Anal. Chem. 1997, 69, 894-897
Selective Determination of Cr(VI) by a Self-Assembled Monolayer-Based Electrode Iva Turyan and Daniel Mandler*
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Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
We have developed a selective electrode for chromium(VI), based on a self-assembled monolayer of 4-(mercapton-alkyl)pyridinium on gold surfaces, which exhibits unique speciation capabilities. Cr(VI) levels as low as 1 parts per trillion can be detected using a 4-(mercaptoethyl)pyridinium monolayer. The different parameters that govern the analytical performance of these electrodes have been studied in detail and optimized. In addition, the organization of the monolayers has been examined by a variety of surface techniques such as XPS, FT-IR, and electrochemistry. Our results show that structuring and understanding the solid-liquid interface at the molecular level are essential for designing probes with superior analytical characteristics. Speciation is one of the major challenges in analytical chemistry.1 Of particular interest is metal speciation. For example, while Cr(III) is essential to our bodies and part of our daily diet, Cr(VI) is highly toxic to humans, causing different disorders, and is classified as a suspected carcinogenic agent.2 Thus, the determination of trace levels of Cr(VI), which are often below 1 ppb in natural waters and biological fluids in the presence of relatively high concentrations of Cr(III), is of particular importance.3 Although the redox speciation of chromium has been accomplished by separate preconcentration of Cr(III) and Cr(VI) fractions using chelating resins, coprecipitation, ion chromatography, and solvent extraction, such procedures are obviously complicated.4 Electroanalytical methods that are potentially sensitive for redox speciation have also been used to determine Cr(VI).5,6 In spite of the fact that Cr(VI) has been preconcentrated and (1) Ure, A. M., Davidson, C. M., Eds. Chemical Speciation in the Environment; Blackie Academic & Professional: Glasgow, 1995. (2) Aitio, A.; Tomatis, L. In On the Carcinogenicity of Nickel and Chromium and Their Compounds, Trace ELements in Health and Disease; Royal Society of Chemistry: Cambridge, U.K., 1991; p 168. (3) Rao, V. M.; Sastri, M. N. J. Sci. Ind. Res. 1982, 41, 607-615. (4) (a) Nahhmush, A. M.; Pyrzynska, K.; Trojanowicz, M. Anal. Chim. Acta 1994, 288, 247-257. (b) de Jong, G. J.; Brinkman, U. A. Th. Anal. Chim. Acta 1978, 98, 243-250. (c) Isshiki, K.; Sohrin, Y.; Karatani, H.; Nakayama, E. Anal. Chim. Acta 1989, 224, 55-64. (5) (a) Elleouet, C.; Quentel, F.; Madec, C. Anal. Chim. Acta 1992, 257, 301308. (b) Dobney, A. M.; Greenway, G. M. Analyst 1994, 119, 293-297. (c) Boussemart, M.; van den Berg, C. M. G.; Ghaddaf, M. Anal. Chim. Acta 1992, 262, 103-115. (d) Boussemart, M.; van den Berg, C. M. G. Analyst 1994, 119, 1349-1353. (e) Scholz, F.; Lange, B.; Draheim, M.; Pelzer, J. Fresenius J. Anal. Chem. 1990, 338, 627-629. (f) Wang, J.; Setiadji, R.; Chen, L.; Lu, J.; Morton, S. G. Electroanalysis 1992, 4, 161-165. (6) (a) Cox, J. A.; Kulesza, P. J. Anal. Chim. Acta 1983, 154, 71-78. (b) Teasdale, P. R.; Spencer, M. J.; Wallace, G. G. Electroanalysis 1989, 1, 541547. (c) Malakhova, N. A.; Chernysheva, A. V.; Brainina, Kh. Z. Zh. Anal. Khim. 1987, 42, 1636-1639. (d) Paniagua, A. R.; Vazquez, M. D.; Tascon, M. L.; Batanero, P. S. Electroanalysis 1993, 5, 155-163.
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determined on mercury5 and chemically modified solid electrodes,6 most of these interfaces exhibit moderate stability and selectivity when employed in natural samples. An especially attractive approach, which is still in its infancy, as a means of assembling selective electrodes involves self-assembled monolayers (SAMs).7 Many of the limitations that are frequently encountered while using thin polymeric films, such as slow diffusion across the film, are completely eliminated in SAMs.7-13 Moreover, SAMs offer highly organized systems in which the solid-liquid interface can be predesigned at the molecular level in order to modify the surface with desired properties. Thus, for example, Rubinstein and co-workers8 demonstrated that a mixed functionalized SAM recognized selectively Cu2+ ions in the presence of other ions. Recently, we presented a highly sensitive electrode for cadmium ions using ω-mercaptocarboxylic acid monolayers.10 At the same time, SAMs have been used for assembling electrochemical biosensors. For instance, Willner,11 Creager,12 and Wilson13 reported on the design of sensors for glucose based on incorporating glucose oxidase into SAMs, whereas Meyerhoff14 developed an immunoassay system in which a monoclonal antibody was immobilized via a SAM. We summarize here our recent findings whereby structuring the solid-liquid interface using a self-assembled monolayer, a highly sensitive and, in particular, selective electrode exhibiting speciation capabilities toward Cr(VI), was developed. EXPERIMENTAL SECTION Electrochemical measurements were carried out on a BAS100B electrochemical analyzer. FT-IR spectra were collected with a nitrogen-purged IFS-66 spectrometer (Bruker) at a resolution of 2 cm-1 equipped with an MCT detector and using a p-polarized beam incident at 80°. A bare gold surface, which was previously exposed to ozone and UV (UVOCS, Montgomeryville, PA), was used as a reference. XPS measurements were performed using (7) Mandler, D.; Turyan, I. Electroanalysis 1996, 8, 207-213. (8) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. (b) Steinberg, S.; Tor, Y.; Sabatani, E.; Rubinstein, I. J. Am. Chem. Soc. 1991, 113, 5176-5182. (c) Steinberg, S.; Rubinstein, I. Langmuir 1992, 8, 1183-1187. (9) (a) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869-8871. (b) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688-691. (c) Takehara, K.; Takemura, H.; Ide, Y.; Okayama, S. J. Electroanal. Chem. 1991, 308, 345-350. (d) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1992, 64, 1998-2000. (e) Wang, J.; Wu, H.; Angnes, L. Anal. Chem. 1993, 65, 1893-1896. (f) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37-41. (g) Rojas, M. T.; Koniger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336-343. (10) Turyan, I.; Mandler, D. Anal. Chem. 1994, 66, 58-63. (11) Willner, I.; Riklin, A. Anal. Chem. 1994, 66, 1535-1539. (12) Creager, S. E.; Olsen, K. G. Anal. Chim. Acta 1995, 307, 277-289. (13) Jung, S.-K.; Wilson, G. S. Anal. Chem. 1996, 68, 591-596. (14) Duan, C.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 1369-1377. S0003-2700(96)00752-4 CCC: $14.00
© 1997 American Chemical Society
Figure 1. (A) Square wave voltammetry of a 4-(2-mercaptoethyl)pyridinium-modified gold electrode (scan rate 90 mV/s) after a preconcentration step in solutions containing different concentrations of Cr(VI): (1) 0, (2) 4.20 × 10-11, (3) 8.27 × 10-11, (4) 3.53 × 10-10, and (5) 5.44 × 10-10 M. (B) Calibration curve for chromium(VI) (obtained from peak currents).
an AXIS-HS spectrometer (Kratos) with an Al KR monochromatized source of 1486.6 eV. The pressure in the analysis chamber was ∼10-9 Torr. All chemicals (highest purity available) were purchased from Aldrich or Merck and used as received. 4-(2-Mercaptoethyl)pyridine was synthesized according to a described procedure.15 Diluted solutions of Cr(VI) in fluoride buffer (0.15 M, pH 4.5) were prepared immediately before use from a standard 1010 µg/ mL atomic absorption chromium(VI) solution (Aldrich). All glassware were carefully cleaned with concentrated perchloric acid and triply distilled water to avoid contaminations. Three different types of gold electrodes have been used. Polycrystalline gold electrodes were prepared by sealing a short rod (3 mm diameter) of gold (99.9995%, Aldrich) in a Teflon tube. The electrodes were polished with alumina paste (1 and 0.05 µm) and sonicated in clean water followed by electrocycling in 0.1 M H2SO4 (+1.4 to -0.7 V vs Hg/Hg2SO4). Disposable screen-printed gold electrodes (0.64 cm2) were obtained from GBF (Braunschweig, Germany). Finally, gold surfaces were also prepared by vapor depositing (at 3 × 10-6 Torr) gold on glass slides (∼1000 Å) which were previously coated with a thin layer of chromium (∼2 nm). These gold surfaces were flame annealed with a hydrogen flame before modification. The monolayer of 4-(2-mercaptoethyl)pyridinium was assembled upon immersing a polished, vapor-deposited, or disposable gold electrode in 5 mM solution of thiol and 0.1 M H2SO4 for 10 min. Preconcentration of Cr(VI) was carried out by dipping a modified electrode in a stirred 0.15 M fluoride buffered solution (pH 4.5) for 5 min (unless the effect of time of preconcentration was studied) under open-circuit potential. Samples for XPS and FT-IR measurements were prepared similarly followed by rinsing with clean water and drying under a nitrogen stream. Electrochemical blank tests were always carried out by immersing the modified surface in the preconcentration solution before adding (15) Bauer, L.; Gardella, L. A., Jr. J. Org. Chem. 1961, 26, 82-85.
Cr(VI) ions. Determination of Cr(VI) was performed in a chromium-free solution (0.15 M NaF, pH 7.8) employing cathodic stripping square wave voltammetry from +0.5 to -0.1 V vs Ag/ AgCl reference electrode. The regeneration procedure involved the elimination of Cr(VI) by electrocycling the electrode between +0.5 and -0.1 V in 0.1 M HClO4. RESULTS AND DISCUSSION Our electrode for Cr(VI) is based on a positively charged 4-(mercapto-n-alkyl)pyridinium (I) monolayer assembled on a gold surface. Pyridinium derivatives form strong and stable com-
HN
(CH2)nSH I
plexes16 with CrO42- and Cr2O72-, suggesting that a pyridiniumbased SAM would effectively extract Cr(VI), while repelling cations, e.g., Cr(III). Figure 1 shows the square wave voltammograms of a gold electrode covered with a 4-(2-mercaptoethyl)pyridinium, In)2, monolayer after preconcentrating chromium(VI) from solutions consisting of different Cr(VI) concentrations. A cathodic wave is detected at Epk ) 0.18 V vs Ag/AgCl, which is attributed to the electrochemical reduction of Cr(VI) to Cr(III). This process, which was previously reported,6a,17 can be clearly detected (at a similar potential) with the modified electrode using cyclic voltammetry in solutions consisting of millimolar concentrations of Cr(VI). (16) (a) Gili, P. Rev. Chim. Miner. 1984, 24, 171-176. (b) Corey, E. J.; Schmidt, G. Tetrahedron Lett. 1979, 5, 399-402. (c) Katime, A. I. A.; Gili, P.; Roman, P. Afinidad 1982, 39, 282-285. (d) Martin-Zarza, P.; Gili, P.; RodriguezRomero, F. V.; Ruiz-Pe´rez, C.; Solans, X. Polyhedron 1995, 14, 2907-2917. (e) Camelot, M. Rev. Chim. Miner. 1969, 6, 853-884. (17) (a) Cox, J. A.; West, J. L.; Kulesza, P. J. Analyst 1984, 19, 927-930. (b) Smart, N. G.; Hitchman, M. L.; Ansell, R. O.; Fortune, J. D. Anal. Chim. Acta 1994, 292, 77-80.
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Figure 2. Cathodic square wave peak currents of a gold electrode, modified with a 4-(2-mercaptoethyl)pyridine monolayer after a preconcentration step in solution consisting of 3.18 × 10-10 M Cr(VI) and various ions, as a function of the -log of the molar concentration of the interfering ion: (+) Fe3+; (]) Ag+; (×) Cu2+; (b) MoO42-; (O) VO4-; ([) MnO4-; (2) Cr3+; (4) Cl-, NO3-, CH3COO-, PO43-, SCN-, ClO4-.
The calibration curve was obtained (by systematically increasing the concentration of Cr(VI) between 4.2 × 10-11 and 5.4 × 10-10 M) after optimizing all the parameters that affect the performance of the electrode, i.e., time of modification and preconcentration, pH of the extraction solution, and nature of electrolyte in which the electrochemical analysis is performed. It should be noted that HCrO4- is the dominant species18 in the pH range where optimum extraction of Cr(VI) was observed. The remarkable detection limit of this electrode, calculated from the standard deviation of the background (signal equals 3σ of the background noise), is 2.3 × 10-11 M (1.2 parts per trillion) chromium(VI), with a relative standard deviation of 10% (n ) 5 for 9.6 × 10-11 M Cr(VI)). For comparison, Cox and Kuleza6a reported a detection limit of ∼5 × 10-8 M for Cr(VI) using a poly(vinylpyridine)-modified electrode. Interestingly, the high sensitivity is followed by high selectivity as well. Figure 2 shows the effect of added cations and anions (into the extracting solution) on the cathodic peak current of Cr(VI). It can be seen that the interface is highly selective toward Cr(VI) under the experimental conditions and the determination of 3.2 × 10-10 M Cr(VI) is not influenced by a 104-109-fold excess of the added cations or anions. The minute interference of negatively charged ions indicates that the preconcentration of Cr(VI) is not a simple ion-exchange process. Only MoO42- shows a considerable interference, which is presumably due to its similar structure and size. Furthermore, the analysis of a sample consisting of 0.1 ppb of Cr(VI) was not affected at all (Figure 2) by the presence of a 103-fold excess of Cr(III), revealing the unique high speciation of the electrode toward Cr(VI). In order to verify our results, the simultaneous analyses of two samples were carried out by graphite furnace atomic absorption and by our electrode. The first sample consisted of 10.1 ppb of only Cr(VI) while the second was a mixture of 5.05 ppb of Cr(VI) and 15 ppb of Cr(III). The analysis by atomic absorption yielded a total Cr concentration of 10.1 and 21.0 ppb for the first and second samples, respectively. On the other hand, the concentrations of Cr(VI) in these samples (18) Pourbaix, M., Ed. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon Press: New York, 1966; p 261.
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detected by our electrode were 10.03 and 5.1 ppb, respectively. These results clearly show the specific and reliable response of the electrode toward Cr(VI). Blank experiments evidently showed that the SAM was absolutely essential for accomplishing this high sensitivity and selectivity. The modified electrode was easily regenerated after analysis without affecting the pyridine monolayer. The regeneration procedure, which involved the electrocycling of the electrode at acidic pH, resulted in Cr(VI) reduction and the complete depletion of Cr(III) from the positively charged film.19 The repetitive analyses of a solution consisting of 10 parts per trillion of Cr(VI) by the same modified electrode demonstrates that the electrode is very stable under the experimental conditions and can be used for numerous experiments. At the same time, highly reproducible analyses were obtained (3.8% standard deviation) using screenprinted disposable gold electrodes, which were modified by following the same procedure. All these results clearly show the potential of this electrode for the analysis of low levels of Cr(VI).20 One of the major advantages in applying SAMs in electroanalytical chemistry stems from their relatively high organization, which makes it possible to correlate the analytical performance and the structure of the interface. Moreover, the interface can be structured and modified at the molecular level by changing the molecule that forms the monolayer. We found that the nature of the organic layer has a major effect on the analytical performance of the electrode. While a monolayer of 4-mercaptopyridine exhibits somewhat less sensitivity toward Cr(VI) as compared with In)2, a 2-mercaptopyridine monolayer is substantially less sensitive than its isomer. We applied a number of surface techniques, such as electrochemistry, FT-IR, XPS, and wettability measurement, as a means of studying the interface and understanding its unique properties in terms of Cr(VI) extraction. Determination of the excess of surface coverage, Γ, by cyclic voltammetry of monolayers composed of 2 and 4-mercaptopyridine, as well as of In)2, shows that 4-substituted pyridines form more densely packed arrays. The monolayers show a distinct cathodic wave at ∼-0.4 V in 0.1 M HClO4 which is due to the cathodic desorption of the layer.21 The charge under this wave was used for estimating the surface coverage. Qualitatively, the excess of surface coverages of In)2 and 4-mercaptopyridine were comparable and higher than that of 2-mercaptopyridine. Nevertheless, it should be noted that the analytical signal recorded with a gold electrode modified with 2-mercaptopyridine was much smaller than the value that would have been obtained if the signal had been dependent only on the excess of surface coverage. These findings are also supported by XPS measurements. The preconcentration of chromium(VI) was studied in the three different systems. We found that while the atomic ratio for Cr/N in an In)2 monolayer equals 0.75, 4-mercaptopyridine and 2-mercaptopyridine monolayers yield ratios of 0.65 and 0.19, respectively. Moreover, XPS confirms that Cr(VI) is indeed preconcentrated as indicated by its two (2p) peaks (at 576 and 586 eV). (19) It is also conceivable that at pH 1.0, where H2CrO4 predominates, the complex between Cr(VI) and positively charged monolayer decomposes, facilitating the regeneration of the electrode. (20) Patent pending. Mandler, D.; I. Turyan, Israel 114831, 1996. (21) (a) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875-5876. (b) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (c) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860-5862.
Figure 3. (a) Reflection-absorption FT-IR spectrum of 4-(2mercaptoethyl)pyridine adsorbed on gold. (b) Transmission spectrum of 4-(2-mercaptoethyl)pyridine in a KBr pellet.
Nevertheless, we cannot determine whether the complex on the surface is formed between one HCrO4- and one pyridinium ring or one Cr2O72- and two pyridinium moieties. A more detailed characterization was accomplished by FT-IR. Figure 3 shows the IR spectra of the pure In)2 and its monolayer. Comparing the two spectra clearly shows the disappearance of bands at 2988, 3026, and 3068 cm-1 in the monolayer. These bands are assigned to the C-H stretching of the ring, whereas the bands at 2852 and 2923 cm-1 correspond to the symmetric and asymmetric alkyl C-H stretching, respectively. This indicates that the chain is disordered similarly to the chains of short alkanethiols on gold, i.e., in a liquid type. At the same time, the ring can be oriented either parallel or perpendicular to the surface. In this orientation the four C-H bonds of the pyridine have only a minor dipole moment contribution normal to the surface and, therefore, are not detected using a p-polarized incident beam. The only conclusion that can be drawn from the clear disappearance of the C-H vibrations of the ring is that the ring is not randomly oriented. (22) The pKb value of the monolayer was determined by wettability and capacity measurements and will be described elsewhere. (23) Cainelli, G.; Cardillo, G. Chromium Oxidation in Organic Chemistry; Springer-Verlag: Berlin, 1984. (24) Luzzio, F. A.; Guziec, F. S., Jr. Org. Prep. Proceed. Int. 1988, 20, 533-584. (25) Sisler, H.; Ming, W.; Ch. L.; Metter, E.; Hurley, F. R. J. Am. Chem. Soc. 1953, 75, 446-448.
The high selectivity of the interface toward Cr(VI) cannot be attributed only to the positive charge of the pyridinium moiety22 as is indicated by the fact that a 2-aminoethanethiol monolayer extracted Cr(VI) very poorly. Interestingly, the extraction of Cr(VI) was followed also by wettability and capacitive measurements. Clear changes in the advancing contact angles (∆θ ) 17 ( 3°) of aqueous buffered solutions on films of In)2 on gold, as well as in the differential capacity of the double layer (∆Cdl ) 11.3 ( 1.8 µF/cm2) were observed upon extracting Cr(VI) ions. All these results suggest that Cr(VI) is efficiently extracted by a pyridinium monolayer. Complexes between pyridine and chromium trioxide, i.e., Collins reagent, in protic media have been widely used for the conversion of alcohols to carbonyl compounds.23 Characterization of these complexes revealed the existence of pyridinium and dichromate ions associated through hydrogen bonding.16 Similar complexes of pyridinium chlorochromate, pyridium fluorochromate, and pyridinium dichromate have also been prepared and extensively used as mild oxidants.24 The existence of hydrogen bonding in all these complexes has been confirmed by X-ray diffraction, IR, and 1H-NMR spectroscopies. Sisler25 reported that the formation of complexes between 2-alkyl-substituted pyridines and CrO3 was substantially less favored than with pyridines substituted in the 4 position. Although we are not aware of any stability constant measurements of complexes of pyridinium and CrO42- or Cr2O72- in aqueous solutions, we believe that the mechanism of complexation between our SAM and Cr(VI) is similar. The fact that the extraction of Cr(VI) is substantially different between 2- and 4-mercaptopyridinium is in accordance with these reports. The high selectivity of our electrode might be due to either the organization of the thiols on the surface, which assists in complexation, or the unique strong complexation between oxochromium(VI) species and 4-substituted pyridine rings. Continuous efforts that aim to shed light on the intimate structure of the interface are being undertaken, in particular, by scanning probe microscopy techniques. ACKNOWLEDGMENT R. Edelshtein (AFM images) and I. Erel (AA analysis) are acknowledged for their assistance. We are indebted to U. Billitewski and M. Stiene from GBF for the disposable gold electrodes. This work was supported by the Bureau for Environmental Control.
Received for review July 26, 1996. Accepted November 22, 1996.X AC9607525 X
Abstract published in Advance ACS Abstracts, February 1, 1997.
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