Characterization of polymeric film of a new manganese phthalocyanine complex octa-substituted with 2-diethylaminoethanethiol, and its use for the electrochemical detection of bentazon

Electrochimica Acta 55 (2009) 37–45

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Characterization of polymeric film of a new manganese phthalocyanine complex octa-substituted with 2-diethylaminoethanethiol, and its use for the electrochemical detection of bentazon Isaac Adebayo Akinbulu, Tebello Nyokong ∗ Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa

a r t i c l e

i n f o

Article history: Received 15 May 2009 Received in revised form 11 July 2009 Accepted 26 July 2009 Available online 3 August 2009 Keywords: Bentazon Manganese diethylaminoethanethiol phthalocyanine Electropolymerization Cyclic voltammetry Electrochemical impedance spectroscopy

a b s t r a c t Manganese acetate octakis-(2-diethyaminoethanethiol) phthalocyanine (AcMnODEAETPc) was newly synthesized and characterized by spectroscopic and electrochemical methods. Solution electrochemistry of the complex showed three redox processes assigned to MnIII Pc−1 /MnIII Pc−2 , MnIII Pc−2 /MnII Pc−2 and MnII Pc−2 /MnII Pc−3 species. The new molecule was polymerized onto a glassy carbon electrode (GCE) to form thin films of different thickness, giving poly-10-AcMnODEAETPc-GCE, poly-20-AcMnODEAETPcGCE and poly-30-AcMnODEAETPc-GCE, where 10, 20 and 30 represent the number of voltammetry scans during polymerization. Three distinct redox processes were observed on the modified electrode in 0.1 M phosphate buffer solution, pH 5, which confirmed the formation of the polymer. The current signal due to the herbicide, bentazon, was dependent on film thickness; the best signal was obtained on poly20-AcMnODEAETPc-GCE while poly-10-AcMnODEAETPc-GCE gave the least signal. However, the signals due to the herbicide were better on the different films compared to the bare electrode. Electrochemical impedance spectroscopy (EIS) technique revealed that differences in film thickness offered different charge transfer resistances, Rct , hence difference in current signals for bentazon oxidation were observed on these films. A Tafel slope of 77 mV/decade, obtained for the herbicide on poly-20-AcMnODEAETPcGCE, denotes a fast one electron transfer followed by a slow chemical step in the electro-oxidation of bentazon. The voltammetry signals of the herbicide on the films indicated the likely involvement of ring-based redox processes in the detection of the herbicide. A plot of background corrected current response, on this film, versus the concentration of bentazon was linear within the range 50–750 ␮M with a detection limit of 2.48 × 10−7 M. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Metallophthalocyanine (MPc) complexes have been used as electrocatalysts for many reactions [1,2]. For use as electrocatalysts, MPcs are used to modify electrodes as thin films. Formation of thin films may be accomplished in various ways, such as electropolymerization and self-assembly techniques; the former was used in the current work because of its ability to form multilayered polymer coatings of the complexes, forming a three-dimensional reaction zone at the electrode surface, thus enhancing sensitivity of the electrode response [3]. The choice of central metal has appreciable influence on both the electrocatalytic behaviour of MPc complexes and the conducting pathways in the polymers formed from these complexes. In the present work, a new manganese phthalocyanine complex, octa-substituted at the peripheral positions with 2-

∗ Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109. E-mail address: [email protected] (T. Nyokong). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.07.074

diethylaminoethanethiol groups (AcMnODEAETPc, Scheme 1), was electropolymerized on glassy carbon electrode. Mn was chosen as central metal because it exhibits variable oxidation states in MPcs. These range from MnI to MnIV , hence MnPcs have potential to be good catalysts, especially for reactions involving multi-electron transfer. In addition, MnPcs have not been extensively explored as electrocatalysts compared to CoPc and FePc derivatives [1,2]. The focus of the current study is to investigate the use of polymeric film of AcMnODEAETPc for electrocatalytic detection of the herbicide, bentazon. Electrochemical impedance spectroscopy (EIS) was employed to study the dependence of current response of the herbicide on film thickness. This technique has been widely used to study the kinetics of electrochemical intercalation in polymer film electrodes [4–7]. Bentazon (Fig. 1) is a selective and contact herbicide used for post-emergent weed control of broad leafed weeds in beans, rice, maize and peanuts [8]. Toxic effects such as trembling, weakness, vomiting, diarrhea and difficulty in breathing have been identified with this herbicide. Its detection and quantification in environmental samples are therefore necessary. The methods that have

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I.A. Akinbulu, T. Nyokong / Electrochimica Acta 55 (2009) 37–45

Scheme 1. Synthetic pathway for the formation of manganese acetate octakis-(2-diethyaminoethanethiol) phthalocyanine (AcMnODEAETPc).

been reported for the quantification of bentazon in environmental samples include gas chromatography [9,10] and high performance liquid chromatography (HPLC) [11,12]. The sensitive nature of these methods is overshadowed by extensive and time-consuming sample preparation, making them unsuitable for routine environmental monitoring of this herbicide. The use of bare glassy carbon electrode for the detection of bentazon in commercial sample has also been reported [13]. Severe poisoning of the electrode, by the oxidation product of the herbicide, places a major constrain on the use bare electrode. The use of conducting polymers of polyaniline and polypyrole, modified with carbon paste, has also been reported [14]. The measured voltammetry signal depended on the amount of bentazon sopped from the solution, which may not truly reflect the actual concentration of the herbicide in a given sample. The use of MPc complexes for the detection of

bentazon has not been reported before, despite the reported electrocatalytic properties of MPc complexes towards many analytes [1,2]. 2. Experimental 2.1. Materials and reagents Bentazon was purchased from Sigma–Aldrich and used as supplied. Stock solution of bentazon (1.04 × 10−2 M) was prepared in freshly distilled ethanol because of its limited solubility in water. Different concentrations of bentazon were prepared by diluting desired volumes of the stock solution in required amounts of 0.1 M phosphate buffer solution of pH 5. All solutions were prepared with ultra pure water of resistivity 18.2 M cm obtained from

I.A. Akinbulu, T. Nyokong / Electrochimica Acta 55 (2009) 37–45

Fig. 1. Molecular structure of bentazon.

a Milli-Q Water system. Electrochemical experiments were carried out in nitrogen-saturated buffer containing small amounts of ethanol from the stock solution of bentazon. Electrode modification was achieved by electropolymerization of AcMnODEAETPc in freshly distilled dimethyl formamide (DMF, SAARCHEM) containing 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4 , Aldrich) as supporting electrolyte. Dichloromethane (DCM), tetrahydrofuran (THF) and methanol (MeOH) were also purchased from SAARCHEM. All other reagents were of analytical grade and were used as received from the suppliers without further purification. 2.2. Electrochemical methods All electrochemical experiments were performed using Autolab potentiostat PGSTAT 302 (Eco Chemie, Utrecht, The Netherlands) driven by the general purpose electrochemical system data processing software (GPES, software version 4.9). Square wave voltammetric analysis was carried out at a frequency of 10 Hz, amplitude: 50 mV and step potential: 5 mV. A conventional three-electrode system was used. The working electrode was a bare glassy carbon electrode (GCE) or GCE modified with AcMnODEAETPc designated as poly-n-MnODEAETPc-GCE, where n represents the number of voltammetry scans during electropolymerization. Ag|AgCl wire (for polymerization only) and platinum wire were used as the pseudo reference and auxiliary electrodes, respectively. The potential response of the Ag|AgCl pseudo-reference electrode was less than the Ag|AgCl (3 M KCl) by 0.015 ± 0.003 V. For studies of bentazon, the Ag|AgCl (3 M KCl) was employed. Prior to electropolymerization, the electrode surface was polished with alumina on a Buehler felt pad and rinsed with excess millipore water. The total effective area of the electrode was estimated using double step chronocoulometry, with hexacyanoferrate (III) (4 mM), in 1 M KCl, as the redox active species. Hexacyanoferrate (III) was used because it has a known diffusion coefficient, Do = 7.6 × 10−6 cm2 s−1 [15]. The total effective area of the electrode was obtained from the slope of the plot of charge, Q, versus the square root of time, t1/2 (Eq. (1)). Q =

2nFAC0 D1/2 t 1/2 1/2

(1)

where Q is the charge in coulombs, n is the number of electron transfer, ∼1, F is Faraday’s constant, t is time in seconds, C0 and D are the bulk concentration and diffusion coefficient of hexacyanoferrate (III), respectively, and A is the total effective area of the electrode. Electropolymerization was achieved by repetitive cyclic voltammetry scanning (10, 20 or 30 cycles) of the GCE (from −1.0 V to +1.0 V versus Ag|AgCl) in 1.0 × 10−4 M of the AcMnODEAETPc monomer, in freshly distilled DMF containing 0.1 M TBABF4 as supporting electrolyte, to form the respective polymeric films. The modified electrode was characterized by scanning in 0.1 M phosphate buffer, pH 5, between −1.0 V and +1.0 V (versus Ag|AgCl). Electrochemical impedance spectroscopy measurements were performed with Autolab FRA software between 1.0 mHz and 10 kHz using a 5 mV rms sinusoidal modulation in 100 ␮M bentazon at its

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half-wave potential of 0.80 V (versus Ag|AgCl). A non-linear least squares (NNLS) method based on the EQUIVCRT programme [16] was used for automatic fitting of the obtained EIS data. All experiments were performed at approximately 25 ◦ C. Spectroelectrochemical data were obtained using a home-made optically transparent thin-layer electrochemical (OTTLE) cell which was connected to a Bioanalytic Systems (BAS) CV 27 voltammograph. UV–vis spectra were recorded on Cary 50 UV/vis/NIR spectrophotometer. IR (KBr discs) was recorded on Bruker Vertex 70-Ram II spectrophotometer. Elemental analysis was performed using Vario Elementar Microcube EL111. 1 H nuclear magnetic resonance (1 H NMR, 400 MHz) was obtained in CDCl3 using Bruker EMX 400 NMR spectrometer. 2.3. Synthesis 2.3.1. 1,2-Bis-(diethylaminoethanethiol)-4,5-dicyanobenzene (2) 1, 2-Dichloro-4,5-dicyanobenzene (1), Scheme 1, was synthesized according to reported procedure [17]. 1,2-Bis-(2diethylaminoethanethiol)-4,5-dicyanobenzene (2) was also synthesized according to literature procedure [18], with slight modification as follows: compound 1 (2.32 g, 11.75 mmol) was dissolved in anhydrous DMF (100 ml) under nitrogen and 2diethylaminoethanethiol hydrochloride (6 g, 35.34 mmol) was added. After stirring for 10 min, finely ground anhydrous K2 CO3 (19.5 g, 141.36 mmol) was added in portions over 2 h with stirring. The reaction mixture was stirred at room temperature for 48 h under nitrogen. Then the solution was poured into ice (600 g). The precipitate was filtered off, washed with water, until the filtrate was neutral. The product was then dried in air. Yield: 3.61 g (80%). IR (KBr) vmax (cm−1 ): 3075, 2970–2812, 2229, 1562, 1459, 1382, 1343, 1275, 1200, 1113, 1066, 991, 733, 529. 1 H NMR (CDCl3 ) ı = 7.54 (s, 2H, Ar-H), 3.12–3.08 (t, 4H, SCH2 ), 2.80–2.77 (t, 4H, NCH2 ), 2.61–2.55 (qnt, 8H, CH2 C), 1.05–1.01 (t, 12H, CH3 ) ppm. 2.3.2. Manganese(III) acetate octakis-(2-diethylaminoethanethiol) phthalocyanine (AcMnODEAETPc) Manganese(III)acetate octakis-(2-diethylaminoethanethiol) phthalocyanine was synthesized according to the procedure recently reported for the corresponding Co complex [18], with some modifications, Scheme 1. A mixture of compound 2 (0.6 g, 1.54 mmol), manganese acetate (0.066 g, 0.38 mmol) and 2(dimethylaminoethanol) (1.2 ml) was refluxed for 12 h under nitrogen. After cooling to room temperature, the mixture was treated with excess MeOH:H2 O (1:1, v/v) in order to precipitate the product. The product was filtered and dried in air. The product was then purified using column chromatography with neutral alumina as column material and DCM/MeOH (20:1) as eluent. Yield: 0.54 g (86.72%) (found: C, 57.11%; H, 6.72%; N, 12.63%; calc. for C80 H120 N16 S8 MnOAc.CH2 Cl2 : C, 58.51%; H, 7.34%; N, 13.16%; UV–vis (THF): max (nm) (log ε): 353 (4.6), 423 (4.5), 465 (4.5), 503 (4.4), 676 (4.3), 750 (5.0); IR (KBr) vmax (cm−1 ); 2966–2805 (CH2 ), 1413, 1377, 1327, 1070, 781, 744, 705, 606. 3. Results and discussion 3.1. Synthesis and spectroscopic characterization Scheme 1 shows the synthetic route involved in the formation of AcMnODEAETPc. The phthalonitrile, compound 2, was obtained via a based-catalysed (K2 CO3 ) nucleophilic aromatic substitution reaction, with 2-diethylaminoethanethiol acting as the nucleophile. Cyclotetramerization of compound 2 to form AcMnODEAETPc occurred in the presence of Mn(CH3 COO)2 . Purification of the

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I.A. Akinbulu, T. Nyokong / Electrochimica Acta 55 (2009) 37–45 Table 1 Electrochemical data of thiol substituted MnPc. Half-wave (E1/2 ) and peak (Ep ) potentials in V versus Ag|AgCl. Values were recorded in DMF containing TBABF4 . Complexa

MII Pc−3 /MII Pc−2

MIII Pc−2 /MII Pc−2

MIII Pc−1 /MIII Pc−2

Ref.b

3 MnTMPyPc MnOPMPcc MnTDMPcc

−0.82

−0.21 −0.057 −0.46 −0.26

+0.78 +1.34 +0.75 +0.83

TW [31] [32] [20]

−1.24 −0.98

a TMPy = Tetra-(2-mercaptopyridine), OPM = Octapentylmercapto, TDM = Tetrakis (dodecyl-mercapto). b TW: this work. c Recorded in DCM containing TBABF4 .

Fig. 2. UV–vis spectra of 6.7 × 10−6 M of AcMnDEAETPc in THF.

complex was achieved by column chromatography on alumina. AcMnODEAETPc is soluble in solvents such as THF, DCM and DMF. The complex was characterized by IR and UV–vis spectroscopies as well as elemental analysis. The results obtained were consistent with the predicted structure. The formation of AcMnODEAETPc was confirmed by the disappearance of the sharp C N vibration at 2229 cm−1 of 2. AcMnODEAETPc showed characteristic Q-band absorption, in THF, at 750 nm (Fig. 2). The absorptions in the 450 nm region are usually associated with charge transfer in MnPc [19]. Fig. 2 shows that the Q-band due to AcMnODEAETPc is appreciably red-shifted compared to that of MPcs in general. This observation is characteristic of MnPc complexes [19]. 3.2. Voltammetric and spectroelectrochemical characterization Fig. 3 shows the cyclic and square wave (inset) voltammetry profiles of ∼1 × 10−3 M of AcMnODEAETPc in freshly distilled dimethyl formamide containing 0.1 M TBABF4 as supporting electrolyte. Three distinct processes, labeled I, II and III, can be identified. Redox couples II (E1/2 = −0.21 V) and III (E1/2 = −0.82 V) versus Ag|AgCl, are reversible with peak separations of E = 80 mV for couple II and E = 90 mV for couple III. E value for ferrocene, at the same scan rate was ∼90 mV versus Ag|AgCl. The cathodic to anodic peak current ratio were near unity for both processes II and III. Plots of peak current (Ip ) versus square root of scan rate (v1/2 ) (not shown) were

Fig. 3. Cyclic and square wave (inset) voltammograms of 10−3 M of AcMnDEAETPc in freshly distilled DMF containing 0.1 M TBABF4 . Scan rate: 100 mV s−1 versus Ag|AgCl.

linear for both processes, thus suggesting diffusion control at the electrode surface. Redox couple II is assigned to MnIII Pc−2 /MnII Pc−2 and couple III to MnII Pc−2 /MnII Pc−3 in comparison with literature [20]. Process I was irreversible (Ep = +0.78 V versus Ag|AgCl), it was assigned to ring oxidation, MnIII Pc−1 /MnIII Pc−2 [20]. The lack of reversibility of the oxidation processes is typical of thio substituted MPc complexes [20]. Assignment of these processes was based on the wellknown electrochemistry of manganese phthalocyanine complexes (Table 1). Processes II and III involved one electron each. Redox processes involving metallophthalocyanine complexes may involve the ring or the central metal (for electroactive metal) and each process is normally one electron transfer process. Multi-electron transfer processes can also occur in MPc complexes and are easily observable by relatively higher peak current. These processes usually occur in the presence of ring substituents containing oxidizable group such as sulphur or nitrogen atom [21]. Thus, the relatively higher peak current observed for couple I (Fig. 3) can be attributed to the presence of sulphur and nitrogen atoms in the substituent (Scheme 1). The redox processes and the number of electron transferred in each case were confirmed using spectroelectrochemistry. Spectroelectrochemical studies were carried out using optically transparent thin-layer electrode cell. Fig. 4a shows the spectral changes observed on the application of potential of couple II (−0.35 V versus Ag|AgCl). The initial spectrum showed very pronounced peak at 653 nm with the usual Q-band at 757 nm. The slight shift in the Q-band from 750 nm (Fig. 2) to 757 nm (Fig. 4a) is due to differences in solvents and the presence of electrolyte. The peak at 653 nm is characteristic of MnPc ␮-oxo complexes [19]. This species is observed in DMF (Fig. 4a) but was not observed in THF (Fig. 2). The peak at 757 nm is due to MnIII Pc species. There was no MnII Pc species before reduction. The presence of the ␮-oxo MnPc species was confirmed by monitoring the spectral transformations of complex 3 in DMF solution when not de-aerated and when de-aerated with dry N2 gas. This was evidenced by the decrease of the peak at 653 nm on bubbling nitrogen. However, attempts to exclude oxygen completely during spectroelectrochemistry were unsuccessful. The cyclic and square wave voltammograms reported above were reported under an atmosphere of nitrogen hence are due to mainly the MnIII Pc species. Upon reduction at potentials of processes II, there was a blue shift in the Q-band from 757 nm to 705 nm and the colour of the complex changed from purple to green, a gradual disappearance of the charge transfer bands in the 500 nm region was also noticed, with an isobestic point at 736 nm. The isobestic point observed was not so clear, which suggests the presence of more than two species, thus confirming the presence of the ␮-oxo complex with the starting MnIII Pc−2 species and the electro-generated species, MnII Pc−2 . The formation of the latter was accompanied by the formation of more ␮-oxo MnPc species. The persistence of ␮-oxo MnPc species has been observed for some MnPc complexes [22].

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Fig. 5. Repetitive cyclic voltammograms of 10−4 M AcMnODEAETPc in DMF containing 0.1 M TBABF4 . Scan rate: 100 mV s−1 versus Ag|AgCl. First scan (dashed line).

with increased intensities, at 543 nm and 590 nm are characteristic of ring-based redox process [25]. This observation confirmed that process III is a ring-based reduction, thus justifying the assignment of the couple to MnII Pc−2 /MnII Pc−3 redox process. This supports the claim that first reduction in MnII Pc−2 occur at the ring, although, other authors have reported metal reduction to the MnI Pc−2 species [19]. A drastic decrease in intensity was also observed for the band (653 nm) associated with the ␮-oxo complex. The reaction in Fig. 4b, did not go to completion hence the number of electrons were not calculated, however, the spectral changes occurring confirm ring-based process. 3.3. Electrode modification and optimization of film thickness Fig. 4. UV–vis spectra changes for AcMnODEAETPc observed during controlled potential electrolysis at (a) −0.31 V (process II), (b) −0.9 V (process III). Electrolyte = DMF containing 0.1 M TBABF4 . Initial spectrum (i), final spectrum (ii).

A shift in the position of the Q-band without a decrease in intensity is typical of metal-based electro reduction process, and a blue shift in the Q-band upon reduction is usually associated with the reduction of MnIII Pc−2 to MnII Pc−2 [19]. These spectral changes thus confirmed that redox couple II in Fig. 3 is due to the reduction of MnIII Pc−2 to MnII Pc−2 . The number of electrons involved (n) was determined to be approximately unity using Eq. (2) [23], Q = nFVC

(2)

where n, F, V and C are the number of electrons transferred, Faraday’s constant, volume and concentration of the electroactive species (AcMnIII ODEAETPc), respectively. Q is the charge obtained at the end of the transformation in Fig. 4a (i.e. for curve ii). Even though the ␮-oxo complex was formed (as discussed below) only one electron was transferred, suggesting that the MnII Pc species forms and then undergoes equilibrium reactions discussed below. It has been suggested that an equilibrium exists between MnPc species in DMF and in the presence of oxygen as shown by Eqs. (3)–(6) [22,24]: PcMnII + O2  PcMnIII (O2 )

(3)

PcMnIII (O2 ) + PcMnII  PcMnIII − O2 − MnIII Pc III

PcMn

III

IV

− O2 − Mn Pc  2PcMn O

IV

II

III

2PcMn O + 2PcMn  2PcMn II

4PcMn

+ O2 → 2PcMn

III

(5) III

− O − Mn Pc

–O–Mn

III

(4)

Pc(net equation)

(6) (7)

Further reduction of AcMnODEAETPc, at the potentials of couple III, resulted in the spectral changes shown in Fig. 4b. A decrease in intensity of the new Q-band (705 nm) and emergence of new bands,

Modification of electrode was accomplished by electropolymerization of the monomer on glassy carbon electrode. Polymer formation involving MPc complexes is usually facilitated by the presence of substituent containing oxidizable group. For instance, oxidation of the amino substituent in MPc(NH2 )4 resulted in the formation of radicals, which attack benzene rings of neighbouring molecules, initiating polymer formation. In the present work, oxidation of the substituent, on both the sulphur group and the nitrogen atom of the amino group, may have facilitated polymer formation. However, the nitrogen atom of the tertiary amino group is more susceptible to oxidation than the sulphur atom, because the lone pairs of electron on this atom are of higher density than that on the sulphur atom. The thickness of the polymeric film, formed during electropolymerization, is determined by the number of cyclic voltammetry scans. In the present work, different number of voltammetry cycles (10, 20 and 30) was employed during electropolymerization. Fig. 5 shows the evolution of cyclic voltammograms obtained during repetitive scanning of 1 × 10−4 M AcMnODEAETPc (20 cycles) in freshly distilled DMF containing 0.1 M TBABF4 supporting electrolyte. Electropolymerization was evident from the emergence of new weak peak at ∼−0.5 V [26], and the shift in peak potentials and increase in peak currents of both the cathodic and anodic components of the couples labeled II and III in Fig. 5. The peak potentials of the anodic components shifted from ∼+0.02 V to ∼+0.14 V (for couple II) and ∼−0.65 V to ∼−0.56 V (for couple III) while the cathodic components shifted from ∼−0.08 V to ∼−0.17 V (for couple II) and ∼−0.77 V to ∼−0.83 V (for couple III). The shift to more positive potential, for the anodic components, and more negative potential, for the cathodic components, with increase in scan number, is a proof of an increase in the electrical resistance of the polymeric film, and overpotential is needed to overcome this resistance [27].

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Fig. 6. Cyclic voltammetry profiles of poly-20-AcMnODEAETPc-GCE (solid line) and bare GCE (dashed line) in 0.1 M phosphate buffer of pH 5. Scan rate: 100 mV s−1 versus Ag|AgCl. Inset: plot of peak current versus scan rate (100–500 mV s−1 ).

3.4. Characterization of modified electrode The modified electrode was dried in a slow stream of nitrogen and characterized in 0.1 M phosphate buffer solution of pH 5, Fig. 6. This pH was used since all voltammetry measurements were carried out at this pH value. The presence of polymeric film of AcMnODEAETPc on the electrode was evident from the cyclic voltammograms shown in Fig. 6. Three distinct redox processes were observed on the modified electrode, in 0.1 M phosphate buffer solution, pH 5. The redox couple labeled I, Ep = ∼+0.80 V (versus Ag|AgCl) is characteristic of ring-based process of the absorbed AcMnODEAETPc, it may be assigned to MnIII Pc−1 /MnIII Pc−2 species. The peak potential for process I (ring oxidation, Pc−1 /Pc−2 ) was very close to that observed for the solution of the monomer (Ep = +0.78 V Ag|AgCl). Process II, E1/2 = ∼−0.24 V (versus Ag|AgCl) was assigned to MnIII Pc−2 /MnII Pc−2 redox species [20]. This process was also observed in the solution of the monomer at E1/2 = −0.21 V versus Ag|AgCl. Redox couple III (E1/2 = −0.67 V versus Ag|AgCl), assigned to MnII Pc−2 /MnII Pc−3 , was observed in monomer solution but at E1/2 = −0.82 V versus Ag|AgCl. The more positive nature of this reduction couple on the modified electrode, relative to that of the monomer solution, suggests better electron transfer process on the polymeric film than in the monomer solution. The peak to peak separation for this couple was 40 mV versus Ag|AgCl. E values for surface-confined redox couples are usually non-zero, compared to zero volt expected for the Nernstian reaction of an ideal reversible surface-confined redox species. However, plot of current versus scan rate was linear (inset in Fig. 6), confirming the presence of surface-confined redox species The surface coverage of the polymer on the electrode was estimated for the different number of voltammetry scans (10, 20 and 30), using Eq. (8), where n is the number of electron transferred ∼1, A is the total effective area of the electrode (0.042 cm2 , using Eq. (1)), F is Faraday’s constant and Q (1.28 × 10−6 C for 10 scans, 2.28 × 10−6 C for 20 scans and 2.71 × 10−6 C for 30 scans) is the integrated background corrected charge for the cathodic peak of couple II (Ep = ∼−0.24 V versus Ag|AgCl), Fig. 6. Values of surface coverage ( MODEATPc ) were calculated to be 3.16 × 10−10 mol/cm2 , 5.63 × 10−10 mol/cm2 and 6.69 × 10−10 mol/cm2 for 10, 20 and 30 cyclic voltammetry scan number, respectively. MODEATPc =

Q nFA

where the rest of the symbols are as described for Eq. (2).

(8)

Fig. 7. Repetitive cycling of poly-20-AcMnODEAETPc-GCE in 0.1 M phosphate buffer, pH 5, first scan (solid line) and fifth scan (dashed line). Scan rate: 100 mV s−1 versus Ag|AgCl.

3.5. Voltammetry signals of bentazon on poly-n-AcMnODEAETPc-GCE Prior to voltammetry measurements, the modified electrode was conditioned in 0.1 M phosphate buffer solution, pH 5 (Fig. 7), by repetitive cycling, until a reproducible scan was obtained. Electrode conditioning improves electrode stability. The conditioning process resulted in the decrease in the currents of process I. The current signal due to bentazon appeared at potentials of process I on the modified electrode where there was no peak after conditioning, hence no contribution from the background peak. The current signals observed in Fig. 8 can thus adequately be assigned to bentazon. No signal is expected to be observed in the blank (Fig. 8e) since voltammetry measurements were recorded with the conditioned electrode. The best current signal (background corrected) due to bentazon was obtained on poly-20-AcMnODEAETPc-GCE (3.88 ␮A), Fig. 8b, while that on poly-30-AcMnODEAETPc-GCE (3.66 ␮A) was slightly smaller (Fig. 8a). Of the modified electrodes, the least signal was obtained on poly-10-AcMnODEAETPc-GCE (2.61 ␮A) (Fig. 8c). However, the current signal of the herbicide on each of the films was better than that observed on the bare electrode (1.3 ␮A) (Fig. 8d). This indicates electrocatalytic behaviour of the film towards the herbicide. The above observations show that the increase in film thickness from poly-10-AcMnODEAETPc-GCE to poly-20-AcMnODEAETPc-GCE translated to enhanced catalytic activity of the film towards the herbicide, while further increase in film thickness did not appreciably affect the signal of the herbicide; hence poly-20-AcMnODEAETPc-GCE was used for further

Fig. 8. Cyclic voltammograms of poly-20-AcMnODEAETPc-GCE in pH 5 phosphate buffer in the absence of bentazon (e) and 100 ␮M bentazon in 0.1 M phosphate buffer, pH 5, on (a) poly-30-AcMnODEAETPc-GCE, (b) poly-20-AcMnODEAETPcGCE, (c) poly-10-AcMnODEAETPc-GCE and (d) bare GCE (dashed line). Scan rate: 100 mV s−1 versus Ag|AgCl.

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Fig. 9. Square wave voltammetry profiles of bentazon (50–750 ␮M) in 0.1 M phosphate buffer, pH 5, on poly-20-AcMnODEAETPc-GCE. Scan rate: 100 mV s−1 versus Ag|AgCl. Inset: calibration curve of bentazon.

voltammetry measurements. This suggests there may have been surface saturation of the electrode after 20 voltammetry cycles. Square wave voltammetry technique was used for quantitative determination of the herbicide because of its more sensitive nature compared to cyclic voltammetry. Fig. 9 shows the square wave voltammetry profiles of bentazon, on poly-20AcMnODEAETPc-GCE, at different concentrations (50–750 ␮M). A linear relationship was obtained between current response and concentration, Fig. 9 (inset), with a detection limit of 2.48 × 10−7 M, using YB + 3ı criteria. The relationship between current response and concentration is expressed linearly by the equation Ip (␮A) = (0.0121 ± 0.00012) ␮M − 0.0904 ± 0.001. The detection limit obtained was less than the value reported using bare glassy carbon electrode (1.0 × 10−5 M) [13]. The modified electrode showed better stability compared to the bare electrode. Reasonable percentage, 63%, of the herbicide signal was recovered on the modified electrode, compared to the bare electrode, 24%, upon repetitive cycling in 100 ␮M solution of the herbicide. 3.6. Electrochemical impedance spectroscopic interpretation of bentazon signals on the films Vorotyntsev et al. [28] proposed a model for the response of conducting polymer film|solution system to low-amplitude variation of the electrode polarization. In this model, consideration was given to both the electro-diffusive transport of electronic and ionic species in the bulk film and non-equilibrium heterogeneous charge transfer. This model directly interpreted the Randles circuit behaviour: the Warburg region and pure capacitance behaviour at low frequencies and the high frequency response of the system which consist of a semicircle due to the resistance (Rb ) of the polymer phase in parallel with its geometric capacitance, Cg . For a very rapid charge transfer at the polymer|electrolyte interface, the semicircle is defined by the charge transfer resistance, Rct , in parallel with the double layer capacitance, Cdl [29]. Apart from the charge transfer semicircle, there is also a Warburg impedance (linear portion of the impedance spectrum observed in the low frequency region) due to the diffusion of species in the polymer. The Randle circuit model is generally applicable to any electrode/material

43

Fig. 10. Impedance spectra obtained from (a) poly-30-AcMnODEAETPc-GCE, (b) poly-20-AcMnODEAETPc-GCE, (c) poly-10-AcMnODEAETPc-GCE and (d) bare GCE in 0.1 M phosphate buffer solution, pH 5, containing 100 ␮M bentazon. Applied potential = 0.80 V versus Ag|AgCl. Inset: suggested Randle equivalent circuit model for the impedance spectra.

system [30]. However, the type and number of circuit elements depend on the shape of the impedance spectrum (Nyquist plot). The strength of this model is the use of the nature of the circuit elements to explain the physical electrochemistry of the modeled electrochemical cell. In the present work, the impedance spectra obtained were more of semicircles (Fig. 10), eliminating the influence of the Warburg impedance, created by the diffusion of the herbicide in the polymer. An equivalent circuit based on these considerations is shown in Fig. 10 (inset); where Rs is the solution resistance while other parameters are as defined previously. Table 2 gives the values of these parameters for the bare electrode and poly-AcMnODEAETPc film of different film thickness. The Rct for the bare electrode (847 k) was far higher than that for any of the films. This explains why the lowest current response was observed on the bare electrode. The lowest charge transfer resistance was observed on poly-20-AcMnODEAETPcGCE (288 k), while 299 k and 345 k were observed on poly-30-AcMnODEAETPc-GCE and poly-10-AcMnODEAETPc-GCE, respectively. The relatively high value of Rct for poly-10AcMnODEAETPc-GCE, compared to that of the other films, supports the fact that the total effective area of the electrode may not have been sufficiently covered after ten successive polymeric cycles. The relatively close values of Rct for poly-20-AcMnODEAETPc-GCE and poly-30-AcMnODEAETPc-GCE justify the slight difference in the current responses of the herbicide observed on these films (Fig. 8). It also shows that beyond the saturation surface coverage, there was no increase in the catalytic activity of the film, despite the increase in film thickness. Generally, the values of Rct reflect the responses of bentazon on the bare electrode and the various films. Rs values were within narrow range (0.9 k and 1.2 k). This is expected since the presence of the films on the electrode is not supposed to significantly change the solution resistance. The values of Cdl were also not significantly different between bare and the different films, but the slight difference is symbolic of the difference in the conducting nature of the films. Importantly, the values of n for the bare electrode and the different films are less than 1, which shows that the bare and the modified electrode are not ideal capacitors but conducting surfaces.

Table 2 Summary of estimated EIS parameters obtained for the electrode at applied potential of 0.80 V (versus Ag|AgCl). Electrodes

Rs (k)

Rct (k)

Cdl (␮F)

n

Phase angle (◦ )

f (Hz)

Bare GCE poly-10-AcMnODEAETPc-GCE poly-20-AcMnODEAETPc-GCE poly-30-AcMnODEAETPc-GCE

0.9 1.1 1.1 1.2

847 345 288 299

10.9 9.0 10.0 9.4

0.60 0.64 0.63 0.60

51.3 56.3 57.4 53.3

20.0 3.2 4.0 4.0

44

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Fig. 11. Typical Bode plots (phase angle versus log f) obtained from (a) poly-30-AcMnODEAETPc-GCE, (b) poly-20-AcMnODEAETPc-GCE, (c) poly-10AcMnODEAETPc-GCE and (d) bare GCE in 0.1 M phosphate buffer solution, pH 5, containing 100 ␮M bentazon. Applied potential = 0.80 V versus Ag|AgCl.

The data obtained from the Bode plot (phase angle versus log f) (Fig. 11) are also indicated in Table 2. Fig. 11 shows well resolved symmetrical peaks at different maxima. The bare electrode gave a maximum value of ∼51◦ at 20 Hz corresponding to the relaxation process of the electrode|solution interface. The shift in the relaxation process to different phase angle (53.3–56.3◦ ) and at lower frequencies (3.2–4.2 Hz) for the different films, compared to the bare electrode, confirmed that the oxidation of bentazon occurred on the films rather than directly on the bare electrode. 3.7. Mechanism of electrocatalysis of bentazon on poly-20-AcMnODEAETPc-GCE The enhanced current responses of bentazon on polyMnODEAETPc, compared to the bare electrode, suggested electrocatalytic behaviour of the film towards the herbicide. Also, the voltammetry signals of bentazon, observed at ∼0.8 V (versus Ag|AgCl) on the film indicated the involvement of MPc ring redox reactions in catalysis (Fig. 8). This value is close to the potential for the first ring oxidation processes on the polymeric film (Fig. 6), hence the involvement of this species in the suggested mechanism. Fig. 12 shows the cyclic voltammetry profiles obtained for poly-20AcMnODEAETPc-GCE in 100 ␮M solution of bentazon at different scan rate (100–500 mV s−1 ), inset is a linear plot of peak current (Ip ) versus the square root of scan rate (v1/2 ), indicating that the catalysis of the herbicide on these films is diffusion-controlled. Plots of peak potential (Ep ) versus log v (Fig. 13) was used to further inves-

Fig. 13. Plot of peak potential versus log v (100–500 mV s−1 ) for 100 ␮M bentazon on poly-20-AcMnODEAETPc-GCE.

tigate the mechanism of electrocatalysis of bentazon on the film, using Eq. (9), for a totally irreversible diffusion-controlled system. The anodic peak potential can be represented by [23]: Ep =

2.3RT log  + K 2(1 − ˛)na F

(9)

where Ep is the oxidation peak potential of bentazon in mV, v is the scan rate in mV s−1 , K is a constant, na is the number of electron involved in the rate determining step, R is the gas constant, T is the temperature and F, Faraday’s constant. Tafel slope is represented as: 2.3RT/(1 − ˛)na F. Tafel slope of 77 mV/decade (Fig. 13) was obtained, a value which is close to 60 mV/decade, suggesting that a fast one electron transfer is followed by a slow chemical step (dimerization of the oxidation product). This is represented by the mechanism shown in Eqs. (10)–(12). Bentazon is represented as RNHNR1 , where R1 is the isopropyl group and R is the remaining portion of the molecule. Based on the reported dimerization of the oxidation product of bentazon [13], oxidation may involve any of the two nitrogen atoms in the molecule (Fig. 1). In this work, the nitrogen of the tertiary amine (nitrogen atom attached to the isopropyl group) is involved in the suggested mechanism. The higher electron density of the lone pair of the nitrogen of the tertiary amine group, compared to that of the nitrogen of the secondary amine, makes it more susceptible to oxidation, hence its involvement in the suggested mechanism. Dimerization resulted in the poisoning of electrode after oxidation [13]. Electrode poisoning was also observed in the present study, but appreciably reduced due to the presence of the polymeric film on the electrode. MnIII Pc−2 → MnIII Pc−1 + e−

(10)

MnIII Pc−1 + RNHNR 1 → MnIII Pc−2 + RNHN• + R 1

(11)

RNHN•

(12)

+

RNHN•

→ RNHN-NHNR MnIII Pc−2

Regeneration of species in Eq. (11) denotes catalysis while Eq. (12) indicates the dimerization of the oxidation product. 4. Conclusions

Fig. 12. Cyclic voltammetry profiles of 100 ␮M bentazon in 0.1 M phosphate buffer, pH 5 at different scan rate (100–500 mV s−1 ) on poly-20-AcMnODEAETPc-GCE. Inset: plots of peak current versus square root of scan rate.

Electrochemical characterizations of the monomeric and polymeric forms of new AcMnODEAETPc are reported. The solution electrochemistry of the complex gave three distinctly defined redox processes, attributed to MnIII Pc−1 /MnIII Pc−2 , MnIII Pc−2 /MnII Pc−2 and MnII Pc−2 /MnII Pc−3 processes. The polymeric form of the new complex is a stable thin film formed on glassy carbon electrode. The film showed all the redox processes observed in the solution of the

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monomer. The film showed electrocatalytic behaviour towards the herbicide, bentazon, with a low detection limit of 2.48 × 10−7 M, hence can be used as electrochemical sensor for this herbicide. The response of the herbicide on this film was dependent on the thickness of the film, which was explained in terms of the difference in charge transfer resistances with different film thickness. The best response was observed on the film formed from 20 successive polymeric cycles (poly-20-AcMnODEAETPc-GCE). Voltammetry signal of bentazon on this film suggested the involvement of the MPc ringbased redox processes in the electro-oxidation of the herbicide. Acknowledgements This work was supported by the Department of Science and Technology (DST) and National Research Foundation (NRF) of South Africa through DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology and Rhodes University. References [1] J. Zagal, F. Bedioui, J.P. Dodelet (Eds.), N4-macrocyclic Metal Complexes, Springer, New York, 2006. [2] K.I. Ozoemena, T. Nyokong, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 3, American Scientific Publishers, California, 2006, p. 157 (Chapter E, and references therein). [3] M.D. Imisides, R. John, P.J. Riley, G.G. Wallace, Electroanalysis 3 (1991) 879. [4] J. Tanguy, N. Mermilliod, M. Hoclet, J. Electrchem. Soc. 134 (1987) 795. [5] I. Rubinstein, E. Sabatini, J. Electrochem. Soc. 134 (1987) 3078. [6] M.J.R. Presa, H.L. Bandey, R.I. Tucceri, M.I. Florit, D. Posadas, A.R. Hillman, Electrochim. Acta 44 (1999) 2073. [7] I. Betova, M. Bojinov, E. Lankinen, G. Sundholm, J. Electroanal. Chem. 472 (1999) 20.

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