Determination of copper(II) ion in food using an ionic liquids-carbon nanotubes-based ion-selective electrode

Accepted Manuscript Title: Determination of copper(II) ion in food using an ionic liquids-carbon nanotubes-based ion-selective electrode Authors: Yunchang Fan, Chen Xu, Rupeng Wang, Guitao Hu, Juan Miao, Kun Hai, Chong Lin PII: DOI: Reference:

S0889-1575(17)30120-5 http://dx.doi.org/doi:10.1016/j.jfca.2017.05.003 YJFCA 2893

To appear in: Received date: Revised date: Accepted date:

5-2-2017 22-4-2017 4-5-2017

Please cite this article as: Fan, Yunchang., Xu, Chen., Wang, Rupeng., Hu, Guitao., Miao, Juan., Hai, Kun., & Lin, Chong., Determination of copper(II) ion in food using an ionic liquids-carbon nanotubes-based ion-selective electrode.Journal of Food Composition and Analysis http://dx.doi.org/10.1016/j.jfca.2017.05.003 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.

Original Research Article Determination of copper(II) ion in food using an ionic liquids-carbon nanotubes-based ion-selective electrode

Yunchang Fana, Chen Xua, Rupeng Wangb, Guitao Hua, Juan Miaoc,*, Kun Haia, Chong Lina

a

College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo

454003, China b

School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003,

China c

Medical College, Henan Polytechnic University, Jiaozuo 454003, China

*Corresponding author. E-mail address: [email protected] Tel.: +863913987823, Fax: +863913987815.

Highlights 

An ionic liquid-based copper(II) ion-selective electrode was developed;



The proposed electrode had good selectivity and sensitivity;



The measurement accuracy of the proposed electrode could be comparable to GF-AAS.

1

ABSTRACT In this work, a copper(II) ion-selective electrode with an ionic liquid (IL), 1-(2-aminoethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide ([C4NH2C2im]NTf2), as active material has been developed. Parameters affecting the performance of the electrode, such as the dosage of the ILs, the amount of carbon nanotubes, and aqueous pH values, were investigated. The experimental results indicated that the optimal composition of the electrode filling material was 47.6% [C4NH2C2im]NTf2, 47.6% tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide (TBPNTf2) and 4.8% carboxylic multi-walled carbon nanotubes (MWCNTs-COOH). Under the optimal conditions, the developed electrode exhibited a good linear response over the concentration range of 10−10 to 10−5 mol L−1 and had a low limit of detection, 7.9 × 10‒11 mol L‒1. No obvious interference from common metal ions was found. The proposed electrode was applied to determine copper (II) ion in food samples and the results were comparable to graphite furnace atomic absorption spectrometry (GF-AAS).

Keywords: Ionic liquids (ILs); Copper(II) ion; Ion-selective electrode; Carbon nanotubes; Food samples; Food analysis; Food composition

1. Introduction Copper is usually regarded as an essential trace element but extremely toxic at higher levels (Haywood et al., 2016). The determination of trace copper in environmental and food samples is therefore of great importance. At present, atomic absorption spectrometry (AAS) (Yin et al., 2016; Dalali et al., 2012), atomic emission spectrometry (AES) (Li et al., 2013; Benzo et al.,

2

2002), stripping voltammetry (Dai et al., 2014; Domingos et al., 2016) and spectrophotometry (Liao et al., 2011; Turkoglu & Soylak, 2005) are usually used for this purpose. Although these methods are widely used in the laboratory, they have drawbacks, such as being time consuming, difficult to use and requiring expensive analytical instruments. The ion-selective electrode method, as an alternative, has been developed, due to its low cost, ease of operation, rapid response and good sensitivity (Ansari et al., 2012; Afkhami er al., 2014; Ghaedi et al., 2012; Baig & Khan, 2015; Issa et al., 2012; Birinci et al., 2016). Ansari et al. (2012) used a pencil graphite electrode coated with copper (II)–carmoisine dye complex in polyaniline matrix as a copper ion-selective electrode. The introduced electrode had high sensitivity (a detection limit of 2.0×10‒6 mol L‒1) and high selectivity. Birinci et al. (2016) developed a new solid contact copper selective electrode with a poly(vinyl chloride) (PVC) membrane containing o-xylylenebis(N,N-diisobutyldithiocarbamate) as ionophore. This electrode exhibited rapid response (< 10 s) and higher sensitivity (a detection limit of 4.9 × 10–7 mol L–1); no serious interference from common ions was found. Ionic liquids (ILs) are composed entirely of ions and stay in the liquid state over a wide temperature range. They have excellent electrical conductivity and high chemical and thermal stability and are ideal electrode materials in electrochemical analysis (Wardak, 2015; Ismaiel et al., 2014). Besides, carbon nanotubes (CNTs) have also been widely applied as fillers in ion-selective electrode methods, due to their high charge transfer capacity, hydrophobicity and chemical stability (Wardak, 2015; Parra et al., 2009; Crespo et al., 2008). The combined use of ILs and CNTs can produce new materials with excellent electrochemical properties. Wardak (2015) developed a new solid contact cadmium selective electrode by using the IL

3

1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6), and multi-walled carbon nanotubes (MWCNTs) as new membrane components; the resultant electrode had high potential stability, low detection limit (2.3×10–9 mol L–1) and excellent selectivity. In view of the fact that copper(II) ion (Cu2+) has strong binding ability to amino groups, it can be expected that amino functionalized ILs may have the ability to selectively identify the copper(II) ion. The aims of this work were thus to construct an amino functionalized IL-CNTs-based copper(II) ion-selective electrode and to investigate the analytical performance of this new electrode. It should be noted that the IL [C4NH2C2im]NTf2, is a liquid at room temperature and cannot be used alone as the electrode filling material. A solid-state compound is thus needed. Paraffin is usually used as the binder to prepare the electrode paste. However, paraffin and [C4NH2C2im]NTf2 are not miscible in each other; besides, paraffin is not conductive and thus weakens the electrochemical response (Ganjali et al., 2009; Isaac & Prabhakaran, 2016). Therefore, a solid-state IL, TBPNTf2, which has good intermiscibility with [C4NH2C2im]NTf2, was used as the binder.

2. Materials and methods 2.1. Reagents and chemicals Copper chloride dihydrate (≥ 99%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Chitosan (CS, with >95 % deacetylation), nitric acid (HNO3, 70%, electronic grade), hydrogen peroxide (H2O2, 30%, guaranteed reagent) and nano graphite powder (99%, 40 nm) were obtained from Aladdin Reagent Co. (Shanghai, China); carboxylic multi-walled

4

carbon nanotubes (MWCNTs-COOH, >95%, diameter 10‒20 nm, length 10‒30 nm, carboxyl content 2.0%) were purchased from Beijing Dk Nano Technology Co., Ltd. (Beijing, China). 2-Bromoethylamine hydrobromide (98%), 1-butylimidazole (98%) and lithium bis(trifluoromethanesulphonyl)imide (LiNTf2, 98%) were obtained from Energy Chemical Co., (Shanghai, China). Tetrabutylphosphonium bromide (TBPBr, 99%) was obtained from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (Lanzhou, China). All the other reagents were of analytical grade unless stated otherwise. Ultrapure water (18.2 MΩ cm) produced by an Aquapro purification system (Aquapro International Co., Ltd., Dover, DE) was used throughout the experiments. Food samples, including buckwheat powder, cowpea, sesame, soybean milk powder, bean curd stick, tofu skin and hazelnut, were purchased from local supermarket.

2.2. Instruments A disintegrator (model: 08A1, Xulang Co., Guangzhou, China) was used to crush the food samples to below 100 mesh. A microwave digestion system (model: MD6H, Aopule Co., Chengdu, China) was used to treat the food samples. A TAS-990AFG GF-AAS (Purkinje General Instrument Co., Beijing, China) equipped with a deuterium lamp background corrector, a copper hollow cathode lamp and a transversely-heated graphite tube was used to determine Cu2+. The detection wavelength was set at 324.8 nm. The heating program of the graphite furnace is listed in Table 1. For the potential measurements of Cu2+, a pHS-3B digital pH/mV meter (Shanghai Leici Instrument Factory, Shanghai, China) was employed. A saturated Hg/Hg2Cl2 was used as the reference electrode. The copper(II) ion-selective electrode and the

5

reference electrode were immersed into 10 mL of Cu2+ solution during the analytical processes.

2.3. Sample preparation All the food samples were treated via the reported microwave digestion method (Zhong et al., 2016; Silva et al., 2010). Briefly, 0.3000 g of a specific sample and 4.0 mL of concentrated HNO3 were added into a polytetrafluoroethylene microwave digestion tank and then 1.0 mL of H2O2 (30%) was added dropwise. After eliminating the bubbles, the digestion tank was sealed by a cap and put into the microwave digestion system. The digestion program was as follows: microwave power, 800 W; the temperature increases from room temperature to 120 oC within 5 min, remains constant for 5 min, thereafter increasing to 150 oC within 5 min and kept at that temperature for 10 min. After cooling down, the digestion solution was diluted to 25 mL with ultrapure water. Before measurements, the pH values of the sample solutions were adjusted to 7.0 with 0.1 mol L–1 NaOH. Blank experiments were conducted in a similar way.

2.4. Synthesis of the amino functionalized IL The amino functionalized IL, 1-(2-aminoethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide ([C4NH2C2im]NTf2), was synthesized by referring to the procedures described in previous works (Fan et al., 2012; Bates et al., 2002). Typically, 0.2 moles of 2-bromoethylamine hydrobromide were dissolved in 30 mL of anhydrous ethanol and then an equimolar amount of 1-butylimidazole was added; after refluxing for 24 h at 75 oC, ethanol was removed by vacuum-rotary evaporation. The resultant sticky liquid was dissolved into 30 mL of ultrapure water and the pH of the resultant solution was adjusted to 11.0 by the

6

addition of solid NaOH. Then, 0.2 moles of LiNTf2 were added and two immiscible phases were formed under stirring for 10 min. After removal of the upper water phase, the bottom (IL) phase was washed several times, each time with 10 mL of ultrapure water until Br‒ free, as indicated by the AgNO3 test of the water washings. After vacuum drying 24 h at 75 °C, [C4NH2C2im]NTf2 was obtained as a light yellow liquid (63% yield). 1H NMR (500 MHz, DMSO-d6): δ (ppm), 0.903‒0.938 (t, 3H), 1.270‒1.316 (m, 2H), 1.769‒1.814 (m, 2H), 2.921‒2.967 (m, 2H), 3.615 (s, 2H), 4.119‒4.142 (t, 2H), 4.163‒4.204 (t, 2H), 7.726 (s, 1H), 7.747 (s, 1H), 9.135 (s, 1H).

2.5. Synthesis of tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide (TBPNTf2) Generally, 0.2 moles of TBPBr were dissolved into 100 mL of water and then 0.2 moles of LiNTf2 were added. After stirring for 10 min, a solid-state product (TBPNTf2) was obtained. The resultant TBPNTf2 was washed several times, each time with 20 mL of ultrapure water until Br‒ free, as indicated by the AgNO3 test of the water washings. After vacuum drying 24 h at 75 °C, TBPNTf2 was obtained as a white solid (92% yield, melting point: 83‒85 oC). 1H NMR (500 MHz, DMSO-d6): δ (ppm), 0.904‒0.932 (t, 12H), 1.385‒1.488 (m, 16H), 2.143‒2.202 (m, 8H). 2.6. Preparation of the copper(II) ion-selective electrode The electrode filling material was prepared as follows: 47.6% (wt%, similarly hereinafter) of [C4NH2C2im]NTf2, 47.6% of TBPNTf2 and 4.8% of MWCNTs-COOH were mixed at 70 oC and subsequently sonicated for 20 min to obtain a homogenous mixture. The working electrode was prepared by packing the abovementioned electrode filling material (17.2 mg) into the end of a glass tube (openings at both ends, 2 mm inner diameter, 3 cm length). Electrical contact was

7

achieved by inserting a copper wire (8 cm length, 0.4 mm diameter) into the glass tube. This copper wire can move up and down to press the conductive material down when renewal of the electrode surface is needed. The electrode surface was smoothed and polished with a weighing paper. The electrode was activated by immersing it into 1.0 × 10–5 mol L–1 of Cu2+ for 3 h before use. 3. Results and discussion 3.1. Selection of the additives For the ion-selective method, besides the active material ([C4NH2C2im]NTf2 in this work), some additives such as multi-walled carbon nanotubes (MWCNTs) (Ghaedi et al., 2013; Afkhami et al., 2014) and graphite powder (Ganjali et al., 2009; Tutulea-Anastasiu et al., 2013) are usually used in order to improve the conductivity. Furthermore, chitosan (CS) is a natural macromolecule containing glucosamine groups, which can coordinate with metal ions and has also been used as additive to increase the sensitivity of the ion-selective electrodes (Panggabean, 2011; Kurniasih et al., 2012). Therefore, MWCNTs-COOH, nano graphite powder (GP) and CS were used additives and their performance was compared. The results shown in Fig. 1 indicate that the addition of MWCNTs-COOH or GP improves the sensitivity of the electrode, due to their good electrical conductivity (Marinho et al., 2012; Hong et al., 2013). The addition of CS decreases slightly the sensitivity of the electrode because of its poor conductivity (Kurniasih et al., 2012). In view of the fact that MWCNTs-COOH exhibited the best sensitivity, it was adopted in this work.

8

3.2. Effect of the component ratio As mentioned above, a mixture of MWCNTs-COOH, [C4NH2C2im]NTf2 and TBPNTf2 was used to fabricate the electrode. Their composition ratio (w/w, similarly hereinafter) thus influenced the performance of the proposed electrode. The effect of the ratio of TBPNTf2 to [C4NH2C2im]NTf2 was firstly studied by fixing the ratio of the mixture of TBPNTf2 and [C4NH2C2im]NTf2 to MWCNTs-COOH at 20:1. Generally, high contents of [C4NH2C2im]NTf2 will result in high sensitivity. As can be seen from Fig. 2 panel A, the electrode with TBPNTf2:[C4NH2C2im]NTf2 = 1:1 exhibited the highest sensitivity. The reasons lie in that [C4NH2C2im]NTf2 is a liquid and has fluidity; higher contents of [C4NH2C2im]NTf2 (e.g., TBPNTf2:[C4NH2C2im]NTf2 = 1:2 and without the addition of TBPNTf2 make the composite more mobile and unstable, subsequently reducing the sensitivity. When the ratio of TBPNTf2 to [C4NH2C2im]NTf2 increases up to 2:1, the sensitivity decreases, due to the decrease in the content of [C4NH2C2im]NTf2. Based on these results, a ratio of [C4NH2C2im]NTf2 to TBPNTf2,

1:1 was regarded as the optimum. Furthermore, the effect of the MWCNTs-COOH dosage on the electrode sensitivity was studied by fixing the ratio of [C4NH2C2im]NTf2 to TBPNTf2 at 1:1 and changing the ratios of the mixture of [C4NH2C2im]NTf2 and TBPNTf2 to MWCNTs-COOH. The results shown in Fig. 2 panel B indicate that the sensitivity of the proposed electrode increases with the increase of the content of MWCNTs-COOH up to 1:20 and then decreases above this level. The reasons lie in that with the increase of the content of MWCNTs-COOH, the electrical conductivity of the electrode increases; however, higher content of MWCNTs-COOH means that the content of the

9

active material, [C4NH2C2im]NTf2, decreases. Therefore, 20:1 was selected as the optimal ratio of the mixture of [C4NH2C2im]NTf2 and TBPNTf2 to MWCNTs-COOH.

3.3. Effect of pH on the electromotive force The effect of aqueous pH on the electromotive force is shown in Fig. 3. It is clear that the electromotive force keeps constant in the pH range of 7.0−11.0, which can be regarded as the working pH range for the electrodes. It is known that the amino group can be protonated in acidic conditions, which reduces the binding affinity between amino group and Cu2+ (Barakat, 2011). This is why the electrode has lower response at pH 5.0−6.0. Furthermore, Cu2+ will form a complex with OH‒ under strong basic conditions (e.g., pH 12.0) (Barakat, 2011), which also results in a decrease of the electrode response. In addition, there is no difference in sensitivity between the copper(II) solution controlling the pH by phosphate buffer and that controlling the pH by HNO3 (0.1 mol L−1) or NaOH (0.1 mol L−1). Based on these observations, pH 7.0 was regarded as the optimal condition and the pH values of copper(II) solutions were adjusted by HNO3 or NaOH solution for the following studies.

3.4. Effect of the conditioning time Generally, a new electrode should be conditioned before use (Turkoglu & Soylak, 2005; Ansari et al., 2012; Afkhami et al., 2014; Baig & Khan, 2015). As can be seen from Fig. 4, the electrode sensitivity increases with the increase of the conditioning time from 1 to 3 h. Compared with the response slope obtained at 3 h of conditioning time, the response slopes obtained at 4 h, 5 h, 6 h and 24 h of conditioning time only decrease by 6.4%, 7.2% 9.4% and 4.8%, respectively.

10

Generally, an error of around 10% is acceptable for the response slopes of the ion-selective electrode method (Juárez-Gómez et al., 2016). Therefore, the sensitivity of the electrode can be regarded as approximately constant in the range of 3 to 24 h of conditioning time. However, the conditioning time of the reported copper (II) ion-selective electrodes is 24 h or more (Ansari et al., 2012; Afkhami et al., 2014; Baig & Khan, 2015; Birinci et al., 2016). Obviously, short conditioning time is one of the advantages of the proposed electrode. The reason is most probably, due to the fast kinetics of the complexation of Cu2+ with [C4NH2C2im]NTf2. Based on this result, 3 h was regarded as the optimum.

3.5. Response time Response time is another important parameter for an ion-selective electrode. According to the IUPAC recommendations (Singh et al., 2014), response time is defined as the required time for the potential of an electrode to reach its steady-state value within ± 1 mV after the electrode is immersed in the Cu2+ solution. The response time of the proposed electrode was measured by changing the concentration of Cu2+ solution successively from 1.0 × 10−10 to 1.0 × 10−5 mol L−1. The experimental results indicated that over the entire concentration range, the proposed electrode reached its equilibrium responses in short time (about 9 s).

3.6. Reproducibility To evaluate the reproducibility of proposed electrode, two methods were adopted: (I) the proposed electrode was used to determine consecutively Cu2+ solutions (1.0 × 10−7 mol L−1) at 3-min intervals for 2 h (40 times). The experimental results showed that the relative standard

11

deviation (RSD) of the potential values for the forty successive determinations was 0.30%; (II) ten independent electrodes were prepared and their response slopes for Cu2+ were determined; the experimental results indicated that these electrodes showed similar response slopes with RSD of 3.7%. These results suggested the proposed electrode had good reproducibility.

3.7. Selectivity Selectivity is one of the most vital characteristics of an ion-selective electrode. In this work, the selectivity coefficients were determined by the fixed interference method (FIM), in which the selectivity coefficients were evaluated graphically for solutions of constant activity of the interfering ion (1.0 × 10−3 mol L−1), and varying activity of Cu2+ (Ghaedi et al., 2012; Ghaedi et al., 2013; Bakker et al., 2000). The intersection of the extrapolated linear portions of this plot indicates the value of a[S1]i that is to be used to calculate the selectivity coefficient from the following equation (Ghaedi et al., 2012, 2013; Bakker et al., 2000): FIM K Cu  2 ,j

i j

(1)

Zi Zj

FIM where K Cu2 , j is the selectivity coefficient; i, j, α and Z denote the target ion (Cu2+ in this

work), an interfering ion, ion activity and charge number of an ion, respectively. The resultant selectivity coefficients are listed in Table 2. The values of selectivity coefficients are all in the order of 10−4 to 10−3, suggesting that the proposed electrode is highly selective towards Cu2+ with respect to a variety of cations.

3.8. The lifetime and the reuse of the proposed electrode To study the lifetime, the proposed electrode was used to determine successively the Cu2+ 12

solution for two weeks. No remarkable changes in response slope were found and the RSD of the response slope was only 2.6%. However, after one month of use, the response slope of the proposed electrode decreased to 70.8% of its initial value. To reuse the proposed electrode, two methods could be adopted: (I) the electrode surface was polished to expose a new fresh layer, this electrode could be used to determine Cu2+ without loss of its sensitivity (the response slope was 95.1% of its initial value). (II) The used electrode was treated by constant potential reduction in NaCl aqueous solution (0.2 mol L‒1) at ‒0.5 V for 30 min. After this treatment, the response slope of the electrode could reach 97.9% of its initial value.

3.9. Working concentration range and detection limit Under the optimized conditions described above, the potentiometric response of the proposed electrode to Cu2+ was studied in the concentration range of 10‒12 to 10‒4 mol L‒1. The calibration curve of the Cu2+-selective electrode is shown in Fig. 5. As can be seen, the potential response of the proposed electrode and the logarithm of the Cu2+ concentration exhibit good linearity with a correlation coefficient (r) of 0.9954 in the concentration range of 10‒10 to 10‒5 mol L‒1. The detection limit of the proposed electrode was calculated according to the IUPAC recommendations (Baig & Khan, 2015; Birinci et al., 2016; Bakker et al., 2000) from the intersection of two extrapolated segments of the calibration curve (Fig. 5) and was found to be 7.9 × 10‒11 mol L‒1. The slope of the calibration curve is 8.17 mV decade–1, which severely deviated from the theoretical value predicted by the Nernst equation (29.5 mV decade–1 for bivalent cations). The possible reason may be that the IL cation, [C4NH2C2im]+, is capable of coordinating with Cu(II) (the ratio of metal to ligand is 1:6) due to the presence of the amino

13

group in [C4NH2C2im]+ (Fan et al., 2012); the behavior of the complexes, Cu([C4NH2C2im])68+, on the electrode surface is similar to octavalent cations. Additionally, Amemiya et al. (2003) reported that for the cation-selective electrodes, the formation of complexes between cations and ionophores tends to give apparently sub-Nernstian responses and the response slopes depend on the stoichiometries of the cation-ionophore complexes. Furthermore, many works have reported the use of ion-selective electrodes to determine Cu2+ (Ansari et al., 2012; Afkhami et al., 2014; Ghaedi et al., 2012; Baig & Khan, 2015). The performance of the proposed electrode was thus compared with the published ones and the results are listed in Table 3. As can be seen, the proposed electrode exhibits high sensitivity and rapid response.

3.10. Determination of Cu2+ in real samples To evaluate the applicability and the accuracy of the proposed electrode, the Cu2+ contents in real food samples were determined by the proposed electrode and GF-AAS method (Ghaedi et al., 2012; Baig & Khan, 2015; Issa et al., 2012; Birinci et al., 2016), respectively. It was found that the contents of Cu2+ in the digestion solutions ranged from 4.5 × 10–7 to 1.9 × 10–6 mol L–1, which are in the linear range of the calibration curve. The contents of Cu2+ in food samples ranged from 2.4 to 9.8 mg kg–1, details are in Table 4. For the statistical treatment of data, t test was performed at 95% confidence level. As shown in Table 4, the calculated t-values at 95% confidence level were smaller than the critical value (2.78), suggesting that there were no significant differences between the results obtained by the developed electrode and GF-AAS method. The recoveries obtained by the proposed electrode are not less than 97%, showing the

14

usability of the proposed electrode for the rapid and low-cost determination of Cu2+ content in complex samples.

4. Conclusions An amino functionalized IL can be used as active material in the development of copper(II) ion-selective electrodes. It was found that the composition of 47.6% [C4NH2C2im]NTf2, 47.6% TBPNTf2 and 4.8% MWCNTs-COOH exhibits the best response slope with a detection limit of 7.9 × 10‒11 mol L–1. The proposed electrode is characterized by fast response, reasonable stability and high sensitivity. Common metal ions do not affect the determination of copper(II) ion, suggesting that the proposed electrode has improved selectivity. The proposed electrode was also applied to determination of Cu2+ in food samples and the results obtained were in good agreement with the GF-AAS method. A comparison between the response characteristics of the proposed electrode and those of previously reported Cu(II) ion-selective electrodes indicates that the present electrode is superior.

Funding This research was supported by the National Natural Science Foundation of China (Nos. 21307028, 21401045) and the Project of Education Department of Henan province (No. 16B610007).

Ethical approval This article does not contain any studies with human or animal subjects

15

Conflict of interest The authors declare that there are no conflict of interest.

16

References Afkhami, A., Khoshsafar, H., Madrakian, T., Shirzadmehr, A. (2014). A new nano-composite electrode as a copper (II) selective potentiometric sensor. J. Iran. Chem. Soc. 11, 1373–1380. Amemiya, S., Bühlmann, P., Odashima, K. (2003). A generalized model for apparently “non-Nernstian” equilibrium responses of ionophore-based ion-selective electrodes. 1. independent complexation of the ionophore with primary and secondary ions. Anal. Chem. 75, 3329–3339. Ansari, R., Delavar, A.F., Aliakbar, A., Mohammad-khah, A. (2012). Solid-state Cu (II) ion-selective electrode based on polyaniline-conducting polymer film doped with copper carmoisine dye complex. J. Solid State Electrochem. 16, 869–875. Baig, U., Khan, A.A. (2015). Polyurethane-based cation exchange composite membranes: preparation, characterization and its application in development of ion-selective electrode for detection of copper(II). J. Ind. Eng. Chem. 29, 392–399. Bakker, E., Pretsch, E., Bühlmann, P. (2000). Selectivity of potentiometric ion sensors. Anal. Chem. 72, 1127–1133. Barakat, M.A. (2011). New trends in removing heavy metals from industrial wastewater. Arab. J. Chem. 4, 361–377. Bates, E.D., Mayton, R.D., Ntai, I., Davis, Jr, J.H. (2002). CO2 capture by a task specific ionic liquid. J. Am. Chem. Soc. 124, 926–927. Benzo, Z., Marcano, E., Gómez, C., Ruiz, F., Salas, J., Quintal, M., Garaboto, A., Murillo, M. (2002). Simultaneous determination of nickel, iron, and copper in margarine by inductively

17

coupled plasma–atomic emission spectroscopy after sample emulsification. J. AOAC Int. 85, 967–970. Birinci, A., Eren, H., Coldur, F., Coskun, E., Andac, M. (2016). Rapid determination of trace level copper in tea infusion samples by solid contact ion selective electrode. J. Food Drug Anal. 24, 485–492. Crespo, G.A., Macho, S., Rius, F.X. (2008). Ion-selective electrodes using carbon nanotubes as ion-to-electron transducers. Anal. Chem. 80, 1316–1322. Dai, X., Qiu, F., Zhou, X., Long, Y., Li, W., Tu, Y. (2014). Amino-functionalized mesoporous silica modified glassy carbon electrode for ultra-trace copper(II) determination. Anal. Chim. Acta 848, 25–31. Dalali, N., Ashouri, M., Nakisa, S. (2012). Solid phase extraction based on modified multi-walled carbon nanotubes packed column for enrichment of copper and lead on-line incorporated with flame atomic absorption spectrometry. J. Iran. Chem. Soc. 9, 181–188. Domingos, R.F., Carreira, S., Galceran, J., Salaün, P., Pinheiro, J.P. (2016). AGNES at vibrated gold microwire electrode for the direct quantification of free copper concentrations. Anal. Chim. Acta 920, 29–36. Fan, Y., Zhang, S., Yang, F., Niu, H., Shi, J. (2012). Extraction behavior of copper(II) ion with a hydrophobic amino-functionalized ionic liquid. Microchim. Acta 177, 237–243. Ganjali, M.R., Khoshsafar, H., Shirzadmehr, A., Javanbakht, M., Faridbod, F. (2009). Improvement of carbon paste ion selective electrode response by using room temperature ionic liquids (RTILs) and multi-walled carbon nanotubes (MWCNTs). Int. J. Electrochem. Sci. 4, 435–443.

18

Ghaedi, M., Khajehsharifi, H., Montazerozohori, M., Tavallali, H., Tahmasebi, K., Khodadoust, S. (2012). Designing and synthesis of bis(2,4-dihydroxybenzylidene)-1,6-diaminohexane and its efficient application as neutral carrier for preparation of new copper selective electrode. Mat. Sci. Eng. C 32, 674–679. Ghaedi, M., Montazerozohori, M., Sahraei, R. (2013). Comparison of the influence of nanomaterials on response properties of copper selective electrodes. J. Ind. Eng .Chem. 19, 1356–1364. Haywood, S., Boursnell, M., Loughran, M.J., Trafford, J., Isherwood, D., Liu, X., Olohan, L., Carter, S.D. (2016). Copper toxicosis in non-COMMD1 Bedlington terriers is associated with metal transport gene ABCA12. J. Trace Elem. Med. Biol. 35, 83–89. Hong, J., Yang, W.Y., Zhao, Y.X., Xiao, B.L., Gao, Y.F., Yang, T., Ghourchian, H., Moosavi-Movahedi, Z., Sheibani, N., Li, J.G., Moosavi-Movahedi, A.A. (2013). Catalase immobilized on a functionalized multi-walled carbon nanotubes–gold nanocomposite as a highly sensitive biosensing system for detection of hydrogen peroxide. Electrochim. Acta 89, 317–325. Isaac, R., Prabhakaran, P. (2016). Artificial neural network optimization of a carbon paste electrode for the detection of zinc ions. Nano Biomed. Eng. 8, 47–53. Ismaiel, A.A., Aroua, M.K., Yusoff, R. (2014). A new electrochemical sensor based on task-specific ionic liquids-modified palm shell activated carbon for the determination of mercury in water samples. Sensors 14, 13102–13113. Issa, Y.M., Ibrahim, H., Shehab, O.R. (2012). New copper(II)-selective chemically modified carbon paste electrode based on etioporphyrin I dihydrobromide. J. Electroanal. Chem. 666,

19

11–18. Juárez-Gómez, J., Ramírez-Silva, M.T., Romero-Romo, M., Rodríguez-Sevilla, E., Pérez-García, F., Palomar-Pardavé, M. (2016). Ion-selective electrodes for mercury determination at low concentrations: construction, optimization and application. J. Electrochem. Soc. 163, B90–B96. Kurniasih, D., Atikah, Sulistyarti, H. (2012). The coated-wire ion selective electrode (CWISE) of chromate using PVC-membrane based on chitosan as a carrier. J. Pure App. Chem. Res. 1, 33–40. Li, W., Simmons, P., Shrader, D., Herrman, T.J., Dai, S.Y. (2013). Microwave plasma-atomic emission spectroscopy as a tool for the determination of copper, iron, manganese and zinc in animal feed and fertilizer. Talanta 112, 43–48. Liao, X., Liang, B., Li, Z., Li, Y. (2011). A simple, rapid and sensitive ultraviolet-visible spectrophotometric technique for the determination of ultra-trace copper based on injection-ultrasound-assisted dispersive liquid–liquid microextraction. Analyst 136, 4580–4586. Marinho, B., Ghislandi, M., Tkalya, E., Koning, C.E., de With, G. (2012). Electrical conductivity of compacts of graphene, multi-wall carbon nanotubes, carbon black, and graphite powder. Powder Technol. 221, 351–358. Panggabean, A.S. (2011). Preparation and characterization ion selective electrode Cd(II) based on chitosan in PVC membrane. Indo. J. Chem. 11, 285–289. Parra, E.J., Crespo, G.A., Riu, J., Ruiz, A., Rius, F.X. (2009). Ion-selective electrodes using multi-walled carbon nanotubes as ion-to-electron transducers for the detection of perchlorate. Analyst 134, 1905–1910. 20

Silva, S.G., Oliveira, P.V., Rocha, F.R.P. (2010). A green analytical procedure for determination of copper and iron in plant materials after cloud point extraction. J. Braz. Chem. Soc. 21, 234–239. Singh, A.K., Sahani, M.K., Bandi, K.R., Jain, A.K. (2014). Electroanalytical studies on Cu(II) ion-selective sensor of coated pyrolytic graphite electrodes based on N2S2O2 and N2S2O3 heterocyclic benzothiazol ligands. Mat. Sci. Eng. C 41, 206–216. Turkoglu, O., Soylak, M. (2005). Spectrophotometric determination of copper in natural waters and pharmaceutical samples with chloro(phenyl) glyoxime. J. Chin. Chem. Soc. 52, 575–579. Tutulea-Anastasiu, M.D., Wilson, D., del Valle, M., Schreiner, C.M., Cretescu, I. (2013). A solid-contact ion selective electrode for copper(II) using a succinimide derivative as ionophore. Sensors 13, 367–4377. Wardak, C. (2015). Solid contact cadmium ion-selective electrode based on ionic liquid and carbon nanotubes. Sensor Actuat. B Chem. 209, 131–137. Yin, Q., Zhu, Y., Ju, S., Liao, W., Yang, Y. (2016). Rapid determination of copper and lead in Panax notoginseng by magnetic solid-phase extraction and flame atomic absorption spectrometry. Res. Chem. Intermed. 42, 4985–4998. Zhong, W.S., Ren, T., Zhao, L.J. (2016). Determination of Pb (lead), Cd (cadmium), Cr (chromium), Cu (copper), and Ni (nickel) in Chinese tea with high-resolution continuum source graphite furnace atomic absorption spectrometry. J. Food Drug Anal. 24, 46–55.

21

Figure captions Fig. 1. Effect of the addition of MWCNTs-COOH, GP and CS on the sensitivity of the proposed electrode; pH 7.0 (50 mmol L‒1, phosphate buffer).

22

Fig. 2. Effect of the component ratio on the sensitivity of the proposed electrode; pH 7.0 (50 mmol L‒1, phosphate buffer).

23

Fig. 3. Effect of pH of aqueous solution on the electromotive force of the proposed electrode; Ccopper(II) = 1.0 × 10–7 mol L–1.

24

Fig. 4. Effect of the conditioning time on the sensitivity of the proposed electrode; pH 7.0.

25

Fig. 5. Calibration curve of the Cu2+-selective electrode.

26

Table 1 Heating program of graphite furnace.

Operation

Temperature (oC)

Ramp (s)

Holding time (s)

Drying

100

10

10

Pyrolysis

800

10

20

Atomization

1800

0

4

Cleaning

2300

0

1

27

Table 2 Selectivity coefficients of Cu2+ selective electrode in the presence of different interfering ions. Interfering ion

Selectivity coefficient

Zn2+

3.0 × 10‒3

Co2+

1.9 × 10‒3

Ca2+

9.3 × 10‒4

Ni2+

9.3 × 10‒4

Pb2+

6.8 × 10‒4

Cr3+

5.2 × 10‒4

Fe3+

4.7 × 10‒4

Mg2+

3.9 × 10‒4

NH4+

2.6 × 10‒4

Mn2+

1.9 × 10‒4

28

Table 3 Comparison of the proposed copper (II) ion-selective electrode with similar electrodes reported in the literature. Reference

Linear range (mol L–1)

Detection limit

Response time (s)

pH range

Conditioning time

(mol L–1) Ansari et al., 2012

5.0 × 10‒6 ‒ 1.0 × 10‒1

2.0 × 10‒6

20

4.0–7.0

24 h

Afkhami et al., 2014

4.5 × 10‒8 ‒ 1.0 × 10‒2

2.34 × 10‒8

10

3.5–6.0

24 h

Ghaedi et al., 2012

5.0 × 10‒8 ‒ 1.0 × 10‒1

4.0 × 10‒8

10

2.5–5.5

Not mentioned

Baig & Khan, 2015

1.0 × 10‒8 ‒ 1.0 × 10‒1

1.0 × 10‒8

Birinci et al., 2016 Ghaedi et al., 2013 This work

1.0 ×

10‒6

‒ 1.0 ×

10‒1

6.0 ×

10‒8

‒ 1.0 ×

10‒1

1.0 ×

10‒10

‒ 1.0 ×

10‒5

14

2.5–6.5

5‒7 days

4.9 ×

10‒7

< 10

3.5–6.0

1 day

4.0 ×

10‒8

< 10

2.0–5.0

Not mentioned

7.9 ×

10‒11

9

7.0–11.0

3h

29

Table 4 Analytical results of Cu2+ in real samples (average ± standard deviation, n (detection number) = 3). sample

found (mg kg‒1) electrode

GF-AAS

buckwheat powder

2.5 ± 0.12

2.3 ± 0.14

cowpea

4.2 ± 0.094

4.1 ± 0.087

sesame

9.8 ± 0.41

soybean milk powder

t test

added (mg kg‒1)

t testa

recovery (%) electrode

GF-AAS

2.0

(97.7 ± 2.8)%

(98.8 ± 2.4)%

1.35

3.3

(101.1 ± 3.9)%

10.5 ± 0.47

1.94

10

(97.7 ± 3.2)%

(98.8 ± 1.9)%

0.51

2.4 ± 0.089

2.6 ± 0.11

2.45

2.7

(97.0 ± 2.7)%

(98.4 ± 2.9)%

0.61

bean curd stick

7.0 ± 0.35

7.5 ± 0.26

1.99

7.3

(105.9 ± 3.1)%

tofu skin

7.6 ± 0.28

7.7 ± 0.14

0.55

8.0

(97.2 ± 4.3)%

hazelnut

9.8 ± 0.32

9.4 ± 0.37

1.42

10

(107.7 ± 4.2)%

a

1.88

The theoretical value of t at 95% confidence level is 2.78

30

(93.3 ± 3.5)%

(100.5 ± 3.6)%

0.52 2.58

1.97

(98.2 ± 4.2)%

0.29

(98.6 ±3.9)%

2.75