Iva Turyan

Selective Determination of Cr(VI) by a Self-Assembled Monolayer-Based Electrode.

We have developed a selective electrode for chromium(VI), based on a self-assembled monolayer of 4-(mercapto-n-alkyl)pyridinium on gold surfaces, which exhibits unique speciation capabilities. Cr(VI) levels as low as 1 parts per trillion can be detected using a 4-(mercaptoethyl)pyridinium monolayer. The different parameters that govern the analytical performance of these electrodes have been studied in detail and optimized. In addition, the organization of the monolayers has been examined by a variety of surface techniques such as XPS, FT-IR, and electrochemistry. Our results show that structuring and understanding the solid-liquid interface at the molecular level are essential for designing probes with superior analytical characteristics.

Self-assembled monolayers on mercury surfaces

Abstract The formation and organization of self-assembled monolayers of alkanethiols and ω-mercaptocarboxylic acids on mercury surfaces have been studied. The organization process was examined in-situ and ex-situ by cyclic voltammetry. Charging current measurements as well as electron transfer experiments with Ru(NH 3 ) 6 3+/2+ indicate that the adsorption of homologous alkanethiols and ω-mercaptocarboxylic acids on mercury is followed by the formation of a densely packed array.

The riboflavin-mediated photooxidation of doxorubicin

Purpose: Previously, it was shown that exposing doxorubicin (ADR) to 365 nm light resulted in the loss of its cytotoxic activity as well as its absorbance at 480 nm. These processes were much enhanced when mediated by riboflavin. In the present study we investigated the quantitative and qualitative aspects of riboflavin-mediated photodegradation of ADR. Methods: ADR solutions containing variable concentrations of riboflavin and other agents were exposed to 365 nm light for variable time periods and then the absorbance spectrum of ADR was measured by a double beam spectrophotometer. These measurements were used to calculate the half-time of the ADR degradation process. The degraded ADR solutions were analyzed by chromatography and mass spectrometry. Results: Analysis of the riboflavin effect indicated that a maximal rate of photolytic degradation of ADR was obtained only after most of the ADR molecules had formed bimolecular complexes with riboflavin. The retardation of lumichrome formation by ADR and the inhibition of ADR bleaching by excess of ascorbic acid suggested that ADR was degraded by a photooxidation process. Similar spectral changes occurred when ADR was exposed to strong oxidizers such as sodium hypochlorite and dipotassium hexachloroiridate. Cyclic voltammetry revealed that the oxidation-reduction process of ADR was not electrochemically reversible and therefore the oxidation potential could not be determined accurately; however its value should be between 0.23 and 0.78 V. Analysis of the photooxidative process revealed that it was not mediated by the formation of singlet oxygen, superoxide anion radicals, hydrogen peroxide or hydroxyl radicals, and it is suggested that ADR was oxidized directly by the excited triplet riboflavin. The mass spectrograms and the HPLC chromatograms of photooxidized ADR indicate that the central ring of ADR was opened and that 3-methoxysalicylic acid was produced by this cleavage. Conclusions: The riboflavin-mediated photodegradation of ADR is an oxidative process resulting in the cleavage of the anthraquinone moiety. 3-Methoxysalicylic acid was identified as one of the resulting fragments. It is possible that some of the large fractions of the ADR metabolites that are nonfluorescent are the result of an in vivo oxidation of ADR and that 3-methoxysalicylic acid may play a role in the different biological activities of ADR.

Comparing Different Approaches for Assembling Selective Electrodes for Heavy Metals

Different approaches for increasing the sensitivity and selectivity of voltammetric probes for heavy metals have been examined. These have involved adsorbed monolayers of host molecules, functionalized self-assembled monolayers and thin polymeric films. Two systems are presented that focus on the selective analysis of Hg(II) and Fe(II). The electrode for Hg(II) is based on a macrocyclic ligand, i.e., Kryptofix-222, while that for Fe(II) relies on the selective complexation by ferroin ligands, such as 1,10-phenanthroline. The monomeric and polymeric approaches were examined and compared for both systems. The principal aim of this manuscript is to highlight the ability of fine-tuning the selectivity characteristics of the solid-liquid interface as a result of considering and optimizing analyte-interface interactions. We find that basically, an interface comprising a monolayer, which is capable of forming a strong and selective complex with the analyte, is likely to exhibit sensitivity that is at least as good as a polymeric film. On the other hand, the stability and durability of the latter usually makes it a superior interface for repetitive analysis and will therefore be the preferred choice for flow analysis systems.

On-Line Analysis of Mercury by Sequential Injection Stripping Analysis (SISA) Using a Chemically Modified Electrode

A highly flexible, automatic sequential-injection stripping analysis (SISA) system has been developed and applied for monitoring the levels of mercury in aqueous solutions using a chemically modified electrode. The preparation of the modified electrode comprises spin coating of an ethanolic solution of poly(4-vinylpyridine) and Kryptofix-222 onto a glassy carbon electrode (GCE) followed by cross-linking the polymer. The analysis is based on the anodic stripping voltammetry of mercury using differential pulse voltammetry. A sequence of 36 operations was needed to complete a full cycle of cleaning, calibration and analysis.

Studying electron transfer at electrochromic tungsten oxide sol-gel films with scanning electrochemical microscopy (SECM)

Electron transfer processes at tungsten oxide films prepared by the sol–gel method were investigated and characterized by scanning electrochemical microscopy (SECM) and cyclic voltammetry. The feedback mode of the SECM was used for studying the localization and mechanism of the heterogeneous charge transfer at tungsten oxide thin films, which were deposited on conducting, i.e., tin oxide, and insulating substrates. Investigating the feedback current as a function of the mediator concentration, thickness of the tungsten oxide film and the nature and concentration of the electrolyte, allow determination of the rate limiting step and conductivity of the film.

Electrochemical study of the Cd(II)/Cd(Hg) system in 2-mercaptoacetic acid solutions

Abstract A detailed study of the Cd(II)/Cd(Hg) system in 2-mercaptoacetic acid, H 2 MA, solutions has been carried out in 1 M NaClO 4 at pH 4.8–12.0. The electrochemical behavior of 2-mercaptoacetic acid at a dropping mercury electrode was studied as well. Since all complexes that are formed in the solution undergo a reversible two-electron reduction, mathematical analysis of the data, provided the composition of the various complexes. Thus, the stability constants of the following complexes: Cd(SCH 2 CO 2 ) 3 4− , Cd(SCH 2 CO 2 ) 2 2− , Cd(SCH 2 CO 2 ), CdH(SCH 2 CO 2 ) 3 3− , CdH 2 (SCH 2 CO 2 ) 3 2− , CdH(SCH 2 CO 2 ) 2 − , CdH 2 (SCH 2 CO 2 ) 2 and CdH(SCH 2 CO 2 ) + were determined and their degree of formation as a function of pH were estimated. In addition, the protonation constants of the ligand were calculated. The present results indicate that H 2 MA forms extremely stable bidentate complexes with cadmium ions. The formation of the various complexes as a function of pH reveals that at pH 10.7–12.0 the complex Cd(SCH 2 CO 2 ) 3 4− predominates, whereas at pH 4.8–6.0 the complexes CdH 2 (SCH 2 CO 2 ) 2 and CdH(SCH 2 CO 2 ) + are dominant. At pH 6.1–10.6 different protonated and unprotonated complexes composed of one to three ligands, coexist.

Chemically modified electrode for the determination of gold — an electrochemical and spectrophotometric study

The voltammetric determination of trace amounts of gold has been studied using a chemically modified electrode. The role played by the host molecule attached to a glassy carbon electrode (GCE), i.e. 8,9,17,18-dibenzo-1,7-dioxa-10,13,16-triazacyclooctadecane (DDTC), was examined by electrochemical and spectrochemical techniques. Cathodic stripping voltammetry (CSV) was applied to study the various parameters affecting the affinity of gold ions by a DDTC modified electrode as well as the interference by other ions. Finally, the method has been successfully applied to determine gold traces in a geological sample. In addition, the formation of an AuCl4−-DDTC complex was followed and its formation constant was determined by spectrophotometry.