Nurgul Karadas - Academia.edu (original) (raw)

Papers by Nurgul Karadas

TURKISH JOURNAL OF CHEMISTRY, 2019

Curcuminoids, reported to have important biological properties, such as antioxidant, anti-Alzheim... more Curcuminoids, reported to have important biological properties, such as antioxidant, anti-Alzheimer, and antidiabetic properties, comprise curcumin (CRM; 1,7-bis [4-hydroxy-3-methoxyphenyl]-1,6-heptadiene-3,5-dione) and its derivatives demethoxycurcumin (DMC; (E,6E)-1-(3,4-dimethoxy-cyclohexyl)-7-(3,4-dimethoxyphenyl)hepta-1,6-diene-3,5dione) and bisdemethoxycurcumin (BDMC; 1,7-bis[4-hydroxyphenyl]-1,6-heptadiene-3,5-dione). Their electrochemical oxidations are thoroughly explored by applying cyclic and differential pulse voltammetric techniques. The dependence of current intensities and potentials on pH, concentration, scan rate, and nature of the buffer was investigated. The outcome is supported by density functional theory computations indicating the transfer of 4-e − /H + , 6-e − /H + , and 8e − /H + couples involved in the oxidation mechanisms of CRM, DMC, and BDMC, respectively, leading to the formation of the same oxidized product.

Macedonian Journal of Chemistry and Chemical Engineering, 2013

In the present paper, rapid, sensitive, selective, accurate and precise analytical methodologies ... more In the present paper, rapid, sensitive, selective, accurate and precise analytical methodologies were developed for the determination of the antidepressant drug mirtazapine (MIR) using voltammetry and liquid chromatography. In cyclic voltammetry (CV), MIR showed one sharp oxidation peak and one additional wave in acidic media in the anodic direction; at pH 5.50, the mirtazapine peak was single and sharp. Under optimized conditions, the peak current showed a linear dependence with concentration in the range between 0.212 and 26.54 µg/mL for glassy carbon (GC) and 1.06 and 26.54 µg/mL for boron-doped diamond (BDD) electrodes using differential pulse (DP) and square wave (SW) voltammetric techniques. The possible oxidation mechanisms are also discussed. A simple and fully validated reverse phase-liquid chromatography (RP-LC) method to test for MIR in tablets was developed using an X-Select RP-18 column (250x4.60 mm ID x 5µm) at 25ºC with methanol:water (30:70, v/v, containing 15 mM o-p...

digitool.library.mcgill.ca

Epidemics, interzones and biosocial change : retroviruses and biologies of globalisation in West ... more Epidemics, interzones and biosocial change : retroviruses and biologies of globalisation in West Africa. Epidemics, interzones and biosocial change : retroviruses and biologies of globalisation in West Africa. Nguyen, Vinh-Kim, 1963-. ...

Journal of Electroanalytical Chemistry, 2019

Mass-sensitive sensing has one main advantage: mass is one of the most fundamental properties of ... more Mass-sensitive sensing has one main advantage: mass is one of the most fundamental properties of any analyte. They also require suitable recognition materials to ensure selectivity. Quartz Tuning Forks (QTFs) have been known as common oscillator components due to their stable resonant frequencies. In recent years, QTFs have started to be implemented as transducers into sensor systems which has the potential on resulting mass-sensitive sensors with high reliability. In this study, a developed QTF mass-sensitive sensor system has been utilized for applying calculations and finally formulizing the frequency to mass-load relationship for the standard 32768Hz QTF. Coulometric copper deposition method was used for a reliable mass-load that could be calculated consequently from the charge reduction measured through the potentiostat. Frequency changes were calculated through the measurements read on the QTF sensor system described. SEM imaging and SEM-EDX element analysis methods were used to assay and confirm the modification performed on the surface.

Current Pharmaceutical Analysis, 2013

Analytical Chemistry, 2015

A silver amperometric detector coupled to liquid chromatography (LC) was used for the determinati... more A silver amperometric detector coupled to liquid chromatography (LC) was used for the determination of 6-thioguanine (6-TG) and two of its metabolites, thiouric acid (TU) and 2-amino-6-mercaptopurine riboside (6-TGR). The silver detector coupled to LC operated at a low applied potential (0.08 V vs Ag/AgCl) and offered a chromatogram with peak responses corresponding to molecules interacting with silver, namely, chloride ions and small soluble biothiols in addition to the organothiol drug compounds investigated. Online electrochemical surface cleaning permitted the improvement of the repeatability and peak shape of the recorded signal compared to direct current amperometric detection (AD) when operating in chloride containing media. The studied molecules were eluted isocratically within 5 min on a reversed-phase C18 column without interference from endogenous biothiols present in urine samples. Diluted urine samples (1:1) were directly injected in the LC setup; a linear calibration curve was obtained between peak area and analyte concentration between 0.1 and 10 μM for all the studied molecules. Limits of detection (LODs) were 0.03, 0.008, and 0.01 μM, and the limits of quantification (LOQs) were 0.1, 0.02, and 0.03 μM for TU, 6-TG, and 6-TGR, respectively. Within-day RSDs were 2%, 0.8%, and 1% and between-day RSDs were 2%, 0.9%, 2% for TU, 6-TG, and 6-TGR, respectively. Recoveries in spiked urine were 99.8%, 99.9%, and 99.0% for TU, 6-TG, and 6-TGR, respectively.

Talanta, 2013

The voltammetric behavior of anticancer drug irinotecan (IRT) was investigated at poly (methylene... more The voltammetric behavior of anticancer drug irinotecan (IRT) was investigated at poly (methylene blue)/ multi-walled carbon nanotube (PMB/MWCNT) modified glassy carbon electrode (GCE). The modified electrode surface was characterized by a scanning electron microscope (SEM). The PMB/MWCNT modified GCE exhibits a distinct shift of the oxidation potential of IRT on the cathodic direction and a considerable enhancement of the peak current compared with bare electrode. The calibration curve was linear between the concentration range 8.0 Â 10 À 6 and 8.0 Â 10 À 5 M with the detection limit of 2.14 Â 10 À 7 M by differential pulse voltammetry in pH 10.0 Britton-Robinson buffer solution. Controlled potential coulometry was applied to find transferred electron numbers due to the oxidation of IRT. In this study, the pK a value of IRT was also determined by the dependence of the retention factor on the pH of the mobile phase. The effect of the mobile phase composition on the ionization constant was studied by measuring the pK a at different acetonitrile-water mixtures, ranging between 35 and 50% (v/v) using the reversed-phase liquid chromatography (RP-LC) method with UV detector. IRT was exposed to thermal, photolytic, hydrolytic and oxidative stress conditions, and the stressed samples were detected by the proposed method. Sensitive, rapid, and fully validated electrochemical and RP-LC methods for the determination of IRT in its dosage form were presented in details.

Talanta, 2014

Electrochemical oxidation of pemetrexed (PMX) was studied on bare, carboxylic acid functionalized... more Electrochemical oxidation of pemetrexed (PMX) was studied on bare, carboxylic acid functionalized multi-walled carbon nanotubes and over-oxidized polypyrrole modified (oo-PPy/MWCNTs-COOH/GCE) glassy carbon electrodes by cyclic and adsorptive stripping differential pulse voltammetric techniques. The oo-PPy/MWCNTs-COOH/GCE is very sensitive to the oxidation of PMX. The results proved that the over-oxidation of the PPy film gave a negative charge density on porous layer that improved the adsorption for PMX. The effects of pH, concentrations of MWCNTs and monomer, the number of cycles for the electropolymerization and the scan rate for sensor preparation were optimized. The MWCNTs-COOH and oo-PPy based sensor showed an excellent recognition capacity toward PMX. The linear responses have been obtained in the range from 8.00 × 10(-7)M to 1.00 × 10(-4)M with 2.04 × 10(-7)M detection limit for the bare GCE and from 1.00 × 10(-8)M to 1.00 × 10(-7)M with 3.28 × 10(-9)M detection limit for the modified GCE. The oxidation of PMX was controlled by the adsorption process on both types of electrode surfaces. The proposed methods were compared with the literature on UV spectrophotometric assay, which was carried out at an absorption maximum of 225 nm. The proposed method and the designed sensors have been successfully applied for the determination of PMX in pharmaceuticals.

Sensors and Actuators B: Chemical, 2013

ABSTRACT

Journal of Pharmaceutical and Biomedical Analysis, 2012

The detailed electrooxidative behavior of benidipine (BEN) has been studied by using glassy carbo... more The detailed electrooxidative behavior of benidipine (BEN) has been studied by using glassy carbon (GC) and boron-doped diamond (BDD) electrodes. Using cyclic voltammetry, depending on the pH values and the working electrodes, BEN showed one or two sharp and irreversible oxidation responses. The voltammetric experiments on some model compounds allowed elucidation of the oxidation mechanism of BEN. Highly sensitive, selective, rapid, and fully validated voltammetric methods for the determination of BEN in tablet dosage form were also presented. Under optimized conditions, the peak current showed a linear dependence with concentration in the range between 3.25 g mL −1 and 54.20 g mL −1 for GC and 1.08 g mL −1 and 54.20 g mL −1 for BDD electrodes by using differential pulse (DPV) and square wave (SWV) voltammetric techniques. In this study, acid dissociation constant (pK a ) value of BEN was determined by using the dependence of the retention factor on the pH of the mobile phase using reverse phase-liquid chromatographic (RP-LC) method. The effect of the composition of the mobile phase on the ionization constant was studied by measuring the pK a at different acetonitrile-water mixtures, ranging between 50 and 65% (v/v). Also simple, accurate, precise and fully validated RP-LC method for the assay of BEN in dosage form has been developed. XTerra RP-18 column at 25 • C with the mobile phase of acetonitrile:water 55:45 (v/v) adjusted to pH 3.0 with 15 mM o-phosphoric acid was used. Isocratic elution was performed in less than 5.0 min with a flow rate of 1.0 mL min −1 . The RP-LC method allowed quantitation over the 0.25-15.00 g mL −1 range for BEN. The proposed voltammetric and RP-LC methods allow a number of cost and time saving benefits. BEN was also exposed to thermal, photolytic, oxidative stress, acid-base catalyzed hydrolyses, and the stressed samples were detected by the proposed RP-LC method.

Chromatographia, 2011

The aim of the present study was to develop a fast, sensitive and reliable method for rapid scree... more The aim of the present study was to develop a fast, sensitive and reliable method for rapid screening of cephalosporin injectable dosage forms namely ceftazidime and ceftizoxime to the detection of counterfeit and substandard drugs that might be illegally commercialized. Ceftazidime, ceftizoxime and cefixime (IS) were separated in a X-Terra RP-18 column (250 9 4.60 mm ID 9 5 l) and DAD detector set at 290 and 260 nm. The mobile phase consisted of a mixture of methanol:water 20:80 (v/v) at a flow rate of 1.0 mL min -1 . Additionally, in order to find the optimum pH value of separation the pK a values of studied compounds were determined by using two different methodologies. Aqueous pK a values of studied compounds have been determined by UV-spectrophotometry and liquid chromatography were used for the determination and direct characterization of the dissociation constants by using the dependence of the capacity factor on the pH of the mobile phase in 20% (v/v) methanol-water binary mixture in which separation was performed. The pH of the mobile phase was adjusted with 25 mM H 3 PO 4 to 3.2. The method was shown to be linear, sensible, accurate, and reproducible over the range of analysis and it can be used to pharmaceutical formulations containing a single active ingredient within a short analysis time.