Reduction Of S2O82 Research Articles

The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction.

Aqueous solutions containing both the strong oxidant, peroxydisulfate (S2O82-), and the strong reductant, oxalate (C2O42-), are thermodynamically unstable due to the highly exothermic homogeneous redox reaction: S2O82- + C2O42- → 2 SO42- + 2 CO2 (ΔG0 = -490 kJ/mol). However, at room temperature, this reaction does not occur to a significant extent over the time scale of a day due to its inherently slow kinetics. We demonstrate that the S2O82-/C2O42- redox reaction occurs rapidly, once initiated by the Ru(NH3)62+-mediated 1e- reduction of S2O82- to form S2O83•-, which rapidly undergoes bond cleavage to form SO42- and the highly oxidizing radical SO4•-. Theoretically, the mediated electrochemical generation of a single molecule of S2O83•- can initiate an autocatalytic cycle that consumes both S2O82- and C2O42- in bulk solution. Several experimental demonstrations of S2O82-/C2O42- autocatalysis are presented. Differential electrochemical mass spectrometry measurements demonstrate that CO2 is generated in solution for at least 10 min following a 30-s initiation step. Quantitative bulk electrolysis of S2O82- in solutions containing excess C2O42- is initiated by electrogeneration of immeasurably small quantities of S2O83•-. Capture of CO2 as BaCO3 during electrolysis additionally confirms the autocatalytic generation of CO2. First-principles density functional theory calculations, ab initio molecular dynamics simulations, and finite difference simulations of cyclic voltammetric responses are presented that support and provide additional insights into the initiation and mechanism of S2O82-/C2O42- autocatalysis. Preliminary evidence indicates that autocatalysis also results in a chemical traveling reaction front that propagates into the solution normal to the planar electrode surface.

Copper doped terbium metal organic framework as emitter for sensitive electrochemiluminescence detection of CYFRA 21-1

Lanthanide metal organic frameworks (L-MOFs) are emerging as promising electrochemiluminescence (ECL) emitters for bioanalysis. This work proposed a copper doped terbium MOF as a luminescent tag for construction of a “signal-on” ECL immunosensing method. The Tb–Cu-PA MOF was prepared using Tb3+ and Cu2+ ions as metal linkers and m-phthalic acid as bridge ligand, and exhibited strong ECL emission with K2S2O8 as a coreactant. The immunosensor was prepared by immobilizing capture antibody on Pd nanoparticles modified Ni–Co layered double hydroxide (Pd-ZIF-67@LDH) nanoboxes, which showed strong electrocatalytic activity toward the reduction of S2O82− for amplifying the ECL signal. Upon the sandwich-typed immunoreactions, Tb–Cu-PA MOF labeled antibody was introduced onto the immunosensor for sensitive ECL detection of target protein. Using cytokeratin 19 fragment 21–1 (CYFRA21-1), a representative lung cancer biomarker, as target model, the ECL immunosensing method showed a linear range of 0.01–100 ng/mL and a detection limit of 2.6 pg/mL (S/N = 3). This immunosensing strategy highlighted the advances of using luminescent and electroactive MOFs in the developments of highly efficient immunosensors for bioanalysis.

High-Efficiency CNNS@NH2-MIL(Fe) Electrochemiluminescence Emitters Coupled with Ti3C2 Nanosheets as a Matrix for a Highly Sensitive Cardiac Troponin I Assay.

Synthesis of a highly efficient electrochemiluminescence (ECL) luminescent material is one of the effective means to improve the sensitivity of the sensor. In this study, an efficient ECL luminescent nanomaterial, carbon nitride nanosheet (CNNS) decorated amino-functional metal-organic frameworks (CNNS@NH2-MIL(Fe)) were synthesized for sensitive ECL detection of cardiac troponin I (cTn-I). The synthesized CNNS@NH2-MIL(Fe) realized the effective mass loading of CNNS, and more importantly, the NH2-MIL(Fe) could expedite the reduction of coreactant S2O82- to produce abundant ECL reaction intermediate SO4•- near CNNS, thus, shortening the distance between SO4•- and the excited state of CNNS with less energy loss to extremely enhance the ECL signal of CNNS. Furthermore, the ECL signal of the immunosensor could be further enhanced when the Ti3C2 nanosheet was used as the matrix to capture primary anti-cTn-I due to the reason that Ti3C2 not only exhibited a large surface area and excellent metallic conductivity, but also could act as a coreaction accelerator to speed up the reduction of S2O82- with plenty of SO4•- generated. Therefore, this proposed ECL immunosensor using CNNS@NH2-MIL(Fe) as a signal probe and Ti3C2 as a sensing matrix exhibited a significantly enhanced ECL signal and had a high sensitivity and excellent selectivity for cTn-I. Consequently, this multiple signal amplification strategy provided an effective method for trace protein ultrasensitive detection in ECL bioanalysis.

Solid molybdenum nitride microdisc electrodes: Fabrication, characterisation, and application to the reduction of peroxodisulfate

A new methodology was developed to fabricate solid molybdenum nitride microdisc electrodes for the first time. The MoN microrods were produced by heating Mo microwires in dry NH3 atmosphere for several hours. They were characterised by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The latter revealed the samples had crystallised in the δ3-MoN phase with a core of γ-Mo2N. Their electrochemical behaviour was probed for the reduction of Ru(NH3)63+. For this fast electron transfer the MoN microdisc electrodes returned similar voltammetric features to Pt microelectrodes. Their amperometric response was further tested with the reduction of peroxodisulfate. In contrast with other electrode materials, the reduction of S2O82− on MoN microdiscs delivered steady state voltammograms with well-defined diffusion controlled plateau. At low sweep rates, the limiting current was consistent with hemispherical diffusion and stable for at least 500 s. The diffusion coefficient of S2O82− derived from these results, 9.5 × 10−6 cm2 s−1, is in excellent agreement with previous work. At high sweep rates, the reduction of peroxodisulfate was found to be complicated by the simultaneous reduction of adsorbates. The results indicate that MoN is an ideal electrode material to monitor the concentration of peroxodisulfate under steady state conditions.

Polyoxometalate/chitosan–electrochemically reduced graphene oxide as effective mediating systems for electrocatalytic reduction of persulfate

Heteropolymolybdate H4SiMo12O40 (SiMo12) and chitosan–electrochemically reduced graphene oxide (CS-ERGO) multilayer composite films have been processed on Indium tin oxide electrode (ITO) through electrochemical growth method. The use of CS favored the adsorption of negatively charged SiMo12 anions and thus facilitated the growth of the multilayered films. The presence of ERGO enhanced the area of conductive surface, which is beneficial for the increase of the coverage value of SiMo12 on the electrode. By combining these two materials, the loading of SiMo12 was significantly increased. Thus, the CS-ERGO/SiMo12 multilayer films provide more catalytically active centers, which results in a high level of catalytic activity towards the reduction of S2O82−. Results of scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry indicated the successful fabrication of CS-ERGO/SiMo12 multilayer film modified electrode. Application of the proposed electrode in electrochemical sensing was further demonstrated by electrocatalytic reduction of persulfate. Amperometry showed that the proposed sensor exhibited a linear detection range for S2O82− from 0.67 to 30.62μM with excellent sensitivity of 0.0448μAμM−1 and a low detection limit of 0.05μM, which outperformed most previously reported electrochemical sensors. In addition, good anti-interference and stability was also demonstrated. These features make the new SiMo12 modified electrode an ideal substrate for S2O82− detection.

Electrochemical-Reduction-Assisted Fabrication of a Polyoxometalate/Graphene Composite Film Electrode and Its Electrocatalytic Performance

A nanocomposite film modified electrode based on electrochemically reduced graphene oxide (ERGO) and polyoxometalate (POM) clusters was prepared. In this process, the Keggin-type heteropolymolybdate H4SiMo12O40 (SiMo12) was chosen as the representative POM. The POM/graphene nanocomposite film was constructed by electrochemical reduction of GO onto the Indium tin oxide electrode, which was used as matrix to form multilayer films by electrochemical growth method with SiMo12. The strong adsorption effect between the SiMo12 clusters and ERGO nanosheets induces the assembly of SiMo12 on ERGO in a uniformly dispersed state, forming a porous nanocomposite film. More importantly, the film modified electrode exhibited high electrocatalytic activities toward the reduction of S2O82− and NO2−. The enhanced electrochemical performances of (SiMo12/ERGO)6 hybrids could be mostly attributed to the synergistic effect of ERGO and SiMo12 clusters. Considering the facile and environment-friendly features of this method, it is sensible as a general route for the incorporation of various types of POMs onto graphene matrices. This may allow the POM/graphene modified electrode for applications in the fields of catalysis and electrochemical sensor.

Interfacial bond-breaking electron transfer in mixed water-ethylene glycol solutions: reorganization energy and interplay between different solvent modes.

We explore solvent dynamics effects in interfacial bond breaking electron transfer in terms of a multimode approach and make an attempt to interpret challenging recent experimental results (the nonmonotonous behavior of the rate constant of electroreduction of S2O82– from mixed water–EG solutions when increasing the EG fraction; see Zagrebin, P.A. et al. J. Phys. Chem. B2010, 114, 311). The exact expansion of the solvent correlation function (calculated using experimental dielectric spectra) in a series predicts the splitting of solvent coordinate in three independent modes characterized by different relaxation times. This makes it possible to construct a 5D free-energy surface along three solvent coordinates and one intramolecular degree of freedom describing first electron transfer at the reduction of a peroxodisulphate anion. Classical molecular dynamics simulations were performed to study the solvation of a peroxodisulphate anion (S2O82–) in oxidized and reduced states in pure water and ethylene glycol (EG) as well as mixed H2O–EG solutions. The solvent reorganization energy of the first electron-transfer step at the reduction of S2O82– was calculated for several compositions of the mixed solution. This quantity was found to be significantly asymmetric. (The reorganization energies of reduction and oxidation differ from each other.) The averaged reorganization energy slightly increases with increasing the EG content in solution. This finding clearly indicates that for the reaction under study the static solvent effect no longer competes with solvent dynamics. Brownian dynamics simulations were performed to calculate the electron-transfer rate constants as a function of the solvent composition. The results of the simulations explain the experimental data, at least qualitatively.

Oscillatory Peroxodisulfate Reduction on Pt and Au Electrodes under High Ionic Strength Conditions, Caused by the Catalytic Effect of Adsorbed OH

Four electrochemical oscillations of different types (named oscillations α, β, γ, and δ), which are different from previously reported Frumkin-type oscillations in low ionic strength electrolytes, appear for peroxodisulfate (S2O82-) reduction on Pt and Au electrodes under high ionic strength conditions. Impedance measurements for oscillation γ that appears in a region of the most positive potentials have shown that it is classified as hidden negative differential resistance (hidden-NDR or HNDR) oscillators. Detailed electrochemical measurements have suggested that the NDR arises from a decrease (with a negative potential shift) in the surface coverage of adsorbed OH, which acts as a catalyst for the dissociative adsorption of S2O82- (the first step of its reduction). The NDR is hidden by the desorption (with the negative potential shift) of adsorbed sulfate (SO42-) produced by the dissociative adsorption and reduction of S2O82-, which works as a site-blocking agent. Mathematical simulation based on this m...

Visible light emission from a porous silicon/solution diode

Reduction of S2O82− ions at the interface between an n-type porous Si electrode and an aqueous solution gives rise to electroluminescence showing evidence of quantization effects. A broad emission band with a maximum at 670 nm is observed similar to the photoluminescence spectrum of the same layers. The results suggest that holes are ‘‘injected’’ into the valence band of the porous semiconductor from an intermediate of the reduction reaction, the SO4−⋅ radical ion. The resulting electron-hole recombination is responsible for the visible light emission.