Water de-nitration by potentiostatic electroreduction is described here. A novel electrocatalyst based on nano-copper particles, supported onto multi-walled carbon nanotubes (MWCNTs), and spray-deposited onto fluorine doped tin oxide-glass substrates, is described. The Cu/MWCNT/FTO electrode has been characterized by several methods and assessed as a working electrode in aqueous nitrate ion electroreduction, in comparison with MWCNT sprayed on FTO (MWCNT/FTO) with no copper. Comparison with earlier reported electrodes is also described. XRD patterns confirm the presence of nano-copper crystallites, in the electrode, with average size ⁓45 nm. Within 2 h of electrolysis, Cu/MWVNT/FTO exhibits more than 65% removal of nitrate at -1.80 V (vs. SCE). In longer time (7 h) the electrode completely converts the nitrate into N₂ (∼65%) and (NH₄⁺) ∼35% with no NO₂^- ions. The kinetics show 0.76 order with respect to nitrate, and a rate constant 4.53 × 10^-2 min^-1 higher than earlier counterparts. The new electrode functions under various conditions of temperature, pH, electrolyte type and concentration and inter-electrode spacing, only at ambient applied potential. Moreover, the electrode exhibits stability under nitrate electroreduction conditions, and can be recovered and reused for multiple times without efficiency loss. XRD and EDS results also confirm the electrode stability after multiple reuse. Compared to earlier systems, the Cu/MWCNT/FTO is environmentally stable, safe, non-costly with high nitrate removal efficiency and selectivity.

Electrochemical Sensors for Determination of Bromate in Water and Food Samples-Review

The application of potassium bromate in the baking industry is used in most parts of the world to avert the human health compromise that characterizes bromates carcinogenic effect. Herein, various methods of its analysis, especially the electrochemical methods of bromate detection, were extensively discussed. Amperometry (AP), cyclic voltammetry (CV), square wave voltammetry (SWV), electrochemiluminescence (ECL), differential pulse voltammetry and electrochemical impedance spectroscopy (EIS) are the techniques that have been deployed for bromate detection in the last two decades, with 50%, 23%, 7.7%, 7.7%, 7.7% and 3.9% application, respectively. Despite the unique electrocatalytic activity of metal phthalocyanine (MP) and carbon quantum dots (CQDs), only few sensors based on MP and CQDs are available compared to the conducting polymers, carbon nanotubes (CNTs), metal (oxide) and graphene-based sensors. This review emboldens the underutilization of CQDs and metal phthalocyanines as sensing materials and briefly discusses the future perspective on MP and CQDs application in bromate detection via EIS.

Electrocatalytic reduction of bromate based on Pd nanoparticles uniformly anchored on polyaniline/SBA-15

A nano-composite electrocatalyst of Pd nanoparticles (Pd-NPs) anchored on polyaniline (PANI) supported by mesoporous SBA-15 (Pd-NPs/PANI/SBA-15), was synthesized using an in situ chemical method. Transmission electron microscopy showed that the Pd-NPs were homogeneously dispersed. Fourier-transform infrared and X-ray photoelectron spectroscopies confirmed that the Pd-NPs in the metallic state (Pd(0)) were predominantly immobilized on nitrogen sites in the PANI chains. The electrochemical performance of Pd-NPs/PANI/SBA-15 for electrocatalytic reduction of bromate (BrO3(-)) in an acidic medium was investigated by cyclic voltammetry (CV) and amperometric measurement. The reduction peak in the CV curves in the region 0.12 to -0.22V (vs. SCE) corresponded to response of BrO3(-) electroreduction, and the reduction peak current was well fitted linearly to the BrO3(-) concentration. It is proposed that the bromate ions diffuse to the Pd-NPs active sites and then the electrocatalytic reduction occurred with the H(+) doped in PANI. Furthermore, by amperometric measurement, Pd-NPs/PANI/SBA-15 showed relatively high sensitivity with respect to BrO3(-) concentration in the range of 8μmolL(-1) to 40mmolL(-1). Continuous CV for 200 cycles proved that Pd-NPs/PANI/SBA-15 had excellent electrocatalytic stability. These results show that Pd-NPs/PANI/SBA-15 is effective for electrocatalytic reduction of BrO3(-) and has great potential for the fabrication of BrO3(-) electrochemical sensor.

Graphene-modified Pd/C cathode and Pd/GAC particles for enhanced electrocatalytic removal of bromate in a continuous three-dimensional electrochemical reactor

Bromate (BrO3(-)) is a carcinogenic and genotoxic contaminant commonly generated during ozonation of bromide-containing water. In this work, the reductive removal of BrO3(-) in a continuous three-dimensional electrochemical reactor with palladium-reduced graphene oxide modified carbon paper (Pd-rGO/C) cathode and Pd-rGO modified granular activated carbon (Pd-rGO/GAC) particles was investigated. The results indicated that the rGO sheets significantly promoted the electrochemical reduction of BrO3(-). With the enhanced electron transfer by rGO sheets, the electroreduction of H2O to atomic H* on the polarized Pd particles could be significantly accelerated, leading to a faster reaction rate of BrO3(-) with atomic H*. The synergistic effect of the Pd-rGO/C cathode and Pd-rGO/GAC particles were also exhibited. The atomic H* involved in various electroreduction processes was detected by electron spin resonance spectroscopy and its role for BrO3(-) reduction was determined. The performance of the reactor was evaluated in terms of the removal of BrO3(-) and the yield of Br(-) as a function of the GO concentration, Pd loading amount, current density, hydraulic residence time (HRT), and initial BrO3(-) concentration. Under the current density of 0.9 mA/cm(2), BrO3(-) with the initial concentration of 20 μg/L was reduced to be less than 6.6 μg/L at the HRT of 20 min. The BrO3(-) reduction was inhibited in the presence of dissolved organic matter. Although the precipitates generated from Ca(2+) and Mg(2+) in the tap water would cover the Pd catalysts, a long-lasting electrocatalytic activity could be maintained for the 30 d treatment. SEM and XPS analysis demonstrated that the precipitates were predominantly deposited onto the Pd-rGO/C cathode rather than the Pd-rGO/GAC particles.

Enhancement of nitrite and nitrate electrocatalytic reduction through the employment of self-assembled layers of nickel- and copper-substituted crown-type heteropolyanions

Multilayer assemblies of two crown-type type heteropolyanions (HPA), [Cu20Cl(OH)24(H2O)12(P8W48O184)](25-) and Ni4(P8W48O148)(WO2)](28-), have been immobilized onto glassy carbon electrode surfaces via the layer-by-layer (LBL) technique employing polycathion-stabilized silver nanoparticles (AgNP) as the cationic layer within the resulting thin films characterized by electrochemical and physical methods. The redox behaviors of both HPA monitored during LBL assembly with cyclic voltammetry and impedance spectroscopy revealed significant changes by immobilization. The presence of AgNPs led to the retention of film porosity and electronic conductivity, which has been shown with impedance and voltammeric studies of film permeabilities toward reversible redox probes. The resulting films have been characterized by physical methods. Finally, the electrocatalytic performance of obtained films with respect to nitrite and nitrate electrocatalytic reduction has been comparatively studied for both catalysts. Nickel atoms trapped inside HPA exhibited a higher specific activity for reduction.

Investigations into the electrochemical, surface, and electrocatalytic properties of the surface-immobilized polyoxometalate, TBA3K[SiW10O36(PhPO)2]

Surface anchoring of an organic functionalized POM, TBA3K[SiW10O36(PhPO)2] was carried out by two methods, the layer-by-layer (LBL) assembly technique by employing a pentaerythritol-based ruthenium(II) metallodendrimer as a cationic moiety and also by entrapping the POM in a conducting polypyrrole film. The redox behavior of the constructed films was studied by using cyclic voltammetry and electrochemical impedance spectroscopy. The surface morphologies of the constructed multilayers were examined by scanning electron microscopy and atomic force microscopy. X-ray photoelectron spectroscopy was conducted to confirm the elements present within the fabricated films. The multilayer assembly was also investigated for its catalytic efficiency towards the reduction of nitrite.

Molybdenum, molybdenum oxides, and their electrochemistry

The electrochemical behaviors of molybdenum and its oxides, both in bulk and thin film dimensions, are critical because of their widespread applications in steels, electrocatalysts, electrochromic materials, batteries, sensors, and solar cells. An important area of current interest is electrodeposited CIGS-based solar cells where a molybdenum/glass electrode forms the back contact. Surprisingly, the basic electrochemistry of molybdenum and its oxides has not been reviewed with due attention. In this Review, we assess the scattered information. The potential and pH dependent active, passive, and transpassive behaviors of molybdenum in aqueous media are explained. The major surface oxide species observed, reversible redox transitions of the surface oxides, pseudocapacitance and catalytic reduction are discussed along with carefully conducted experimental results on a typical molybdenum glass back contact employed in CIGS-based solar cells. The applications of molybdenum oxides and the electrodeposition of molybdenum are briefly reviewed.

Development of an automated on-line electrochemical chlorite ion sensor

A sensor system for the automatic, in-line, determination of chlorite ion is reported. Electroanalytical measurements were performed in electrolyte-free liquids by using an electrochemical probe (EC), which enables in-line detection in high-resistance media such as disinfected water. Cyclic voltammetry scan rate studies suggest that the current arising from the oxidation of chlorite ion at an EC probe is mass-transfer limited. By coupling FIA with an EC probe amperometric cell, automated analysis was achieved. This sensor is intended to fulfill the daily monitoring requirements of the EPA DBP regulations for chlorite ion. Detection limits of 0.02-0.13 mg/L were attained, which is about one order of magnitude below the MRDL. The sensor showed no faradaic signal for perchlorate, chlorate, or nitrate. The lifetime and stability of the sensor were investigated by measuring calibration curves over time under constant-flow conditions. Detection limits of <0.1 mg/L were repeatedly achieved over a period of three weeks.

Bromate ion removal by electrochemical reduction using an activated carbon felt electrode

The electrochemical removal of bromate ion (BrO3-) was investigated using a two-compartment electrolytic flow cell with activated carbon felt electrodes. Bromate ion removal and corresponding Br- increase was observed during electrochemical treatment whereas the activated carbon felt used possessed no catalytic effect on BrO3- reduction. The BrO3-reduction rate was accelerated at lower pH, which also improved current efficiency. Transition of chemical equilibrium of the BrO3- reductive reaction was theorized as the reason for pH dependency of the BrO3- reduction.The electrochemicaltreatment of BrO3- -contaminated tap water resulted in a rapid decrease in BrO3- concentration from 100 to 48 microg/L with a contact time of 9.2 s. Thus, electrochemical treatment allowed the rapid removal of BrO3-. However, competitive hydrogen evolution at the cathodes reduced current efficiency of BrO3- reduction. Standard potentials of corresponding anodic and cathodic reactions suggested that electrolysis at a terminal voltage less than 1.229 V would promote BrO3- reduction without hydrogen evolution. However, the activated carbon felt electrode did not function well at a terminal voltage of 1.0 V. Accordingly, the development of an electrode material with high catalytic activity will be required to improve current efficiency.

Electrochemistry and electrocatalysis of polyoxometalate-ordered mesoporous carbon modified electrode

In this work, we have developed a convenient and efficient method for the functionalization of ordered mesoporous carbon (OMC) using polyoxometalate H6P2Mo18O62 x H2O (P2Mo18). By the method, glassy carbon (GC) electrode modified with P(2)Mo(18) which was immobilized on the channel surface of OMC was prepared and characterized for the first time. The large specific surface area and porous structure of the modified OMC particles result in high heteropolyacid loading, and the P(2)Mo(18) entrapped in this order matrix is stable. Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption-desorption isotherm and X-ray diffraction (XRD) were employed to give insight into the intermolecular interaction between OMC and P(2)Mo(18). The electrochemical behavior of the modified electrode was studied in detail, including pH-dependence, stability and so on. The cyclic voltammetry (CV) and amperometry studies demonstrated that P(2)Mo(18)/OMC/GC electrode has high stability, fast response and good electrocatalytic activity for the reduction of nitrite, bromate, idonate, and hydrogen peroxide. The mechanism of catalysis on P(2)Mo(18)/OMC/GC electrode was discussed. Moreover, the development of our approach for OMC functionalization suggests the potential applications in catalysis, molecular electronics and sensors.

Bromate analysis in groundwater and wastewater samples

Bromate (BrO(3)(-)) is a disinfection by-product formed during ozonation of potable water supplies containing bromide (Br(-)). Bromate has been classed by the World Health Organisation as a 'possible human carcinogen', leading to implementation of 10-25 microg L(-1)(as BrO(3)(-)) drinking water limits in legislative areas including the United States and European Union. Techniques have been developed for bromate analysis at and below regulatory limits, with Ion Chromatography (IC) coupled with conductivity detection (IC-CD), post-column reaction and ultra-violet (UV) detection (IC-PCR), or inductively coupled plasma-mass spectrometry detection (IC-ICPMS) in widespread use. The recent discovery of bromate groundwater contamination in a UK aquifer has led to a requirement for analysis of bromate in a groundwater matrix, for environmental monitoring and development of remediation strategies. The possibility of bromate-contaminated water discharge into sewage treatment processes, whether accidental or as a pump-and-treat strategy, also required bromate analysis of wastewater sources. This paper summarises techniques currently available for trace bromate analysis in potable water systems and details studies to identify a methodology for routine analysis of groundwater and wastewater samples. Strategies compared were high performance liquid chromatography (HPLC) with direct UV or PCR/UV detection, IC-CD, IC-PCR, and a simple spectrophotometric technique. IC-CD was the most cost-effective solution for simultaneous analysis of bromate and bromide within groundwater samples, having a 5 microg L(-1) detection limit of both anions with limited interference from closely-eluting species. Wastewater samples were successfully analysed for bromate only using HPLC with PCR/UV detection, with detection limits below 20 microg L(-1)(as BrO(3)(-)) and low interference. HPLC with direct UV detection was unsuitable for bromate analysis within the concentration range 50-5000 microg L(-1) which was required for this project, but column choice was shown to be a major factor in determining limits of detection. Spectrophotometry could not reproducibly determine bromate concentration, although the technique showed promise as a quick field method for high-level groundwater bromate analysis.