Anodic stripping voltammetric determination of copper(II) using a functionalized carbon nanotubes paste electrode modified with crosslinked chitosan

Sensors and Actuators B 142 (2009) 260–266

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Anodic stripping voltammetric determination of copper(II) using a functionalized carbon nanotubes paste electrode modified with crosslinked chitosan Bruno C. Janegitz a , Luiz H. Marcolino-Junior b , Sérgio P. Campana-Filho c , Ronaldo C. Faria a , Orlando Fatibello-Filho a,∗ a b c

Departamento de Química, Centro de Ciências Exatas e de Tecnologia, Universidade Federal de São Carlos, São Carlos, SP, Brazil Departamento de Química, Universidade Federal do Paraná, Curitiba, PR, Brazil Instituto de Química de São Carlos, Universidade de São Paulo, SP, Brazil

a r t i c l e

i n f o

Article history: Received 26 May 2009 Received in revised form 29 July 2009 Accepted 17 August 2009 Available online 25 August 2009 Keywords: Carbon nanotubes Crosslinked chitosan Copper determination Anodic stripping voltammetry

a b s t r a c t The development and application of a functionalized carbon nanotubes paste electrode (CNPE) modified with crosslinked chitosan for determination of Cu(II) in industrial wastewater, natural water and human urine samples by linear scan anodic stripping voltammetry (LSASV) are described. Different electrodes were constructed using chitosan and chitosan crosslinked with glutaraldehyde (CTS-GA) and epichlorohydrin (CTS-ECH). The best voltammetric response for Cu(II) was obtained with a paste composition of 65% (m/m) of functionalized carbon nanotubes, 15% (m/m) of CTS-ECH, and 20% (m/m) of mineral oil using a solution of 0.05 mol L−1 KNO3 with pH adjusted to 2.25 with HNO3 , an accumulation potential of −0.3 V vs. Ag/AgCl (3.0 mol L−1 KCl) for 300 s and a scan rate of 100 mV s−1 . Under these optimal experimental conditions, the voltammetric response was linearly dependent on the Cu(II) concentration in the range from 7.90 × 10−8 to 1.60 × 10−5 mol L−1 with a detection limit of 1.00 × 10−8 mol L−1 . The samples analyses were evaluated using the proposed sensor and a good recovery of Cu(II) was obtained with results in the range from 98.0% to 104%. The analysis of industrial wastewater, natural water and human urine samples obtained using the proposed CNPE modified with CTS-ECH electrode and those obtained using a comparative method are in agreement at the 95% confidence level. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The carbon nanotubes (CNTs), since their discovery by Iijima in 1991 [1], have attracted considerable attention due to their fascinating and unique structural, mechanical, electrical and electrochemical properties. The carbon nanotubes have been widely used for the development of chemically modified electrodes providing the electrochemists with special properties, as good conductivity, and enhanced sensing capabilities of sensors and biosensors [2–7]. Moreover, extensive efforts have been devoted to design novel CNTs modified electrodes to improve the voltammetric determinations of organic [8,9] and inorganic [10] compounds. A pretreatment of the CNTs is usually necessary to eliminate metallic impurities, and/or to improve the electron transfer properties and/or to allow further functionalization. The pretreatment consists in expose the CNTs to an acidic solution of sulfuric [11], nitric [12–15] or hydrochloric [16,17] acid, or mixture of these acids at

∗ Corresponding author at: Departamento de Química, Universidade Federal de São Carlos, Caixa Postal 676, CEP 13560-970, São Carlos, SP, Brazil. Tel.: +55 16 33518098; fax: +55 16 33518350. E-mail address: [email protected] (O. Fatibello-Filho). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.08.033

room temperature, under refluxing or under sonication for different times [15]. Chitosan, a polysaccharide, which occurs in the cell walls of some fungi, is the N-deacetylated derivative of chitin, an abundant biopolymer extracted from the outer shells of crustaceans such as shrimps, crabs and lobsters [18,19]. It is a copolymer composed by 2-amino-2-deoxy-d-glucopyranose and 2-acetamido-2-deoxy-dglucopyranose units linked by ␤(1–4) glycosidic bonds, which has a high content of hydroxyl and amino groups along its chains, conferring a high metal-chelating ability to chitosan, which is widely studied as material for wastewater treatment [20] and electrochemical determinations of metals ions in aqueous solution [21]. In addition to the aforementioned, chitosan has been used for protein immobilization and for the development of biosensors [22–24]. Some properties of chitosan such as its mechanical strength, chemical stability, hydrophilicity, pore size and biocompatibility can be improved by physical [25–29] and/or chemical [30,31] modifications. The chemical modifications of the chitosan by covalently attaching of selected molecules to the amino or hydroxyls groups can improve the ion-transport and ion-exchange proprieties of the biopolymer. For example, Cruz et al. [32] described a anion permselectivity behavior and an increase in the ion transport of the chitosan film chemically modified with an anionic dye and

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crosslinked with glutaraldehyde. In the same way the use of a bifunctional crosslinking agent, such as glutaraldehyde (GA) or epichlorohydrin (ECH), can modifies the chitosan proprieties and enhances its metal ion adsorption capabilities [33]. Due to interesting proprieties of CNTs, the combination of the modified chitosans with CNTs can be an attractive strategy to design electrochemical sensors [24,31,34] The copper can be tolerated by human beings at relatively large concentration but excessive intake of this element manifests certain diseases in humans as Menke’s syndrome and Wilson’s disease [35,36] and it is also toxic to aquatic organisms even at very small concentrations in natural water. There are several studies of new materials for removing toxic metals by sorption or biosorption as emerging technique for the treatment of wastewater [20,37]. In this work, functionalized carbon nanotubes paste electrodes modified with chitosan and crosslinked chitosan with GA or ECH were evaluated for Cu(II) determination by LSASV in industrial wastewater, natural waters and urine samples. 2. Experimental 2.1. Reagents and solutions The multiwalled carbon nanotubes, of 110–170 nm in diameter and 5–9 ␮m in length were purchased from Sigma (St. Louis, MO, USA). Chitosan with weight-average molecular weight of 700,000 g mol−1 and 82% of deacetylation degree obtained by Ndeacetylation of chitin after a multistep process by freeze–pump out–thaw cycles as previously described [38]. The water used in all experiments was the ultrapure water with resistivity not less than 18.2 M cm obtained with a Millipore Milli-Q system (Billerica, USA). All other chemical reagents were used as received. 2.2. Apparatus The voltammetric measurements were performed with a threeelectrode system, including the modified carbon nanotubes paste electrodes as working electrode, a platinum plate as counter electrode, and Ag/AgCl (3.0 mol L−1 KCl) as reference electrode. Voltammetric measurements were carried out using a Autolab Ecochemie model PGSTAT12 (Utrecht, Netherlands) potentiostat/galvanostat controlled by GPES 4.9 software. X-ray diffraction patterns were obtained using a Rigaku-DMax 2500PC (Tokyo, Japan) diffractometer with Cu K␣ radiation in the range 5◦ ≤ 2 ≤ 75◦ at 0.03◦ min−1 . The functionalization of carbon nanotubes was verified by fourier transform infrared spectroscopy (FTIR) using a Bomem model MB 102 (Quebec, Canada), spectrophotometer. Comparative studies of Cu(II) determination were carried out with flame atomic absorption spectrometry Varian Spectra AA640 (Mulgrave, Australian) spectrometer (FAAS) equipped with a deuterium lamp for background correction. This equipment was operated according to the recommendations of the manufacturer for maximum sensitivity using an air–acetylene flame. The measurements were performed at 327 nm, using a copper hollow cathode lamp. 2.3. Functionalization of the multiwalled carbon nanotubes The carbon nanotubes were initially submitted to a chemical pretreatment using a mixture of concentrated sulfuric and nitric acids 3:1 (v/v) for 12 h at room temperature. After this, the suspension was filtered, the solid was washed with ultrapure water until pH 6.5–7.0 and then it was dried at 120 ◦ C for 5 h [39–41].

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2.4. Preparation of crosslinked chitosan The chemical modification of chitosan with GA was carried out by suspending 3 g of chitosan in 50 mL of 2.5% (w/v) GA aqueous solution and keeping it under constant stirring at room temperature for 12 h. After this, the GA-crosslinked chitosan was filtered, rinsed with ultrapure water to remove unreacted GA and dried at room temperature [33,35]. The chemical modification of chitosan with ECH was carried out by suspending 3 g of chitosan in 50 mL of aqueous solution containing 0.01 mol L−1 ECH and 0.067 mol L−1 NaOH at 40 ◦ C and keeping the suspension under magnetic stirring for 2 h [33,41]. Thus, the ECH-crosslinked chitosan was filtered, rinsed with ultrapure water to remove unreact ECH and dried at room temperature. 2.5. Preparation of functionalized carbon nanotubes paste electrodes modified with chitosan and crosslinked chitosan The chemically modified electrodes were prepared mixing chitosan or chitosan crosslinked with GA or ECH presenting particles with size from 20 to 80 ␮m, functionalized multiwalled carbon nanotubes and mineral oil in different compositions. The mixture was then manually homogenized for at least 30 min. After that, modified carbon nanotubes paste was packed into an electrode body, consisting of a plastic cylindrical tube (o.d. 8 mm, i.d. 6 mm) equipped with a copper rod (surface area 0.16 cm2 ) serving as an external electrical contact. Then, the paste was pressed into the cavity of the electrode body with the copper rod and the surface was smoothed against clean paper. 2.6. Analytical procedure The Cu(II) determinations with different electrodes were evaluated by differential pulse anodic stripping voltammetry (DPASV), square wave anodic stripping voltammetry (SWASV) and linear sweep anodic stripping voltammetry. The parameters used in the different voltammetric techniques for copper measurements were evaluated. The standard addition method was applied for Cu(II) determination in real samples of industrial wastewater, natural waters and human urine using the proposed modified electrodes. Then, small volumes of a standard Cu(II) solution were added and the respective voltammograms were recorded [42]. The recovery studies were realized with a solution with wellknown concentration and considered true was added and the percentage recovery was calculated. Various known amounts of Cu(II) were added to the samples and were subsequently analyzed by proposed electrode. 3. Results and discussion 3.1. Study of the CNPE and CNPE modified with chitosan and crosslinked chitosan for Cu(II) measurements Fig. 1 shows the FTIR spectra of CNTs non-functionalized (inset) and functionalized CNTs, which exhibit a intense absorption at 1617 and 1645 cm−1 corresponding to the carboxylate and carboxylic acid group, respectively. The FTIR spectra a significant increase in the peaks intensity was observed for the carbon nanotubes pretreated with mixture of nitric and sulfuric acid as compare with untreated carbon nanotubes. This variation in the intensity of the peaks can be related to the increase in the carboxylic and carboxylate groups indicating the functionalization of the CNTs. These results are in accordance with the previous reports [43,44].

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Fig. 1. FTIR spectrum of the non-functionalized (inset) and functionalized carbon nanotubes.

The influence of the pretreatment of the carbon nanotubes on the Cu(II) electrochemical responses was evaluated. The electrodes were prepared using carbon nanotubes as received and carbon nanotubes pretreated in a proportion of 80% (m/m) of nanotubes and 20% (m/m) of mineral oil. The measurements were carried out in a solution of 9.0 × 10−4 mol L−1 Cu(II), 0.05 mol L−1 KNO3 with pH adjust for 2.25 with HNO3 by LSASV using an accumulation potential of −0.2 V vs. Ag/AgCl (3.0 mol L−1 KCl) applied for 270 s and a scan rate of 25 mV s−1 in a potential range from −0.2 to 0.6 V. The selected parameters used in this initial study were the same employed previously [45]. The voltammograms obtained are presented in Fig. 2. As shown, the electrode prepared with functionalized carbon nanotubes showed a larger and well-defined anodic peak current as compared with the non-functionalized carbon nanotubes. The pretreatment of the carbon nanotubes was used for eliminate impurities as carbonaceous materials and metallic compounds and to introduce carboxyl and carboxylate groups at the ends and/or at the sidewall of the nanotube structure that can increase the conduction of electrons [46,47]. Therefore, the functionalization lead to the improvement in the electroanalytical response for Cu(II) of the electrode modified with the pretreated CNTs [48].

Fig. 2. Voltammograms of CNPE prepared with non-functionalized (A) and functionalized (B) carbon nanotubes in the presence of 9.0 × 10−4 mol L−1 Cu(II). The experimental conditions were 0.05 mol L−1 KNO3 (pH 2.25), accumulation potential of −0.2 V for 270 s and scan rate of 25 mV s−1 .

Fig. 3. Voltammograms of CNPE (A), CNPE-CTS (B), CNPE-CTS-GA (C) and CNPE-CTSECH (D) in the presence of 9.0 × 10−5 mol L−1 Cu(II). The experimental conditions were 0.05 mol L−1 KNO3 (pH 2.25), accumulation potential of −0.2 V for 270 s and scan rate of 25 mV s−1 .

The carbon nanotubes paste electrode modified with chitosan (CNPE-CTS), chitosan crosslinked with GA (CNPE-CTS-GA) or ECH (CNPE-CTS-ECH) containing respectively 60% (m/m) of carbon nanotubes, 20% (m/m) of modifier and 20% (m/m) mineral oil were constructed and the analytical response for Cu(II) by LSASV was evaluated. The measurements were carried out using a solution with 9.0 × 10−5 mol L−1 of Cu(II) and 0.05 mol L−1 of KNO3 (pH 2.25), an accumulation potential of −0.2 V vs. Ag/AgCl (3.0 mol L−1 KCl) was applied for 270 s and a scan rate of 25 mV s−1 in a potential range from −0.2 to 0.6 V. As shown in the Fig. 3, the anodic peak current response increased in the following order: CNPE < CNPECTS < CNPE-CTS-GA < CNPE-CTS-ECH indicating that the chitosan lead to the improvement in the electroanalytical responses for Cu(II) as compared with the CNPE without chitosan. The CNPE prepared with chemically modified CTS present the highest current responses for Cu(II) as compared with unmodified CTS. The role of CTS crosslinked in the current enhanced is not clear at present. However, the crosslinking of CTS can improve the ion transport by a mechanism involving pore and membrane diffusion as recently described [23,32] that can be responsible for the enhanced of Cu(II) response. Comparing the type of crosslinking the use of the ECH reagent led to a highest current response, which can be related to the presence of amine groups along the polymer chains, once the epichlorohydrin covalently binds chitosan chains by reacting with its hydroxyl groups [49] that can be increase the adsorption of Cu(II). Therefore, the CNPE-CTS-ECH electrode was selected for the further studies due to the best performance responses for Cu(II). Fig. 4 shows the X-ray diffraction patterns of CTS, CTS-GA and CTS-ECH powder. It can be seen that the crystallinity of the crosslinked chitosan showed a slight increase when compared with CTS indicated by the increase in the relative intensity of the peaks at 10◦ , 20◦ and 21.5◦ corresponding respectively to the (0 2 0), (1 1 0) and (1 2 0) reflections [50,51]. The obtained results were different from those obtained by Beppu et al. [50] and Koyama and Taniguchi [52] since the authors observed a decrease in the crystallinity of the crosslinking chitosan when compared with the chitosan. However, the crosslinking was carried out with glutaraldehyde in a previously formed chitosan membrane and the crosslinking can induce a chemical modification to produce a less organized structures. Our studies the crosslinking was carried out in homogeneous conditions by suspending the chitosan powder in GA or ECH solution. The improvement of the Cu(II) electrochemical response obtained with CNPE-CTS-GA and CNPE-CTS-ECH as compared with CNPE-

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Fig. 4. X-ray diffraction analysis of CTS (A); CTS-GA (B) and CTS-ECH (C).

CTS can be related to the variation of the ion transport due to the change in the mechanism that can involving pore diffusion and the X-ray analysis measurement indicated the change in the chitosan structure after crosslinking treatments [29,53–55]. The particle size of chitosan and crosslinked chitosan particle size is also an important factor influencing the analytical performance and can interfere in efficiency of CNPE, so that particles size used in this work were controlled (20–80 ␮m). Smaller particles are interesting because crosslinked chitosan uptake can increase with a decrease in the particle size since the effective surface area is higher for the same mass of smaller particles [56,57]. The effect of the CTS-ECH composition of the CNPE on electrochemical Cu(II) measurements was evaluated varying the mass ratio of carbon nanotubes, CTS-ECH and mineral oil. The percentage of mineral oil was fixed at 20% (m/m) and a CTS-ECH/carbon nanotube mass ratio was varied from 0.14 to 0.60. Fig. 5 presents the anodic peak current for Cu(II) stripping measurement with respect to different composition of the CNPE-CTS-ECH. It can be seen that the electrode with a CTS-ECH/carbon nanotubes mass ratio lower than 0.23 showed a poor analytical response for Cu(II) stripping measurement. This effect can attributed to the low concentration of CTS-ECH disposable on the electrode surface for the

Fig. 5. Anodic peak currents obtained by LSASV as function of the amount of CTSECH in the CNPE for 9.0 × 10−5 mol L−1 Cu(II) solution. The experimental conditions were 0.05 mol L−1 KNO3 (pH 2.25), accumulation potential of −0.2 V for 270 s and scan rate of 25 mV s−1 .

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Cu(II) adsorption. The composition of 65% (m/m) of carbon nanotubes, 15% of CTS-ECH and 20% of mineral oil, that provides a CTS-ECH/carbon nanotubes mass ratio of 0.23 showed the highest analytical response for Cu(II) measurement with a well defined peak current. Therefore, this composition was selected for further studies. The influence of the dissolved oxygen on the copper stripping analysis with CNPE-CTS-ECH was appraised and no interference was observed in 9.0 × 10−5 mol L−1 Cu(II) (data not shown). Thus, all the measurements were realized without need of time-consuming de-oxygenation steps. The influence of the supporting electrolyte on Cu(II) measurements was evaluated in HCl, HNO3 , NaNO3 , KNO3 and acetate buffer solutions as supporting electrolytes. The CNPE-CTS-ECH electrode presents stripping responses for Cu(II) with a well defined anodic peak current in a solution of 0.05 mol L−1 KNO3 at a pH of 2.25 adjusted with HNO3 . Moreover, the use of acid solution prevents the hydrolysis of the Cu(II) ions. Hence, the solution of 0.05 mol L−1 KNO3 (pH 2.25) was selected as the supporting electrolyte for subsequent studies. The measurements of Cu(II) were evaluated using the SWASV, DPASV and LSASV. The SWASV and DPASV showed responses with higher background current and poor sensitivity (data not shown). The LSASV showed responses with well defined anodic peak current, low background current and good accuracy as compared with results obtained with SWASV and DPASV. Although SWASV and DPASV are more sensitive techniques due to the great discrimination of the capacitive current, in this specific case the voltammetric profiles showed low repeatability and reproductivity for Cu(II) determination. Then, LSASV was chosen for further experiments. The dependence of the anodic peak currents with the scan rate using LSASV was appraised in the interval from 10 to 120 mV s−1 . It was observed that the peak current increased linearly with the increase in the scan rate up to 100 mV s−1 . The higher scan rates a significant increase on the magnitude of the peak current was not observed. Therefore, a scan rate of 100 mV s−1 was chosen for further studies. Fig. 6 shows the influence of the accumulation potential on the peak current of Cu(II), which was investigated over the potential range from −0.5 to −0.1 V. At more negative accumulation potentials, Cu(II) is reduced more completely, thus increasing the peak current. The peak current increased as the accumulation potential became more negative up to −0.3 V. When the accumulation poten-

Fig. 6. Study of the anodic peak currents obtained by LSASV as function of the accumulation time and potential (inset) in the presence of 9.0 × 10−5 mol L−1 Cu(II). The experimental conditions were 0.05 mol L−1 KNO3 (pH 2.25), accumulation potential of −0.3 V for 300 s and scan rate of 100 mV s−1 .

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Table 1 Interference study for the determination of 1.0 × 10−6 mol L−1 Cu(II) with CNPE-CTSECH. Species

Concentration (mol L−1 )

Interference (%)

Ca2+ , K+ , Cl− , Mg2+ , CO3 2− SO4 2− , PO4 3− Hg2+ Pb2+ Co2+ Ni2+ Cr3+ Cd2+ Urea Fructose, l-histidine, l-cysteine, maltose, d-galactose, l-tyrosine, glycine

1.0 × 10−5

No interference

1.0 × 10−5 1.0 × 10−5 1.0 × 10−5 1.0 × 10−5 1.0 × 10−5 1.0 × 10−5 1.0 × 10−3 1.0 × 10−5

1.0 −1.5 1.2 −2.1 1.7 3.0 −1.7 No interference

tial shifted in the negative direction, the peak current improved very slightly and the background current increased. The best definition of the peak current was obtained at a potential of −0.3 V, achieving a great analytical signal and selectivity of Cu(II) determination. The accumulation time was performed in the range from 0 to 330 s. The peak height initially increased rapidly on increasing the preconcentration period and started to level off at 300 s. At the same accumulation potential, a longer accumulation time will cause Cu(II) to be reduced completely and consequently lead to a higher peak current. However, when the accumulation time is too long, the reduced Cu0 covers the entire effective electrode surface, and the peak current does not change with increasing accumulation time. Thus, an accumulation potential of −0.3 V and accumulation time of 300 s were selected for further studies. 3.2. Interference studies In the interference study of the CNPE-CTS-ECH in the Cu(II) determination, possible interferences were chosen based on the major constituents of the samples evaluated. The selectivity of the CNPE-CTS-ECH was evaluated by LSASV measurements in 1 × 10−6 mol L−1 Cu(II) solution spiked with 10-fold excess of different metal ions and organic substances commonly present in natural waters, industrial wastewaters and human urine into Cu(II) solutions during the accumulation step The results of all species evaluated the electrode response presented less than 5% of interference and the values are shown in Table 1. Therefore, the commonly inorganic and organic species present in samples studies, at the concentration evaluated, do not affect the Cu(II) determination using the CNPE-CTS-ECH.

Fig. 7. Voltammograms obtained with CNPE-CTS-ECH in blank solutions (A) and in different Cu(II) concentrations 7.90 × 10−8 , 1.90 × 10−6 , 4.30 × 10−6 , 7.90 × 10−6 , 1.20 × 10−5 and 1.60 × 10−5 mol L−1 (B-G) and the calibration curve (inset). The experimental conditions were 0.1 mol L−1 KNO3 (pH 2.25), accumulation potential of −0.3 V for 300 s and scan rate of 100 mV s−1 .

tion and the total concentration of Cu(II) was analyzed using the CNPE-CTS-ECH. Table 2 summarizes the results obtained with the recovery studies. The mean of the recovery was calculated as the ratio, expressed as a percentage, of the total copper concentration founded and the copper concentration added to the samples. The Cu(II) recovery studies showed values in range from 98% to 104%. Consequently, the developed CNPE-CTS-ECH presented a good accuracy for Cu(II) determinations in the samples matrix studies. The copper content in different samples of industrial wastewaters as real sample and urine as spiked sample was analyzed by LSASV with the CNPE-CTS-ECH. The standard addition method was used for Cu(II) determinations in the samples using the CNPECTS-ECH and the comparative methods by FAAS. Each sample was analyzed in triplicate and the results obtained are shown in Table 3. As seen the results obtained with the proposed procedure were Table 2 Recovery analysis of Cu(II) in human urine, natural water and industrial wastewater samples. Cu(II) concentration (␮mol L−1 ) Sample

Added

Found

Recovery (%)

Human urine

0.50 5.0 10.0

0.49 ± 0.03 5.1 ± 0.1 9.9 ± 0.1

98.0 102 99.0

Natural water

0.50 5.0 10.0

0.49 ± 0.03 5.1 ± 0.1 9.9 ± 0.1

98.0 102 99.0

0.50 1.0 5.0

0.52 ± 0.09 1.0 ± 0.1 4.9 ± 0.1

104 100 98.0

3.3. Analytical characteristics and application Fig. 7 shows the stripping voltammograms under optimized conditions of the Cu(II) in range from 7.90 × 10−8 to 1.60 × 10−5 mol L−1 obtained with CNPE-CTS-ECH and the respective calibration curve (inset). The peak current increases proportionally with the concentration of Cu(II) with a linear regression equation of Ipa (␮A) = 2.1 (± 0.2)+ 7.0 (±0.2) × 106 Cu(II)] (mol L−1 ), correlation coefficient of 0.9982 and a detection limit of 1.00 × 10−8 mol L−1 . The CNPE-CTS-ECH showed good precision with a relative standard deviation of 3.1% for 10 determinations of 1.00 × 10−6 mol L−1 Cu(II) using one surface for the whole runs. In addition, a relative standard deviation of 5.8% was obtained for the LSASV measurements 1.0 × 10−6 mol L−1 Cu(II) using ten different CNPE-CTS-ECH electrodes prepared in the same way. The recovery studies were carried out with samples of natural water, human urine and industrial wastewaters. The different samples were spiked with known amount of Cu(II) standard solu-

Industrial wastewater

Table 3 Determination of Cu(II) in different samples. Samples

Cu(II) concentration (␮mol L−1 ) Comparative methoda

Proposed method

Relative error (%)

Human urine

0.50 ± 0.03 2.4 ± 0.2

0.52 ± 0.09 2.3 ± 0.1

4.0 −4.1

Industrial wastewater

3.5 ± 0.2 10.7 ± 0.2

3.6 ± 0.1 11.1 ± 0.1

a

Determined by FAAS.

1.0 3.6

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265

Table 4 Comparison of the figure of merits of the CNPE-CTS-ECH developed in the present work and the others electrodes described in the literature. Modifiera

Electrodeb

Methodc

Concentration range (M)

Detection limit (M)

Interferences

Accumulation time

Ref.

2-Aminothiazole Alirazin red S (KRS)-K2 S2 O8 PCHA Calix[4]arene DPNSG CTS-ECH

(MCP) (CP) (MCP) (MCP) (MCP) (MCNP)

(DPASV) (SV) (DPASV) (DPASV) (POT) (LSASV)

7.5 × 10−8 to 2.5 × 10−6 8.0 × 10−10 to 3.0 × 10−8 1.0 × 10−8 to 1.0 × 10−6 5 × 10−8 to 16 × 10−7 1.0 × 10−7 to 1.0 × 10−2 7.9 × 10−8 to 1.6 × 10−5

3.1 × 10−8 1.6 × 10−10 5.00 × 10−10 1.1 × 10−6 8.0 × 10−8 1.0 × 10−8

Ni2+ , Zn2+ , Cd2+ , Pb2+ , Fe2+ Ni2+ , Fe3+ , Cd2+ , Pb2+ , Fe2+ , Cr3+ , Co2+ Hg2+ , Cr3+ , Fe3+ , Ag+ , Au3+ , Al3+ Zn2+ , Ag+ , Cd2+ , Pb2+ Ag+ , Cd2+ , Pb2+ No

20 min 3 min 10 min 10 min – 5 min

[63] [59] [60] [64] [65] This work

a b c

PCHA, N-phenylcinnamohydroxamic acid, DPNSG, nanoporous silica gel with dipyridyl group, CTS-ECH, chitosan crosslinked with epichloridrin. MCP, modified carbon paste; CP, carbon paste; MCNP, modified carbon nanotube paste. DPASV, differential pulse anodic stripping voltammetry; SV, stripping voltammetry; LSASV, linear scan anodic stripping voltammetry.

not statistically different from the comparative method, at a 95% confidence level. Thus, the CNPE-CTS-ECH can be employed for quantification of Cu(II) in both samples. The main advantage of this procedure is the use of a carbon nanotubes paste electrode as working electrode is due the good response, accuracy and to easy and fast construction by simple mixing of carbon nanotubes powder, CTS-ECH and mineral oil. The analytical characteristics, analytical properties and selectivity achieved by the presented electrode are compared in Table 4 with some recently reported electrodes for copper determination. It can be seen from this table that Co2+ [58,59], Hg2+ [60–62], Pb2+ [59,63–65], Cd2+ [59,63–65], Ni2+ [58,63] and Cr3+ [59,60] presented significant interference in copper(II) electrodes. Previously reported copper(II) electrodes have a very small value of selectivity coefficient in most cases, and the linear range and detection limit of the proposed electrode is somewhat similar to those recently published papers. However, there are a few reports in the literature [66,67] which presented a better selectivity than that obtained in this work. 4. Conclusions The paste electrode prepared with functionalized carbon nanotubes and chitosan crosslinked with epichlorohydrin showed a sensitive, precise and accurate response for Cu(II) determinations in industrial wastewaters, natural water and human urine samples. The CNPE-CTS-ECH can be easily prepared and the detection limits of 1.00 × 10−8 mol L−1 were achieved under the optimized conditions without need of time-consuming de-oxygenation steps. Acknowledgments The authors are grateful to Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP; Proc. 07/08365-7 and 06/05146-0), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenac¸ão de Aperfeic¸oamento de Pessoal de Nível Superior (CAPES) for financial support. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [2] L. Kavan, L. Dunsch, H. Kataura, Electrochemical tuning of electronic structure of carbon nanotubes and fullerene peapods, Carbon 42 (2004) 1011–1019. [3] B.S. Sherigara, W. Kutner, F. D’Souza, Electrocatalytic properties and sensor applications of fullerenes and carbon nanotubes, Electroanalysis 15 (2003) 753–772. [4] X. Tan, M. Li, P. Cai, L. Luo, X. Zou, An amperometric cholesterol biosensor based on multiwalled carbon nanotubes and organically modified sol-gel/chitosan hybrid composite film, Anal. Biochem. 337 (2005) 111–120. [5] Z.G. Qiang Zhao, Qiankun Zhuang, Electrochemical sensors based on carbon nanotubes, Electroanalysis 14 (2002) 1609–1613. [6] J. Li, J.E. Koehne, A.M. Cassell, H. Chen, H.T. Ng, Q. Ye, W. Fan, J. Han, M. Meyyappan, Inlaid multi-walled carbon nanotube nanoelectrode arrays for electroanalysis, Electroanalysis 17 (2005) 15–27.

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Biographies Bruno Campos Janegitz received the MS degree from the Federal University of São Carlos, São Carlos, SP, Brazil in 2009, and actually is PhD student in the same University. His research interests include electroanalytical chemistry, nanostructured electrode materials and modified electrode surfaces, electrochemical sensors and biosensors. Luiz Humberto Marcolino-Junior received his PhD degree in analytical chemistry (2007) and post doctorate (2008) from the Federal University of São Carlos, São Carlos, SP, Brazil. He is an active professor of analytical chemistry in the Department of Chemistry at the Federal University of Paraná, Curitiba, PR, Brazil. His research interests include electroanalytical chemistry, nanostructured electrode materials and modified electrode surfaces and electrochemical sensors. Sergio P. Campana-Filho received his PhD degree in physical chemistry from the São Paulo University, São Carlos, SP, Brazil in 1990. In 2004–2005 he developed a post doctorate stage in the Universitá Claude Bernard Lyon, France. He is an active professor of the Chemistry Institute of São Carlos, São Paulo University, São Carlos, SP, Brazil. The main focuses of his scientific research are the chemistry and physical chemistry of polysaccharides from microbial, vegetable and animal sources, mainly bacterial polysaccharides, cellulose, chitin/chitosan and derivatives. His work is focused on the extraction processes, chemical derivatization, physical chemistry characterization and properties/applications of these polysaccharides and derivatives. Ronaldo Sensi Faria is an active professor in the Department of Chemistry at the Federal University of São Carlos, São Carlos, SP, Brazil. He received his master and doctor degrees in analytical chemistry from the same University in 1995 and 2000, respectively. He has experience in the area of analytical chemistry, with emphasis in electroanalytical chemistry, acting mainly in the development of modified electrodes and quartz crystal microbalance technique. Orlando Fatibello-Filho received his PhD degree in analytical chemistry from São Paulo University and post doctorate in 1989 from the University of New Orleans, USA. He spent 1 year (2008–2009) as visiting professor in the Department of Chemistry at University of Coimbra, Coimbra, Portugal. Currently, he is a full professor of Analytical Chemistry in the Department of Chemistry at the Federal University of São Carlos, Brazil. His research interests include new nanostructured electrode materials and modified electrode surfaces, electrochemical sensors and biosensors, flow-injection systems and their applications in the determination of analytes in pharmaceutical formulations, environmental and food samples.