Environ Monit Assess (2014) 186:4807–4817 DOI 10.1007/s10661-014-3739-0
Maize tassel-modified carbon paste electrode for voltammetric determination of Cu(II) Mambo Moyo & Jonathan O. Okonkwo & Nana M. Agyei
Received: 19 September 2013 / Accepted: 18 March 2014 / Published online: 6 April 2014 # Springer International Publishing Switzerland 2014
Abstract The preparation and application of a practical electrochemical sensor for environmental monitoring and assessment of heavy metal ions in samples is a subject of considerable interest. In this paper, a carbon paste electrode modified with maize tassel for the determination of Cu(II) has been proposed. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to study morphology and identify the functional groups on the modified electrode, respectively. First, Cu(II) was adsorbed on the carbon paste electrode surface at open circuit and voltammetric techniques were used to investigate the electrochemical performances of the sensor. The electrochemical sensor showed an excellent electrocatalytic activity towards Cu(II) at pH 5.0 and by increasing the amount of maize tassel biomass, a maximum response at 1:2.5 (maize tassel:carbon paste; w/w) was obtained. The electrocatalytic redox current of Cu(II) showed a linear response in the range (1.23 μM to 0.4 mM) with the correlation coefficient of 0.9980. The limit of detection and current–concentration sensitivity were calculated to be 0.13 (±0.01) μM and 0.012 (±0.001) μA/μM, respectively. The sensor gave good recovery of Cu(II) in M. Moyo (*) : J. O. Okonkwo Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, Private Bag X680, Arcadia, Pretoria 0001, South Africa e-mail: [email protected] N. M. Agyei Department of Chemistry, University of Limpopo, P.O. Box 235, Medunsa 0204, South Africa
the range from 96.0 to 98.0 % when applied to water samples. Keywords Maize tassel . Carbon paste . Cu(II) . Voltammetry . Electrochemical sensor
Introduction During recent decades, rapid urbanization and industrialization of the world has led to the accumulation of a vast number of contaminants in the environment. Trace metals hold a superlative position in that list and they have become a public health concern because of their toxicity, nonbiodegradability and persistence in the environment (Verma and Singh 2005; Liu et al. 2007; Gómez et al. 2011). The toxicity of these metals is enhanced through bioaccumulation in animal and plant tissues. The assessment of damage caused by these metals has increased in demand in recent years (Gupta et al. 2010). Copper is an essential element which is required by all organisms as a catalytic cofactor for biological processes such as respiration, oxidative stress protection, and normal cell growth and development (Yuce et al. 2010). Several manifestations of copper deficiency in animals appear to be related to decreased tissue concentration of copper-containing enzymes (Mazloum-Ardakani et al. 2009). In general, a daily copper intake of 1.5–2 mg is essential. However, severe oral intoxication will affect mainly the blood and kidneys. Therefore, the search for portable, rapid, and onsite methods for copper monitoring in biological
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samples and decontamination of industrial waste waters before being discharged into natural water bodies has gathered momentum. Several analytical techniques have been developed to detect and quantify trace metals in a variety of matrices, using atomic absorption spectrometry (Sardans et al. 2010), ultraviolet spectrophotometry (Dayou and Yuan 2007), atomic fluorescence spectrometry (Cotton and Jenkins 1970), x-ray fluorescence (Teixeira et al. 2007), inductively coupled plasma spectrometry (Silva et al. 2009), and isotope dilution inductively coupled plasma mass spectrometry (Christopher and Thompson 2013). However, these techniques are commonly used for measurements of trace metal ions in the laboratory and usually are unfit for field analysis and for rapidly monitoring trace metals in contaminated sites. Expensive instrumentation and complicated sample preparation processes are also required with the aforementioned techniques. Recently, several interesting electrochemical determinations based on modified electrodes have been developed (Pauliukaite et al. 2002; Xiaoli et al. 2005; Adam et al. 2005; Sherigara et al. 2007; Sun et al. 2007; Xu et al. 2008; Injang et al. 2008; Song et al. 2010; Nezamzadeh-Ejhieh and Masoudipour 2010; Svobodova-Tesarova et al. 2011; Nezamzadeh-Ejhieh and Nematollahi 2011; Nascimento et al. 2011; Nezamzadeh-Ejhieh and Esmaeilian 2012; Nezamzadeh-Ejhieh and Hashemi 2012; Devnani and Satsangee 2013). Chemically modified electrodes (CMEs) are of increasing interest in modern electroanalysis as compared to the conventional procedure of using either hanging mercury drop electrode or the mercury film-coated electrode. Mercury is toxic and its difficulties in handling, storage, and disposal may strictly restrict its use an electrode material (Daniele et al. 2008). For rapid detection of trace metals in aqueous solutions, chemically modified electrodes have become very popular since they allow the accumulation of analytes on the electrode surface. For example, carbon paste electrode is mixed directly with a modifier, and hence increasing the sensitivity and selectivity of the determination of trace metal ions. Various biomaterials used in combination with carbon paste have been extensively reviewed (Moyo et al. 2012). Maize tassel, the male inflorescence of the maize plant forms at the top of the stem and provides the pollen for fertilizing the “ear” (also known as a cob). Maize tassel has no value after fertilization, and farmers
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involved in seed production usually cut off the tassel after pollination period. Adsorption studies have also been reviewed to confirm that maize tassel has a high sorption capacity for trace metals (Zvinowanda et al. 2009). Hence, the development of a highly selective maize tassel electrochemical sensor based on the aforementioned properties would enable easy and low-cost detection of trace metals such as copper in aqueous samples. This article reports on studies of the electrochemical behavior of Cu(II) incorporated in maize tasselmodified carbon paste electrode (MT-CPE) using cyclic voltammetry (CV) and square wave voltammetry (SWV). Furthermore, the optimum conditions such as possible accumulation time of the analyte onto the electrode, solution pH, and the effect of interferents such as Cd(II), Zn(II), and Pb(II) ions on Cu(II) determination were investigated.
Materials and methods Chemicals and reagents All solutions were prepared with ultrapure water (18 M Ω cm−1) from a Labo Star DI/UV4 water purification system, Germany. Stock solutions of Cu(II), Pb(II), Cd(II), and Zn(II) (1 mM) were prepared from the corresponding analytical grade metal nitrates (SigmaAldrich, South Africa). Standard solutions of metal ions were prepared freshly by diluting stock solutions. Extra pure graphite fine powder, 2 μm (Sigma-Aldrich) and mineral oil (Sigma-Aldrich) were used for preparing the carbon paste electrode (CPE). Electrolyte solutions were prepared from analytical grade NaNO3. To set the pH of preconcentration solutions, 0.1 M KOH (Minema chemicals, South Africa) and 0.1 M hydrochloric acid (HCl) (Merck, South Africa) were used. Maize tassel preparation The maize tassel was sampled from Tshwane University of Technology farm in Pretoria, South Africa. Maize tassel was plucked off the woody parts of the maize plant, thoroughly washed with water, and air-dried at room temperature. The material was then milled using laboratory mill 3 100 (Stockholm, Sweden). The milled maize tassel was then fractionated using 100-μm analytical sieves and washed twice with 0.01 M HCl in
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order to remove any metal impurities that might be on the biomass. The acid-washed biomass was then washed twice with deionized water prior to electrode modification. Preparation of maize tassel-modified electrochemical sensor The MT-CPE was prepared by hand mixing of maize tassel powder and graphite powder at a ratio (w/w) of 1:6, 1:2.5, 1:1, and 1:0.4 and ground into very small particles in an agate mortar. About 30 mg of the mineral oil was added to the mixture and mixed thoroughly until a uniformly wetted paste was obtained. The paste was then packed onto the end of a carbon paste electrode (BASi, USA) and the surface smoothened on a weighing paper. The bare CPE, used for comparison, was prepared in the same way without adding maize tassel to the mixture. To renew the electrode, a layer of paste was removed from its surface and a new layer of paste was applied; the surface was then smoothened again on the weighing paper.
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Scheme 1 shows summary of the steps involved before voltammetric analysis. Before the voltammetric measurements, the solution was purged with high-purity nitrogen for 120 s. Cyclic voltammetry and square wave voltammetry were scanned from +600 to −800 mV, step E (mV)= 4, square wave amplitude (mV)=25, and square wave frequency (Hz)=15.
Results and discussion Surface morphological studies Figure 1 shows SEM images of CPE, MT, and MT-CPE. As shown in Fig. 1a, the surface of the CPE consisted of isolated and irregular carbon flakes, while Fig. 1b exhibits flattish shapes originating from the fibrous nature of the maize tassel. The flattish rod like shapes of maize tassel could be seen embedded in the carbon powder (Fig. 1c) and evenly distributed on the surface of the graphite particles.
Characterization
Electrochemical characterization
The morphology of the maize tassel, graphite, maize tassel-graphite paste was characterized by scanning electron microscopy (SEM, JEOL JSM-6700 F operated at 8 kV). The functional groups on the maize tassel were identified using Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer 100 made in Waltham, MA, USA). The electrochemical measurements of the MTCPE and CPE were performed with a Bioanalytical Systems (USA) CV-50 W with a three-compartment electrochemical cell comprising platinum wire auxiliary electrode, Ag/AgCl (saturated KNO3), and MT-CPE or bare CPE as working electrodes in a redox probe. The electroactive surface area of the electrodes was determined using Randles-Sevcik equation. Cyclic voltammetry of each cycle involved the following three steps: (1) preparation of electrode, (2) immersion of the electrode into a stirred solution containing 1 mM Cu(II) for a required time to allow adsorption of copper ions on maize tassel in the electrode (preconcentration step at open circuit), and (3) immersing the electrode in cell containing 0.05 M NaNO3 and recording voltammograms for measuring copper ions at electrode (measuring step). After going through the aforementioned steps, the electrode was rinsed carefully with ultrapure water.
Cyclic voltammetry is a useful technique to depict the electrochemical surface characteristics of modified carbon paste electrode. Figure 2 compares the activity of CPE and MT-CPE in 1 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) containing 0.1 M KCl. On the CV curve of the CPE (curve a), a pair of redox peaks corresponding to the redox reaction of ferricyanide as shown Eq. 1 were observed. FeðCNÞ6 4− ↔FeðCNÞ6 3− þ e−
ð1Þ
A further increase, in peak current was observed when the MT-CPE (curve b) was used. Based on cyclic voltammetry result, it can be concluded that MT was incorporated in the carbon powder and allowed increased electron transfer of the redox probe. The electroactive area, A (cm2), can be estimated from the slope of a plot of the voltammetric peak current, Ip versus the square root of sweep rate, v ½ using the Randles–Sevcik equation (Ip =2.69 x 105 n 3/2 AD ½ Co v ½), where Ip refers to the peak current. For K3Fe(CN)6, n=1; and represents the number of electrons transferred, Do (7.6 x 10−6 cm2 s−1), the diffusion coefficient, v the scan rate, and Co the concentration of the redox probe (Rezaei and Damiri 2008; Wan et al.
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Scheme 1 Summary of the stages involved from sample preparation up to adsorption after packing the homogenized paste into the carbon electrode
2010). The surface area of the CPE and MT-CPE were calculated as 13.8 and 58.6 mm2, respectively. Adding MT into the CPE led to a further enhancement of electroactive surface area of the modified electrode. Voltammetric response of Cu(II) at the MT-CPE The activity and active surface area of a CPE and the MT-CPE were investigated using cyclic voltammetry in 0.05 M NaNO3. Figure 3 shows cyclic voltammograms of Cu(II) on modified electrode in 0.05 M NaNO3 at scan rate of 100 mV s−1. The CPE (a) exhibits a virtually flat and featureless voltammetric response while a cathodic peak with identified peak potential and peak current was obtained on the MT-CPE (b). This peak was due to reduction of adsorbed Cu(II). Thus, these results provided conclusive evidence for the accumulation of Cu(II) ions at the MT-CPE. Influence of scan rate The variation of the peak current (Ipc) and the peak potential (Epc) with the scan rate (v) in the range of
20–100 mV s−1 was studied. Figure 4 shows cyclic voltammograms of 0.14 mM Cu2+ with different scan rates in 0.05 M NaNO3. A linear relationship (insert of Fig. 4) was found between the peak current, Ipc and the square root of the scan rate with the linear regression as I pc (μA) = (2.638 ± 0.021) v 1/2 (mVs −1 ) 1/2 − (1.146 ± 0.041) (R = 0.9986). This phenomenon indicates diffusion-controlled reaction. The peak potential moves slightly in positive direction when scan rate increases implying that the electron transfer is not very fast. However, at sweep rates more than 100 mV s−1, peak current decreased. Accumulation mechanism of Cu(II) onto the MT-CPE The accumulation process of Cu(II) at the modified electrode surface is based on the adsorption phenomenon and can be represented by: CuðIIÞaqueous þ MTsurface ¼ ðCuðIIÞ−MTÞadsorption
ð2Þ
Biomaterials are preferred for quantitative work because they contain different functional groups such as hydroxyl, carboxyl, sulfhydry,l and amino groups in
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Fig. 1 SEM images of the typical morphologies of a graphite, b MT, and c MT-CPE
their cell walls (Alphat et al. 2008). The removal of cations from aqueous solution is probably due to the electrostatic attraction between the negatively charged groups created after ionization of the aforementioned groups on the maize tassel surface. The FTIR spectra of MT before and after adsorption are shown in Fig. 5. A strong band at 3,332.8 cm−1 attributed to either –OH or –NH groups showed a slight shift to 3,314.6 cm−1. The band at 1,727.5 cm−1 could be as a result of a carbonyl group and a slight increase in frequency to 1,740.5 cm−1 was observed. The results from the spectral analysis suggested that the hydroxyl and carboxylic groups could be involved in the adsorption reaction.
Experimental parameter optimization Voltammetric behavior of Cu(II) on modified CPE was evaluated in terms of the effect of amount of maize tassel, accumulation time and the pH of the accumulation medium. Finally, the calibration curve was plotted, and the influence of various substances as potential interference compounds on the determination of Cu(II) was studied under the optimum conditions. Influence of maize tassel ratio in carbon paste Electrochemical sensors were prepared using varying ratios of maize tassel. Carbon paste electrodes with
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Fig. 2 CVs of CPE (a), MT-CPE in 1.0 mM K3Fe(CN)6/ K4Fe(CN)6 (1:1) containing 0.1 M KCl, scan rate 100 mV s−1
ratios 1:6; 1:2.5; 1:1; and 1:0.4 (w/w) of maize tassel and graphite were used. The electrochemical response increased with increasing amount of the maize tassel in the carbon paste up to 1:2.5 (w/w). Increasing the amount of the maize tassel in the carbon paste beyond this ratio resulted in a decrease in current response. This might be because the maize tassel and graphite on the electrode surface was so thick that it increased the diffusion distance of Cu(II) hindering mass transfer and electron transfer. Furthermore, the background of voltammetric signal was rather large and signal noisy. This was in agreement with other reported work on agro-wastes such as orange peel (Elviña and Mojica 2005), banana tissue (Mojica et al. 2007), plant refuse (Devnani and Satsangee 2013), and microbial organisms such as Circinella sp. (Alphat et al. 2008). According to these results, a carbon paste composition of 1:2.5 (w/w) maize
Fig. 3 CVs in 0.05 M for both CPE (a) and MT-CPE (b) after 10 min chemical preconcentration in [Cu(II)]=0.140 mM, with scan rate 100 mV s−1 and sweep range from 600 mV to −800 mV
Fig. 4 CVs obtained in 0.05 M NaNO3 for MT-CPE after 10 min chemical preconcentration in [Cu(II)]=0.140 mM, sweep range from +600 mV to −800 mV in different scan rate (from inner to outer) 20, 40, 60, 80, 100 mV s−1. The insert shows linear relationship between the peak currents and the square roots of scan rate, Ipc (μA)=(2.638 ±0.021) v1/2(mVs−1)1/2 − 1.146±0.041 (R= 0.9986)
tassel and graphite was selected as the most appropriate to give reproducible results of the sensor.
Influence of accumulation time Accumulation time is an important factor to define the optimum conditions for the determination of different pollutants. The influence of the accumulation time on the peak current is illustrated in Fig. 6. The experiment
Fig. 5 FTIR spectra of raw maize tassel, after treatment with Cu(II)
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observed at pH 6 greater than or equal to pH 6. These results are in good agreement with previous reports for the first class of metal ions (Gardea-Torresdey et al. 1988; Yuce et al. 2010). Overall, the results demonstrated that weak acidic solution was most suitable for MTCPE and Cu(II) detection, and pH 5 was therefore chosen for further experiments. Analytical characterization of the electrochemical sensor
Fig. 6 SWV of the sensor at the different chemical accumulation times. Inset: plot of peak currents against chemical accumulation time. The data were obtained in 0.05 M NaNO3, [Cu(II)]= 0.140 mM, sweep range from +600 mV to −800 mV, scan rate 100 mV s−1
was performed with 0.14 mM Cu(II) solutions. The results showed that the reduction current increased with accumulation time in the range of 0–25 min. However, after a specific accumulation period, the peak current tended to level off, illustrating saturation of the MT-CPE surface. Hence, 10 min was chosen as an optimum time and, therefore, was used in further experiments. Influence of accumulation medium pH Both the MT-CPE sensor and the electrochemical reaction of Cu(II) could be influenced by the pH of the accumulation solution. Copper belongs to the first class of metal ions that tightly and rapidly bind at pH greater than or equal to 5 and can be stripped off at pH less than or equal to 2 (Gardea-Torresdey et al. 1988). The effect of pH on Cu(II) reduction was examined in the range of 2.0–6.0, as shown in Fig. 7. In the range of pH 2.0–6.0, the reduction peak current increased with increasing pH at first and reached a maximum value at 5. The lower peak current at low pH especially 2 is an indication of possible competition of protons for the exchange sites on the surface of maize tassel or the maize tassel is positively charged because the pH is lower than the isoelectric point. The functional groups of biomaterial cell wall are surrounded by protons and repulsive forces preventing metal ions such as copper to reach the cell wall (Kalyani et al. 2004). The reduction current decreased with further increase in pH as shown in Fig. 7. Precipitation of large quantity of copper ions was
The voltammetric determination of a series of standard solutions of Cu(II) were performed under the optimized working conditions previously described. As can be seen (Fig. 8), the reduction peak current increased proportionally with the Cu(II) concentration in the range of 1.23 μM to 0.4 mM with regression equation of I/μA = (12.006 ± 0.001) [Cu (II)]/μA/mM-(0.0631 ± 0.021) (R=0.9980). The limit of detection was estimated using IUPAC procedure (Analytical Methods Committee 1987) and its estimated value was 0.13 μM. The value obtained was lower than of the organofunctionalised silica modified carbon paste electrode (Cesarino et al. 2007). The MT-CPE showed good precision with a relative standard deviation of 5.2 % for 5 determinations of 0.2 mM Cu(II) using one modified surface for the whole runs. In addition, a relative standard deviation of 4.21 %
Fig. 7 SWV of the MT-CPE sensor at different pH values. Inset: Plot of peak currents against different chemical preconcentration solution pH: (from a to e): 2, 3, 4, 5, 6. The data were obtained in 0.05 M NaNO3 containing [Cu(II)]=0.140 mM. Sweep range from +0.6 to −0.8 V
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Environ Monit Assess (2014) 186:4807–4817 Table 1 Change in cathodic peak current of [Cu(II)]=0.2 mM in the presence of electroactive ions Interfering ion
Concentration/mM
ΔIpc / %a
Cd2+
0.03
−0.59
0.125
−1.3
0.16
−4.0
Zn2+
Pb2+
a
0.31
−9.0
0.03
−0.6
0.125
−1.8
0.16
−5.0
0.31
−10.0
0.03
−0.7
0.125
−12
0.16
−15
0.31
−16
Change in cathodic peak current in percentage
application of the described electrode has the advantages of low cost of fabrication, easy preparation and simple operation, and can help in identification and quantification of trace metal ions. Interference studies
Fig. 8 a SWVof the sensor after incubation with different [Cu(II)] (from a to i): 0.00123, 0.019, 0.023, 0.05, 0.1, 0.14, 0.2, 0.3, 0.4 mM under optimal conditions. b The calibration equation is I/μA = (12.006 ± 0.001) [Cu (II)]/μA/mM − (0.0631 ± 0.021) (R= 0.9980)
was obtained for the SWV measurements 0.2 mM Cu(II) using seven different MT-CPE electrodes prepared in the same way. Compared with some existing biomaterial based sensors, our developed biomaterial sensor provides a wider linear range of Cu(II) than that based on amperometric sensor on which the preparation is based on immobilization of Saccharomyces cerevisiae (Lehmann et al. 2000). The linear range is also comparable to other modifiers such as Circinella sp. (Alphat et al. 2008), silica (Cesarino et al. 2007), and natural zeolite (Alphat et al. 2005). However, more time is required for biomass preparation (Yüce et al. 2010) and high cost of the modifier are some of the drawbacks for the other methods (Lehmann et al. 2000; Mazloum-Ardakani et al. 2009). Thus, the
The presence of other electroactive ions, Zn(II), Cd(II), and Pb(II) in the analyte solution either enhances or decreases the peak current of Cu(II). These ions were chosen because they are usually present in natural water samples and they can be adsorbed by maize tassel (Zvinowanda et al. 2009). For electrochemical studies, this is a problem as these ions might exhibit redox peaks in roughly the same potential range of Cu(II). The response of the maize tassel sensor was evaluated in the presence of 0.2 mM Cu(II) and interfering metal ions at concentrations of 0.03 to 0.31 mM. Studies were
Table 2 Recovery analysis of Cu(II) in tap water samples Sample
[Cu(II)]/μM Added
Tape water
Found
Recovery / %
0.5
0.48±0.1
96.0
10.2
9.8±0.1
96.1
15.0
14.7±0.02
98.0
Results are expressed as mean value±S.D. based on three replicates (n=3) determinations
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conducted by SWV using optimized conditions obtained before. Based on the results in Table 1, 0.03 mM of Cd(II), Zn(II), and Pb(II) had minimal effect on the determination of Cu(II). The results also show that Cd(II) (0.31 mM), Zn(II) (0.31 mM), and Pb(II) (0.125 to 0.31 mM) decreased the peak current of copper by competing with the analyte for the ion-exchange sites in the MT-CPE. However, at all the studied concentrations of the aforementioned electroactive ions, the determination of the Cu(II) was not affected completely since the obtained peaks of Cu(II) were observed clearly. Simultaneous analysis of some of these cations can be performed and the interference effect can be eliminated by the standard addition of known concentration of Cu(II) in the matrix, when higher concentrations of interfering ions are present (Alphat et al. 2008). Preliminary application of the sensor To demonstrate the feasibility of the new sensor for possible applications, preliminary application of the sensor was examined by determining the concentration of Cu(II) in samples of tap water. The different samples were spiked with known amount of Cu(II) standard solution and analyzed using the MT-CPE. Table 2 summarizes the results obtained with the recovery studies. The recoveries were in the range of 96.0– 98.0 %, which indicated the efficacy of the MT-CPE sensor for practical analysis. It can be concluded that the fabricated MT-CPE presented a good accuracy for Cu(II) determinations in the samples matrices studied.
Conclusions The present study proposes an easy to make and lowcost electrochemical sensor for the determination of Cu(II) using maize tassel which is normally discarded as a waste. The MT-modified carbon paste is capable of displaying enhanced voltammetric response of Cu(II). The optimum conditions such as pH 5, accumulation time (10 min), and carbon paste composition of 1:2.5 (w/w) maize tassel and graphite were found to give good sensor response. The SWV reduction current for the MT-CPE presented a linear response from 1.23 μM to 0.4 mM. The MT-CPE sensor showed good recovery from 96.0 to 98.0 % when used in water analysis by using standard addition method. It is expected that with its good adsorptive behavior and easy fabrication, the
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sensor could be easily extended to detect Cu(II) and other trace metal ions in other landfill leachates. Acknowledgement The authors would like to acknowledge financial support from Tshwane University of Technology and the Food Science and Technology Department of the University of Pretoria for the use of the milling equipment.
Conflict of interest The authors declare that they have no conflict of interest
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Environ Monit Assess (2014) 186:4807–4817 Pauliukaite, R., Metelka, R., Švancara, I., Królicka, A., Bobrowski, A., Vytřas, K., et al. (2002). Carbon paste electrodes modified with Bi2O3 as sensors for the determination of Cd and Pb. Analytical and Bioanalytical Chemistry, 374(6), 1155–1158. Rezaei, B., & Damiri, S. (2008). Multiwalled carbon nanotubes modified electrode as a sensor for adsorptive stripping voltammetric determination of hydrochlorothiazide. Sensors, 8, 1523–1529. Sardans, R., Montes, F., & Peñuelas, J. (2010). Determination of As, Cd, Cu, Hg and Pb in biological samples by modern electrothermal atom ic absorption spectrometry. Spectrochimica Acta Part B, 65, 97–112. Sherigara, B. S., Shivaraj, Y., Mascarenhas, R. J., & Satpati, A. K. (2007). Simultaneous determination of lead, copper and cadmium onto mercury film supported on wax impregnated carbon paste electrode: assessment of quantification procedures by anodic stripping voltammetry. Electrochimica Acta, 52, 3137–3142. Silva, E. L., dos Santos Roldan, P., & Giné, M. F. (2009). Simultaneous preconcentration of copper, zinc, cadmium, and nickel in water samples by cloud point extraction using 4-(2-pyridylazo)- resorcinol and their determination by inductively coupled optic emission spectrometry. Journal of Hazardous Materials, 171, 1133–1138. Song, W., Zhang, L., Shi, L., Li, D., Li, Y., & Long, Y. (2010). Simultaneous determination of cadmium(II), lead(II) and copper(II) by using a screen-printed electrode modified with mercury nano-droplets. Microchimica Acta, 169, 321–326. Sun, D., Wan, C., Li, G., & Wu, K. (2007). Electrochemical determination of lead (II) using a montmorillonite calciummodified carbon paste electrode. Microchimica Acta, 158, 255–260. Svobodova-Tesarova, E., Baldrianova, L., Stoces, M., Svancara, I., Vytras, K., Hocevar, S. B., et al. (2011). Antimony powdermodified carbon paste electrodes for electrochemical stripping determination of trace heavy metals. Electrochimica Acta, 56(19), 6673–6677. Teixeira, L. S. G., Rocha, R. B. S., Sobrinho, E. V., Guimarães, P. R. B., Pontes, L. A. M., & Teixeira, J. S. R. (2007). Simultaneous determination of copper and iron in automotive gasoline by X-ray fluorescence after pre-concentration on cellulose paper. Talanta, 72, 1073–1076. Verma, N., & Singh, M. (2005). Biosensors for heavy metals. BioMetals, 18, 121–129. Wan, Q., Yu, F., Zhu, L., & Wang, X. (2010). Bucky-gel coated glassy carbon electrodes, voltammetric detection of fentomolar level lead ions. Talanta, 82, 1820–1824. Xiaoli, C., Guodong, L., Liyu, L., Wassana, Y., & Yuehe, L. (2005). Electrochemical sensor based on carbon paste electrode modified with nanostructured cryptomelane-type manganese oxides for detection of heavy metal. Sensor Letters, 3, 16–21. Xu, H., Zeng, L. P., Xing, S. J., Xian, Y. Z., Shi, G. Y., & Jin, L. T. (2008). Ultrasensitive voltammetric detection of trace lead, and cadmium using MWCNTs-Nafion-bismuth composite electrodes. Electroanalysis, 20, 2655–2662.
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4817 Zvinowanda, C. M., Okonkwo, O. J., Sekhula, M. M., Agyei, N. M., & Sadiku, R. (2009). Application of maize tassel for the removal of Pb, Se, Sr, U and V from borehole water contaminated with mine wastewater in the presence of alkaline metals. Journal of Hazardous Materials, 164, 884–891.
Maize tassel-modified carbon paste electrode for voltammetric determination of Cu(II) Mambo Moyo & Jonathan O. Okonkwo & Nana M. Agyei
Received: 19 September 2013 / Accepted: 18 March 2014 / Published online: 6 April 2014 # Springer International Publishing Switzerland 2014
Abstract The preparation and application of a practical electrochemical sensor for environmental monitoring and assessment of heavy metal ions in samples is a subject of considerable interest. In this paper, a carbon paste electrode modified with maize tassel for the determination of Cu(II) has been proposed. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to study morphology and identify the functional groups on the modified electrode, respectively. First, Cu(II) was adsorbed on the carbon paste electrode surface at open circuit and voltammetric techniques were used to investigate the electrochemical performances of the sensor. The electrochemical sensor showed an excellent electrocatalytic activity towards Cu(II) at pH 5.0 and by increasing the amount of maize tassel biomass, a maximum response at 1:2.5 (maize tassel:carbon paste; w/w) was obtained. The electrocatalytic redox current of Cu(II) showed a linear response in the range (1.23 μM to 0.4 mM) with the correlation coefficient of 0.9980. The limit of detection and current–concentration sensitivity were calculated to be 0.13 (±0.01) μM and 0.012 (±0.001) μA/μM, respectively. The sensor gave good recovery of Cu(II) in M. Moyo (*) : J. O. Okonkwo Department of Environmental, Water, and Earth Sciences, Tshwane University of Technology, Private Bag X680, Arcadia, Pretoria 0001, South Africa e-mail: [email protected] N. M. Agyei Department of Chemistry, University of Limpopo, P.O. Box 235, Medunsa 0204, South Africa
the range from 96.0 to 98.0 % when applied to water samples. Keywords Maize tassel . Carbon paste . Cu(II) . Voltammetry . Electrochemical sensor
Introduction During recent decades, rapid urbanization and industrialization of the world has led to the accumulation of a vast number of contaminants in the environment. Trace metals hold a superlative position in that list and they have become a public health concern because of their toxicity, nonbiodegradability and persistence in the environment (Verma and Singh 2005; Liu et al. 2007; Gómez et al. 2011). The toxicity of these metals is enhanced through bioaccumulation in animal and plant tissues. The assessment of damage caused by these metals has increased in demand in recent years (Gupta et al. 2010). Copper is an essential element which is required by all organisms as a catalytic cofactor for biological processes such as respiration, oxidative stress protection, and normal cell growth and development (Yuce et al. 2010). Several manifestations of copper deficiency in animals appear to be related to decreased tissue concentration of copper-containing enzymes (Mazloum-Ardakani et al. 2009). In general, a daily copper intake of 1.5–2 mg is essential. However, severe oral intoxication will affect mainly the blood and kidneys. Therefore, the search for portable, rapid, and onsite methods for copper monitoring in biological
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samples and decontamination of industrial waste waters before being discharged into natural water bodies has gathered momentum. Several analytical techniques have been developed to detect and quantify trace metals in a variety of matrices, using atomic absorption spectrometry (Sardans et al. 2010), ultraviolet spectrophotometry (Dayou and Yuan 2007), atomic fluorescence spectrometry (Cotton and Jenkins 1970), x-ray fluorescence (Teixeira et al. 2007), inductively coupled plasma spectrometry (Silva et al. 2009), and isotope dilution inductively coupled plasma mass spectrometry (Christopher and Thompson 2013). However, these techniques are commonly used for measurements of trace metal ions in the laboratory and usually are unfit for field analysis and for rapidly monitoring trace metals in contaminated sites. Expensive instrumentation and complicated sample preparation processes are also required with the aforementioned techniques. Recently, several interesting electrochemical determinations based on modified electrodes have been developed (Pauliukaite et al. 2002; Xiaoli et al. 2005; Adam et al. 2005; Sherigara et al. 2007; Sun et al. 2007; Xu et al. 2008; Injang et al. 2008; Song et al. 2010; Nezamzadeh-Ejhieh and Masoudipour 2010; Svobodova-Tesarova et al. 2011; Nezamzadeh-Ejhieh and Nematollahi 2011; Nascimento et al. 2011; Nezamzadeh-Ejhieh and Esmaeilian 2012; Nezamzadeh-Ejhieh and Hashemi 2012; Devnani and Satsangee 2013). Chemically modified electrodes (CMEs) are of increasing interest in modern electroanalysis as compared to the conventional procedure of using either hanging mercury drop electrode or the mercury film-coated electrode. Mercury is toxic and its difficulties in handling, storage, and disposal may strictly restrict its use an electrode material (Daniele et al. 2008). For rapid detection of trace metals in aqueous solutions, chemically modified electrodes have become very popular since they allow the accumulation of analytes on the electrode surface. For example, carbon paste electrode is mixed directly with a modifier, and hence increasing the sensitivity and selectivity of the determination of trace metal ions. Various biomaterials used in combination with carbon paste have been extensively reviewed (Moyo et al. 2012). Maize tassel, the male inflorescence of the maize plant forms at the top of the stem and provides the pollen for fertilizing the “ear” (also known as a cob). Maize tassel has no value after fertilization, and farmers
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involved in seed production usually cut off the tassel after pollination period. Adsorption studies have also been reviewed to confirm that maize tassel has a high sorption capacity for trace metals (Zvinowanda et al. 2009). Hence, the development of a highly selective maize tassel electrochemical sensor based on the aforementioned properties would enable easy and low-cost detection of trace metals such as copper in aqueous samples. This article reports on studies of the electrochemical behavior of Cu(II) incorporated in maize tasselmodified carbon paste electrode (MT-CPE) using cyclic voltammetry (CV) and square wave voltammetry (SWV). Furthermore, the optimum conditions such as possible accumulation time of the analyte onto the electrode, solution pH, and the effect of interferents such as Cd(II), Zn(II), and Pb(II) ions on Cu(II) determination were investigated.
Materials and methods Chemicals and reagents All solutions were prepared with ultrapure water (18 M Ω cm−1) from a Labo Star DI/UV4 water purification system, Germany. Stock solutions of Cu(II), Pb(II), Cd(II), and Zn(II) (1 mM) were prepared from the corresponding analytical grade metal nitrates (SigmaAldrich, South Africa). Standard solutions of metal ions were prepared freshly by diluting stock solutions. Extra pure graphite fine powder, 2 μm (Sigma-Aldrich) and mineral oil (Sigma-Aldrich) were used for preparing the carbon paste electrode (CPE). Electrolyte solutions were prepared from analytical grade NaNO3. To set the pH of preconcentration solutions, 0.1 M KOH (Minema chemicals, South Africa) and 0.1 M hydrochloric acid (HCl) (Merck, South Africa) were used. Maize tassel preparation The maize tassel was sampled from Tshwane University of Technology farm in Pretoria, South Africa. Maize tassel was plucked off the woody parts of the maize plant, thoroughly washed with water, and air-dried at room temperature. The material was then milled using laboratory mill 3 100 (Stockholm, Sweden). The milled maize tassel was then fractionated using 100-μm analytical sieves and washed twice with 0.01 M HCl in
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order to remove any metal impurities that might be on the biomass. The acid-washed biomass was then washed twice with deionized water prior to electrode modification. Preparation of maize tassel-modified electrochemical sensor The MT-CPE was prepared by hand mixing of maize tassel powder and graphite powder at a ratio (w/w) of 1:6, 1:2.5, 1:1, and 1:0.4 and ground into very small particles in an agate mortar. About 30 mg of the mineral oil was added to the mixture and mixed thoroughly until a uniformly wetted paste was obtained. The paste was then packed onto the end of a carbon paste electrode (BASi, USA) and the surface smoothened on a weighing paper. The bare CPE, used for comparison, was prepared in the same way without adding maize tassel to the mixture. To renew the electrode, a layer of paste was removed from its surface and a new layer of paste was applied; the surface was then smoothened again on the weighing paper.
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Scheme 1 shows summary of the steps involved before voltammetric analysis. Before the voltammetric measurements, the solution was purged with high-purity nitrogen for 120 s. Cyclic voltammetry and square wave voltammetry were scanned from +600 to −800 mV, step E (mV)= 4, square wave amplitude (mV)=25, and square wave frequency (Hz)=15.
Results and discussion Surface morphological studies Figure 1 shows SEM images of CPE, MT, and MT-CPE. As shown in Fig. 1a, the surface of the CPE consisted of isolated and irregular carbon flakes, while Fig. 1b exhibits flattish shapes originating from the fibrous nature of the maize tassel. The flattish rod like shapes of maize tassel could be seen embedded in the carbon powder (Fig. 1c) and evenly distributed on the surface of the graphite particles.
Characterization
Electrochemical characterization
The morphology of the maize tassel, graphite, maize tassel-graphite paste was characterized by scanning electron microscopy (SEM, JEOL JSM-6700 F operated at 8 kV). The functional groups on the maize tassel were identified using Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer 100 made in Waltham, MA, USA). The electrochemical measurements of the MTCPE and CPE were performed with a Bioanalytical Systems (USA) CV-50 W with a three-compartment electrochemical cell comprising platinum wire auxiliary electrode, Ag/AgCl (saturated KNO3), and MT-CPE or bare CPE as working electrodes in a redox probe. The electroactive surface area of the electrodes was determined using Randles-Sevcik equation. Cyclic voltammetry of each cycle involved the following three steps: (1) preparation of electrode, (2) immersion of the electrode into a stirred solution containing 1 mM Cu(II) for a required time to allow adsorption of copper ions on maize tassel in the electrode (preconcentration step at open circuit), and (3) immersing the electrode in cell containing 0.05 M NaNO3 and recording voltammograms for measuring copper ions at electrode (measuring step). After going through the aforementioned steps, the electrode was rinsed carefully with ultrapure water.
Cyclic voltammetry is a useful technique to depict the electrochemical surface characteristics of modified carbon paste electrode. Figure 2 compares the activity of CPE and MT-CPE in 1 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) containing 0.1 M KCl. On the CV curve of the CPE (curve a), a pair of redox peaks corresponding to the redox reaction of ferricyanide as shown Eq. 1 were observed. FeðCNÞ6 4− ↔FeðCNÞ6 3− þ e−
ð1Þ
A further increase, in peak current was observed when the MT-CPE (curve b) was used. Based on cyclic voltammetry result, it can be concluded that MT was incorporated in the carbon powder and allowed increased electron transfer of the redox probe. The electroactive area, A (cm2), can be estimated from the slope of a plot of the voltammetric peak current, Ip versus the square root of sweep rate, v ½ using the Randles–Sevcik equation (Ip =2.69 x 105 n 3/2 AD ½ Co v ½), where Ip refers to the peak current. For K3Fe(CN)6, n=1; and represents the number of electrons transferred, Do (7.6 x 10−6 cm2 s−1), the diffusion coefficient, v the scan rate, and Co the concentration of the redox probe (Rezaei and Damiri 2008; Wan et al.
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Scheme 1 Summary of the stages involved from sample preparation up to adsorption after packing the homogenized paste into the carbon electrode
2010). The surface area of the CPE and MT-CPE were calculated as 13.8 and 58.6 mm2, respectively. Adding MT into the CPE led to a further enhancement of electroactive surface area of the modified electrode. Voltammetric response of Cu(II) at the MT-CPE The activity and active surface area of a CPE and the MT-CPE were investigated using cyclic voltammetry in 0.05 M NaNO3. Figure 3 shows cyclic voltammograms of Cu(II) on modified electrode in 0.05 M NaNO3 at scan rate of 100 mV s−1. The CPE (a) exhibits a virtually flat and featureless voltammetric response while a cathodic peak with identified peak potential and peak current was obtained on the MT-CPE (b). This peak was due to reduction of adsorbed Cu(II). Thus, these results provided conclusive evidence for the accumulation of Cu(II) ions at the MT-CPE. Influence of scan rate The variation of the peak current (Ipc) and the peak potential (Epc) with the scan rate (v) in the range of
20–100 mV s−1 was studied. Figure 4 shows cyclic voltammograms of 0.14 mM Cu2+ with different scan rates in 0.05 M NaNO3. A linear relationship (insert of Fig. 4) was found between the peak current, Ipc and the square root of the scan rate with the linear regression as I pc (μA) = (2.638 ± 0.021) v 1/2 (mVs −1 ) 1/2 − (1.146 ± 0.041) (R = 0.9986). This phenomenon indicates diffusion-controlled reaction. The peak potential moves slightly in positive direction when scan rate increases implying that the electron transfer is not very fast. However, at sweep rates more than 100 mV s−1, peak current decreased. Accumulation mechanism of Cu(II) onto the MT-CPE The accumulation process of Cu(II) at the modified electrode surface is based on the adsorption phenomenon and can be represented by: CuðIIÞaqueous þ MTsurface ¼ ðCuðIIÞ−MTÞadsorption
ð2Þ
Biomaterials are preferred for quantitative work because they contain different functional groups such as hydroxyl, carboxyl, sulfhydry,l and amino groups in
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Fig. 1 SEM images of the typical morphologies of a graphite, b MT, and c MT-CPE
their cell walls (Alphat et al. 2008). The removal of cations from aqueous solution is probably due to the electrostatic attraction between the negatively charged groups created after ionization of the aforementioned groups on the maize tassel surface. The FTIR spectra of MT before and after adsorption are shown in Fig. 5. A strong band at 3,332.8 cm−1 attributed to either –OH or –NH groups showed a slight shift to 3,314.6 cm−1. The band at 1,727.5 cm−1 could be as a result of a carbonyl group and a slight increase in frequency to 1,740.5 cm−1 was observed. The results from the spectral analysis suggested that the hydroxyl and carboxylic groups could be involved in the adsorption reaction.
Experimental parameter optimization Voltammetric behavior of Cu(II) on modified CPE was evaluated in terms of the effect of amount of maize tassel, accumulation time and the pH of the accumulation medium. Finally, the calibration curve was plotted, and the influence of various substances as potential interference compounds on the determination of Cu(II) was studied under the optimum conditions. Influence of maize tassel ratio in carbon paste Electrochemical sensors were prepared using varying ratios of maize tassel. Carbon paste electrodes with
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Fig. 2 CVs of CPE (a), MT-CPE in 1.0 mM K3Fe(CN)6/ K4Fe(CN)6 (1:1) containing 0.1 M KCl, scan rate 100 mV s−1
ratios 1:6; 1:2.5; 1:1; and 1:0.4 (w/w) of maize tassel and graphite were used. The electrochemical response increased with increasing amount of the maize tassel in the carbon paste up to 1:2.5 (w/w). Increasing the amount of the maize tassel in the carbon paste beyond this ratio resulted in a decrease in current response. This might be because the maize tassel and graphite on the electrode surface was so thick that it increased the diffusion distance of Cu(II) hindering mass transfer and electron transfer. Furthermore, the background of voltammetric signal was rather large and signal noisy. This was in agreement with other reported work on agro-wastes such as orange peel (Elviña and Mojica 2005), banana tissue (Mojica et al. 2007), plant refuse (Devnani and Satsangee 2013), and microbial organisms such as Circinella sp. (Alphat et al. 2008). According to these results, a carbon paste composition of 1:2.5 (w/w) maize
Fig. 3 CVs in 0.05 M for both CPE (a) and MT-CPE (b) after 10 min chemical preconcentration in [Cu(II)]=0.140 mM, with scan rate 100 mV s−1 and sweep range from 600 mV to −800 mV
Fig. 4 CVs obtained in 0.05 M NaNO3 for MT-CPE after 10 min chemical preconcentration in [Cu(II)]=0.140 mM, sweep range from +600 mV to −800 mV in different scan rate (from inner to outer) 20, 40, 60, 80, 100 mV s−1. The insert shows linear relationship between the peak currents and the square roots of scan rate, Ipc (μA)=(2.638 ±0.021) v1/2(mVs−1)1/2 − 1.146±0.041 (R= 0.9986)
tassel and graphite was selected as the most appropriate to give reproducible results of the sensor.
Influence of accumulation time Accumulation time is an important factor to define the optimum conditions for the determination of different pollutants. The influence of the accumulation time on the peak current is illustrated in Fig. 6. The experiment
Fig. 5 FTIR spectra of raw maize tassel, after treatment with Cu(II)
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observed at pH 6 greater than or equal to pH 6. These results are in good agreement with previous reports for the first class of metal ions (Gardea-Torresdey et al. 1988; Yuce et al. 2010). Overall, the results demonstrated that weak acidic solution was most suitable for MTCPE and Cu(II) detection, and pH 5 was therefore chosen for further experiments. Analytical characterization of the electrochemical sensor
Fig. 6 SWV of the sensor at the different chemical accumulation times. Inset: plot of peak currents against chemical accumulation time. The data were obtained in 0.05 M NaNO3, [Cu(II)]= 0.140 mM, sweep range from +600 mV to −800 mV, scan rate 100 mV s−1
was performed with 0.14 mM Cu(II) solutions. The results showed that the reduction current increased with accumulation time in the range of 0–25 min. However, after a specific accumulation period, the peak current tended to level off, illustrating saturation of the MT-CPE surface. Hence, 10 min was chosen as an optimum time and, therefore, was used in further experiments. Influence of accumulation medium pH Both the MT-CPE sensor and the electrochemical reaction of Cu(II) could be influenced by the pH of the accumulation solution. Copper belongs to the first class of metal ions that tightly and rapidly bind at pH greater than or equal to 5 and can be stripped off at pH less than or equal to 2 (Gardea-Torresdey et al. 1988). The effect of pH on Cu(II) reduction was examined in the range of 2.0–6.0, as shown in Fig. 7. In the range of pH 2.0–6.0, the reduction peak current increased with increasing pH at first and reached a maximum value at 5. The lower peak current at low pH especially 2 is an indication of possible competition of protons for the exchange sites on the surface of maize tassel or the maize tassel is positively charged because the pH is lower than the isoelectric point. The functional groups of biomaterial cell wall are surrounded by protons and repulsive forces preventing metal ions such as copper to reach the cell wall (Kalyani et al. 2004). The reduction current decreased with further increase in pH as shown in Fig. 7. Precipitation of large quantity of copper ions was
The voltammetric determination of a series of standard solutions of Cu(II) were performed under the optimized working conditions previously described. As can be seen (Fig. 8), the reduction peak current increased proportionally with the Cu(II) concentration in the range of 1.23 μM to 0.4 mM with regression equation of I/μA = (12.006 ± 0.001) [Cu (II)]/μA/mM-(0.0631 ± 0.021) (R=0.9980). The limit of detection was estimated using IUPAC procedure (Analytical Methods Committee 1987) and its estimated value was 0.13 μM. The value obtained was lower than of the organofunctionalised silica modified carbon paste electrode (Cesarino et al. 2007). The MT-CPE showed good precision with a relative standard deviation of 5.2 % for 5 determinations of 0.2 mM Cu(II) using one modified surface for the whole runs. In addition, a relative standard deviation of 4.21 %
Fig. 7 SWV of the MT-CPE sensor at different pH values. Inset: Plot of peak currents against different chemical preconcentration solution pH: (from a to e): 2, 3, 4, 5, 6. The data were obtained in 0.05 M NaNO3 containing [Cu(II)]=0.140 mM. Sweep range from +0.6 to −0.8 V
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Environ Monit Assess (2014) 186:4807–4817 Table 1 Change in cathodic peak current of [Cu(II)]=0.2 mM in the presence of electroactive ions Interfering ion
Concentration/mM
ΔIpc / %a
Cd2+
0.03
−0.59
0.125
−1.3
0.16
−4.0
Zn2+
Pb2+
a
0.31
−9.0
0.03
−0.6
0.125
−1.8
0.16
−5.0
0.31
−10.0
0.03
−0.7
0.125
−12
0.16
−15
0.31
−16
Change in cathodic peak current in percentage
application of the described electrode has the advantages of low cost of fabrication, easy preparation and simple operation, and can help in identification and quantification of trace metal ions. Interference studies
Fig. 8 a SWVof the sensor after incubation with different [Cu(II)] (from a to i): 0.00123, 0.019, 0.023, 0.05, 0.1, 0.14, 0.2, 0.3, 0.4 mM under optimal conditions. b The calibration equation is I/μA = (12.006 ± 0.001) [Cu (II)]/μA/mM − (0.0631 ± 0.021) (R= 0.9980)
was obtained for the SWV measurements 0.2 mM Cu(II) using seven different MT-CPE electrodes prepared in the same way. Compared with some existing biomaterial based sensors, our developed biomaterial sensor provides a wider linear range of Cu(II) than that based on amperometric sensor on which the preparation is based on immobilization of Saccharomyces cerevisiae (Lehmann et al. 2000). The linear range is also comparable to other modifiers such as Circinella sp. (Alphat et al. 2008), silica (Cesarino et al. 2007), and natural zeolite (Alphat et al. 2005). However, more time is required for biomass preparation (Yüce et al. 2010) and high cost of the modifier are some of the drawbacks for the other methods (Lehmann et al. 2000; Mazloum-Ardakani et al. 2009). Thus, the
The presence of other electroactive ions, Zn(II), Cd(II), and Pb(II) in the analyte solution either enhances or decreases the peak current of Cu(II). These ions were chosen because they are usually present in natural water samples and they can be adsorbed by maize tassel (Zvinowanda et al. 2009). For electrochemical studies, this is a problem as these ions might exhibit redox peaks in roughly the same potential range of Cu(II). The response of the maize tassel sensor was evaluated in the presence of 0.2 mM Cu(II) and interfering metal ions at concentrations of 0.03 to 0.31 mM. Studies were
Table 2 Recovery analysis of Cu(II) in tap water samples Sample
[Cu(II)]/μM Added
Tape water
Found
Recovery / %
0.5
0.48±0.1
96.0
10.2
9.8±0.1
96.1
15.0
14.7±0.02
98.0
Results are expressed as mean value±S.D. based on three replicates (n=3) determinations
Environ Monit Assess (2014) 186:4807–4817
conducted by SWV using optimized conditions obtained before. Based on the results in Table 1, 0.03 mM of Cd(II), Zn(II), and Pb(II) had minimal effect on the determination of Cu(II). The results also show that Cd(II) (0.31 mM), Zn(II) (0.31 mM), and Pb(II) (0.125 to 0.31 mM) decreased the peak current of copper by competing with the analyte for the ion-exchange sites in the MT-CPE. However, at all the studied concentrations of the aforementioned electroactive ions, the determination of the Cu(II) was not affected completely since the obtained peaks of Cu(II) were observed clearly. Simultaneous analysis of some of these cations can be performed and the interference effect can be eliminated by the standard addition of known concentration of Cu(II) in the matrix, when higher concentrations of interfering ions are present (Alphat et al. 2008). Preliminary application of the sensor To demonstrate the feasibility of the new sensor for possible applications, preliminary application of the sensor was examined by determining the concentration of Cu(II) in samples of tap water. The different samples were spiked with known amount of Cu(II) standard solution and analyzed using the MT-CPE. Table 2 summarizes the results obtained with the recovery studies. The recoveries were in the range of 96.0– 98.0 %, which indicated the efficacy of the MT-CPE sensor for practical analysis. It can be concluded that the fabricated MT-CPE presented a good accuracy for Cu(II) determinations in the samples matrices studied.
Conclusions The present study proposes an easy to make and lowcost electrochemical sensor for the determination of Cu(II) using maize tassel which is normally discarded as a waste. The MT-modified carbon paste is capable of displaying enhanced voltammetric response of Cu(II). The optimum conditions such as pH 5, accumulation time (10 min), and carbon paste composition of 1:2.5 (w/w) maize tassel and graphite were found to give good sensor response. The SWV reduction current for the MT-CPE presented a linear response from 1.23 μM to 0.4 mM. The MT-CPE sensor showed good recovery from 96.0 to 98.0 % when used in water analysis by using standard addition method. It is expected that with its good adsorptive behavior and easy fabrication, the
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sensor could be easily extended to detect Cu(II) and other trace metal ions in other landfill leachates. Acknowledgement The authors would like to acknowledge financial support from Tshwane University of Technology and the Food Science and Technology Department of the University of Pretoria for the use of the milling equipment.
Conflict of interest The authors declare that they have no conflict of interest
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