Novel electrochemical sensor based on N-doped carbon nanotubes and Fe 3 O 4 nanoparticles: Simultaneous voltammetric determination of ascorbic acid, dopamine and uric acid Diana M. Fernandes a,⇑ , Marta Costa a , Clara Pereira a , Belén Bachiller-Baeza b , Inmaculada Rodríguez-Ramos b , Antonio Guerrero-Ruiz c , Cristina Freire a,⇑ a REQUIMTE/Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal b Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie 2, Cantoblanco, 28049 Madrid, Spain c Dpto. de Química Inorgánica y Química Técnica, Facultad de Ciencias, UNED, Senda del Rey 9, 28040 Madrid, Spain article info Article history: Received 28 April 2014 Accepted 23 June 2014 Available online 4 July 2014 Keywords: Fe 3 O 4 magnetic nanoparticles Carbon nanotubes Dopamine Ascorbic acid Uric acid abstract A new modified electrode based on N-doped carbon nanotubes functionalized with Fe 3 O 4 nanoparticles (Fe 3 O 4 @CNT-N) has been prepared and applied on the simultaneous electrochemical determination of small biomolecules such as dopamine (DA), uric acid (UA) and ascorbic acid (AA) using voltammetric methods. The unique properties of CNT-N and Fe 3 O 4 nanoparticles individually and the synergetic effect between them led to an improved electrocatalytic activity toward the oxidation of AA, DA and UA. The overlapping anodic peaks of these three biomolecules could be resolved from each other due to their lower oxidation potentials and enhanced oxidation currents when using the Fe 3 O 4 @CNT-N modified elec- trode. The linear response ranges for the square wave voltammetric determination of AA, DA and UA were 5–235, 2.5–65 and 2.5–85 lmol dm 3 with detection limit (S/N = 3) of 0.24, 0.050 and 0.047 lmol dm 3 , respectively. These results show that Fe 3 O 4 @CNT-N nanocomposite is a promising candidate of cutting- edge electrode materials for electrocatalytic applications. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Since their discovery, carbon nanomaterials have been attract- ing considerable experimental and theoretical interest because of their unique structures and properties which make them suitable and very attractive for a great number of applications in several research fields [1–7]. Among the most studied carbon nanomaterials are carbon nanotubes (CNTs) [1,2]. Due to their outstanding electrical, mechanical, optical and thermal properties, CNTs have huge potential for several technological areas in the energy, medicine, information and chemical industries, where they can be used as catalyst supports, chemical sensors, gas adsorbents, among other things [1,2]. CNTs are also extensively used in electrochemistry due to their wide potential window, low cost and electrocatalytic activity for a variety of redox reactions. Their applications in electroanalysis depend in great part on their microstructure and surface chemistry [1,7]. For all these reasons, they are promising building blocks for hybrid materials. Many workers have tried to control the CNT structures by chemical doping with foreign atoms [8,9]. For instance, N doping has been successfully employed to modify the structural or electri- cal properties of CNTs [8,9], giving rise to metallic behavior [8], affecting the lattice alignment [9], and regulating the growth mechanism. Additionally, N-doped CNTs have a bamboo-like struc- ture and exhibit an enhanced biocompatibility and sensitivity when compared to pristine CNTs [10,11]. New hybrid materials based on CNTs and nanoparticles have also been developed quickly in the last decade not only because they display the individual properties of CNTs and the nanoparticles, but also because they can exhibit additional synergistic properties. These nano-sized materials offer many advantages due to their size and unique phys- ical properties [12]. Several types of magnetic CNTs have been syn- thesized by decorating CNTs with different magnetic nanoparticles (MNPs) in order to provide additional advantages [13–15]. Magnetite (Fe 3 O 4 ) is one of the most commonly used magnetic nanomaterials because of its biocompatibility, catalytic activity, high saturation magnetization, low toxicity and easy preparation [16]. Some reports can be found concerning the application of http://dx.doi.org/10.1016/j.jcis.2014.06.050 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding authors. Fax: +351 220402659 (D.M. Fernandes), fax: +351 220402659 (C. Freire). E-mail addresses: diana.fernandes@fc.up.pt (D.M. Fernandes), acfreire@fc.up.pt (C. Freire). Journal of Colloid and Interface Science 432 (2014) 207–213 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis