Modified Carbon Paste Electrode Based on Multi-Walled Carbon Nanotubes ( MWCNTs ) and Ionic Liquid for Potentiometric Determination of Cu 2 + Ions in Real Samples

Carbon paste electrode based on multi-wall carbon nanotube (MWCNT) and ionic liquid was prepared and used for the potentiometric determination of copper ion. A multi-walled carbon nanotubes (MWCNTs) was used in the composition of the carbon paste to enhance conductivity and transduction of chemical signal to electrical signal. Furthermore, unification of 1,4-diamino anthraquinone as an ionophore to this composition caused to significantly improve d selectivity toward Cu (II) ions. The electrode show a nernstian slope of 29.5 ±0.2 mV per decade for Cu ion over a wide concentration range from 1.0×10 to 1.0×10 M.The lower detection limits is 8.0×10 -7 M. The electrode has a fast response time (<5 s), a satisfactory relatively long life time and reproducibility. The proposed sensor revealed a fairly good selectivity toward Cu ion in comparison to other common cations and the working pH range was 3.0-12. In a final manner, these electrode have been successfully implemented for the determination of Cu ions content in real samples.


INTRODUCTION
Copper is one of the most widespread element in the environment of industrialized countries.Due to many applications of copper in industry [1], in many biological systems [2], and in medicinal and environmental samples, also is an essential element and is toxic at high concentration.Researchers have attempted to develop sensors for copper determination with high selectivity and sensitivity [3][4][5][6].
Nanoscience and nanotechnology have a more attention in the recent years because wide application in industrials especially sensors filed [17][18][19][20][21]. Nano-materials such as nanoparticles, carbon nanotubes or nanocomposite connected with biomolecules are being used for several bioanalytical applications [22][23][24].Electrochemical analysis is taking advantages from all the possibilities offered by nanomaterials easy to be detected by conventional electrochemical methods [25,26].
Using ion-selective sensor is very convenient for the analysis of samples having difficult matrices, such as soil and industrial wastewater [27].Potentiometric electrodes based on ion-selective electrodes are especially suited for such determination because they offer advantages such as selectivffity, sensitivity, good precision, simplicity, and low cost.This work is to introduce a very simple and inexpensive (potentiometric) method in determination of copper(II) in a wide concentration range.In this study, Cu (II) carbon paste electrode (CPE) based on 1,4-diamino anthraquinone ionophore, Fig1, was constructed in order to determine copper ion concentration.Then the electrode was modified by MWCNTs to achieve the better electrode response.The modified electrode was successfully used as an indicator for potentiometric determination of Cu (II) in water sample.

Reagents and solutions
All analytical reagent grade chemicals and distilled water were used for preparing all aqueous solutions.The graphene powder synthesized according to the method of T.Hartono et al [28] with the ionic liquid (1-butyl-3-methyl imidazolum hexa fluorophosphate) was of high purity and was used for the preparation of the carbon pastes .chlorideand nitrate salts of the cations used are all purchased from Merck.The multi-walled carbon nanotubes (MWCNTs) with 10-40 nm diameters and 1-25 µm length were purchased from Research Institute of the Petroleum Industry (Iran).

Apparatus
The glass cell in which carbon paste electrode was placed into contained an Ag/AgCl electrode (Azar electrode, Iran) as a reference electrode.The Cu 2+ CPE was used as an indicator electrode.Both electrodes were connected to a pH/mV meter (Metrohm-691, Switzerland).
The following cell was assembled for the conduction of the EMF (electromotive force) measurements: Carbon paste electrode | sample Cu 2+ ion solution | Ag/AgCl-KCl (satd.)

Electrode Preparation
General procedure to prepare the carbon paste electrode was as follows: Different amounts of 1,4-diamino anthraquinone as an ionophore with an appropriate amount of graphene powder, ionic liquid, Plasticizer(DBP) and MWCNTs were thoroughly mixed.After homogenization of the mixture, the paste was thoroughly packed into the teflon tube and a electrice wire was inserted into the opposite end of the CPE to set up electrical contact.The external surface of the carbon paste was smoothed with soft paper.A new surface was produced by scraping out the old surface and replacing the new carbon paste.The electrode was finally conditioned for 24 h by soaking it in a 1.0×10 -3 mol L -1 Cu(NO3)2 solution [29,30].

Carbon paste electrode composition
The sensing element of a potentiometric ion-selective electrode has important role in selectivity behavior of the electrode.The influence of the percent of materials in the carbon paste composition was investigated and the results are summarized in Table 1.
The typical CPE with optimized composition (electrode no.2) shows a Nernstian slope of 29.5 mVper decade.The electrode composed of 12% Ionic Liquid, 4% Ionophore, 70% Graphene Powder11% Plasticizer and 3% MWCNT was found to be optimal for Cu 2+ carbon paste electrode.These results show that the 1,4-diamino anthraquinone is an excellent ionophore for Cu 2+ ion, also, addition of MWCNTs to the paste improves the linear range and the response time of the electrode.This new carbon paste electrode was selected for further examination.

pH effect on the electrode response
In order to study the effect of pH on the response of the optimal modified Cu 2+ sensor (no.2), the potential was measured for a fixed concentration of Cu 2+ ion solutions (10 -3 mol L - 1 ) at different pH values.The pH was varied from (1-14) by addition of concentrated HNO3 or NaOH.The change in potential as a function of pH is shown in Figure 3.The potential was constant and quantitative in the pH range of 3.0-12.At pH<3.0, the electrode response increased rather irregularly with increasing analyte acidity.At such high acidic solutions, the observed increase in potential indicates that the protonated ionophore possesses a poor response to the Cu 2+ ions and strong response to H3 O + ions in the solution.As can be seen in Figure 3 the potential decreased sharply at higher pHs (>12) due to the formation of Cu (OH) 2 which not reacts with the ligand.
In addition, Fluctuations at pH greater than 12 might be due to the formation of soluble or insoluble Cu2+ hydroxy complexes.The fluctuations at a pH value of 3.0were attributed to the protonation of ligand in the carbon paste.

Response time
Response time is an important factor for any sensor.For electrochemical sensors, this parameter is evaluated by measuring the average time required to achieve a potential within ±0.1 mV of the final steady-state potential upon successive immersion of a series of interested ions, each having a ten-fold difference in concentration.The measurements of potential versus time were carried out with the Cu (II) nitrate solutions from lower (1.0×10 -6 M) to higher (1.0×10 -1 M) concentrations (Figure4).For the proposed modified Hg 2+ sensor, the response time was less than 5s.

Selectivity
Selectivity is the most important characteristic of any sensor, and describes an ion selective electrode's specificity toward the target ion in the presence of interfering ions, the potentiometric selectivity coefficients of the proposed carbon paste electrode were evaluated by separate solution method (SSM) [5] and the results are depicted in Table 2.

Determination of copper in spiked tap and well water
To assess the applicability of the proposed sensor to real samples, Cu(II) was measured in tap and well water.Each sample was analyzed in triplicate, using this sensor by the standard addition method.The results, in Table 3, show an average recovery of 92.8% with relative standard deviation (RSD) of 1.03%.The results of this study confirm that the sensor can be successfully used for routine analysis with portable equipment.

CONCLUSIONS
In this work, Cu 2+ carbon paste electrode based on MWCNT and 1,4-diamino anthraquinone as ionophore is introduced.By incorporating ionophore in composite of this electrode, high selectivity and sensitivity to copper ion were observed.The modified CPEs show better potentiometric response than typical CPEs in terms of sensitivity, Nernstian slope, linear range, and response stability.Since the electrode shows excellent sensitivity, selectivity and stability, it may find application in the analysis of real samples.
The electrode has a great potential to be used in the development of portable analyzers for monitoring Cu (II) ions in very polluted natural waters.

Figure 3 .
Figure 3.Effect of pH on the potential response of the Cu 2+ carbon paste electrode in the test solution of Cu 2+ ion (10 -3 mol L -1 )

Figure 4 .
Figure 4. Dynamic response time of the proposed carbon paste electrode (no.2)

Table 1 .
The optimization of the carbon paste ingredients

Table 2 .
Selectivity coefficients of various interfering species for proposed sensor (no.2)The potential of a cell comprising an ion-selective electrode and a reference electrode is measured with two separate solutions, one containing the ion (A) at the activity   , the other one containing the ion (B) at the same activity   =   .If the measured values are   and   , respectively, the value of is calculated from the equation:These results seem to indicate that interference effects upon the performance of the electrode assembly are negligible.

Table 3 .
Recovery of copper ions from different water samples