1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 DOI: 10.1002/elan.201700167 Magnetite Nanoparticles Bonded Carbon Quantum Dots Magnetically Confined onto Screen Printed Carbon Electrodes and Their Performance as Electrochemical Sensor for NADH. Thiago C. Canevari,* [a, b] Fernando H. Cincotto, [c] Delmarcio Gomes, [b] Richard Landers, [d] and Henrique E. Toma* [b] Abstract: Hybrid magnetite/carbon quantum dots (MagNP/C-dots) were prepared and their characterization performed by high resolution transmission electron micro- scopy (HR-TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Because of their suitable magnetization and electrochemical properties, they were used as versatile electrode modifiers after magnetically confining onto screen printed carbon electro- des (SPE), with the aid of a miniature external magnet. The reported strategy introduces a convenient procedure for assembling modified electrodes, since the nanopar- ticles can be easily released by removing the magnet. The non-enzymatic magnetic biosensor showed excellent per- formance in the determination of NADH at the concen- tration range 2 3 10 À7 to 5 3 10 À6 mol L À1 , exhibiting a sensitivity of 0.15 mmol L À1 and detection limit of 20 nmol L À1 . The MagNP/C-dots /SPE sensor was also successfully applied for the determination of NADH in serum samples. The interference of typical biological molecules has also been investigated. Keywords: Bioelectrochemistry · NADH · Nanoelectrochemistry · Magnetic nanoparticles · Sensors 1 Introduction Nanoparticles are being increasingly used as electrode modifiers, because they can expand the active area, introduce new functionalities, and improve the electron transfer processes [1]. In particular, magnetite nano- particles (MagNP) are being used in (bio) electrochemical sensors [2, 3], because their magnetic properties allow a rapid and efficient modification procedure for the elec- trode surface, by applying an external magnet. In addition, they usually exhibit functional groups on their surface, such as hydroxyl groups, favoring the binding of molecules to their surface, and facilitating the electron transfer process at the electrode-solution interface. Similarly to the magnetite nanoparticles, polycrystalline spinel ferrites can also be employed in the construction of electro- chemical sensors. [4–5] A particularly relevant substrate in electroanalytical chemistry is nicotinamide adenine dinucleotide, NADH. It is a very important coenzyme in biological reactions, and the ratio of the reduced/oxidized forms, NADH/ NAD, can indicate some critical changes in the organism, such as the decrease of the energy production due to their influence on the ATP level, in addition to showing the emergence of Parkinson’s diseases and insomnia [6]. For this reason, NADH analysis is an important issue, and is usually carried out by electrochemistry techniques, ex- ploiting a great variety of electrodes, such as those modified with SWCNTs, oxidized reduced graphene oxide, polymers, dyes [7,8] and ferrite nanocomposite [4]. In general, the modified electrodes have advantages over the conventional electrodes such as gold (Au), platinum (Pt) and glassy Carbon (GCE) [9, 10], e. g. by allowing to perform under less positive oxidation potentials, and exhibiting a greater sensitivity associated with an im- proved electron transfer kinetics at the electrode-solution interface. In general, the electrochemical techniques are also less expensive in relation to other analytical methods such as mass spectroscopy, chromatography and fluorescence [11]. In this work, we are showing that the association of superparamagnetic MagNP (Fe 3 O 4 ) nanoparticles with carbon quantum dots, can be an interesting alternative to improve the NADH electrochemical analysis. This partic- [a] T. C. Canevari Engineering School, Presbyterian University Mackenzie, 01302-907 S¼o Paulo, SP, Brazil; Tel/Fax: (+ 55) 11 3091-3887 E-mail: tccanevari@gmail.com [b] T. C. Canevari, D. Gomes, H. E. Toma Institute of Chemistry, University of S¼o Paulo, 05508-000, S¼o Paulo, SP, Brazil; E-mail: henetoma@iq.usp.br [c] F. H. Cincotto Department Chemistry, Federal University of S¼o Carlos, 13565-905, S¼o Carlos, SP, Brazil; [d] R. Landers Institute of physic, University of Campinas, 13560-970, Cam- pinas, SP, Brazil; Full Paper www.electroanalysis.wiley-vch.de # 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2017, 29, 1 – 9 1 These are not the final page numbers! ÞÞ