DEVELOPMENT OF A CHEMICALLY MODIFIED ELECTRODE BASED ON CARBON PASTE AND FUNCTIONALIZED SILICA-GEL FOR PRECONCENTRATION AND VOLTAMMETRIC DETERMINATION OF MERCURY(II)

A mercury-sensitive chemically modified electrode (CME) based on modified silica gel-containing carbon paste was developed. The functional group attached to the silica gel surface was 3-(2-thiobenzimidazolyl)propyl, which is able to complex mercury ions. This electrode was applied to the determination of mercury(II) ions in aqueous solution. The mercury was chemically preconcentrated on the CME prior to voltammetric determination by anodic stripping in the differential-pulse mode. A calibration graph covering the concentration range from 0.08 to 2 mg l-1 was constructed. The precision for six determinations of 0.122 and 0.312 mg l-1 Hg(II) was 3.2 and 2.9% (relative standard deviation), respectively. The detection limit for a 5-min preconcentration period was 0.013 mg l-1. A study for foreign ions was also made.

Characterization of electrodes chemically modified with Mn(III) porphyrin/polypyrrole films as catalytic surfaces for an azo dye

In this study we describe the electrochemical behavior of 5,10,15,20-tetrakis(2'-aminophenylporphyrin)manganese(III) chloride supported on a glassy carbon electrode, as well as the electrochemical preparation and characterization of thin films based on pyrrole-3-carboxylic acid. The electrocatalytic action of the electrode modified with the Mn(III) porphyrin toward an azo dye was tested, and the characteristic strong interaction between the incorporated metalloporphyrin and RR120 dye was verified. Copyright (c) 2006 Society of Porphyrins & Phthalocyanines.

Stripping voltammetry of mercury(II) with a chemically modified carbon paste electrode containing silica gel functionalized with 2,5-dimercapto-1,3,4-thiadiazole

The accumulation voltammetry of mercury(II) was investigated at a carbon paste electrode chemically modified with silica gel functionalized with 2,5-dimercapto-1,3,4-thiadiazole (DTTPSG-CPE). The repetitive cyclic voltammogram of mercury(II) solution in the potential range -0.2 to +0.8 V (vs. Ag/AgCl), (0.02 mol L-1 KNO3; nu=20 mV s(-1)) show two peaks one at about 0.0 V and other at 0.31 V. However, the cathodic wave peak, around 0.0 V, is irregular and changes its form in each cycle. This peak at about 0.0 V is the reduction current for mercury(II) accumulated in the DTTPSG-CPE. The anodic wave peak at 0.31 V is well-defined and does not change during the cycles. The resultant material was characterized by cyclic and differential pulse anodic stripping voltammetry performed with the electrode in differents supporting electrolytes. The mercury response was evaluated with respect to pH, electrode composition, preconcentration time, mercury concentration, cleaning solution, possible interferences and other variables. The precision for six determinations (n=6) of 0.05 and 0.20 mg (L)-(1) Hg(II) was 2.8 and 2.2% (relative standard deviation), respectively. The method was satisfactory and used to determine the concentration of mercury(II) in natural waters contaminated by this metal.

Evaluation of a highly sensitive amperometric biosensor with low cholinesterase charge immobilized on a chemically modified carbon paste electrode for trace determination of carbamates in fruit, vegetable and water samples

A highly sensitive amperometric biosensor for determination of carbamate pesticides directly in water, fruit and vegetable samples has been evaluated, electrochemically characterized and optimized. The biosensor strip was fabricated in screen printed technique on a ceramic support using silver-based paste for reference electrode, and platinum-based paste for working and auxiliary electrodes. The working electrode was modified by a layer of carbon paste mixed with cobalt(II) phthalocyanine and acetylcellulose. Cholinesterase (ChE) enzymes with low enzymatic charge were immobilized on this layer. The operational simplicity of the biosensor consists in that a small drop (similar to 50 mu l) of substrate or sample is deposited on a horizontally positioned biosensor strip representing the microelectrochemical cell. The working potential of the biosensor was 370 mV versus Ag/AgI on a ship reference electrode preventing the interference of electroactive species which are oxidable at more positive potentials. The biosensor was applied to investigate the degradation of two reference ChE inhibitors in freeze dried water under different storage conditions and for direct determination of some N-methylcarbamates (NMCs) in fruit and vegetable samples at ppb concentration levels without any sample pretreatment. A comparison of the obtained results for the total carbamate concentration was done against those obtained using HPLC measurements. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Preconcentration and determination of mercury(II) at a chemically modified electrode containing 3-(2-thioimidazolyl)propyl silica gel

A mercury-sensitive chemically modified graphite paste electrode was constructed by incorporating modified silica gel into a conventional graphite paste electrode. The functional group attached to the (3-chloropropyl) silica gel surface was 2-mercaptoimidazole, giving a new product denoted by 3-(2-thioimidazolyl)propyl silica gel, which is able to complex mercury ions. Mercury was chemically adsorbed on the modified graphite paste electrode containing 3-(2-thioimidazolyl)propyl silica (TIPSG GPE) by immersion in a Hg(II) solution, and the resultant surface was characterized by cyclic and differential pulse anodic stripping voltammetry. One cathodic peak at 0.1 V and other anodic peak at 0.34 V were observed on scanning the potential from -0.1 to 0.8 V (0.01 M KNO3; ν = 2.0 mV s-1 νs. Ag/AgCl). The anodic peak at 0.34 V show an excellent sensitivity for Hg(II) ions in the presence of several foreign ions. A calibration graph covering the concentration range from 0.02 to 2 mg L-1 was obtained. The detection limit was estimated to be 5 μg L-1. The precision for six determinations of 0.05 and 0.26 mg L-1 Hg(II) was 3.0 and 2.5% (relative standard deviation), respectively. The method can be used to determine the concentration of mercury(II) in natural waters contaminated by this metal. 2005 © The Japan Society for Analytical Chemistry.

Ascorbate electro-oxidation by modified electrodes: Polypyrrole and polypyrrole/Ni(OH)(2) composite thin films

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Electrochemical modified electrodes based on metal-salen complexes

A general view of the electroanalytical applications of metal-salen complexes is discussed in this review. The family of Schiff bases derived from ethylenediamine and ortho-phenolic aldehydes (N,N'-ethylenebis(salicylideneiminato) - salen) and their complexes of various transition metals, such as Al, Ce, Co, Cu, Cr, Fe, Ga, Hg, Mn, Mo, Ni, and V have been used in many fields of chemical research for a wide range of applications such as catalysts for the oxygenation of organic molecules, epoxidation of alkenes, oxidation of hydrocarbons and many other catalyzed reactions; as electrocatalyst for novel sensors development; and mimicking the catalytic functions of enzymes. A brief history of the synthesis and reactivity of metal-salen complexes will be presented. The potentialities and possibilities of metal-Salen complexes modified electrodes in the development of electrochemical sensors as well as other types of sensors, their construction and methods of fabrication, and the potential application of these modified electrodes will be illustrated and discussed.

Phytase immobilization on modified electrodes for amperometric biosensing

Phytase (myo-inositol hexaphosphate phosphohydrolase) and phytic acid (myo-inositol hexaphosphate) play an important environmental role, in addition to being a health issue in food industry. Phytic acid is antinutritional due to its ability to chelate metal ions and may also react with proteins decreasing their bioavailability. In this work, we produced biosensors with phytase immobilized in Layer-by-Layer (LbL) films, which could detect phytic acid with a detection limit of 0.19 mmol L-1, which is sufficient to detect phytic acid in seeds of grains and vegetables. The biosensosrs consisted of LbL films containing up to eight bilayers of phytase alternated with poly(allylamine) hydrochloride (PAH) deposited onto an indium-tin oxide (ITO) substrate modified with Prussian Blue. Amperometric detection was conducted in an acetate buffer solution (at pH 5.5) at room temperature, with the biosensor response attributed to the formation of phosphate ions. In subsidiary experiments with the currents measured at 0.0 V (vs. SCE), we demonstrated the absence of effects from some interferents, pointing to a good selectivity of the biosensor. (c) 2007 Elsevier B.V. All rights reserved.

Adsorption of Cu(II) and Co(II) complexes on a silica gel surface chemically modified with 2-mercaptoimidazole

The isotherms of adsorption of MeX2 (Me = Cu2+, Co2+; X = Cl-, Br-, ClO4-) by silica gel chemically modified with 2-mercaptoimidazole (SiMI) were studied in acetone and ethanol solutions, at 25 degrees C. Covalently attached 2-mercaptoimidazole molecule to silica gel surface adsorbs MeX2 from solvent by forming a surface complex. The metal is bonded to the surface through the nitrogen atom of attached 2-mercaptoimidazole. At low loading, the electronic and ESR spectral parameters indicated that the Cu2+ complexes are in a distorted-tetragonal symmetry field. The d-d electronic transition spectra showed that for Cu(ClO4)(2) complex, the peak of absorption did not change for any degree of metal loading and for Cl- and Br- complexes, the peak maxima shifted to higher energy with lower metal loading. The CoX2(X = Cl-, Br-, ClO4-) analogues possess a distorted-tetrahedral field.

Study of copper complexes adsorbed on a silica gel surface chemically modified with 2-amino-1,3,4-thiadiazole

The isotherms of adsorption of CuX2 (X=Cl-, Br-, ClO4-) by silica gel chemically modified with 2-amino-1,3,4-thiadiazole were studied in acetone and ethanol solutions: at 298 K. The following equilibria constants (in 1 mol(-1)) were determined: (a) CuCl2: 3.5 x 10(3) (ac), 2.0 x 10(3) (eth); (b) CuBr2: 2.8 x 10(3) (ac), 2.0 x 10(3) (eth); (c) Cu(ClO4)(2): 1.8 x 10(3) (ac), 1.0 x 10(3) (eth); ac = acetone, eth = ethanol. The electron spin resonance spectra of the surface complexes indicated a tetragonal distorted structure in the case of lower degrees of metal loading on the chemically modified surface. The d-d electronic transition spectra showed that for the ClO4-, complex, the peak of absorption did not change for any degree of metal loading, and for Cl- and Br- complexes, the peak maxima shifted to a higher energy region with a lower metal loading. (C) 1998 Elsevier B.V. B.V. All rights reserved.

Study of copper complexes absorbed on a silica gel surface chemically modified with 5-amino-1,3,4-thiadiazole-2-thiol

The isotherms of adsorption of CuX2 (XCl-, Br-, ClO4-) by silica gel chemically modified with 5-amino-1,3,4-thiadiazole-2-thiol were studied in acetone and ethanol solutions, at 25 degrees C. The following equilibria constants (in L mol(-1)) were determined: (a) CuCl2, 3.2 x 10(3) (ac), 2.5 x 10(3) (eth); (b) CuBr2, 2.9 x 10(3) (ac), 2.3 x 10(3) (eth); (c) Cu(ClO4)(2), 1.8 x 10(3) (ac), 1.2 x 10(3) (eth); ac, acetone; eth, ethanol. The electron spin resonance spectra of the surface complexes indicated a tetragonal-distorted structure in the case of lower degrees of metal loading on the chemically modified surface. The d-d electronic transition spectra showed that for the ClO4- complex, the peak of absorption did not change for any degree of metal loading and for Cl- and Br- complexes, the peak maxima shifted to higher energy with lower metal loadings. (C) 1998 Academic Press.

Adsorption of CuX2 (X = Cl-, Br-, and ClO4-) on a silica gel surface chemically modified with thiazolidine-2-thione from acetone and ethanol solutions

The isotherms of adsorption of CuX2 (X = Cl-, Br, ClO4-,) by silica gel chemically modified with thiazolidine-2-thione were studied in acetone (ac) and ethanol (eth) solutions at 25 degrees C. The following equilibrium constants (in 1 mol(-1)) were determined: a) CuCl2, 1.9 x 10(3) (ac), 1.6 x 10(3) (eth); b) CuBr2, 1.7 x 10(3) (ac), 1.2 x 10(3) (eth); c) Cu(ClO4)(2), 1.1 x 10(3) (ac), 1.0 x 10(3) (eth). The electron spin resonance spectra of the surface complexes indicate a tetragonal distorted structure in the case of lower degrees of metal loading on the chemically modified surface. The d-d electronic transition spectra show that for the ClO4- complex, the peak of absorption did not change for any degree of metal loading, and for Cl- and Br complexes, the peak maxima shift to higher energy with lower metal loading.

Adsorption of copper(II) and cobalt(II) complexes on a silica gel surface chemically modified with 3-amino-1,2,4-triazole

The isotherms of adsorption of MX2 (M = Cu2+, Co2+; X = Cl-, Br-, ClO4) by silica gel chemically modified with 3-amino-1,2,4-triazole (SiATR) were studied in acetone and ethanol solutions, at 25 degrees C. The 3-amino-1,2,4-triazole molecule, covalently bound to the silica gel surface, adsorbs MX2 from solvent by forming a surface complex. At low loading, the electronic and electron spin resonance spectral parameters indicated that the Cu2+ complexes have distorted tetragonal symmetry. The CoX2 (X = Cl-, Br-) analogues exhibit a distorted-tetrahedral geometry, whilstthe (SiATR)mCo)ClO4)(2) complex has a tetragonally distorted octahedral geometry, with four equatorial nitrogen atoms around the cobalt. (C) 1998 Elsevier B.V. B.V. All rights reserved.

Synthesis and characterization of TiO2 chemically modified by Pd(II) 2-aminothiazole complex for the photocatalytic degradation of phenol

An investigation was made into the photocatalytic activity of in situ synthesized TiO2 chemically modified by Pd(II) 2-aminothiazole complex for phenol degradation at different pH values. At longer reaction times, the bare titania presented far poorer pbotoactivity than the modified catalysts in the entire range of pH studied. The catalyst complexed with Pd(II) was more efficient than the metal-free Pd, irrespective of pH and reaction time, suggesting that metal plays an important role. A cooperative mechanism is proposed, involving the possible photoactivation of both TiO2 and sensitizer. (C) 2007 Elsevier B.V. All rights reserved.

Electrochemical Behavior of a Glassy Carbon Electrode Chemically Modified with Nickel Pentacyanonitrosylferrate in Presence of Sulfur Compounds

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)