Electrocatalytic activity of cobalt Schiff base complex immobilized silica materials towards oxygen reduction and hydrazine oxidation

Abstract

In this study, mesoporous silica spheres (MSS) are selected as a host framework to encapsulate Co(salen) and Co(salophen) (where Co(salen) is [N,N′-bis(salicylaldehyde) ethylenediimino cobalt(III)] and Co(salophen) is [N,N′-bis(salicylaldehyde)-1,2 phenylenediimino cobalt(III)]), represented as MSS–Co(salen) and MSS–Co(salophen) respectively. The prepared materials were characterized by various physicochemical methods such as diffuse reflectance, UV-vis, FT–IR and electrochemical techniques. Host–guest interactions with a feeble coordination bond (between one apical Si–O⁻ and the Co³⁺ metal ion) lead to the successful anchoring of Co(salen) and Co(salophen) to this silica framework. These constructed catalyst materials were applied to oxygen reduction studies as well as to the oxidative analytical determination of hydrazine (HZ). Both oxygen reduction (in 0.05 M HClO₄) and HZ oxidation (in neutral pH condition) showed significantly low overpotential on MSS–Co(salen) and MSS–Co(salophen) modified glassy carbon (GC) electrodes (GC/MSS–Co(salen) and GC/MSS–Co(salophen), respectively). Additionally, HZ was determined by two different electrochemical techniques, namely cyclic voltammetry (CV) and amperometry. From amperometry, GC/MSS–Co(salen) exhibits a linear calibration range from 10.0 to 210.0 μM for the HZ determination and GC/MSS–Co(salophen) exhibits two segmented linear calibration ranges from 10.0 to 310.0 μM. The CV technique demonstrates similar two segmented calibration plots (from 1.0 to 400.0 μM) for both GC/MSS–Co(salen) and GC/MSS–Co(salophen).

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1. Introduction

Now-a-days hydrazine (HZ) and its derivatives find widespread usage in pesticides, rocket fuel, textile agents, pharmaceutical intermediates and photographic developer.^1–3 Severe exposure of it to the environment causes an execrable effect on human beings, which encompasses irritation of eyes and nose, nausea, faintness and pulmonary edema. Moreover, its quite neurotoxic nature produces carcinogenic and mutagenic² effects, which may damage kidney, liver, lungs and central nervous system.⁴ The entire treatise leads to a considerable analytical interest for the detection of HZ. Among the several techniques, electrochemical techniques put forward the opening of cheap, portable, sensitive and rapid methodologies to detect HZ.^5,6 Electrochemical oxidation of HZ has been extensively studied in the last few years.^7–9 However, at traditional carbon electrodes it becomes kinetically sluggish and a relatively high overpotential is required for the process.¹⁰ Hence, one promising approach is to decrease its oxidation potential using chemically modified electrodes (CMEs). These CMEs contain a specifically selected redox mediator immobilized on the conventional electrodes, which can facilitate the voltammetric as well as amperometric detection by reducing overpotential, increasing current, and minimizing surface fouling (in case of any). Similarly, oxygen reduction (OR) has also been considered as import since it is used in H₂O₂ producing industries as well as taking place at the cathode in fuel cells and metal-air batteries.¹¹ The increasing exploration for the development of low cost and metal-based electrocatalysts has led to several CMEs with immobilized electrocatalyst for OR.¹¹ Based on previous studies of free Schiff bases and their complexes, which have biological,¹² catalytic,¹³ optical¹⁴ and photochromic¹⁵ activity, Co(salen)/Co(salophen) complexes (where Co(salen) is [N,N′-bis(salicylaldehyde) ethylenediimino cobalt(III)] and Co(salophen) is [N,N′-bis(salicylaldehyde)-1,2 phenylenediimino cobalt(III)]) have been chosen as the effective redox mediators for amperometric/voltammetric study in this work. The high surface area, uniform pore size, and lined-silanol groups confirm MCM-41 type mesoporous silica spheres (MSS) as potential hosts for a variety of guest chemical species including metal nanoparticles and transition metal complexes.^16–23 Therefore, the immobilization of catalyst molecules on these solid supports can be satisfactorily employed in the areas of catalysis.^{16,17,21,22,24} In consequence of this, in our present work we discuss in detail the preparation, characterization and electrocatalytic activities of Co(salen)/Co(salophen) incorporated mesoporous silica spheres i.e. MSS–Co(salen)/Co(salophen) towards the reduction of oxygen in acidic media and the oxidative recognition of HZ at neutral pH.

2. Experimental

2.1 Material synthesis

Tetraethoxysilane (TEOS, >98%) (Sigma-Aldrich), cetyltrimethylammonium bromide (CTAB) (Himedia), ammonia, poly(vinyl alcohol), ethylenediamine, salicylaldehyde, 1,2-phenylenediamine (SD fine chem limited, India) were used without further purification. Other reagents were of analytical grade and solutions were prepared with triply distilled water. Schiff base ligand salenH₂ (N,N-bis(salicylaldehyde) ethylenediamine) was prepared and purified according to literature procedure.²⁵ In general, salicylaldehyde was added to an Erlenmeyer flask containing 95% ethanol, placed in a sand bath, and heated to boiling whilst constantly stirred. To this boiling solution, ethylenediamine was added and the solution was allowed to stir for another 5 min. The flask was removed from the heat and allowed to cool slowly. Bright yellow flaky crystals were collected on a Buchner funnel followed by washing with cold ethanol and thorough air drying. Similarly, for the synthesis of salophenH₂ (N,N′-bis(salicylaldehyde)-1,2 phenylenediamine) the known method²⁶ was followed and it was recrystallized from ethanol. Then, the corresponding cobalt complexes, Co(III)(salen) [N,N′-bis(salicylaldehyde) ethylenediimino cobalt(III)] and Co(III)(salophen) [N,N′-bis(salicylaldehyde)-1,2 phenylenediimino cobalt(III)] were synthesized using the requisite amount of salenH₂ or salophenH₂ and the metal ion precursor cobalt(II) acetate. SalenH₂ was added into ethanol in a round-bottom flask that had been fitted with a magnetic stir bar and condenser and was allowed to reflux for about 30 min. The flask was immersed in a water bath of constant temperature between 60–70 °C. Alongside in a beaker, cobalt(II) acetate was dissolved in hot water. Subsequent to the complete dissolution of SalenH₂, this cobalt(II) acetate solution was added and the whole mixture was refluxed for another 5 min. Finally, the so formed Co(salen) was washed with ethanol and diethyl ether and then dried, followed by recrystallization from ethanol. A similar procedure was adopted for the synthesis of Co(salophen) also. For the structure of the complexes, see Fig. S1 (ESI†).

The synthesis procedure of the mesoporous silica spheres (MSS) was analogous to our previous study.¹⁶ CTAB was dissolved in 3 : 3.3 : 1 ethanol : water : ammonia (25%) mixture. TEOS in ethanol was prepared separately and added to the CTAB solution. The reaction mixture was stirred for 2 h. The formed white precipitate was collected, washed with plenty of water and ethanol and then dried in vacuum for at least 24 h. The CTAB was removed from the synthesized material by acid extraction in ethanol. To the 100 mg of the so formed MSS material in dimethyformamide (DMF), 0.24 mmol of the catalysts (Co(salen)/Co(salophen)) were mixed. The suspension was stirred for about 24 h, filtered, washed with excess of DMF and vacuum dried. The products are named as MSS–Co(salen) or MSS–Co(salophen), respectively (Scheme 1).

Scheme 1 Scheme representing the synthesis of MSS, MSS–Co(salen) and MSS–Co(salophen) materials.

2.2 Characterization

Fourier transform infrared (FT–IR) spectra of the solid samples were recorded from Spectrum RX1, Perkin Elmer spectrometer using KBr pellets over the range of 400–4000 cm⁻¹. The electronic spectra/diffuse reflectance of the neat and encapsulated complexes were recorded using UV-vis spectrophotometers (2802 PC UNICO, USA and UV 1700 Pharma Spec., Shimadzu). Cyclic voltammetry (CV) and other electrochemical studies were carried out with CHI 660C Electrochemical work station (CH instruments, USA). A three electrodes, one compartment electrochemical cell was used for all the studies. Glassy carbon electrodes (GCs) modified with MSS (GC/MSS), MSS–Co(salen) (GC/MSS–Co(salen)) and MSS–Co(salophen) (GC/MSS–Co(salophen)) materials were used as the working electrodes. Pt wire and a KCl saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All the above electrochemical studies were performed at room temperature (25 °C) and under nitrogen atmosphere unless otherwise mentioned.

2.3 Preparation of modified electrodes

The procedure for the preparation of the modified electrodes was similar to our previous studies.^16,17,21,22 Before the modification of the GC surface, it was polished with neutral alumina powder on a Buehler-felt pad, washed with water followed by methanol and sonicated for 5 min in water. In general, 1.0% colloidal solutions of the respective materials, containing 0.01% poly(vinyl alcohol) (for effective binding of the silica materials with the GC surface), were used to modify the GC surface. 5 μL of the respective colloidal solutions were coated onto 0.07 cm² of the GC to obtain GC/MSS, GC/MSS–Co(salen) and GC/MSS–Co(salophen).

2.4 Measurement of the catalytic rate constant

Based on the chronoamperometric currents, the catalytic rate constant for the electrocatalytic reaction can be calculated (eqn (1)).²⁷

I_cat/I_L = (π k_catCt)^1/2 (1)

Here, I_cat and I_L are the currents at the modified electrodes in the presence and absence of oxygen, respectively. C, k_cat, and t are the bulk concentration of oxygen (1.2 × 10⁻⁶ M²⁸), catalytic rate constant (M⁻¹ s⁻¹) and time (s), respectively. The plot of I_cat/I_Lversus t^1/2 yielded a straight line, which allows the determination of the k_cat values.

3. Results and discussion

3.1 Diffuse reflectance and UV-vis absorption

Diffuse reflectance spectra of MSS–Co(salen) and MSS–Co(salophen) in addition to UV-vis electronic spectra of the corresponding cobalt complex in DMF are shown in Fig. 1, where spectra a and b denote the UV-vis absorbance of Co(salen) and Co(salophen) species in DMF solution and spectra a′ and b′ represent the diffuse reflectance of the MSS–Co(salen) and MSS–Co(salophen) materials, respectively. In particular, the characteristic intense absorption band at about 440–465 nm is due to cobalt d → d transition²⁹ and 380–410 nm is ascribed mainly to d → π*,³⁰pπ → d,³¹ and intra ligand (n → π*)³² transfers. The bands at 320–350 nm are assigned as n → π* transitions involving the molecular orbital of the C=N chromophore and the benzene ring.³³ Thus, the results obtained from diffuse reflectance spectra and solution absorbance spectra are alike, which validates the successful inclusion of the cobalt complex in MSS through host–guest interactions by a number of coordinate bonds between apical Si–O⁻ and the metal ion.

Fig. 1 UV-vis absorbance spectra of 1.30 mM Co(salen) (a) and 1.16 mM Co(salophen) (b) in DMF and diffuse reflectance spectra of MSS–Co(salen) (a′) and MSS–Co(salophen) (b′) materials. 0.24 mmol of the metal complexes was initially used for the preparation of the MSS–Co(salen) and MSS–Co(salophen) materials.

3.2 FT-IR spectral analysis

The FT-IR spectra of the parent MSS, MSS–Co(salen), and MSS–Co(salophen) materials are depicted in Fig. 2a–c, respectively. The MSS material does not exhibit any bands between 1670 to 1300 cm⁻¹, whereas the silica encapsulated cobalt complexes show several characteristic bands in that region, which are generally ascribed as the C–C and C–O stretching modes of quasi-aromatic chelates in the Co(salen) and Co(salophen) complexes.³⁴ The MSS, MSS–Co(salen), and MSS–Co(salophen) materials show a broad band in the range 3400–3750 cm⁻¹. This band may be attributed to the O–H stretching vibration of free and hydrogen bonded (at around 3678 cm⁻¹) silanol groups of the parent MSS.³⁵ Nevertheless, it should be noted that trace amounts of adsorbed water may also contribute in this region. Furthermore, three strong bands at 1085, 800, and 464 cm⁻¹, in the mid-IR region, could be assigned as the asymmetric stretching, symmetric stretching and bending vibrations, respectively, of Si–O–Si bridges (SiO₂ tetrahedra).³⁵ The characteristic bands for free Co(salen) (1626, 1602, 1531, 1470, 1447 and 1390 cm⁻¹)³⁶ and Co(salophen) (1621, 1543, 1465, 1442, 1387, 1356, 1309 and 1245 cm⁻¹)³⁷ are present in their respective silica encapsulated materials also. This entire study clearly depicts the successful incorporation of cobalt complexes in MSS.

Fig. 2 FT-IR spectra of the synthesized MSS (a), MSS–Co(salen) (b), and MSS–Co(salophen) (c) materials.

3.3 Electrochemical characterization

Fig. 3A and B show the potentiodynamic responses of GC/MSS–Co(salen) and GC/MSS–Co(salophen) electrodes with different scan rates in 0.05 M HClO₄. In the acidic medium, the [Co(III)]⁺ to [Co(II)] redox couple furnished E_1/2 (E_1/2 = (E_pa + E_pc)/2) values of 0.41 and 0.38 V, respectively, for the above materials.^38,39 The redox couple due to [Co(II)] to [Co(I)]⁻ is not well exhibited at GC/MSS–Co(salen). It shows an irreversible reduction process with a reduction potential at around −0.4 V (Fig. 3A). However, well defined redox peaks (due to [Co(II)] to [Co(I)]⁻) are observed for the GC/MSS–Co(salophen) electrodes with an E_1/2 value at around −0.05 V (Fig. 3B). The insets of Fig. 3A and B show the linear variation of the peak currents (corresponding to the [Co(III)]⁺/[Co(II)] redox couple of the respective materials) with the square root of the scan rate. This implies that semi-infinite diffusion conditions prevail at all of the scan rates studied. This result is similar to the observation of Rubinstein and Bard for the Ru(bpy)₃²⁺ (where bpy = 2,2′-bipyridine) immobilized on Nafion film.⁴⁰ At neutral conditions (0.1 M pH 7.0 phosphate buffer solution), the redox couple due to [Co(II)] to [Co(III)]⁺ is observed at E_1/2 values of 0.16 and 0.14 V for GC/MSS–Co(salen) and GC/MSS–Co(salophen) (Fig. 4A and B), respectively.^38,39 The insets of Fig. 4A and B represent the respective I_pa and I_pc against the square root of the scan rate, which is expected when semi-infinite diffusion conditions prevail.⁴⁰

Fig. 3 Cyclic voltammograms of (A) GC/MSS–Co(salen) and (B) GC/MSS–Co(salophen) in 0.05 M HClO₄ with different scan rates. Inset: variation of I_pa and I_pc with the square root of the scan rates.

Fig. 4 Cyclic voltammograms of (A) GC/MSS–Co(salen) and (B) GC/MSS–Co(salophen) in 0.1 M pH 7.0 phosphate buffer solution (scan rate: 50 mV s⁻¹). Inset: variation of I_pa and I_pc with the square root of the scan rates. For clarity, cyclic voltammograms with different scan rates are not given. Arrows indicate the peak positions.

3.4 Electrocatalytic oxygen reduction

OR occurs at the cathode in a fuel cell, which is potentially an efficient and emission free energy source. Co(II) complexes act as catalysts for OR electrocatalytically⁴¹ as well as photocatalytically.^42,43 On the electrode surface, Co(III) is first reduced to Co(II) (electrode reduction), then it forms an adduct with dioxygen (eqn (2)). The adduct decomposes by producing Co(III) and hydrogen peroxide. Co(II) is again generated by the electrode reduction of Co(III) thus making a catalytic cycle. The overall reduction processes may be prearranged as follows (eqn (2) and (3)):

MSS–[Co(II)S] + O₂ → MSS–[Co(III)-O₂⁻] (2)

MSS–[Co(III)S-O₂⁻] + 2H⁺ + e⁻ → MSS–[Co(III)S]⁺ + H₂O₂ (3)

where S represents (salen) and (salophen).

Fig. 5A and B depict the respective CV responses obtained from the GC/MSS–Co(salen), GC/MSS–Co(salophen) and GC/MSS electrodes in 0.05 M HClO₄ in the presence and absence of O₂. In the absence of oxygen no characteristic peaks are observed (curves a and b in Fig. 5A and B). In the presence of oxygen, OR started at the GC/MSS electrode at −0.23 V (curve a′ in Fig. 5A and B) whereas at the GC/MSS–Co(salen) and GC/MSS–Co(salophen) electrodes (curve b′ in Fig. 5A and B, respectively) this OR process started at about −0.05 V. Thus, MSS–Co(salen) and MSS–Co(salophen) decrease the OR overpotential by 180 mV. The catalytic efficiency is also analyzed by determining the catalytic rate constants for the individual electrodes. The observed k_cat value (4.8 × 10⁴ M⁻¹ s⁻¹) for GC/MSS–Co(salen) can be compared with other catalytic OR reactions reported using a similar method. Comparison reveals that MSS–Co(salen) could be more efficient for OR reactions than MPS–ZnPc, Au–MPS, Au–MPS–ZnPc and Ag–MPS–ZnPc materials (where MPS is mercaptopropyl functionalized MSS type material and ZnPc is zinc phthalocyanine),¹⁶ however, less efficient than the MPS–CoPc and Au–MPS–CoPc materials (where CoPc is cobalt phthalocyanine).²² MSS–Co(salophen) also showed a comparable k_cat value (0.3 × 10⁴ M⁻¹ s⁻¹) with the MPS materials, however, less than the MSS–Co(salen). The salophen ligand being more planar than salen, it can form a thermodynamically stable adduct with O₂, which concurrently validates our observed better catalytic activity of Co(salen), which forms a less thermodynamically stable adduct.⁴⁴ Comparison of the catalytic efficiency and the rate constant may not be highly accurate since the concentration of the catalytic materials is different.

Fig. 5 Cyclic voltammograms of (A) GC/MSS (a, a′) and GC/MSS–Co(salen) (b, b′) and (B) GC/MSS (a, a′) and GC/MSS–Co(salophen) (b, b′) in 0.05 M HClO₄ under N₂ (a, b) and O₂ (a′, b′) saturated conditions. Scan rate: 50 mV s⁻¹.

3.5 Electrocatalytic HZ oxidation

Since HZ at the modified electrodes is electrochemically quite sensitive, we predict here the electrochemical responses of GC/MSS–Co(salen) and GC/MSS–Co(salophen) toward the analytical determination of HZ. Fig. 6A and B shows the CV responses of all modified electrodes in the presence and absence of 1.0 mM HZ. At GC/MSS, in the absence of HZ, no characteristic oxidation peak was predicted (curve a of Fig. 6A and B). GC/MSS–Co(salen) (curve b of Fig. 6A) and GC/MSS–Co(salophen) (curve b of Fig. 6B) show peaks due to the [Co(II)] to [Co(III)]⁺ redox couple (also refer Fig. 4A and B). In the presence of HZ, the GC/MSS electrode (curve a′ of Fig. 6A and B) shows oxidation current, however, there is no defined peak to show the oxidation potential of HZ. The GC/MSS–Co(salen) (curve b′ of Fig. 6A) and GC/MSS–Co(salophen) (curve b′ of Fig. 6B) electrodes show well defined anodic peaks attributed to the oxidation of HZ. On a first look at the magnitude of the oxidation current under identical concentrations of HZ, GC/MSS–Co(salophen) (curve b′ of Fig. 6B) seems to catalyze HZ more effectively compared to the GC/MSS–Co(salen). However, careful analysis reveals that the reverse is true. On GC/MSS–Co(salophen), HZ oxidation starts around 0.05 V and the corresponding anodic peak is observed at 0.31 V with high current. At the same time, the onset potential (0.03 V) and the peak potential (0.29 V) for HZ oxidation at GC/MSS–Co(salen) (curve b′ of Fig. 6A) is less positive (by 20 mV) than at the GC/MSS–Co(salophen) electrode, indicating the efficient oxidation of HZ at GC/MSS–Co(salen). The low current at GC/MSS–Co(salen) could be due to the low amount of adsorbed Co(salen) on MSS when compared with Co(salophen). It should be noted that the comparison may not be accurate because the concentrations of adsorbed Co(salen) and Co(salophen) are different. The CV responses obtained for a series of HZ solutions with various concentrations (from 1.0 to 900.0 μM) are illustrated in Fig. S2–S4 (ESI†). This property of increase in HZ oxidation current with increase in concentration is used to construct a sensor for HZ determination. Calibration curves are made from the CV responses, which is linear in the concentration range 1.0 to 400.0 μM with two linear segments (first linear range from 1.0 to 200.0 μM and next linear range from 200.0 to 400.0 μM) for both GC/MSS–Co(salen) (Fig. 6C) and GC/MSS–Co(salophen) (Fig. 6D). The following mechanism can be proposed for the oxidation of HZ on these modified electrodes in neutral pH (eqn (4) and (5)). Electrode oxidation maintains the Co(III) species at the electrode surface, which reacts with HZ to produce a possible intermediate N₂H₃. This intermediate then decomposes to produce N₂ as the final product.

MSS–[Co(III)S]⁺ + N₂H₄ + H₂O → N₂H₃ + H₃O⁺ + MSS–[Co(II)S] (4)

N₂H₃ + 3H₂O → N₂ + 3H₃O⁺ + 3e⁻ (5)

where S represents (salen) and (salophen).

Fig. 6 Cyclic voltammograms of (A) GC/MSS (a, a′) and GC/MSS–Co(salen) (b, b′) and (B) GC/MSS (a, a′) and GC/MSS–Co(salophen) (b, b′) in 0.1 M pH 7.0 phosphate buffer with the presence (a′, b′) and absence (a, b) of 1.0 mM HZ. Scan rate: 50 mV s⁻¹. (C) and (D) show the variation of anodic oxidation current with different concentrations of HZ for GC/MSS–Co(salen) and GC/MSS–Co(salophen) electrodes, respectively. The solid lines indicate the linear portions of the calibration plots and n represents the number of points used for the corresponding linear fit.

3.6 Amperometric detection of hydrazine

To further understand the sensing ability of these electrodes, amperometric responses were recorded with incremental additions of HZ to the supporting electrolyte solution (0.1 M pH 7.0 phosphate buffer). HZ was added at regular intervals of time and each addition increased the overall HZ concentration by 10 μM. The electrolyte solution was kept under constant stirring and the working electrode was applied with 0.4 V, which is a few mV more positive than the HZ oxidation peak potential in order to oxidize HZ at the maximum rate. The obtained amperometric responses for HZ determination at the GC/MSS–Co(salen) and GC/MSS–Co(salophen) electrodes are shown in Fig. 7A and B, respectively. They exhibit a quick response by considerable increase in current with increasing HZ concentrations. These well-defined amperometric responses demonstrate the stability and high catalytic efficiency of the materials. Inset in Fig. 7A is the corresponding calibration plot with a linear response within the concentration range of 10.0 to 210.0 μM for GC/MSS–Co(salen). GC/MSS–Co(salophen) (inset in Fig. 7B) shows two segmented linear calibration ranges (first one is from 10.0 to 50.0 μM and the other is from 50.0 to 310.0 μM).

Fig. 7 Current-time responses obtained at (A) GC/MSS–Co(salen) and (B) GC/MSS–Co(salophen) electrodes on successive injection of HZ to the supporting electrolyte solution (0.1 M pH 7.0 phosphate buffer). A constant stirring was maintained with an applied potential of 0.4 V. Inset: the corresponding calibration plots.

4. Summary

Immobilization of cobalt Schiff base complexes into/onto the channels of the MSS has been successfully completed in this work. Different characterization techniques such as UV-vis diffuse reflectance spectroscopy, FT–IR, and electrochemical studies evidence this. The MSS–Co(salen)/Co(salophen) materials were identified as electrocatalysts for oxygen reduction and HZ oxidation. Furthermore, these materials were also utilized to construct amperometric sensors for trace level HZ determination. MSS–Co(salen) exhibits better activity for the reduction of oxygen and also for the oxidation of HZ compared to MSS–Co(salophen). The GC/MSS–Co(salen) and GC/MSS–Co(salophen) electrodes can be repeatedly used for the electrocatalytic studies with small or no changes in catalytic activity. Efficient catalytic reduction of oxygen by MSS–Co(salen) was noted, with a high catalytic rate constant value.

Acknowledgements

The authors thank the Council of Scientific and Industrial Research (CSIR, 01(2098)/07/EMR-II), New Delhi, India for their munificent funding. We also acknowledge Prof. S. B. Rai, Department of Physics, BHU, Varanasi, India, for FT–IR and Mr Chandronil Chongdar for his assistance during the initial stage of this study in the synthesis, characterization and HZ oxidation. MP acknowledges CSIR for SRF.

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Footnote

† Electronic supplementary information (ESI) available: Cyclic voltammograms of GC/MSS–Co(salen) and GC/MSS–Co(salophen) electrodes with different concentrations of HZ. See DOI: 10.1039/c2cy20465g

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