DOI: 10.1002/elan.201900036 Differential Pulse Voltammetry Determination of OfloxacininHumanSerumandUrineBasedonaNovel Tryptophan-graphene Oxide-Carbon Nanotube Electrochemical Sensor Yuqiu Zhu, [a] Chengkun Li, [a] Lumin Wang, [a] Miao Chen, [a] Jingang Yu, [a] Qi Liu,* [a] and Xiaoqing Chen* [a, b] Abstract: The highly sensitive determination of ofloxacin (OFL) in human serum and urine was achieved on a novel tryptophan-graphene oxide-carbon nanotube (Trp-GO- CNT) composite modified glassy carbon electrode (Trp- GO-CNT/GCE). The Trp-GO-CNT composite was fab- ricated, and its morphologies and surface functional groups were characterized by field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared (FT-IR) spectroscopy. The electrochemical prop- erties of Trp-GO-CNT/GCE were investigated by cyclic voltammetry (CV) and electrochemical impedance spec- troscopy (EIS). The superior electrochemical behaviors of Trp-GO-CNT/GCE toward OFL can be mainly assigned to the excellent electrocatalytic activity of Trp, the great conductivity and high surface area of GO and CNT, and the synergistic effect between Trp, GO and CNT. Under optimum conditions, a wide and valuable linear range (0.01–100 μM), a low detection limit (0.001 μM, S/N = 3), a good linear relationship (R2 > 0.999), good stability and repeatability were obtained for the quantitative determi- nation of OFL. Furthermore, the Trp-GO-CNT electro- chemical sensor was successfully applied to the determi- nation of OFL in human serum and urine samples, and satisfactory accuracy and recovery could be obtained. Keywords: Electrochemical detection · Ofloxacin · Tryptophan-graphene oxide-carbon nanotube composite · Differential pulse voltammetry · Quantitative determination 1 Introduction Ofloxacin (OFL), one of the synthetic fluoroquinolones, has been widely used against many pathogenic gram- negative and gram-positive bacteria through inhibition of their DNA gyrase [1]. Especially, OFL is widely used for bactericide due to its low cost, wide bactericidal spectrum and rapid absorption [2]. Therefore, fast and accurate analysis of OFL is very important for understanding the patient‘s medical process in clinical treatment. Further- more, OFL is not only applied in human medicine but also widely used in the veterinary industry to prevent and treat various infectious diseases, and even used as growth- promoting reagents. However, drug residues which enter the food chain have caused some potential hazards such as bacterial resistance, hypersensitivity reaction and toxic effect [3]. Especially, synthetic fluoroquinolones are usually overused in long-term care, which is harmful to the pregnancy and breastfeeding mothers, and the grow- ing children and adolescents [4]. Therefore, the maximum residue levels (MRLs) for fluoroquinolones and with- drawal periods have been enacted in more and more countries. Developing a reliable and simple method to detect the trace amount of OFL in the environment or human serum is impending. Up to now, various techniques such as fluorimetric detection [5], UV visible spectrophotometry [6], high performance liquid chromatography (HPLC) [7], liquid chromatography-mass spectrometry (LC-MS) [8], capil- lary electrophoresis [9] and microbiological assay [10] have been developed for the determination of OFL. However, these methods have exhibited some drawbacks such as time-consuming in sample detection, complex and expensive instruments required, and complicated sample pretreatments needed. Recently, electrochemical detec- tion methods have attracted considerable interest because they are relatively more facile and stable, less time- consuming, more environment-friendly and less costly besides the excellent detection limits [11,12]. Previously reported studies have indicated that the fabrication of modified electrodes is essential for electrochemical analy- sis [13]. Therefore, it is highly desirable to develop novel electrodes for the precise detection of OFL. [a] Y. Zhu, C. Li, L. Wang, M. Chen, J. Yu, Q. Liu, X. Chen College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China E-mail: iliuqi@csu.edu.cn xqchen@csu.edu.cn [b] X. Chen Key Laboratory of Hunan Province for Water Environment and Agriculture Product Safety, Central South University, Changsha 410083, Hunan, China Supporting information for this article is available on the WWW under https://doi.org/10.1002/elan.201900036 Full Paper www.electroanalysis.wiley-vch.de © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2019, 31, 1 – 9 1 These are not the final page numbers! ��