Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond Nitrogen-doped carbon nanotubes towards electrochemical sensing: Efect of synthesis temperature Marines Steinmetz a,1 , Dhésmon Lima a,1 , Raíssa Ribeiro Lima Machado b , Uttandaraman Sundararaj c , Mohammad Arjmand d , Aline Bruna da Silva b , João Paulo Santos b , Christiana Andrade Pessôa a , Karen Wohnrath a, ⁎ a Department of Chemistry, Universidade Estadual de Ponta Grossa, Av. General Carlos Cavalcanti, 4748, 84030-900 Ponta Grossa, Paraná, Brazil b Department of Materials Engineering, Centro Federal de Educação Tecnológica de Minas Gerais, Av. Amazonas, 5253, 30421-169 Belo Horizonte, Minas Gerais, Brazil c Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr N.W., T2N 1N4 Calgary, Alberta, Canada d School of Engineering, University of British Columbia, 1137 Alumni Avenue, V1V 1V7 Kelowna, British Columbia, Canada ARTICLE INFO Keywords: Nitrogen-doped carbon nanotube Chemical vapor deposition Electrochemical sensing Dopamine Epinephrine ABSTRACT Nitrogen-doped carbon nanotubes (N-CNTs) were synthesized at various temperatures ranging from 650 to 950 °C, at 100 °C intervals, by chemical vapor deposition technique (CVD). The synthesized N-CNTs were em- ployed as modifers of glassy carbon paste electrodes (GCPE), which were further used as electrochemical sensors for the determination of dopamine (DA) and epinephrine (EP), two important catecholamines that display biological roles as neurotransmitters and hormones. The results revealed that electrodes modifed with the N-CNTs synthesized at 650 °C (GCPE/N-CNT 650 ) and 950 °C (GCPE/N-CNT 950 ) presented better electro- catalytic activities and sensing capabilities than the others (GCPE/N-CNT 750 and GCPE/N-CNT 850 ). N-CNT 950 had the highest graphitization and the highest powder conductivity, whereas N-CNT 650 had the lowest gra- phitization and the lowest powder conductivity. However, both N-CNT 950 and N-CNT 650 had the highest ni- trogen contents, 3.5 and 3.8 at.%, respectively, which probably enhanced their number of electroactive sites to interact with DA and EP molecules. This result is of high signifcance, because the use of the lowest temperature (650 °C) in the CVD process, yielded N-CNTs with improved electrocatalytic activity and less energy con- sumption during CVD. 1. Introduction Carbon nanotubes (CNTs) have been recognized as good candidates to improve the performance of electrochemical sensors due to their exceptional electrical, physical, and chemical properties [1–4]. CNTs exhibit large surface area, high thermal conductivity, chemical stabi- lity, and rapid electron transfer capability, which are responsible for their widespread use as electrode materials aiming biomolecule detec- tion [3,5,6]. Such properties of CNTs are associated with their unique tubular and porous structure, presence of defect sites (such as hepta- gons, pentagons, and Stone-Wales defects), active sites at end-caps, and carbon bonding type [4,6]. Despite their remarkable properties, CNTs exhibit very low dispersibility and high tendency to agglomerate in usual polar and non-polar solvents [7,8]. Strategies to overcome this issue include chemical functionalization of CNT with polymers, in- corporation in composite matrices, and doping with foreign atoms [8–10]. The latter strategy has been widely employed not only to increase CNT dispersibility, but also to enhance their sensing capability [11] and conductivity [12]. Among the several techniques for heteroatom in- sertion into CNTs, nitrogen doping stands out due to the close atomic radius between nitrogen and carbon atoms [13,14]. Several types of nitrogen bonding can be formed during the doping process, e.g., pyr- idinic, pyrrolic, and graphitic [15,16]. Nitrogen has fve valence elec- trons; thus, its insertion into carbon-based structures creates drastic changes in the energy band arrangements. This leads to creation of additional energy levels [17,18], besides the formation of chemically active sites [10,19]. Nitrogen doping introduces a substantial amount of defects into CNT structure, providing more reactive sites on their sur- face, which infuences their electrochemical properties [11,19,20]. For instance, nitrogen-doped CNTs (N-CNTs) generally exhibit enhanced electrical transfer properties, increased surface reactivity, and https://doi.org/10.1016/j.diamond.2020.108093 Received 8 July 2020; Received in revised form 19 August 2020; Accepted 11 September 2020 ⁎ Corresponding author. E-mail address: karen.woh@gmail.com (K. Wohnrath). 1 Marines Steinmetz and Dhésmon Lima made equal contributions to this work. Diamond & Related Materials 110 (2020) 108093 Available online 18 September 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved. T