Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. −0.44 V (vs. Ag|AgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed.     

Surface-enhanced Raman scattering of 4-aminobenzenethiol on silver nanoparticles substrate

Active surface-enhanced Raman scattering (SERS) silver nanoparticles substrate was prepared by multiple depositions of Ag nanoparticles on glass slides. The substrate is based on five depositions of Ag nanoparticles on 3-aminopropyl-trimetoxisilane (APTMS) modified glass slides, using APTMS sol–gel as linker molecules between silver layers. The SERS performance of the substrate was investigated using 4-aminobenzenethiol (4-ABT) as Raman probe molecule. The spectral analyses reveal a 4-ABT Raman signal enhancement of band intensities, which allow the detection of this compound in different solutions. The average SERS intensity decreases significantly in 4-ABT diluted solutions (from 10⁻⁴ to 10⁻⁶ mol L⁻¹), but the compound may still be detected with high signal/noise ratio. The obtained results demonstrate that the Ag nanoparticles sensor has a great potential as SERS substrate.     

Direct electrochemical response of Myoglobin using a room temperature ionic liquid, 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate,as supporting electrolyte

Direct electrochemical response of Myoglobin (Myb) at the basal plane graphite (BPG) electrode was observed when a room temperature ionic liquid (RTIL), 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate ([HEMIm][BF₄]), was used as the supporting electrolyte. In a 0.17 M [HEMIm]BF₄ aqueous solution, a couple of well-defined redox peaks of Myb could be obtained, whose anodic and cathodic peak potentials were at −0.158 and −0.224 V (vs. Ag/AgCl), respectively. Both anodic and cathodic peak currents increased linearly with the potential scan rate. Compared with the supporting electrolyte of phosphate buffer, [HEMIm][BF₄] played an obvious promotion for the direct electron transfer between Myb and the BPG electrode. Further investigation suggested that Myb was adsorbed tightly on the surface of the BPG electrode in the presence of [HEMIm][BF₄] to form a stable, approximate monolayer Myb film. Myb adsorbed on the BPG electrode surface showed a remarkable electrocatalytic activity for the reduction of oxygen in a [HEMIm][BF₄] aqueous solution. Based on these, a third-generation biosensor could be constructed to directly detect the concentration of oxygen in aqueous solution with a limit of detection of 2.3 × 10⁻⁸ M.     

Surface poisoning during electrocatalytic monosaccharide oxidation reactions at gold electrodes in alkaline medium

In the present study, the surface poisoning of electrocatalytic monosaccharide oxidation reactions at gold electrodes were investigated. In the cyclic voltammetric studies, the electrocatalytic oxidation of aldohexose and aldopentose type monosaccharides, aminosugars, acetyl-glucosamine and glucronamide were observed at gold plate electrodes in alkaline medium. However, in controlled-potential electrolytic studies ranging −0.3 to −0.2 V in reaction solutions, current flows during electrolyses decreased quickly with time, except when glucosamine was used as a substrate.Results from surface enhanced infrared adsorption (SEIRA) spectroscopic measurements at an evaporated gold electrode for the electrocatalytic oxidation of glucose in 0.1 mol dm⁻³ NaOH at −0.3 V and Gaussian simulated spectra indicated that the gluconic acid as a 2-electron oxidation product and/or its analogs adsorbed onto the gold surface. Electrochemical quartz crystal microbalance (EQCM) measurement results, along with surface adsorption results from surface poisoning at the gold electrode during electrolytic reactions, suggested that gluconic acid and/or its analogs adsorbed vertically onto electrode surfaces in a full monolayer packing-like conformation. In the case of the electro oxidation of glucosamine in 0.1 mol dm⁻³ NaOH at −0.2 V, the obtained SEIRA spectra and EQCM results, clearly indicated that the glucosaminic acid as a 2-oxidation glucosamine product did not strongly bind onto the gold electrode surface.     

Sulfur doped reduced graphene oxide as metal-free catalyst for the oxygen reduction reaction in anion and proton exchange fuel cells

Sulfur doped reduced graphene oxide (S-rGO) is investigated for catalytic activity towards the oxygen reduction reaction (ORR) in acidic and alkaline electrolytes. X-ray photoelectron spectroscopy shows that sulfur in S-rGO is predominantly integrated as thiophene motifs within graphene sheets. The overall sulfur content is determined to be approximately 2.2 at.% (elemental analysis). The catalytic activity of S-rGO towards the ORR is investigated by both rotating disc electrode (RDE) and polymer electrolyte fuel cell (PEFC) measurements. RDE measurements reveal onset potentials of 0.3 V and 0.74 V (vs. RHE) in acidic and alkaline electrolyte, respectively. In a solid electrolyte fuel cell with S-rGO as cathode material, this is reflected in an open circuit voltage of 0.37 V and 0.78 V and a maximum power density of 1.19 mW/cm² and 2.38 mW/cm² in acidic and alkaline polymer electrolyte, respectively. This is the first report investigating the catalytic activity of a sulfur doped carbon material in both acidic and alkaline liquid electrolyte, as well as in both proton and anion exchange polymer electrolyte fuel cells.     

Vibrational spectroscopic characterization of the phosphate mineral hureaulite – (Mn,Fe)5(PO4)2(HPO4)2·4(H2O)

This research was done on hureaulite samples from the Cigana claim, a lithium bearing pegmatite with triphylite and spodumene. The mine is located in Conselheiro Pena, east of Minas Gerais. Chemical analysis was carried out by Electron Microprobe analysis and indicated a manganese rich phase with partial substitution of iron. The calculated chemical formula of the studied sample is: (Mn_3.23, Fe_1.04, Ca_0.19, Mg_0.13)(PO₄)_2.7(HPO₄)_2.6(OH)_4.78. The Raman spectrum of hureaulite is dominated by an intense sharp band at 959 cm⁻¹ assigned to PO stretching vibrations of HPO₄²⁻ units. The Raman band at 989 cm⁻¹ is assigned to the PO₄³⁻ stretching vibration. Raman bands at 1007, 1024, 1047, and 1083 cm⁻¹ are attributed to both the HOP and PO antisymmetric stretching vibrations of HPO₄²⁻ and PO₄³⁻ units. A set of Raman bands at 531, 543, 564 and 582 cm⁻¹ are assigned to the ν₄ bending modes of the HPO₄²⁻ and PO₄³⁻ units. Raman bands observed at 414, and 455 cm⁻¹ are attributed to the ν₂ HPO₄²⁻ and PO₄³⁻ units. The intense A series of Raman and infrared bands in the OH stretching region are assigned to water stretching vibrations. Based upon the position of these bands hydrogen bond distances are calculated. Hydrogen bond distances are short indicating very strong hydrogen bonding in the hureaulite structure. A combination of Raman and infrared spectroscopy enabled aspects of the molecular structure of the mineral hureaulite to be understood.     

Electrochemistry of Actinides and Selected Fission Products in the Head End of Spent Nuclear Fuel Reprocessing

The data on the uranium metal corrosion rate in the solutions of nitric acid (0,1 – 4 M) and effect of complex forming agents on uranium corrosion properties are presented. The increase of HNO₃ concentration caused the shift of corrosion potential from 38 mV to 446 mV and the increase of the corrosion rate from 0,02 to 0,62 mg.cm^-2h^-1. Transpassivation potential of U metal was found weakly effected by HNO₃ concentration varying from 448 to 470 mV/Ag/AgCl. The addition of HCOOH to the electrolytes containing less than 3 M HNO₃ found to shift the values of corrosion potentials about 500 mV towards negative direction reducing the passivation of U metal. The data on the kinetics of oxidative dissolution of PuO₂ using Ag(II) and Am(VI,V) as mediators and the effect of the mediator generation techniques are discussed. The electrochemical properties of UC in the solutions 2 – 4 M HNO₃, results of the quantitative determination of “oxidizable carbon” in dissolver solutions are presented. The results of corrosion and dissolution studies of Tc metal and Tc - Ru alloys containing from 19 to 70 at.% Ru in 0.5 0– 6 M HNO3 indicate the formation of passive films of Tc(IV) – Ru(III,IV) hydroxides at the electrode surface in the solutions containing less than 2 M HNO₃ at the potentials less than 650 mV/Ag/AgCl. The increase of HNO3 concentration to values exceeding 3 M and the shift of the electrode potential towards positive direction causes the transition of the Tc and Tc-Ru alloys to transpassive state. The values of transpassivation potentials increase with the increasing with HNO₃ concentration. Quantitative dissolution of Tc metal without application of oxidation potential becomes possible in the electrolytes, containing more than 4 M HNO₃. The rate of Tc – Ru alloys dissolution is noticed to slow down with the increase of Ru content in the alloy.     

Raman spectroelectrochemical study of Toluidine Blue,adsorbed and electropolymerized at a gold electrode

The redox dye Toluidine Blue (TB), adsorbed and electropolymerized at a roughened gold electrode, has been studied by Raman spectroscopy at λ_ex of 676.4 nm in an electrochemical cell. Most of the spectral bands have been assigned based on density functional theory (DFT) calculations. The number and position of the bands, as well as their intensity depend on electrode potential and solution pH, allowing one to discern different redox forms of TB or its polymer. Raman spectra, obtained in an acidic solution (pH 1.0) at a low electrode potential (−0.2 to 0.0 V vs. Ag/AgCl) are of low intensity, and correspond to the reduced (leucoform) of TB. At a higher potential (0.2–1.0 V), the intensity increases significantly, presumably due to resonance enhancement, and new bands appear, corresponding to oxidized form of TB. In a pH-neutral solution, the changes in spectra with varying electrode potential are less expressed. The observed changes in band positions and intensities have been analyzed.     

Direct electrochemistry of hemoglobin and its electrocatalytic effect based on its direct immobilization on carbon ionic liquid electrode

Direct electrochemistry of hemoglobin (Hb) has been achieved by its direct immobilization on carbon ionic liquid electrode (CILE). CILE was immersed in a solution containing Hb and ionic liquid, octylpyridinium chloride ([OcPy][Cl]), to directly immobilize Hb on CILE. Cyclic voltammetry of modified electrode exhibited quasi-reversible peaks corresponding to Hb. The oxidation and reduction peak potentials of immobilized Hb in acetate buffer solution, pH 5.0 and at a scan rate of 0.1 V s⁻¹ were obtained at about –150 mV and –290 mV, respectively. The average surface coverage of the electroactive Hb adsorbed on the electrode surface was calculated as 8.4 × 10⁻¹¹ mol cm⁻². Hb retained its bioactivity on modified electrode and showed excellent electrocatalytic activity towards oxygen, hydrogen peroxide and nitrite. Hydrogen peroxide can be determined in the range of 1.0 × 10⁻⁴–5.0 × 10⁻³ M.     

Controlled-potential electrosynthesis of glucosaminic acid from glucosamine at a gold electrode

The electrocatalytic oxidation of d-glucosamine (2-amino-2-deoxy-d-glucose) in alkaline and neutral solutions was examined using a carbon felt electrode modified with 2 nm core sized gold nanoparticles (Au_2 nm nanoparticles) and a gold plate electrode. The electrocatalytic voltammetric oxidation curves of d-glucosamine were obtained in both solutions. The voltammetric responses for the electrocatalytic oxidation at a Au_2 nm nanoparticle-modified electrode in both alkaline and neutral solutions were almost the same to those at a gold plate electrode. The oxidized product was identified to be d-glucosaminic acid (2-amino-2-deoxy- d-gluconic acid) generated by the 2-electron oxidation product of d-glucosamine by electrospray ionization time-of-flight mass spectra (ESI TOF-MS). The HPLC results also indicated that the oxidation product was d-glucosaminic acid.The controlled-potential electrolysis of d-glucosamine was performed at the Au_2 nm nanoparticle-modified carbon felt electrodes in both alkaline and neutral solutions. In the alkaline solution, at a potential of −0.2 V, d-glucosaminic acid was formed with a current efficiency of 100%. In the neutral solution, electrolysis at 0.4 V on d-glucosaminic acid was obtained with current efficiencies of 70%.     

Electro-reduction of cuprous chloride powder to copper nanoparticles in an ionic liquid

Cyclic voltammetry of the CuCl powder in a cavity microelectrode revealed direct electro-reduction in solid state in 1-butyl-3-methylimidazolium hexafluorophosphate. Potentiostatic electrolysis of the salt powder (attached to a current collector) in the ionic liquid produced Cu nanoparticles as confirmed by X-ray diffraction, energy dispersive X-ray analysis, scanning and transmission electron microscopy. The particle size decreased down to 10 nm when the electrode potential was shifted from −0.9 V to −1.8 V (versus Ag/Ag⁺). The electro-reduction and the nanoparticle formation mechanisms were investigated in the ionic liquid and also in aqueous 0.1 mol L⁻¹ KClO₄ in which larger Cu particles were obtained.     

Effect of the reference solution in the measurement of ion activity coefficients using cells with transference at T = 298.15 K

This work reports individual activity coefficients of ions at T = 298.15 K in aqueous solutions obtained from voltage values of the respective half-cell ion-selective-electrode and a single-junction Ag–AgCl reference electrode, filled with different reference solutions at different concentrations. For potassium and chloride ions in KCl aqueous solutions, reference solutions of KCl, NaCl, or CsCl were used. For sodium and chloride ions in aqueous NaCl solutions, reference solutions of CsCl were used. Experimental runs were performed at molalities (1, 2, and 3) m of the reference solution. The concentration of the sample solution was increased, starting from around 1 · 10^?3 m, up to the molality of the reference solution. The values of activity coefficients are calculated using the Henderson equation to estimate the liquid-junction potential. Results show that the ionic activity coefficients are independent of the nature and concentration of reference solution.     

Direct electrochemical response of cytochrome c on a room temperature ionic liquid,N-butylpyridinium tetrafluoroborate,modified electrode

Cytochrome c (Cyt c) and a room temperature ionic liquid (RTIL), 2-methyl-N-butylpyridinium tetrafluoroborate ([2-MBPy][BF₄]), were immobilized on the surface of a basal plane graphite (BPG) electrode. Direct electron transfer from Cyt c to BPG electrode was clearly observed. A pair of well-defined and reversible redox peaks could be obtained in a 0.13 M [2-MBPy][BF₄] aqueous solution. The anodic and cathodic peak potentials of Cyt c were at −0.064 and −0.020 V (vs. Ag/AgCl), respectively. The results showed that [2-MBPy][BF₄] promoted the direct electron transfer between Cyt c and the BPG electrode. Cyt c immobilized on BPGE can catalyze the reduction of hydrogen peroxide. Based on this, a biosensor can be constructed to detect quantitatively hydrogen peroxide.     

Amperometric enzyme biosensor for hydrogen peroxide via Ugi multicomponent reaction

A novel strategy based on the Ugi multicomponent reaction was employed for immobilizing horseradish peroxidase on sodium alginate-coated gold electrode. The electrode was employed for constructing an amperometric biosensor device using 1 mM hydroquinone as electrochemical mediator. The electrode showed linear response (poised at −300 mV vs Ag/AgCl) toward H₂O₂ concentration between 70 μM and 8.8 mM at pH 7.0. The biosensor reached 95% of steady-state current in about 12 s and its sensitivity was 33.8 mA/M cm². The electrode retained full initial activity after 30 days of storage at 4 °C in 50 mM sodium phosphate buffer, pH 7.0.     

Surface-enhanced Raman scattering of 4,4′-bipyridine on silver by density functional theory calculations

Surface-enhanced Raman scattering (SERS) of 4,4′-bipyridine (BPy) on silver foil substrate was measured using the 488, 514.5, and 1064 nm excitation lines. Density functional theory (DFT) methods were used to calculate the structure and vibrational spectra of Ag–BPy, Ag₃–BPy and Ag₄–BPy complexes with B3LYP/6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set. The Raman bands of BPy were assigned on the basis of the calculation of potential energy distribution. The calculated spectra of Ag–BPy and Ag₄–BPy complexes were much closer to the experimental results of BPy adsorbed on silver surface than that of Ag₃–BPy complexes. The vibrational frequencies that are sensitive to the planar or non-planar structure of BPy and to the dihedral angle of two pyridyl rings were discussed. The DFT results showed that the angles between two pyridyl rings for Ag–BPy and Ag₄–BPy were skewed by about 38.44° and 37.1°, respectively. The energy gaps of the HOMO and LUMO from DFT were 415–912 nm for BPy–Ag complexes. The relative intensities of SERS bands changed with different excitation laser lines. Thus, a chemical enhancement mechanism should play an important role in the SERS of BPy on silver substrate.     

Determination of secnidazole in tablets and human serum by cathodic adsorptive stripping voltammetry

The electrochemical reduction of secnidazole was carried out in BR buffer solutions in the pH range 2.0–11.8 by dc polarography. The polarograms exhibited two irreversible reduction waves in acidic media and one wave in alkaline media, corresponding to the reduction of nitro group in the drug. The cathodic adsorptive voltammetric behavior was studied on glassy carbon electrode to optimize an analytical method for determination of secnidazole. The drug was determined in the range between 4.0 × 10^?6 and 1.2 × 10^?4 mol L^?1. The proposed method was successfully applied to the determination of the drug content in tablets with mean recovery and relative standard deviation of 100.91% and 1.82%, respectively. It was also applied to human serum with a good precision and accuracy.     

Quasi-reversible two-electron reduction of oxygen at gold electrodes modified with a self-assembled submonolayer of cysteine

The electrochemical reduction of molecular oxygen (O₂) has been performed at gold electrodes modified with a submonolayer of a self-assembly (sub-SAM/Au) of a thiol compound (typically cysteine (CYST)) in O₂-saturated 0.5 M KOH. At bare gold electrode O₂ reduction reaction proceeds irreversibly, while this reaction is totally hindered at gold electrodes with a compact structure of CYST over its surface. The partial reductive desorption of the compact CYST monolayer was achieved by controlling the potential of the CYST/Au electrode, leading to the formation of a submonolayer coverage of the thiol compound over the Au electrode surface (sub-SAM/Au), at which the CYST molecules selectively block the Au(1 0 0) and Au(1 1 0) fractions (the so-called rough domains) of the polycrystalline Au while the Au(1 1 1) component (the so-called smooth domains) remains bare (i.e., uncovered with CYST). This sub-SAM/Au electrode extraordinarily exhibits a quasi-reversible two-electron reduction of molecular oxygen (O₂) in alkaline medium with a peak separation (ΔE_p) between the cathodic and anodic peak potentials (E_p^c,E_p^a) of about 60 mV. The ratio of the anodic current to the cathodic one is close to unity. The formal potential (E^o′) of this reaction is found to equal −150 mV vs. Ag/AgCl/KCl(sat.).     

Spontaneous formation of an electroactive co-polymeric film derived from nordihydroguaiaretic acid and 4,4′-diaminobibenzyl

Binary solution of nordihydroguaiaretic acid (NDGA) and 4,4′-diaminobibenzyl (DABB) undergoes rapid oxidation by ambient oxygen to form a thin film of poly-NDGA-co-DABB on the surface of the reaction chamber and on immersed substrates. Electrochemistry of thus formed films was studied in 0.1 M sulfuric acid and in phosphate buffer (pH 7.4). Electrochemical behavior of the co-polymeric film is characterized by two redox couples, the predominant one being observed at more negative potentials comparing to parent NDGA i.e. 0.28 vs. 0.49 V (vs. Ag/AgCl) in 0.5 M H₂SO₄. The peak potentials were shifted toward lower values with solution pH at the rate of 59 mV/pH unit indicating a 2e/2H⁺ transition as expected for quinone-containing films. The poly-NDGA-co-DABB film exhibits catalytic activity toward electroreduction of nitrite to nitric oxide in acidic electrolytes. This reaction can be used to quantify nitrite in a broad concentration range with low detection limit (0.3 μM, S/N = 3).     

Co-deposition of arsenic and arsine on Pt,Cu, and Fe electrodes

An electrochemical investigation of arsenic in aqueous solutions was carried out in order to assess the possibility of removing it by reduction from As(III) to its elemental form. Arsine evolution was significant at potentials below −0.650 V_SCE on Cu, and below −0.728 V_SCE on Pt. As(V) could also be removed on Cu, with a larger evolution of arsine. When a potential equal to −0.560 V_SCE was applied to an iron electrode, arsenic deposition took place simultaneously with iron dissolution, and arsine evolution was negligible.     

Investigation of the electrocatalytic activity of boron-doped diamond electrodes modified with palladium or gold nanoparticles for oxygen reduction reaction in basic medium

The paper reports on the electrocatalytic activity of boron-doped diamond (BDD) electrodes electrochemically modified with palladium (Pd) or gold nanoparticles (Au NPs) towards oxygen reduction reaction (ORR) in alkaline medium. The BDD/Pd NP interface shows a well-defined diffusion-controlled voltammetric oxygen reduction peak at −0.25 V vs. Ag/AgCl. This is more positive than the ORR peak at −0.59 V vs. Ag/AgCl observed on BDD/Au-NP composite electrodes. The ORR proceeds via a four-electron process in both cases.