Electroanalysis of thiocyanate using a novel glassy carbon electrode modified by aryl radicals and cobalt tetracarboxyphthalocyanine

Electrochimica Acta 53 (2007) 480–486

Electroanalysis of thiocyanate using a novel glassy carbon electrode modified by aryl radicals and cobalt tetracarboxyphthalocyanine Fungisai Matemadombo a , Philippe Westbroek b , Tebello Nyokong a,∗ a

Rhodes University, Department of Chemistry, PO Box 94, 6140 Grahamstown, South Africa b Ghent University, Department of Textiles, Technologiepark 907, B-9052 Ghent, Belgium Received 20 March 2007; received in revised form 25 June 2007; accepted 26 June 2007 Available online 25 July 2007

Abstract Electrochemical grafting of 4-nitrobenzenediazonium tetrafluoroborate onto a glassy carbon electrode (GCE) results in the formation of a nitrophenyl radical, which reacts with the surface to form a covalent bond (grafting) and results in a nitrophenyl modified electrode. The nitro group is electrochemically reduced to a NH2 group. Cobalt tetracarboxyphthalocyanine (CoTCPc) complex is then attached to the NH2 group using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as coupling agents. The new CoTCPc modified electrode was characterized using cyclic voltammetry and then employed for the catalytic oxidation of thiocyanate. © 2007 Elsevier Ltd. All rights reserved. Keywords: Cobalt tetracarboxyphthalocyanine; Thiocyanate; Aryl radical; Cyclic voltammetry; 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide; NHydroxysuccinimide

1. Introduction The deliberate alteration of electrode surfaces, through the incorporation of an appropriate surface modifier, may solve electroanalytical problems whilst forming the basis for new analytical applications and different sensing devices [1]. Chemically modified electrodes may improve analytical applications [2–5] by accelerating electron transfer reactions or allowing preferential accumulation. Glassy carbon is popular as an electrode material [1] due to its excellent mechanical and electrical properties, wide usable potential range, relatively reproducible performance (depending largely on solution purity and electrode pre-treatment [6]) and low cost [3] in comparison to Au or Pt electrodes. However, glassy carbon electrode (GCE) fabrication is difficult due to its hardness and fragility, limiting its use to the dimensions and forms obtained commercially [7]. Also, as glassy carbon has some amorphous characteristic, it is not always homogenous [7]. Oxidation of carbon surfaces results in oxygen functional groups (e.g. carboxy or hydroxyl) [8–11]. The precise nature

∗

Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109. E-mail address: [email protected] (T. Nyokong).

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.06.064

of these oxygen functionalized groups has proven problematic to study, aggravated by carbon surface corrosion [12–14]. It is consequently necessary to develop convenient and efficient modification techniques for the augmentation of glassy carbon surfaces. Metallophthalocyanines (MPcs) exhibit a series of oxidative and reductive electron transfer processes and hence may be used as versatile electron relays for the activation of redox processes [15,16] when immobilized onto electrode surfaces. Immobilization of MPcs on electrodes by polymerization or by the formation of self-assembled monolayers results in reproducible thin films. However, their formation requires the synthesis of ring substituted MPc complexes, which is very time consuming [17–19]. Methods for electrode modification using readily available MPc complexes are thus being developed. For example formation of self-assembled monolayers (SAMs) of simple MPc complexes onto pre-formed SAMs on gold have been reported [20,21]. In this work we present a new approach for the modification of a glassy carbon electrode (GCE) using a simple MPc complex, cobalt tetracarboxyphthalocyanine (CoTCPc, Fig. 1). A glassy carbon electrode is first modified by grafting of an aryl radical from nitrobenzenediazonium tetrafluoroborate (1, Scheme 1) [22], followed by reduction of the NO2 group to NH2 . The CoTCPc, synthesized according to literature meth-

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Fig. 1. Molecular structure of cobalt tetracarboxyphthalocyanine (CoTCPc, 4).

ods [23], is then attached to the NH2 group using a combination of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as coupling agents, Scheme 2. This work represents the first report of the coordination of a metallophthalocyanine (MPc) to an electrode pre-modified with aryl radicals. MPc complexes (especially CoPc derivatives) are excellent electrocatalysts for many analytes. The most effective methods of electrode modification employed using MPc complexes include electropolymerization and the formation of self-assembled monolayers, both requiring synthesis of derivatised MPc species, which as stated above are time consuming. The method presented in this work provides a new way of forming a stable electrode with readily available MPc complexes without the need for complicated and lengthy synthesis.

Scheme 1. Aryl radical modification of a glassy carbon electrode. GCE = glassy carbon electrode.

The CoTCPc functionalized GCE was used for the electroanalysis of thiocyanate. Thiocyanate is a vital biological and environmental molecule [24–27]. Medically, thiocyanates interfere with thyroxine synthesis in the thyroid gland thereby hindering iodine uptake by the thyroid gland and thiocyanate levels are used to detect the extent of cigarette smoking [25,26]. Environmentally, thiocyanate levels are used to monitor HCN (related chemical of thiocyanate) from fire atmospheres. Thiocyanate is harmful to aquatic life. The detection of thiocyanate is therefore important and hence in this work CoTCPc functionalized GCE is used to electroanalyze thiocyanate.

Scheme 2. Diagrammatic representation of carboxylic acid group (of 4) activation by EDC and NHS (5), followed by reaction with an amine on GCE-3 and then the formation of GCE-7. GCE = glassy carbon electrode. NHS = N-hydroxysuccinimide. EDC = 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide.

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2. Experimental 2.1. Materials and equipment Acetonitrile (ACN) and tetrabutylammonium tetrafluoroborate (NBu4 BF4 ) were purchased from Aldrich. Potassium thiocyanate was purchased from Riedel-de Ha¨en. 4-Nitrobenzenediazonium tetrafluoroborate was purchased from Sigma. Ethanol, distilled before use, was obtained from NCP Alcohols. N-Hydroxysuccinimide (NHS) was purchased from Fluka. 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) was purchased from Sigma–Aldrich. Cobalt tetracarboxyphthalocyanine (CoTCPc) was synthesized according to literature methods [23]. Potassium ferricyanide (K3 Fe(CN)6 ), dimethylformamide (DMF), HClO4 , pH 4 buffer tablets, NaOH, NaH2 PO4 and H3 PO4 were procured from Saarchem. Na2 HPO4 was purchased from PAL Chemicals. Acetone was purchased from Protea Chemicals and distilled before use. Deionized water was obtained from a Milli-Q water system. UV–vis spectra were recorded on a Varian 500 UV–vis/NIR spectrophotometer. Cyclic voltammetry studies were recorded using a BioAnalytical System (BAS) B/W 100 Electrochemical Workstation. 2.2. Electrochemical methods For cyclic voltammetry (CV), a conventional three electrode system consisting of a glassy carbon electrode (3.00 mm diameter) as a working electrode, Ag|AgCl pseudo reference electrode and a platinum wire counter electrode was employed. All electrochemical experiments were performed at room temperature. All solutions were purified of oxygen by bubbling nitrogen prior to experiments and the electrochemical cell was kept under nitrogen atmosphere throughout the analyses. The glassy carbon electrode was polished with aqueous slurry of alumina on a Buehler-felt pad followed by washing thoroughly with deionized water. The GCE was rinsed with deionized water, then with acetone before transferring into the electrochemical cell. For the formation of the nitrophenyl grafted GCE (GCE2, Scheme 1), cyclic voltammograms of the unmodified GCE were recorded in a fresh solution of 0.01 mol dm−3 of 1 in ACN containing 0.1 mol dm−3 NBu4 BF4 . In order to transform NO2 to NH2 , the cyclic voltammogram of GCE-2 was recorded in a protic aqueous solution containing 0.1 mol dm−3 KCl in EtOH:H2 O (1:9, v:v). This was followed by continuous cyclic voltammetric multi-cycling (52 scans) in the same protic aqueous solution resulting in GCE-3. Finally the attachment of CoTCPc to GCE-3 (forming GCE-7, Scheme 2), was performed by immersing GCE-3 into a solution of 1 mM CoTCPc (4) containing 5 mmol dm−3 EDC and 12.5 mmol dm−3 NHS in EtOH:DMSO (1:1, v:v) solution for 24 h to form GCE-7.

electrochemical reduction of 4-nitrobenzenediazonium tetrafluoroborate (diazonium salt, 1, Scheme 1) [22]. Such aryl radicals are unstable and react immediately with the electrode surface. This leads to the formation of strong covalent carbon–carbon bonds between the carbon of the GCE and the nitrophenyl group (step 1, Scheme 1). Fig. 2 shows the cyclic voltammograms for scan numbers 1, 2 and 50 of a GCE recorded in a fresh solution of 0.01 mol dm−3 of 1, in ACN containing 0.1 mol dm−3 NBu4 BF4 . The broad peak, at approximately −1.0 V (Fig. 2, scan 1), corresponds to the one electron reduction of diazonium salt 1, Eq. (1) [28]: (NO2 )ArN2 + + e− → (NO2 )Ar • + N2

(1)

where Ar represents the aryl group (C6 H4 ) and Ar• the aryl radical. The irreversible peak of scan 1 (Fig. 2), at approximately −0.20 V, is due to adsorbed species of 1 as has been observed before [28]. This fact was confirmed by a plot of peak current (Ip ) versus scan rate at different scan rates of this peak (Fig. 2, inset): the straight line obtained proved that the peak (at −0.20 V) is due to a surface reaction. Due to the adsorbed state of species 1, the activation energy for reduction, which is related to the applied overpotential, is much smaller compared to the activation energy of unadsorbed species (broad reduction wave around −1.0 V). From Fig. 2 it may be noted that there is a clear difference (i.e. inhibition of redox processes) between scan number 1 and scan number 2. This observed inhibition is due to the layer of GCE bound nitrophenyls, blocking any electron transfer, which explains the absence of redox peaks that were observed in scan number 1 [28]. The cyclic voltammogram of GCE-2, after derivatization (by cyclic voltammetry multi-scanning) in a solution of 0.01 mol dm−3 of 1 in ACN containing 0.1 mol dm−3 NBu4 BF4 (for 50 scans) and then rinsing thoroughly with acetone, was recorded in a blank solution of 0.1 mol dm−3 NBu4 BF4 in ACN. The result is shown in Fig. 3. The quasi-reversible cyclic voltammetric peak around −1.1 V of Fig. 3 is due to the 4-nitrophenyl

3. Results 3.1. Electrode modification using CoTCPc The initial part in the modification of glassy carbon electrodes of this work involves aryl radicals being generated from the

Fig. 2. Cyclic voltammograms of a GCE recorded in 0.01 mol dm−3 of 1, in ACN containing 0.1 mol dm−3 NBu4 BF4 . Numbers refer to scan numbers. Scan rate = 200 mV/s. Inset: plot of peak current (Ip ) vs. scan rate for scan 1.

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hydroxylamine is converted into ArNH2 and the peak currents decrease with scan number from 2 to 52 (due to inhibition by the layers of NH2 ) and finally disappear when the conversion is complete (scan number 52, Fig. 4). The transformation of ArNO2 to ArNH2 may be represented by the following equation [29,31,32]: GCE-ArNO2 + 6e− + 6H+ → GCE-ArNH2 + 2H2 O

Fig. 3. Cyclic voltammogram (scan number 5) of GCE-2, after derivatization and then rinsing thoroughly with acetone, recorded in blank (0.1 mol dm−3 NBu4 BF4 in ACN). Scan rate = 200 mV/s.

group redox process, Eq. (2) [28]: GCE-ArNO2 + e− ↔ GCE-ArNO2 −•

(2)

where GCE = GCE surface and Ar = C6 H4 . The redox process in Fig. 3 occurs near the potential of nitrobenzene itself [28], and was observed even after several scans. The electrochemical reduction of the NO2 to NH2 to form the aryl amino functionalized GCE-3 (step 2, Scheme 1) [22] was carried out as follows. GCE-2 was thoroughly rinsed with acetone to remove any unadsorbed species then transferred to a solution of 0.1 mol dm−3 KCl in EtOH:H2 O (1:9, v:v). The cyclic voltammetric results of GCE-2 in the 0.1 mol dm−3 KCl in EtOH:H2 O (1:9, v:v) solution are shown in Fig. 4. Scan number 1 of Fig. 4 presents a broad reduction peak at −1.15 V and an oxidation peak at Ep ≈ −0.4 V. The former peak (at −1.15 V) is related to the four electron reduction of nitrophenyl to phenylhydroxylamine [29,30]. Then, upon reversal of the scan direction, some of the formed phenylhydroylamine is oxidized at −0.4 V to nitrosobenzene. The formation of ArNH2 from the reduction of the nitrosobenzene corresponds to the reduction peak at −0.55 V during the second scan [29,30]. With each scan, more phenyl-

Fig. 4. Cyclic voltammogram of GCE-2 recorded in 0.1 mol dm−3 KCl in EtOH:H2 O (1:9, v:v). Numbers refer to scan numbers. Scan rate = 200 mV/s.

(3)

where Ar = C6 H4 . The fact that no reductive peak at −0.55 V was observed in the first scan can be explained by the absence of nitrosobenzene that is formed for the first time at the end of scan number 1. The formation of amide bonds between the amino modified GCE-3 and CoTCPc (4) was facilitated by the use of EDC and NHS as coupling agents. Jiang et al. [33] have estimated that using the mixture of EDC and NHS, about 60% of carboxylic acid groups are NHS-activated, 30% EDC-activated leaving only 10% not activated. Scheme 2 shows the mechanistic activation of carboxylic acid groups of CoTCPc by EDC and NHS forming a reactive intermediate 6, followed by the reaction of 6 with amine on GCE-3 and the formation of an amide bond [34]. The reaction of 6 with an amine (from GCE-3) results in GCE-7. Several coupling times (4, 8, 12 and 24 h) were studied but the best results, especially with respect to surface coverage, were obtained after a 24 h coupling time. Characterization of these electrodes in a K3 Fe(CN)6 (Fe(III)/Fe(II) couple) solution revealed indeed a maximum surface coverage and the smallest background currents obtained after a 24 h coupling time. For all modified electrode surfaces (GCE-2, GCE-3 and GCE-7) the electrochemical reactions related to the hexacyanoferrate redox couple were inhibited, therefore an experimental proof for the coupling of CoTCPc at the surface electrode was not obtained. Fig. 5 shows the cyclic voltammograms of GCE-7 recorded in a blank (1 mol dm−3 HClO4 ) solution showing the dependence of peak current on scan rate. Inset of Fig. 5 shows the linear dependence of peak current on scan rate of GCE-7 in blank (1 mol dm−3 HClO4 ) solution, typical of adsorbed species. The peak couple at approximately 150 mV of Fig. 5 is assigned to [Co(III)Pc−2 /Co(II)Pc−2 ].

Fig. 5. Cyclic voltammograms of GCE-7 recorded in blank (1 M HClO4 ) solution showing the dependence of peak current on scan rate (25–200 mV s−1 ). Inset: scan rate study demonstrating the dependence of peak current on scan rate of GCE-7 in blank (1 mol dm−3 HClO4 ) solution.

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Surface coverage (Γ ) may be calculated using the following equation (4) [1]: ip =

n2 F 2 AΓ (υ) 4RT

(4)

where ip is the peak current (A), n the number of electrons, A the area of the electrode (cm2 ), υ the scan rate (V/s) and the other symbols have their usual meanings. The Γ value of GCE-7 was estimated from the background corrected peak current (ip ) under the [Co(III)Pc−2 /Co(II)Pc−2 ] couple (Fig. 5). The Γ value of GCE-7 was 4.2 × 10−9 mol/cm2 and is four times higher than the 1 × 10−10 mol/cm2 range reported for metallophthalocyanines lying flat on the surface [35–37], thus confirming the expected perpendicular orientation for CoTCPc in GCE-7. 3.2. Electroanalysis 3.2.1. Electrocatalytic activity of GCE-7 towards thiocyanate Fig. 6 shows the cyclic voltammogramms of GCE-7 recorded in the absence (i) and presence (ii) of thiocyanate in pH 4 buffer. No peaks were observed on unmodified GCE, GCE2 or GCE-3 recorded in thiocyanate in pH 4 buffer (Figs. not shown). But on GCE-7 an oxidation peak was clearly observed at Ep ≈ 0.75 V (Fig. 6ii). The observation of the peak proves electrocatlytic activity of GCE-7 towards the oxidation of thiocyanate. Previous studies [38] showed the peak for the electrocatalyzed oxidation of thiocyanate occurring at similar potentials (0.75–0.78 V versus Ag|AgCl) as reported in this work. The Tafel slope was determined by using the standard equation (Eq. (5)) for a totally irreversible process [39]: Ep =

2.3RT log υ + K 2(1 − α)nF

(5)

Fig. 7. Plot of Ep vs. log v for GCE-7 in 7.2 × 10−4 mol dm−3 thiocyanate in pH 4 buffer solution.

where α is the transfer coefficient, υ the scan rate, n the number of electrons involved in the rate determining step and K is the intercept. A plot of Ep versus log v for GCE-7 in thiocyanate in pH 4 buffer solution (Fig. 7) gave a linear relationship. A Tafel slope (calculated using Eq. (5)) of 68 mV dec−1 , indicates that a fast one electron transfer is followed by a slow chemical step. The calculated α value of approximately 0.5 (α = 0.6) signifies that there is an equal probability of forming either products or reactants from the reaction activated transition state. The heterogeneous electron transfer coefficient, k, was obtained from the following equation:    2.3 (1 − α)nFD RT 0 K=E + 0.78 + log (6) (1 − α)nF 2 k2 RT 

where E0 is the formal potential, D the diffusion coefficient and the other symbols have their usual meaning. Using D = 2.15 × 10−5 cm2 s−1 [40], k was calculated as 0.507 cm s−1 . The total number of electrons (nt ) involved in the electrocatalytic oxidation of thiocyanate was calculated using Eq. (7), valid for a totally irreversible electrode process [39]: ip = 2.99 × 105 nt [(1 − α)n]1/2 ACo D1/2 υ1/2

Fig. 6. Cyclic voltammograms of GCE-7 recorded in pH 4 buffer (i) and 7.2 × 10−4 mol dm−3 thiocyanate in pH 4 buffer solution (ii). Scan rate = 200 mV/s.

(7)

where A is the area of the electrode (cm2 ), Co the concentration of the electroactive reactant (mol dm−3 ) and D is the diffusional coefficient of thiocyanate (D = 2.15 × 10−5 cm2 s−1 [40]). The total number of electrons transferred, calculated using Eq. (7), was calculated as 1 (nt = 1.3). Fig. 8 shows the linear relationship (R2 = 0.990) between peak current and thiocyanate ion concentration (mmol range) of GCE-7. The linear range was 0.001–0.01 mol dm−3 and a sensitivity of 67 ␮A/mol dm−3 was obtained. GCE-7 is useful for analyses of thiocyanate concentrations up to 1 × 10−2 mol dm−3 (beyond which the linear relationship is lost). Due to the linear relationship of Fig. 8, the graph may be used as an analytical tool for determining the concentration of thiocyanate in solution in this range. The detection limit was found to be 6.5 × 10−6 mol dm−3 (3␦ criteria), a value which is comparable to other types of thiocyanate selective electrodes [25,26,41].

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4. Conclusion In this work, a GCE is modified by electrochemical grafting of 4-nitrobenzenediazonium tetrafluoroborate (1) forming nitrophenyl radicals on the GCE. This is followed by the electrochemical reduction of the NO2 group to a NH2 group to form the aryl radical amino functionalized GCE-3. Through the use of EDC coupling, assisted by NHS, CoTCPc is coordinated to the modified electrode. The new CoTCPc electrode was successfully employed in the electrocatalytic oxidation of thiocyanate confirming the attachment of the phthalocyanine complex onto the aryl radical grafted GCE. Acknowledgements Fig. 8. Plot of peak current vs. SCN− concentration (0–4.2 mM).

3.2.2. Possible mechanisms Fig. 9 shows the UV–vis spectra of Co(II)TCPc (i) and Co(II)TCPc + SCN− (ii) recorded in DMF. The 5 nm shift in Q band, between Co(II)TCPc (λmax = 680 nm) and Co(II)TCPc + SCN− (λmax = 685 nm) confirms a coordination of SCN− to the Co(II)TCPc complex. Shifts in Q band are typical of axial ligation in MPc complexes [42]. Based on the observations above, the following mechanism is proposed for the catalytic oxidation of SCN− : Co(II)Pc + SCN− → [(SCN− )Co(II)Pc]−

(8)

[(SCN− )Co(II)Pc]− → [(SCN− )Co(III)Pc] + e−

(9)

Acid

[(SCN− )Co(III)Pc] −→ Co(II)Pc + HSCN

(10)

Eq. (8) is proposed since shifts in spectra typical of axial ligand exchange were observed on addition of SCN− to solutions of CoTCPc (Fig. 9). Eq. (9) is proposed because oxidation of thiocyanate occurs at potentials following the formation of the Co(III)Pc. The total number of electrons involved was found to be unity hence suggesting the formation of HSCN in acid media rather than the more common (SCN)2 . However the reduction peak due to HSCN reported before [34] was not observed in this work.

Fig. 9. UV–vis spectra of Co(II)TCPc (i) and Co(II)TCPc + SCN− (ii) recorded in DMF.

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