A self assembled monolayer of cysteamine (CA) was prepared on the surface of gold disc electrode ... more A self assembled monolayer of cysteamine (CA) was prepared on the surface of gold disc electrode and further modified with nitrilotriacetic acid (NTA). Characterization of prepared Au/CA/NTA electrode was done with cyclic voltammetry and electrochemical impedance spectroscopy in the presence of potassium hexacyanoferrate(II)/(III). Thus prepared electrode was tested for determination of lead. The response of modified electrode to Pb2+ concentration was measured by differential pulse adsorptive cathodic stripping voltammetry. Under the optimized working conditions (accumulation time, pH of accumulation and stripping solutions) the dependence of the stripping peak current response on concentration of Pb2+ was linear in the range of 0.002 – 10 µM with a correlation coefficient of 0.9839. The interference of Co2+, Ni2+ Mg2+, Mn2+ and Ca2+ was found to be significantly reduced for analyte preconcentration from borate buffer pH = 7.0. Lead determination in potable and thermal water sample...
Solid-state optical sensor membranes based on immobilised chromogenic radicals for the assessment... more Solid-state optical sensor membranes based on immobilised chromogenic radicals for the assessment of antioxidant activity have been studied. Two stable lipophilic chromogenic radicals, DPPH (2,2-diphenyl-1-picrylhydrazyl radical) and galvinoxyl radical, GV, (2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy radical), were immobilised in plasticised PVC films and screened for suitability as indicators of antioxidative activity. The spectrophotometric characterisation of the polymer films containing immobilised free radicals was performed, and the response of the immobilised free radicals toward standard antioxidants was studied. It has been demonstrated that the immobilised radicals retain their reactivity towards antioxidants and the results suggest that the reactivity of immobilised radicals is comparable to standard solution-based DPPH assays. Polymer films containing immobilised DPPH radical respond to standard antioxidants in aqueous solution...
A novel enzymatic amperometric method is described for the determination of oxalic acid in urine.... more A novel enzymatic amperometric method is described for the determination of oxalic acid in urine. An amperometric biosensor was made by immobilizing oxalate oxidase on the surface of a chromium(III) hexacyanoferrate-modified graphite electrode by using a bovine serum albumin and glutaraldehyde cross-linking procedure. The enzyme biocatalyzes oxalate decomposition in the presence of oxygen into carbon dioxide and hydrogen peroxide. The oxalate concentration, which is proportional to the amount of hydrogen peroxide, was determined amperometrically by measuring the current resulting in the reduction of hydrogen peroxide at a very low working potential (0.05 V versus the Hg ¿Hg2Cl2¿ 3M KCl electrode), which minimized the influence of the possible interferences present in human urine. All experiments were performed with succinic buffer, pH 3.8, containing 0.1M KCl and 5.4mM ethylenediaminetetraacetic acid. In an aqueous solution of pure oxalic acid, the biosensor showed good linearity in...
A Trolox equivalent antioxidant capacity (TEAC) decolourisation assay was adapted to a flow injec... more A Trolox equivalent antioxidant capacity (TEAC) decolourisation assay was adapted to a flow injection analysis (FIA) system and a simple and rapid method for antioxidant activity evaluation was developed. To avoid the time consuming step of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS) radical cation preparation by chemical oxidation of ABTS, as in the original TAEC assay, and hence, to shorten the analysis time, the ABTS radical cation was generated on-line by electrochemical oxidation of ABTS in the flow-through electrolysis cell forming a part of the FIA system. The proposed method was optimised with respect to a flow rate, injection volume and ABTS radical cation/carrier ratio. Under the optimised conditions linear calibration graphs for Trolox were obtained over the range 10-100 microM, with a limit of detection 1.6 microM. Good reproducibility (relative standard deviation 1.95%) and sample throughput (32 samples per hour) were achieved. The developed method was applied to the evaluation of the antioxidant activity of pure compounds and samples of some common beverages. In both cases a good correlation between the results obtained by the proposed method and TEAC values evaluated by the classic TAEC decolourisation assay was obtained (r(2)= 0.996 for pure compounds and r(2)= 0.957 for beverage samples).
ABSTRACT A novel electrochemical method for selective determination of antioxidant activity based... more ABSTRACT A novel electrochemical method for selective determination of antioxidant activity based on 2,2-diphenyl-1-picrylhydrazyl / 2,2-diphenyl-1-picrylhydrazin (DPPH⋅/ DPPH) redox couple and biamperometric technique is proposed. Two identical glassy carbon disk electrodes were mounted in a classic electrochemical cell and the tested working potential difference was between 50 and 200 mV. The DPPH⋅/ DPPH redox couple exhibited a high degree of reversibility. Selectivity of detector was tested by different antioxidant compounds in ethanol-phosphate buffer solution, pH 7.40. The results of antioxidant activity of pure antioxidant compounds and of actual samples of beverages, expressed as Trolox equivalents and determined by proposed biamperometric and spectroscopic measurements, were in good correlation (R=0.9959). The detection limit for Trolox established by the applied biamperometry was 0.05 μM while the resulting current was linear up to 25 μM of Trolox.
Sažetak: In this study, AgBr nanoparticles were prepared by controlled single jet method in the p... more Sažetak: In this study, AgBr nanoparticles were prepared by controlled single jet method in the presence of gelatine and polyvinylpyrrolidone (PVP). In order to obtain better insight about the effect of PVP and its compatibility with gelatine on the formation of AgBr ...
A self assembled monolayer of cysteamine (CA) was prepared on the surface of gold disc electrode ... more A self assembled monolayer of cysteamine (CA) was prepared on the surface of gold disc electrode and further modified with nitrilotriacetic acid (NTA). Characterization of prepared Au/CA/NTA electrode was done with cyclic voltammetry and electrochemical impedance spectroscopy in the presence of potassium hexacyanoferrate(II)/(III). Thus prepared electrode was tested for determination of lead. The response of modified electrode to Pb2+ concentration was measured by differential pulse adsorptive cathodic stripping voltammetry. Under the optimized working conditions (accumulation time, pH of accumulation and stripping solutions) the dependence of the stripping peak current response on concentration of Pb2+ was linear in the range of 0.002 – 10 µM with a correlation coefficient of 0.9839. The interference of Co2+, Ni2+ Mg2+, Mn2+ and Ca2+ was found to be significantly reduced for analyte preconcentration from borate buffer pH = 7.0. Lead determination in potable and thermal water sample...
Solid-state optical sensor membranes based on immobilised chromogenic radicals for the assessment... more Solid-state optical sensor membranes based on immobilised chromogenic radicals for the assessment of antioxidant activity have been studied. Two stable lipophilic chromogenic radicals, DPPH (2,2-diphenyl-1-picrylhydrazyl radical) and galvinoxyl radical, GV, (2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy radical), were immobilised in plasticised PVC films and screened for suitability as indicators of antioxidative activity. The spectrophotometric characterisation of the polymer films containing immobilised free radicals was performed, and the response of the immobilised free radicals toward standard antioxidants was studied. It has been demonstrated that the immobilised radicals retain their reactivity towards antioxidants and the results suggest that the reactivity of immobilised radicals is comparable to standard solution-based DPPH assays. Polymer films containing immobilised DPPH radical respond to standard antioxidants in aqueous solution...
A novel enzymatic amperometric method is described for the determination of oxalic acid in urine.... more A novel enzymatic amperometric method is described for the determination of oxalic acid in urine. An amperometric biosensor was made by immobilizing oxalate oxidase on the surface of a chromium(III) hexacyanoferrate-modified graphite electrode by using a bovine serum albumin and glutaraldehyde cross-linking procedure. The enzyme biocatalyzes oxalate decomposition in the presence of oxygen into carbon dioxide and hydrogen peroxide. The oxalate concentration, which is proportional to the amount of hydrogen peroxide, was determined amperometrically by measuring the current resulting in the reduction of hydrogen peroxide at a very low working potential (0.05 V versus the Hg ¿Hg2Cl2¿ 3M KCl electrode), which minimized the influence of the possible interferences present in human urine. All experiments were performed with succinic buffer, pH 3.8, containing 0.1M KCl and 5.4mM ethylenediaminetetraacetic acid. In an aqueous solution of pure oxalic acid, the biosensor showed good linearity in...
A Trolox equivalent antioxidant capacity (TEAC) decolourisation assay was adapted to a flow injec... more A Trolox equivalent antioxidant capacity (TEAC) decolourisation assay was adapted to a flow injection analysis (FIA) system and a simple and rapid method for antioxidant activity evaluation was developed. To avoid the time consuming step of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS) radical cation preparation by chemical oxidation of ABTS, as in the original TAEC assay, and hence, to shorten the analysis time, the ABTS radical cation was generated on-line by electrochemical oxidation of ABTS in the flow-through electrolysis cell forming a part of the FIA system. The proposed method was optimised with respect to a flow rate, injection volume and ABTS radical cation/carrier ratio. Under the optimised conditions linear calibration graphs for Trolox were obtained over the range 10-100 microM, with a limit of detection 1.6 microM. Good reproducibility (relative standard deviation 1.95%) and sample throughput (32 samples per hour) were achieved. The developed method was applied to the evaluation of the antioxidant activity of pure compounds and samples of some common beverages. In both cases a good correlation between the results obtained by the proposed method and TEAC values evaluated by the classic TAEC decolourisation assay was obtained (r(2)= 0.996 for pure compounds and r(2)= 0.957 for beverage samples).
ABSTRACT A novel electrochemical method for selective determination of antioxidant activity based... more ABSTRACT A novel electrochemical method for selective determination of antioxidant activity based on 2,2-diphenyl-1-picrylhydrazyl / 2,2-diphenyl-1-picrylhydrazin (DPPH⋅/ DPPH) redox couple and biamperometric technique is proposed. Two identical glassy carbon disk electrodes were mounted in a classic electrochemical cell and the tested working potential difference was between 50 and 200 mV. The DPPH⋅/ DPPH redox couple exhibited a high degree of reversibility. Selectivity of detector was tested by different antioxidant compounds in ethanol-phosphate buffer solution, pH 7.40. The results of antioxidant activity of pure antioxidant compounds and of actual samples of beverages, expressed as Trolox equivalents and determined by proposed biamperometric and spectroscopic measurements, were in good correlation (R=0.9959). The detection limit for Trolox established by the applied biamperometry was 0.05 μM while the resulting current was linear up to 25 μM of Trolox.
Sažetak: In this study, AgBr nanoparticles were prepared by controlled single jet method in the p... more Sažetak: In this study, AgBr nanoparticles were prepared by controlled single jet method in the presence of gelatine and polyvinylpyrrolidone (PVP). In order to obtain better insight about the effect of PVP and its compatibility with gelatine on the formation of AgBr ...
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Papers by Stjepan Milardović