Reşat Apak’s research while affiliated with Istanbul University-Cerrahpaşa and other places

Three different Mn‐based peroxidase‐mimetic nanoparticles (NPs) were synthesized as nanozymes: (i) manganese (II)‐ferrite (MnFe2O4) fabricated by co‐precipitating MnCl2.4H2O and FeCl3.6H2O; (ii) MnO2 synthesized from precipitation of MnSO4 in basic medium; (iii) manganese oxide obtained by reducing KMnO4 with tris(2‐hydroxyethyl) amine, all characterized using STEM, EDXS, FTIR and XPS techniques. These NPs acted as peroxidases, revealed by using a DNA probe, on which the damage generated by peroxidase mimetic NPs was monitored spectrophotometrically by CUPRAC and 3,3′,5,5′‐tetramethylbenzidine (TMB) methods. Results were confirmed by RP‐HPLC and voltammetry, the latter involving an AuNPs‐modified glassy carbon electrode (GCE), multi‐walled carbon nanotubes (MWCNT) and Nafion combination. Colorimetric assay with CUPRAC rather than TMB better agreed with chromatographic/voltammetric findings in detecting both peroxidase activity and DNA hazard. Mn(III)‐containing manganese oxide from permanganate reduction had stronger peroxidase activity. The proposed method demonstrated protective effects of antioxidants (morin, catechin and quercetin) against oxidative DNA damage for the first time.

A green process, both in terms of solvent (natural deep eutectic solvents, NADES) and extraction method (microwave-assisted extraction, MAE) for the recovery of bioactive components from sunflower pomace (SFP, a by-product of oil extraction process), was designed to contribute to their sustainable valorization. For the extraction of valuable phenolics from sunflower pomace, nine potential NADES were prepared. Among them, choline chloride-urea-water (CC-U-W) at a molar ratio of 1:2:4 was selected for extraction, showing better yield and physicochemical properties. Operational parameters (extraction temperature, time, water ratio, and solvent-to-solid ratio) for MAE were optimized and modeled utilizing response surface methodology. Under optimal conditions, the antioxidant properties of SFP extract were evaluated by CUPRAC, ABTS, and DPPH methods. The total phenolic contents of extracts were evaluated by the Folin–Ciocalteu method. Phenolics were characterized by using the HPLC–PDA system. The results showed that SFP NADES extract had potential antioxidant activity which was higher than that of traditional solvents. In order to better evaluate SFP extract as a valuable food ingredient, SFP extracts at a ratio of 5, 10, and 20% added smoothie-like beverages with strawberries and yogurt were prepared. Fortification of beverages with NADES extract enhanced antioxidant efficiency, increasing total antioxidant capacity in a range of 12.4 to 68.6% and free radical scavenging capacity between 0.4 to 67.9% during both the initial and the in vitro gastrointestinal digestion stages. The addition of SFP NADES extract to food samples made a positive contribution in terms of the antioxidant activity of the final product. The results of this study revealed that sunflower by-products can be evaluated as a potential antioxidant source which is easily accessible, and the proposed extraction process has an important potential to recover bioactive compounds with high efficiency.

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 µM for lactate under optimal conditions with a detection limit of 0.17 µM. 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. Graphical Abstract