Poly(para toluene sulphonic acid) and gold nanoparticles modified glassy carbon electrode for simultaneous voltammetric sensing of xanthine and hypoxanthine

A voltammetric sensor has been developed for the individual as well as simultaneous determination of xanthine (XA) and hypoxanthine (HX) based on an electroactive-polymerised layer of para toluene sulphonic acid and gold nanoparticles composite modified glassy carbon electrode ([p(PTSA)]/AuNPs/GCE)]. Under optimized conditions, an enhancement in the oxidation currents with well-separated and well-resolved peak position and a lower shift in the peak potentials were observed. By square wave voltammetry, the simultaneous determinations of XA and HX were achieved in the linear ranges 6.00 × 10-4 M to 3.00 × 10-6 M and 5.00 × 10-4 M to 1.00 × 10-5 M with detection limits of 4.09 × 10-7 M and 4.10 × 10-7 M, respectively. The mechanistic aspects were unveiled from linear sweep voltammetric studies and found that the electrode processes were diffusion-controlled. Finally, the sensor was successfully employed for the simultaneous determination of spiked amount of XA and HX in synthetic urine and serum samples.

High redox potential promotes oxidation of pyrite under neutral conditions: Implications for optimizing pyrite autotrophic denitrification

Pyrite autotrophic denitrification (PAD) represents an important natural attenuation process of nitrate pollution and plays a pivotal role in linking nitrogen, sulfur, and iron cycles in a variety of anoxic environments. However, there are knowledge gaps about the oxidation mechanism of pyrite under anaerobic neutral conditions. This study explored the performance of PAD in the presence of EDTA and revealed the mechanism of anaerobic pyrite oxidation and microbial mineral transformation. It was demonstrated that ~200 mV was the electrochemical threshold for converting pyrite into bioavailable forms in PAD conditions, and accelerated pyrite oxidation by Fe³⁺-EDTA complexes can improve the performance of PAD effectively. Furthermore, genus related to sulfur and nitrogen cycle (Sulfurimonas, Denitrobacter) were found at higher abundances in cultures containing EDTA. The analysis of metagenomic binning showed that the microbial community in PAD culture with EDTA addition exhibited higher levels of functional diversity and redundancy. These results will further the understanding of the oxidation mechanism of pyrite under anaerobic neutral conditions and the corresponding microbial activities, and provide insights into the practical application of PAD.

Stabilization of uranyl(v) by dipicolinic acid in aqueous medium

Preparation of a stable U(v) complex in an aqueous medium is a challenging task owing to its disproportionation nature (conversion into more stable U(vi) and U(iv) species) and sensitivity to atmospheric oxygen. The stable uranyl (UO₂²⁺)/dipicolinic acid (DPA) complex ([U^(VI)O₂(DPA)(OH)(H₂O)]^-) was formed at pH 10.5-12.0, which was confirmed by potentiometric and spectrophotometric titrations, and NMR, ESI-MS and EXAFS spectroscopy. The complex [U^(VI)O₂(DPA)(OH)(H₂O)]^- can be electrochemically reduced on the Pt electrode at -0.9 eV (vs. Ag/AgCl) to [U^(V)O₂(DPA)(OH)(H₂O)]^2- in aqueous medium under an anaerobic environment. According to cyclic voltammetric analysis, a pair of oxidation and reduction waves at E'₀ = -0.592 V corresponds to the [U^(VI)O₂(DPA)(OH)(H₂O)]^-/[U^(V)O₂(DPA)(OH)(H₂O)]^2- redox couple and the formation of [U^(V)O₂(DPA)(OH)(H₂O)]^2- was confirmed by the electron stoichiometry (n = 0.97 ± 0.05) of the reduction reaction of [U^(VI)O₂(DPA)(OH)(H₂O)]^-. The pentavalent uranyl complex [U^(V)O₂(DPA)(OH)(H₂O)]^2- was further characterized via UV-vis-NIR absorption spectrophotometry and X-ray absorption (XANES and EXAFS) spectroscopy. The [U^(V)O₂(DPA)(OH)(H₂O)]^2- complex is stable at pH 10.5-12.0 in anaerobic water for a few days. DFT calculation shows the strong complexing ability of DPA stabilizing the unstable oxidation state U(v) in aqueous medium.

Influence of sulfuric acid concentration in the simultaneous voltammetric determination of uranium and plutonium in nuclear fuels

Interfacial coupled chemical reaction between U(iv) (formed at the electrode surface) and Pu(iv) (diffuses from the bulk towards the electrode) regenerates U(vi) at the electrode-solution interface and causes enhancement in the U(vi) reduction current, thus creating problems in the simultaneous voltammetric determination of U and Pu. Despite such interference between U(iv) and Pu(iv), the simultaneous voltammetric determination of U and Pu in FBTR Mark-1 fuel samples in sulfuric acid (1 M H₂SO₄) on a poly(3,4-ethylenedioxythiophene) (PEDOT)-poly(styrenesulfonate) (PSS)-modified glassy-carbon (GC) electrode (PEDOT-PSS/GC) has been reported. However, the reported method is applicable only for FBTR mark-1 fuel samples, in which the ratio [Pu]/[U] > 2 is always maintained. For nuclear samples having [Pu]/[U] < 2 (e.g., PFBR fuel), the simultaneous voltammetric determination of U and Pu is extremely challenging. Herein, we report a modified version of the earlier method for the simultaneous determination of U and Pu in nuclear samples ((U, Pu)C and (U, Pu)O₂), irrespective of the [Pu]/[U] ratio. The effect of acidity (H₂SO₄ conc.) on the coupled chemical reaction between U(iv) and Pu(iv) was examined. It was observed that an increase in the acidity of H₂SO₄ minimized the coupled chemical reaction, and at 5 M H₂SO₄, change in the Pu(iv) concentration did not have any effect on the U(vi) reduction current. The coupled chemical reaction between U(iv) and Pu(iv) ceased at 5 M H₂SO₄ and hence, the simultaneous voltammetric determination of U and Pu was possible on PEDOT-PSS/GC, irrespective of the [Pu]/[U] ratio in 5 M H₂SO₄. The method was applied for both (U, Pu)O₂ (PFBR) and (U, Pu)C (FBTR) samples and was compared with the well-established biamperometric method. The present method shows accuracy and precision comparable to biamperometry and did not show any interference from the commonly encountered impurities in nuclear samples. Thus, both FBTR and PFBR nuclear fuels having different [Pu]/[U] ratios can be analyzed by the present approach and it is a strong competitor to replace the well-established biamperometric method for routine sample analysis.

Electron Transfer between Electrically Conductive Minerals and Quinones

Long-distance electron transfer in marine environments couples physically separated redox half-reactions, impacting biogeochemical cycles of iron, sulfur and carbon. Bacterial bio-electrochemical systems that facilitate electron transfer via conductive filaments or across man-made electrodes are well-known, but the impact of abiotic currents across naturally occurring conductive and semiconductive minerals is poorly understood. In this paper I use cyclic voltammetry to explore electron transfer between electrodes made of common iron minerals (magnetite, hematite, pyrite, pyrrhotite, mackinawite, and greigite), and hydroquinones—a class of organic molecules found in carbon-rich sediments. Of all tested minerals, only pyrite and magnetite showed an increase in electric current in the presence of organic molecules, with pyrite showing excellent electrocatalytic performance. Pyrite electrodes performed better than commercially available glassy carbon electrodes and showed higher peak currents, lower overpotential values and a smaller separation between oxidation and reduction peaks for each tested quinone. Hydroquinone oxidation on pyrite surfaces was reversible, diffusion controlled, and stable over a large number of potential cycles. Given the ubiquity of both pyrite and quinones, abiotic electron transfer between minerals and organic molecules is likely widespread in Nature and may contribute to several different phenomena, including anaerobic respiration of a wide variety of microorganisms in temporally anoxic zones or in the proximity of hydrothermal vent chimneys, as well as quinone cycling and the propagation of anoxic zones in organic rich waters. Finally, interactions between pyrite and quinones make use of electrochemical gradients that have been suggested as an important source of energy for the origins of life on Earth. Ubiquinones and iron sulfide clusters are common redox cofactors found in electron transport chains across all domains of life and interactions between quinones and pyrite might have been an early analog of these ubiquitous systems.

Simultaneous determination of guanine and adenine in the presence of uric acid by a poly(para toluene sulfonic acid) mediated electrochemical sensor in alkaline medium

Simultaneous sensing of guanine and adenine in presence of uric acid in alkaline medium by polymer modified electrode.

Electrochemistry of actinides on reduced graphene oxide: craving for the simultaneous voltammetric determination of uranium and plutonium in nuclear fuel

Schematic representation of the interference of Pu(iv) in the voltammetric determination of U in a mixed U–Pu solution in 1 M H₂SO₄.

Electrochemical studies of U(VI)/U(IV) redox reaction in 1M H2SO4 at single-walled carbon nanotubes (SWCNTs) modified gold (Au) electrode

Electrochemistry of U(VI)/U(IV) couple in 1 M H2SO4 was studied on bare and SWCNT modified gold (Au) electrodes by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The gold electrode modified with single-walled carbon nanotubes (SWCNTs-Au) was characterized by scanning electron microscopy (SEM). Electrocatalysis of U(VI)/U(IV) redox reaction was observed on SWCNT-Au. The lower charge transfer resistance at SWCNT-Au promoted the rate of electron transfer reaction of U(VI)/U(IV) couple. These results are interesting to develop electroanalytical methodologies for uranium determination using SWCNT modified electrodes. To the best of our knowledge, this is the first study on the electrocatalysis of uranium on SWCNT modified electrode.