Novel electrochemical sensor based on N-doped carbon nanotubes and Fe3O4 nanoparticles: Simultaneous voltammetric determination of ascorbic acid, dopamine and uric acid

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Highlights

Abstract

A new modified electrode based on N-doped carbon nanotubes functionalized with Fe3O4 nanoparticles (Fe3O4@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 Fe3O4 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 Fe3O4@CNT-N modified electrode. 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 μmol dm−3 with detection limit (S/N = 3) of 0.24, 0.050 and 0.047 μmol dm−3, respectively. These results show that Fe3O4@CNT-N nanocomposite is a promising candidate of cutting-edge electrode materials for electrocatalytic applications.

Introduction

Since their discovery, carbon nanomaterials have been attracting 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], [2], [3], [4], [5], [6], [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 electrical 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 structure 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 physical properties [12]. Several types of magnetic CNTs have been synthesized by decorating CNTs with different magnetic nanoparticles (MNPs) in order to provide additional advantages [13], [14], [15]. Magnetite (Fe3O4) 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 Fe3O4 in sensing applications [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Most of reports involve the immobilization of enzymes on the surface of the nanoparticles which offer numerous advantages including enhancement of the enzymatic activity and reduction in the mass-transfer processes associated with the recognition of substrates by enzymes. The Fe3O4 based biosensors have been used for the detection of sulfite, phenolic compounds, glucose, lactate, and H2O2 [23], [24], [25], [26], [27].

Dopamine (DA) is one of the naturally occurring catecholamines which acts as an important neurotransmitter in mammalian brain [28] and also plays a significant role in the functioning of the central nervous system. Serious diseases such as Schizophrenia and Parkinsonism may result by loss of DA containing neurons [29]. Uric acid (UA) is an end product from purine derivatives in human metabolism [30]. Such as for DA, abnormal levels of UA are a marker of several illnesses such as gout, renal disease [31], [32] and cardiovascular problems [32], [33], [34], [35]. Ascorbic acid (AA) is one of the most important vitamins, due to its antioxidant and pH regulator properties [36]. AA exists in mammalian brain and it is also often added to various food products and pharmaceuticals. AA has been used for the prevention and treatment of common cold, mental illness, infertility, cancer and AIDS.

Several methods have been used to determine DA, UA and AA such as chromatography, spectroscopy and electrochemistry [30], [37], [38], [39], [40], [41]. Since the three molecules are electroactive compounds they can be detected by electrochemical methods based on oxidation processes; however, in general at bare electrodes their reactions are irreversible and therefore require high over-potentials. Additionally, their direct redox reactions take place at very similar potentials at bare electrodes, which results in rather poor selectivity. Consequently, modification of electrode surface with suitable electrocatalysts should improve their sensitivity and the selectivity. Therefore, the design of new composite materials that can be successfully immobilized on electrode surfaces to provide increased sensitivity and selectivity is extremely important. Magnetite has already been incorporated into carbon materials such as multi-walled carbon nanotubes [19], graphene nanoribbons [17], graphene sheets [21] and reduced graphene oxide sheets [18] for the determination of dopamine, ascorbic acid or uric acid.

This work reports the synthesis and characterization of a magnetic nanocomposite based on N-doped CNTs (CNT-N) functionalized with Fe3O4 nanoparticles and more importantly the preparation of a novel Fe3O4@CNT-N modified electrode for the simultaneous electrochemical determination of AA, DA and UA. This nanocomposite material combines the beneficial characteristics of N-doped CNTs (c.a. high electrical conductivity) and Fe3O4 MNPs sensing properties.

Section snippets

Materials, reagents and methods

Sodium sulfate (99.5% Prolabo), sulfuric acid (95–97% Merck), hydrochloric acid (37%, Fisher Scientific), monobasic potassium phosphate (⩾99%, Sigma), dibasic sodium phosphate (⩾99%, Sigma Aldrich), sodium chloride (⩾99.5%, Sigma Aldrich), potassium hexacyanoferrate (III) (⩾99%, Merck), potassium chloride (⩾99.5%, Merck), iron(II) chloride tetrahydrate (analytical grade, Merck), anhydrous iron(III) chloride (analytical grade, Merck), 1-amino-2-propanol (MIPA, 93%, Aldrich) and absolute ethanol

Characterization of Fe3O4@CNT-N hybrid nanomaterial

The TEM micrographs (Fig. 1) reveal that the pristine CNT-N nanomaterial exhibits a periodical multiwall bamboo-like structure (external diameter = 105 nm; inner diameter = 83 nm) [42], which is preserved upon its functionalization with the Fe3O4 MNPs. Additionally, the TEM images of Fe3O4@CNT-N (Fig. 1(b)) show that the CNTs are covered with nearly-spherical nanoparticles of 3–5 nm average particle size, confirming the successful immobilization of the MNPs on the CNTs surface.

The surface chemical

Conclusions

The preparation of glassy carbon electrode with Fe3O4@CNT-N nanocomposite was proved to be easy to perform and fast, leading to robust and electrochemically stable electrode. The fabricated nanocomposite electrode displayed higher electrocatalytic activity toward the oxidation of ascorbic acid, dopamine and uric acid than bare electrode. The oxidation of these three biomolecules at Fe3O4@CNT-N modified electrodes showed three well defined oxidation peaks with large peak separation, less

Acknowledgments

The authors thank Fundação para a Ciência e a Tecnologia (FCT) for financial support through Grant Nos. PEst-C/EQB/LA0006/2013 and PTDC/CTM-POL/0813/2012, to Operation NORTE-07-0124-FEDER-000067–Nanochemistry and MICINN of Spain (Projects CTQ-2011-29272-C04-01 and -03) for financial support. DMF also thanks FCT for her PD Grant SFRH/BPD/74877/2010.

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