Mônica F. Belian

Flow-injection electrochemical determination of citric acid using a cobalt(II)–phthalocyanine modified carbon paste electrode

The electrocatalytic activity of cobalt(II)-phthalocyanine immobilized in a carbon paste electrode for citric acid oxidation under a diffusion-controlled electrochemical process was employed to develop a selective flow injection amperomettric analysis method. Under optimized conditions, rapid and reliable determinations of citric acid (65 samples per hour) were achieved for analyses of commercial fruit juices with minor pretreatment of the samples. A linear response was accomplished ranging from 0.25 to 15 mmol L(-1) (r(2)=0.9979) with an estimated detection limit of 0.117 mmol L(-1). Alternatively, a very sensitive and selective assay was developed for the micromolar range, obtaining a linear response ranging from 3.81 to 91.2 μmol L(-1) (r(2)=0.9993) with an estimated detection limit of 2.54 μmol L(-1), by using square wave voltammetric methodology. The speed and simplicity of the proposed approaches compares advantageously to the all other citric acid determination methods.

Synthesis and Characterization of Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) Complexes With 2,6-Pyridinedicarboxilic Acid, Chelidamic Acid, and Chelidonic Acid

The synthesis and characterization of 21 complexes of Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) with 2,6-pyridinedicarboxilic acid, (dipicolinic acid [DPA]), 4-hidroxypiridine-2,6-dicarboxilic acid (chelidamic acid [DAMIC]), and chelidonic acid (DONIC) is reported. The complexes were characterized by CHN elemental analysis, and infrared and ultraviolet spectroscopies. The obtained electronic spectra were employed to calculate the 10 Dq values. The observed sequence in 10 Dq values is: Fe (III) > Cr (III) > Ni (II) > Co (II) for DPA and DONIC complexes. For DAMIC complexes, Co (II) > Ni (II).

Ferricyanide Confined in a Protonated Amine-Functionalized Silica Film on Gold: Application to Electrocatalytic Sensing of Nitrite Ions

ABSTRACT An amine-functionalized porous sol–gel silica film was shown to be an effective platform to immobilize small anionic redox mediators of high solubility on solid electrodes by electrostatic interaction. The highly soluble mediator hexacyanoferrate was used as a model. The film was grown and firmly anchored on a gold electrode surface via thiol groups of a self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane. Film growth and thickness were controlled by electrochemical modulation of pH at the electrode/solution interface in a sol of a hydrolyzed solution of tetraethoxysilane and 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane by the application of a negative potential to the electrode. Protonation of the amine groups made the amine-functionalized surface useful to immobilize hexacyanoferrate on gold. Thus, the immobilization is pH dependent, being highly effective in strongly acidic medium. Cyclic voltammetry and scanning electron microscopy were used to characterize the film and to optimize the experimental conditions. The stability of the film was demonstrated by applying the catalytic properties of the hexacyanoferrate containing surface for nitrite sensing using a flow injection analysis (FIA) system. Under the optimized conditions, the sensor exhibited high sensitivity, low detection limit, easy handling, and stability with a linear range from 1.0 to 40.0 µmol L−1 and a detection limit of 0.53 µmol L−1 based on a signal-to-noise ratio of 3. The sensor was successfully applied to nitrite determination in water samples using FIA with excellent recoveries.

Luminescent solid phase for sialic acid determination: a promising sensor for milk-adulterated samples

This article presents the synthesis, characterization and spectroscopic study of silica modified with thenoyltrifluoroacetonate (SilTTA) and coordinated to an europium (III) ion, for the determination of sialic acid (NANA). Elemental analysis and infrared spectroscopy suggest silica functionalization, as well as coordination of beta-diketone to the lanthanide ion. The emission spectra of compound-free and coordinated Eu-SilTTA to NANA showed significant changes with respect to the maximum emission and spectral profile, suggesting that the NANA ion is coordinated to the Eu(III). The values of the phenomenological intensity parameters show an increase in polarizability around the Eu(III) in the case of Eu-SilTTA coordinated to NANA, as expected, since water molecules are less polarizable than sialic acid. The results of the batch assay showed that luminescent silica can be used for sialic acid determination in milk-adulterated samples, with a correlation coefficient of 0.9992; and a detection limit of 0.4 mg/L; relative standard deviation (RSD%) = 0.0028.