Self-assembled hybrids with xanthate functionalized carbon nanotubes and electro-exfoliating graphene sheets for electrochemical sensing of copper ions

    Self-assembled hybrids with xanthate functionalized carbon nanotubes and electro-exfoliating graphene sheets for electrochemical sensing of copper ions Feng Yang, Duhong He, Baozhan Zheng, Dan Xiao, Li Wu, Yong Guo PII: DOI: Reference:

S1572-6657(16)30005-4 doi: 10.1016/j.jelechem.2016.01.005 JEAC 2446

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

12 August 2015 11 December 2015 7 January 2016

Please cite this article as: Feng Yang, Duhong He, Baozhan Zheng, Dan Xiao, Li Wu, Yong Guo, Self-assembled hybrids with xanthate functionalized carbon nanotubes and electro-exfoliating graphene sheets for electrochemical sensing of copper ions, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.01.005

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Self-assembled hybrids with xanthate functionalized carbon

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nanotubes and electro-exfoliating graphene sheets for

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electrochemical sensing of copper ions

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College of Chemistry, Sichuan University, Chengdu, 610064, P. R. China.

Analytical & Testing Center, Sichuan University, Chengdu, 610064, P. R. China.

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Feng Yang,a Duhong He,a Baozhan Zheng,a Dan Xiaoa, c Li Wu,b* and Yong Guoa*1

College of Chemical Engineering, Sichuan University, Chengdu 610064, P. R. China.

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Abstract:

Recently, graphene and carbon nanotubes (CNTs) hybrids stimulated the development of

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advanced composites. Especially, the electrochemical activities of CNTs or corresponding hybrids have been greatly improved via functionalization. In this work, we developed a self-assembled

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graphene and carbon nano-tube hybrids with interconnected network of carbon structures, which was applied to fabricate a modified carbon paste electrode (CPE) for electrochemical sensor with

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well-response and specificity. In square wave stripping voltammetry (SWSVs), the relative current of the proposed sensor was linearly proportional to the concentration of Cu2+ in the range of 0.02 to 11.10 μM and 31.10 to 111.1 μM with a detection limit (DL = 3S/k) of 0.0095 μM, which was better than the reported for other carbon-based materials. These advantages might be attributed to the

*Corresponding authors： College of Chemistry, Sichuan University, Chengdu, 610064, P. R.

China. Fax: + 86-28-85412907; Tel: +

86-28-85416218; E-mail: [email protected] (Yong Guo) Analytical & Testing Center, Sichuan University, Chengdu, 610064, P. R. China. Fax: + 86-28-85412316; Tel: + 86-28-85412956; E-mail: [email protected] (Li Wu)

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ACCEPTED MANUSCRIPT cooperation of the large surface area for the adsorption and the electron transfer of hybrids for Cu2+. The proposed electrochemical sensor exhibited excellent analytical performance with high sensitivity

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and selectivity. Finally, it was applied to detect Cu2+ in the tap water and pond water with high

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accuracy and good recovery. Results indicate the hybrids prepared here may provide an effective

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strategy for electrochemical sensors.

Keywords: Electro-exfoliating graphene, Functionalized carbon tube, Self-assembled hybrids,

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Electrochemical sensor

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1 Introduction

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Heavy metal ions (such as manganese, zinc, cobalt and copper among others) play an important

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role in living organisms.[1] However, excessive levels or even small doses of very toxic metals may

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cause serious problems on the environment or human health. Typical analysis methods for heavy metals (e.g. spectroscopic techniques[2-4], colorimetric method[5], fluorescent spectrometry[6, 7]

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etc.) are time-consuming and expensive. With the development of nanomaterial science, the

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importance of the electrochemical detection of heavy metals is shown due to its high sensitivity, low cost, straightforward operation and ease of miniaturization. Especially, the developed nano-carbon

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materials based electrochemical systems are giving new inputs to the novel heavy metal sensors with

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interest for applications in environmental and safety and other fields.[8, 9]

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Recently, graphene and carbon nanotube (CNTs) hybrids have emerged as a new class of promising materials attractive for practical applications in transparent electrodes[10],

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capacitors[11, 12], electrochemical sensors[13-15] , electro-catalyst[16, 17], solar cells[18], batteries[19, 20] or field-effect transistors [21], etc. Generally, the graphene and CNTs hybrids were studied as the conductive substrates in sensor fields[15] or as the electrochemical sensing of H2O2.[14] While, the functionalized CNTs composited with graphene-based materials as the electrochemical sensing of metal ions with well-selectivity and high-sensitivity was few reported. However, the development of CNTs in sensor fields are limited due to the selectivity, dispersity and conductivity hinder.[22, 23] In order to overcome this disadvantage, it is essential to treat CNTs by chemical functionalization or hybrid approach. It is a promising strategy for integration of graphene materials and carbon 3

ACCEPTED MANUSCRIPT nanotubes into hybrid materials with desirable properties, which inherit the advantages of both graphene and CNTs.[14-16]

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Graphene and graphene oxide were researched as dispersant and conductive agent in the

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hybrids to improve the electrochemical properties of the hybrids. In addition, excellent flexibility and strength of composites are reinforced via interconnecting CNTs with graphene

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sheets.[24] Meanwhile, graphene would promote fast charge or/and electron transport pathways between CNTs and graphene sheets.[25] The surface area of hybrids would be

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improved due to the effective intercalation and distribution of CNTs in between the graphene sheets.[25, 26] Hence, it is important to combine the merits of carbon tube and graphene to

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overcome the deficiencies and promote the applications as electrochemical sensors. As for electrochemical sensor, extremely high quality graphene with little defects and oxygenated

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groups is necessary. However, graphene oxide contains several oxygen-containing functional groups which increase the dispersity of the graphene oxide, but enhance the sheet

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resistances.[27-29] Therefore, it is the important to control the oxygen groups and defects of the graphene sheets.

In this work, we combined the merits of the xanthate functionalized carbon nanotube (hyxCNTs) and electrochemical cleavage graphene sheets (EGNs) to successfully fabricate the hybrids of EGNs/hyxCNTs. The xanthated hydroxyl-CNTs (hyxCNTs) act as the active material and electrochemical cleavage graphene sheets (EGNs) act as the dispersant and linkers to construct the electrochemical sensor. Results indicated that we have successfully proposed the electrochemical sensor with a highly selective and sensitive sensor for Cu 2+. 4

ACCEPTED MANUSCRIPT 2 Experimental sections

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2.1 Preparation of dispersed graphene

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The dispersed graphene sheets (EGNs) were prepared by electrochemical exfoliating

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approach.[30] In a typical electrochemical synthesis of dispersed graphene sheets (EGNs), high purity graphite rods (JCG 70, Sinosteel Shanghai Advanced Graphite Material Co., Ltd.) were

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adopted as both anode and cathode as the carbon sources and deionized water was used as the

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electrolyte. The two graphite rods were inserted into the deionized water with 50 V static potentials via a direct current power supply. With the electro-exfoliation processing, the current gradually

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increased and the transparent solution turned to a homogeneous dark-brown solution, which the

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graphite was gradually exfoliated to form graphene. After reaction for several days, the obtained

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solution was filtered to remove the graphite particles. The resulting filtrate was centrifuged at 5000 rpm for 3 min to remove the poor dispersed sheets. Finally, water was removed by reduced pressure

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distillation and the concentrated graphene sheets (1.0 mg mL-1) solution was stored for use. The purified electrochemical cleavage graphene (EGNs) dispersion was stable without precipitation, even after several weeks.

2.2 Synthesis of xanthated carbon nanotubes Carbon nanotubes (1.50 g) were dispersed into 200.0 mL sulfonitric mixture acids (concentrated H2SO4/HNO3 3/1) to form a stable dispersion by violent ultrasonication in an ultrasonic bath (KS-3000 ULTRASONIC CLEANCER) for 30 min. The resultant suspension was then treated with microwave radiation at 700 W (Tmax 160 ℃) for 5 min in a microwave reactor (Sineo UWave-1000, 5

ACCEPTED MANUSCRIPT Shanghai Sineo Microwave Chemistry Technology Co., Ltd). The mixture was cooled to room temperature and dispersed in water, which was purified by dialysis in a dialysis bag (retain molecular

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weight 3500 Da) with deionized water until the filtrated solution being neutral. The water of mixture

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was removed by reduced pressure distillation to obtain the purified hydroxyl-CNTs (hyCNTs). It was dried at 45 ℃ in vacuum oven for two weeks.

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The hyCNTs (0.35 g) were dispersed into 300.0 mL pH = 13.00 sodium hydroxide solution to obtain the stable suspension with stirring and ultrasonication-assisted. The mixture was stirred for

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overnight and it was treated in round-bottom flasks (250 mL) at oil-bath 95 ℃ under stirring for 3 h to form the alkoxide (CNTs-C-O-) group. These alkoxide groups are easier to be converted into the

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xanthate group. Then the mixture cooled to room temperature and added dropwise 0.50 mL ethyl alcohol and 0.50 mL carbon disulfide (CS2). The mixture was stirred for overnight. It should be

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noted that the dosage of CS2 was added dropwise into the reaction system, because an excess of it can be adsorbed on to the surface of hyCNTs, which will segregate sodium hydroxide from hyCNTs

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and hinder the xanthation. Ethanol can increase the solubility of CS 2 in reaction system to facilitate xanthation process and improve the efficiency of xanthation (Reaction 2). The black system gradually turned to a homogeneous dark green. The reactions can be represented as followings: hyCNTs + NaOH → CNTs-ONa + H2O CNTs-ONa + CS2 → CNTs-O-CS2Na

(1) (2)

After allowing the mixture to settle for 30 min, the jacinth supernatant was decanted and centrifuged washing with acetone, alcohol and deionized water, successively. Finally, the mixture was further purified via dialyzing over deionized water in a dialysis bag (retain molecular weight 6

ACCEPTED MANUSCRIPT 3500 Da) for one weeks. The xanthated hyCNTs was designated as hyxCNTs. The as-synthesized

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hyxCNTs was dried at 45 ℃ in vacuum oven for one week.

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2.3 Fabrication of modified electrodes

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The EGNs/hyxCNTs-modified carbon paste electrode was prepared by mixing 15.0 mg EGNs/hyxCTs (m/m 3/20), carbon powder (150.0 mg), and paraffin oil (250 μL) in a mortar. The

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homogeneous paste was packed tightly into a polypropylene tube with polished bottom; a copper

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wire was inserted through the opposite end to ensure electrical contact (Fig. 1d). The electrode surface was polished on a polishing paper. For the comparison, the blank electrode (without any

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active material) and hyxCNT-modified electrode were prepared according to the above procedure.

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The electrode surface was renewed by squeezing out an amount of paste, and polished it on a

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polishing paper.

2.4 Electrochemical measurements

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All electrochemical measurements were performed on an AUTOLAB PGSTAT302 (Serial no.: AUT2683, Metrohm, Netherlands). A conventional three-electrode system was employed with a modified-CPE as the working electrode, saturated calomel electrode (SCE) as the reference, and a platinum electrode as the counter electrode in HNO3 solution system. The responses activities of the modified-electrodes for the Cu2+ were researched by using cyclic voltammetry (CV) and square wave stripping voltammetry (SWSV). The CVs were recorded in pH = 2.00 HNO 3 in the presence of 20 μM Cu2+ by scanning in the potential range of – 0.45 to 0.25 V. In square wave stripping voltammetry, the deposition treatment was carried out at – 0.45 V for 180 s under stirring. After 5 s 7

ACCEPTED MANUSCRIPT duration, an anodic square wave scan was applied to the working electrode at the potential range

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from - 0.45 to 0.25 V.

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Other experimental sections, including “Chemicals and Reagents” and “Characterizations” are

3 Results and discussions

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3.1 Fabrication and characterizations of materials

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obtained in Supplementary Material.

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We combined the merits of the xanthate functionalized carbon nanotube (hyxCNTs) and electrochemical cleavage graphene sheets (EGNs) to successfully fabricate hybrids. Firstly,

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the oxygen containing functional groups were introduced to CNTs by mixture acids oxidation, and then oxygen groups reacted with carbon disulfide in pH = 13.00 sodium

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hydroxide system to prepare xanthate functionalized hyCNTs (hyxCNTs) (Fig. 1.a). As shown in Fig. S1 and Fig. S2, the enhanced O-peak and the appeared of S-peaks indicated

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that we have successfully prepared hyxCNTs. The structure of CNTs was characterized by X-ray diffraction (XRD) and Raman (Fig. S3).

Fig. 1 Schematic protocols for the fabrication of EGNs/hyxCNTs and modified carbon paste electrode. (a)The synthesis procedure of hyxCNTs; (b) The electrochemical cleavage process to form EGNs from graphite; (c) Schematic for the hyxCNTs patching on the graphene sheets under ultrasonic treatment for 30 min. EGNs/hyxCNTs represented of strong interaction between EGNs and hyxCNTs via π-π stacking.[31, 32]; (d) The procedure for the fabrication of modified carbon paste electrode.

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ACCEPTED MANUSCRIPT Secondly, the dispersity and conductivity of graphene strongly depend on the degree of defect content, functional groups and crystallinity etc. Graphene sheets, which are synthesized by chemical

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oxidation methods, are easily broken and exhibit high resistances. Hence, the stable dispersible

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graphene aqueous was prepared via electrochemical exfoliated graphite rod in two electrodes system (Fig. 1b). The graphite rod and EGNs were characterized by XPS, XRD and Raman (Fig. S4 and Fig.

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S5). The results demonstrate that (i) parts of oxygen-containing functional groups are introduced to EGNs (Fig. S4); (ii) it still remains the structure of graphene as shown in Fig. S5a; (iii) there are

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some defects and functional groups on the surface of EGNs (Fig. S5b), but they are much less than graphene oxide being prepared by chemical oxidation.[33] The detailed discussions were obtained in

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Supplementary Material.

The protocol for the fabrication of EGNs/hyxCNTs and modified-electrode were depicted in Fig.

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1. The resultant electrochemical cleavage graphene sheets (EGNs) were used for self-assembly with hyxCNTs to form EGNs/hyxCNTs. To confirm the dispersity of EGNs for hyxCNTs in aqueous

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solution, we investigated the dispersity of hyxCNTs with different ratio of EGNs (Fig. S6). As shown in Fig. S6, the dispersity of hyxCNTs was gradually improved with the increase of EGNs and the agglomeration was gradually disappeared. The different materials modified electrodes were fabricated as shown in Fig. 1d and the electrochemical properties of electrodes were investigated and discussed in the following experiments.

Fig. 2 Morphology structural characterizations of EGNs, hyxCNTs and EGNs/hyxCNTs. TEM images of EGNs (a), hyxCNTs (b) and EGNs/hyxCNTs (c). The insets: the corresponding magnified TEM.

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ACCEPTED MANUSCRIPT Fig. 2 shows the transmission electron microscope (TEM) images of EGNs, hyxCNTs and EGNs/hyxCNTs. The TEM images of EGNs/hyxCNTs clearly reveal that most voids within the

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hyxCNTs network are covered with EGNs, which was consistent with SEM images (Fig. S7). The

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bonding between hyxCNTs and EGNs appears to be fairly strong as the hyxCNTs bundles are always found adhering to the surface of EGNs. The morphology of EGNs/hyxCNTs is consistent with the

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structural model for the EGNs/hyxCNTs as shown in Fig. 1c. Here, EGNs serve as patches, which link the different hyxCNTs bundles by filling the inter-bundle void spaces. Hence, the hybrids retain

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the structural integrity and continuity even during electron transfer process.[34] The observation reveals that EGNs can be connected with hyxCNTs and well-dispersed them in aqueous solution.

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Results are consistent with Fig. S6 which exhibits the dispersity of hyxCNTs and EGNs/hyxCNTs

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and the solution system of EGNs/hyxCNTs is homogeneous dispersion.

3.2 Electroactivity of modified carbon paste electrodes

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In order to further study the electrochemical activity of the different modified electrodes, electrochemical experiments were carried out. Firstly, we employed the cyclic voltammetry (CV) to compare the CV behaviors of different active materials modified electrodes in the presence of 20 μM Cu2+ in pH = 2.00 HNO3 solution (Fig. 3a). As shown in Fig. 3a, there are no obvious redox peaks at the blank electrode and EGNs-modified electrode. For the hyxCNTs- and EGNs/hyxCNTs-modified electrode, there are anodic peaks. While, the cyclic voltammogram (CV) of EGNs/hyxCNTs modified-CPE exhibits more obvious peaks, and the peak current and the background current of EGNs/hyxCNTs are also enlarged. In addition, the oxidation peak current increases with the addition of Cu2+ into the electrochemical system (Fig. 3b). These results indicate that the EGNs/hyxCNTs 10

ACCEPTED MANUSCRIPT modified-CPE has significantly advantage compared with other modified electrodes. Thus, based on the increase of the oxidation peak current, the EGNs/hyxCNTs modified-CPE can be used as a

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potential electrochemical sensor for detecting Cu2+.

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Fig. 3 (a) Cyclic voltammograms (CVs) of blank electrode (black line), EGNs-modified electrode (red line), hyxCNTs modified-electrode (blue line), and EGNs/hyxCNTs modified-CPE (olive line) at a scan rate of 30 mVs-1

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(20 μM Cu2+, pH = 2.00 HNO3); (b) CVs of EGNs/hyxCNTs modified with various Cu2+ concentrations of 0, 20,

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40, 60, 80, 100 μM in pH = 2.00 HNO3; (c) CVs of the EGNs/hyxCNTs modified electrode in pH = 2.00 HNO3 at different scan rates (from 5 to 60 mV s-1) in the presence of 20 μM Cu2+ (Inset: the plot of anodic peak currents

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versus scan rates).

In addition, we also employed CVs to investigate the electrochemical behaviour of Cu2+ on the

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surface of modified electrode. As shown in Fig. 3c, the CVs of EGNs/hyxCNTs modified-CPE were explored in pH = 2.00 HNO3 system (20 μM Cu2+) with different scan rates from 5 to 60 mV s-1. It

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indicated that the anodic peak current is linearly proportional to the scan rates over the range from 5 to 60 mVs-1 (inset of Fig. 3c). In addition, the linear regression equation of ipa(μA) = 0.388 + 0.23v (R2 = 0.986) is extracted. These electrochemical characteristic indicate that the electrochemical reaction is a surface adsorption controlled process.[35, 36] This could be ascribed to xanthogenate hyxCNTs with electrical activity. The functionalized carbon nano-tubes with xanthate (-CS2-) groups have been used as a chelating agent in the determination of metal ions, which is ascribed to the formation of the complex between sulfur atoms and copper ion (as shown in Fig. 4I). The electrochemical reaction will occur when the Cu2+ interact with xanthate of hyxCNTs.[36] As shown 11

ACCEPTED MANUSCRIPT in Fig. 4, the electrochemical process includes two steps: (i) adsorption and electrochemical reductive process (enriching process); (ii) electrochemical oxidative process (stripping process). The

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adsorption and electrochemical reductive half-reactions of the modified electrode can be described

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by the Eq. (1) and (2):

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EGNs/hyxCNTs-Cu2+ (surface) + 2e- → EGNs/hyxCNTs-Cu0 (surface)

(2)

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EGNs/hyxCNTs (surface) + Cu2+ → EGNs/hyxCNTs-Cu2+ (surface)

What’s the role of EGNs in hybrids? In order to gain insight into the effect of EGNs in the

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hybrids, electrochemical impedance spectroscopy (EIS), CV and square wave stripping voltammetry (SWSV) techniques were used as shown in Fig. S8. As is known to all, the semicircle diameter

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equals the electron transfer resistance (Rct) and it controls the electron transfer kinetics of the redox probe at the electrode surface.[35] As shown in Fig. S8a, minimum electron transfer resistance was

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measured for the EGNs/hyxCNTs-modified CPE. Namely, the Rct decrease after the EGNs attachment due to the enhanced conduction for electron transfer between the functionalized carbon

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nanotubes (active sites with Cu0/Cu2+) and substrate (carbon powders). And the result is consistent with the electrochemical responses (Fig. S8b, c and Fig. 3). Hence, these results demonstrate that the EGNs improve the electrical conductivity of the hybrids electrode, and this results are consistent with previous report.[37] Additionally, the introduction of EGNs will disperse the functionalized carbon nanotubes to enlarge the electroactive surface area of the hybrids, which would have a large number of active reaction sites for electrochemical reactions. As discussed above, the EGNs not only act as the dispersant, but also as the conductive agent in EGNs/hyxCNTs modified-CPE (as shown in Fig. 4). 12

ACCEPTED MANUSCRIPT The active sites (the xanthate on surface of hyxCNTs) can selectively combine with the copper ions (Fig. 4). The hyxCNTs network acts as the active matrix and conductive frame, while the EGNs

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sheets serve as intermediate layers connecting the hyxCNTs to exhibit an effective electrochemical

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performance for Cu2+ sensing. Therefore, we speculated that the electrochemical process includes the two steps: (I) the adsorption and electro-deposition step, which is a physical absorption and

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electro-reduction reaction; (II) stripping step which is an electro-oxidation reaction and releases Cu2+ into the reaction system. This schematic and inference are consistent with electron microscopes

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results (as shown in Fig. 2) and electrochemical response results (Fig. 3a, Fig. 5a and Fig. S8).

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Fig. 4 The process for EGNs/hyxCNTs modified-CPE behaviour for Cu2+ detection. The electrochemical processes

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including adsorption and electro-deposition step (I) and stripping step (II).

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3.3 Optimization of conditions for Cu2+

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To fabricate a highly sensitive electrochemical sensor, the electrochemical techniques and some parameters were optimized. Firstly, we compared the current responses of EGNs/hyxCNTs modified-electrode in the presence of 20 μM Cu2+ (pH = 2.00 HNO3) by using differential-pulse voltammetry (DPV), differential-pulse stripping voltammetry (DPSV), square wave stripping voltammetry technique (SWSV) (Fig. S9). Compared with DPV and DPSV, the current response of the SWSV has a significant advantage. Hence, the attractive ability of the proposed-electrochemical sensor of the selective determination of Cu2+ was studied using SWSV technique. Secondly, in order to monitor and characterize the modified electrode with the stepwise fabricated products, the SWSV responses of different activity materials modified-electrodes for Cu2+ 13

ACCEPTED MANUSCRIPT (20 μM) in pH = 2.00 HNO3 were investigated (Fig. 5a). As shown in Fig. 5a, tiny responses are detected on the blank electrode and EGNs modified-electrode. Compared with hyCNTs and

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hyxCNTs modified-electrodes, the EGNs/hyxCNTs modified-electrode exhibits the strongest

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response for Cu2+ under the same conditions. It indicates that the EGNs/hyxCNTs modified-electrode exhibits the best electrochemical behaviour for Cu2+. These results indicate that the electro-exfoliated

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graphene sheets (EGNs) as the dispersant additive and conductive agent can improve performance of EGNs/hyxCNTs-modified electrode as the sensor for Cu2+, which is consistent with the results of

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CVs in Fig. 3a.

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Fig. 5 (a) SWSVs of the blank electrode (black line), hyCNTs (red line), hyxCNTs (blue line), EGNs (magenta line)

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and EGNs/hyxCNTs (olive line) modified-electrode in pH = 2.00 HNO3 system (dash line: without 20 μM copper

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ion; solid line: with 20 μM copper ion); Plots of the SWSVs relative peak currents against pHs (b), deposition potential (c), deposition time (d), frequency (e) and amplitude (f) obtained from the EGNs/hyxCNTs modified-CPE

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for 20 μM Cu2+ in pH = 2.00 HNO3 (Insets: the corresponding SWSV curves).

Finally, some experimental parameters (including the pH, deposition potential, deposition time, frequency and amplitude) were optimized as shown in Fig. 5b - f. As shown in Fig. 5b, the relative peak current decreases with the pH increasing after 2.00. Therefore, the optimum pH value of electrolyte was 2.00. The effect of deposition potential on the SWSV response for Cu2+ is investigated in the range from – 0.55 to 0 V (Fig. 5c). The relative peak current begins to stabilize after – 0.35 V. Finally, we took - 0.45 V as the deposition potential. It is found that the relative peak currents linearly increased with the increasing of the deposition time, which leads to the higher 14

ACCEPTED MANUSCRIPT response and the more analysis time. The deposition time of 180 s for determination of Cu 2+ was chosen as the efficiency being considered. As shown in Fig.4e and Fig. 4f, the relative peak currents

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were increased with the increasing of frequency and amplitude, while the SWSV curves were

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gradually occurred deformation. The frequency and amplitude were set at 25 Hz and 40 mV, respectively.

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3.4 SWSVs detection of EGNs/hyxCNTs modified electrochemical sensor Under the optimized conditions, typically SWSVs responses of the blank electrode, hyxCNTs

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modified-CPE and EGNs/hyxCNTs modified-CPE were measured with successive addition of Cu2+ to a stirred pH = 2.00 HNO3 system (Fig. S10). As shown in Fig. S10, the current responses of

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EGNs/hyxCNTs-modified electrode are the strongest than hyxCNTs-modified electrode and blank

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electrode.

Fig. 6 SWSVs responses of EGNs/hyxCNTs modified-CPE toward copper ions with different concentrations (from

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0.02 to 911.1 μM) in a stirred pH = 2.00 HNO3 solution (insets: the corresponding SWSVs responses of the proposed electrode toward lower-concentration copper ion); (b) The resultant linear calibration curve of EGNs/hyxCNTs for copper ions (inset: the calibration curve of relative current versus corresponding copper ion concentration). Deposition potential: - 0.45 V, deposition time: 180 s, frequency: 25 Hz, amplitude: 40 mV, error bars: SD, n = 4.

For EGNs/hyxCNTs modified-CPE, the oxidation peak currents increase with the introduction of copper ions (Fig. 6a) and the relative peak currents showed a good linear correlation in two ranges of concentration gradients (0.02 to 11.10 μM and 31.1 to 111.1 μM). The calibration curves of the 15

ACCEPTED MANUSCRIPT electrochemical sensor were shown in the inset of Fig. 6b. In the range from 0.02 to 11.10 μM, the linear regression equation was expressed as y = 1.038 + 0.200 CCu2+ (μM) (R2 = 0.998) and the

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detection limit of the proposed electrode was down to 0.0095 μM. For the range from 31.10 to 111.1

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μM, the linear regression equation was expressed as y = 3.085 + 0.103 CCu2+ (μM) (R2 = 0.995). The current deviates from linearity at higher Cu2+ concentrations, most possibly due to the saturation of

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active sites at the surface of the proposed electrode. Compared with previous research, there are obviously superiority in linear range and detection limit, which was better than the reported for the

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xanthated carbon nano-spheres (3.55 × 10-8 M)[36] and graphene-based materials (1.87 × 10-8

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M)[38].

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3.5 Specificity of the proposed sensor

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Fig. 7 The specificity of the EGNs/hyxCNTs modified-CPE. (a) The relative responses (I/I0 -1) of EGNs/hyxCNTs modified CPE to heavy ions (20 μM); (b) The relative current intensity of EGNs/CNTs modified CPE to 20 μM

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Cu2+ in the absence (black histogram) and presence of interfering metal ions (magenta histogram, 30 μM) and the mixture of all the interfering metal ions (magenta histogram). Error bars: SD, n = 4.

The specificity of a sensor is a very important characteristic in analyzing real samples. Some heavy metal ions (such as Ca2+, Mg2+, Ba2+, Zn2+, Mn2+, Cd2+, Fe2+, Fe3+, Ni2+, Hg2+, Pb2+) as the potential interfering electroactive species were investigated one by one to evaluate the selectivity of the proposed electrochemical sensor (Fig. 7). There are no significant responses except Cu 2+ as shown in Fig. 7a. When copper ions (20 μM) were added, a significant relative response (I/I0 -1) is observed. Hence, the proposed sensor exhibited a good selectivity of Cu 2+. The well-response and 16

ACCEPTED MANUSCRIPT specificity of the sensor can be attributed to the xanthated groups (–CS2-) as the chelating agent, which adsorb Cu2+ to the surface of the electrode. The interference experiments were also carried out

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by adding 20 μM Cu2+ and 30 μM interfering heavy metal ions as indicated in Fig. 7b. For all the

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interfering species, EGNs/hyxCNTs modified-CPE did not show obvious fluctuation in the sensor response. These results demonstrate that selective determination of Cu2+ on EGNs/hyxCNTs

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investigate

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reproducibility

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modified-CPE can be achieved with high sensitivity and selectivity. the

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sensor,

four

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modified-electrodes were parallelly prepared and one modified-electrode was renewed (the surface of it) for the repeated measurements of a single concentration of copper ion (20 μM). All the

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electrodes generated reproducible relative currents with relative standard deviation (RSD) of 3.71% was obtained. In addition, the stability of the sensor was investigated by detecting the

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electrochemical signal after two weeks. The relative current could retain 98.54% of its initial response for copper ion (20 μM). The results suggest that the electrochemical sensor has satisfactory

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reproducibility and acceptable stability.

3.6 Applications

To evaluate the reliability and practical applicability of the electrochemical sensor, different amounts of target copper ion were added into the real sample (Pond water: the Lily Pond of Sichuan University, Chengdu, China; Top water) and the SWSVs signals of the obtained samples were subsequently performed. The samples were also analyzed by ICP-OES method (a commercially available technique) for comparison. The results of the determination and recovery tests were displayed in Table S1. The recoveries are in the range of 95.88% – 106.5%. The results demonstrate 17

ACCEPTED MANUSCRIPT that the proposed electrochemical sensor offers an excellent and accurate method for the practical

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determination of Cu2+ in real samples.

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Conclusions

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Summary, we proposed a simple strategy to fabricate a copper ion electrochemical sensor with high sensitivity and selectivity. We employed electrochemical exfoliation graphite to prepare the

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graphene sheets with well-dispersity and conductivity. The xanthate functionalized modification

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improved the specificity and selectivity of carbon tube. The dispersity, stability and conductivity of functionalized carbon tubes were improved with the introducing of EGNs. The EGNs/hyxCNTs

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modified-CPE displayed a linear response to Cu2+ in the concentration range from 0.02 to 11.10 μM

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and from 31.1 to 111.1 μM with a detection limit down to 0.0095 μM. The proposed approach is

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expected to design new chemical sensors based on functionalized carbon nano-tube and graphene hybrids for applications in electrochemical sensor fields.

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Acknowledgements

Authors greatly acknowledge financial support for the project from the Natural Science Foundation of China (NO. 21345001) and the projects of international science and technology cooperation and exchange of Sichuan province of China (2014HH0021).

References [1] G. Aragay, A. Merkoçi, Nanomaterials application in electrochemical detection of heavy metals, Electrochim. Acta, 84 (2012) 49-61. [2] A.F. Barbosa, V.M.P. Barbosa, J. Bettini, P.O. Luccas, E.C. Figueiredo, Restricted access carbon 18

ACCEPTED MANUSCRIPT nanotubes for direct extraction of cadmium from human serum samples followed by atomic absorption spectrometry analysis, Talanta, 131 (2015) 213-220.

IP

T

[3] S. Lee, J.J. Gonzalez, J.H. Yoo, J.R. Chirinos, R.E. Russo, S. Jeong, Application of femtosecond

SC R

laser ablation inductively coupled plasma mass spectrometry for quantitative analysis of thin Cu(In,Ga)Se2 solar cell films, Thin Solid Films, 577 (2015) 82-87.

NU

[4] X. Liao, B. Liang, Z. Li, Y. Li, A simple, rapid and sensitive ultraviolet-visible spectrophotometric technique for the determination of ultra-trace copper based on

MA

injection-ultrasound-assisted dispersive liquid-liquid microextraction, Analyst, 136 (2011) 4580-4586.

TE

D

[5] T.G. Thomas, K. Sreenath, K.R. Gopidas, Colorimetric detection of copper ions in

5358-5362.

CE P

sub-micromolar concentrations using a triarylamine-linked resin bead, Analyst, 137 (2012)

[6] Z. Hu, J. Hu, Y. Cui, G. Wang, X. Zhang, K. Uvdal, H.-W. Gao, A facile "click" reaction to

AC

fabricate a FRET-based ratiometric fluorescent Cu2+ probe, J. Mater. Chem. B, 2 (2014) 4467-4472.

[7] F. Wang, Z. Gu, W. Lei, W. Wang, X. Xia, Q. Hao, Graphene quantum dots as a fluorescent sensing platform for highly efficient detection of copper(II) ions, Sensor. Actuat. B: Chem., 190 (2014) 516-522. [8] F.R. Baptista, S.A. Belhout, S. Giordani, S.J. Quinn, Recent developments in carbon nanomaterial sensors, Chem. Soc. Rev., 44 (2015) 4433-4453. [9] P. Greil, Perspectives of nano-carbon based engineering materials, Adv. Eng. Mater., 17 (2015) 19

ACCEPTED MANUSCRIPT 124-137. [10] V.C. Tung, L.-M. Chen, M.J. Allen, J.K. Wassei, K. Nelson, R.B. Kaner, Y. Yang,

IP

T

Low-temperature solution processing of graphene−carbon nanotube hybrid materials for

SC R

high-performance transparent conductors, Nano Lett., 9 (2009) 1949-1955. [11] L.-Y. Lin, M.-H. Yeh, J.-T. Tsai, Y.-H. Huang, C.-L. Sun, K.-C. Ho, A novel core-shell

NU

multi-walled carbon nanotube@graphene oxide nanoribbon heterostructure as a potential supercapacitor material, J. Mater. Chem. A, 1 (2013) 11237-11245.

MA

[12] C. Yang, J. Shen, C. Wang, H. Fei, H. Bao, G. Wang, All-solid-state asymmetric supercapacitor based on reduced graphene oxide/carbon nanotube and carbon fiber paper/polypyrrole electrodes,

TE

D

J. Mater. Chem. A, 2 (2014) 1458-1464.

[13] X. Dong, Y. Ma, G. Zhu, Y. Huang, J. Wang, M.B. Chan-Park, L. Wang, W. Huang, P. Chen,

CE P

Synthesis of graphene-carbon nanotube hybrid foam and its use as a novel three-dimensional electrode for electrochemical sensing, J. Mater. Chem., 22 (2012) 17044-17048.

AC

[14] S. Woo, Y.-R. Kim, T.D. Chung, Y. Piao, H. Kim, Synthesis of a graphene–carbon nanotube composite and its electrochemical sensing of hydrogen peroxide, Electrochim. Acta, 59 (2012) 509-514. [15] Q. Sheng, M. Wang, J. Zheng, A novel hydrogen peroxide biosensor based on enzymatically induced deposition of polyaniline on the functionalized graphene–carbon nanotube hybrid materials, Sensor. .Actuat. B: Chem., 160 (2011) 1070-1077. [16] G.-L. Tian, M.-Q. Zhao, D. Yu, X.-Y. Kong, J.-Q. Huang, Q. Zhang, F. Wei, Nitrogen-doped graphene/carbon nanotube hybrids: in situ formation on bifunctional catalysts and their superior 20

ACCEPTED MANUSCRIPT electrocatalytic activity for oxygen evolution/reduction reaction, Small, 10 (2014) 2251-2259. [17] Y. Li, W. Zhou, H. Wang, L. Xie, Y. Liang, F. Wei, J.-C. Idrobo, S.J. Pennycook, H. Dai, An

IP

T

oxygen reduction electrocatalyst based on carbon nanotube-graphene complexes, Nat. Nano, 7

SC R

(2012) 394-400.

[18] M.-H. Yeh, L.-Y. Lin, C.-L. Sun, Y.-A. Leu, J.-T. Tsai, C.-Y. Yeh, R. Vittal, K.-C. Ho,

NU

Multiwalled carbon nanotube@reduced graphene oxide nanoribbon as the counter electrode for dye-sensitized solar cells, J. Phys. Chem. C, 118 (2014) 16626-16634.

MA

[19] M.-Q. Zhao, X.-F. Liu, Q. Zhang, G.-L. Tian, J.-Q. Huang, W. Zhu, F. Wei, Graphene/single-walled carbon nanotube hybrids: one-step catalytic growth and applications for

TE

D

high-rate Li–S batteries, ACS Nano, 6 (2012) 10759-10769. [20] X. Li, P. Huang, Y. Zhou, H. Peng, W. Li, M. Qu, Z. Yu, A novel Li4Ti5O12/graphene/carbon

CE P

nano-tubes hybrid material for high rate lithium ion batteries, Mater. Lett., 133 (2014) 289-292. [21] S.H. Chae, W.J. Yu, J.J. Bae, D.L. Duong, D. Perello, H.Y. Jeong, Q.H. Ta, T.H. Ly, Q.A. Vu, M.

AC

Yun, X. Duan, Y.H. Lee, Transferred wrinkled Al2O3 for highly stretchable and transparent graphene–carbon nanotube transistors, Nat. Mater., 12 (2013) 403-409. [22] I. Dumitrescu, P.R. Unwin, J.V. Macpherson, Electrochemistry at carbon nanotubes: perspective and issues, Chem. Comm., (2009) 6886-6901. [23] C. Gao, Z. Guo, J.-H. Liu, X.-J. Huang, The new age of carbon nanotubes: An updated review of functionalized carbon nanotubes in electrochemical sensors, Nanoscale, 4 (2012) 1948-1963. [24] T.-K. Hong, D.W. Lee, H.J. Choi, H.S. Shin, B.-S. Kim, Transparent, Flexible Conducting Hybrid Multilayer Thin films of multiwalled carbon nanotubes with graphene nanosheets, ACS 21

ACCEPTED MANUSCRIPT Nano, 4 (2010) 3861-3868. [25] R. Lv, E. Cruz-Silva, M. Terrones, Building complex hybrid carbon architectures by covalent

IP

T

interconnections: graphene–nanotube hybrids and more, ACS Nano, 8 (2014) 4061-4069.

SC R

[26] Z. Fan, J. Yan, L. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian, F. Wei, A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in

NU

supercapacitors, Adv. Mater. 22 (2010) 3723-3728.

[27] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi, B.H.

MA

Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature, 457 (2009) 706-710.

Rev., 39 (2010) 228-240.

TE

D

[28] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc.

CE P

[29] S.Z. Butler, S.M. Hollen, L. Cao, Y. Cui, J.A. Gupta, H.R. Gutiérrez, T.F. Heinz, S.S. Hong, J. Huang, A.F. Ismach, E. Johnston-Halperin, M. Kuno, V.V. Plashnitsa, R.D. Robinson, R.S. Ruoff,

AC

S. Salahuddin, J. Shan, L. Shi, M.G. Spencer, M. Terrones, W. Windl, J.E. Goldberger, Progress, challenges, and opportunities in two-dimensional materials beyond graphene, ACS Nano, 7 (2013) 2898-2926. [30] Y.-L. Zhang, L. Wang, H.-C. Zhang, Y. Liu, H.-Y. Wang, Z.-H. Kang, S.-T. Lee, Graphitic carbon quantum dots as a fluorescent sensing platform for highly efficient detection of Fe3+ ions, RSC Adv., 3 (2013) 3733-3738. [31] D. Yu, L. Dai, Self-assembled graphene/carbon nanotube hybrid films for supercapacitors, J. Phys. Chem. Lett., 1 (2009) 467-470. 22

ACCEPTED MANUSCRIPT [32] D. Cai, M. Song, C. Xu, Highly conductive carbon-nanotube/graphite-oxide hybrid films, Adv. Mater., 20 (2008) 1706-1709.

IP

T

[33] F. Yang, M. Zhao, B. Zheng, D. Xiao, L. Wu, Y. Guo, Influence of pH on the fluorescence

SC R

properties of graphene quantum dots using ozonation pre-oxide hydrothermal synthesis, J. Mater. Chem., 22 (2012) 25471-25479.

NU

[34] F. Tristán-López, A. Morelos-Gómez, S.M. Vega-Díaz, M.L. García-Betancourt, N. Perea-López, A.L. Elías, H. Muramatsu, R. Cruz-Silva, S. Tsuruoka, Y.A. Kim, T. Hayahsi, K. Kaneko, M.

MA

Endo, M. Terrones, Large area films of alternating graphene–carbon nanotube layers processed in water, ACS Nano, 7 (2013) 10788-10798.

TE

D

[35] L. Zhang, C. Zhou, J. Luo, Y. Long, C. Wang, T. Yu, D. Xiao, A polyaniline microtube platform for direct electron transfer of glucose oxidase and biosensing applications, J. Mater. Chem. B, 3

CE P

(2015) 1116-1124.

[36] J. Zhang, C. Cheng, Y. Huang, L. Qian, B. Zheng, H. Yuan, Y. Guo, D. Xiao, Facile synthesis of

AC

functionalizated carbon nanospheres for determination of Cu2+, Analyst, 138 (2013) 2073-2079. [37] C. Li, Z. Li, H. Zhu, K. Wang, J. Wei, X. Li, P. Sun, H. Zhang, D. Wu, Graphene nano-“patches” on a carbon nanotube network for highly transparent/conductive thin film applications, J. Phys. Chem. C, 114 (2010) 14008-14012. [38] X. Gong, Y. Bi, Y. Zhao, G. Liu, W.Y. Teoh, Graphene oxide-based electrochemical sensor: a platform for ultrasensitive detection of heavy metal ions, RSC Adv., 4 (2014) 24653-24657.

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Fig. 1 Schematic protocols for the fabrication of EGNs/hyxCNTs and modified carbon paste

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electrode. (a)The synthesis procedure of hyxCNTs; (b) The electrochemical cleavage process to form

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EGNs from graphite; (c) Schematic for the hyxCNTs patching on the graphene sheets under ultrasonic treatment for 30 min. EGNs/hyxCNTs represented of strong interaction between EGNs

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and hyxCNTs via π-π stacking.[31, 32]; (d) The procedure for the fabrication of modified carbon

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paste electrode.

Fig. 2 Morphology structural characterizations of EGNs, hyxCNTs and EGNs/hyxCNTs. TEM

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images of EGNs (a), hyxCNTs (b) and EGNs/hyxCNTs (c). The insets: the corresponding magnified

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TEM.

Fig. 3 (a) Cyclic voltammograms (CVs) of blank electrode (black line), EGNs-modified electrode

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(red line), hyxCNTs modified-electrode (blue line), and EGNs/hyxCNTs modified-CPE (olive line) at a scan rate of 30 mVs-1 (20 μM Cu2+, pH = 2.00 HNO3); (b) CVs of EGNs/hyxCNTs modified with various Cu2+ concentrations of 0, 20, 40, 60, 80, 100 μM in pH = 2.00 HNO3; (c) CVs of the EGNs/hyxCNTs modified electrode in pH = 2.00 HNO3 at different scan rates (from 5 to 60 mV s-1) in the presence of 20 μM Cu2+ (Inset: the plot of anodic peak currents versus scan rates).

Fig. 4 The process for EGNs/hyxCNTs modified-CPE behaviour for Cu2+ detection. The electrochemical processes including adsorption and electro-deposition step (I) and stripping step (II).

Fig. 5 (a) SWSVs of the blank electrode (black line), hyCNTs (red line), hyxCNTs (blue line), EGNs 24

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peak currents against pHs (b), deposition potential (c), deposition time (d), frequency (e) and

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amplitude (f) obtained from the EGNs/hyxCNTs modified-CPE for 20 μM Cu2+ in pH = 2.00 HNO3 (Insets: the corresponding SWSV curves).

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Fig. 6 SWSVs responses of EGNs/hyxCNTs modified-CPE toward copper ions with different

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concentrations (from 0.02 to 911.1 μM) in a stirred pH = 2.00 HNO3 solution (insets: the corresponding SWSVs responses of the proposed electrode toward lower-concentration copper ion);

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(b) The resultant linear calibration curve of EGNs/hyxCNTs for copper ions (inset: the calibration

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curve of relative current versus corresponding copper ion concentration). Deposition potential: - 0.45

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V, deposition time: 180 s, frequency: 25 Hz, amplitude: 40 mV, error bars: SD, n = 4.

Fig. 7 The specificity of the EGNs/hyxCNTs modified-CPE. (a) The relative responses (I/I0 -1) of

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EGNs/hyxCNTs modified CPE to heavy ions (20 μM); (b) The relative current intensity of EGNs/CNTs modified CPE to 20 μM Cu2+ in the absence (black histogram) and presence of interfering metal ions (magenta histogram, 30 μM) and the mixture of all the interfering metal ions (magenta histogram). Error bars: SD, n = 4.

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Graphical abstract

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 The functionalized carbon nano-tubes were synthesized by two steps (including oxidation and

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xanthation) to improve the specificity and selectivity of cupric ions.

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 The electrochemical exfoliated water-dispersed graphene sheets were used as dispersant and linkers to form hybrids with functionalized carbon nano-tubes by self-assembly. The

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electrochemical processes and mechanism of the proposed sensor were discussed.

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 We have successfully constructed an electrochemical sensor with well-response and selectivity,

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owing to the functionalization and the interconnected network of carbon structures.

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