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.201700825 Graphene Quantum Dots-Based Electrochemical Biosensor for Catecholamine Neurotransmitters Detection Sylwia Baluta, [a] Anna Lesiak, [a, b] and Joanna Cabaj* [a] Abstract: An useful electrochemical sensing approach was developed for epinephrine (EP) detection based on graphene quantum dots (GQDs) and laccase modified glassy carbon electrodes (GC). The miniature GC bio- sensor was designed and constructed via the immobiliza- tion of laccase in an electroactive layer of the electrode coated with carbon nanoparticles. This sensing arrange- ment utilized the catalytic oxidation of EP to epinephrine quinone. The detection process was based on the oxidation of catecholamine in the presence of the enzyme – laccase. With the optimized conditions, the analytical performance demonstrated a high degree of sensitivity 2.9 mAmM 1 cm 2 , selectivity in a broad linear range (1– 120 3 10 6 M) with detection limit of 83 nM. Moreover, the method was successfully applied for EP determination in labeled pharmacological samples. Keywords: cyclic voltammetry · electrochemical biosensor · epinephrine · GQDs · laccase 1 Introduction Nanomaterials due to their unique properties, reasonable costs, and controllable shape have become an interesting group of materials in the electroanalytical measurements and sensing technologies. Special attention should be given to nanoparticles based on carbon, especially gra- phene quantum dots (GQDs), which possess planar structure of pristine or modified graphitic carbon [1]. As a new carbon-based material, GQDs possess many advan- tages due to the additional unique properties that arise from their nanoscale size. GQDs are expected to be suitable for various applications. The unique features of these well-defined nanoparticles include biocompatibility, chemical inertness, low toxicity, mechanical stiffness, high stability, strength and elasticity, and very high electrical and thermal conductivity [2–5]. Because of the large surface area, electrodes using GQDs are applied in capacitors [6] and batteries [7, 8], and the conductivity of GQDs is higher than that of graphene oxide (GO) [9]. As a result of these, electrode modification with GQDs preserves the biological activity of the immobilized enzyme, and simultaneously enhances the electrochemical signal [10]. The nanoscale dimension of GQDs permits the wiring of redox-centers in enzymes, especially oxidor- eductases, e. g. laccase [11–13]. The effective protein immobilization onto the electrode surface is crucial in enzyme-based biosensor fabrication. The structure of the support used for laccase immobilization should preserve the enzyme functionality. Herein, a semiconducting material based on GQDs was used. GQDs are a durable, long-lasting material, to which protein binds effectively, and which improves the electrical contact between the enzyme and the electrode significantly. Consequently, nanoparticles based on carbon are often used in biosensor construction for the detection of very wide range of chemical specimens. Therefore, such systems can be employed for e.g. ascorbic acid, glucose or heavy metal ions sensing (Pb 2 + , Hg 2 + ) with or without protein [14–17]. However, in most cases GQDs are functionalized with some chemical groups to enhance the connection between the biological material and the surface, or analyte and to improve the determination of molecules [18–20]. In the bio-system for EP determination presented below it was essential to design a very simple and accurate method for neurotransmitter detection, with effective binding surface protein which could be used in future for medical purposes. Due to the afore mentioned properties of GQDs, biosensors based on nanoscale materials and enzymes are often used for neurotransmitters (dopamine, epinephrine, norepinephrine) detection [21–24]. Recently, there has been an increasing interest in the application of methods based on nanoparticles to deter- mine the level of neurotransmitters with special focus on catecholamine detection. Accordingly, a successful deter- mination of dopamine (DA), epinephrine (EP) [27–28] as well as norepinephrine (NE) [29–30] levels using tools based on nanomaterials [25–26] has also been reported. Mostly, in these procedures enzymes are successfully used as a biologically active material. However, there are only a few studies concerning nanoparticles, which are pivotal in the detection of neurotransmitters. Mazloum-Ardakani et al. reported the use of Nickel Oxides nanoparticles/ [a] S. Baluta, A. Lesiak, J. Cabaj Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzez˙e Wyspian´skiego 27, 50-370 Wrocław, Poland E-mail: joanna.cabaj@pwr.edu.pl [b] A. Lesiak Faculty of Experimental Physics, Wrocław University of Sci- ence and Technology, Wybrzez˙e Wyspian´ skiego 27, 50-370 Wrocław, Poland Full Paper www.electroanalysis.wiley-vch.de # 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2018, 30, 1 – 11 1 These are not the final page numbers! ÞÞ