Full length articleSynthesis and electrochemical properties of Co3O4-rGO/CNTs composites towards highly sensitive nitrite detection
Graphical abstract
A highly sensitive nitrite electrochemical sensor was fabricated by using carbon nanotubes and reduced graphene oxide composite film (rGO/CNTs), which was functionalized with cobalt oxide nanostructures (Co3O4).
Introduction
It is well knows that nitrite is extensively used as an additive and corrosion inhibitor in food industry and industrial water [1]. Recently, the detection of nitrite has attracted much attention due to its harmful effects on human health and physiological systems. For example, nitrite can promote the irreversible oxidization of hemoglobin to methemoglobin, thereby reducing the oxygen transport capability of blood [2]. In addition, it can react with amines and lead to the generation of carcinogenic N-nitrosamines in stomach, which causes cancer and hypertension [3]. Therefore, the content of nitrite in daily cuisine must be strictly controlled. According to the World Health Organization and the European Commission's Scientific Committee on Food, the daily intake of nitrite should be <0.07 mg nitrite ions per kilogram body weight [4]. Besides, the European Union stipulates that the maximum permissible concentration of nitrite is 0.1 mg·L−1 (~2.2 μM) in drinking water [5]. Therefore, there is immense need of developing accurate and rapid analytical methods to determine the amount of nitrite to supervise the quality of food and water. Among the numerous nitrite-detection techniques [[6], [7], [8], [9], [10]], electrochemical detection method offers an opportunity for rapid, simple, low-cost, and high-precision methodologies. However, it is still challenge for improvement the sensitivity and other properties of the electrochemical nitrite sensors.
In order to improve the electrochemical sensing properties, great efforts have been focused on electrochemical sensors by modifying the electrode with electron transfer mediators. Transition metal oxide, as a typical mediator, have been widely used in many fields such as electronics and advanced catalysts owing to their unique properties of good catalytic activity, high earth-abundance and chemically reactive facets [11]. Especially, nanostructured transition metal oxides play an important role to improve the performance of electrochemical sensor towards the stability and sensitivity. In the family of transition metal oxides, cobalt oxide (Co3O4) nanostructures exhibit attractive behaviors due to its electro-catalytic activity towards the determination of compounds such as glucose, glutathione, carbohydrate and hydrogen peroxide [[12], [13], [14], [15]]. The most probable reason is that the difference in oxygen defect, oxygen holes and oxygen adsorbed in different state of cobalt in Co3O4 (a mixed valance state of Co(II) and Co(III)) result in the high activity and selectivity of this metal oxide catalysts [16]. The Co3O4 nanostructures have been also used for nitrite detection. Thangamuthu's group [17] prepared Co3O4 disordered circular sheet via the precipitation method. They investigated the electrochemical nitrite sensing property of Co3O4 by modifying on glassy carbon electrode. The sensor demonstrates good properties. Moreover, to overcome the low intrinsic electronic conductivity and enhance its electrochemical performance and applications in electrochemical sensors, Co3O4 can be assembled together with conductive carbon-based materials to form a new composite material [18].
Among various carbonaceous materials, carbon nanotubes (CNTs) and graphene have been widely used as a promising electrode. CNTs have attracted much attention due to their unique chemical and electronic properties since discovery in 1991. It has been widely used in the preparation of electrochemical sensors owing to their good chemical stability and the fast electron transfer ability. Meanwhile, the CNTs are conducive to the dispersion of nanoparticles and the enhancement of catalytic performance. The single atomic layer of graphene has also been used to fabricate chemically modified electrodes owning to its excellent electronic, thermal and mechanical properties, and high theoretical surface area [19]. Many related research results have been reported by predecessors. Such as, Erden's group [20] reported an amperometric hydrogen peroxide biosensor based on Co3O4 nanoparticles and CNTs modified glassy carbon electrode, which showed excellent performance. Haldorai et al. [21] used a 3D coordination complex as a precursor to synthesize reduced graphene oxide/Co3O4 nanospindle and the composite was successfully applied to the electrochemical detection of nitrite. These previous works prove that the CNTs and graphene exhibit excellent performance in biosensors.
In addition, to obtain better catalytic performance, researchers have also developed different sensing materials by combining CNTs with graphene as support material for metallic nanoparticles. Fei et al. [22] developed an acetamiprid electrochemical sensor based on Au nanoparticles decorated multi-walled carbon nanotube-reduced graphene oxide nanoribbons. Mani et al. [23] fabricated a hydrogen peroxide and nitrite sensor using reduced graphene oxide-multiwalled carbon nanotubes‑platinum nanoparticles nanocomposite. Assembling the graphene and CNTs via non-covalent π-π stacking interactions could collectively harvest higher electrical conductivities and provide a larger specific area compared with either pristine graphene or CNTs. In this kind of electrochemical detection systems, CNTs can bridge defects to promote electron transfer as well as increase electrochemical performance.
Herein, we reported a one-step annealing process to synthesize Co3O4-rGO/CNTs composites using GO/CNTs as the support material of zeolitic imidazolate framework-67 (ZIF-67). In order to improve the electrocatalytic properties of the sensors, the weight ratio of GO to CNTs and the mass percentage of Co3O4 were optimized for nitrite determination. The experimental results show that the optimized Co3O4-rGO/CNTs/GCE electrochemical sensor exhibited high sensitivity, low detection limit, good selectivity and stability for nitrite analyses. The as-prepared electrochemical sensor was further applied for the determination of nitrite in real samples.
Section snippets
Reagents and chemicals
Graphite (325 meshes) was obtained from Qingdao Huatai Lubrication Sealing Technology Co. Ltd. Multi-wall carbon nanotubes (MWCNT, outside diameter: 30–50 nm, impurities: >98 wt%, length: <10 μm) was purchased from Chengdu organic chemicals Co. Ltd. Chinese academy of sciences. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, ≥99.0%), 2-methylimidazole(C4H6N2, ≥98.0%) and sodium nitrite (NaNO2, ≥99.0%) were obtained from Sigma-Aldrich (Shanghai, China). Potassium permanganate (KMnO4, >99.5%) was used
Characterization of the Co3O4-rGO/CNTs composites
The crystal structure of the as-prepared pure Co3O4 crystals and Co3O4-rGO/CNTs composites (77.4 wt%, 53.3 wt% and 40.6 wt%, respectively) is analyzed by XRD as presented in Fig. 2. For pristine Co3O4, the diffraction peaks can be assigned to the diffraction planes of (220), (311), (400), (511) and (440), which are indexed to the Co3O4 (JCPDS 42–1467). Moreover, the sharp diffraction peaks confirm the well crystallization of the as-prepared Co3O4 sample [26]. With the addition of rGO/CNTs, the
Conclusions
In summary, a novel electrochemical sensor based on Co3O4-rGO/CNTs has been established successfully for the determination of nitrite. The cyclic voltammetry and amperometric measurements were introduced to optimize the composition of rGO/CNT and the mass percentages of Co3O4 for nitrite detection. Experimental results indicate that a strong electrocatalytic property was obtained with GO:CNTs = 1:1 and Co3O4 mass percentages of 53.3 wt% at the Co3O4-rGO/CNTs modified electrodes. Under optimal
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (51803148, 51672100), Higher school science and technology innovation project of Shanxi (2016137), Natural Science Foundation of Shanxi Province (201801D221180, 201801D221188), Talent project of Shanxi Province (201605D211036), and the Scientific Research Starting Foundation for Professors and Doctors from Huizhou University.
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