Laser engraved nitrogen-doped graphene sensor for the simultaneous determination of Cd(II) and Pb(II)

Accepted Manuscript Laser engraved nitrogen-doped graphene sensor for the simultaneous determination of Cd(II) and Pb(II)

Xueni Lin, Zhiwei Lu, Wanlin Dai, Baichen Liu, Yuxin Zhang, Junye Li, Jianshan Ye PII: DOI: Reference:

S1572-6657(18)30605-2 doi:10.1016/j.jelechem.2018.09.016 JEAC 12599

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

22 June 2018 4 September 2018 5 September 2018

Please cite this article as: Xueni Lin, Zhiwei Lu, Wanlin Dai, Baichen Liu, Yuxin Zhang, Junye Li, Jianshan Ye , Laser engraved nitrogen-doped graphene sensor for the simultaneous determination of Cd(II) and Pb(II). Jeac (2018), doi:10.1016/ j.jelechem.2018.09.016

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ACCEPTED MANUSCRIPT Laser engraved nitrogen-doped graphene sensor for the simultaneous determination of Cd(II) and Pb(II)

College of Chemistry and Chemical Engineering, South China University of

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Xueni Lina, Zhiwei Lua, Wanlin Daia, Baichen Liua, Yuxin Zhanga, Junye Lib, Jianshan Yea,*

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Technology, Guangzhou 510641, P.R. China

The Affiliated High School of South China Normal University, Guangzhou 510630,

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P.R. China

Corresponding author at: College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, P.R. China. Tel.: +86 20 87113241. E-mail: [email protected] (J. Ye)

ACCEPTED MANUSCRIPT Abstract Recently, graphene-based nanomaterials have attracted widespread attention as new electrode-modified materials for different electrochemical sensing platforms. However, graphene-based nanomaterials modified heavy metal sensors often have the disadvantages in that

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their detection ranges are not wide enough. To solve this problem, an easy and feasible approach

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derived from laser engraving technique to fabricate a high-quality 3D porous graphene-based

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heavy metal sensor with a quite wide detection range was established. The nitrogen-doped laser

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engraved graphene (N@LEG) was synthesized via introducing polyaniline (PANI) and polyvinylpyrrolidone (PVP) as N-dopant. Herein, by coupling the unique electrochemical

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properties and the 3D porous structure framework with large electrochemical active surface areas of LEG with the strong metal ions affinity of N atoms, N@LEG modified glassy carbon electrode

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(N@LEG/GCE) with in-situ bismuth film modification has been successfully utilized for the

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simultaneous determination of Cd(II) and Pb(II) using square wave anodic stripping voltammetry (SWASV). After optimizing conditions, the proposed sensor exhibits quite wide linear ranges

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varying from 5 to 10 g L-1 and 10 to 380 g L-1 for Cd(II), and 0.5 to 10 µg L-1 and 10 µg L-1 to 380

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µg L-1 for Pb(II), respectively. In addition, the detection limits are calculated to be 1.08 g L-1 (S/N = 3) for Cd(II) and 0.16 g L-1 (S/N = 3) for Pb(II), which are nearly 3 times and 60 times less than the guideline values of the drinking water presented by the World Health Organization (WHO), for Cd(II) and Pb(II), respectively. Furthermore, the results describe satisfactory advantages of remarkable sensitivity, selectivity, anti-interference, repeatability, reproducibility and stability. Besides, the fabricated sensor is proved to be a potential perspective to detect Cd(II) and Pb(II) in actual water samples.

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Keywords: Electrochemical determination; Modified electrode; Electrochemical impedance

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spectroscopy (EIS); Cyclic voltammetry (CV); Raman; Heavy metal.

ACCEPTED MANUSCRIPT 1. Introduction At present, due to the toxicity, stability and non-biodegradability of heavy metal ions (HMI) even trace amounts, which have a harmful impact on various environmental components, involving terrestrial and aquatic biota, the effective determination of them is of great significant[1].

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The routine techniques used widely for the HMI determination, including atomic absorption

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spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS), etc., have

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several defects such as time-consuming, high-cost, requirement for cumbersome instrumentation

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and in particular unsuitability for on-spot monitoring[2, 3]. Compared with spectroscopic methods, electrochemical methods, especially anodic stripping voltammetry (ASV) have been generally

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regarded as a simple and efficient tool to detect HMI owing to their superiority of low-cost, satisfactory sensitivity, fast response, portable instruments, easy to operate and the possibility of

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on-site analysis[2, 4-7]. ASV measurements often contain a pre-concentration step and a stripping

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step on working electrode surface[8]. Therefore, the choice of good performance working electrode modified materials makes contributions to improving the electrochemical analysis.

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Graphene, a kind of carbon nanomaterials with a unique 2D hexagonal structure of

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sp2-hybridized carbon, has caused extensive attention recently due to its high specific area, unique electrical conductivity, self-assembly behavior and mechanical flexibility[9, 10]. Studies have shown that the porous structure of graphene could possess a large specific area as well as have access to an effective multidimensional pathway for mass transfer and electron transport[11]. Graphene-based sensors have been widely applied to the detection of glucose, dopamine and heavy metal ions. Although most graphene-based heavy metal sensors have low detection limits,[5, 12-14] there are few reports on graphene-based heavy metal sensors with fairly wide linear 1

ACCEPTED MANUSCRIPT ranges[15]. So it is an urgent requirement to find a method for the preparation of graphene-based heavy metal sensors with considerable wide detection ranges. On the other hand, functionalization or edge/basal plane doping plays a vital role in the electrical properties of graphene and its derivatives. Thus, various doped graphene-based nanomaterials, especially doped with

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heteroatoms (e.g., N, B, S and Cl), have been greatly expanded the applications of battery[16],

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supercapacitor[17, 18], non-metal electrocatalysis[19] and electrochemical sensing[20, 21].

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Among them, N-doping has attracted wide concern on account of N atoms comparatively strong electron affinity to improve the electrochemical activity and excellent compatibility with the

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carbon matrix[1]. Besides, N-doping could lead to defect sites by replacing C atoms, which

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allowed potential active sites to ameliorate electrochemical activities of graphene-based nanomaterials[22, 23]. In addition, PANI, one of the most vital conducting polymers, has been

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utilized for the application of electrochemical sensors owing to its satisfactory conductivity, ease

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of synthesis and fairly non-poisonous[24]. So far, the nitrogen-containing polymer, polyvinylpyrrolidone (PVP), also has been considered as N-dopant[1]. Moreover, PVP can prevent

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the nanoparticle aggregation during the synthesis.[15] Up to now, N-doped graphene-based

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electrochemical sensors have been reported for the determination of heavy metals, but their detection range is narrower.[14, 25] In this work, an N-doped graphene-based heavy metal sensor was prepared by introducing PANI and PVP as N-dopant to improve the detection ranges. Although there are many strategies for the synthesis of graphene, they require either high temperature treatments or complex and time-consuming chemical preparation routes,[26, 27] which hinder their application due to the poor efficiency and time-consuming operations. Hence, it is an urgent need to develop a method capable of fabricating graphene at low-cost, high-efficiency, 2

ACCEPTED MANUSCRIPT and large-scale to meet actual demand. In recent years, LEG technique has achieved rapid development and the LEG has opened up a new horizon for electrochemical applications. With a standard Light Scribe DVD drive, El-Kady et al.[28] have produced the reduced graphene oxide patterns via direct laser scribing graphene oxide, which opened up a new direction for the direct

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on-chip manufacture of micro supercapacitors. Arul et al.[29] have also investigated the

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mechanism of forming graphene features by scribing with a variety of laser sources. Lin et al.[30]

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has successfully simplified the preparation of graphene via a one-step laser scribing on a flexible commercial-available PI sheet with a CO2 infrared laser. The resultant sheets were named as laser

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scribed graphene (LSG) with remarkable electron conductivity and large surface area and could be

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directly used as supercapacitor electrodes without any additives. Using the process similar to that reported previously using CO2 laser, the graphene layer could be formed on the pinewood, a

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natural and renewable material[31]. With the advantages of a 3D mesoporous framework,

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outstanding conductivity and electroactivity, LEG has been applied to supercapacitors[32], sensors[33], and so on[34]. Therefore, an easy and rapid method, laser engraving technique, was

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adopted to prepare a graphene-based heavy metal sensing platform with 3D porous structures. It

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was because of the 3D porous structure framework with such a large electrochemical active surface area of LEG that made contributions on the rather wide detection range. Our group have reported a glassy carbon electrode modified with a bismuth film and laser etched graphene for simultaneous voltammetric sensing of Cd(II) and Pb(II)[35]. Besides, to the best of our knowledge, there is no report on the application of laser engraving technique to prepare N-doped graphene-based electrodes for the detection of HIM in real water samples.

In this paper, we fabricated 3D porous graphene-based electrode matrices by the simple and 3

ACCEPTED MANUSCRIPT rapid laser engraving technique, and then synthesized N@LEG via introducing PANI and PVP as N-dopant. Consequently, by combining the unique electrochemical properties and large electrochemical active surface area of LEG with the strong metal ions affinity of N atoms, newly-designed N@LEG/GCE shows excellent responses for the simultaneous determination of

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Cd(II) and Pb(II) with such a wide detection range in virtue of square wave anodic stripping

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voltammetry (SWASV). Moreover, the facile and fast preparation and the extremely wide linear

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range indicate that the sensor is promising and ideal for the applications of HMI sensing. Additionally, the acquisition of 3D porous N@LEG sheds light on the preparation of graphene and

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ranges based on laser engraving technique.

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graphene-based heavy metal sensors with comparable detection limits and fairly wide linear

Scheme.1 Schematic illustration of the fabrication process of N@LEG electrodes for the electrochemical

simultaneous detection of Cd(II) and Pb(II).

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ACCEPTED MANUSCRIPT 2. Experimental 2.1 Materials and Reagents

Commercial PI sheets (thickness: 0.125 mm) were purchased from Shenzhen Golden Green

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Leaf Technology Co., Ltd. Potassium ferricyanide (K3[Fe(CN)6]), Potassium ferrocyanide (K4Fe(CN)6·3H2O), potassium chloride (KCl), acetic acid (CH3COOH, HAc), sodium acetate

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(CH3COONa, NaAc) and hydrochloric acid (HCl) were purchased from Sinopharm Chemical

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Reagent Co., Ltd. 0.1 M NaAc/HAc buffer with different pH was prepared by NaAc and HAc.

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Nafion (5%), aniline, PVP, and ammonium persulfate (APS) were from Sigma-Aldrich. Nafion (5%) was diluted to 0.125% with alcohol. Standard stock solutions of Cd(II), Pb(II) and Bi(III)

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were obtained from Aladdin Industrial Corporation. Ultra-pure water from a water purification

2.2 Instrumentations

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were of analytical grade.

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system GWA-UN1-10 (18.25 MΩ cm) was used throughout all the experiments. All chemicals

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The SEM images were conducted by a Zeiss LEO1530VP scanning electron microscopy.

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XRD patterns were executed with a Bruker D8 Advance diffractometer with a Cuκα radiation. Raman spectra were implemented with a Jobin Yvon LabRam Aramis spectrometer. XPS measurements were accomplished on an ESCALAB 250Xi spectrometer. Laser engraving was carried out on a laser engraving machine (K3020) with glass sealed off CO2 laser. Fourier transform infrared (FTIR) spectral studies were performed with traditional KBr pellet method on a Bruker VERTEX 70 spectrometer.

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ACCEPTED MANUSCRIPT 2.3 Laser engraving and fabrication of nanocomposite

Firstly, the PI sheets were cleaned by rinsing with alcohol and ultra-pure water and then dried in an oven at 60 °C before laser irradiation. Then, laser engraving was executed on PI sheets by the design software at a power of 5 W and a scanning speed of 50 mm/s in atmospheric conditions

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to form a black carbonized layer on PI films. In the meanwhile, in order to compound PANI on the

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once-etched PI films, 12.4 g APS and 10.0 g PVP were dissolved in 70.0 mL ultra-pure water and

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then cooled in an ice water bath for about 30 min (solution I). After that，the once-etched PI films

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were moved to a 500ml beaker. Then 2 mol/L HCl was added until the black carbonized area was submerged and 5ml aniline was added. The mixture was placed in an ice water bath under

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continuous stirring with the solution I added dropwise and then left overnight. The resulting product was named as once-etched PI films compounded with polyaniline (OE-PI@PANI).

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Afterwards, the OE-PI@PANI was cleaned by ultra-pure water and then dried in an oven. Finally,

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the OE-PI@PANI was laser engraved again with a larger power (6.2 W) in the same area to make graphitization more sufficiently. The fully graphitized material which had been broken away from

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the PI film was scraped off to obtain the desired product. Since the PANI combined on the

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once-etched PI film was decomposed by the instantaneous high temperature generated during the laser engraving process[11], the obtained nanomaterials were defined as N@LEG. The direct twice-etched graphene (TE-LEG) was prepared in the similar ways except the procedure of combining with the PANI. The PANI was prepared using the method above aside from integrating with the PI sheets.

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2.4 Preparation of modified electrode

Before the modification, 4 mg N@LEG was ultrasonically dispersed in 2 mL 0.125% Nafion-alcohol solution (V:V=1:1) for more than 2 hours to acquire a homogeneous suspension.

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The bare GCE was polished with alumina powder and then sonicated with alcohol and ultra-pure water in turns for 2 minutes. Then the treated GCE was dried under an infrared lamp. After that, 4

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µL of N@LEG suspension was dropped onto the surface of GCE and then dried under an infrared

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lamp to acquire the N@LEG/GCE. Moreover, PANI-GCE and TE-LEG-GCE were obtained by

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the process mentioned above for comparison.

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2.5 Electrochemical measurements

Electrochemical measurements were implemented on an IGS6030 electrochemical

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workstation (Guangzhou ingsens sensor technology Co., Ltd, China) with a conventional

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three-electrode system consisting of a GCE working electrode, an Ag/AgCl saturated KCl reference electrode, and a platinum counter electrode. The CVs were implemented with the

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potential varied from -0.2 V to 0.6 V at a scan rate of 100 mV s-1. And the EIS measurements were

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carried out with the frequency from 0.1 Hz to 106 Hz and the amplitude of 5 mV. Both of them were executed in 5.0 mM Fe(CN)63-/4- solution containing 0.1 M KCl. Furthermore, SWASV was employed for the simultaneous determination of Cd(II) and Pb(II) with a deposition potential at -1.3 V for 210 s and a potential from -1.0 V to 0 V with 25 mV amplitude, 10 Hz frequency and 5 mV increment potential in 0.1 M NaAc/HAc buffer (pH=4.5) in the presence of 200 µg L-1 Bi(III). Prior to every experiment, a cleaning step by polarizing at 0.5 V for 60 s was adopted.

2.6 Preparation of samples 7

ACCEPTED MANUSCRIPT Real lake water samples were filtered twice through a 0.22 µm filter membrane before the measurements. Generally, 5.0 mL of lake water sample was mixed with 5.0 mL of 0.1 M NaAc/HAc buffer (pH 4.5) with 200 µg L-1 Bi(III)and then analyzed under the optimized SWASV.

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3. Results and discussion

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3.1 Surface characterization of N@LEG

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The surface morphologies of N@LEG were characterized by SEM. As shown in Fig. 1(a), it is clearly observed that there are rather compact 3D porous structures with different sizes on the

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surface of N@LEG. At higher magnification (Fig. 1(b)), there are typical 3D graphene

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frameworks of the stacked graphene sheets caused by laser scanning on PI films. This is due to the fact that the focused laser beams generated the instantaneous high temperature that causes the PI

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decomposed into gaseous products so that the momentary gas spray resulted in the formation of

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porous structures during the decomposition and graphitization processes.[11] It is because of the presence of the 3D porous stacked graphene sheets structures which provide such large specific

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surface areas and abundant reactive sites that facilitate the accumulation of metal ions and make

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contributions on the very wide detection range.

Fig. 1 (a) SEM image of N@LEG; (b) magnified SEM image of N@LEG.

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ACCEPTED MANUSCRIPT In addition, the structure information of N@LEG was further analyzed by Raman spectroscopy. As exhibited in Fig. 2(a), the peaks of OE-PI@PANI are quite different from pure PANI or TE-LEG in peak numbers. As for the pure PANI, four prominent peaks located at 1163 cm-1, 1338 cm-1 and 1480 cm-1, which derive from the stretching vibration of C=C, C–N and C–H

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[36]. Besides, the Raman spectrum of the TE-LEG depicts three representative peaks of graphene,

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i.e. a D peak at 1350 cm-1, a G peak at 1585 cm-1 and 2D peak at 2700 cm-1 approximately[37].

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Since all of the above characteristic peaks appear in the Raman spectra of the OE-PI@PANI nanocomposite, it is certain that PANI was successfully combined with PI films after once laser

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engraved. Furthermore, just as the graphene obtained by directly laser etching PI sheets

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twice-TE-LEG, N@LEG also exhibits the D-band (at 1350 cm-1) corresponding to defects in disordered graphene flakes, the G-band (at 1585 cm-1) related to the hexagonal clamping mode of

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graphite carbon, and 2D-band (at 2700 cm-1) deriving from the layer stacking pattern of carbon

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atoms[38]. Additionally, the intensity ratio ID/IG, which is employed to evaluate the degree of defects and disorder in the sp2 hybridized graphitic carbon, are about 0.71 for N@LEG and 0.27

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for LEG, revealing the crystal defect and disorder degree ascended after compounding with PANI

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[39]. Despite that, the distinct 2D peak further illustrates the generation of 3D multilayer graphene [37], which is in accordance to with the SEM results. Nevertheless, the characteristic peaks of PANI does not appear in N@LEG, which might be due to the pyrolysis of PANI caused by the instantaneous high temperature generated during laser engraving[11]. The successful synthesis of 3D multilayer graphene with porous frameworks plays an important role in the very wide detection range of heavy metal sensors.

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Fig. 2 (a) Raman spectra of PANI, OE-PI@PANI, TE-LEG and N@LEG; (b) XRD patterns of N@LEG; (c) XPS

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survey spectra of TE-LEG and (d) N@LEG.

Furthermore, the structural information of N@LEG was also investigated by XRD. As shown

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in Fig. 2(b), N@LEG presents a distinct peak at 2θ =25.83° consistent with the typical C (002) plane of crystalline graphene, while the peak at 42.20° for the 100 plane is corresponding to an

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in-plane structure[40]. The XRD results are in agreement with Raman, which further proves the production of graphene.

To analyze the surface properties and the variation of the components of the TE-LEG and N@LEG nanocomposite, XPS was also performed. The XPS survey spectra of TE-LEG and N@LEG (Fig. 2(c) and Fig. 2(d)) reveal that both of them consist of C, N, O elements. The N element contents on the surface of TE-LEG and N@LEG are 0.09 at.% and 2.15 at.%, respectively. 10

ACCEPTED MANUSCRIPT Compared with TE-LEG, the N content of N@LEG increases, which is ascribed to the fact that the transient high temperature generated during the laser engraving process causes the PANI on the OE-PI@PANI film to be thermally decomposed[11]. The presence of N-doped graphene has a significant effect on the considerable wide detection range. The high-resolution curve-fitted XPS

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spectra and related analysis of TE-LEG and N@LEG in the C 1s, O 1s and N 1s regions are

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shown in Fig. S1(a-d) in Electronic Supplementary Materials (ESM). Moreover, the FTIR spectra

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of PANI, TE-LEG and N@LEG are displayed in Fig. S2. The FTIR results further justify the

3.2 Electrochemical behaviors of N@LEG.

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material as nitrogen-doped graphene. The specific graph and explanation are exhibited in ESM.

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Electrochemical characterizations of N@LEG were first studied by adopting [Fe(CN)6]3-/4- as electrochemical redox probes. Fig. 3(a) show the CVs of bare-GCE (curve a), PANI-GCE (curve

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b), TE-LEG/GCE (curve c) and N@LEG/GCE (curve d) modified electrodes in 5.0 mM Fe(CN)63-/4- containing 0.1 M KCl. A pair of well-defined reversible redox peaks is exhibited at

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the bare GCE (curve a). Compared with the bare GCE (48.58 µA), modification with the PANI (61.68 µA) onto GCE surface will increase the anodic peak current of [Fe(CN)6]3-/4-, resulting

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from the good electrical conductivity of PANI which accelerates the electron transfer rate[24]. In addition, a larger peak current of [Fe(CN)6]3-/4- also attains with the modification of TE-LEG (60.66 µA) onto GCE, on account of the porous 3D graphene nanosheets that provide large specific areas for electrochemical reactions. Nevertheless, it is worth noting that N@LEG/GCE presents the highest peak currents (139.6 µA), which are more than 2-fold higher than other electrodes. This phenomenon indicates the fact that N@LEG has the best ability to facilitate the electron transfer as well as makes the fastest response to redox reactions. 11

ACCEPTED MANUSCRIPT Besides, the Nyquist plots are shown in Fig. S3. The resistance of 300 Ω on bare-GCE (curve a), 270 Ω PANI-GCE (curve b), and 260 Ω on TE-LEG/GCE (curve c) and 165 Ω on N@LEG/GCE (curve d) are observed respectively. The resistances change of these modified electrodes are consistent with the CV results, which not only verify that N@LEG was successfully

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modified on the GCE surface but also indicate the excellent electrical conductivity of N@LEG. In

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addition, the electrochemical surface area study (ESCA) related graphs and explanations are

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shown in Fig. S4 (ESM). The effective area was calculated to be 0.03 cm2 of bare-GCE and 0.06 cm2 of LEGCNs-GCE, respectively. The increased electrochemical activity surface area of

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LEGCNs-GCE (twice as large as bare-GCE) indicates that LEGCNs are beneficial to the

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improvement of the electrochemical performance and the adsorption of heavy metal ions. Moreover, the sensitivity is calculated to be 37.47 µA·µM-1·cm-2 for Cd(II) and

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224.47µA·µM-1·cm-2 for Pb(II), respectively.

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Furthermore, SWASV was performed for trace heavy metals determination in 0.1 M NaAc/HAc buffer (pH 4.5) accompanied by an in situ-incorporated Bi film with a concentration of

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200 µg L-1. The stripping voltammograms of 200µg L-1 Cd(II) and Pb(II) at different film-coated

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GCEs and bare GCE are presented in Fig. 3(b) (Bare-GCE: short dot line; PANI: dash dot line; TE-LEG: dash line; N@LEG: solid line). At the PANI modified GCE, the smallest stripping peaks of Cd(II) and Pb(II) are observed, suggesting that the PANI has a worst detection sensitivity of Cd(II) and Pb(II) so that it had better not be used to electrochemically detect them when compared to other electrodes. In contrast, at TE-LEG modified GCE, the stripping responses of Cd(II) and Pb(II) are much higher than bare GCE result from the large effective specific surface area which is beneficial to the adsorption of the probe metal ions. Most importantly, comparison with other 12

ACCEPTED MANUSCRIPT electrodes, N@LEG/GCE possesses not only the distinguishable and completely separated stripping peaks at almost -0.80 V (Cd(II)) and -0.55V (Pb(II)) but also the highest stripping signals, which revealed that N@LEG has wonderful affinity as well as excellent SWASV performance towards Cd(II) and Pb(II) due to the synergistic effect of LEG and N-doped. The analyses above

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certify that there are some merits at N@LEG/GCE: (1) 3D porous stacked graphene nanosheets

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structure provide a considerable large specific area to accelerate the electron transfer at interfaces;

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(2) multilayer stacked graphene nanosheets facilitate electron transfer and improve the conductivity; (3) The presence of N heteroatoms contributes to the structural stability and the

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affinity of metal ions. Thus, N@LEG is considered as a potential electrochemical sensor platform

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for the simultaneous detection of Cd(II) and Pb(II).

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Fig. 3 (a) CVs of bare GCE (curve a), PANI (curve b), TE-LEG (curve c) and N@LEG (curve d) modified electrodes in 5 mM Fe(CN)63-/4- solution with 0.1 M KCl. Scan rate: 100 mV s-1; (b) SWASV responses of 200 µg L-1 Cd(II) and Pb(II) in 0.1 M NaAc/HAc buffer (pH 4.5) on the N@LEG (curve a), TE-LEG (curve b), bare GCE

(curve c) and PANI (curve d) modified electrodes. Deposition potential: -1.3 V. Deposition time: 210s. Bi(III) concentration: 200 µg L-1.

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ACCEPTED MANUSCRIPT 3.3 Optimization of the determination conditions.

In order to improve the sensitivity of N@LEG/GCE in the determination of Cd(II) and Pb(II), several correlative factors were optimized via SWASV, such as (a) deposition potential; (b) deposition time; (c) pH; (d) the concentration of Bi(III). The optimal experimental conditions were

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listed as follows: (a) Deposition potential: -1.3 V; (b) Deposition time: 210 s; (c) Optimal pH: 4.5;

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(e) Bi(III) concentration: 200 µg·L-1. The specific charts and explanations are shown in Fig. S5 in

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Electronic Supplementary Materials (ESM)).

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3.4 Electrochemical determination of Cd(II) and Pb(II) using SWASV

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SWASV was employed to discuss the limit of detection and linear range of the N@LEG/GCE for the simultaneous determination of Cd(II) and Pb(II) under the optimized experimental

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conditions. Fig. 4(a) depicts the SWASV curves of different concentrations of Cd(II) and Pb(II) in

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0.1 M NaAc/HAc buffer (pH 4.5) containing 200 µg L-1 Bi(III) at N@LEG/GCE. The stripping peaks of Cd(II) and Pb(II) increase accompanied by the successive enhancement in probe metal

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ions concentration. And well-defined response peaks are observed at around -0.55 V and -0.80 V belonging to Cd(II) and Pb(II), respectively. The corresponding regression equations of Cd(II) and

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Pb(II) contain two wide linear ranges (Fig. 4(b)):

For Cd(II):

Ip (µA) = 0.04C (µg·L-1) + 1.64; (5 to 10 µg·L-1, R2 = 0.996) Ip (µA) = 0.14C (µg·L-1) +0.33; (10 to 380 µg·L-1, R2 = 0.991)

For Pb(II):

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(1)

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ACCEPTED MANUSCRIPT Ip (µA) = 0.13C (µg·L-1) + 1.86; (0.5 to 10 µg·L-1, R2 = 0.999)

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Ip (µA) = 0.20C (µg·L-1) + 1.06; (10 to 380 µg·L-1, R2 = 0.997)

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The reason for such wide linear ranges is due to the 3D porous stacked graphene sheets

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structures of N@LEG which provide large specific surface areas and abundant reactive sites that facilitate the accumulation of metal ions and the influence of N-doped graphene. Moreover, the

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limits of detection (LOD) (based on S/N = 3) is calculated according to the following formula:

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LOD =3 σB/b, in which σB is the standard deviation of the population of the blank responses and b

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is the slope of the regression line[1, 41, 42]. The LOD are 1.08 µg L-1 for Cd(II) and 0.16 µg L-1 for Pb(II), which are nearly 3 times and 60 times less than the guideline values of the drinking

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water presented by the World Health Organization (WHO), for Cd(II) and Pb(II), respectively. A comparison with the previously reported modified electrodes is given in Table 1. Apparently, the

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new-designed N@LEG/GCE demonstrates lower or comparable detection limits and much wider linear ranges. In addition, N@LEG/GCE requires neither expensive instruments nor complicated

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and tedious processes. Thus, the fabricated sensor is applicable for sensitive and rapid determination of Cd(II) and Pb(II) with the merits of simple fabrication, easy operation and

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low-cost.

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Fig. 4 (a) SWASV curves of Cd(II) and Pb(II) at 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12.5, 15, 20, 40, 60, 80, 100, 120,

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140,180, 220, 260, 300, 340 and 380 µg L-1 and (b) Corresponding calibration plots for Cd(II) at 6, 7, 8, 9, 10, 12.5, 15, 20, 40, 60, 80, 100, 120, 140,180, 220, 260, 300, 340 and 380 µg L-1 and Pb(II) at 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9,

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10, 12.5, 15, 20, 40, 60, 80, 100, 120, 140,180, 220, 260, 300, 340 and 380 µg L-1, respectively. (c-d) SWASV

responses and the corresponding calibration plots on N@LEG/GCE for the individual determination of Cd(II) at

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10, 50, 100, 150, 200 and 250 µg L-1, respectively. (e-f) SWASV responses and the corresponding calibration plots on N@LEG/GCE for the individual determination of Pb(II) at 10, 50, 100, 150, 200 and 250 µg L-1, respectively. Deposition potential: -1.3 V. Deposition time: 210s. Bi(III) concentration: 200 µg·L-1. Supporting electrolyte: 0.1

M acetate buffer (pH 4.5).

16

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Table 1 Comparison of our designed electrode for the determination of Cd(II) and Pb(II) with other modified

electrodes.

Linear range(µg L-1) Methods

References Pb(Ⅱ)

Cd(Ⅱ)

Pb(Ⅱ)

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Cd(Ⅱ)

Bi/Nafion/RGO-GNPs-GCE

SWASV

1–90

1–90

0.08

0.12

[13]

Bi/poly(p-ABSA)-GCE

DPASV

1–110

1–130

0.63

0.80

[43]

BiOCl/MWCNT-GCE

SWASV

5–50

RI

Electrodes

LOD(µg L-1)

1.20

0.57

[44]

RGO/Bi-CPE

DPASV

20–120

2.80

0.55

[45]

CB-15-crown-5-GCE

DPASV

15.7–191.9

10.9–186.5

4.70

3.30

[46]

Nafion/Bi/NMC-GCE

DPASV

2–10,10–100

0.5–10,10–100

1.50

0.05

[1]

Bi/Au-GN-Cys-GCE

SWASV

0.5-40

0.10

0.05

[12]

N@LEG/GCE

SWASV

1.08

0.16

This work

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5–50

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20–120

D

0.5-40

0.5–10,10–380

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5–10,10–380

RGO: reduced graphene oxide; GNPs: gold nanoparticles; Poly(p-ABSA): poly(p-aminobenzene sulfonic acid; DPASV: differential pulse

4-carboxybenzo-15-crown-5;

NMC:

nitrogen

doped

microporous

carbon

(NMC);

Au-GN-Cys:

gold

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CB-15-crown-5:

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anodic stripping voltammetry; BiOCl: bismuth-oxychloride; MWCNTs: multiwalled carbon nanotubes; CPE: carbon paste electrode;

nanoparticle-graphene-cysteine composite

3.5 Interference, repeatability, reproducibility and stability of N@LEG/GCE

Since the mutual interference between Cd(II) and Pb(II) during the simultaneous determination is so crucial for practical applications, an individual detection of Cd(II) or Pb(II) was conducted by altering one’s concentration whereas the concentration of another one was kept constant in the meanwhile. As seen in Fig. 4(c) and Fig. 4(d), the stripping signals of Cd(II) 17

ACCEPTED MANUSCRIPT ascend linearly over the range from 10 to 250 µg L-1 while the concentration of Pb(II) is kept at 50 µg L-1. However, the responses of Pb(II) fundamentally remain changeless at the same time. Analogous results are observed with the enhancement of the concentration of Pb(II) from 10 to 250 µg L-1 but fixing the concentration of Cd(II) in the meanwhile (Fig. 4(e) and Fig. 4(f)). As

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similar detection sensitivities of Cd(II) and Pb(II) are obtained (0.09 versus 0.04 for Cd(II) and

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0.19 versus 0.13 for Pb(II)) when compared with the results found in simultaneous determination,

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the results suggest that there is little mutual interference between Cd(II) and Pb(II).

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Aside from the mutual interference, the presence of non-target metal ions also disturbs the SWASV determination of target metal ions. To evaluate the anti-interference performance of the

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proposed sensor, various interfering metal ions (including Zn(II), Co(II), Ni(II), Cu(II), Mg(II), Cr(II), Fe(III), Mn(II), As(II), Hg(II) and Al(II)) were added to 0.1 M NaAc/HAc buffer (pH 4.5)

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containing 50 µg L-1 Cd(II) and Pb(II) by SWASV under the optimum conditions. Fig. 5(a) shows

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that 50-fold Zn(II), Co(II), Ni(II), Mg(II), Cr(II), Fe(III), Mn(II), As(II) and Al(II) have no distinct influences on response signals and the tolerance ratio was measured to be 4.15% at most. However,

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10-fold Cu(II) results in less than 30.33% peak current decrease, while 10-fold Hg(II) is calculated

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to be less than 26.90% peak current increase. This suppression effect is likely due to the formation of intermetallic compounds as well as competition for active sites on the surface of the electrode, reported previously[47-49]. Since the Cu2[Fe(CN)6] has a larger pKsp valueand is therefore more insoluble. And the hexacyanoferrate(II) was not found to have any effect on the Cd(II) and Pb(II) peaks except at concentrations above 0.1 M when a small drop in response was noted. Therefore, the Cu(II) interference can be reduced by the addition of 0.1 mM ferrocyanide to the sample extract solutions before analysis to form an insoluble and stable copper-ferrocyanide complex with 18

ACCEPTED MANUSCRIPT the help of a ligand[49]. In addition, it is obvious that the stripping peak currents of Cd(II) and Pb(II) are significantly increased when Hg(II) coexist, which is attributed to competition the formation of an amalgam as well as the competition between electrodeposited bismuth and Hg for surface sites on the electrode[47, 50]. Actually, in order to improve the sensitivity, the hanging

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mercury drop electrode (HMDE) and the mercury film electrode have been widely used in the

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electrochemical determination of heavy metal ions[48, 51]. Thus, the formation of mercury film

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and followed by the Metal-Hg intermetallic compound formation on the surface of the electrode increase the peak current for heavy metal ions. Overall, the presence of Hg strengthens the

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sensitivity in the detection of heavy metal ions[52]. Thus, the above results indicated the

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N@LEG/GCE had satisfactory anti-interference ability.

In addition, in the interference experiments, we also studied the effects of several organic

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substances with 300-fold concentration. The 300-fold concentration of testing substances was

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introduced into the sample which contained 50 µg·L-1 Cd(II) and Pb(II) and 200 µg·L-1 Bi(III). As shown in Fig. 5(b), the presence of chloroform (CH3Cl) has almost no effect on the response

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signals, the tolerance ratio of which is 1.3%. However, the complexing agents such as

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ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and citric acid, have an inhibitory effect on the test results, which will weaken or even eliminate the stripping signals of Cd(II) and Pb(II), especially the EDTA, whose presence causes the signals to almost disappear). On the contrary, the surface active substances such as sodium dodecyl benzene sulfonate (SDBS) and sodium laurylsulfonate (SDS) have an increased effect on the detection signal, whose tolerance ratio is 12.6% and 15.8%. As for formaldehyde (CH3CHO), both of the stripping signals of Cd(II) and Pb(II) weaken and the tolerance ratio is 16.6%. Therefore, this sensor is not suitable for 19

ACCEPTED MANUSCRIPT systems containing complexing agents or complex organic substances.

The repeatability was appraised by the relative standard deviation (RSD %) of ten repetitive experiments of SWASV (Fig. S6). The RSDs are 3.85% and 3.47% of each of Cd(II) and Pb(II), which manifest the wonderful repeatability performance of the N@LEG/GCE. The reproducibility

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was next judged with five different N@LEG/GCE and the RSDs are 7.12% for Cd(II) and 6.87%

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for Pb(II). The results indicate that the preparation procedures are reliable and the prepared sensor

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has excellent reproducibility. The stability was evaluated after two weeks of storage at room

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temperature. The RSDs are 5.54% for Cd(II) and 6.15% for Pb(II), so the prepared sensor

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D

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possesses long-time stability.

Fig. 5 (a-b) Selectivity of N@LEG/GCE for simultaneous detection of Cd(II) and Pb(II). Deposition potential: -1.3

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V. Deposition time: 210s. Cd(II) and Pb(II) concentration: 50 µg·L-1. Bi(III) concentration: 200 µg·L-1. Supporting

electrolyte: 0.1 M acetate buffer (pH 4.5).

3.6 Real sample analysis

To quantify of Cd(II) and Pb(II) in real samples and estimate the applicability of N@LEG/GCE, standard addition method was carried out in real lake water sample. The lake 20

ACCEPTED MANUSCRIPT water was treated by diluting with 0.1 M acetate buffer (pH 4.5) containing 200 µg L-1 Bi(III) (v:v=1:1). The results are listed in Table 2 and the recoveries of Cd(II) and Pb(II) vary from 97.80% to 113.65%. Furthermore, the results obtained are in accordance with the AAS method. Hence, N@LEG/GCE sensor has excellent accuracy and recovery, indicating the promising

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application for the determination of Cd(II) and Pb(II) in practical samples.

Found by AAS(µg L-1)

Sample Cd(Ⅱ)

Pb(Ⅱ)

Cd(Ⅱ)

Pb(Ⅱ)

5.58

6.12

/

/

5.82

10

10

10.35

20

20

18.90

30

30

Pb(Ⅱ)

4.85

/

/

10.62

9.78

106.20

97.80

19.87

22.73

21.07

113.65

105.35

31.32

29.55

33.73

98.50

112.43

10.11

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D

Lake water

CE

29.76

Recovery/%

Cd(Ⅱ)

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Cd(Ⅱ) Pb(Ⅱ)

Found by proposed method(µg L-1)

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Added (µg L-1)

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Table 2 Recovery for the determination of Cd(II) and Pb(II) in lake water (n = 3) .

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4. Conclusions

In the present work, the laser engraving technique provides a facile and novel method for efficient and large-scale fabrication of high-quality 3D porous graphene-based electrode matrices. By introducing PANI as N-dopant after once laser engraving PI sheets, the resulting OE-PI@PANI was subjected to the secondary etching so that the surface PANI was decomposed by the instantaneous high temperature generated during the laser engraving process[11], thereby synthesizing the N@LEG. The N@LEG possesses 3D porous stacked graphene morphologies 21

ACCEPTED MANUSCRIPT with large specific surface areas and the unique metal ions affinity of N atoms, which are conducive to the accumulation of HMI and the improvement of the quite wide detection ranges. Moreover, the simultaneous detection of Cd(II) and Pb(II) is achieved in terms of SWASV with high selectivity, extremely wide linear range and low detection limit. The quite wide linear ranges

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vary from 5 to 380 g L-1 for Cd(II), and 0.5 to 380 µg L-1 for Pb(II), respectively. In addition, the

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detection limits were calculated to be 1.08 g L-1 (S/N = 3) for Cd(II) and 0.16 g L-1 (S/N = 3) for

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Pb(II), which were nearly 3 times and 60 times less than the guideline values of the drinking water presented by the World Health Organization (WHO), for Cd(II) and Pb(II), respectively. In the

sensitivity is

calculated

to be 37.47 µA·µM-1·cm-2

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addition,

for

Cd(II)

and

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224.47µA·µM-1·cm-2 for Pb(II), respectively. The facile preparation process and satisfactory analytical performance of the proposed sensor indicate a promising application towards detection

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of Cd(II) and Pb(II) in real water. In the future, abundant electrochemical sensors with such wide

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detection ranges can be fabricated according to the above proposed method as well as expand

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in-field miniaturized or wearable sensing platforms.

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Acknowledgments

This work was supported by National Natural Science Foundation of China (NSFC, No. 21875070), Guangdong Nature Science Foundation (Project No. 2017A030312005), and Guangzhou Science Technology and Innovation Commission (No. 201508020010). We also wish to thank the engineer Qian Liu from Shiyanjia lab for help with the XPS analysis.

22

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Graphical abstract

Schematic illustration of the fabrication process of the nitrogen-doped laser engraved graphene

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modified glassy carbon electrode (N@LEG-GCE) for the simultaneous determination of cadmium

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(Cd(II)) and lead (Pb(II)) by square wave anodic stripping voltammetry (SWASV).

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Highlights 1. The 3D porous nitrogen-doped laser engraved graphene (N@LEG) was easily fabricated by laser engraving technique.

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2. The sensor possessed very wide detection ranges due to the large specific surface areas of 3D

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porous stacked graphene sheets structures and the influence of nitrogen-doped graphene.

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3. The sensor exhibited quite wide linear ranges varying from 5 to 10 g L-1 and 10 to 380 g L-1 for

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Cd(II), and 0.5 to 10 µg L-1 and 10 µg L-1 to 380 µg L-1 for Pb(II), respectively.

31