Fe₂O₃ Nanoparticle/SWCNT Composite Electrode for Sensitive Electrocatalytic Oxidation of Hydroquinone

Abstract

This article reports a composite electrode of Fe₂O₃ nanoparticles/SWCNTs for hydroquinone electrochemical sensing. The synthesis includes the wet-chemical synthesis of Fe₃O₄ nanoparticles and the assembly of Fe₃O₄ nanoparticles onto single walled carbon nanotubes. Fe₃O₄/NH₂-CNTs composite is formed via the electrostatic attraction between positively-charged ammonia-terminated CNTs and negatively-charged Fe₃O₄ nanoparticles. Subsequent heat treatment oxidizes Fe₃O₄ to Fe₂O₃. The composite can be applied onto FTO glass to form Fe₂O₃/CNTs/FTO electrode and the electrode shows good performance in electrochemical sensing of hydroquinone as compared to Fe₂O₃/FTO and CNTs/FTO electrodes. SEM and TEM results show the iron oxide NPs were uniformly dispersed onto the surface of SWCNTs. The iron oxide NPs which uniformly anchored on the SWCNTs could accelerate the electron transfer rate which was evidenced by electrochemical impedance spectroscopy. The enhancement of electrochemical response further confirms the synergy between Fe₂O₃ NPs and SWCNTs. Differential pulse voltammetry was successfully used to quantify hydroquinone within the concentration range of 1.0∼80.0 μM under optimal conditions. The detection limit of Fe₂O₃/CNTs/FTO electrode for hydroquinone was 0.50 μM (S/N = 3). The electrode was further applied to test for hydroquinone in tap water and the Fe₂O₃/CNTs/FTO electrode also presents good stability and high reproducibility, proofing the potential of Fe₂O₃/CNTs electrode as a promising electrochemical sensor. This work presents an electrode using FTO substrate for the first time. It shows that FTO can be a potential alternative to glassy carbon electrode with lower cost, but without compromising too much on the sensitivity.

Introduction

Hydroquinone (HQ, 1,4-benzenediol) is an isomer of phenolic compounds as well as an undesirable contaminant in medicine, food and the environment. HQ was first obtained by Pelletier and Caventou through dry distillation of quinic acidin 1820 [1]. Early study testified that a high concentration of HQ could lead to acute myeloid leukemia and cause kidney damage [2]. Due to the high toxicity and low degradability of HQ in the ecological environment, the development of reliable analytical methods with high sensitivity has been a hot topic in the field of environmental pollutant analysis. Up to now, many analytical methods including high performance liquid chromatography [3], fluorescence [4], chemiluminescence [5] and electrochemical methods [6] have been reported for achieving the selective determination of HQ. The determination of HQ using these techniques shows high sensitivity, but these methods generally involve expensive instruments, lengthy sample preparation, and complicated analysis procedures. Those limit their application in the rapid detection and analysis. Compared with the above strategies, the electrochemical sensor is an easy and efficient approach. It may also offer advantages like low cost, fast analysis, high sensitivity and selectivity. [7] Moreover, electrochemical oxidation of HQ is possible due to its electro-active group of hydroxyl in benzene ring. Yang et al. investigated the electrochemical behaviors of HQ and catechol (CC) on poly(p-aminobenzoic acid) modified glassy carbon electrode (GCE) [8]. Yin et al. applied a graphene-chitosan composite film modified GCE for the determination of HQ, CC and resorcinol [9]. Guo et al. fabricated electrospun carbon nanofibers modified electrode for the simultaneous determination of CC and HQ [10]. Unfortunately, the dihydroxybenzene isomers have the poor electrochemical response and the oxidation products often cause electrode surface fouling, rendering the directly detection at conventional working electrodes. Hence, it is highly desired to design electrodes that are selective and sensitive for HQ analysis. Nanosized semiconductor materials have been employed as electrode materials for the determination of HQ [11], [12], [13], [14]. However, most of these materials are unable to distinguish two isomers simultaneously. Recently, various functionalized nano-materials comprising nanoporous materials, [7], carbon nanoparticles [11], carbon nanofibers [12], carbon nanotubes [13], [14], and even graphene [18] have been proposed for the simultaneous determination of dihydroxybenzene isomers. Gan et al. developed GO-mesoporous MnO₂ nanocomplex to the simultaneous determination of HQ and CC with low detection limit [15]. Meng et al. constructed a TiO₂-CNTs composite electrode for determination of HQ and CC electrochemically [16]. Their electrochemical sensor showed separated peaks of HQ and CC and also was able to lower the overpotential significantly. Li's group demonstrated that the electrochemical performance of TiO₂/Au/CNT nanocomposite electrode is better than that of TiO₂/CNT composites or CNTs for HQ detection, which attributed to the synergic effect of nano-TiO₂, nano-Au and CNTs [17]. Fe₂O₃ is versatile in catalysis and has been extensively used in photovoltaics, photocatalysis, sensors, and biomedicine, owing to its high chemical stability, low toxicity, and easy availability [18], [19]. The scale and distribution of particles on the electrode surface play critical roles in the catalytic ability for sensor application. Unfortunately, the surface energy of metal oxide increases with the decrease of the particle size. It leads to aggregations and further limits the catalytic sites of metal oxide. To ensure an even dispersion, Fe₂O₃ nanoparticles must be anchored onto suitable supports. In addition, the fast electron–hole pair recombination of Fe₂O₃ also can be inhibited by introducing suitable supports. The support materials are usually regarded as an electron acceptor, thus enhancing rapid electron transfer while limiting electron–hole pair recombination. Numerous carbon support materials, such as carbon nanotubes (CNTs) [20], graphene [21], [22], and mesoporous carbons [23] have been applied as particle supports. Particularly, CNTs have stimulated a vast amount of research since being discovered by Iijima in 1991 [24], due to its unique and outstanding properties such as large surface area and high electrical conductivity [25], [26], [27]. Moreover, its excellent electronic properties suggest that CNTs have the ability to promote electron–transfer reaction when used as an electrode and in the construction of various biosensors with high sensitivity and selectivity for dihydroxybenzene isomers [28], [29], [30], [31], [32], [33]. However, the intrinsically tendency of CNTs to bundle and aggregate in aqueous media is a major impediment to the extension and utilization of CNTs. To address this problem, the surface of CNTs could be functionalized via chemical modification to prevent aggregation by charge stabilization mechanism. Functionalized CNTs can improve the dispersing ability in aqueous and provide more reaction sites for some electrochemical reactions. Functionalization process also benefits the formation of covalent and ionic bonds as well as non-covalent van Der Waals interactions, which could improve the immobilization of nanoparticles onto the surface of CNTs.

In this paper, we present a simple strategy to fabricate CNTs supported Fe₂O₃ nanoparticles, which are subsequently deposited onto FTO glass to make the Fe₂O₃/CNTs/FTO electrode for sensing HQ electrochemically. The iron oxide nanoparticles carry negative charges, while ammonia-terminated SWCNTs have positive charges due to the protonation of amino to form –NH₃⁺. Thus, Fe₂O₃ NPs can be assembled onto NH₂-SWCNTs by electrostatic attraction. The electrochemical oxidation of HQ at the Fe₂O₃/CNTs/FTO electrode was investigated using cyclic voltammetry and differential pulse voltammetry. A sensitive and selective sensor was further developed based on the catalytic property of Fe₂O₃/CNTs/FTO electrode towards the oxidization of HQ. The Fe₂O₃/CNTs/FTO electrode has a good respond and anti-interference ability for determination of HQ in neutral medium.