High-sensitivity determination of lead and cadmium based on the Nafion-graphene composite film
Introduction
As is well known, lead and cadmium pose severe risks to human health with toxic effects on living organism. For example, the toxicity of lead in humans mainly comes from its detrimental mimicking action by occupying the calcium binding sites on numerous calcium-dependent proteins in cells (such as, calmodulin and enzyme protein kinase C), thus resulting in the corresponding impairment of physiological functions [1], [2]. Accordingly, exploring the sensitive, rapid and simple analytical method for precise monitoring of Pb2+ and Cd2+ is urgently needed. The usual methods adopted for the concentration assessments of the metal ions are mainly focused on the use of atomic absorption or inductively coupled plasma (ICP) atomic emission spectroscopy, ICP-mass spectrometry and electrochemical (EC) techniques. However, spectroscopy methods are somewhat cumbersome and not suitable for the in situ measurement due to the ponderous and complicated instruments. On the contrary, EC techniques have attracted growing interests due to high-sensitivity, portability and low cost. Among all the EC methods, electrochemical stripping voltammetric analysis provides a powerful tool for the determination of metal ions [3], which possess high-sensitivity for the metal analysis due to the built-in preconcentration step and is very suitable for on-site and in situ analysis [4], [5]. Most commonly, mercury based electrodes including hanging drop mercury electrode and mercury film electrodes were adopted because of excellent reproducibility and high-sensitivity [6]. However, due to the dramatic toxicity of mercury, numerous attempts have been made to replace it with new mercury-free and reliable electrode. Recently, the bismuth film electrode (BFE) has drawn the increasing attentions in the field of the stripping technique due to the remarkably low toxicity and the ability to form alloy with many metals as well as its wide potential window, which has proved to be equal to or even superior to that of MFEs [7], [8], [9]. On the other hand, in order to alleviate interferences and improve the sensitivity of the sensing interface, some strategies have also been exploited for the stripping analysis, such as, the chemical modified electrode (CME), heated electrodes, microwaved electrodes, and insonated electrodes [10], [11], [12]. Among these, CME capable of accumulating target analytes from dilute aqueous solution has been developed as the fascinating and effective way for the anodic stripping voltammertry (ASV) determination of heavy metal ions. Prominent examples include functionalized mesoporous silica electrode [13], carbon nanotubes (CNT) modified electrode [14], [15], ordered mesoporous carbon [16], acetylene black paste electrode [17], nanocrystalline diamond thin-film electrode [18], and thick-film modified graphite-containing electrode [19]. Although all the above material showed improved stripping signals, especially the CNT modified electrode prepared by Jin's group [15], new materials are still needed to develop highly sensitive and antifouling metal ions sensing platform.
Graphene, a single atomic plane of graphite packed into a dense honeycomb crystal structure, was novel and fascinating carbon material since experimentally produced in 2004 [20]. Much research effort has been made to explore its fascinating applications in electroanalytical chemistry or electrochemistry as novel electrode material for various purposes due to the excellent electrical and mechanical properties [21], [22], [23], [24], [25]. For instance, Li's group [21], [22] has demonstrated the use of the graphene nanosheets to develop a high-performance electrochemical sensor for dopamine and electrocatalytic oxidation of methanol. Worden et al. [24], [25] has also fabricated an excellent glucose biosensor based on the exfoliated graphite nanoplatelets as a viable and inexpensive filler alternative to CNT. Recently, our group [26] developed a Cd2+ sensing platform with the MFE based on the graphene nanosheets. Although, the sensing platform showed ultra-sensitivity for the detection of Cd2+, its wide use was limited due to the toxicity of the mercury.
In this work, graphene nanosheets were used in combination with in situ BFE to fabricate a sensitive and mercury-free electrochemical platform for the analysis of the Pb2+ and Cd2+. Herein, Nafion not only acts as an effective solubilizing agent for graphene nanosheets (Nafion-G), but also as an antifouling coating to reduce the influence of the surface-active macromolecules. This electrochemical sensing interface exhibited excellent stripping performance for the analysis of Pb2+ and Cd2+ combining the advantageous of the graphene nanosheets (higher electrical conductivity, huge specific area) together with the unique features of the in situ plating BFE.
Section snippets
Reagents
Nafion (5 wt.% in low aliphatic alcohols), was purchased from Aldrich (Milwaukee, WI), and then diluted to 1 wt.% Nafion with 2-propanol. Stock solutions of Pb2+ and Cd2+ were prepared by diluting the corresponding standard stock solutions prepared with CdCl2 and Pb(NO3)2, respectively. Bi(NO3)3 was used for the BFE by diluting the corresponding standard stock solution. A 0.1 M acetate buffer (pH 4.5) prepared by mixing appropriate amounts of CH3COOH and CH3COONa, was served as to prepare solution
Morphologic characterization of the graphene nanosheets
The structure and morphology of the resulting graphene deposited on the mica were characterized by AFM. The results (Fig. 1) found that the graphene sheets were almost single-layer. And the average thickness of single-layer graphene sheets was <1 nm. This unique nanostructure may be favourable as potential material for the electrochemical determination of Pb2+ and Cd2+.
Electrochemical characterization of the Nafion-G composite film
Electrochemical experiments were used to characterize the Nafion-G composite film. Different cyclic voltammograms (CVs) at bare,
Conclusions
A new and highly enhanced sensing platform based on the Nafion-graphene composite film was established for the simultaneous determination of Cd2+ and Pb2+ by anodic stripping voltammetry. The composite film combining the advantages of graphene (e.g., strong adsorptive capability and huge specific surface area due to the nanosized graphene sheet and nanoscale thickness of these sheets and good conductivity) and the cationic exchange capacity of Nafion enhanced the sensitivity for metal ion
Acknowledgements
This work is supported by 863 Project 2007AA061501, 973 Project 2009CB930100 as well as the support of 2006BAE03B08 from MOST and the Innovation Method fund of China (2008IM040100).
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