Anal. Chem. 2006, 78, 4194-4199
Multiwalled Carbon Nanotube Chemically Modified Gold Electrode for Inorganic As Speciation and Bi(III) Determination Antonella Profumo,*,† Maurizio Fagnoni,‡ Daniele Merli,† Eliana Quartarone,§ Stefano Protti,‡ Daniele Dondi,‡ and Angelo Albini‡
Dipartimento di Chimica Generale, 27100 Pavia Italy, Dipartimento di Chimica Organica, Universita` degli Studi di Pavia, Via Taramelli 10, Pavia, Italy, and Dipartimento di Chimica Fisica, Universita` degli Studi di Pavia, Via Taramelli 16, Pavia, Italy
A chemically modified gold electrode has been conveniently prepared by binding multiwalled carbon nanotubes (MWCNTs) to which thiol functions have been tethered. The electrode has been characterized by atomic force microscopy and oxidative desorption experiments and gives excellent results for trace determination of As(III) and Bi(III) in natural and high-salinity waters, overcoming the limitation typical of solid electrodes. A mechanism for As(III) preconcentration at the electrode is proposed and supported by results obtained by two similar chemically modified electrodes (CMEs), the first one prepared with single-walled carbon nanotubes and the second one with a monolayer of (biphenyl)dimethanethiol. The performance obtained with the MWCNTs-CME largely overcomes that obtained by using other devices. A sensitive determination of trace metals in water is of obvious importance for toxic elements, e.g., As, where speciation is mandatory in view of the specific toxicity of As(III). Electrochemical methods based on stripping voltammetry on gold electrodes or gold-coated glassy carbon electrodes have been recently developed and operate in strongly acidic media (3 M HCl).1,2 Shortcomings with solid electrodes result from their analytical response (strongly dependent on pretreatment) and from the formation of a gold oxide film on the surface. These effects limit both sensitivity and reproducibility and hinder their use for routine applications.3 In the effort to overcome these limitations, we took into consideration carbon nanotube (CNT)-modified electrodes that have been found to mediate electron-transfer reactions with electroactive species in solutions4 and to alleviate surface fouling effects. Moreover, they exhibit low background current, chemical inertness, and wide potential window. Indeed, a few CNT-based electrochemical sensors and biosensors have been introduced for * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +39-0382-528544. Tel: +39-0382-987581. † Dipartimento di Chimica Generale. ‡ Dipartimento di Chimica Organica. § Dipartimento di Chimica Fisica. (1) Kopanika, M.; Novotny, L. Anal. Chim. Acta 1998, 368, 211-218. (2) Rasul, S. B.; Munir, A. K. M.; Hossain, Z. A.; Khan, A. H.; Alauddin, M.; Hussam, A. Talanta 2002, 58, 33-43. (3) Huang, H.; Dasgupta, P. K. Anal. Chim. Acta 1999, 380, 27-37. (4) Zhao, Q.; Gan, Z.; Qiankun, Z. Electroanalysis 2002, 14, 1609-1613.
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Scheme 1. MWCNTs CME Preparation Scheme
the determination of important biomolecules5,6 and metal ions such as lead and cadmium.7 These are based on single-walled CNTs (SWCNTs), which are attached to the electrode surface either by binding after functionalization or by incorporation in a polymeric film. We surmised that a breakthrough could be obtained by using multiple-wall CNTs (MWCNTs), which are less stable and have more structural defects than SWCNTs. These are less costly and available at higher purity and, due to the large number of structural defects, are susceptible to a more extensive derivatization. As shown below, it was possible to develop a CNT chemically modified electrode (CNTs-CME) bearing SH groups and to apply it to the determination at a microgram per liter level of inorganic As(III). In the following, we report the preparation of this MWCNT chemically modified electrode (Scheme 1), its (5) Wang, J. Electroanalysis 2005, 17, 7-14. (6) Davis, J. J.; Coleman, K. S.; Azamian, B. R.; Bagshaw, C. B.; Green, M. L. H. Chem. Eur. J. 2003, 9, 3732-3739. (7) Lu, G.; Lin, Y.; Tu Y.; Ren, Z. Analyst 2005, 130, 1098-1101. 10.1021/ac060455s CCC: $33.50
© 2006 American Chemical Society Published on Web 04/28/2006
Scheme 2. SWCNTs CME Preparation Scheme
Chart 1. Biphenyldimethanethiol Self-Assembled Monolayer (BPDMT SAM)
characterization, and analytical application for the determination of As(III), total inorganic As (after As(V) reduction), and Bi(III)s a likely contaminant, e.g., in washings from arsenic minesseven in high-salinity waters. To assess the role of nanotube structure in the electron transfer and the function of external SH groups in the As and Bi deposition on the electrode surface, its response was compared with those obtained with two similar CMEs, viz. SWCNTs CME and single-layer biphenyldimethanethiol (BPDMT, see Scheme 2 and Chart 1). Thus, purified MWCNTs were oxidized introducing carboxyl functionalities and the corresponding chloride esterified with mercaptoethanol to form the thioester (Scheme 1), which was found to adsorb on a gold electrode to form the desired CNT-CME (see in the text). The CME was examined by atomic force microscopy (AFM) and characterized by electrochemical techniques. EXPERIMENTAL SECTION Instrumentation. Measurements were carried out with an Amel 433/W polarographic analyzer equipped with a standard three-electrode cell with chemically modified gold electrodes as
working electrode (2.0-mm diameter), a platinum wire as auxiliary electrode, and a Ag/AgCl/KCl (4 M KCl saturated with AgCl) reference electrode. AFM images were obtained with an Autoprobe CP microscope (Park Instruments-VEECO), operating in noncontact mode, by means of sharpened silicon tip onto highresolution rectangular cantilevers (resonant frequency, 150 kHz; force constant, 5.5 N/m). For each analyzed sample, scans of 5 µm × 5 µm and 2.5 µm × 2.5 µm were carried out with a scan rate ranging from 0.5 to 1.5 Hz. A standard second-order flattening processing of the images was performed in order to correct the scanner nonlinearity. Inductively coupled plasma-mass spectrometer (ICPMS) measurements were carried out on a Perkin-Elmer model Elan DRC-e instrument, following the standard procedures suggested by the manufacturer. Microwave (MW) digestion was performed in Teflon bombes on a CEM microwave digestor operating at 330 MHz. Materials and Reagents. Reagents of the purest grade available were purchased from Fluka and Carlo Erba and used as received; acetic buffer was purified before use on Chelex 100 (batch procedure: 30 mL of acetic buffer was stirred overnight with 500 mg of Chelex 100, purified as previously reported8). As(III) standard solution (1000 ppm) was prepared by weighing the appropriate amount of arsenic trioxide (Fluka, ACS grade) and dissolving in the minimum amount of 8 M NaOH, and then pH was adjusted to ∼3 with HCl. This solution was stable for at least 1 month and was used daily to prepare standard solutions of 10 and 1 ppm As. As(V) standard solution was prepared by dissolving sodium arsenate heptahydrate (Fluka) in the appropriate volume of water. This standard is stable for at least 1 month. Both Sigma-Aldrich commercially available SWCNTs (50% purity, diameter × length 1.2-1.5 nm × 2-20 µm, bundles) and MWCNTs (95% purity, o.d. × wall thickness × length 20-30 nm × 1-2 nm × 0.5-2 µm) were used. All glassware was carefully cleaned first with concentrated nitric acid and then with Milli-Q water in order to avoid contamination. For determination of total As content, As(V) was preliminarily reduced as follows. To 10 mL of the natural water sample (pH 4-8) 2 mL of saturated H2SO3 solution was added. The resulting solution was MW digested for 20 min at 320-MHz power; N2 was bubbled in the solution for 300 s to remove excess SO2. Acetic buffer was added, and the sample was analyzed by the described procedure. Electrode Preparation. The electrode gold disk cross section exposed (diameter 2.0 mm) was abraded with successively finer grades alumina (from 1 to 0.05 µm), rinsed with water, and briefly cleaned in an ultrasonic bath to remove any trace alumina from the surface. (a) Multiwalled Carbon Nanotube Chemically Modified Electrode. Carboxylic derivative of CNTs was obtained from commercial available MWCNTs by refluxing in 4 M HNO3, as described in the literature.9 The thus obtained oxidized MWCNTs (20 mg) were refluxed in SOCl2 (10 mL) for 12 h. The resulting mixture was decanted, and excess SOCl2 was removed in vacuo. A solution of mercaptoethanol (2 mL, 30 mmol) and of triethylamine (1 mL, 7 mmol) in CH2Cl2 (10 mL) was added, and the mixture was refluxed for 24 h. The suspension was centrifuged (8) Cheng, C. J.; Akagi, T.; Haragugi, H. Anal. Chim. Acta 1987, 198, 173181. (9) Taab, H. A.; Haenel, M. Chem. Ber. 1973, 106, 2190-2192.
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and the solid repeatedly washed with methanol to give derivatized MWCNTs (10 mg; Scheme 1). The selective acylation of OH groups rather than of the SH group has been largely documented in the literature.10,11 The MWCNTs-CME (1) was prepared by dipping the cleaned gold electrode in a sonicated suspension of 3 mg of derivatized nanotubes in 1 mL of DMSO for 48 h (in Scheme 1, the overall procedure is reported). (b) Single-Walled Carbon Nanotube Chemically Modified Electrode. Previous studies12,13 have shown that the chemical treatment of SWCNTs with strong acids leads to the formation of carboxylic (and phenolic) groups at the nanotube ends (as well as sidewall defect sites), beside reducing the length of CNTs. This allows the covalent immobilization of the SWCNTs on the appropriate surface. In this case, a cystamine monolayer was assembled on the gold electrode and the SWCNTs (dispersed by sonicating 3 mg of the material in 1 mL of DMF) were linked to the SAM surface in the presence of the coupling reagent, 1,3dicyclohexylcarbodiimide14 (DCC, 3 mg) as shown in Scheme 2 to obtain a product of structure 2b. To prepare β-thioester derivatized SWCNTs (see 2, Scheme 2), mercaptoethanol was coupled to the carboxyl groups at the free edges of the 2b by using DCC14 (2 mM mercaptoethanol solution in 1 mL of DMF + 3 mg of DCC). (c) Self-Assembled Monolayer (SAM) Electrodes. The preparation of biphenyldimethanethiol monolayer (BPDMT-SAM) on a gold electrode (3, Chart 1) was achieved by dipping the gold electrode, cleaned according to the above procedure, in a 5 mM ethanolic solution of biphenyldimethanethiol15 for 12 h: the electrode was then rinsed with ethanol and water before use. A mercaptoethanol monolayer-covered gold electrode was also prepared for comparison purposes. Electrode Characterization. The new CME was characterized under various aspects and its use for the determination of metals was tested. The effective gold electrode area was determined electrochemically16 by using 0.1 M ferrocene in acetonitrile with an Ag/AgCl acetonitrile nonaqueous reference electrode (BAS) in 0.1 M tetrabutylammonium perchlorate. The anodic and cathodic peak currents of the ferrocene redox couple were obtained by cyclic voltammetry (CV) as a function of the square root of the scan rate (10-500 mV/s). The electrode area was calculated by assuming the diffusion coefficient (D) 2 × 10-5 cm2/ s, according to the modified form of the Randles-Sevcik equation
Ip ) (2.69 × 105)n3/2AD1/2C∞ν1/2
where C∞ is the concentration of ferrocene in the bulk solution (10) Alvaro, M.; Atienzar, P.; De La Cruz, P.; Delgado, J. L.; Garcia, E.; Langa, F. Chem. Phys. Lett. 2004, 386, 342-345. (11) Reynolds, D. D.; Fields, D. L.; Johnson, D. L. J. Org. Chem. 1961, 26, 51195122. (12) Chang, K. H.; Tao, Y. S.; Li, W. S. Synlett 2004, 37-40. (13) Lyn, Y.; Rao, A. M.; Sadanadan, B.; Kenik, E. A.; Sun, Y.-P. J. Phys. Chem. B 2002, 106, 1294-1298. (14) Valentini, F.; Amine, A.; Orlanducci, S.; Terranova, M. L.; Palleschi, G. Anal. Chem. 2003, 75, 5413-5421. (15) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113-2117. (16) Profumo, A.; Merli, D.; Pesavento, M. Anal. Chim. Acta 2005, 539, 245250.
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Figure 1. BPDMT Langmuir rate law.
(mol/cm3), n is the electron stoichiometry (n ) 1), Ip is the peak current (ampere), A is the electrode area (cm2), D is the diffusion coefficient (2 × 10-5 cm2/s at T ) 298 K), and ν is the scan rate (V/s) In this way, the area was estimated as 0.0309 (7) cm2, i.e., within 10% of the geometrically gold electrode area. This indicated that the surface was well polished and with low roughness. The interfacial capacitance of the SAM electrode was determined by CV17 (from the current values measured at 150 mV versus Ag/AgCl in 0.1 M acetate buffer pH 4.0). The rate of surface coverage is expressed by the following relationship: ϑ (t) ) [1 - exp(-Kt)], where ϑ (t) is the surface coverage at any instant of time t and K is the rate constant of the adsorption. ϑ can be obtained by using the expression, ϑ ) (C0 - Ct)/(C0 - Cf), C0 being the bare electrode capacitance, Ct the capacitance at any time t, and Cf the capacitance of the fully covered monolayer.18 (see Figure 1 for adsorption trend). The surface coverage Γ (mol/cm2) for BPDMT-SAM was evaluated by using reductive desorption of thiol from the electrode surface. The film desorbed in alkaline (pH >11) solution through threeelectron oxidative and one-electron reductive paths,19 respectively in eqs 1 and 2.
AuSCH2(C6H4)2CH2SH + 2H2O f Au(0) + -
O2SCH2(C6H4)2CH2SH + 4H+ + 3e- (1)
AuSCH2(C6H4)2CH2SH + e- f Au(0) + -
SCH2(C6H4)2CH2SH (2)
The value of Γ was calculated as Γ ) Q/nFA, where Q is the charge required to electrolyze the thiol SAM, n is the number of electrons involved in the electron-transfer process (n ) 1), F is the Faraday constant, and A is the electrode area. The values of Q were determined by integration of the area under the cathodic peak in the current/potential curves obtained in 0.5 M KOH (CV, Ei -200 mV, Ef -1600 mV, scan speed 100 mV/s) after compensating for the charging current. The large cathodic wave with a peak current near -1.1 V in the first scan of the thiol layer arose from the one-electron reduction according to reaction 2. During the second voltammetric scan, a peak at -1,1 V was observed (17) D’Annibale, A.; Regoli, R.; Sangiorgio, P.; Ferri, T. Electroanalysis 1999, 11, 505-510. (18) Bauer, H. H. Electrodics; Wiley: New York, 1972; p 81. (19) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter M. D. Langmuir 1991, 7, 2687-2693.
Figure 2. (a) AFM images of a representative portion of MWCNTs CME surface. (b) AFM image of a representative portion of SWCNTs CME surface. Table 1. Electrochemical Conditions for As(III) and Bi(III) Determination
Figure 3. CV desorption test of MWCNTs from electrode surface. Conditions: 0.05 M nitric acid, CV, scan speed 100 mV/s, Ei -400 mV, and Ef 1800 mV.
and was attributed to the partial readsorption onto the electrode of the thiolate formed according to eq 2 when the potential was cycled to more positive values. RESULTS CME Characterization. CNT-modified electrodes were characterized by AFM (Figure 2). Figure 2a shows the AFM analysis of MWCNT-modified polycrystalline gold surface. It could be appreciated that MWCNTs with an average length of 100 nm and a diameter of ∼20 nm were horizontally placed on the metal surface. Analysis of representative portions of the surface was consistent with ∼108 CNTs/mm2. The AFM analysis performed on a gold-sputtered quartz slide gave very similar results (not shown). To document the nature of the interactions between modified MWCNTs and electrode surface, a desorption test by CV was performed. The results in Figure 3 show that an oxidative cleavage (R-S-Au f R-SO3- + Au) had occurred.20 Thus, an inflection at 1300 mV, due to gold oxide formation, replaced the peak at 1200 mV when a second scan immediately followed the first one. It may be noticed that the gold oxide reduction peak was the same in both scans. These results supported that a covalent bond was established between functionalized MWCNTs and the gold surface (see Scheme 1). CV experiments showed that this MWCNTs-CME had a capacitance of 42(3) (n ) 3) µF/cm2, as compared to a capacitance of 84(4) µF/cm2 (n ) 3) for the bare gold electrode and a nearly ideal electrochemical response to the Fe(CN)63-/Fe(CN)64- redox probe. (20) Arrigan, D. W. M.; Le Bihan, M. Analyst 1999, 124, 1645-1649.
parameter
As(III)
Bi(III)
Edep Ef tdep scan speed
-800 mV 500 mV 120 s 100 mV/s
-400 mV 500 mV 120 s 100 mV/s
Figure 2b shows the AFM images of the SWCNT-modified polycrystalline gold surfaces determined by coupling of the CNTs to the surface previously modified with cysteamine. As illustrated in Figure 2b, needlelike protrusions were clearly detected on the surface. This supported that SWCNTs were placed perpendicularly to the gold surface, as indeed was expected because the carboxylic groups in these CNTs were located at both tips (see Scheme 2). Notice that as many as eight carboxyl groups might be present at each end of a ∼1.3-nm-diameter SWCNT.20 Therefore, eight amide bonds could be generated between each nanotube and the modified gold surface, explaining the preferred standing placement of the SWCNTs on the electrode surface, as opposed to the lying positioned MWCNTs. Analysis of representative portions of the electrode surface was consistent with the presence of ∼108 CNTs/ mm2 (medium length 10-50 nm). BPDMT-SAM Electrode. [1,1′-Biphenyl]-4,4′-dimethanethiol assembled in a monolayer followed the Langmuir rate law.21,22 BPDMT was bonded to the gold surface through one of the thiol groups, leaving the latter one at the SAM-environment interface (see Chart 1). The BPDMT-SAM capacitance was 17(3) µF/cm2. The BPDMT-SAM was completely blind to the Fe(CN)63-/Fe(CN)64- redox couple. The surface coverage Γ was 3.2(5) × 10-10 mol/cm2. As(III) and Bi(III) Determination. The CNTs-CME demonstrated to be effective for trace determination of As(III) and Bi(III) in acetic buffer (pH 4.0; 0.1 M). The standard addition method was used for the determination of both analytes; electrochemical parameters and working conditions are reported in Table 1. Under these conditions, the bare gold electrode gave only weak and poorly reproducible responses to either As or Bi. (a) As Determination. The data for As are shown in Table 2, where it appears that with the MWCNTs-CME a largely improved sensitivity and an excellent reproducibility (6 data points for each (21) Sur, U. K.; Subramanian, R.; Lakshminarayanan, V. J. Colloid Interface Sci. 2003, 266, 175-182. (22) Hu, K.; Bard, A. J. Langmuir 1998, 14, 4790-4794.
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Table 2. Analytical Results for As(III) Determination
As(III)a
R2
Ep vs Ag/AgCl (mV)
linearity (180 s dep) (µg/L)
I(µA) ) 0.04(2) (µg/L) +2.3(4)
0.92
b
nonlinear
I(µA) ) 0.236(8) (µg/L) - 0.05(5) I(µA) ) 0.074(4) (µg/L) + 0.08(5) I(µA) ) 0.12(2) (µg/L) + 0.03(5) I(µA) ) 0.056(3) (µg/L) - 0.07(6)
0.998 0.993 0.994 0.995
75 93.3 96 47
0.3-50 8-50 2-50 5-150
electrode bare gold or mercaptoethanol SAM MWCNTs-CME SWCNTs CME (2b) SWCNTs CME (2) BPDMTs SAM
calibration curve for
a Differential pulse stripping, t dep ) 120 s, Edep ) -800 mV, Ei ) -600 mV, and Ef ) 700 mV, scan speed 100 mV/s; acetic buffer 0.1 M, and 6 data points in the range 10-50 µg/L. b Ep varies from 40 to 80 mV.
Table 3. Arsenic Determination in Natural Water Samples: Results Obtained by ICPMS and MWCNTs CME Electrodes MWCNT CME
ICPMS
sample
As(III) (µg/L)
total inorg As (µg/L)
total inorg As (µg/L)
low-salinity natural water low-salinity natural water + 5 µg/L As(III) tap water + As(III) 1 µg/L tap water + As(V) 5 µg/L (MW digested + SO2) tap water + MMAa10 µg/L (MW digested + SO2) seawater seawater + DMAc 10 µg/L + As(III) 10 µg/L (MW digested + SO2)
