Emerson Schwingel Ribeiro

Electrochemical property of methylene blue redox dye immobilized on porous silica–zirconia–antimonia mixed oxide

The mixed oxide SiO2/ZrO2 obtained by the sol–gel processing method adsorbs Sb(V) from acid solution, forming a new phase on the matrix surface designated as SiO2/ZrO2/Sb2O5. The amount of Zr(IV) in the matrix was 8.1 wt.%, corresponding to 0.89 mmol g−1, and the amount of Sb(V) incorporated in the matrix was 6.3 wt.% (0.52 mmol g−1). This material presented a specific surface area, SBET=590 m2 g−1. The immobilized Sb2O5 is a good cation exchanger and adsorbs methylene blue (MB) cationic dye with an average surface density δ=1.1×10−11 mol cm−2 (about 0.07 molecule nm−2). The dye is strongly entrapped in the pores of the matrix and it is not leached off in KCl solutions as concentrated as 1.0 M. A carbon paste of the SiO2/ZrO2/Sb2O5/MB material showed a reversible redox pair with midpoint potential Em=−0.1 V versus SCE. This midpoint potential is not affected by solution pH change between 2 and 7. The immobilized dye mediated NADH catalytic oxidation at ca. 50 mV.

Novel on-line sequential preconcentration system of Cr(III) and Cr(VI) hyphenated with flame atomic absorption spectrometry exploiting sorbents based on chemically modified silica.

In the present study, a flow injection system using dual mini-columns, SiO(2)/Al(2)O(3)/TiO(2) and silica gel functionalized with [3-(2-aminoethylamino)propyl] trimethoxysilane (SiO(2)/AAPTMS) for the sequential preconcentration of Cr(III) and Cr(VI), respectively, from water samples with FAAS detection was proposed. A two-level full factorial design (2(4)) and desirability function were employed for the optimization of variables related to the system performance. The detection limits of 0.66 and 0.27 μg L(-1) for Cr(III) and Cr(IV), respectively, were obtained under the optimized preconcentration conditions (flow rate of 7.0 mL min(-1)), pH 5.0, buffer concentration (acetate buffer) of 0.01 mol L(-1), and eluent (2.5 mol L(-1) HCl) flow rate of 5.0 mL min(-1). The other parameters including preconcentration factor (PF), consumptive index (CI), and concentration efficiency (CE) were found to be 17.62/32.98, 1.13/0.6 mL, and 6.2/11.54 min(-1) for Cr(III)/Cr(VI), respectively. The developed method was applied to the Cr(III) and Cr(VI) determination in water samples [tap, lake and mineral water, artificial saliva and parenteral solutions (physiological serum, water for injection, and glucose physiological solution)]. The method accuracy was checked by the analysis of standard reference materials (trace elements in water).

Cobalt(II) hematoporphyrin IX and protoporphyrin IX complexes immobilized on highly dispersed titanium(IV) oxide on a cellulose microfiber surface: electrochemical properties and dissolved oxygen reduction study

Hematoporphyrin IX (8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21H-23H-porphine-2,18-dipropionic acid) and protoporphyrin IX (8,13-divinyl-3,7,12,17-tetramethyl-21H-23H-porphine-2,18-dipropionic acid) were efficiently immobilized on a cellulose/titanium (IV) oxide composite fiber surface by the reaction of the porphyrin COOH groups with TiO2, presumably by forming the COOTi chemical bond. Furthermore, Co(II) was incorporated into the porphyrin ring, with this reaction being followed by UV–vis spectra in the solid state and confirmed by the change of the absorption bands due to a local symmetry change from D2h to D4h upon metallation of the porphyrin ring. Electrochemical studies by using cyclic voltammetry and chronoamperometry techniques, showed that the immobilized complexes catalyzed O2 reduction at −0.18 V for hematoporphyrin and −0.20 V for protoporphyrin in 1 mol l−1 KCl solution at pH 6. The cathodic current peak intensities plotted against O2 concentrations in the range from 0.5 to 13 mg l−1, showed a linear correlation. Rotating disk experiments were carried out in order to estimate the number of electrons involved on the process. It was observed that for both modified electrodes, dissolved O2 was reduced to H2O2 in a two-electron process.

Iron(II) tetrasulphophthalocyanine complex adsorbed on a silica gel surface chemically modified by 3-n-propylpyridinium chloride

Abstract Iron (II) tetrasulphophthalocyanine complex was adsorbed on a 3- n -propylpyridinium chloride functionalized silica gel surface by means an ion exchange reaction. The amount of the complex bonded to the substrate surface, by electrostatic interaction, is 25 μmol g −1 and the specific surface area S BET =320 m 2 g −1 . A carbon paste electrode made with the material can electrocatalytically reduce dissolved oxygen to water at a potential Ep c =−140 mV in 0.5 M KCl solution. The plot of the cathodic current against dissolved oxygen concentrations between 0.65 and 14.4 mg dm −3 showed a linear correlation at 298 K and pH 5. The electrode presented a reproducible response and chemically it was stable under various oxidation–reduction cycles.

Silica-alumina-niobia (SiO2/Al2O3/Nb2O5) matrix obtained by the sol-gel processing method: new material for online extraction of zinc ions

In the present work, a new material, SiO2/Al2O3/Nb2O5 (designated as SiAlNb), was evaluated as an adsorbent in a flow injection spectrophotometric method for online preconcentration and determination of trace amounts of Zn2+ ions. The preconcentration method is based on Zn2+ adsorption onto the surface of SiAlNb in alkaline medium (pH 9.0). The elution step is carried out using HNO3 solution, followed by reaction of the Zn2+ ions with 1-(2-piridylazo)-2-naphtol (pan) in ammoniacal buffer solution (pH 9.3) containing Tween-80. The [Zn(pan)2] complex formed is determined at 560 nm. The method presented a linear range between 7.6 and 180.0 µg L-1 (r = 0.9992) and limits of detection and quantification of 2.3 and 7.6 µg L-1, respectively. According to the Langmuir linear model, the maximum adsorption capacity was found to be 7.0 mg of Zn2+ g-1 of SiAlNb. The proposed method was successfully applied to the Zn2+ determination in water samples (lake, mineral, tap) and certified reference material (TORT-2 Lobster Hepatopancreas).

O emprego de quitosana quimicamente modificada com anidrido succínico na adsorção de azul de metileno

The adsorption capacity of a-chitosan and its modified form with succinic anhydride was compared with the traditional adsorbent active carbon by using the dye methylene blue, employed in the textile industry. The isotherms for both biopolymers were classified as SSA systems in the Giles model, more specifically in L class and subgroup 3. The dye concentration in the supernatant in the adsorption assay was determined through electronic spectroscopy. By calorimetric titration thermodynamic data of the interaction between methyene blue and the chemically modified chitosan at the solid/liquid interface were obtained. The enthalpy of the dye/chitosan interaction gave 2.47 ± 0.02 kJ mol-1 with an equilibrium constant of 7350 ± 10 and for the carbon/dye interaction this constant gave 5951 ± 8. The spontaneity of these adsorptions are reflected by the free Gibbs energies of -22.1 ± 0.4 and -21.5 ± 0.2 kJ mol-1, respectively, found for these systems. This new adsorbent derived from a natural polysaccharide is as efficient as activated carbon. However 97% of the bonded dye can be eluted by sodium chloride solution, while this same operation elutes only 42% from carbon. Chitosan is efficient in dye removal with the additional advantage of being cheap, non-toxic, biocompatible and biodegradable.

Cobalt(II) Tetrasulfophthalocyanine Complex Adsorbed on a Silica Gel Surface Chemically Modified with 3‐N‐Propylpyridinium Chloride: Oxalic Acid Oxidation Study

Cobalt(II) tetrasulfophthalocyanine complex was adsorbed on the 3-N-propylpyridinium chloride functionalized silica gel surface by means of an ion exchange reaction. The specific surface area of the modified silica was SBET= 320 m2 g–1 and the amount of the complex bonded to the substrate surface was 17 mol g–1. A carbon paste electrode made with electrocatalytically oxidized oxalic acid at an anodic potential E=850 mV in 0.5 M KCl solution at pH 2. The anodic peak current intensities were linearly correlated with oxalic acid concentrations between 3.97 and 19.2×10–2 M. The electrode showed a very reproducible response and was chemically very stable under various oxidation-reduction cycles. The electrode was applied in the determination of oxalic acid in a spinach sample.

Electrochemical study and complete factorial design of Toluidine Blue immobilized on SiO2/Sb2O3 binary oxide

SiO2/Sb2O3 of specific surface area SBET = 788 m2 g−1 and 4.7 wt % of Sb was prepared by the sol–gel method. Toluidine Blue (TB+) was immobilized on SiO2/Sb2O3 by ion exchange reactions and the amount of dye bonded to the substrate surface was 13.72 μmol g−1 for SiO2/Sb2O3. This material was used to modify carbon paste electrodes and the electrochemical properties of Toluidine Blue (TB+) immobilized on a silica surface modified with antimonium trioxide were investigated by cyclic voltammetry. The electron mediator property of toluidine blue was optimized using a factorial design, consisting of four factors each at two levels. Factorial analysis was carried out by searching for better reversibility of the redox process, that is, the lowest separation between anodic and cathodic peak potentials and a current ratio near unity. The aqueous phase pH does not appear to influence the peak separation, ΔE, and the |Ipa//Ipc| current ratio response. The other factors studied, the scan rate, type of electrolyte and electrolyte concentration are important for this chemically modified electrode system demonstrating significant influences on the reversibility of electron transfer. The experimental observations and data analyses on this system indicate that the smallest peak separation occurs using 20 mV s−1 and 1.0 mol L−1 KCl while values of |Ipa//Ipc| close to unity are found for 20 mV s−1 with 1.0 mol L−1 concentrations of either KCl or CH3COONa. The electrodes presented reproducible responses and were chemically stable for various oxidation-reduction cycles.

Electrochemical properties and dissolved oxygen reduction study on an iron(III)-tetra(o-ureaphenyl)porphyrinosilica matrix surface

Electrochemical reduction of dioxygen on an iron(III)-tetra-o-ureaphenylporphyrino-silica matrix surface ((o)-FeTUPPS) obtained by attachment of the iron porphyrin tetraurea to a silica matrix through the sol-gel process was studied. Cyclic voltammetry and chronoamperometry techniques using as working electrode a carbon paste electrode modified with (o)-FeTUPPS, showed that (o)-FeTUPPS catalyzed O2 reduction at −0.34 V vs SCE in 0.5 mol.L−1KCl solution at pH 6. The electrode showed a linear and reproducible response to O2 concentrations in the range ca. 0.5 and 11 mg.L−1 and was chemically stable under various oxidation-reduction cycles. From rotating disk experiments it was concluded that dissolved O2 was reduced at the electrode surface to H2O2 in a two-electron process.

Cationic dyes immobilized on cellulose acetate surface modified with titanium dioxide: factorial design and an application as sensor for NADH

The electrochemical properties of meldola blue and toluidine blue cationic dyes immobilized on cellulose acetate surface modified with titanium dioxide were investigated by cyclic voltammetry. The materials synthesized were employed as carbon paste electrodes. The redox mediator properties of the meldola blue and toluidine blue chemically modified electrodes were optimized using a factorial design, consisting of two levels and four factors with two pseudo-central points (n= 20 experiments). The factorial analysis was carried out by searching for better reversibility of the redox process, such as the lowest separation between anodic and cathodic potential peaks and a current ratio near unity. The factors that presented significant effects on the overall optimization of the system to achieve the best conditions of the reversibility of electron transfer were the main factors scan rate and type of electrode (meldola blue or toluidine blue), besides the interaction factors KCl concentration × type of electrode (B×D) and the pH × [KCl] concentration × scan rate (A×B×C) interaction. The best electrochemical reversibility conditions obtained were: using the CA-TiO2-MB electrode, 1.0 mol L-1 KCl as supporting electrolyte, at scan rate of 10.0 mV s-1. Afterwards, the CA-TiO2-MB modified electrode was tested as an amperometric sensor for the determination of NADH, with a detection limit of 0.1 µmol L-1.