Margarita Stilianova Stoytcheva - Academia.edu (original) (raw)

Papers by Margarita Stilianova Stoytcheva

Colloids and Surfaces B: Biointerfaces, 2009

A poly-amidosulfonic acid and multi-wall carbon nanotubes composite (PASA/MWNTs) modified electro... more A poly-amidosulfonic acid and multi-wall carbon nanotubes composite (PASA/MWNTs) modified electrode has been constructed by electropolymerization on glassy carbon electrode (GCE). The electrochemical behaviors of hydroquinone (HQ) and catechol (CC) were investigated using cyclic and differential pulse voltammetries (DPVs) at the prepared electrode. Separation of the reductive peak potentials for HQ and CC was about 120 mV in pH 6.0 phosphate buffer solution (PBS), which makes it suitable for simultaneous determination of these compounds. In the presence of 1.0 x 10(-4)mol L(-1) isomer, the reductive peak currents of DPV are proportional to the concentration of HQ in the range of 6.0 x 10(-6) to 4.0 x 10(-4)mol L(-1), and to that of CC in the range of 6.0 x 10(-6) to 7.0 x 10(-4)mol L(-1). When simultaneously changing the concentration of both HQ and CC, the linear concentration range of HQ (or CC) is 6.0 x 10(-6) to 1.0 x 10(-4)mol L(-1) (or 6.0 x 10(-6) to 1.8 x 10(-4)mol L(-1)), and the corresponding detection limits are 1.0 x 10(-6)mol L(-1). The proposed method has been applied to simultaneous determination of HQ and catechol in water sample, and the results are satisfactory.

OmniaScience Monographs, Apr 3, 2014

The biosensor is an analytical compact device that embodies a biological detection element and a ... more The biosensor is an analytical compact device that embodies a biological detection element and a chemical transducer that converts the biological signal into an electrical output that is proportional to the concentration of the compund that is desired to assess. The biosensors are high-technology monitoring tools that offer electrochemical measures in a continuous, fast and highly sensitive fashion. Their potential use in real-life applications span from medical and health tasks to environmental monitoring problems. This chapter focuses particularly on Glucose Oxidase Amperometric Biosensors (GOAB) modeling, in order to understand their dynamics and behavior by automated computerized algorithms. In this contribution, we step aside from traditional modeling techniques –mathematical equations and simulation models– by using statistical learning methods. We model the continuous amperometric response of a GOAB by means of several classic and statistical learning methods. Specifically, k...

ECS Transactions, 2019

The Reversible Hydrogen Electrode (RHE) potential depends on the pH and the H2 pressure which det... more The Reversible Hydrogen Electrode (RHE) potential depends on the pH and the H2 pressure which determines the disolved hydrogen concentration according to the Henry law. RHE was applied as hydrogen sensor for real time bacterial hydrogen production rate evaluation employing the photosynthetic Rodobacter Capsulatus strain IR3 in 10 ml reactor containing RCV medium. The reliable determination of the produced H2 volume for determination of the process rate requires big reactor volumes since the H2 production rate is small, measured in mL/hour and the precise measurements of small gas volumes is very complicated. The RHE potential monitoring however allows real time determination of the generated H2 and hence the process rate without consumption of great reagent amounts.

ECS Transactions, 2010

ABSTRACT Some bacterial and hybrid (bacterial/enzymatic) electrochemical sensors recently develop... more ABSTRACT Some bacterial and hybrid (bacterial/enzymatic) electrochemical sensors recently developed by the authors were evaluated for potential in situ metal ions and organophosphorus pesticides determinations. The analytical performances of the ...

ECS Transactions, 2010

The controversial statements reported till now about the local conductivity of the solution at it... more The controversial statements reported till now about the local conductivity of the solution at its interface with the corroded specimen affecting the SVET results were verified by the simultaneous application of a combination of two measuring techniques: SVET and conductometry. Bare and chromated AA 6061-T6 aerospatial aluminum alloy samples in 5% NaCl solutions were employed in the experiments. It was confirmed that the local conductivity variations are negligible above the chromated specimens obviously because of the pit sealing while about 4% changes were observed 5 minutes after the immersion of the bare samples. An approach for fast automatic detection of pit sites on the entire specimen surface, based on high resolution digital optical scanning coupled with image recognition software was developed and applied in the SVET/Conductivity experiments.

ECS Transactions, 2009

In this work are reported new results on the type of the immobilized acetylcholinesterase inhibit... more In this work are reported new results on the type of the immobilized acetylcholinesterase inhibition by specific and non-specific inhibitors, as well as the values of the characteristic enzyme reaction kinetic parameters in the absence and in the presence of inhibitors, identified applying an electrochemical approach. It was found that: -The kinetics of the process of acetylthiocholine hydrolysis catalyzed by the immobilized acetylcholinesterase is complicated by mass transfer constraints, at low substrate concentrations; -The inhibition of the immobilized acetylcholinesterase by specific inhibitors (trichlorfon, oxophosphol, galantamine, caffeine, atropine, nicotinic acid, and eserine) is of a competitive type, while quinine and thiamine acts as non-competitive inhibitors; -The character of acetylcholinesterase inhibition in heterogeneous phase by non-specific inhibitors (Hg2+, Cu2+, Cd2+, Mn2+, Fe3+, and AsO33-) is reversible and non-competitive. The reactivity of the inhibitors d...

ECS Transactions, 2010

A noise suppression approach was developed and applied for pitting corrosion studies of chromatiz... more A noise suppression approach was developed and applied for pitting corrosion studies of chromatized 6061-T6 airspace aluminum alloy by Scanning Vibrating Electrode Technique (SVET). The proposed approach is based on the power line frequency application as a SVET probe driving frequency combined with a synchronized double integration of the SVET probe signal yielding a free of noise signal product. Noise suppression up to -58 dB was achieved allowing precise measurement of the ionic current above the corroded metal surface proving the opportunity to replace the high cost lock-in amplifier.

ECS Transactions, 2010

A simple and rapid on-line Differential Pulse Polarographic (DPP) method for direct Co2+ determin... more A simple and rapid on-line Differential Pulse Polarographic (DPP) method for direct Co2+ determination in Zn plant electrolyte was developed and tested in real solutions. 1-nitroso-2-naphtol was applied as Co2+ to Co3+ oxidizing and chelate forming agent in ammonia buffer (NH4OH/HCl in 8 to 1 ratio) supporting electrolyte. The great DPP difference between Co3+ and Zn2+ peak potentials allows the complete elimination of the Zn2+ interference ensuring reliable results in the linear concentration range from 10 ppb to 20 ppm. Disposable mercury surface electrodes were applied as working electrodes.

Lecture Notes in Computer Science, 2014

ABSTRACT Biosensors are small analytical devices incorporating a biological element for signal de... more ABSTRACT Biosensors are small analytical devices incorporating a biological element for signal detection. The main function of a biosensor is to generate an electrical signal which is proportional to a specific analyte i.e. to translate a biological signal into an electrical reading. Nowadays its technological attractiveness resides in its fast performance, and its highly sensitivity and continuous measuring capabilities; however, its understanding is still under research. This paper focuses to contribute to the state of the art of this growing field of biotechnology specially on Glucose Oxidase Biosensors (GOB) modeling through statistical learning methods from a regression perspective. It models the amperometric response of a GOB with dependent variables under different conditions such as temperature, benzoquinone, PH and glucose, by means of well known machine learning algorithms. Support Vector Machines(SVM), Artificial Neural Networks (ANN) and Partial least squares (PLS) are the algorithms selected to do the regression task.

ECS Transactions, 2019

Activated by Au nanoparticles, Boron Doped Diamond (BDD) electrode was modified electrochemically... more Activated by Au nanoparticles, Boron Doped Diamond (BDD) electrode was modified electrochemically by TiO2 and applied for As(III) determination in natural ground waters. The modification with TiO2 results in up to 49.7% sensitivity increase compared with the non modified electrode achieved at the maximum TiO2 coverage percentage of about 73% of the BDD surface.

ECS Transactions, 2011

Cerium based conversion coatings were immersion deposited on AA6061-T6 and characterized to deter... more Cerium based conversion coatings were immersion deposited on AA6061-T6 and characterized to determine the effect of NO2- as an additive in the conversion solution on the anticorrosion behavior. The NO2- addition to the cerium chloride and hydrogen peroxide solution improved the corrosion resistance and coating morphology. Electrochemical measurements were performed by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in 0.01 M NaCl aqueous solution and the coating morphology was analyzed by Atomic Force Microscopy (AFM).

Journal of the Serbian Chemical Society, 2010

In this study, the Cr(VI) removal potential of waste mycelium from the industrial xylanase-produc... more In this study, the Cr(VI) removal potential of waste mycelium from the industrial xylanase-producing strain Aspergillus awamori was evaluated. It was determined by FTIR analysis that amino groups from the major fungal wall constituents, chitin and chitosan, played a key role in the metal binding process. The effect of pH, initial ion concentration, temperature and amount of biomass on the removal was also studied. The removal efficiency increased with decreasing pH and increasing temperature and amount of biomass. The mechanism of Cr(VI) removal by A. awamori can be explained by a two-stage process involving an initial adsorption stage followed by a reducing stage. The removal process was described by a second-order polynomial and the optimal process parameters for attaining Rmax 94.4 % in 48 h were predicted, i.e., pH 1.5 and T 40?C. From both economic and ecological points of view, a promising possibility for the utilization of waste industrial mycelium of A. awamori as a low-cost...

Biosensors and Bioelectronics, 2006

The sensors applied recently for determination of cholinesterase activity are mostly enzymatic am... more The sensors applied recently for determination of cholinesterase activity are mostly enzymatic amperometric sensors, in spite of their disadvantages: short lifetime at ambient temperature, instability of the response, interferences, as well as passivation of the electrode surface. In the present paper a new approach for determination of cholinesterase activity was proposed, overcoming the main drawbacks of the analysis performed with amperometric enzymatic sensors. Instead of the immobilization of enzymes on a conducting electrode surface, whole cells of Arthrobacter globiformis, containing choline oxidase were fixed on a Clark type oxygen probe. Current proportional to bacteria respiration is registered as a sensor response. The application of whole cells of bacteria as a sensing element permits to achieve high stability of the response and long lifetime of the sensor at ambient temperature, due to the conservation of the enzyme in its natural micro-environment inside the immobilized cells. The proposed sensor keeps its functionality more than 7 weeks stored in deionized water at ambient temperature. For the first 2 weeks the amplitude of the response decreases with only 10% and at the end of the studied 7 weeks period the response was 50% of the initial. The other advantages of the proposed sensor are: the dissolved oxygen is used as a mediator which concentration can be reliably and interferences free measured by the aim of a Clark type oxygen probe applied as a transducer; reproducible bacterial membranes can be elaborated by filtration of resuspended bacterial culture after preliminary determination of its activity; application of membranes containing lyophilized bacteria capable to be conserved infinitely long time and activated just before their application; negligible cost compared with the sensors based on immobilized enzymes. The steady-state response of the proposed bacterial sensor to choline obtained in 200 s is linear in the investigated concentration range up to 2 × 10 −4 mol dm −3 , with detection limit of 8 × 10 −8 mol dm −3 and sensitivity of 4 × 10 −1 A cm 3 mol −1 , at pH 6, temperature of 25 • C and stirring rate of 300 rpm. Choline is formed as a result of the catalytic hydrolysis (depending on the cholinesterase activity) of the substrate acetylcholine. Linear calibration graph for cholinesterase activity determination was obtained in the range up to 11 mU cm −3 , with a slope of 1.97 × 10 −2 A cm 3 mU −1 , at pH 6, temperature of 25 • C and stirring rate of 300 rpm. The tests with reconstituted lyophilized serum with known activity used as a control sample confirm the accuracy of the proposed method. The relative error of the determination was only 2.82%.

Analytica Chimica Acta, 1995

The infhtence of the temperature, pH, p-benzoquinone and enzyme concentrations and speed of rotat... more The infhtence of the temperature, pH, p-benzoquinone and enzyme concentrations and speed of rotation of the electrode on the glucose determination with a p-benzoquinone mediated graphite sensor with covalently linked glucoseoxidase has been investigated. The results were applied to glucose analysis in concrete samples: blood serum, foodstuffs and pharmaceuticals. It was found that the proposed bioelectrocatalytic method for the determination of glucose is not inferior to the approved methods with respect to its accuracy; it is more rapid in comparison with the photometric method and it can be applied to determinations of low and high glucose concentrations.

ECS Meeting Abstracts

Simultaneous Determination of Cr(VI) and Fe(II) in Waste Waters by Differential Alternative Pulse... more Simultaneous Determination of Cr(VI) and Fe(II) in Waste Waters by Differential Alternative Pulses Voltammetry. [ECS Meeting Abstracts 1102, 2455 (2011)]. Roumen Zlatev, Margarita Stoytcheva, Benjamin Valdez, Marcela Ovalle.

Talanta, Jan 15, 2017

A simple, rapid, and reagent-less calorimetric method for H2O2 quantification, applicable automat... more A simple, rapid, and reagent-less calorimetric method for H2O2 quantification, applicable automatically on-line was developed, analytically characterized and tested with real SC-1 alkaline etching solutions used in the semiconductor technology. Being based on H2O2 catalytic decomposition by immobilized solid catalyst, the proposed method possesses excellent specificity toward the H2O2 without any interference. The total quantification time was found to be less than 60s, RSD lies in the range from 4.7% to 1.8% for the linear concentration range from 2.8 10(-2) to 2molL(-1) respectively and the LOD was determined to be as low as 9.3 10(-3)molL(-1).

Sensors, 2016

Biosensors are small analytical devices incorporating a biological recognition element and a phys... more Biosensors are small analytical devices incorporating a biological recognition element and a physico-chemical transducer to convert a biological signal into an electrical reading. Nowadays, their technological appeal resides in their fast performance, high sensitivity and continuous measuring capabilities; however, a full understanding is still under research. This paper aims to contribute to this growing field of biotechnology, with a focus on Glucose-Oxidase Biosensor (GOB) modeling through statistical learning methods from a regression perspective. We model the amperometric response of a GOB with dependent variables under different conditions, such as temperature, benzoquinone, pH and glucose concentrations, by means of several machine learning algorithms. Since the sensitivity of a GOB response is strongly related to these dependent variables, their interactions should be optimized to maximize the output signal, for which a genetic algorithm and simulated annealing are used. We report a model that shows a good generalization error and is consistent with the optimization.

Materials Performance, 2010

Phosphoric acid (H 3 PO 4 ) is produced by a reaction between phosphate ore and sulfuric acid (H ... more Phosphoric acid (H 3 PO 4 ) is produced by a reaction between phosphate ore and sulfuric acid (H 2 SO 4 ), forming a thick slurry. Erosion-corrosion-resistant alloys are selected for fabrication of turbine agitators and slurry pumps. The simultaneous process of mechanical erosion and electrochemical corrosion is reported.

Materials Performance, 2010

Colloids and Surfaces B: Biointerfaces, 2009

A poly-amidosulfonic acid and multi-wall carbon nanotubes composite (PASA/MWNTs) modified electro... more A poly-amidosulfonic acid and multi-wall carbon nanotubes composite (PASA/MWNTs) modified electrode has been constructed by electropolymerization on glassy carbon electrode (GCE). The electrochemical behaviors of hydroquinone (HQ) and catechol (CC) were investigated using cyclic and differential pulse voltammetries (DPVs) at the prepared electrode. Separation of the reductive peak potentials for HQ and CC was about 120 mV in pH 6.0 phosphate buffer solution (PBS), which makes it suitable for simultaneous determination of these compounds. In the presence of 1.0 x 10(-4)mol L(-1) isomer, the reductive peak currents of DPV are proportional to the concentration of HQ in the range of 6.0 x 10(-6) to 4.0 x 10(-4)mol L(-1), and to that of CC in the range of 6.0 x 10(-6) to 7.0 x 10(-4)mol L(-1). When simultaneously changing the concentration of both HQ and CC, the linear concentration range of HQ (or CC) is 6.0 x 10(-6) to 1.0 x 10(-4)mol L(-1) (or 6.0 x 10(-6) to 1.8 x 10(-4)mol L(-1)), and the corresponding detection limits are 1.0 x 10(-6)mol L(-1). The proposed method has been applied to simultaneous determination of HQ and catechol in water sample, and the results are satisfactory.

OmniaScience Monographs, Apr 3, 2014

The biosensor is an analytical compact device that embodies a biological detection element and a ... more The biosensor is an analytical compact device that embodies a biological detection element and a chemical transducer that converts the biological signal into an electrical output that is proportional to the concentration of the compund that is desired to assess. The biosensors are high-technology monitoring tools that offer electrochemical measures in a continuous, fast and highly sensitive fashion. Their potential use in real-life applications span from medical and health tasks to environmental monitoring problems. This chapter focuses particularly on Glucose Oxidase Amperometric Biosensors (GOAB) modeling, in order to understand their dynamics and behavior by automated computerized algorithms. In this contribution, we step aside from traditional modeling techniques –mathematical equations and simulation models– by using statistical learning methods. We model the continuous amperometric response of a GOAB by means of several classic and statistical learning methods. Specifically, k...

ECS Transactions, 2019

The Reversible Hydrogen Electrode (RHE) potential depends on the pH and the H2 pressure which det... more The Reversible Hydrogen Electrode (RHE) potential depends on the pH and the H2 pressure which determines the disolved hydrogen concentration according to the Henry law. RHE was applied as hydrogen sensor for real time bacterial hydrogen production rate evaluation employing the photosynthetic Rodobacter Capsulatus strain IR3 in 10 ml reactor containing RCV medium. The reliable determination of the produced H2 volume for determination of the process rate requires big reactor volumes since the H2 production rate is small, measured in mL/hour and the precise measurements of small gas volumes is very complicated. The RHE potential monitoring however allows real time determination of the generated H2 and hence the process rate without consumption of great reagent amounts.

ECS Transactions, 2010

ABSTRACT Some bacterial and hybrid (bacterial/enzymatic) electrochemical sensors recently develop... more ABSTRACT Some bacterial and hybrid (bacterial/enzymatic) electrochemical sensors recently developed by the authors were evaluated for potential in situ metal ions and organophosphorus pesticides determinations. The analytical performances of the ...

ECS Transactions, 2010

The controversial statements reported till now about the local conductivity of the solution at it... more The controversial statements reported till now about the local conductivity of the solution at its interface with the corroded specimen affecting the SVET results were verified by the simultaneous application of a combination of two measuring techniques: SVET and conductometry. Bare and chromated AA 6061-T6 aerospatial aluminum alloy samples in 5% NaCl solutions were employed in the experiments. It was confirmed that the local conductivity variations are negligible above the chromated specimens obviously because of the pit sealing while about 4% changes were observed 5 minutes after the immersion of the bare samples. An approach for fast automatic detection of pit sites on the entire specimen surface, based on high resolution digital optical scanning coupled with image recognition software was developed and applied in the SVET/Conductivity experiments.

ECS Transactions, 2009

In this work are reported new results on the type of the immobilized acetylcholinesterase inhibit... more In this work are reported new results on the type of the immobilized acetylcholinesterase inhibition by specific and non-specific inhibitors, as well as the values of the characteristic enzyme reaction kinetic parameters in the absence and in the presence of inhibitors, identified applying an electrochemical approach. It was found that: -The kinetics of the process of acetylthiocholine hydrolysis catalyzed by the immobilized acetylcholinesterase is complicated by mass transfer constraints, at low substrate concentrations; -The inhibition of the immobilized acetylcholinesterase by specific inhibitors (trichlorfon, oxophosphol, galantamine, caffeine, atropine, nicotinic acid, and eserine) is of a competitive type, while quinine and thiamine acts as non-competitive inhibitors; -The character of acetylcholinesterase inhibition in heterogeneous phase by non-specific inhibitors (Hg2+, Cu2+, Cd2+, Mn2+, Fe3+, and AsO33-) is reversible and non-competitive. The reactivity of the inhibitors d...

ECS Transactions, 2010

A noise suppression approach was developed and applied for pitting corrosion studies of chromatiz... more A noise suppression approach was developed and applied for pitting corrosion studies of chromatized 6061-T6 airspace aluminum alloy by Scanning Vibrating Electrode Technique (SVET). The proposed approach is based on the power line frequency application as a SVET probe driving frequency combined with a synchronized double integration of the SVET probe signal yielding a free of noise signal product. Noise suppression up to -58 dB was achieved allowing precise measurement of the ionic current above the corroded metal surface proving the opportunity to replace the high cost lock-in amplifier.

ECS Transactions, 2010

A simple and rapid on-line Differential Pulse Polarographic (DPP) method for direct Co2+ determin... more A simple and rapid on-line Differential Pulse Polarographic (DPP) method for direct Co2+ determination in Zn plant electrolyte was developed and tested in real solutions. 1-nitroso-2-naphtol was applied as Co2+ to Co3+ oxidizing and chelate forming agent in ammonia buffer (NH4OH/HCl in 8 to 1 ratio) supporting electrolyte. The great DPP difference between Co3+ and Zn2+ peak potentials allows the complete elimination of the Zn2+ interference ensuring reliable results in the linear concentration range from 10 ppb to 20 ppm. Disposable mercury surface electrodes were applied as working electrodes.

Lecture Notes in Computer Science, 2014

ABSTRACT Biosensors are small analytical devices incorporating a biological element for signal de... more ABSTRACT Biosensors are small analytical devices incorporating a biological element for signal detection. The main function of a biosensor is to generate an electrical signal which is proportional to a specific analyte i.e. to translate a biological signal into an electrical reading. Nowadays its technological attractiveness resides in its fast performance, and its highly sensitivity and continuous measuring capabilities; however, its understanding is still under research. This paper focuses to contribute to the state of the art of this growing field of biotechnology specially on Glucose Oxidase Biosensors (GOB) modeling through statistical learning methods from a regression perspective. It models the amperometric response of a GOB with dependent variables under different conditions such as temperature, benzoquinone, PH and glucose, by means of well known machine learning algorithms. Support Vector Machines(SVM), Artificial Neural Networks (ANN) and Partial least squares (PLS) are the algorithms selected to do the regression task.

ECS Transactions, 2019

Activated by Au nanoparticles, Boron Doped Diamond (BDD) electrode was modified electrochemically... more Activated by Au nanoparticles, Boron Doped Diamond (BDD) electrode was modified electrochemically by TiO2 and applied for As(III) determination in natural ground waters. The modification with TiO2 results in up to 49.7% sensitivity increase compared with the non modified electrode achieved at the maximum TiO2 coverage percentage of about 73% of the BDD surface.

ECS Transactions, 2011

Cerium based conversion coatings were immersion deposited on AA6061-T6 and characterized to deter... more Cerium based conversion coatings were immersion deposited on AA6061-T6 and characterized to determine the effect of NO2- as an additive in the conversion solution on the anticorrosion behavior. The NO2- addition to the cerium chloride and hydrogen peroxide solution improved the corrosion resistance and coating morphology. Electrochemical measurements were performed by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in 0.01 M NaCl aqueous solution and the coating morphology was analyzed by Atomic Force Microscopy (AFM).

Journal of the Serbian Chemical Society, 2010

In this study, the Cr(VI) removal potential of waste mycelium from the industrial xylanase-produc... more In this study, the Cr(VI) removal potential of waste mycelium from the industrial xylanase-producing strain Aspergillus awamori was evaluated. It was determined by FTIR analysis that amino groups from the major fungal wall constituents, chitin and chitosan, played a key role in the metal binding process. The effect of pH, initial ion concentration, temperature and amount of biomass on the removal was also studied. The removal efficiency increased with decreasing pH and increasing temperature and amount of biomass. The mechanism of Cr(VI) removal by A. awamori can be explained by a two-stage process involving an initial adsorption stage followed by a reducing stage. The removal process was described by a second-order polynomial and the optimal process parameters for attaining Rmax 94.4 % in 48 h were predicted, i.e., pH 1.5 and T 40?C. From both economic and ecological points of view, a promising possibility for the utilization of waste industrial mycelium of A. awamori as a low-cost...

Biosensors and Bioelectronics, 2006

The sensors applied recently for determination of cholinesterase activity are mostly enzymatic am... more The sensors applied recently for determination of cholinesterase activity are mostly enzymatic amperometric sensors, in spite of their disadvantages: short lifetime at ambient temperature, instability of the response, interferences, as well as passivation of the electrode surface. In the present paper a new approach for determination of cholinesterase activity was proposed, overcoming the main drawbacks of the analysis performed with amperometric enzymatic sensors. Instead of the immobilization of enzymes on a conducting electrode surface, whole cells of Arthrobacter globiformis, containing choline oxidase were fixed on a Clark type oxygen probe. Current proportional to bacteria respiration is registered as a sensor response. The application of whole cells of bacteria as a sensing element permits to achieve high stability of the response and long lifetime of the sensor at ambient temperature, due to the conservation of the enzyme in its natural micro-environment inside the immobilized cells. The proposed sensor keeps its functionality more than 7 weeks stored in deionized water at ambient temperature. For the first 2 weeks the amplitude of the response decreases with only 10% and at the end of the studied 7 weeks period the response was 50% of the initial. The other advantages of the proposed sensor are: the dissolved oxygen is used as a mediator which concentration can be reliably and interferences free measured by the aim of a Clark type oxygen probe applied as a transducer; reproducible bacterial membranes can be elaborated by filtration of resuspended bacterial culture after preliminary determination of its activity; application of membranes containing lyophilized bacteria capable to be conserved infinitely long time and activated just before their application; negligible cost compared with the sensors based on immobilized enzymes. The steady-state response of the proposed bacterial sensor to choline obtained in 200 s is linear in the investigated concentration range up to 2 × 10 −4 mol dm −3 , with detection limit of 8 × 10 −8 mol dm −3 and sensitivity of 4 × 10 −1 A cm 3 mol −1 , at pH 6, temperature of 25 • C and stirring rate of 300 rpm. Choline is formed as a result of the catalytic hydrolysis (depending on the cholinesterase activity) of the substrate acetylcholine. Linear calibration graph for cholinesterase activity determination was obtained in the range up to 11 mU cm −3 , with a slope of 1.97 × 10 −2 A cm 3 mU −1 , at pH 6, temperature of 25 • C and stirring rate of 300 rpm. The tests with reconstituted lyophilized serum with known activity used as a control sample confirm the accuracy of the proposed method. The relative error of the determination was only 2.82%.

Analytica Chimica Acta, 1995

The infhtence of the temperature, pH, p-benzoquinone and enzyme concentrations and speed of rotat... more The infhtence of the temperature, pH, p-benzoquinone and enzyme concentrations and speed of rotation of the electrode on the glucose determination with a p-benzoquinone mediated graphite sensor with covalently linked glucoseoxidase has been investigated. The results were applied to glucose analysis in concrete samples: blood serum, foodstuffs and pharmaceuticals. It was found that the proposed bioelectrocatalytic method for the determination of glucose is not inferior to the approved methods with respect to its accuracy; it is more rapid in comparison with the photometric method and it can be applied to determinations of low and high glucose concentrations.

ECS Meeting Abstracts

Simultaneous Determination of Cr(VI) and Fe(II) in Waste Waters by Differential Alternative Pulse... more Simultaneous Determination of Cr(VI) and Fe(II) in Waste Waters by Differential Alternative Pulses Voltammetry. [ECS Meeting Abstracts 1102, 2455 (2011)]. Roumen Zlatev, Margarita Stoytcheva, Benjamin Valdez, Marcela Ovalle.

Talanta, Jan 15, 2017

A simple, rapid, and reagent-less calorimetric method for H2O2 quantification, applicable automat... more A simple, rapid, and reagent-less calorimetric method for H2O2 quantification, applicable automatically on-line was developed, analytically characterized and tested with real SC-1 alkaline etching solutions used in the semiconductor technology. Being based on H2O2 catalytic decomposition by immobilized solid catalyst, the proposed method possesses excellent specificity toward the H2O2 without any interference. The total quantification time was found to be less than 60s, RSD lies in the range from 4.7% to 1.8% for the linear concentration range from 2.8 10(-2) to 2molL(-1) respectively and the LOD was determined to be as low as 9.3 10(-3)molL(-1).

Sensors, 2016

Biosensors are small analytical devices incorporating a biological recognition element and a phys... more Biosensors are small analytical devices incorporating a biological recognition element and a physico-chemical transducer to convert a biological signal into an electrical reading. Nowadays, their technological appeal resides in their fast performance, high sensitivity and continuous measuring capabilities; however, a full understanding is still under research. This paper aims to contribute to this growing field of biotechnology, with a focus on Glucose-Oxidase Biosensor (GOB) modeling through statistical learning methods from a regression perspective. We model the amperometric response of a GOB with dependent variables under different conditions, such as temperature, benzoquinone, pH and glucose concentrations, by means of several machine learning algorithms. Since the sensitivity of a GOB response is strongly related to these dependent variables, their interactions should be optimized to maximize the output signal, for which a genetic algorithm and simulated annealing are used. We report a model that shows a good generalization error and is consistent with the optimization.

Materials Performance, 2010

Phosphoric acid (H 3 PO 4 ) is produced by a reaction between phosphate ore and sulfuric acid (H ... more Phosphoric acid (H 3 PO 4 ) is produced by a reaction between phosphate ore and sulfuric acid (H 2 SO 4 ), forming a thick slurry. Erosion-corrosion-resistant alloys are selected for fabrication of turbine agitators and slurry pumps. The simultaneous process of mechanical erosion and electrochemical corrosion is reported.

Materials Performance, 2010