Electrochemical determination of trace copper(II) with enhanced sensitivity and selectivity by gold nanoparticle/single-wall carbon nanotube hybrids containing three-dimensional l-cysteine molecular adapters

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Abstract

This paper described how a inorganic–organic hybrids electrochemical sensor, a three-dimensional l-cysteine self-assembled monolayers (SAM) functionalized gold nanoparticles/single-walled carbon nanotubes/glassy carbon electrode (l-cys/AuNPs/SWCNTs/GCE), was applied to improve the performance of detecting trace Cu(II) in stripping voltammetry. The AuNPs/SWCNTs nanohybrid has a three-dimensional porous nanostructure with large active surface area, and the l-cysteine adapters on AuNPs/SWCNTs surface can greatly enhance the sensitivity and selectivity in detecting Cu(II). The differential pulse anodic stripping voltammetry (DPASV) response of the Cu(II) at the l-cys/AuNPs/SWCNTs/GCE was ca. 6.7 and 6.5 times larger than that at the AuNPs/SWCNTs/GCE and l-cys/AuNPs/GCE without SWCNTs, respectively. The sensor demonstrated a wide linear response range and a lower detection limit of 0.02 nM with a signal-to-noise of 3 using 3 min of preconcentration time. The interference experiments showed that Ag(I), Pb(II), Cd(II) and Hg(II) had little influence on Cu(II) signal. The high sensitivity and excellent selectivity in contrast to the values reported previously in the area of electrochemical Cu(II) detection, demonstrated the analytical performance of the as-prepared sensor toward Cu(II) was superior to the existing electrodes. The as-prepared sensor was further applied to determine the Cu(II) in the real environmental water sample, and the results agreed satisfactorily with the certified values.

Introduction

Copper is an essential trace element required for the human body to maintain normal, however, the extensive and excessive usage of copper may lead to toxic effect and result in serious environmental contamination [1], [2]. High level of copper in environment could lead to accumulation within the food chain and harm to human health. So, it is important to detect the content of Cu(II) in the water and environment. Various methods including fluorescence, atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, inductively coupled plasma optical emission spectroscopy and electrochemistry have been reported for the determination of Cu(II). In comparison with other methods, electrochemical method is particularly suited for the detection of Cu(II) due to its particular advantages, e.g. high sensitivity, inherent simplicity, miniaturization, and low cost. Many electrochemical strategies are based on the modification of the working electrode surface with monomolecular-level ligands through the self-assembled approach [3], [4], [5], such as l-aspartic acid/l-cysteine/gold nanoparticle modified microelectrode [6], l-cysteine modified flexible PDMS-gold electrode [7], 4-amino-6-hydroxy-2-mercaptopyrimidine modified gold electrode [8], (3-mercaptopropyl) sulfonate modified nanoporous gold electrode [9], meso-2,3-dimercaptosuccinic acid modified gold electrode [10], d,l-penicillamine and thiodimethylglyoxime modified gold electrode [11], etc. Among various modifiers, application of chemisorbed organosulfur SAM on gold surface for the adsorptive stripping analysis of Cu(II) has received much attention in recent years [10], [11], [12], [13], such as, the SAM of l-cysteine on gold electrode surfaces for Cu(II) are well studied [5], [7], [14]. At these l-cysteine modified electrode interface, Cu(II) ions are generally thought to interact with nitrogen and oxygen or sulfur atoms in the SAM [4], [15], and excellent results in selectivity, sensitivity, wide linear range of concentration and limit of detection could achieved after the SAM of l-cysteine functionalized the sensing interface. However, a number of drawbacks are inherent in this method, for example, the l-cysteine SAM was always assembled on planar gold interface. The major drawback of planar gold electrode is that its low roughness surface and low adsorption capacity of l-cysteine molecular, which could lead to slow electron transport, low sensitivity and selectivity during voltammetry. Thus, their sensitive performance and practical applications may be limited. Therefore, there is still an urgent demand for constructing particular electrode substrate materials with sufficient roughness surface area, high adsorption capacity of organosulfur molecular and high electron transport in this field.

As far as the various substrate materials are concerned, nanostructured inorganic material is an excellent candidate material because of its large active surface area, significant mechanical strength and good chemical stability [16]. Gold nanoparticles supported on the single-walled carbon nanotubes (AuNPs/SWCNTs) are reinforcing electrode substrate material, owing to its unique three-dimensional nanostructure, large active surface area, fast electron transfer rate, and excellent catalytic and sensing characteristics [17], [18], [19]. When the organosulfur groups were assembled onto AuNPs/SWCNTs surface, it formed a three-dimensional SAM cluster. The three-dimensional SAM could provide a high ratio of active complexation sites to Cu(II), which would enhance the sensitivity and selectivity in determination.

In this work, we reported a novel and sensitive method for the detection of Cu(II) by immobilizing l-cysteine SAM on the surface of a three-dimensional AuNPs/SWCNTs nanohybrid modified GCE (AuNPs/SWCNTs/GCE). For constructing a saturation SAM, the AuNPs/SWCNTs/GCE was immersed in a 10.0 mM l-cysteine solution for 12 h at room temperature. The three-dimensional l-cys/AuNPs/SWCNTs/GCE sensor exhibited a higher sensitivity and selectivity, faster binding kinetics to Cu(II) due to its higher ratio of complexation sites, larger surface-to-volume ratios and larger affinity of l-cysteine adapter to Cu(II). The as-prepared sensor not only could strikingly improve the sensitivity and selectivity for Cu(II) analysis, but also show excellent stability and good repeatability and, thus, could be used for enrichment and detection of Cu(II) in environment.

Section snippets

Reagents

l-Cysteine, Gold(III) chloride trihydrate (99.99%) and Cu(II) nitrate were purchased from Sigma-Aldrich and used as supplied. Potassium ferricyanide (K3Fe(CN)6) and ethanol were supplied by Shanghai Chemicals Ltd. SWCNTs used in this study were purchased from Chengdu Organic Chemicals Co. Ltd. (China). All other reagents were commercially available as analytical reagent grade. Stock solutions of Cu(II) were prepared by dissolving Cu(II) nitrate 98% (Sigma–Aldrich) in the minimum required amount

Characterization of l-cys/AuNPs/SWCNTs/GCE

Fig. 1 illustrates the preparation procedures of the l-cys/AuNPs/SWCNTs/GCE. The preparation procedures were summarized as three steps: dropped the pretreated SWCNTs on the fresh surface of GCE; electrodeposited AuNPs on the surface of SWCNTs/GCE; assembled l-cysteine SAM on AuNPs/SWCNTs/GCE surface. Purification and disentanglement of SWCNTs was prepared according to our previous report [20]. 10 μL of dispersed SWCNTs in acetone was dropped onto the surface of a freshly polished GCE and dried

Conclusions

In summary, we have demonstrated a novel and sensitive method for the detection of Cu(II) by immobilizing l-cysteine SAM on the surface of a three-dimensional AuNPs/SWCNTs nanohybrids modified GCE. The AuNPs/SWCNTs/GCE with l-cysteine molecule adapters exhibits a higher sensitivity and selectivity, faster binding kinetics to Cu(II) due to their larger surface-to-volume ratios, higher ratio of coordination sites and larger affinity of l-cysteine to Cu(II). The results demonstrate that the

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 20907035 and 21201132), the Anhui Provincial Natural Science Foundation (Nos. 1208085QB19 and KJ2011A269) and the Science & Research Program of Anhui Province (Nos. 1206c0805031 and 1106c0805007).

Xu-Cheng Fu received his Ph.D. degree in materials physical and chemistry in 2011 from the Institutes of Physical Science of CAS. He is presently an associate professor of Chemistry in West Anhui University. His research activities are nanomaterials and development of new nanomaterials for the electrochemistry sensor application.

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    Xu-Cheng Fu received his Ph.D. degree in materials physical and chemistry in 2011 from the Institutes of Physical Science of CAS. He is presently an associate professor of Chemistry in West Anhui University. His research activities are nanomaterials and development of new nanomaterials for the electrochemistry sensor application.

    Ju Wu received her M.S. in material chemistry in 2008 from Jiangnan University, Wuxi, China. She is an associate professor in West Anhui University. Her areas of research are focused on nanomaterials and their potential electrochemical applications.

    Ju Liu is presently an undergraduate student of West Anhui University. His main research interest is focused on electrochemical analysis.

    Cheng-Gen Xie received his Ph.D. degree in inorganic chemistry in 2007 from Anhui University, Hefei, China. He is presently a professor of Chemistry in West Anhui University. His current areas of research include nanomaterials and their applications in sensors.

    Yi-Shu Liu received his M.S. in inorganic chemistry in 2002 from Anhui Normal University, Wuhu, China. He is presently an associate professor of Chemistry of West Anhui University. His current interests are electrochemical analysis and nanomaterial.

    Yu Zhong received his Ph.D. degree in materials physical and chemistry in 2012 from Institutes of Physical Science of CAS. He is presently an associate professor of Chemistry of West Anhui University. His current research interests are focused on the design and synthesis of nanoaterials for electrochemical application.

    Jin-Huai Liu is a professor at the Institutes of Physical Science of CAS. He is the chief scientist of National Major Project of Fundamental Research (973 Project) from 2010 to 2015. His current research interests include biomimetic nanomaterials, biosensors and chemical sensors, semiconductor sensors.

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