Electrochemical mediatorless detection of norepinephrine based on MoO 3 nanowires Kunda J. Samdani, Dong Woo Joh, Manas K. Rath, Kang Taek Lee* Department of Energy Science and Engineering, DGIST, Daegu, 42988, Republic of Korea A R T I C L E I N F O Article history: Received 8 May 2017 Received in revised form 30 August 2017 Accepted 31 August 2017 Available online 1 September 2017 Keywords: MoO 3 nanowires Norepinephrine Electrochemical biosensor Mediatorless A B S T R A C T Achieving direct electron transfer between a biomolecule and modified electrodes is paramount for fabricating advanced biosensor devices. In this report, 1-dimensional (1D) MoO 3 nanowires (NWs) were synthesized in a systematic growth evolution study. These MoO 3 NWs, with the glassy carbon electrodes (GCEs), were further used as a mediatorless biosensor electrode for the detection of norepinephrine (NE) by cyclic voltammetry and chronoamperometry techniques. The MoO 3 NWs/GCE had a magnificent response time of 2 s in the electrochemical detection of NE, with a detection limit of 0.11 mM. This excellent bio-electrochemical performance is attributed to its high catalytic activity and 1D microstructure, providing a path for electron transport and increasing their sensitivity. The MoO 3 NWs/GCE also had a promising diffusion constant (D) value of 3.34  10 5 cm 2 /s and a heterogeneous rate constant (k) of 8.03  10 4 cm/s. The modified electrode possessed high stability, reproducibility, and selectivity. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Catecholamines act as neurotransmitters (NTs) and/or hor- mones in the central nervous system [1], influencing the activity of cells. They are involved in several physiological mechanisms and are related to some of the most common human pathologies, such as Parkinson's disease, Alzheimer disease, schizophrenia and hyperactivity [2,3]. Among them, norepinephrine (NE) is an important catechol- amine neurotransmitter in the mammalian central nervous system [4]. NE mediates increase in heart rate, blood pressure, pupil dilation, and dilation of the airways, and the constriction of blood vessels. NE is critical for attention and focus, learning, memory, and the sleep-wake cycle; thus, it is also used as a performance- enhancing drug in athletes, rendering it a prohibited substance by the World Anti-Doping Agency [5]. NE faciliates the conversion of glycogen to glucose, increasing in energy production [4]. Thus, a routine and reliable method must be developed for its detection. In recent years, several methods have been reported for measuring NE, including flow injection [6], HPLC [7], gas chromatography [8], spectrophotometry [9], fluorometry [10], and chemiluminescence [11]. However, most such methods are complicated, requiring derivatization and suffering from low sensitivity and high cost. These drawbacks have been overcome though the use of electrochemical methods with various modified electrodes [12,13], simplifying the fabrication with the desired composition, in turn improving their sensitivity [14,15]. As a result, this approach has become a better method than other instrument- based protocols [16,17]. There is widespead an interest in the use of nanowires (NWs) as the basis of biosensing applications [18]. As a wire decreases in diameter to the nanometer scale, the ratio of surface atoms compared to interior atoms rises dramatically, increasing in the conductivity on the wire surface and in the wire interior [19]. Thus, NWs are attractive bio-electrochemical transducer components. There are several reports on NWs as effective electrodes for biosensors. Hahm et al. [20] used Si NWs for the direct detection of DNA, and Komathi et al. [21] developed a titanium dioxide nanowire bridged 3D graphene-based cholesterol biosensor. Furthermore, nanostructure materials have potential use in mediatorless (direct) electron transfer between biomolecules and modified electrodes. Usually, biosensors that employ electron mediators have many limitations, such as high cost, potential cytotoxicity, and low selectivity. Thus, achieving direct electron transfer between biomolecules and electrodes is critical for fabricating advanced bio-electrochemical devices [22]. Among the various transition metal oxides, molybdenum trioxide (MoO 3 ) nanostructures possess the most desirable * Corresponding author at: DGIST, 333, Techno Jungang Daero, Hyeong- pungMyeon, DalseongGun, Daegu, 42988, Republic of Korea. E-mail address: ktlee@dgist.ac.kr (K.T. Lee). http://dx.doi.org/10.1016/j.electacta.2017.08.187 0013-4686/© 2017 Elsevier Ltd. All rights reserved. Electrochimica Acta 252 (2017) 268–274 Contents lists available at ScienceDirect Electrochimica Acta journal homepa ge: www.elsev ier.com/locate/electacta