Monodispersed Au nanoparticles decorated graphene as an enhanced sensing platform for ultrasensitive stripping voltammetric detection of mercury(II)

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Abstract

We demonstrate a new highly sensitive and selective Hg(II) sensor with a graphene-based nanocomposite film as the enhanced sensing platform. The platform was constructed by homogenously distributing monodispersed Au nanoparticles (AuNPs) onto the two-dimensional (2D) graphene nanosheet matrix. Its surface structure and electrochemical performance were systematically investigated. Such a nanostructured composite film platform could combine with the advantages of AuNPs and graphene nanosheets, greatly facilitate electron-transfer processes and the sensing behavior for Hg(II) detection, leading to a remarkably improved sensitivity and selectivity. The detection limit was found to be as low as 6 ppt (S/N = 3), much below the guideline value from the World Health Organization (WHO). The interference from other heavy metal ions such as Cu2+, Cr3+, Co2+, Fe3+, Zn2+ and I ions associated with mercury analysis could be effectively inhibited. The performance of new sensor was also evaluated by the direct detection of Hg(II) in river water specimens, suggesting it is very promising for practical environmental monitoring applications.

Introduction

Mercury(II), a neurotoxicant, is considered to be one of the highly toxic heavy metal ions [1], [2], [3], [4]. High exposure to mercury would result in kidney and respiratory failure, damage in the gastrointestinal tract and nervous system [5]. Yearly, about 10,000 tons of mercury is released into the environment by human activities [6], resulting in a highly severe threat on human life. Obviously, the development of a rapid determination and reliable quantification of a trace level of mercury has become increasingly important for public security and health protection. Although atomic absorption spectrometry (AAS) [7], [8], [9], atomic fluorescence spectrometry (AFS) [10], ion coupled plasma mass spectrometry (ICPMS) [11], [12], and microwave induced plasma atomic emission spectroscopy (MIP-AES) could be used to monitor Hg(II) [13], [14], they are very expensive and not suitable for in situ measurements. As an alternative to these spectroscopic techniques, electrochemical analysis, particularly stripping voltammetry, has attracted significant interests for trace analysis of heavy metals, because of their excellent sensitivity, short analysis time, low power consumption and cheap equipments [15], [16], [17], [18], [19], [20]. Among many solid electrodes so far reported for the detection of Hg(II), gold was found to be a superior substrate as a working electrode because of its high affinity for mercury, which could enhance the preconcentration effect [9], [21], [22]. In recent years, nanotechnology-based sensors have become one of the most active areas in environmental analysis [23], [24]. Owing to unique capabilities, such as high surface area, increased mass transport, low detection limit, and better signal-to-noise ratio, nanosized metal particles, especially Au nanoparticles (AuNPs) assembled on various supports as modified electrodes have emerged as a promising alternative for the electroanalysis of Hg(II) [17], [25], [26], [27]. For instance, Compton et al. loaded AuNPs onto glassy carbon microspheres and then electrically wired the AuNPs-modified glassy carbon microsphere film to the underlying macroelectrode using carbon nanotubes [17]. Raj et al. constructed AuNPs-based ensembles onto the thiol functionalized sol-gel silicate network by a colloidal chemical approach [26]. Recently, our group constructed a bimetallic Au–Pt inorganic-organic hybrid nanocomposite as a highly sensing platform for the detection of Hg ions [28]. These studies demonstrated that AuNPs-based nanocomposites combined with stripping voltammetry could well meet the requirements of field detections of Hg ions in the environment. However, it is still a great challenge to homogenously distribute monodispersed AuNPs on the supporting skeleton for sensitive and selective detection of Hg(II).

Graphene, a single layer of sp2 hybridized carbon atoms packed into a dense honeycomb two-dimensional lattice, has attracted tremendous attention from both experimental and theoretical scientific communities since experimentally produced in 2004 [29], [30], [31]. Due to its novel properties, such as exceptional thermal and mechanical properties, high electrical conductivity, graphene provides potential applications in synthesizing nanocomposites, nanoelectronics, electromechanical reasonators, and ultrasensitive sensors [32], [33], [34]. Recently, as an advanced nanoelectrocatalyst, graphene-based materials have been highly concerned for constructing electrochemical sensors, such as electrochemical determination of glucose [35], [36], β-nicotinamide adenine dinucleotide [37], dopamine [38] and cytochrome c [39], etc. More recently, Dong et al. explored the graphene-based DNA biosensors [40]. Compared with that the “classical” graphite and glassy carbon, the graphene-modified electrode exhibits greatly enhanced electrochemical reactivity for the four free bases of DNA. Obviously, graphene-based materials hold great promise on fabricating enhanced electrochemical-sensing platforms. However, graphene-based nanocomposite film has never been used for the determination of Hg(II).

Herein, we report on an ultrasensitive Hg(II) sensor by using nanocomposite film of monodispersed Au nanoparticles and graphene as the powerful platform. Uniform AuNPs were homogenously distributed onto the graphene nanosheet matrix, constructing a monodispersed AuNPs-based ensemble. It is worth noting that the as-prepared composite matrix in our work combines the advantages of the graphene nanosheets (unique electrical conductivity, enlarged active surface area) together with AuNPs (extraordinarily catalytic activity, good conductivity). This should greatly facilitate the rapid, stable and sensitive measurement of Hg(II). The performance of this novel platform for stripping determination of Hg(II) is investigated in detail. Encouragingly, such a nanostructured composite film offers a remarkably improved sensitivity and selectivity, and exhibits fine applicability for the detection of Hg(II) in practical water samples.

Section snippets

Apparatus

The general morphology of the products was characterized by the scanning electron microscopy (SEM, JSM-5600). An inductively coupled plasma mass spectrometer (ICPMS, Elan Drc-E, PerkinElmer, USA) was used for the measurements of the concentration of Hg in the real sample. The Hg(II) concentration was determined from a response curve generated using a series of standard solutions of known Hg(II) concentration. The response curve for calibration was generated using peak intensities.

Characterization of the modified electrode Surface

Fig. 1a shows the SEM image of the as-synthesized chi-graphene powder, indicating the flake-like shape. The surface exhibits a few thin wrinkles. As indicated in the magnified SEM image (inset of Fig. 1a), the observation for the edge of the nanosheet confirms the layered structure of graphene. With the successive deposition onto chi-graphene/GCE, uniform AuNPs of ∼30 nm in average diameter formed randomly on the sheet (Fig. 1b). Obviously, the generated AuNPs were homogenously distributed onto

Conclusion

In summary, we have successfully constructed a novel and high-performance platform for the stripping analysis of Hg(II). This platform is based on an AuNPs-graphene hybrid nanocomposite. By using graphene nanosheet as a supporting skeleton, monodispersed AuNPs are homogenously distributed onto the 2D graphene nanosheet matrix. Such a nanostructured composite film greatly facilitates electron-transfer processes and the sensing behavior for Hg(II) detection, leading to a remarkably improved

Acknowledgements

This work was supported by National Science Foundation of China (Grant 20803026), Natural Science Foundation of Hubei Province (Grant 2008CDB032), and the Key Project of Ministry of Education of China (Grant 108097).

Jingming Gong was born on February 27, 1977. She received her PhD degree in 2004 in Chemistry from University of Science and Technology of China (USTC). Presently, she is an associate professor at Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University. Her research interests include electroanalytical chemistry, biosensor, environmental science and biomaterials.

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    Jingming Gong was born on February 27, 1977. She received her PhD degree in 2004 in Chemistry from University of Science and Technology of China (USTC). Presently, she is an associate professor at Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University. Her research interests include electroanalytical chemistry, biosensor, environmental science and biomaterials.

    Ting Zhou was born on January 9, 1987. She received her BS degree from Nanyang Normal University (China) in 2008. She is currently studying for her Master degree on the fabrication of sensor toward the determination of heavy metal ions in College of Chemistry, Central China Normal University.

    Dandan Song was born on September 25, 1957. Currently, she is a professor in College of Chemistry, Central China Normal University. She majors on the environmental analysis.

    Lizhi Zhang was born on February 6, 1973. Currently, he is a professor at Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University. His research interests include photocatalysis, environmental science and nanomaterials.

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