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    A new colorimetric lactate biosensor based on CUPRAC reagent using binary enzyme (lactate-pyruvate oxidases)-immobilized silanized magnetite nanoparticles
    (Springer Wien, 2024) Ayaz, Selen; Ersan, Teslime; Dilgin, Yusuf; Apak, Resat
    A novel optical lactate biosensor is presented that utilizes a colorimetric interaction between H2O2 liberated by a binary enzymatic reaction and bis(neocuproine)copper(II) complex ([Cu(Nc)2]2+) known as CUPRAC (cupric reducing antioxidant capacity) reagent. In the first step, lactate oxidase (LOx) and pyruvate oxidase (POx) were separately immobilized on silanized magnetite nanoparticles (SiO2@Fe3O4 NPs), and thus, 2 mol of H2O2 was released per 1 mol of the substrate due to a sequential enzymatic reaction of the mixture of LOx-SiO2@Fe3O4 and POx-SiO2@Fe3O4 NPs with lactate and pyruvate, respectively. In the second step, the absorbance at 450 nm of the yellow-orange [Cu(Nc)2]+ complex formed through the color reaction of enzymatically produced H2O2 with [Cu(Nc)2]2+ was recorded. The results indicate that the developed colorimetric binary enzymatic biosensor exhibits a broad linear range of response between 0.5 and 50.0 mu M for lactate under optimal conditions with a detection limit of 0.17 mu M. The fabricated biosensor did not respond to other saccharides, while the positive interferences of certain reducing compounds such as dopamine, ascorbic acid, and uric acid were minimized through their oxidative removal with a pre-oxidant (NaBiO3) before enzymatic and colorimetric reactions. The fabricated optical biosensor was applied to various samples such as artificial blood, artificial/real sweat, and cow milk. The high recovery values (close to 100%) achieved for lactate-spiked samples indicate an acceptable accuracy of this colorimetric biosensor in the determination of lactate in real samples. Due to the increase in H2O2 production with the bienzymatic lactate sensor, the proposed method displays double-fold sensitivity relative to monoenzymatic biosensors and involves a neat color reaction with cupric-neocuproine having a clear stoichiometry as opposed to the rather indefinite stoichiometry of analogous redox dye methods.
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    A New Redox Mediator (Cupric-Neocuproine Complex)-Modified Pencil Graphite Electrode for the Electrocatalytic Oxidation of H2O2: A Flow Injection Amperometric Sensor
    (Wiley-V C H Verlag Gmbh, 2020) Emir, Gamze; Dilgin, Yusuf; Apak, Resat
    This paper introduces the cupric-neocuproine complex ([Cu(Nc)(2)](2+)) as a new redox mediator for the electrocatalytic oxidation of H2O2. For this purpose, [Cu(Nc)(2)](2+) was modified onto a disposable pencil graphite electrode (PGE) surface by using Nafion (Nf) as a cation-exchange membrane for the effective adsorption of complex. The modified electrode was prepared by immersing Nf-adsorbed PGE into [Cu(Nc)(2)](2+) complex solution (0.40 mM of Cu2+ and 0.80 mM of Nc prepared at pH 4.76 in 0.10 M acetate buffer). SEM images and EDX spectra of electrodes were presented for the elucidation of their surface morphologies. Cyclic voltammetric results showed that H2O2 was electrocatalytically oxidized at [Cu(Nc)(2)](2+)/Nf/PGE, because both the increase in current and the potential shift to a more negative direction were observed for oxidation of H2O2 at [Cu(Nc)(2)](2+)/Nf/PGE in comparison to bare PGE. Then, the electrocatalytic activity of [Cu(Nc)(2)](2+)/Nf/PGE toward H2O2 oxidation was utilized for amperometric determination of H2O2 in flow injection analysis (FIA) system. Flow injection (FI) amperometric studies showed two linear calibration ranges (1.0-1000.0 mu M (R-2=0.9958) and 1000.0-10000.0 mu M (R-2=0.9902) with a detection limit of 0.4 mu M H2O2 which is considered good for electroanalytical determinations, especially for electrocatalytic oxidation. To test the applicability of the developed FI amperometric hydrogen peroxide sensor, H2O2 contents of some relevant samples were determined.
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    A novel flow injection amperometric method for sensitive determination of total antioxidant capacity at cupric-neocuproine complex modified MWCNT glassy carbon electrode
    (Springer Wien, 2022) Ayaz, Selen; Uzer, Aysem; Dilgin, Yusuf; Apak, Resat
    A novel amperometric method is presented for the determination of total antioxidant capacity in flow injection analysis (FIA) system using copper(II)-neocuproine complex modified on Nafion-functionalized multi-walled carbon nanotube-glassy carbon electrode ([Cu(Ncp)(2)(2+)]/Nf@f-MWCNT/GCE). Cyclic voltammetric studies showed that the modified electrode exhibits a very well-formed reversible redox couple for Cu(II)-/Cu(I)-complex. In addition, the [Cu(Ncp)(2)(2+)]/[Cu(Ncp)(2)(+)] redox pair shows very good electrocatalytic activity towards the oxidation of polyphenolic compounds (PPhCs) such as trolox, catechin, and quercetin due to the enhancement of the anodic peak current of the redox couple in the presence of these analytes. This electrocatalytic oxidation current at the [Cu(Ncp)(2)(2+)]/Nf@f-MWCNT/GCE was used for flow injection (FI) amperometric determination of PPhCs. FI amperometric-time curves recorded under optimized conditions (applied potential: + 0.6 V vs. Ag/AgCl/KCl(0.10 M), flow rate: 2 mL/min) showed that the proposed electrode had a wide linear range (LR) with a very low detection limit (LOD) for PPhCs. LR and LOD were 0.5-800 and 0.2 mu M for trolox, respectively and 0.50-250 and 0.14 mu M, respectively, for both quercetin and catechin. This sensitive method was successfully applied to the amperometric measurement of total antioxidant capacity (TAC) of some herbal teas, giving compatible results with the spectrophotometric CUPRAC method. The proposed method gave higher rank to fast-reacting antioxidants; it was equally precise but had a wider linear range and lower LOD than the spectrophotometric CUPRAC assay (e.g., LOD for ascorbic acid and gallic acid were 0.07 and 0.08 mu M, respectively), and similar electroanalytical methods using the CUPRAC reagent.
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    Amperometric sensor for total antioxidant capacity measurement using Cu (II)-neocuproine/carrageenan-MWCNT/GCE
    (Elsevier, 2024) Sen, Furkan Burak; Elmas, Ervanur; Dilgin, Yusuf; Bener, Mustafa; Apak, Resat
    Determination of total antioxidant capacity (TAC) in foods is very important for combating oxidative stressinduced diseases. For this purpose, an amperometric sensor was developed for TAC determination by manufacturing a new electrode consisting of a copper(II)-neocuproine (Cu(II)-Nc) and carrageenan-multi-walled nanotube suspension functionalized on a glassy carbon electrode, named as Cu(II)-Nc/Car-MWCNT/GCE. Cyclic voltammetric studies showed that the modified electrode exhibits a very well-formed reversible redox couple for the Cu(II)-Nc/Cu(I)-Nc complex. For amperometric sensor preparation, the Cu(II)-Nc cationic chelate was electrostatically retained on the electrode with the anionic groups of Car (a sulfated biopolymer of low cost) and immobilized on the surface. Cu(II)-Nc was reduced to Cu(I)-Nc on the electrode with the addition of antioxidants under constant potential, thereby functioning as an electron-transfer mediator. Thus, the individual antioxidant compounds were not measured at their own peak potentials but were indirectly measured at a common potential through their ability to reduce cupric-to-cuprous neocuproine. The reoxidation current of Cu(I)-Nc showing a proportional increase with antioxidant concentration was recorded, resulting in increased sensitivity and selectivity. This anodic current intensity of Cu(I)-Nc correlated with the total antioxidant capacity (TAC) of real samples such as plant extracts, reflecting the combined reducing ability of all antioxidants in the sample. Calibration graphs of antioxidants were obtained by optimizing the working conditions. The current intensity of Cu (I)-Nc arising from trolox varied linearly with concentration in the range of 4.98-84.39 mu M. The limit of detection and limit of quantification values for trolox were found to be 0.59 and 1.99 mu M, respectively. The developed method selectively responded to food antioxidants, and was not affected by potential interferent ions and molecules commonly found in foods. The TAC values of real samples such as herbal teas, plant extracts, and fruit juices were calculated and compared with those found by the conventional spectrophotometric CUPRAC method.
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    Flow injection amperometric determination of hydrazine at a cupric-neocuproine complex/anionic surfactant modified disposable electrode
    (Elsevier, 2020) Ayaz, Selen; Dilgin, Yusuf; Apak, Resat
    Hydrazine (N2H4) is a hazardous chemical widely used as rocket propellant and industrial intermediate, therefore its sensitive and accurate analysis in environmental samples is very important. In this study, a new approach for flow injection amperometric detection of N2H4 is proposed based on its electrocatalytic oxidation at a bis(neocuproine)copper (II) chelate complex/sodium dodecyl sulfate-modified graphite pencil electrode (Cu (II)-Ncp/SDS/GPE). The proposed electrode was prepared by consecutive adsorption of anionic surfactant (SDS) and Cu(II)-Ncp onto pencil leads and the surface morphologies of electrodes prepared were elucidated by recording of their SEM images. Cyclic voltammograms (CVs) showed that Cu(II)-Ncp exhibited an excellent redox mediator property for the electrocatalytic oxidation of N2H4, because oxidation potential of N2H4 at bare GPE shifted to more negative direction with the use of Cu(II)-Ncp/SDS/GPE. Then, flow injection (FI) electroanalysis of N2H4 was performed by utilizing the electrocatalytic oxidation of N2H4 at Cu(II)-Ncp/SDS/GPE. A wide linear range (0.25-250 mu M) with a limit of detection (LOD) of 0.07 mu M and a sensitivity of 262 mu AmM(-1)cm(-2) were obtained for the FI amperometric detection of N2H4. Satisfactory recoveries were obtained for N2H4 detection in water samples, showing that this method can be accurately applied to real water samples.
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    Introductory Note to the Special Issue of Analytical Letters for the International Conference: 10th Aegean Analytical Chemistry Days (AACD 2016)
    (Taylor & Francis Inc, 2018) Apak, Resat; Dilgin, Yusuf; Arda, Aysem
    This special issue of 'Analytical Letters' is devoted to selected papers based on contributions from the 10th Aegean Analytical Chemistry Days (AACD 2016) International Meeting. The first AACD Meeting was held in Izmir-Turkey in 1998 for enhancing the scientific collaboration of analytical chemists from Aegean countries primarily comprising Turkey and Greece, but these series of conferences later evolved into a successful international event covering the entire field of analytical chemistry practiced in almost all countries. Selected papers from most of these conferences were published in the special issues of reputable journals. Specifically in this special edition, the authors have extended their conference proceedings papers and added their latest experimental results. The papers have undergone the standard review process. The AACD 2016 Conference, providing a forum among analytical chemistry researchers from many different countries, was held in eanakkale, jointly hosted by Eanakkale Onsekiz Mart University (COMU) and Istanbul University (IU). AACD 2016 combined invited lectures, parallel oral sessions and poster presentations, altogether comprising 14 invited lectures, 45 contributed oral presentations and 343 posters, and three company presentations. The selected best 40 posters were orally presented for 5 min at the 'flash poster presentation' session. The Conference Organizers are thankful to the AACD Continuation Committee, invited speakers and all presenters, authors and reviewers who were willing to share their expertise with us by submitting/reviewing the valuable contributions to this special issue, and finally to the Chief Editor of 'Analytical Letters' and Taylor & Francis who handled the complete review process.