An efficient electrochemical sensor based on three-dimensionally interconnected mesoporous graphene framework for simultaneous determination of Cd(II) and Pb(II)
Introduction
Bioaccumulation of heavy metals like cadmium and lead tends to bring out long-lasting damage towards human health and eco-environment for the high toxicity and long durability of the metal ingredients [1], [2]. Faced with the increasing discharge of heavy metal pollutants, considerable efforts have been contributed to efficient removal and selective, sensitive determination of toxic metals [1], [3], [4]. Among a variety of analytical methods, differential pulse anodic stripping voltammetry (DPASV) has been widely recognized as a powerful tool for measuring cadmium and lead in trace amounts [5]. Compared with the standard spectrometric methods for cadmium and lead detection, such as inductively coupled plasma atomic emission spectroscopy, atomic absorption spectroscopy and inductively coupled plasma mass spectrometry [4], the DPASV approach has great potential for on-site metal monitoring owing to its low cost, high sensitivity and good portability [5], [6].
The chemical modified electrodes (CMEs) play important roles in toxic metals analyses via DPASV method [3], [7], [8]. Recently, a variety of carbonaceous materials including graphene [3], [9], [10], [11], carbon nanotubes [12], [13] and porous carbons [14], [15], [16], [17], have been developed and applied as electrode components in electrochemical determination of heavy metal ions, owing to their wide potential window, high conductivity and chemical inertness [18], [19]. In these studies, the design concept for better analytical performance of the CMEs is to improve the accumulation capability of modifying materials for diluted analytes. In this regard, porous carbon, especially ordered mesoporous carbon (OMC), has drawn growing interests for its uniform and tunable pores [16], [20], [21], [22]. However, the amorphous structure and suboptimal conductivity of OMC may limit its performance in electrochemical processes [23]. Compared with the OMC, the ordered graphitized mesoporous carbon (GMC) possesses well-aligned porous framework, high conductivity and chemical stability [24]. Lately, the GMC has been demonstrated to be good electrode modifier for applications in biosensor, oxygen reduction reaction and lithium-ion battery [23], [24], [25], [26]. However, to the best of our knowledge, the application of GMC as efficient electrode modifier for metal detection was rare, probably because the fabrication of GMC with well-defined mesoporosity and high surface area is still challenging. It is noted that the preparation of GMC by heating OMC at high temperatures (>2500 °C) usually leads to reduced surface area and regressive structural ordering. On the other hand, the synthetic progress using graphitization catalysts like metal or metal oxide may result in partially graphitic framework due to the relatively low temperature (<1500 °C) [23], [26].
In this work, a kind of GMC that highly ordered mesoporous graphene framework (described as MGF) was directly fabricated via a transformation of self-assembled Fe3O4 nanocrystals (NCs) superlattices, where the Fe3O4 NCs served as both the template and the graphitization catalyst. The obtained MGF with three-dimensionally interconnected mesoporosity and abundant structural graphitic domains was further used for simultaneous electrochemical sensing of Cd(II) and Pb(II). The MGF modified electrode exhibited improved electroactivity compared with bare glassy carbon electrode (GCE) and OMC modified one that was fabricated with OMC prepared under relatively low carbonization temperature in our lab. The “alloy” ability of in-situ plated bismuth film and the mechanical stability of Nafion film also contribute synergistically to the high repeatability, good sensitivity and low detection limit of the prepared Bi/MGF-Nafion/GCE. Compared with the electrodes based on a nitrogen doped microporous carbon (NMC) and a metal-organic framework derived nanoporous carbon (NPC) we reported [14], [15], the present MGF based electrode exhibits improved properties. For instance, it processes better reusability than the NMC based electrode, benefiting from its efficient regeneration through a mild conditioning treatment. Compared with the NPC based ones, a distinctly improved detection limit and linear range were obtained with the present MGF based electrode. The present Bi/MGF-Nafion/GCE was also proved to be promising in real water samples analyses.
Section snippets
Chemicals and characterization apparatus
All chemicals were of analytical grade and used as received, except for specifically detailed. Oleic acid (90%), 1-octadecene (90%) and Nafion (5 wt% in low aliphatic alcohols) were purchased from Aldrich. Sodium oleate (97%) was obtained from TCI. Ethanol, isopropanol, hexane and FeCl3·6H2O (98%) were purchased from Aladdin. N, N-dimethylformamide (DMF, Tianjin Oubokai Chemical Industry) was used for the preparation of 0.1 wt% Nafion-DMF suspension. The stock solutions of Bi(III), Pb(II) and
Characterization of MGF
Typical TEM images at low and high magnification in Fig. 1a, b demonstrate the as-prepared MGF exhibits a highly ordered mesoporous architecture, with pore walls comprising few-layer graphene. Raman spectrum with well-resolved G (1583 cm−1) and 2D (2697 cm−1) bands in Fig. 1c also confirms the highly graphitic nature of the MGF [28], [29]. Typical type-IV N2 adsorption-desorption isotherm with a large H2-type hysteresis loop at P/P0 = 0.47 suggests the presence of uniform mesopores in MGF (Fig. 1d)
Conclusion
In summary, a well-organized MGF with high surface area, uniform interconnected three-dimensionally mesopores and well-stacking graphene layers was synthesized by transforming self-assembled Fe3O4 NCs superlattices. Being combined with bismuth and Nafion films, the as-prepared MGF served as a efficient electrode component to fabricate a Bi/MGF-Nafion/GCE for simultaneous determination of Cd(II) and Pb(II). The proposed electrochemical sensor shows high repeatability and stability, good
Acknowledgements
The authors acknowledge the support from the ‘1000 Talent Program’ (The Recruitment Program of Global Experts), National Basic Research Program of China (973 program: 2014CB845602), Shanghai International Science and Technology Cooperation Project (15520720100), the National Natural Science Foundation of China (21473247 and 21373052) as well as the ‘Young Creative Sci-Tech Talents Cultivation Project (2013711012, 2013711016)’ of the Xinjiang Uyghur Autonomous Region.
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