Accepted Manuscript Title: Constructed ILs coated porous magnetic nickel cobaltate hexagonal nanoplates sensing materials for the simultaneous detection of cumulative toxic metals Authors: Yuanyuan Dong, Lei Zhang PII: DOI: Reference:

S0304-3894(17)30200-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2017.03.034 HAZMAT 18450

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

26-9-2016 13-3-2017 15-3-2017

Please cite this article as: Yuanyuan Dong, Lei Zhang, Constructed ILs coated porous magnetic nickel cobaltate hexagonal nanoplates sensing materials for the simultaneous detection of cumulative toxic metals, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.03.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Constructed ILs coated porous magnetic nickel cobaltate hexagonal nanoplates sensing materials for the simultaneous detection of cumulative toxic metals Yuanyuan Dong, Lei Zhang College of Chemistry, Liaoning University, 66 Chongshan Middle Road, Shenyang, Liaoning, 110036, People’s Republic of China

*

Corresponding author. Tel.: +86 24 62207809; Fax: +86 24 62202380.

E-mail address: [email protected] (L. Zhang). 1

Graphical Abstract

Highlights • A novel sensor material based on ionic liquids@nickel cobaltate was constructed. • Various morphologies of NiCo2O4 were synthesized for electrocatalytic comparison. • ILs@NiCo2O4-P was used to detect cumulative toxic metals for the first time. • The sensor displayed well reproducibility, excellent selectivity and sensitivity. • The method was applied to detect practical samples with satisfactory results.

Abstract: The different morphologies of magnetic nickel cobaltate (NiCo2O4) electrocatalysts, consisting of nanoparticles (NiCo2O4-N), nanoplates (NiCo2O4-P) and microspheres (NiCo2O4-S) were fabricated. It was found that the electrocatalytic properties of the sensing materials were strongly dependent on morphology and specific surface area. The porous NiCo2O4 hexagonal nanoplates coupled with ILs as modified materials (ILs@NiCo2O4-P) for the simultaneous determination of thallium (Tl+), lead (Pb2+) and copper (Cu2+), exhibited high sensitivity, long-time stability and good repeatability. The enhanced electrocatalytic activity was attributed to relatively 2

large specific surface area, excellent electronic conductivity, and unique porous nanostructure. The analytical performance of the constructed electrode on detection of Tl+, Pb2+ and Cu2+ was examined using differential pulse anodic stripping voltammetry (DPASV). Under optimal conditions, the electrode showed a good linear response to Tl+, Pb2+and Cu2+ in the concentration range of 0.1−100.0, 0.1−100.0 and 0.05−100.0 μg/L, respectively. The detection limits (S/N = 3) were 0.046, 0.034 and 0.029 μg/L for Tl+, Pb2+ and Cu2+, respectively. The fabricated sensor was successfully applied to detect trace Tl+, Pb2+ and Cu2+ in various water and soil samples with satisfactory results. Hence, this work provided a promising material for electrochemical determination of cumulative toxic metals individually and simultaneously. Keywords: Electrochemical sensor; Simultaneous determination; Porous NiCo2O4 hexagonal nanoplates; Ionic liquids; Cumulative toxic metal ions.

1. Introduction The cumulative toxic metals, such as thallium (Tl+), lead (Pb2+) and copper (Cu2+) are considered as a severe threat to the environment and human health even at trace level [1, 2]. Once absorbed, the accumulation of these metal ions in the human body can pose serious disorders to human organs and greatly threat the health of human [3]. Among them, the toxicity of Tl+ is much higher than that of Pb2+ and Cu2+ in the biosphere [4]. Tl+ is known to have mutagenic, carcinogenic, and teratogenic activity in animals and man [5]. Pb2+ toxicity is a particularly serious problem which may 3

cause irreversible health effects to the hepatic, blood, central nervous and renal systems [6]. Cu2+ is a vital element widely distributed in all sorts of food, shellfish and animals, but the intake of great amounts can be toxic [7]. Thus, it is important to know the amount of these metal ions in practical samples which is mainly due to their toxicological effects depending on their chemical form and concentration. Routine detection methods for metals include atomic absorption spectrophotometry (AAS) [8], atomic fluorescence spectrometry (AFS) [9], inductively-coupled-plasma mass spectrometry (ICP-MS) [10] and colorimetry [11]. These methods generally can provide high sensitivity and selectivity for detecting metal ions. However, the commonly used spectrometric methods are more expensive, time consuming and not suitable for the in situ analysis of metal ions because of the dependence on complicated equipment. By contrast, electrochemical method, especially differential pulse stripping anodic voltammetry (DPASV) is an effective method to detect metal ions due to its short analysis time, low cost, excellent sensitivity and the ability to accurately detect multiple elements at trace level [12, 13]. The working electrode materials play a crucial role in carrying out the stripping analysis of metal ions. For instance, Afkhami et al. reported electrochemical determination of Hg2+ and Pb2+ using MWCNTs and a new synthesized Schiff base modified carbon paste electrode [14]. Chen et al. developed a functionalized polypyrrole nanotube arrays sensor for the detection of Cu2+ [15]. An ideal electrode material is considered to possess good performance, such as effective preenrichment, 4

large electrochemically active area, excellent conductivity, good chemical stability, and low background current over a wide potential range [16, 17]. Thus, there is an urgent need but it remains an important challenge to rationally design and construct electro-active sensing materials. The spinel NiCo2O4, a binary oxide, is currently popular as an electrode material due to its high electrochemical activity, outstanding electronic conductivity, multiple convertible valence states, high abundance, low cost, environmental benignity and easily controllable morphologies, which is one of the promising mixed-metal oxides for electrode materials [18, 19]. It is found that the electrochemical properties of the electrode materials depend largely on the morphology, crystallinity and porosity of the electrode materials and especially, porous structure is beneficial for transfer of electrons. So far, various morphologies NiCo2O4 [20, 21] and many NiCo2O4-based hybrid materials including several NiCo2O4/RGO [22], MWCNT/NiCo2O4 nanosheet [23] and NiCo2O4/NiO composites [24], have been synthetized and studied. Particularly, these composite exhibited quite encouraging electrochemical performance, which have been used for energy storage in supercapacitor and lithium ion battery. Generally, carbon-based mterials, not only acts as a support for nanocrystals but also as a conductive material for the transfer of electrons in composite catalyst systems. However, ionic liquids (ILs) conductive binder is a promising material used in the electrochemistry field, which has shown the advantages of fast electron transfer and sensitivity, high conductivity and excellent 5

antifouling ability for electroanalysis [25, 26]. Composite nanomaterials which are made up of ILs in combination with bimetal oxide have attracted significant attention for their novel electronic and catalytic properties [27, 28]. By substituting carbon-based material with suitable ILs conductive binder, low cost and easy-to-fabricate electrodes have been produced. However, only a few studies have reported on the application of the porous NiCo2O4 nanoplates couple with 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]-ILs) as sensing materials for the detection of metal ions. Therefore, more research work needs to be finished in this field. In this work, a new sensing material has been fabricated by combining the porous NiCo2O4 hexagonal nanoplates and the conductivity of [BMIM][PF6]-ILs (ILs@NiCo2O4-P) for the electrochemical simultaneous determination of Tl+, Pb2+ and Cu2+ by DPASV. ILs@NiCo2O4-P composite is designed with the ultimate aim of enhancing the electronic conductivity, increasing electroactive sites, preventing interparticle agglomeration and accumulating targets on the surface of electrode. The electrocatalytic activity of ILs@NiCo2O4-P modified electrode was compared with ILs@NiCo2O4-N, ILs@NiCo2O4-S, and their single nickel cobaltate such as NiCo2O4-N, NiCo2O4-P and NiCo2O4-S. Remarkably, the as-synthesized ILs@NiCo2O4-P showed enhanced electrochemical performances in the determination of metal ions. Finally, the fabricated sensor was successfully applied to detect trace Tl+, Pb2+ and Cu2+ in real samples with satisfactory results. 6

2. Experiment Section. 2.1. Materials and Chemicals. CoCl2·6H2O, Ni(Ac)2·4H2O, Tl2SO4, Pb(NO3)2 and Cu(NO3)2 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shenyang, China). Cetyltrimethylammonium bromide (CTAB) were obtained from Aoboxing Biochemical Technology Co., Ltd. (Beijing, China). ILs (1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6]) was purchased from Chengjie Chemical Co., Ltd. (Shanghai, China). The 5% Nafion solution (D520) was from HESEN Co., Ltd. (Shanghai, China). Acetate buffer (HAc–NaAc) solutions of 0.1 M for different pHs were prepared by mixing stock solutions of 0.1 M NaAc and HAc. All reagents were of analytical grade, and all solutions were prepared by using ultrapure water. 2.2. Apparatus. Electrochemical measurements were performed with a CHI660D electrochemical workstation (Chenhua Instrument Company, Shanghai, China) using the typical three-electrode system, which contains an Ag/AgCl (3M KCl) reference electrode, a platinum wire auxiliary electrode and a modified or bare glassy carbon electrode (GCE) as working electrode. Scanning electron microscopy (SEM, Hitachi SU8000, Japan) and transmission electron microscopy (TEM, JEM-2010, Japan) were applied to observe the morphology, size and microstructure of the materials. X-ray diffraction (XRD) patterns were recorded on Siemens D5000 Diffractometer (Germany) using 7

Cu Kα radiation. Fourier transform infrared spectroscopy (FTIR, Avatar 330, Nicolet, Thermo Electron Scientific, Madison, WI) was used to characterize NiCo2O4-P and ILs@NiCo2O4-P within a 400−4000 cm−1 spectral domain using KBr pellet technique with a resolution of 4 cm−1 at room temperature. The micromeritics Tristar 3020 apparatus (Norcross, GA) was used to estimate the BET surface areas from N2 adsorption-desorption isotherms. S−3C Model pH meter (Shanghai Precision Scientific Instrument Co., China) was used to measure the pH of solutions. 2.3 Synthesis of different morphology of NiCo2O4 and ILs@NiCo2O4 composite. NiCo2O4-P was synthesized via a facile one-step hydrothermal process followed by further calcination treatment. Typically, 0.249 g Ni(Ac)2·4H2O, 0.476 g CoCl2·6H2O and 0.1 g CTAB were dissolved in 40.0 mL deionized water under magnetic stirring for 60 min to obtain a light pink colored solution. Followed, 3.2 g NaOH was added to above solution and stirred for 10 min. The resulting mixture was then poured into a Teflon-lined stainless steel autoclave, sealed and kept at 160 °C for 20 h. Then, the product was cooled to room temperature, collected by centrifugation and washed repeatedly with deionized water and ethanol, and dried at 60 ◦C overnight. Finally, the as-prepared gray precursors were calcined at 400 ◦C for 2 h in air atmosphere in order to obtain black powders. For comparison, other two different morphology of NiCo2O4 (NiCo2O4-S and NiCo2O4-N) were synthesized by previous reported methods [29, 30]. ILs@NiCo2O4-P composite was fabricated via simple physical method. 15.0 mg 8

NiCo2O4-P and 15.0 μL ILs were added into a mortar and ground for 30 min, then the fabricated products were collected. For comparison, other two different morphology of ILs@NiCo2O4 composites were fabricated by the above method. 2.4. Preparation of the modified electrodes. Prior to use, the bare GCE was polished with alumina powder (0.05, 0.3, and 1.0 μm) on polishing cloth, respectively, and subsequently rinsed with deionized water and ethanol by ultrasonication to remove the adsorbed substance and then dried in air. For the preparation of the modified electrode, a 3.0 mg/mL well-distributed ILs@NiCo2O4-P dispersion was prepared with 1% nafion solution by ultrasonication for 30 min. Then the 8.0 μL ILs@NiCo2O4-P dispersion was cast onto the surface of the GCE, and allowed to dry at the infrared lamp to obtain the ILs@NiCo2O4-P/GCE. For comparison, different morphology NiCo2O4/GCE and ILs@NiCo2O4/GCE was also prepared. 2.5. Electrochemical measurements. DPASV was chosen for metal ions detection in 5.0 mL 0.1 M pH 4.0 HAc–NaAc solution under optimized conditions. Standard solutions of Tl+, Pb2+ and Cu2+ were added to the electrochemical cell and accumulated at the potential of −1.1 V for 120 s. Subsequently, the DPASV responses were recorded in the potential range of −1.2 to +0.4 V with a step potential of 4 mV and an amplitude of 50 mV. Before analysis, the 5.0 mL acetate buffer solution was deoxygenated by purging with highly purified nitrogen for 10 min. All electrochemical experiments were performed at room 9

temperature.

3. Results and discussion 3.1. Characterization of Electrode Materials. It is well known that the structures feature of NiCo2O4 significantly affect its electrocatalytic activity. The different morphologies of NiCo2O4 electrocatalysts, consisting of nanoparticles, nanoplates and microspheres were synthetized. Fig. 1A displayed that NiCo2O4-N easily aggregated together, showing the irregular spherical formation of 20–30 nm in diameter. The severe aggregation will affect the electrocatalytic activity of NiCo2O4-N. As shown in Fig. 1B, the morphology of the three dimensional (3D) hierarchical NiCo2O4-S was observed by SEM, the obtained microspheres are uniform with a diameter of about 2 μm. It was clearly observed that the microspheres have dandelion-like hierarchical structures with a large number of small nanorods radially grown on the surface. Fig. 1C showed the morphology of regular hexagonal NiCo2O4-P precursors, however, no changes was found for the shapes of hexagonal nanoplates NiCo2O4 after calcinations treatment in Fig.1D. In detail, the diameter and average thickness of the nanoplates were 100 nm and 25 nm, respectively. The morphology and microstructure of the NiCo2O4-P were further characterized by TEM. The TEM image in Fig.1E also showed a uniform distribution of hexagonal-shaped NiCo2O4-P. Further, the high magnification image of NiCo2O4-P (Fig. 1F) revealed that many pores could be clearly distinguished on the hexagonal nanoplates, which were beneficial for the improvement of electrochemical

10

performance [31]. Obviously, the pores were presumably generated from the decomposition of hydroxide to oxide during calcination at 400 ◦C [32]. The crystalline phases of NiCo2O4-N, NiCo2O4-P and NiCo2O4-S were studied by XRD patterns, as shown in Fig. 2A. The obtained diffraction peaks at 2θ values of 19.0◦, 31.3◦, 36.9◦, 44.9◦, 59.4◦ and 65.3◦ matched well with the (111), (220), (311), (400), (511) and (440) crystal planes of NiCo2O4 with a cubic spinel structure, which were consistent with the standard JCPDS (JCPDS No. 73-1702), suggesting the successful synthesis of NiCo2O4 [33]. The products were highly pure because no peaks of other crystallized phases could be observed from these patterns, and XRD spectra results showed that NiCo2O4 with three different morphologies had shown similar diffraction peaks. Typically, the BET surface area is an important factor for the electroactive materials, which can significantly affect their electrocatalytic activity. The specific surface areas of NiCo2O4-P, NiCo2O4-S and NiCo2O4-N were investigated by N2 adsorption/desorption analyses, as shown in Fig. 2B. The BET surface areas of NiCo2O4-N, NiCo2O4-P and NiCo2O4-S were 23.1 m2g−1, 103.7 m2g−1 and 65.8 m2g−1, respectively. Among them, the BET surface area of NiCo2O4-P was the largest, which may contribute to the excellent catalytic activity and adsorption capacity. 3.2 The selection and electrochemical activity of sensing materials. For comparison, the electrochemical behavior of Tl+, Pb2+ and Cu2+ on the three different morphologies of NiCo2O4 modified electrodes was studied in pH 4.0 11

HAc-NaAc buffer solution by DPASV, and the results were presented in Fig. 3. The sensitivity was enhanced when the GCE was further modified with NiCo2O4-N, NiCo2O4-S or NiCo2O4-P. The highest peak currents in this series of sensing materials was observed on the NiCo2O4-P/GCE. The increase in sensitivity could be due to the high surface area and porous structure of the NiCo2O4-P. It is well known that high specific surface area can provide more electroactive sites and adsorbed more analytes. Furthermore, these pores can also allow electrons to transport within their internal pore channels, which would enhance electrocatalytic activity. Thus, NiCo2O4-P was selected as electrode material in the following work. Additionally, it was found when ILs was anchored onto the surface of NiCo2O4 ( as form ILs@NiCo2O4-N, ILs@NiCo2O4-S, ILs@NiCo2O4-P), ILs@NiCo2O4/GCE (Fig. 3, solid line) afforded the further bigger oxidation current than those at NiCo2O4/GCE under the same conditions (dotted line), and ILs can improve dispersion of NiCo2O4 (Fig. S1, SEM, TEM). Remarkably, ILs@NiCo2O4-P exhibited the highest electrocatalytic activity in determination of three metal ions. The presence of ILs can prohibit NiCo2O4-P aggregating, and help to accumulate the target analytes on the surface of modified electrode. Moreover, ILs as modifiers /binder can form a compatible film with the higher conductivity and accelerate the electron transportation. The structural information of ILs@NiCo2O4-P was further confirmed through FTIR. As shown in Fig. S2 (curve a), the absorption bands located at 3424 and 1627 cm-1 were respectively attributed to -OH stretching and bending modes of absorbed water 12

molecules. Furthermore, the absorption band at around 1190 cm-1 was corresponding to C-N stretching vibration which was due to the deeply trapped CTAB molecules derived from the synthesis process of NiCo2O4-P. The stretching vibration of Metal–O (Co−O or Ni−O) appeared at 689 cm-1 and 569 cm-1 [34]. It could be seen from curve b, the characteristic absorption peaks of [BMIM][PF6]-ILs at 846 cm−1, 1461 cm−1, 3168 cm−1 and 2930 cm−1 corresponding to PF6−, N−H and C−H stretching vibrations. As expected, all of the main absorption peaks of NiCo2O4-P and ILs appeared in the FTIR spectrum of ILs@NiCo2O4-P, suggesting that ILs was successfully coated onto the NiCo2O4-P surface. Therefore, ILs@NiCo2O4-P/GCE can be selected to use for the investigation of the electrochemical behaviors of Tl+, Pb2+ and Cu2+ in this study. 3.3. Electrochemical Characterization of the ILs@NiCo2O4-P/GCE. The electrodes were electrochemically characterized using cyclic voltammetry (CV) technique in neutral solution of 0.1 mM of [Fe(CN)6]3-/4- containing 1.0 M KCl (Fig. 4A), and the results were compared to those obtained from the bare GCE. Comparing with the bare GCE, a pair of well-defined redox peaks was observed on the ILs/GCE with the peak-to-peak separation (ΔEp) of 0.065 V, which was attributed to good conductivity of ILs. After casting NiCo2O4-P on GCE, the modified electrode displayed higher currents, which can be ascribed to the better electrochemical catalytic behavior, low mass transfer resistance and large surface area. The highest currents were observed with the ILs@NiCo2O4-P/GCE. This revealed that the synergistic effect of NiCo2O4-P and ILs may improve the electrochemical sensitivity 13

and the conduction pathways on the electrode surface, contributing to the acceleration of the electron transfer process. Interestingly, the ΔEp of the ILs@NiCo2O4-P/GCE (ΔEp = 0.076 V) decreased compared to ΔEp of the NiCo2O4-P/GCE (ΔEp = 0.129 V), further indicating that the presence of NiCo2O4-P and ILs improved electron transfer kinetics. Electrochemical impedance spectra (EIS) was also employed to assess the interface properties of the modified electrodes (Fig. 4B). In a typical EIS, the electron-transfer resistance (Rct) is equal to the diameter of the semicircle, which controls the redox process of the probe. The Rct value of bare GCE (curve a), ILs/GCE (curve b), NiCo2O4-P/GCE (curve c) and ILs@NiCo2O4-P/GCE (curve d) were got as 530 Ω, 286 Ω, 166 Ω and 119 Ω, respectively. The electron transfer resistance was gradually decreased, indicating that the existence of ILs, NiCo2O4-P and ILs@NiCo2O4-P on the surface of electrode could greatly decrease the resistance of electrode interface due to the good conductivity, suggesting that the ILs@NiCo2O4-P composite could be used as an excellent electron transfer medium to promote electron transfer. These results were consistent with above CV data. The effective surface area of electrode can be studied by chronocoulometry in 0.1 mM [Fe(CN)6]3-/4- containing 1.0 M KCl (as shown in Fig. 4C). Based on the slope of plot of Q versus t1/2 shown in the Fig. 4D, the effective surface area of electrode could be calculated according to the following equation [35]:

Q(t ) 

2nFAcD 1/2t 1/2  Qdl  Qads π 1/2 14

(1)

where A is effective surface area of the electrode, c is the concentration of substrate, n is the number of electrons transferred, D is the standard diffusion coefficient of [Fe(CN)6]3-/4- (7.6×10−6 cm2 s-1 [36]), Qdl is the double-layer charge, and Qads is the Faradaic charge. F is the Faraday constant. By calculating, effective surface area of electrode was 0.0083, 0.0355, 0.0688 and 0.0973 cm2 for bare GCE, ILs/GCE, NiCo2O4-P/GCE and ILs@NiCo2O4-P/GCE, respectively. The effective surface area of the ILs@NiCo2O4-P/GCE was nearly 11.7 times as the bare GCE. The results showed that the modification of ILs@NiCo2O4-P could significantly increase electrode effective surface area. The enlarged effective electrode surface could increase the electrode active sites, magnify the current response of targets, improve the adsorption capacity, thence, enhanced the sensitivity. 3.4. Electrochemical detection of Tl+, Pb2+ and Cu2+. Fig. 5 showed DPASV of 50.0 μg L-1 Tl+, Pb2+ and Cu2+ at the bare/GCE (a), ILs/GCE (b), NiCo2O4-P/GCE (c) and ILs@NiCo2O4-P/GCE (d) in 0.1 M HAc–NaAc solution (pH 4.0) at −1.1 V for 120 s. From Fig. 5, weak peak current responses were observed on the bare/GCE, because it is difficult to deposit target metals onto the surface of bare GCE. The peak currents of metal ions at the ILs/GCE were higher than that at bare GCE. This might be that the high conductive ILs decreased the electron transfer barrier and accelerated the accumulation rate of metal ions on the electrode surface. An obvious increase in the peak currents could be observed at NiCo2O4-P/GCE, indicating that the higher surface area of the NiCo2O4-P increased 15

the effective surface area of the electrode and provided more electroactive sites for the deposition of metal ions. The highest peak currents were observed on the ILs@NiCo2O4-P/GCE with well-defined peaks at −0.784 V, −0.532 V, and −0.036 V accountable for Tl+, Pb2+ and Cu2+, respectively. Besides, a small and weak peak was observed between the peak of Cu2+ and Pb2+, which may be due to the formation of Cu−Pb intermetallic compounds or catalytic hydrogen evolution at the Cu-coated surface [37]. The enhanced electrochemical performance could be attributed to the strong synergistic effect between the NiCo2O4-P (e.g. large surface area, good adsorbability and conductivity) and ILs (e.g. high ionic conductivity and good dispersibility). These results suggested that ILs@NiCo2O4-P/GCE was suitable for the simultaneous electrochemical detection Tl+, Pb2+ and Cu2+. 3.5. Optimization of detection conditions. In order to achieve the maximum sensitivity with ILs@NiCo2O4-P/GCE towards Tl+, Pb2+ and Cu2+, various experimental conditions (pH value, deposition potential and time) were optimized in 0.1 M HAc–NaAc solution containing 10.0 μg/L each of Tl+, Pb2+ and Cu2+. The pH value have a significant effect on the voltammetric response, therefore, it is necessary to select a appropriate pH. The electrochemical responses of the three metal ions from pH 3.0 to 6.0 were studied by DPASV and the results were shown in Fig. S3A. The results obtained showed that the peak currents of Tl+, Pb2+ and Cu2+ increased with pH rising and reached a maximum at pH 4.0, and then decreased with 16

the continuous increase of pH to 6.0. Therefore, pH 4.0 was selected as optimal pH for all sensing experiments. Accumulation can affect the amount of metal ions adsorbed on the electrode surface, and then affect sensitivity determination. Fig. S3B illustrated the effect of deposition potential in the potential range of −1.3 V to −0.8 V. When the deposition potential shifted to more negative, metal ions are more easily reduced on the electrode, and the maximum peak current was observed at −1.1 V for Tl+, Pb2+ and Cu2+. When the deposition potential was more negative, decrease in peak currents were observed due to the competitive generation of H2. Thus, a deposition potential of −1.1 V was selected for the subsequent experiment. The deposition time is another critical factor which also has a significant effect on both sensitivity and the detection limit. Thus, the dependence of peak currents on the deposition time was studied from 30 to 210 s. As shown in Fig. S3C, with the increase of deposition time, the peak currents of three metal ions enhanced, and the maximum were found at 120 s due to the increased amount of metal ions during accumulation on the electrode surface. On further increase in deposition time, the peak currents of three metal ions decreased gradually, indicating that the electrode surface became saturated by the metal ions [38]. Thus, 120 s was chosen to be the optimal deposition time. 3.6. Analytical performance. Under the optimal conditions, the ILs@NiCo2O4-P/GCE was applied for individual and simultaneous determination of TI+, Pb2+ and Cu2+ by DPASV. Fig. S4 17

showed the DPASV responses of the three metal ions respectively, and the corresponding calibration curve was derived accordingly (inset in Fig. S4). Fig. S4A presented a well-defined peak of Tl+ which the peak currents exhibited a good linear relationship with the concentration from 0.1 to 100.0 μg/L, but there are two linear sections in the regression line which fit the following equations: for concentrations in the range from 0.1 to 10.0 μg/L: I p = 0.4966c + 0.2538 (R2 = 0.999); for concentrations in the range from 10.0 to 100.0 μg/L: I p = 0.2640c + 1.9731 (R2 = 0.999). The limit of detection (LOD) was calculated (S/N = 3) as 0.040 μg/L. Besides, the DPASV responses of Pb2+ and Cu2+ over concentrations that ranged from 0.1 to 100.0 μg/L and from 0.05 to 100.0 μg/L were shown in Fig. S4B and C, respectively. The linearization equations for Pb2+ and Cu2+ were I p = 0.6670c + 0.2198 (R2 = 0.999), I p = 0.4824c + 0.3069 (R2 = 0.995) for the range of 0.1(0.05) to 10.0 μg/L, and I p = 0.3412c + 2.5302 (R2 = 0.997), I p = 0.2653c + 2.0977 (R2 = 0.998) for the range of 10.0 to 100.0 μg/L. The LOD (S/N = 3) of 0.031 μg/L was for Pb2+ and 0.027 μg/L for Cu2+. Therefore, the results obtained showed that the ILs@NiCo2O4-P/GCE could act as a good platform for the electrochemical determination of three metal ions. Simultaneous determination of Tl+, Pb2+ and Cu2+ was investigated at the same concentration with the deposition potential of −1.1 V for 120 s and the results were shown in Fig. 6. As shown, the separation between the voltammetric peaks of three metal ions was sufficiently large, therefore, the simultaneous and selective determination of mixed metal ions using the ILs@NiCo2O4-P/GCE was feasible. The 18

peak currents increased proportionally with increasing concentrations of TI+, Pb2+ and Cu2+. The calibration curves of these targets were presented in Fig. 6B. In the concentration range of 0.1(0.05) to 10.0 μg/L: the linearization equations were evaluated as I p = 0.4646c + 0.1971 (R2 = 0.998), I p = 0.7893c + 0.2619 (R2 = 0.997) and I p = 0.5247c + 0.1369 (R2 = 0.998) for Tl+, Pb2+ and Cu2+; from 10.0 to 100.0 μg/L: I p = 0.1963c + 2.7351 (R2 = 0.993), I p = 0.2262c + 5.0349 (R2 = 0.994) and I p = 0.2657c + 1.7068 (R2 = 0.998), respectively. The LODs were calculated (S/N =

3) to be 0.046 μg/L for Tl+, 0.034 μg/L for Pb2+ and 0.029 μg/L for Cu2+. Obviously, the sensitivities in simultaneous determination of Tl+, Pb2+ and Cu2+ were significantly decreased due to formation of intermetallic compound and the competitive adsorption on the surface of electrode. However, compared with other electrode materials reported previously in electrochemical determination of metal ions (Table 1), both the LODs and the linear range of the proposed electrode showed promising advantages. 3.7. Reproducibility, stability and selectivity. In order to evaluate the reproducibility of the ILs@NiCo2O4-P/GCE, a series of 12 time parallel experiments for 50.0 μg/L Tl+ in 0.1 M pH 4.0 HAc–NaAc solution was performed. Fig. 7 showed the peak currents were almost constant on ILs@NiCo2O4-P/GCE with a relative standard deviation (RSD) of 3.94%. The results indicated that ILs@NiCo2O4-P/GCE had good stability for repeated electrochemical detection. Moreover, the electrode signal retained to 93% of its initial response after 19

the modified electrode was stored for two weeks at room temperature. The interference study was examined by adding some possible interferents into HAc–NaAc solution containing 50.0 μg/L each of Tl+, Pb2+ and Cu2+ under the optimized conditions. The experimental results revealed that 500-fold many common inorganic ions such as Cl−, K+, PO43−, SO42−, NO3−, Na+, Mg2+, Ca2+ and Al3+ had no remarkable influences on the peak currents of Tl+, Pb2+ and Cu2+. This may be attributed to the fact that they were normally inactive by voltammetry. Zn2+ has been found no interference the concentration was increased to 20-fold. Hg2+ could increase the stripping peak currents of metal ions due to the formation of amalgam at the electrode surface. In addition, 5-fold concentrations of Cd2+ was found to significantly influence the peak current of Tl+ because of the proximity of their corresponding oxidation potentials. 3.8. Real sample analysis. To further evaluate the practicality and feasibility of the ILs@NiCo2O4-P/GCE, river water (from Xinkai River, Shenyang, China), sea water (from Dalian, China), and snow water and soil samples (from discard smelter, Fushun, China) were selected as real samples for quantitative analysis. Air-dried soil samples were stirred for 60 min with deionised water at room temperature, and then the sample solutions were filtrated. The soil suspension and water samples were filtered with a 0.45 μm membrane, and the pH was adjusted with HAc and NaAc. The experimental results were shown in Table 2. The spiked sample recoveries were approximately 20

92.9–105.9%, 89.2–104.9% and 88.5–106.4% for Tl+, Pb2+ and Cu2+, respectively, at different spiked levels (10.0 and 50.0 μg/L). These results showed that the proposed ILs@NiCo2O4-P/GCE had significant potential practical applications.

4. Conclusions In this study, a novel ILs@NiCo2O4-P composite as sensing material has been successfully prepared via a facile hydrothermal reaction followed by further calcination treatment and coating process using ILs. The present approach is an easy method to modify electrode. Owing to the porous characteristics of NiCo2O4-P, and the synergistic effect of NiCo2O4-P and ILs, the ILs@NiCo2O4-P composite displays distinctly enhanced electrocatalytic activity towards the electrochemical process of Tl+, Pb2+ and Cu2+. Furthermore, the proposed electrode was successfully used to determinate metals ions in practical samples with satisfactory results. It is believed that the ILs@NiCo2O4-P/GCE has promising applications in the convenient and fast determination of metal ions in environmental systems.

Acknowledgements This project was supported by science and technology foundation of ocean and fisheries of Liaoning province (201408, 201406), General project of scientific research of the education department of Liaoning province (L2015206), Liaoning scientific instruments service sharing information platform ability construction funds (201507A003), Liaoning provincial department of education innovation team projects (LT2015012) and the foundation of 211 project for innovative talent training, 21

Liaoning university. The authors also thank their colleagues and other students who participated in this study.

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27

Figure captions Fig. 1. SEM images of (A) NiCo2O4-N, (B) NiCo2O4-S, (C) NiCo2O4-P precursor and (D) pure NiCo2O4-P; TEM images of (E) NiCo2O4-P and the magnified view of the hexagonal nanoplates (F). Fig. 2. (A) XRD patterns and (B) N2 adsorption–desorption isotherm of three different morphology of NiCo2O4 samples. Fig. 3. (A) DPASVs of 50.0 μg/L Tl+, Pb2+ and Cu2+ on GCE modified by NiCo2O4 and ILs@NiCo2O4 composites with different morphologies in pH 4.0 HAc-NaAc solution. Deposition potential, −1.1 V; deposition time, 120 s. Fig. 4. (A) CVs of bare GCE (a), ILs/GCE(b), NiCo2O4-P/GCE (c) and ILs@NiCo2O4-P/GCE (d) in 0.1 mM [Fe(CN)6]3-/4- containing 1.0 M KCl; (B) EIS curves of bare GCE (a), ILs/GCE(b), NiCo2O4-P/GCE (c) and ILs@NiCo2O4-P/GCE (d) in 10 mM [Fe(CN)6]3-/4- containing 0.1 M KCl; (C) Plot of Q–t curves of bare GCE (a), ILs/GCE (b), NiCo2O4-P/GCE (c) and ILs@NiCo2O4-P/GCE (d) in 0.1 mM [Fe(CN)6]3-/4- containing 1.0 M KCl. (D) Plots of Q–t1/2 curves. Fig. 5. DPASVs of 50.0 μg/L Tl+, Pb2+ and Cu2+ in 0.1 M HAc−NaAc solution (pH 4.0) on the bare GCE (a), ILs/GCE(b), NiCo2O4-P/GCE (c) and ILs@NiCo2O4-P/GCE (d). Deposition potential, −1.1 V; deposition time, 120 s. Fig. 6. (A) DPASV responses of the ILs@NiCo2O4-P/GCE for the simultaneous analysis of Tl+, Pb2+ and Cu2+ under different metal ions concentrations (0.1-100

28

μg/L for Tl+ and Pb2+; 0.05-100 μg/L for Cu2+) in 0.1 M HAc–NaAc solution (pH 4.0), respectively. (B) The corresponding calibration plots toward Tl+, Pb2+ and Cu2+ respectively. Deposition potential, −1.1 V; deposition time, 120 s. Error bar: n=3. Fig. 7. The stability of 12 times repetitive measurements of DPASV responses for 50.0 μg/L Tl+ on the ILs@NiCo2O4-P/GCE in 0.1 M NaAc−HAc solution (pH 4.0).

Table caption Table 1 Comparison of different electrodes for determination of Tl+, Pb2+ and Cu2+. Table 2 Results for the determination of Tl+, Pb2+ and Cu2+ in real samples.

29

Fig. 1. SEM images of (A) NiCo2O4-N, (B) NiCo2O4-S, (C) NiCo2O4-P precursor and (D) pure NiCo2O4-P; TEM images of (E) NiCo2O4-P and the magnified view of the hexagonal nanoplates (F).

30

(A)

400

511

440

Intensity(a.u.)

NiCo2O4-P

NiCo2O4-S

NiCo2O4-N

10

20

30

40 50 2θ(degree)

60

400

(B)

-1

220

3

111

Quantity Adsorbed (cm g STP)

311

70

300

2 -1

NiCo2O4-P

SBET=103m g

NiCo2O4-S

SBET=65.8m g

2 -1

2 -1

NiCo2O4-N SBET=23.1m g 200

100

80

0 0.0

0.2

0.4 0.6 0.8 Relative pressure (p/p0)

1.0

Fig. 2. (A) XRD patterns and (B) N2 adsorption–desorption isotherm of three different morphology of NiCo2O4 samples.

31

40

ILs@NiCo2O4-P NiCo2O4-P

35

ILs@NiCo2O4-S

2+

Pb

Current/μA

30

Tl

+

2+

Cu

NiCo2O4-S ILs@NiCo2O4-N NiCo2O4-N

25 20 15 10 5

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Potential/V

Fig. 3. (A) DPASVs of 50.0 μg/L Tl+, Pb2+ and Cu2+ on GCE modified by NiCo2O4 and ILs@NiCo2O4 composites with different morphologies in pH 4.0 HAc-NaAc solution. Deposition potential, −1.1 V; deposition time, 120 s.

32

700

(B)

20

d c

10

b

c

b a

a

400 300

-10 -20

a - bare GCE b - ILs/GCE c - NiCo2O4-P/GCE

-30 -40

200

a - bare GCE b - ILs/GCE

100

c - NiCo2O4-P/GCE

d - ILs@NiCo2O4-P/GCE

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

d - ILs@NiCo2O4-P/GCE

0

-0.2

Potential/V

0

200

400

40

(C)

d

35

30 25

c

20

Charge/μC

30

Q/μC

d

500

0

35

(A)

600

Zˊˊ/ohm

Current/μA

30

25 20

15

b

600

800 1000 1200 1400

Zˊ/ohm

(D) Yd=58.4x+4.03 Yc=41.3x+2.17 Yb=21.3x+3.73 Ya=4.98x+1.41

d

c b

15

10

Equation

y = a + b*x

Weight

No Weighting

Residual Sum of Squares

7.26202

Pearson's r

0.98954

Adj. R-Square

0.97225

10

5

a

a

0

0.00

0.05

0.10

0.15

t/s

0.20

0.25

0.30

0.1

0.2

0.3

Time/s

0.4

1/2

0.5

0.6

Intercept

4.0348

B

Slope

58.448

C

Intercept

2.1748

C

Slope

41.298

D

Intercept

D

Slope

E

Intercept

1.408

E

Slope

4.984

ILs@NiCo2O4-P/GCE (d) in 0.1 mM [Fe(CN)6]3-/4- containing 1.0 M KCl; (B) EIS curves of bare GCE (a), ILs/GCE(b), NiCo2O4-P/GCE (c) and ILs@NiCo2O4-P/GCE (d) in 10 mM [Fe(CN)6]3-/4- containing 0.1 M KCl; (C) Plot of Q–t curves of bare GCE (a), ILs/GCE (b), NiCo2O4-P/GCE (c) and ILs@NiCo2O4-P/GCE (d) in 0.1 mM [Fe(CN)6]3-/4- containing 1.0 M KCl. (D)

33

0.97734

B

Fig. 4. (A) CVs of bare GCE (a), ILs/GCE(b), NiCo2O4-P/GCE (c) and

Plots of Q–t1/2 curves.

0.99147 Value

5

0

2.94879

3.73 21.246

45 40

Current/μA

35 30

(a) bare GCE (b) ILs/GCE (c) NiCo2O4/GCE (d) ILs@NiCo2O4/GCE 2+ Pb + Tl

Cu

2+

25 20 15 10 5 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

Potential/V

0.0

0.2

0.4

Fig. 5. DPASVs of 50.0 μg/L Tl+, Pb2+ and Cu2+ in 0.1 M HAc−NaAc solution (pH 4.0) on the bare GCE (a), ILs/GCE(b), NiCo2O4-P/GCE (c) and ILs@NiCo2O4-P/GCE (d). Deposition potential, −1.1 V; deposition time, 120 s.

34

50

(A)

Pb

30

2+

(B)

yCu =0.2657x+1.7068 2+

Cu

+

2+

yPb =0.2262x+5.0349 2+

Current/μA

Current/μA

Tl 30 20 10

20

yTl =0.1963x+2.7351 +

15 8 yCu =0.5247x+0.1369 yPb =0.7893x+0.2619 6 y =0.4646x+0.1971 2+

2+

Current/μA

40

25

10 5

Tl

+

4 2+

Cu 2

Pb Tl

2+

+

0 0

0 -1.2

0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential/V

0

20

40

60

2

4

6

8

Concentration(μg/L)

80

Concentration(μg/L)

10

100

Fig. 6. (A) DPASV responses of the ILs@NiCo2O4-P/GCE for the simultaneous analysis of Tl+, Pb2+ and Cu2+ under different metal ions concentrations (0.1-100 μg/L for Tl+ and Pb2+; 0.05-100 μg/L for Cu2+) in 0.1 M HAc–NaAc solution (pH 4.0), respectively. (B) The corresponding calibration plots toward Tl+, Pb2+ and Cu2+ respectively. Deposition potential, −1.1 V; deposition time, 120 s. Error bar: n=3.

35

25

RSD=3.94%

20 30 25

10 Current/μA

Curremt/μA

15

5 0

20 15 10 5

-5

-1.2

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

Potential/V

-10 0

2

4

6

8

Cycle number

10

12

Fig. 7. The stability of 12 times repetitive measurements of DPASV responses for 50.0 μg/L Tl+ on the ILs@NiCo2O4-P/GCE in 0.1 M NaAc−HAc solution (pH 4.0).

36

Table 1 Comparison of different electrodes for determination of Tl+, Pb2+ and Cu2+. Modified electrode

Linear range (μg/L) Tl+

Detection limit (μg/L)

Pb2+

L-MWCNT-CPE

0.52−145.0

NH2-CMS/GCE

82.9−248.6

Cu2+

Tl+

Pb2+

Cu2+

0.12 25.4−76.3

11.19

Refs.

[14] 0.07

[39]

Ag-Au/GCE

1.8−180.0

1.0−10.0

0.18

0.1

[40]

IL/G/CPE

0.26−40.9

0.26−41.4

0.073

0.093

[41]

Sb/GCE

5.0−100.0

2.0−100.0

2.0−50.0

1.0

1.5

0.5

[42]

Pd1.5/PAC-900/GCE

103.6−1844.1

31.8−317.8

10.36

4.19

[43]

SNAC/GCE

18.7−1181.0

5.7−305.0

1.18

1.47

[44]

0.1−100.0

0.05−100.0

0.034

0.029

This work

ILs@NiCo2O4/GCE

0.1−100.0

37

0.046

Table 2 Results for the determination of Tl+, Pb2+ and Cu2+ in real samples. Added

Founda (μg/L)

(μg/L)

Tl+

Pb2+

Cu2+

Tl+

Pb2+

Cu2+

Tl+

Pb2+

Cu2+

0.0

ndc

3.27

nd

-

-

-

-

-

-

10.0

9.33

13.76

9.65

93.3

104.9

96.5

1.61

2.22

2.36

50.0

46.46

50.15

47.92

92.9

93.76

95.8

2.36

3.22

3.54

0.0

nd

2.68

nd

-

-

-

-

-

-

10.0

10.02

12.39

10.64

100.2

97.1

106.4

1.34

3.11

2.94

50.0

50.49

49.59

51.97

101.0

93.8

103.9

3.11

3.42

1.24

0.0

nd

1.03

0.55

-

-

-

-

-

-

10.0

9.70

9.95

10.16

97.0

89.2

96.1

2.24

3.55

3.11

50.0

50.63

47.58

44.82

101.3

93.1

88.54

2.76

2.91

1.99

0.0

1.77

5.73

6.92

-

-

-

-

-

-

10.0

12.36

15.38

15.77

105.9

96.5

88.5

2.69

2.35

2.79

50.0

51.89

56.98

55.10

100.2

102.5

96.4

2.17

3.53

2.78

RSDb%

Recovery%

sample

Sea Water

River Water

Snow Water

Soil

a

Mean of three measurement.

b

Relative standard deviation for n=3.

c

nd: not detected.

38