INTRODUCTION:

Heavy metal contamination in the environment is a major concern worldwide because of the toxicity of these metals and their potential threat to human health. Currently quantification of heavy metals relies upon collection of liquid discrete samples for subsequent laboratory analysis using techniques such as ICP-MS, AAS, GC, HPLC, FT-IR and GS/MS¹. Electrochemical detection has several advantages over these methods in their simplicity, fast response and suitability for the preparation of inexpensive and portable instrumentations², chemically modified electrodes [CME] with surfaces designed for reacting and binding of target analyte hold great promise for chemical sensing^3-5.

In recent decades, these have been a strong demand for integrated chemical systems that make use of the intrinsic properties of selected materials to particular redox process occurring electrode/solution interfaces. This has led to the emergence of a wide range of chemically modified electrode (CME)⁶, which have found numerous applications in various fields and especially in electroanalysis^7-11.

It is now well established that the analytical scope of solid electrodes, including trace metals analysis can be greatly enhanced through a deliberate modification of their surface^4,5. The use of properly designed modified electrodes can add a new dimension to the preconcentration-voltammetric scheme, since electrostatic interactions, covalent bond or complexation can be used to analytes on an electrode surface^12-13. Complexing electrode materials appear particularly attractive for trace metal analysis. Because the metal complex formation is used in the cyclic voltammetry experiment at open circuit, the use of an electrode modified with a ligand characterized by a high affinity for a given metal cation will ensure highly sensitive and selective measurements, and can reduce interfaces compared to conventional voltammetric techniques.

Various ligands and surface manipulation strategies have been explored for the accumulation- voltammetry of metal cations, including adsorbed and self-assembled monomolecular layers of ligand on gold electrodes^14-16, composite electrode materials prepared by mixing ligands with carbon paste^17,18 or complexing polymer film modified electrodes^19-21.

However, there are no reports about sensing of lead and copper ions by using LMGCE. In these study we developed a more simple and sensitive electrochemical method for sensing of copper and lead ions. The results show that LMGCE as a working electrode exhibits higher sensitivity toward the potential and current values of the lead and copper ions.

EXPERIMENTAL:

Reagents and materials:

N-methyl piperazine (spectro chem.),formaldehyde solution (rankem), hydroquinone (Himedia), ethanol (CHINA), potassium nitrate (sd fine-chem), lead nitrate (Rankem), copper sulphate pentahydrate (Reachem). All chemicals were of analytical grade and were used without further purification.

Electrochemical equipment:

All electrochemical experiments were performed in potassium nitrate medium( 0.5M, pH 6.5) with a 600D electrochemical analyzer (CH Instruments ) using a conventional three- electrode system. The working electrode was a glassy carbon electrode; the counter electrode was a platinum wire and Ag/AgCl electrode served as reference. Solutions were degassed with N₂before each measurement and kept under a N₂ atmosphere during the entire experiments period. All experiments were performed thrice to get concordant value. All experiments were run at 25^oC.

Synthesis of (2,4 bis (4-methyl piperazi-1yl-methyl) 1-4 hydroquinone (L):

The ligand was synthesized by taking mixture of formaldehyde solution (3.5ml, 0.035mol), and N-methyl piperazine (3.8ml, 0.035mol) in ethanol (15ml) and stirred for 24hours. Then p- hydroxy phenol (2g, 0.017mol) in ethanol (10ml) solution was added and stirred for 12 hours. The resulting solution was refluxed in oil bath at 50⁰C for 18hours. The excess of ethanol was evaporated and stand for 24hours. The brown color solid was obtained. The curde sample was crystallized from ethanol and the prepared compound was consistent with the spectral datas.. IR spectrum (KBr disc, /cm-1) 3340 (-OH) br, H¹ NMR (δ ppm in CDCl₃ ) ~ 2.52 ( br, s, 16H methylene protons), ~3.68 (s, 4H, benzylic protons), ~7.8 (s, 2H, Ar-H).

Preparation of (2,4 bis (4-methyl piperazi-1yl-methyl) 1-4 hydroquinone (L) modified glassy carbon electrode:

Glassy carbon electrode was polished with alumina slurry and then rinsed with triple distilled water. 20µl of ethanolic ligand solution was dipped on the surface of a 3mm diameter glassy carbon electrode and kept in dry to evaporation of ethanol. Then LMGCE was obtained. LMGCE was cleaned after the measurement by physical removal of the film, followed by polishing with alumina slurry.

RESULTS AND DISCUSSION:

Electrode process of lead ions on LMGCE:

For this study Pb(II) ions and electrocatalytic activity of the ligand were evaluated on LMGCE in 0.5M KNO₃ medium. Cyclic voltammogram with reduction and oxidation Pb signals were recorded in the potential window -1 V to -0.2V vs Ag/AgCl/ sat KCl at 4 x 10^-4M Pb(NO₃)₂ solution was shown in Fig.1. The detection limit of lead ions on GCE is 1 x 10^-4 M Pb(NO₃)₂ which is confirmed by the Fig.2. Whereas cyclic voltammogram of lead nitrate on LMGCE showed two anodic (A1 andA2) and one cathodic peak(C). According to Fig.1 Pb(II) signals on LMGCE are strongly affected by electro catalytic activity of the ligand (L). The separation of anodic peaks can be caused by a specific catalytic effect of nitrate ions (NO₃^-), which consist in the strong attraction of positive charged species Pb²⁺, Pb⁺ from solutions on the one hand and on the displacement of NO₃^- from an electrode surface on the other^22,23.

We propose that C corresponds to the reduction of Pb(II) (i.e Pb(II) +2e^- Pb(0)), A1 corresponds to the oxidation of Pb(0) (i.e Pb(0) Pb(I) + e^-), A2 corresponds to the oxidation of Pb(I) (i.e Pb(I) Pb(II) + e^-). We remark that the peak A1 corresponds to the oxidation of the product formed in the peak C and A2 is related to the product formed in the peak A1 which is could be due to the specific catalytic effect of nitrate ion. The cyclic voltammograms shows that both the cathodic as well as anodic peaks are enhanced compared to the unmodified one. This happens due to the increased uptakes of lead ions by LMGCE through electro catalytic activity. Cyclic voltammetric parameters were shown in table (I).

Fig.1 Cyclic voltammograms of 4 x 10^-4M Pb(NO₃)₂on bare GCE(1) and LMGCE(2) in 0.5M KNO3 solution at 20mV/s.

Fig.2 Cyclic voltammograms of 1 x 10^-4M Pb(NO₃)₂on bare GCE(1) and LMGCE(2) in 0.5M KNO₃ solution at 20mV/s.

Log (γ_a/γ_c) = log (α/(1-α))

Fig 3 Cyclic voltammograms of 1 x 10^-3M Pb(NO₃)₂on LMGCE at varying scar rates 10, 20, 40 and 80mV/s respectively.

γ_a/γ_c = α/1-α

Fig. 4. A plot of Epc Vs logv for 1 x 10^-3M Pb(NO₃)₂ on LMGCE.

Sweep rate effect:

Cyclic voltammograms were recorded for 1mM Pb²⁺ ion solution (Fig.3) at different scan rates. A plot of Epc Vs logv (Fig.4) is not passing through origin which suggests that the reaction is not diffusion controlled one. Also, a shift in the peak potential is observed with the value of Ipc/V^1/2decreasing with an increase in the scan rate (Table.II).

Table. II

	10mV	20mV	40mV	80mV
Ipa X 10⁵	-0.689	-0.855	-1.213	-1.672
Ipc₁ X 10⁵	1.326	-	-	-
Ipc₂ X 10⁵	1.756	2.325	3.026	3.495
Epa₁	-0.492	-0.491	-0.487	-0.484
Epc₁	-0.627	-	-	-
Epc₂	-0.665	-0.676	-0.687	-0.694

Cyclic Voltammetric parameters for 1mM Pb²⁺ at different sweep rates in 0.5M KNO₃ solution on LMGCE

The scan rate was varied from 10mV/s to 80mV/s, the peak current values were increased with scan rate. On increasing the scan rate, the reduction peak shifts to more negative potentials, while the oxidation peak shifts to more positive potentials. The anodic and cathodic potentials are linearly dependent on the logarithm of scan rate (v) when ΔEp (200/n), which is in agreement with the laviron theory. A plot of Epc versus log v (Fig 4) yields a straight line with slope of -2.3RT/αnF, so that α can be estimated as 0.013 from the slope of the straight line based on the following equation.

Chronoamperometry:

In potential step methods, the potential of the working electrode is changed instantaneously between potentials E₁ and E₂ and the current-time is recorded. Chronoamperometry has no unique analytical utility, but it is useful for the evaluation of diffusion co-efficient of the redox species. The Cottrell equations describes relationship between diffusion co-efficient and bulk concentration^{24, 25}

I = nFACD^1/2/П^1/2t^1/2

Where, D and C are diffusion co-efficient (cm²/s) and the bulk concentration at the lead ion (1mM) respectively. The level of Cottrell current, measured for 1s, increased when after modification. The chronoamperomogram was shown in fig.5. A plot of I (Vs) t^1/2 was shown in Fig.6. From the slope of the straight line, we can calculate the diffusion co-efficient of 8.3 x 10^-7cm²/s and 6.7 x 10^-7 cm²/s for lead ions in before and after modification of GCE respectively.

Fig 5. Chronoamperomogram for 1 x 10^-4 M Pb(NO₃)₂ on plain GCE (1) and LMGCE (2).

Fig 6. A plot of Current Vs 1/t^1/2 for 1 x 10^-4 M Pb(NO₃)₂ on plain GCE (1) and LMGCE (2).

Chronocoulometry:

Fig.7 shows the chronocoulomogram of lead ion at 1mM concentration in before and after modification of GCE. A plot of Q (vs) t^1/2was shown in fig.8. It was found that GCE has a total charge transferred of 2.7mC/cm² in Pb²⁺ ion, while low charge transferred of 0.4mC/cm² in Pb²⁺ ion is observed on LMGCE. This shows that Pb²⁺/ LMGCE less reducible in Pb²⁺ion in aqueous medium.

Effect of potential cycling:

The stability of LMGCE and its effect on the lead redox current were assessed by continuous potential cycling. Fig.9 shows that the current associated with enhanced from the first cycle onward and the potential remains same; reflecting the stability of LMGCE in lead redox system.

Fig 7. Chronocoulomogram for 1 x 10^-4 M Pb(NO₃)₂ on plain GCE (1) and LMGCE (2).

Fig 8. A plot of Charge Vs t^1/2 for 1 x 10^-4 M Pb(NO₃)₂ on plain GCE (1) and LMGCE (2).

Fig 9. Cyclic voltammogram for 1 x 10^-4M Pb(NO₃)₂ on LMGCE, at 1-10 cycles in 0.5M KNO₃ solution. Scan rate 20mV/s.

Effect of potential cycling:

Fig.10 Cyclic voltammograms of 4 x 10^-4M CuSO₄on bare GCE(1) and LMGCE(2) in 0.5M KNO₃ solution at 20mV/s.

Electrode process of copper ions on LMGCE:

For this study, Cu(II) ions and their redox reaction enhanced with the ligand were evaluated on ligand modified glassy carbon electrode in 0.5M KNO₃ medium. Cyclic voltmmogram with reduction and oxidation Cu signals were recorded in the potential window from -1V to -0.2V Vs. Ag/AgCl/sat KCl at 4 x 10^-4M CuSO₄ solution. Whereas cyclic voltammograms of copper suphate in LMGCE showed two anodic peaks (A1 and A2) two cathodic peaks (C1 andC2), CuSO₄in bare GCE gave two anodic (A1and A2) and one cathodic signal (C) (Fig.10). Cyclic voltammetric parameters were shown in table (III). According to (Fig.10) Cu(II) signals on LMGCE are strongly affected by electro catalytic activity of the ligand towards the changing the peak current values. The detection limit of lead ions on GCE is 1 x 10^-4 M CuSO₄which is confirmed by the Fig.11. The separation of cathodic and anodic peaks can be caused by a specific catalytic effect of nitrate anions (NO₃^-), which consists in the stronger attraction of positive charged species such as Cu(II), Cu(I) from solutions on the one hand and on the displacement of NO₃^- from an electrode surface on the other which is confirmed from the electrochemical behavior of copper ion in ammonium fluoride electrolyte was shown in Fig.12. According to the Fig.12 copper ion shows one anodic and one cathodic peak..We propose that peak C1 corresponds to the reduction Cu(II) (i.e Cu(II) + e^- Cu(I)), peak C2 corresponds to the reduction Cu(I) (i.e Cu(I) + e^-

Cu(0)), A1 corresponds to the oxidation of Cu(0) (i.e Cu(0)

Cu(I) + e^-), A2 corresponds to the oxidation of Cu(I) (i.e Cu(0) Cu(I)+e¯). The cyclic voltammogram shows that both the cathodic as well as anodic peaks are enhanced compared to the unmodified one. This happens due to the increased uptakes of copper ions by LMGCE through electro catalytic activity of the ligand contain highly electronegative nitrogen atom. The peak potential was not significantly changed. LMGCE was considered to be sufficiently suitable for trace analysis of heavy metals. Analytically the peak A2 is taken for comparative purpose since the copper ion is completely undergoing dissolution rather than at A1. So the LMGCE used for further studies.

Fig.11 Cyclic voltammograms of 4 x 10^-4M CuSO₄on bare GCE (1) and LMGCE(2) in 0.5M KNO₃ solution at 20mV/s.

Fig.12 Cyclic voltammograms of 1 x 10^-3M CuSO₄on bare GCE in 0.5M NH₄F solution at 20mV/s.

Sweep rate effect:

Cyclic voltammograms were recorded for 1mM Cu²⁺ ion solution (Fig.13) at different scan rates. A plot of Epc Vs logv (Fig.14) is not passing through origin which suggests that the reaction is not diffusion controlled one. The cyclic voltammetry parameters were shown in table IV. Also, a shift in the peak potential is observed with the value of Ipc/V^1/2decreasing with an increase in the scan rate. The scan rate was varied from 10mV/s to 80mV/s.

The peak current value increased with rising of scan rate. On increasing the scan rate the reduction and oxidation peaks were shift to move more negative potentials. The transfer co-efficient of copper ion (α) can be estimated as 0.078 and 0.024 from the slope of the straight lines based on the following equations.

Log (γ_a/γ_c) = log (α/(1-α))

Fig 13. Cyclic voltammograms of 1 x 10^-4M CuSO₄on LMGCE at varying scar rates 10, 20, 40 and 80mV/s respectively.

γ_a/γ_c = α/1-α (Laviron theory).

Fig 14. A plot of Epc Vs logv for 1 x 10^-4M Pb(NO₃)₂ on LMGCE.

Table IV: Cyclic Voltammetric parameters for 1mM Cu²⁺ at different sweep rates in 0.5M KNO₃ solution after modification of GCE.

	10mV	20mV	40mV	80mV
Ipa₁ X 10⁵	-0.859	-0.908	-1.932	-2.032
Ipa₂ X 10⁵	-2.848	-4.867	-6.651	-9.149
Ipc₁ X 10⁵	0.8959	1.035	1.248	1.380
Ipc₂ X 10⁵	0.939	1.118	1.285	1.425
Epa₁	-0.001	-0.012	-0.014	-0.009
Epa₂	0.240	0.269	0.195	0.211
Epc₁	-0.158	-0.170	-0.198	-0.241
Epc₂	-0.082	-0.094	-0.112	-0.132

Chronoamperometry:

The level of Cottrell current measured for 1s, increased (Fig. 15) in LMGCE for copper ions. From the slope of plot (Fig.16), we can calculate the diffusion co-efficient of 3.5 x 10^-6 to 3.06 x 10^-6 cm²/s for copper ions in before and after modification of GCE respectively.

Fig. 15. Chronoamperomogram for 1 x 10^-4 M CuSO₄on plain GCE (1) and LMGCE (2).

Fig 16. A plot of Current Vs 1/t^1/2 for 1 x 10^-4 M CuSO₄ on plain GCE (1) and LMGCE (2).

Chronocoulometry :

Chronocoloumetry charge curve (Fig.17) was enhanced in LMGCE for copper ion at 1mM concentration. A plot of Q (vs) t^1/2was shown in fig.18. It was found that GCE has a total charge transferred of 7.8 mC/cm²in Cu²⁺ ion while high charge transferred of 9.04mC/cm² in Cu²⁺ ion is observed on LMGCE. This shows that Cu²⁺/LMGCE is highly reducible in aqueous medium.

Fig 17. Chronocoulomogram for 1 x 10^-4 M CuSO₄on plain GCE (1) and LMGCE (2).

Fig 18. A plot of Charge Vs t^1/2 for 1 x 10^-4 M CuSO₄ on plain GCE (1) and LMGCE (2).

Effect of potential cycling:

Cyclic voltammograms in Fig (19) were obtained on an LMGCE by the consecutive potential sweep at a scan rate of 20mV/s, in which serial lines were obtained for 0.5M KNO₃ containing 1 x 10^-3M CuSO₄. From the fig.19, the reduction peak current, and oxidation peak current were high at first cycle compared to the other cycle and remains almost constant onward reflecting the stability of the LMGCE in copper redox system.

Fig 19. Cyclic voltammogram for 1 x 10^-4M CuSO₄ in 0.5M KNO₃ solution on LMGCE, at 1-10 cycles. Scan rate 20mV/s.

Effect of heavy metal ions:

The interference study of cadmium, mercury ions in lead and copper redox system on LMGCE was performed in 0.5M KNO₃ medium (Fig.20 and21). The tolerance limit was defined as the maximum concentration of the potential interfering substance, during the determination of lead and copper at 1mM during addition of cadmium(0.1mM, 0.2mM, 0.3mM), mercury (0.1mM, 0.2mM, 0.3mM).The peak current and potential values were not significantly changed when addition of cadmium and mercury ions.

Fig.20. Cyclic voltmmogram of 1mM of Pb²⁺ ion in presence of Cd²⁺ ion (0.1mM, 0.2mM, 0.3mM) and Hg²⁺ ion (0.1mM, 0.2mM, 0.3mM).

Fig.21. Cyclic voltmmogram of 1mM of Cu²⁺ ion in presence of Cd²⁺ ion (0.1mM, 0.2mM, 0.3mM) and Hg²⁺ ion (0.1mM, 0.2mM, 0.3mM).

CONCLUSION:

The result presented here demonstrates the use of LMGCE is highly sensitive and stable in electrochemical measurement. Cathodic as well as anodic peak currents were significantly enhanced indicating an electrocatalysis process due to the presence of 2,4 bis (4-methyl piperazi-yl-methyl) 1-4’ hydroqunone (L). The reduction and oxidation peak current of Pb(NO₃)₂ and CuSO₄ is higher at first cycle and remains stable onwards. It is therefore evident that the LMGCE posses some degree of stability. Potential use of LMGCE as a useful electrode material is therefore clearly evident. The chrononoamperometry and chronocoulometry studies were supported to the cyclic voltammetry results. The interference study was used for detection of lead and copper ions in presence of other heavy metal ions.

ACKNOWLEDGEMENTS:

The authors thanks to UGC-MRP for providing financial support for accomplishment of this work.

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Received on 28.10.2011 Modified on 14.11.2011

Asian J. Research Chem. 4(12): Dec., 2011; Page 1920-1927

	Ipa₁X 10^-5	Ipa₂X 10^-5	Ipc X 10^-5	Epa₁	Epa₂	Epc
Before modification of GCE	-0.2251	-0.725	0.9624	-0.4781	-0.5947	-0.7164
After modification of GCE	-1.353	-1.079	1.382	-0.4895	-0.5815	-0.7726

Sensing of Lead and Copper Metal Ions by Substituted N-Methyl Piperazine Compound on Glassy Carbon Electrode.