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Improved electro-oxidation of triclosan at nano-zinc oxide-multiwalled carbon nanotube modified glassy carbon electrode
Sensors and Actuators B 209 (2015) 898–905
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Improved electro-oxidation of triclosan at nano-zinc oxide-multiwalled carbon nanotube modified glassy carbon electrode Mambo Moyo ∗ , Lehutso R. Florence, Jonathan O. Okonkwo Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, 175 Nelson Mandela Drive, Arcadia Campus, Pretoria 0001, South Africa
a r t i c l e
i n f o
Article history: Received 11 August 2014 Received in revised form 14 November 2014 Accepted 12 December 2014 Available online 23 December 2014 Keywords: Triclosan Nano-zinc oxide Multiwalled carbon nanotube Voltammetry
a b s t r a c t An electrochemical sensor for oxidation of triclosan at a nano-zinc oxide-multiwalled carbon nanotube (nZnO–MWCNT) composite modified glassy carbon (GC) electrode was examined by cyclic voltammetry (CV), differential pulse voltammetry (DPV), chronoamperometry and square wave voltammetry (SWV) in a pH 7.0 phosphate buffer. By combining the benefits of nZnO–MWCNT composite and GC electrode, the resulting modified electrode exhibited outstanding electrocatalytic behavior towards oxidation of triclosan by giving higher currents and lower oxidation peak potential compared to the bare GC electrode, nZnO and MWCNT modified electrodes. The effects of various parameters on the voltammetric response of triclosan were investigated. Under optimized conditions, the resulting sensor offered an excellent response for triclosan in the concentration range from 1.5 g L−1 to 2.0 mg L−1 with detection limit of 1.3 g L−1 and a coefficient of determination (R2 ) of 0.9931. The diffusion coefficient of triclosan was determined to be 1.65 × 10−6 cm2 s−1 . The electrochemical sensor showed satisfactory stability, selectivity and reproducibility when stored under room conditions. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The extensive use of triclosan, (5-chloro-2-(2,4dichlorophenoxy) phenol, the structure of which is shown in Fig. 1), as a common antibacterial agent has increased in personal care product manufacturing industries in recent years. However, triclosan has been classified as an emerging pollutant and is used in items such as toothpaste, disinfecting soaps, cosmetics, household sponges, socks and underwear [1–4]. Triclosan can enter sewage systems and then transported to wastewater sewage treatment plants as it is an ingredient in many consumer products. The determination of persistent ingredients of personal care products in different environmental matrices, such as sewage sludge, discharge effluent, receiving surface waters and sediments have been reviewed [5]. Concentration of triclosan in ng L−1 levels has been reported at water, sea and lakes [6,7]. Triclosan and its degradation products closely resemble highly toxic contaminants – dioxins in chemical structure. Also, under UV irradiation and high pH, triclosan is known to degrade to products of a chemical structure similar to that of dioxin compounds (e.g. 2,7/2,8-dichlorodibenzop-dioxin) [8] and they exhibit even higher toxicity. The effects
∗ Corresponding author. Tel.: +27 839847553. E-mail address: [email protected] (M. Moyo). http://dx.doi.org/10.1016/j.snb.2014.12.059 0925-4005/© 2014 Elsevier B.V. All rights reserved.
of triclosan on humans include mild itching and allergic redness on sensitive skins. For the aforementioned reasons, it is highly desirable to fabricate highly sensitive, selective, rapid and reliable analytical methods for its detection. Current analytical techniques that have been reported for the quantification of triclosan in different environmental matrices include gas chromatography-atomic emission detection, gas chromatography–ion trap mass spectromliquid chromatography-ultraviolet detector, liquid etry, chromatography–mass spectroscopy, chemiluminescence and voltammetric methods [3,4,9–18]. These methods are very sensitive and reliable. However, they are expensive techniques that require substantial period for sample pretreatment, and they can be performed only by qualified and experienced technicians. Electrochemical methods offer the opportunity for portable, sensitive and rapid methodologies. In recent years, chemically modified electrodes have been used to accelerate electron transfer for the electro-oxidation of triclosan. The electrochemical behavior of triclosan was investigated by Pemberton and Hart at a screenprinted carbon electrode in which the pKa value of 7.9 and the irreversible oxidation reaction of triclosan was confirmed [15]. A detection limit of 1.2 M using differential pulse voltammetry was obtained. After electrostatically accumulated triclosan at an ultrathin carbon nanoparticle composite film modified electrode, Amiri et al. reported the oxidation reaction of triclosan to be irreversible
M. Moyo et al. / Sensors and Actuators B 209 (2015) 898–905
Cl
HO
Cl
O
Cl
Fig. 1. Chemical structure of triclosan.
[13]. Based on cyclic and differential pulse voltammetry of triclosan at a multiwalled carbon nanotube film modified glassy carbon electrode, Yang et al. estimated a detection limit of 57 nM [19]. More recently, a detection limit of 9.2 nM triclosan was reported at a carbon nanodots and chitosan modified glassy carbon electrode [20]. One promising approach to further improve the oxidation of triclosan is the use of materials such as metal oxide nanoparticles and carbon nanotubes (CNTs) in composites by considering their unique properties, special structure and excellent mechanical strength. Among the different metal oxide nanoparticles, zinc oxide nanopowder (nZnO) is an attractive semiconductor, with a wide band gap (3.37 eV). ZnO has a large excitonic energy, lowcost synthesis, biocompatibility, good electrochemical activities, non-toxicity, high-electron communication features and high mechanical strength [21]. The nZnO has been used previous for the fabrication of different sensors and biosensors [22–26]. It is well known that CNTs are suitable materials for modification of electrodes and support in sensor applications because of good porosity, large aspect ratio, high electrical conductivity and high chemical stability [27,28]. Consequently, CNTs can be used to immobilize nZnO onto electrodes. The CNTs will not only help to stabilize and bind the nZnO onto the electrode but also to ensure an extremely large surface area and fast electron transfer. In continuation of our studies concerning the modification of glassy carbon electrode [29–32], we decided to develop a simple and cheap nanocomposite using nZnO and MWCNTs with the intent to exploit the synergistic effects associated with these components. The nanocomposite was then used to modify electrodes using the drop coating method. The electrochemical oxidation of triclosan using modified electrodes was investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV) and chronoamperometry. 2. Materials and methods 2.1. Reagents and materials All reagents were of analytical grade and were used without further purification. Stock solutions of triclosan were prepared by
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dissolving a measured amount in minimum amount of 0.1 M NaOH and diluting with water. N,N-dimethylformamide (DMF), nZnO <50 nm, multi-walled carbon nanotube (MWCNT), K3 Fe(CN)6 , K4 Fe(CN)6 were purchased from Sigma–Aldrich (South Africa). Phosphate buffers at different pH values were prepared by mixing standard stock solutions of 0.10 M Na2 HPO4 and 0.10 M NaH2 PO4 and adjusting the pH with 0.1 M H3 PO4 or NaOH from Merck, South Africa. All solutions were prepared using Milli-Q water (resistivity > 18 M cm−1 ).
2.2. Instruments and measurements Electrochemical measurements for CV, DPV, SWV and chronoamperometry were performed using a BASi CV50W voltammetric analyser (Bioanalytical Systems, West Lafayette, Indiana, USA) equipped with a conventional threeelectrode cell consisting of a modified glassy carbon (GC) working electrode, a Ag/AgCl reference electrode and a Pt wire auxiliary electrode. The SWV was recorded with an amplitude potential of 25 mV and frequency of 15 Hz. The phosphate buffer was kept in anaerobic conditions by purging it with high-purity nitrogen for at least 15 min before and continuously during the experiments. The pH measurements were carried out with a Crison 2001 micro-pH-meter (Spain).
2.3. Preparation of nZnO–MWCNT modified glassy carbon electrode Prior to modification, the GC electrode was sequentially polished to a mirror finish using a BASi polishing kit containing 1.0, 0.3 and 0.05 m diamond slurry, and then rinsed thoroughly in ultra-pure water before being ultrasonically rinsed in ethanol and ultra-pure water for 10 min to remove any adsorbed species on the electrode surface. The cleaned GC electrode was dried in air. The composite was prepared by dispersing nZnO–MWCNT (1:3, w/w) with the aid of ultrasonic agitation for 1 h in 1 mL of DMF to give to give 1 mg mL−1 dispersion. A 5 L aliquot of this dispersion was then applied to the GC electrode surface. The electrode was dried in air in an inverted beaker so that a uniform film was formed. The resultant electrode is hereafter denoted as nZnO–MWCNT/GC electrode and was stored at room temperature when not in use. For comparison the nZnO/GC and MWCNT/GC electrodes were also prepared. The schematic diagram of the stepwise fabrication procedure and electrocatalytic oxidation is shown in Scheme 1.
Scheme 1. The fabrication procedure of the electrochemical sensor.
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3. Results and discussion 3.1. Electrochemical characterization of modified electrode Cyclic voltammetry is a useful technique to depict the electrochemical surface characteristics of modified GC electrodes. Fig. 2 represents the CVs of (a) bare GC electrode, (b) nZnO/GC electrode (c) MWCNT/GC electrode and (d) nZnO/MWCNT/GC electrode, which were recorded in a 1 mM [Fe(CN)6 ]3−/4− solution containing 0.1 M KCl at 100 mV s−1 . It can be seen in Fig. 2 (curve a–d) that a pair of peaks corresponding to the redox reaction of ferricyanide were observed. The peak current at nZnO/GC electrode increased when compared to the bare GC electrode. Compared with nZnO/GC electrode, the peak currents of MWCNT/GC electrode further increased. However, the nZnO/MWCNTs/GC electrode exhibited the highest peak current, illustrating the highest electrocatalytic activity resulting from its highest electroactive surface area. Furthermore, the peak-to-peak potential separation between the cathodic and anodic waves of the nZnO/MWCNTs/GC electrode is smaller as compared to the bare GC electrode, nZnO/GC electrode and MWCNT/GC electrode. 3.2. Surface area electrode study For an electrochemically reversible process, Randles–Sevcik equation relates the oxidation peak current (Ipa ) to potential (v) [32]: Ipa = 2.69 × 105 AC0 n3/2 D1/2 v1/2
(1)
where A denotes the electroactive surface area, C0 , the concentration of the species being oxidised, n the number of electrons transferred, D the diffusion coefficient. Accordingly, the slope of an Ipa vs. v1/2 plot will yield an estimate of A. In this work, we have conducted cyclic voltammetry of 1 mM K3 Fe(CN)6 and 1 mM K4 Fe(CN)6 at a nZnO–MWCNT/GC electrode in 0.1 M KCl supporting electrolyte (D for [Fe(CN)6 ]4− = 7.6 × 10−6 cm2 s−1 ) [33] over a range of potential scan rates and the results obtained are shown in Fig. S1a. In this way, A for the modified electrode was estimated to be 0.70 cm2 . When the experiment was repeated at a bare GC electrode, a nZnO/GC electrode and an MWCNT/GC electrode (Fig. S1b–d) the corresponding surface areas of 0.34 cm2 , 0.40 cm2 and 0.55 cm2 were obtained, respectively.
Fig. 2. Cyclic voltammograms of (a) GC electrode, (b) nZnO/GC electrode, (c) MWCNT/GC electrode and (d) nZnO–MWCNT/GC electrode in 0.1 M KCl containing 1 mM [Fe(CN)6 ]3−/4− . Scan rate: 100 mV s−1 .
Fig. 3. A. Cyclic voltammograms (A) and differential pulse voltammograms (Inset B) for Blank (curve a), nZnO–MWCNT/GC electrode, (curve b) GC electrode, (curve c), nZnO/GC electrode, (d) MWCNT/GC electrode, (curve e), and in the presence of triclosan (PBS, pH 7.0) with scan rate of 100 mV s−1 .
3.3. Electrocatalytic oxidation of triclosan at the modified electrode The oxidation of triclosan at modified GC electrodes was characterized by cyclic voltammetry. Fig. 3A, the featureless cyclic voltammogram (curve a) of 0.1 M PBS (pH 7) alone at a nZnO–MWCNT/GC electrode shows only a background current at 100 mV s−1 . Upon addition of 4 mg L−1 triclosan into PBS, an irreversible oxidation peak at 600 mV was obtained at electrode, as shown in curve b. This is most likely caused by the oxidation of the phenolic function group of triclosan [15]. When this experiment was repeated using a bare GC electrode (curve c), an nZnO/GC electrode (curve d) and a MWCNT/GC electrode (curve e), the oxidation potential was observed to shift negatively from 680 mV, to 670 mV and 643 mV, respectively. The increase in peak currents and shift of the oxidation peak potential toward the lesser negative value obtained at nZnO–MWCNT/GC electrode in contrast to other electrodes, indicated that the coexistence of nZnO and MWCNT allowed fast electrode kinetics on the GC electrode. To further verify the oxidation of triclosan, the differential pulse voltammograms (DPVs) for triclosan at the modified and unmodified electrodes were also recorded (Fig. 3B, inset). As Fig. 3B shows, the DPV for blank (curve a) at nZnO–MWCNT/GC electrode did not show any anodic peak in the potential range studied. In order to study the effects of modifiers on the oxidation of 0.5 mg L−1 triclosan, cyclic voltammograms are depicted in Fig. 3A, DPVs for triclosan at the nZnO–MWCNT/GC electrode (curve b), GC electrode (curve c), nZnO/GC electrode (curve d) and MWCNT/GC electrode (curve e) were also obtained. The DPV for GCE showed a small anodic peak while for nZnO/GC electrode, MWCNT/GC electrode, and nZnO–MWCNT/GC electrode showed sharp and well defined peak shapes. The peak for nZnO–MWCNT/GC electrode appeared at about 600 mV as observed in cyclic voltammetry.
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Fig. 4. Continuous cyclic voltammograms for nZnO–MWCNT/GC electrode in 4 mg L−1 of triclosan (PBS, pH 7.0). (a) 1st, (b) 2nd, (c) 3rd with scan rate of 100 mV s−1 .
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Fig. 5. Cyclic voltammograms of triclosan at nZnO–MWCNT/GC electrode in pH 7.0 at different scan rates (a–g): 5, 10, 20, 30, 60, 80 and 100 mV s−1 . Inset plot of (A) the square root of the scan rate (v1/2 /V s−1 ) vs. the peak current (I/A); (B) the logarithm of the scan rate (ln v/mV s−1 ) vs. the peak potential (Ep ); error bar = ±S.D. and n = 3.
3.4. Behavior of triclosan on the modified electrode Further study was done to see the effect of successive scans on triclosan using the nZnO–MWCNT/GC electrode (Fig. 4). The oxidation peak current remarkably decreases (from curve a to curve c) due to electrode fouling caused by the strong adsorption of the reaction products which blocks electrode surface and reducing the effective reaction sites at the modified electrode [19,20]. 3.5. Optimization of experimental variables To improve the analytical characteristics of the developed sensor, optimization of experimental conditions such as scan rate, pH, ratio of MWCNT:nZnO used, concentrations of MWCNT:nZnO and volume of MWCNT:nZnO injected were carried out.
In the present study, Ep linearly depends on the logarithm of the potential scan rate (ln v) (Fig. 5, inset B) according to Eq. (3): Ep = 0.045(±0.012) ln v + 0.54(±0.18); R2 = 0.992(n = 7)
(4)
From Eqs. (3) and (4): RT = 0.045 ˛nF
(5)
The value of ˛n obtained was 0.57. From the foregoing research n was confirmed as 1, hence the value of ˛ = 0.57. The obtained value of ˛ was nearer 0.5, which have been reported for a totally irreversible process [37]. This further indicated the electrochemical oxidation of triclosan to be irreversible.
(3)
3.5.2. Effect of pH The performance of electrochemical sensors is affected by the solution pH of the supporting electrolyte [38]. The voltammetric oxidation of triclosan was examined in the range of 2–9 in 0.1 M PBS (Fig. 6A). As shown in Fig. 6A, the oxidation peak potential shifted towards less positive with the increase of solution pH, indicating that protons are involved in the oxidation of triclosan. The Epa of triclosan is proportional to solution pH in the range of 2–9. The linear regression equation was Epa = −0.055 (±0.0061) pH + 0.88 ± 0.036 (R2 = 0.9935, n = 6) at 95% confidence interval, degrees of freedom (n − 2) [35]. The regression equation demonstrated that one proton and one electron are involved in the electron transfer process (Fig. 6B), since the dEp /dpH value is almost equal to 55 mV/pH. Consequently, the oxidation of triclosan at the nZnO–MWCNT/GC electrode is a one-electron coupled one proton transfer mechanism as depicted in Scheme 2 [3,13,39]: The plot of Ipa vs. pH (Fig. 6B) shows that as the Ipa steadily increases, the pH also increases and reaches a maximum at pH 7.0. Beyond pH 7, the Ipa for oxidation of triclosan decreases, probably due to the dissociation of the phenolic moiety to produce the corresponding anion. Accordingly, with subsequent experiments, a solution of pH 7.0, adjusted by PBS was used.
where E0 is formal potential, k0 is standard heterogeneous rate constant, and ˛ is transfer coefficient. Other symbols have their usual significance.
3.5.3. Influence of MWCNT:nZnO amount on sensor fabrication The CVs current responses of the nZnO–MWCNT/GC electrode sensor loading variable amount of MWCNT:nZnO to 4 mg L−1
3.5.1. Effects of scan rate The nature of the reaction on the nZnO–MWCNT/GC electrode was ascertained using sweep rate studies in the range 5–100 mV s−1 (Fig. 5). A positive shift in Epa is observed with increase in scan rate for the catalytic oxidation of triclosan confirming the irreversible nature of the process. Furthermore, the behavior also suggests a kinetic limitation in the reaction between redox sites of nZnO–MWCNT and triclosan. Fig. 5 (inset A) shows the plot of anodic peak current (Ipa ) of triclosan as a function of v1/2 . The Ipa is linearly proportional to v1/2 indicating a typical diffusioncontrolled electrochemical behavior [32,34]. The linear regression equation at 95% confidence interval, degrees of freedom (n − 2) [35] can be expressed as: (2)Ipa (A) = 0.63(±0.035)v1/2 (mV s−1 ) + 0.061 ± (0.22); R2 = 0.9986 (n = 7) For an electrode process that is diffusion-controlled and irreversible, the relationship of Ep vs. ln v obeys the following equation [36]: Ep = E 0 +
RTk0 RT RT ln + ln v ˛nF ˛nF ˛nF
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Fig. 6. (A) Cyclic voltammograms for nZnO–MWCNT modified GC electrode in 0.1 M PBS containing 4 mg L−1 triclosan at different pH: (a) 2, (b) 4, (c) 5, (d) 6, (e) 7, and (f) 9. (B) Dependence of anodic peak potential and anodic peak current as a function of solution pH. Scan rate 100 mV s−1 ; error bar = ±S.D. and n = 3.
Fig. 7. Current responses of the nZnO–MWCNT/GC electrode sensor from (A) Loading variable ratios of MWCNT:nZnO (B) Different concentrations of MWCNT:nZnO in 4 mg L−1 triclosan in 0.10 M PBS (pH 7.0). Inset in B: Current responses of MWCNT:nZnO volumes injected on the GC electrode; error bar = ±S.D. and n = 3.
3.6. Chronoamperometric study triclosan in 0.1 M PBS (pH 7.0), scan rate, 100 mV s−1 were investigated and results are shown in Fig. 7A. It is observed that the best results were obtained at a concentration ratio of 3:1 (MWCNT:nZnO). Furthermore, a total concentration of 1 mg L−1 was deduced as one giving high Ipa for triclosan (Fig. 7B). The volume of MWCNT:nZnO applied at the respective optimized total concentration on the GC electrode for studying the electrochemical behavior of triclosan was investigated (Fig. 7B, inset). When the volume of MWCNT:nZnO was increased from 2 to 5 L, the Ipa also increased possibly due to the increased amount of composite causing the effective surface area and aggregation effect to increase gradually, thereby increasing the concentration of triclosan on the surface of the electrode, which aids the catalytic reaction. On the other hand, when the volume of MWCNT:nZnO solution was increased from 5 to 8 L, the Ipa decreased. The reason might be that the MWCNT:nZnO film on the electrode surface was so thick that it increased the diffusion distance of triclosan hindering mass transfer and electron transfer. Consequently, 5 L was the optimized loading of MWCNT:nZnO for the detection of triclosan.
Scheme 2. Proposed mechanism of the electrochemical oxidation of triclosan.
Chronoamperometry was used for the estimation of the diffusion co-efficient of triclosan by using the nZnO–MWCNT/GC electrode (Fig. 8). The diffusion coefficient (D) for triclosan was determined using the Cottrell equation [39]: I = nFD1/2 AC0 −1/2 t −1/2
(6)
where A is the surface area, C is the concentration of triclosan. From the slope of the plot of Cottrell plot (I vs. t−1/2 ) (inset) for
Fig. 8. Chronoamperometric response of nZnO–MWCNT/GC electrode in the pH 7.0 (PBS) containing 0.5 mg L−1 of triclosan for a potential step of 0.95 V. (Inset) Plots of I vs. t−1/2 obtained from chronoamperometric measurements; error bar = ±S.D. and n = 3.
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Table 1 Comparison of the response of the fabricated sensor for detection of triclosan with other sensors based on different composites. Electrode
Linear range
Detection limit
Reference
Screen-printed carbon electrode Mercury electrode MWCNT CNDS-CS/GCE Glassy carbon rod nZnO–MWCNT/GC electrode
1.2 M–1.0 mM 2.5 g L−1 –60 g L−1 50 g L−1 –1.75 mg L−1 2.9 g L−1 –0.29 g L−1 10–600 5 g L−1 ; 1–8 mg L−1 . 1.5 g L−1 –2.0 mg L−1
1.2 M 1.9 g L−1 16.5 g L−1 2.7 g L−1 5 g L−1 1.25 g L−1
[15] [18] [20] [21] [35] Present study
CNDS: carbon nanodots; CS: chitosan; MWCNT:multiwalled carbon nanotubes; nZnO: zinc oxide nanoparticles; GCE: glassy carbon electrode.
Table 2 Interference study for the oxidation of 0.1 mg L−1 triclosan with the nZnO–MWCNT/GC electrode sensor. Possible interference
Ratio
Na+ K+ Mg2+ Cu2+ PO4 3− SO4 2−
1:10 1:10 1:10 1:10 1:10 1:10
Triclosan(without interferant)
a
t-Testa
Peak current (A)
6.23 5.68 6.56 5.69 5.24 6.59
± ± ± ± ± ±
0.02 0.01 0.37 0.16 0.01 0.01
Triclosan(with interferant) 6.21 5.66 6.50 5.65 5.21 6.54
± ± ± ± ± ±
0.02 0.09 0.20 0.05 0.03 0.02
1.20 1.67 1.03 0.21 0.85 0.48
Theoretical t-value at 95% confidence level for four degrees of freedom is 2.78.
nZnO–MWCNT/GC electrode estimation and determination of D was found to be 1.65 × 10−6 cm2 s−1 . 3.7. Response characteristics of the nZnO–MWCNT/GC electrode sensor The relationship between the anodic peak current and concentration of triclosan was studied using SWV under optimal experimental conditions for obtaining the voltammetric traces (Fig. 9). The anodic peak current increased with the increase of triclosan concentration. The calibration plot Fig. 9 (inset) was linear over the concentration range 1.5 g L−1 –2.0 mg L−1 . The linear regression equation at 95% confidence interval, degrees of freedom (n − 2) can be expressed as Ip (A) = 0.0036 (±0.00027) g L−1 + 2.2 (±0.27) with a coefficient of determination (R2 = 0.9931, n = 6) [35]. The detection limit (LOD) was 1.3 g L−1 , calculated using the formula as 3s/b, where s is the standard deviation of the peak currents (n = 3) and b is the slope of the calibration plot. The performance of the developed nZnO–MWCNT/GC electrode sensor compared
with other reported triclosan sensors are summarized in Table 1. From the results, it can be concluded that the nZnO–MWCNT/GC electrode sensor exhibited a wider range and lower detection limit. The study indicated that the synergistic effect of nZnO nanoparticles and MWCNT contributed to the excellent electrochemical performance of the hybrid film sensor. Consequently, the nZnO–MWCNT/GC electrode sensor could be an excellent platform for the detection of triclosan in different environmental matrices.
3.8. Stability and reproducibility of the sensor The stability of the nZnO–MWCNT/GC electrode sensor was also investigated by storing the same electrode at room temperature. For detection of 0.1 mg L−1 triclosan using SWV, there was a small decrease in anodic peak current of 5% in the first week. A 90% response current was still retained after one month. Consequently, the stability of the proposed electrode was good enough for continual operation. The intra-assay precision was evaluated by assaying triclosan level for five times on the same sensor. The relative standard deviation (R.S.D%) for the intra-assay was 3.1% at 0.1 mg L−1 triclosan. The same triclosan concentration was used for the inter-assay precision on five sensors. The fabricated sensors showed a relative standard deviation (R.S.D%) of 3.5%. The obtained results demonstrated that the nZnO–MWCNT/GC electrode has good stability, repeatability and reproducibility, and could be used for triclosan detection.
3.9. Interference study and preliminary application of the sensor
Fig. 9. Square wave voltammograms for nZnO–MWCNT/GC electrode sensor in the solutions with different concentrations of triclosan (a) blank, (PBS pH 7.0) (b) 1.5 g L−1 (c) 250 g L−1 , (d) 500 g L−1 , (e) 750 g L−1 , (f) 1 000 g L−1 , (g) 2 000 g L−1 . Inset: Plot of Ipa vs. triclosan concentration; error bar = ±S.D. and n = 3.
The addition of some single charged cation interferents (Na+ ; K+ ), doubly charged cations (Cu2+ ; Mg2+ ), and anions (SO4 2− ; PO4 3− ) [40] which might appear in same matrices with triclosan, were studied by the mixed method, using the ratio of 1:10 for analyte and interference, respectively. The peak currents for oxidation of triclosan in the absence and presence of an interferant were compared statistically by the Student’s t-test at 95% confidence limits for four degrees of freedom [35]. The results showed that the calculated values (Table 2) did not exceed the theoretical value. Therefore, there is no significant difference between the currents in the absence and presence of interferents, indicating that the developed sensor had good selectivity.
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Table 3 Recovery test for triclosan in tap water. Sample (tap water)
Triclosan added (g L−1 )
Triclosan found (g L−1 )a
1 2 3 4 5
20 50 100 300 650
20.3 48.2 97.3 303.5 648.7
± ± ± ± ±
0.2 0.1 0.02 0.01 0.1
Recovery (%) 101.5 96.4 97.3 98.8 99.8
Quantitative determination of triclosan from tap water sources spiked with known quantity of triclosan. a Average of three determinations.
3.10. Application to simulated sample analysis To demonstrate the feasibility of the fabricated sensor for possible environmental applications, preliminary application of the sensor was examined by determination of triclosan in fives tap water samples by standard addition method under the optimized conditions. The results are given in Table 3. The recoveries were in the range of 96.4–101.5%, which indicated the efficacy of the sensor for practical analysis. 4. Conclusion In the present paper, we have described a simple, inexpensive and efficient method for the fabrication of nZnO–MWCNT-modified electrode which can be used as a sensor. The irreversible oxidation of triclosan was investigated in PBS (pH 7.0) and was found to be pH dependent. The relationship between square wave voltammetric responses and triclosan concentrations was linear over the range of 1.5 g L−1 –2.0 mg L−1 and the detection limit is about 1.3 g L−1 . Good recoveries were obtained and ranged from 96.4 to 101.5%. The properties of the developed biosensor such as good stability, repeatability and reproducibility provide promise that the sensor could be used as a management tool for determining the quality of household and industrial waste waters for the existence of other ingredients in the synthesis of personal care products. Acknowledgment The authors would like to acknowledge financial support from Directorate Research and Innovation, Tshwane University of Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.12.059. References [1] H. Tungudomwongsa, J. Leckie, T. Mill, Photocatalytic oxidation of emerging contaminants: kinetics and pathways for photocatalytic oxidation of pharmaceutical compounds, J. Adv. Oxid. Technol. 9 (2006) 59–64. [2] M. Adolfsson-Erici, M. Pettersson, J. Parkkonen, J. Sturve, Triclosan, a commonly used bactericide found in human milk and in the aquatic environment in Sweden, Chemosphere 46 (2002) 1485–1489. [3] L. Vidal, A. Chisvert, A. Canals, E. Psillakis, A. Lapkin, F. Acosta, K.J. Edler, J.A. Holdaway, F. Marken, Chemically surface-modified carbon nanoparticle carrier for phenolic pollutants: extraction and electrochemical determination of benzophenone-3 and triclosan, Anal. Chim. Acta 616 (2008) 28–35. [4] Y. Liu, Q.J. Song, L. Wang, Development and characterization of an amperometric sensor for triclosan detection based on electropolymerized molecularly imprinted polymer, Microchem. J. 91 (2009) 222–226. [5] M. Aaron, Peck analytical methods for the determination of persistent ingredients of personal care products in environmental matrices, Anal. Bioanal. Chem. 386 (2006) 907–939.
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Cai, Triclosan determination in water related to wastewater treatment, Talanta 72 (2007) 1650–1654. [11] R. Sanches-Silva, J. Sendón-García, P. López-Hernández, Paseiro-Losada, Determination of triclosan in foodstuffs, J. Sep. Sci. 28 (2005) 65–72. [12] S.G. Chu, C.D. Metcalfe, Simultaneous determination of triclocarban and triclosan in municipal biosolids by liquid chromatography tandem mass spectrometry, J. Chromatogr. A 1164 (2007) 212–218. [13] M. Amiri, S. Shahrokhian, E. Psillakis, F. Marken, Electrostatic accumulation and determination of triclosan in ultrathin carbonnanoparticle composite filmelectrodes, Anal. Chim. Acta 593 (2007) 117–122. [14] M.J. Bonné, K.J. Edler, J.G. Buchanan, D. Wolverson, E. Psillakis, M. Helton, W. Thielemans, F. Marken, Thin-film modified electrodes with reconstituted cellulose-PDDAC films for the accumulation and detection of triclosan, J. Phys. Chem. 112 (2008) 2660–2666. [15] R.M. Pemberton, J.P. Hart, Electrochemical behaviour of triclosan at a screenprinted carbon electrode and its voltammetric determination in toothpaste and mouthrinse products, Anal. Chim. Acta 390 (1999) 107–115. [16] P. Raghupathy, J. Mathiyarasu James Joseph, K.L.N. Phani, V. Yegnaraman, Hydrotrope-driven disruption of micellar encapsulants for voltammetric detection of triclosan, J. Electroanal. Chem. 584 (2005) 210–214. [17] L.H. Wang, S.C. Chu, C.Y. Chin, An electrochemical study of the reduction of triclosan (Irgasan DP-300) on carbon fiber ultramicroelectrodes, Bull. Electrochem. 20 (2004) 225–229. [18] A. Safavi, N. Maleki, H.R. Shahbaazi, Electrochemical determination of triclosan at a mercury electrode, Anal. Chim. Acta 494 (2003) 225–233. [19] J. Yang, P. Wang, X. Zhang, K. Wu, Electrochemical sensor for rapid detection of triclosan using a multiwall carbon nanotube film, J. Agric. Food Chem. 57 (2009) 9403–9407. [20] H. Dai, G. Xu, L. Gong, C. Yang, Y. Lin, Y. Tong, J. Chen, G. Chen, Electrochemical detection of triclosan at a glassy carbon electrode modifies with carbon nanodots and chitosan, Electrochim. Acta 80 (2012) 362–367. [21] S. Baskoutas, G. Bester, Transition in the optical emission polarization of ZnO nanorods, J. Phys. Chem. C 115 (2011) 15862. [22] C. Zhang, G. Wang, Y. Ji, M. Liu, Y. Feng, Z. Zhang, B. Fang, Enhancement in analytical hydrazine based on gold nanoparticles deposited on ZnO-MWCNTs films, Sens. Actuators B 150 (2010) 247–253. [23] B. Fang, C.H. Zhang, W. Zhang, G.F. Wang, A novel hydrazine electrochemical sensor based on a carbon nanotube-wired ZnO nanoflower-modfied electrode, Electrochim. Acta 55 (2009) 178–182. [24] A. Umar, M.M. Rahman, M. Vaseem, Y.B. Hahn, Ultra-sensitive cholesterol biosensor based on low-temperature grown ZnO nanoparticles, Electrochem. Commun. 11 (2009) 118–121. [25] B. Haghighi, S. 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Moyo, J.O. Okonkwo, Horseradish peroxidase biosensor based on maize tassel-MWCNTs composite for cadmium detection, Sens. Actuators B 193 (2014) 515–521. [31] M. Moyo, Horseradish peroxidase biosensor to detect zinc ions in aqueous solutions, Open J. Appl. Biosensor 3 (2014) 1–7. [32] M. Moyo, J.O. Okonkwo, N.M. Agyei, Maize tassel-modified carbon paste electrode for voltammetric determination of Cu(II), Environ. Monit. Assess. 186 (2014) 4807–4817. [33] Q. Wan, F. Yu, L. Zhu, X. Wang, Bucky-gel coated glassy carbon electrodes, voltammetric detection of fentomolar level lead ions, Talanta 82 (2010) 1820–1824. [34] A. Afkhami, F. Soltani-Felehgari, T. Madrakian, H. Ghaedi, Surface decoration of multi-walled carbon nanotubes modified carbon paste electrode with gold nanoparticles for electro-oxidation and sensitive determination of nitrite, Biosens. Bioelectron. 51 (2014) 379–385.
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Biographies
Mambo Moyo received his Diploma in Education from University of Zimbabwe in 1997. He obtained a BScEd Degree in Chemistry from the Bindura University of Science Education, Zimbabwe, 2004. He obtained his MSc degree in Analytical Chemistry from University of Zimbabwe in 2008. He is currently in his final year doing PhD degree in Environmental Chemistry Management from Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, Pretoria, South Africa. His area of expertise includes biosensors, carbon paste electrodes, biosorption, water and dust analysis and chemistry education.
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Florence Lehutso is a post graduate student in the Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, Pretoria, South Africa. She is currently working on the removal of triclosan from sewage treatments plants, around Gauteng province, South Africa.
Jonathan O. Okonkwo (PhD) is a Research and innovation professor, Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, Pretoria, South Africa. Prof. Okonkwo research areas include, adsorption technology; speciation of toxic metals in aquatic environment; soil remediation, green chemistry and persistent organic pollutants.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Improved electro-oxidation of triclosan at nano-zinc oxide-multiwalled carbon nanotube modified glassy carbon electrode Mambo Moyo ∗ , Lehutso R. Florence, Jonathan O. Okonkwo Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, 175 Nelson Mandela Drive, Arcadia Campus, Pretoria 0001, South Africa
a r t i c l e
i n f o
Article history: Received 11 August 2014 Received in revised form 14 November 2014 Accepted 12 December 2014 Available online 23 December 2014 Keywords: Triclosan Nano-zinc oxide Multiwalled carbon nanotube Voltammetry
a b s t r a c t An electrochemical sensor for oxidation of triclosan at a nano-zinc oxide-multiwalled carbon nanotube (nZnO–MWCNT) composite modified glassy carbon (GC) electrode was examined by cyclic voltammetry (CV), differential pulse voltammetry (DPV), chronoamperometry and square wave voltammetry (SWV) in a pH 7.0 phosphate buffer. By combining the benefits of nZnO–MWCNT composite and GC electrode, the resulting modified electrode exhibited outstanding electrocatalytic behavior towards oxidation of triclosan by giving higher currents and lower oxidation peak potential compared to the bare GC electrode, nZnO and MWCNT modified electrodes. The effects of various parameters on the voltammetric response of triclosan were investigated. Under optimized conditions, the resulting sensor offered an excellent response for triclosan in the concentration range from 1.5 g L−1 to 2.0 mg L−1 with detection limit of 1.3 g L−1 and a coefficient of determination (R2 ) of 0.9931. The diffusion coefficient of triclosan was determined to be 1.65 × 10−6 cm2 s−1 . The electrochemical sensor showed satisfactory stability, selectivity and reproducibility when stored under room conditions. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The extensive use of triclosan, (5-chloro-2-(2,4dichlorophenoxy) phenol, the structure of which is shown in Fig. 1), as a common antibacterial agent has increased in personal care product manufacturing industries in recent years. However, triclosan has been classified as an emerging pollutant and is used in items such as toothpaste, disinfecting soaps, cosmetics, household sponges, socks and underwear [1–4]. Triclosan can enter sewage systems and then transported to wastewater sewage treatment plants as it is an ingredient in many consumer products. The determination of persistent ingredients of personal care products in different environmental matrices, such as sewage sludge, discharge effluent, receiving surface waters and sediments have been reviewed [5]. Concentration of triclosan in ng L−1 levels has been reported at water, sea and lakes [6,7]. Triclosan and its degradation products closely resemble highly toxic contaminants – dioxins in chemical structure. Also, under UV irradiation and high pH, triclosan is known to degrade to products of a chemical structure similar to that of dioxin compounds (e.g. 2,7/2,8-dichlorodibenzop-dioxin) [8] and they exhibit even higher toxicity. The effects
∗ Corresponding author. Tel.: +27 839847553. E-mail address: [email protected] (M. Moyo). http://dx.doi.org/10.1016/j.snb.2014.12.059 0925-4005/© 2014 Elsevier B.V. All rights reserved.
of triclosan on humans include mild itching and allergic redness on sensitive skins. For the aforementioned reasons, it is highly desirable to fabricate highly sensitive, selective, rapid and reliable analytical methods for its detection. Current analytical techniques that have been reported for the quantification of triclosan in different environmental matrices include gas chromatography-atomic emission detection, gas chromatography–ion trap mass spectromliquid chromatography-ultraviolet detector, liquid etry, chromatography–mass spectroscopy, chemiluminescence and voltammetric methods [3,4,9–18]. These methods are very sensitive and reliable. However, they are expensive techniques that require substantial period for sample pretreatment, and they can be performed only by qualified and experienced technicians. Electrochemical methods offer the opportunity for portable, sensitive and rapid methodologies. In recent years, chemically modified electrodes have been used to accelerate electron transfer for the electro-oxidation of triclosan. The electrochemical behavior of triclosan was investigated by Pemberton and Hart at a screenprinted carbon electrode in which the pKa value of 7.9 and the irreversible oxidation reaction of triclosan was confirmed [15]. A detection limit of 1.2 M using differential pulse voltammetry was obtained. After electrostatically accumulated triclosan at an ultrathin carbon nanoparticle composite film modified electrode, Amiri et al. reported the oxidation reaction of triclosan to be irreversible
M. Moyo et al. / Sensors and Actuators B 209 (2015) 898–905
Cl
HO
Cl
O
Cl
Fig. 1. Chemical structure of triclosan.
[13]. Based on cyclic and differential pulse voltammetry of triclosan at a multiwalled carbon nanotube film modified glassy carbon electrode, Yang et al. estimated a detection limit of 57 nM [19]. More recently, a detection limit of 9.2 nM triclosan was reported at a carbon nanodots and chitosan modified glassy carbon electrode [20]. One promising approach to further improve the oxidation of triclosan is the use of materials such as metal oxide nanoparticles and carbon nanotubes (CNTs) in composites by considering their unique properties, special structure and excellent mechanical strength. Among the different metal oxide nanoparticles, zinc oxide nanopowder (nZnO) is an attractive semiconductor, with a wide band gap (3.37 eV). ZnO has a large excitonic energy, lowcost synthesis, biocompatibility, good electrochemical activities, non-toxicity, high-electron communication features and high mechanical strength [21]. The nZnO has been used previous for the fabrication of different sensors and biosensors [22–26]. It is well known that CNTs are suitable materials for modification of electrodes and support in sensor applications because of good porosity, large aspect ratio, high electrical conductivity and high chemical stability [27,28]. Consequently, CNTs can be used to immobilize nZnO onto electrodes. The CNTs will not only help to stabilize and bind the nZnO onto the electrode but also to ensure an extremely large surface area and fast electron transfer. In continuation of our studies concerning the modification of glassy carbon electrode [29–32], we decided to develop a simple and cheap nanocomposite using nZnO and MWCNTs with the intent to exploit the synergistic effects associated with these components. The nanocomposite was then used to modify electrodes using the drop coating method. The electrochemical oxidation of triclosan using modified electrodes was investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV) and chronoamperometry. 2. Materials and methods 2.1. Reagents and materials All reagents were of analytical grade and were used without further purification. Stock solutions of triclosan were prepared by
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dissolving a measured amount in minimum amount of 0.1 M NaOH and diluting with water. N,N-dimethylformamide (DMF), nZnO <50 nm, multi-walled carbon nanotube (MWCNT), K3 Fe(CN)6 , K4 Fe(CN)6 were purchased from Sigma–Aldrich (South Africa). Phosphate buffers at different pH values were prepared by mixing standard stock solutions of 0.10 M Na2 HPO4 and 0.10 M NaH2 PO4 and adjusting the pH with 0.1 M H3 PO4 or NaOH from Merck, South Africa. All solutions were prepared using Milli-Q water (resistivity > 18 M cm−1 ).
2.2. Instruments and measurements Electrochemical measurements for CV, DPV, SWV and chronoamperometry were performed using a BASi CV50W voltammetric analyser (Bioanalytical Systems, West Lafayette, Indiana, USA) equipped with a conventional threeelectrode cell consisting of a modified glassy carbon (GC) working electrode, a Ag/AgCl reference electrode and a Pt wire auxiliary electrode. The SWV was recorded with an amplitude potential of 25 mV and frequency of 15 Hz. The phosphate buffer was kept in anaerobic conditions by purging it with high-purity nitrogen for at least 15 min before and continuously during the experiments. The pH measurements were carried out with a Crison 2001 micro-pH-meter (Spain).
2.3. Preparation of nZnO–MWCNT modified glassy carbon electrode Prior to modification, the GC electrode was sequentially polished to a mirror finish using a BASi polishing kit containing 1.0, 0.3 and 0.05 m diamond slurry, and then rinsed thoroughly in ultra-pure water before being ultrasonically rinsed in ethanol and ultra-pure water for 10 min to remove any adsorbed species on the electrode surface. The cleaned GC electrode was dried in air. The composite was prepared by dispersing nZnO–MWCNT (1:3, w/w) with the aid of ultrasonic agitation for 1 h in 1 mL of DMF to give to give 1 mg mL−1 dispersion. A 5 L aliquot of this dispersion was then applied to the GC electrode surface. The electrode was dried in air in an inverted beaker so that a uniform film was formed. The resultant electrode is hereafter denoted as nZnO–MWCNT/GC electrode and was stored at room temperature when not in use. For comparison the nZnO/GC and MWCNT/GC electrodes were also prepared. The schematic diagram of the stepwise fabrication procedure and electrocatalytic oxidation is shown in Scheme 1.
Scheme 1. The fabrication procedure of the electrochemical sensor.
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3. Results and discussion 3.1. Electrochemical characterization of modified electrode Cyclic voltammetry is a useful technique to depict the electrochemical surface characteristics of modified GC electrodes. Fig. 2 represents the CVs of (a) bare GC electrode, (b) nZnO/GC electrode (c) MWCNT/GC electrode and (d) nZnO/MWCNT/GC electrode, which were recorded in a 1 mM [Fe(CN)6 ]3−/4− solution containing 0.1 M KCl at 100 mV s−1 . It can be seen in Fig. 2 (curve a–d) that a pair of peaks corresponding to the redox reaction of ferricyanide were observed. The peak current at nZnO/GC electrode increased when compared to the bare GC electrode. Compared with nZnO/GC electrode, the peak currents of MWCNT/GC electrode further increased. However, the nZnO/MWCNTs/GC electrode exhibited the highest peak current, illustrating the highest electrocatalytic activity resulting from its highest electroactive surface area. Furthermore, the peak-to-peak potential separation between the cathodic and anodic waves of the nZnO/MWCNTs/GC electrode is smaller as compared to the bare GC electrode, nZnO/GC electrode and MWCNT/GC electrode. 3.2. Surface area electrode study For an electrochemically reversible process, Randles–Sevcik equation relates the oxidation peak current (Ipa ) to potential (v) [32]: Ipa = 2.69 × 105 AC0 n3/2 D1/2 v1/2
(1)
where A denotes the electroactive surface area, C0 , the concentration of the species being oxidised, n the number of electrons transferred, D the diffusion coefficient. Accordingly, the slope of an Ipa vs. v1/2 plot will yield an estimate of A. In this work, we have conducted cyclic voltammetry of 1 mM K3 Fe(CN)6 and 1 mM K4 Fe(CN)6 at a nZnO–MWCNT/GC electrode in 0.1 M KCl supporting electrolyte (D for [Fe(CN)6 ]4− = 7.6 × 10−6 cm2 s−1 ) [33] over a range of potential scan rates and the results obtained are shown in Fig. S1a. In this way, A for the modified electrode was estimated to be 0.70 cm2 . When the experiment was repeated at a bare GC electrode, a nZnO/GC electrode and an MWCNT/GC electrode (Fig. S1b–d) the corresponding surface areas of 0.34 cm2 , 0.40 cm2 and 0.55 cm2 were obtained, respectively.
Fig. 2. Cyclic voltammograms of (a) GC electrode, (b) nZnO/GC electrode, (c) MWCNT/GC electrode and (d) nZnO–MWCNT/GC electrode in 0.1 M KCl containing 1 mM [Fe(CN)6 ]3−/4− . Scan rate: 100 mV s−1 .
Fig. 3. A. Cyclic voltammograms (A) and differential pulse voltammograms (Inset B) for Blank (curve a), nZnO–MWCNT/GC electrode, (curve b) GC electrode, (curve c), nZnO/GC electrode, (d) MWCNT/GC electrode, (curve e), and in the presence of triclosan (PBS, pH 7.0) with scan rate of 100 mV s−1 .
3.3. Electrocatalytic oxidation of triclosan at the modified electrode The oxidation of triclosan at modified GC electrodes was characterized by cyclic voltammetry. Fig. 3A, the featureless cyclic voltammogram (curve a) of 0.1 M PBS (pH 7) alone at a nZnO–MWCNT/GC electrode shows only a background current at 100 mV s−1 . Upon addition of 4 mg L−1 triclosan into PBS, an irreversible oxidation peak at 600 mV was obtained at electrode, as shown in curve b. This is most likely caused by the oxidation of the phenolic function group of triclosan [15]. When this experiment was repeated using a bare GC electrode (curve c), an nZnO/GC electrode (curve d) and a MWCNT/GC electrode (curve e), the oxidation potential was observed to shift negatively from 680 mV, to 670 mV and 643 mV, respectively. The increase in peak currents and shift of the oxidation peak potential toward the lesser negative value obtained at nZnO–MWCNT/GC electrode in contrast to other electrodes, indicated that the coexistence of nZnO and MWCNT allowed fast electrode kinetics on the GC electrode. To further verify the oxidation of triclosan, the differential pulse voltammograms (DPVs) for triclosan at the modified and unmodified electrodes were also recorded (Fig. 3B, inset). As Fig. 3B shows, the DPV for blank (curve a) at nZnO–MWCNT/GC electrode did not show any anodic peak in the potential range studied. In order to study the effects of modifiers on the oxidation of 0.5 mg L−1 triclosan, cyclic voltammograms are depicted in Fig. 3A, DPVs for triclosan at the nZnO–MWCNT/GC electrode (curve b), GC electrode (curve c), nZnO/GC electrode (curve d) and MWCNT/GC electrode (curve e) were also obtained. The DPV for GCE showed a small anodic peak while for nZnO/GC electrode, MWCNT/GC electrode, and nZnO–MWCNT/GC electrode showed sharp and well defined peak shapes. The peak for nZnO–MWCNT/GC electrode appeared at about 600 mV as observed in cyclic voltammetry.
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Fig. 4. Continuous cyclic voltammograms for nZnO–MWCNT/GC electrode in 4 mg L−1 of triclosan (PBS, pH 7.0). (a) 1st, (b) 2nd, (c) 3rd with scan rate of 100 mV s−1 .
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Fig. 5. Cyclic voltammograms of triclosan at nZnO–MWCNT/GC electrode in pH 7.0 at different scan rates (a–g): 5, 10, 20, 30, 60, 80 and 100 mV s−1 . Inset plot of (A) the square root of the scan rate (v1/2 /V s−1 ) vs. the peak current (I/A); (B) the logarithm of the scan rate (ln v/mV s−1 ) vs. the peak potential (Ep ); error bar = ±S.D. and n = 3.
3.4. Behavior of triclosan on the modified electrode Further study was done to see the effect of successive scans on triclosan using the nZnO–MWCNT/GC electrode (Fig. 4). The oxidation peak current remarkably decreases (from curve a to curve c) due to electrode fouling caused by the strong adsorption of the reaction products which blocks electrode surface and reducing the effective reaction sites at the modified electrode [19,20]. 3.5. Optimization of experimental variables To improve the analytical characteristics of the developed sensor, optimization of experimental conditions such as scan rate, pH, ratio of MWCNT:nZnO used, concentrations of MWCNT:nZnO and volume of MWCNT:nZnO injected were carried out.
In the present study, Ep linearly depends on the logarithm of the potential scan rate (ln v) (Fig. 5, inset B) according to Eq. (3): Ep = 0.045(±0.012) ln v + 0.54(±0.18); R2 = 0.992(n = 7)
(4)
From Eqs. (3) and (4): RT = 0.045 ˛nF
(5)
The value of ˛n obtained was 0.57. From the foregoing research n was confirmed as 1, hence the value of ˛ = 0.57. The obtained value of ˛ was nearer 0.5, which have been reported for a totally irreversible process [37]. This further indicated the electrochemical oxidation of triclosan to be irreversible.
(3)
3.5.2. Effect of pH The performance of electrochemical sensors is affected by the solution pH of the supporting electrolyte [38]. The voltammetric oxidation of triclosan was examined in the range of 2–9 in 0.1 M PBS (Fig. 6A). As shown in Fig. 6A, the oxidation peak potential shifted towards less positive with the increase of solution pH, indicating that protons are involved in the oxidation of triclosan. The Epa of triclosan is proportional to solution pH in the range of 2–9. The linear regression equation was Epa = −0.055 (±0.0061) pH + 0.88 ± 0.036 (R2 = 0.9935, n = 6) at 95% confidence interval, degrees of freedom (n − 2) [35]. The regression equation demonstrated that one proton and one electron are involved in the electron transfer process (Fig. 6B), since the dEp /dpH value is almost equal to 55 mV/pH. Consequently, the oxidation of triclosan at the nZnO–MWCNT/GC electrode is a one-electron coupled one proton transfer mechanism as depicted in Scheme 2 [3,13,39]: The plot of Ipa vs. pH (Fig. 6B) shows that as the Ipa steadily increases, the pH also increases and reaches a maximum at pH 7.0. Beyond pH 7, the Ipa for oxidation of triclosan decreases, probably due to the dissociation of the phenolic moiety to produce the corresponding anion. Accordingly, with subsequent experiments, a solution of pH 7.0, adjusted by PBS was used.
where E0 is formal potential, k0 is standard heterogeneous rate constant, and ˛ is transfer coefficient. Other symbols have their usual significance.
3.5.3. Influence of MWCNT:nZnO amount on sensor fabrication The CVs current responses of the nZnO–MWCNT/GC electrode sensor loading variable amount of MWCNT:nZnO to 4 mg L−1
3.5.1. Effects of scan rate The nature of the reaction on the nZnO–MWCNT/GC electrode was ascertained using sweep rate studies in the range 5–100 mV s−1 (Fig. 5). A positive shift in Epa is observed with increase in scan rate for the catalytic oxidation of triclosan confirming the irreversible nature of the process. Furthermore, the behavior also suggests a kinetic limitation in the reaction between redox sites of nZnO–MWCNT and triclosan. Fig. 5 (inset A) shows the plot of anodic peak current (Ipa ) of triclosan as a function of v1/2 . The Ipa is linearly proportional to v1/2 indicating a typical diffusioncontrolled electrochemical behavior [32,34]. The linear regression equation at 95% confidence interval, degrees of freedom (n − 2) [35] can be expressed as: (2)Ipa (A) = 0.63(±0.035)v1/2 (mV s−1 ) + 0.061 ± (0.22); R2 = 0.9986 (n = 7) For an electrode process that is diffusion-controlled and irreversible, the relationship of Ep vs. ln v obeys the following equation [36]: Ep = E 0 +
RTk0 RT RT ln + ln v ˛nF ˛nF ˛nF
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Fig. 6. (A) Cyclic voltammograms for nZnO–MWCNT modified GC electrode in 0.1 M PBS containing 4 mg L−1 triclosan at different pH: (a) 2, (b) 4, (c) 5, (d) 6, (e) 7, and (f) 9. (B) Dependence of anodic peak potential and anodic peak current as a function of solution pH. Scan rate 100 mV s−1 ; error bar = ±S.D. and n = 3.
Fig. 7. Current responses of the nZnO–MWCNT/GC electrode sensor from (A) Loading variable ratios of MWCNT:nZnO (B) Different concentrations of MWCNT:nZnO in 4 mg L−1 triclosan in 0.10 M PBS (pH 7.0). Inset in B: Current responses of MWCNT:nZnO volumes injected on the GC electrode; error bar = ±S.D. and n = 3.
3.6. Chronoamperometric study triclosan in 0.1 M PBS (pH 7.0), scan rate, 100 mV s−1 were investigated and results are shown in Fig. 7A. It is observed that the best results were obtained at a concentration ratio of 3:1 (MWCNT:nZnO). Furthermore, a total concentration of 1 mg L−1 was deduced as one giving high Ipa for triclosan (Fig. 7B). The volume of MWCNT:nZnO applied at the respective optimized total concentration on the GC electrode for studying the electrochemical behavior of triclosan was investigated (Fig. 7B, inset). When the volume of MWCNT:nZnO was increased from 2 to 5 L, the Ipa also increased possibly due to the increased amount of composite causing the effective surface area and aggregation effect to increase gradually, thereby increasing the concentration of triclosan on the surface of the electrode, which aids the catalytic reaction. On the other hand, when the volume of MWCNT:nZnO solution was increased from 5 to 8 L, the Ipa decreased. The reason might be that the MWCNT:nZnO film on the electrode surface was so thick that it increased the diffusion distance of triclosan hindering mass transfer and electron transfer. Consequently, 5 L was the optimized loading of MWCNT:nZnO for the detection of triclosan.
Scheme 2. Proposed mechanism of the electrochemical oxidation of triclosan.
Chronoamperometry was used for the estimation of the diffusion co-efficient of triclosan by using the nZnO–MWCNT/GC electrode (Fig. 8). The diffusion coefficient (D) for triclosan was determined using the Cottrell equation [39]: I = nFD1/2 AC0 −1/2 t −1/2
(6)
where A is the surface area, C is the concentration of triclosan. From the slope of the plot of Cottrell plot (I vs. t−1/2 ) (inset) for
Fig. 8. Chronoamperometric response of nZnO–MWCNT/GC electrode in the pH 7.0 (PBS) containing 0.5 mg L−1 of triclosan for a potential step of 0.95 V. (Inset) Plots of I vs. t−1/2 obtained from chronoamperometric measurements; error bar = ±S.D. and n = 3.
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Table 1 Comparison of the response of the fabricated sensor for detection of triclosan with other sensors based on different composites. Electrode
Linear range
Detection limit
Reference
Screen-printed carbon electrode Mercury electrode MWCNT CNDS-CS/GCE Glassy carbon rod nZnO–MWCNT/GC electrode
1.2 M–1.0 mM 2.5 g L−1 –60 g L−1 50 g L−1 –1.75 mg L−1 2.9 g L−1 –0.29 g L−1 10–600 5 g L−1 ; 1–8 mg L−1 . 1.5 g L−1 –2.0 mg L−1
1.2 M 1.9 g L−1 16.5 g L−1 2.7 g L−1 5 g L−1 1.25 g L−1
[15] [18] [20] [21] [35] Present study
CNDS: carbon nanodots; CS: chitosan; MWCNT:multiwalled carbon nanotubes; nZnO: zinc oxide nanoparticles; GCE: glassy carbon electrode.
Table 2 Interference study for the oxidation of 0.1 mg L−1 triclosan with the nZnO–MWCNT/GC electrode sensor. Possible interference
Ratio
Na+ K+ Mg2+ Cu2+ PO4 3− SO4 2−
1:10 1:10 1:10 1:10 1:10 1:10
Triclosan(without interferant)
a
t-Testa
Peak current (A)
6.23 5.68 6.56 5.69 5.24 6.59
± ± ± ± ± ±
0.02 0.01 0.37 0.16 0.01 0.01
Triclosan(with interferant) 6.21 5.66 6.50 5.65 5.21 6.54
± ± ± ± ± ±
0.02 0.09 0.20 0.05 0.03 0.02
1.20 1.67 1.03 0.21 0.85 0.48
Theoretical t-value at 95% confidence level for four degrees of freedom is 2.78.
nZnO–MWCNT/GC electrode estimation and determination of D was found to be 1.65 × 10−6 cm2 s−1 . 3.7. Response characteristics of the nZnO–MWCNT/GC electrode sensor The relationship between the anodic peak current and concentration of triclosan was studied using SWV under optimal experimental conditions for obtaining the voltammetric traces (Fig. 9). The anodic peak current increased with the increase of triclosan concentration. The calibration plot Fig. 9 (inset) was linear over the concentration range 1.5 g L−1 –2.0 mg L−1 . The linear regression equation at 95% confidence interval, degrees of freedom (n − 2) can be expressed as Ip (A) = 0.0036 (±0.00027) g L−1 + 2.2 (±0.27) with a coefficient of determination (R2 = 0.9931, n = 6) [35]. The detection limit (LOD) was 1.3 g L−1 , calculated using the formula as 3s/b, where s is the standard deviation of the peak currents (n = 3) and b is the slope of the calibration plot. The performance of the developed nZnO–MWCNT/GC electrode sensor compared
with other reported triclosan sensors are summarized in Table 1. From the results, it can be concluded that the nZnO–MWCNT/GC electrode sensor exhibited a wider range and lower detection limit. The study indicated that the synergistic effect of nZnO nanoparticles and MWCNT contributed to the excellent electrochemical performance of the hybrid film sensor. Consequently, the nZnO–MWCNT/GC electrode sensor could be an excellent platform for the detection of triclosan in different environmental matrices.
3.8. Stability and reproducibility of the sensor The stability of the nZnO–MWCNT/GC electrode sensor was also investigated by storing the same electrode at room temperature. For detection of 0.1 mg L−1 triclosan using SWV, there was a small decrease in anodic peak current of 5% in the first week. A 90% response current was still retained after one month. Consequently, the stability of the proposed electrode was good enough for continual operation. The intra-assay precision was evaluated by assaying triclosan level for five times on the same sensor. The relative standard deviation (R.S.D%) for the intra-assay was 3.1% at 0.1 mg L−1 triclosan. The same triclosan concentration was used for the inter-assay precision on five sensors. The fabricated sensors showed a relative standard deviation (R.S.D%) of 3.5%. The obtained results demonstrated that the nZnO–MWCNT/GC electrode has good stability, repeatability and reproducibility, and could be used for triclosan detection.
3.9. Interference study and preliminary application of the sensor
Fig. 9. Square wave voltammograms for nZnO–MWCNT/GC electrode sensor in the solutions with different concentrations of triclosan (a) blank, (PBS pH 7.0) (b) 1.5 g L−1 (c) 250 g L−1 , (d) 500 g L−1 , (e) 750 g L−1 , (f) 1 000 g L−1 , (g) 2 000 g L−1 . Inset: Plot of Ipa vs. triclosan concentration; error bar = ±S.D. and n = 3.
The addition of some single charged cation interferents (Na+ ; K+ ), doubly charged cations (Cu2+ ; Mg2+ ), and anions (SO4 2− ; PO4 3− ) [40] which might appear in same matrices with triclosan, were studied by the mixed method, using the ratio of 1:10 for analyte and interference, respectively. The peak currents for oxidation of triclosan in the absence and presence of an interferant were compared statistically by the Student’s t-test at 95% confidence limits for four degrees of freedom [35]. The results showed that the calculated values (Table 2) did not exceed the theoretical value. Therefore, there is no significant difference between the currents in the absence and presence of interferents, indicating that the developed sensor had good selectivity.
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Table 3 Recovery test for triclosan in tap water. Sample (tap water)
Triclosan added (g L−1 )
Triclosan found (g L−1 )a
1 2 3 4 5
20 50 100 300 650
20.3 48.2 97.3 303.5 648.7
± ± ± ± ±
0.2 0.1 0.02 0.01 0.1
Recovery (%) 101.5 96.4 97.3 98.8 99.8
Quantitative determination of triclosan from tap water sources spiked with known quantity of triclosan. a Average of three determinations.
3.10. Application to simulated sample analysis To demonstrate the feasibility of the fabricated sensor for possible environmental applications, preliminary application of the sensor was examined by determination of triclosan in fives tap water samples by standard addition method under the optimized conditions. The results are given in Table 3. The recoveries were in the range of 96.4–101.5%, which indicated the efficacy of the sensor for practical analysis. 4. Conclusion In the present paper, we have described a simple, inexpensive and efficient method for the fabrication of nZnO–MWCNT-modified electrode which can be used as a sensor. The irreversible oxidation of triclosan was investigated in PBS (pH 7.0) and was found to be pH dependent. The relationship between square wave voltammetric responses and triclosan concentrations was linear over the range of 1.5 g L−1 –2.0 mg L−1 and the detection limit is about 1.3 g L−1 . Good recoveries were obtained and ranged from 96.4 to 101.5%. The properties of the developed biosensor such as good stability, repeatability and reproducibility provide promise that the sensor could be used as a management tool for determining the quality of household and industrial waste waters for the existence of other ingredients in the synthesis of personal care products. Acknowledgment The authors would like to acknowledge financial support from Directorate Research and Innovation, Tshwane University of Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.12.059. References [1] H. Tungudomwongsa, J. Leckie, T. Mill, Photocatalytic oxidation of emerging contaminants: kinetics and pathways for photocatalytic oxidation of pharmaceutical compounds, J. Adv. Oxid. Technol. 9 (2006) 59–64. [2] M. Adolfsson-Erici, M. Pettersson, J. Parkkonen, J. Sturve, Triclosan, a commonly used bactericide found in human milk and in the aquatic environment in Sweden, Chemosphere 46 (2002) 1485–1489. [3] L. Vidal, A. Chisvert, A. Canals, E. Psillakis, A. Lapkin, F. Acosta, K.J. Edler, J.A. Holdaway, F. Marken, Chemically surface-modified carbon nanoparticle carrier for phenolic pollutants: extraction and electrochemical determination of benzophenone-3 and triclosan, Anal. Chim. Acta 616 (2008) 28–35. [4] Y. Liu, Q.J. Song, L. Wang, Development and characterization of an amperometric sensor for triclosan detection based on electropolymerized molecularly imprinted polymer, Microchem. J. 91 (2009) 222–226. [5] M. Aaron, Peck analytical methods for the determination of persistent ingredients of personal care products in environmental matrices, Anal. Bioanal. Chem. 386 (2006) 907–939.
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Biographies
Mambo Moyo received his Diploma in Education from University of Zimbabwe in 1997. He obtained a BScEd Degree in Chemistry from the Bindura University of Science Education, Zimbabwe, 2004. He obtained his MSc degree in Analytical Chemistry from University of Zimbabwe in 2008. He is currently in his final year doing PhD degree in Environmental Chemistry Management from Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, Pretoria, South Africa. His area of expertise includes biosensors, carbon paste electrodes, biosorption, water and dust analysis and chemistry education.
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Florence Lehutso is a post graduate student in the Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, Pretoria, South Africa. She is currently working on the removal of triclosan from sewage treatments plants, around Gauteng province, South Africa.
Jonathan O. Okonkwo (PhD) is a Research and innovation professor, Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, Pretoria, South Africa. Prof. Okonkwo research areas include, adsorption technology; speciation of toxic metals in aquatic environment; soil remediation, green chemistry and persistent organic pollutants.