Investigation on acid functionalization of double-walled carbon nanotubes of different lengths on the development of amperometric sensors Ana P. Lima a , Ariadne C. Catto b , Elson Longo c , Edson Nossol a , Eduardo M. Richter a , Rodrigo A.A. Munoz a, * a Institute of Chemistry, Federal University of Uberl^ andia, Av. Jo~ ao Naves de  Avila, 2121, Uberl^ andia, MG, Brazil b Department of Physics, Federal University of S~ ao Carlos, 13565-905, S~ ao Carlos, SP, Brazil c LIEC, Department of Chemistry, Federal University of S~ ao Carlos, 13565-905, S~ ao Carlos, SP, Brazil article info Article history: Received 17 October 2018 Accepted 8 January 2019 Available online 9 January 2019 Keywords: Carbon Catechol Dopamine Surface area Phenolic compounds abstract Double-walled carbon nanotubes (DWCNTs) of different length were submitted to acid functionalization and investigated as chemical modifiers on glassy-carbon electrode (GCE) for the sensing of dopamine and catechol. Acid functionalization introduced oxygenated groups and defects on the structure of DWCNTs, as detected by infrared, Raman and X-ray photoelectron spectroscopy. However, cyclic vol- tammetric experiments showed higher current responses on non-functionalized (NF) DWCNTs. The decrease in response was stronger for shorter length nanotubes (S-DWCNT) modified GCE, which was attributed to the reduction of electroactive area of functionalized nanotubes after acid treatment with HNO 3 /H 2 SO 4 . Brunauer-Emmett-Teller (BET) analyses confirmed the decrease in surface area of func- tionalized (F) DWCNTs, especially on FeS-DWCNT. Amperometric measurements also showed decrease in sensitivity and higher detection limit values on the FeS-DWCNT, which is also due to the decrease in electroactive area. As conclusion, DWCNT is a potential carbon-based material to the development of highly sensitive amperometric sensors and acid functionalization is not likely required due to the higher surface area provided by modification with the untreated material. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction The use of carbon nanostructures to provide the modification of surfaces has been intensively investigated in the development of electrochemical sensors. Carbon nanotubes (CNTs) are one of the major carbonaceous nanomaterials investigated and applied for this purpose [1e4]. The reason to select such a chemical modifier is related to the claimed electrocatalytic activity of CNTs, which is mainly attributed to defects located along the CNT structure [5,6]; however, additional factors may contribute, such as metallic im- purities originating from their production [7] and change of the mass transport regime to thin-layer diffusion [8,9]. Most investigations found in the literature are devoted to multi- walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) while few reports demonstrating the use of double-walled carbon nanotubes (DWCNTs) can be found. Pumera [10] presented the first application of DWCNTs towards the detec- tion of NADH with improved performance in comparison with SWCNTs. The insertion of DWCNTs into carbon paste electrodes was presented for the simultaneous determination of epinephrine, uric acid and folic acid [11], and ascorbic acid, dopamine and uric acid [12] as well as for the electrochemical reduction of oxygen [13]. Ferrocene encapsulated inside DWCNTs was proposed as an improved sensor for dopamine detection in the presence of excess of ascorbic acid [14]. DWCNTs have been proposed as a platform for biosensors and immunosensors [15, 16] and combined with gra- phene oxide to provide a more sensitive electrochemical sensor to nitrite [17]. The work by Pumera [10] demonstrated that DWCNTs functionalized with concentrated nitric acid (to generate carboxylic groups) provided improved electrochemical response for the oxidation of NADH. Other previous works did not evaluate the ef- fect of acid functionalization on the structure of DWCNTs. More- over, to our knowledge there are no reports aimed to investigate the effect of sizes of DWCNTs on the electrochemistry of different * Corresponding author. E-mail address: munoz@ufu.br (R.A.A. Munoz). Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta https://doi.org/10.1016/j.electacta.2019.01.042 0013-4686/© 2019 Elsevier Ltd. All rights reserved. Electrochimica Acta 299 (2019) 762e771