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Öğe Sensitive voltammetric determination of testosterone in pharmaceuticals and human urine using a glassy carbon electrode in the presence of cationic surfactant(Elsevier, 2014-05-10) Levent, Abdulkadir; Altun, Ahmet; Yardım, Yavuz; Şentürk, ZühreIn this work, the electrochemical investigation of testosterone, a steroid hormone from the androgen group, was carried out in aqueous and aqueous/surfactant solutions using a glassy carbon (GC) electrode. In cyclic voltammetry, the compound showed one irreversible and adsorption-controlled reduction peak. Addition of cationic surfactant (cetyltrimethylammonium bromide, CTAB) was found to enhance the reduction current signal of testosterone, whereas, anionic (sodium dodecylsulfate, SDS) and non-ionic (Tween 80) surfactants exhibited opposite effect. Using square-wave adsorptive stripping voltammetry, the current showed a linear dependence with concentration in the range between 10 and 70 nM in Britton–Robinson buffer, pH 5.0 containing 3 mM CTAB. A detection limit of 1.18 nM (0.34 ng mL−1), and relative standard deviation of 4.12% for a concentration level of 35 nM (n = 11) were calculated. This method was successfully applied for the analysis of testosterone in oil-based pharmaceutical preparations and urine samples without any separation.Öğe Graphene/Nafion composite film modified glassy carbon electrode for simultaneous determination of paracetamol, aspirin and caffeine in pharmaceutical formulations(ScienceDirect, 2016-05-16) Yiğit, Aydın; Yardım, Yavuz; Çelebi, Metin; Levent, Abdulkadir; Şentürk, ZühreA graphene-Nafion compositefilm was fabricated on the glassy carbon electrode (GR-NF/GCE), and usedfor simultaneous determination of paracetamol (PAR), aspirin (ASA) and caffeine (CAF). The electro-chemical behaviors of PAR, ASA and CAF were investigated by cyclic voltammetry and square-waveadsorptive anodic stripping voltammetry. By using stripping one for simultaneous determination of PAR,ASA and CAF, their electrochemical oxidation peaks appeared atþ0.64, 1.04 and 1.44 V, and good linearcurrent responses were obtained with the detection limits of 18 ng mL 1(1.2 10 9M), 11.7 ng mL 1(6.5 10 8M) and 7.3 ng mL 1(3.8 10 8M), respectively. Finally, the proposed electrochemical sensorwas successfully applied for quantifying PAR, ASA and CAF in commercial tablet formulations.Öğe Voltammetric behavior of benzo[a]pyrene at boron-doped diamond electrode: A study of its determination by adsorptive transfer stripping voltammetry based on the enhancement effect of anionic surfactant, sodium dodecylsulfate(Elsevier, 2011-07-15) Yardım, Yavuz; Keskin, Ertuğrul; Levent, Abdulkadir; Şentürk, ZühreBenzo[a]pyrene (BaP), a member of the polycyclic aromatic hydrocarbon (PAH) class, is one of the most potent PAH carcinogens. The electrochemical oxidation of BaP was first studied by cyclic voltammetry at the boron-doped diamond electrode in non-aqueous solvent (dimethylsulphoxide with lithium perchlorate). The compound was irreversibly oxidized in a single step at high positive potential, resulting in the well-resolved formation of a couple with a reduction and re-oxidation wave at much lower potentials. Special attention was given to the use of adsorptive stripping voltammetry together with a medium exchange procedure in aqueous and aqueous/surfactant solutions over the pH range of 2.0–8.0. The technique in aqueous solutions had little value in practice because of too small oxidation peak current. This problem was solved when surfactants were added into the sample solution, by which the oxidation peak currents of BaP were found enhanced dramatically. The employed surfactants were sodium dodecylsulfate (anionic, SDS), cetyltrimethylammonium bromide (cationic, CTAB) and Tween 80 (non-ionic). Using square-wave stripping mode, the compound yielded a well-defined voltammetric response in Britton–Robinson buffer, pH 2.0 containing 2.5 × 10−4 M SDS at +1.07 V (vs. Ag/AgCl) (after 120 s accumulation at +0.10 V). The process could be used to determine BaP in the concentration range of 16–200 nM (4.04–50.46 ng mL−1), with a detection limit of 2.86 nM (0.72 ng mL−1). This method was also applied to determine BaP in model water sample prepared by adding its different concentrations into tap water.
