Electrochemical Determination of Uranyl Ions Using a Self-Assembled

Notably, the cathodic wave shifts toward more negative potentials by ∼190 mV with respect to the peak of the uranyl-calix[6]arene modified electrode...
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Anal. Chem. 2009, 81, 8627–8631

Technical Notes Electrochemical Determination of Uranyl Ions Using a Self-Assembled Monolayer Amit Becker,† Haim Tobias,‡ and Daniel Mandler*,† Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, and Department of Chemistry, Nuclear Research Centre, Negev, P.O. Box 9001, Be’er-Sheva 84190, Israel Uranyl, UO22+, was electrochemically determined by a phosphate based self-assembled monolayer. A pretreated gold electrode with 2-mercatpoethanol was chemically modified by POCl3 or POBr3 to obtain the surface phosphate active sites. The different stages were characterized by reflection-absorption Fourier transform-infrared (FT-IR) spectroscopy, capacity, and X-ray photoelectron spectroscopy (XPS). The electrochemical determination of UO22+ was accomplished, after preconcentration under open circuit potential, by square wave voltammetry. On site electrochemical methods for rapid detection of uranyl ions, UO22+, in aqueous solutions have potential applications in a variety of fields in chemistry. The ability to measure trace levels of uranyl is essential in major fields such as geochemical exploration of uranium, workplace safety regulations, and water quality assessment. In recent years, a variety of physical and chemical techniques for determination of uranium in water have been in use: radiospectrometry,1-3 inductively coupled plasma mass spectrometry,4-7 and complexometric titration,8 to name a few. Nevertheless, these techniques require utilization of costly and complicated apparatus, which is usually not suitable for in situ operation or for field tests. Electroanalytical methods, on the other hand, are simpler in means of equipment and expenditure and yet are potent tools for analysis of uranium in watery environments. Electroanalysis has a major advantage over the aforementioned techniques, as it allows speciation of uranium in solution by direct * Corresponding author. Phone: +972 2 6585831. Fax: +972 2 658 5319. E-mail: [email protected]. † The Hebrew University of Jerusalem. ‡ Nuclear Research Centre, Negev. (1) Ku, T. L.; Knaus, K. G.; Mathieu, G. G. Deep-Sea Res. 1977, 24, 1005– 1007. (2) Ide, H. M.; Moss, W. D.; Minor, M. M.; Campbell, E. E. Health Phys. 1979, 37, 405–409. (3) Holzbecher, J.; Ryan, D. E. Anal. Chim. Acta 1980, 119, 405–408. (4) Boomer, D. W.; Powell, M. J. Anal. Chem. 1987, 59, 2810–2813. (5) Allain, P.; Berre, S.; Premel-Cabic, A.; Mauras, Y.; Delaporte, T.; Cournot, A. Anal. Chim. Acta 1991, 251, 183–185. (6) Lorber, A.; Karpas, Z.; Halicz, L. Anal. Chim. Acta 1996, 334, 295–301. (7) Caddia, M.; Iversen, B. S. J. Anal. At. Spectrosc. 1998, 13, 309–313. (8) Marsh, S. F.; Betts, M. R.; Rein, J. E. Anal. Chim. Acta 1980, 119, 401– 404. 10.1021/ac901092t CCC: $40.75  2009 American Chemical Society Published on Web 09/18/2009

measurement, and as such may generally be regarded as a nondestructive detection method. The most common electrochemical technique for determination of low concentrations of uranyl in aqueous media is adsorptive stripping voltammetry (AdSV), vastly used for quantification of uranium present in natural waters.9 During the last few decades, AdSV methods have been improved and expanded by introducing a range of complexing reagents or chelates to analyze uranyl samples. The uranium accumulation at the electrode surface was thus enhanced, resulting in more efficient electrodeposition process and higher sensitivity. The following reagents have been used as ligands that form complexes with uranium, which adhered to the electrode surface: 2-thenoyltrifluoroacetone-tributylphosphine oxide,10 2,5-dichloro-3,6 dihydroxy-1,4-benzoquinone,11 salicyclic acid tri-n-butyl phosphate system,12 catechol,13,14 Cupferron,15,16 potassium hydrogen phtalate,17 2,6-pyridinedicarboxylic acid18 and DTPA, and propyl gallate.19 Though the impressive reported detection limits for the described AdSV methods are in the relatively low range of nanomolar levels, several reservations should be stated; primarily, the relatively long equilibration time of the uranyl complexation process and, in some cases, the rather long preconcentration period. Mercury, onto which the uranyl complexes are adsorbed, is not recommended because of environmental considerations. In addition, strenuous treatments of the analyzed uranyl samples are often needed, such as dilution, pH adjustment, or CO2 purging. Strongly complexing media, such as carbonate rich waters, are also problematic, as it competes with the uranyl complexing reagents resulting in less effective uranium accumulation endowing lower sensitivity. (9) Wang, J. Stripping Analysis: Principles, Instrumentation, and Applications; VCH Publishers, Inc.: Deerfield Beach, FL, 1985. (10) Mlakar, M.; Branica, M. Marine Chem. 1994, 46, 61–66. (11) Sander, S.; Henze, G. Fresenius J. Anal. Chem. 1994, 349, 654–658. (12) Mlakar, M.; Branica, M. J. Electroanal. Chem. 1988, 256, 269–279. (13) Van Den Berg, C. M. G.; Huang, Z. Q. Anal. Chim. Acta 1984, 164, 209– 222. (14) Lam, N. K.; Kalvoda, R.; Kopanica, M. Anal. Chim. Acta 1983, 154, 79– 86. (15) Wang, J.; Setiadji, R. Anal. Chim. Acta 1992, 264, 205–211. (16) Kefala, G.; Economou, A.; Voulgaropoulos, A. Electroanalysis 2006, 18, 223–230. (17) Farghaly, O. A.; Ghandour, M. A. Talanta 1999, 49, 31–40. (18) Gholivand, M. B.; Rashidi Nassab, H.; Fazeli, H. Talanta 2005, 65, 62–66. (19) Wang, J.; Wang, J.; Tian, B.; Jiang, M. Anal. Chem. 1997, 69, 1657–1661.

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Scheme 1. Schematics of Assembling the Electrode for Preconcentrating Uranyl by a Self-Assembled Monolayer

With an aim at developing uranyl selective electrodes with high specificity and immediate response capabilities, an alternative approach has been adopted by electrochemists in the last 3 decades. Electrochemical sensors based on chemically modified electrodes with high affinity toward uranyl ions are considered to be favored in terms of robustness and response time. Since the modification process of the electrode surface is carried out before the interaction with uranyl is taking place, the main challenge for researchers is to find an agent that can be efficiently bound to the electrode surface, and, at the same time, can interact rapidly with the uranyl ions, assuring fast detection. Recent electrode modification approaches include ion imprinted polymers with specific binding sites for uranyl20,21 and selective host molecules such as calixarenes, bound to the electrode surface.22-28 Phosphate-based uranyl selective electrode, i.e., chemically modified electrodes with phosphate/phosphine derivatives terminal groups, are likely to serve as more adequate on-site rapid detection tools for uranium. The known properties of phosphates to form fast and stable complexes with uranyl29 has driven researchers to develop a uranyl sensor based on organophosphorus compounds which are usually anchored to the electrode surface through an aliphatic residue. Since the pivotal work by Manning et al.,30 who studied the function of several organo-phosphorus compounds bound to a poly(vinyl chloride) (PVC) matrix as uranyl sensors, many groups have dealt with the attempts to design a phosphate-based uranyl selective electrode, by different approaches. Notable studies were published by Goldberg and Meyerstein,31 who applied phosphite ligands as uranyl complexing agents bound to PVC based electrode, Chang et al.32 who introduced tri-n-octylphosphine oxide based electrodes, and Slater et al.33 who have employed organo-phosphorus based electrodes to form bis salts with uranyl ions. Multistage (20) Joyce, M. J.; Port, S. N.; Saunders, G. D.; Walton, P. H. International Patent EP 1019555 (W09915707), 2000. (21) Metilda, P.; Mary Gladis, J.; Prasada Rao, T. Anal. Chim. Acta 2004, 512, 63–73. (22) Sonoda, M.; Nishida, M.; Ishii, D.; Yoshida, I. Anal. Sci. 1999, 15, 1207– 1213. (23) Lu, Q.; Callahan, J. H.; Collins, G. E. Chem. Commun. 2000, 1913–1914. (24) Duncan, D. M.; Cockayne, J. S. Sens. Actuators, B 2001, 73, 228–235. (25) Ramkumar, J.; Nayak, S. K.; Maiti, B. J. Membr. Sci. 2002, 196, 203–210. (26) Evans, C. J.; Nicholson, G. P. Sens. Actuators, B 2004, 105, 204–207. (27) Schmeide, K.; Heise, K. H.; Bernhard, G.; Keil, D.; Jansen, K.; Praschak, D. J. Radioanal. Nucl. Chem. 2004, 261, 61–67. (28) Becker, A.; Tobias, H.; Porat, Z.; Mandler, D. J. Electroanal. Chem. 2008, 621, 214–221. (29) Blake, C. A., Jr.; Baes, C. F., Jr.; Brown, K. B. Ind. Eng. Chem. 1958, 50, 1763–1767. (30) Manning, D. L.; Stokely, J. R.; Magouyrk, D. W. Anal. Chem. 1974, 46, 1116–1119. (31) Goldberg, I.; Meyerstein, D. Anal. Chem. 1980, 52, 2105–2108. (32) Chien-Shu, L.; Fu-Chung, C.; Yu-Chai, Y. Anal. Chem. 1982, 54, 2333– 2336. (33) Moody, G. L.; Slater, J. M.; Thomas, J. D. R. Analyst 1988, 113, 699–703.

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preparation selective electrodes such as a phosphine oxide and phosphoric acid ester34 selective electrode based on calixarene and tri-n-octyl phosphine oxide35 and cysteamine functionalized with phosphate groups36 were recently reported as well. The current study is aimed to offer a phosphate monolayer based uranyl selective electrode, which is easily prepared by two simple and rapid synthetic steps. The first involves the formation of a self-assembled monolayer (SAM) of 2-mercaptoethanol (ME) on gold, while the second step comprises the modification of the SAM with POCl3 or POBr3 to yield phosphate moieties (Scheme 1). Electrochemical capacity measurements, reflection-absorption Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to verify the layer formation stages. The modified electrode was utilized for the preconcentration of uranyl ions from dilute solutions followed by determination using square wave voltammetry (SWV). EXPERIMENTAL SECTION Electrochemical measurements were carried out with a 750B potentiostat (CH Instruments, Austin, TX) using a conventional three-electrode cell. The working electrode was either a homemade gold disk (2 or 3 mm diameter) embedded in a Teflon cylinder or evaporated gold plates (10 mm × 20 mm) having a thin (∼5 nm) chromium layer underneath to increase the adhesion. A high-purity graphite rod (6 mm diameter) and Ag/AgCl (KCl saturated) were used as counter and reference electrodes, respectively. External reflection-absorption infrared spectra (RAFT-IR) were recorded using an Equinox 55 (Bruker) spectrometer equipped with a nitrogen-cooled mercury cadmium telluride (MCT) detector at a resolution of 2 cm-1. Typically 3000 scans were collected versus a clean gold surface reference. The spectra were acquired with a grazing angle accessory having an incident angle of 80° to the normal. X-ray photoelectron spectroscopy was performed with a Kratos Axis Ultra spectrometer with an Al KR-monochromatized source of 1486.71 eV. The pressure in the analysis chamber was 1 × 10-9 Torr. Uranyl nitrate hexahydrate (Merck, 99%) and sodium perchlorate (Sigma-Aldrich, 99%) were purchased and used as received. 2-Mercaptoethanol (ME) was obtained from Sigma-Aldrich. POCl3 and POBr3 (BDH, U.K.) were used without additional treatment. Triethylamine (TEA) (98%, Merck) was dried by distillation after mixing with K2CO3. The distilled TEA was left over a 5 Å molecular sieve. Tetrahydrofuran (THF) from Frutarom was dried over sodium metal (Sigma-Aldrich) in the presence of a low concentration of benzophenone (Sigma-Aldrich, 99%,), (34) Florido, A.; Casas, I.; Garcia-Raurich, J.; Arad-Yellin, R.; Warshawsky, A. Anal. Chem. 2000, 72, 1604–1610. (35) Ramkumar, J.; Maiti, B. Sens. Actuators, B 2003, 96, 527–532. (36) Shervedani, R. K.; Mozaffari, S. A. Surf. Coat. Technol. 2005, 198, 123– 128.

Figure 1. Reflection-absorption FT-IR spectra of a gold surface: (A) C-H vibration region after treatment with 2-mercaptoethanol and (B) mid-IR region after treatment with 2-mercaptoethanol (red line) and after modifying with POCl3 (black line).

which served as a water indicator. Deionized water (Barnstead Easypure UV system) was used for preparing the different solutions. Gold electrodes were cleaned prior to modification with ME. The gold disks were electrocycled in 1 M H2SO4 between the oxidation and reduction of water while the gold plates were annealed at inert conditions according to Nogues and Wanunu37 as the conventional Piranha mixture protocol damaged the evaporated samples. Modification by ME was accomplished by immersing the gold electrodes in 0.01 M of the thiol for 60 min. Then, the treated electrodes were rinsed with deionized water and dried in a desiccator. Modification of the ME/Au surfaces was carried out by immersing in 10 mL of THF which was dried according to the following procedure: 2 g of benzophenone were dissolved in 100 mL of THF, followed by the addition of about 0.5 g of pressed pieces of sodium metal. Before the sodium metal was used, it was rinsed in hexane after removing from the kerosene medium under which it was stored [Caution: sodium metal is highly reactive. It should always be handled under a covering layer of mineral oil and an atmosphere of dry nitrogen/argon]. The mixture was then stirred overnight under nitrogen until it turned dark-blue, and finally the THF was redistilled from the mixture and kept sealed in a flask containing molecular sieve. The final modification procedure included the addition of 14 µL of dry TEA and excess (3 equivalents) of POBr3 or POCl3. The samples were left to react under nitrogen overnight (Scheme 1). Then, they were washed with deionized water and stored under nitrogen atmosphere. Finally, the modified electrode was tested by placing it in 1 × 10-5, 5 × 10-6, and 1 × 10-6 M solutions of uranyl nitrate for different time periods. The extraction of uranyl was studied by SWV, and a calibration curve was plotted according to the voltammetric measurements. RESULTS AND DISCUSSION The formation of a phosphate based self-assembled monolayer followed Scheme 1. Specifically, a SAM of 2-mercaptoethanol was initially assembled on a gold surface. The hydroxyl terminated groups served as the nucleophiles for the phosphate attachment.38 Figure 1 shows the reflection-absorption FT-IR spectra of a gold surface after immersion in ME and after modification with POBr3. Two vibrational ranges are shown. Figure 1A shows two distinct C-H stretching peaks that are assigned to the symmetric (37) Nogues, C.; Wanunu, M. Surf. Sci. 2004, 573, L383–L389. (38) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141–149.

Figure 2. (A) P 2p XPS spectrum after treatment with 2-mercaptoethanol followed by modification with POCl3. (B) U 4f XPS spectrum after immersion in 1 µM UO22+ for 60 min followed by washing with water.

(2855 cm-1) and asymmetric (2926 cm-1) vibrations of the methylene groups. As expected no methyl vibrations can be seen. The wavenumber of the methylenes provides information about the organization of the SAM.38,39 Highly organized SAMs absorb at 2850 and 2918 cm-1, whereas less organized layers show a blue-shift toward higher energies. Hence, it is evident that ME SAMs are disorganized as would be expected for such short aliphatic thiols.40 Furthermore, our results are in excellent agreement with those reported by Spori et al.41 The introduction of phosphate moieties strongly affects the FT-IR spectrum at longer wavenumbers (Figure 1B). The spectrum before and after the addition of POCl3 is not simple to analyze; however, the appearance of three peaks at 1038, 1104, and 1282 cm-1 is evident. The first two peaks are attributed to the asymmetric vibrations of P-O-C, and their width depends on the nature of the groups attached to the carbon atom. The energy of the peaks in our case is blue-shifted, which is probably due to the attachment to the gold surface. Bertilsson and Liedberg performed FT-IR studies of assemblies of longer hydroxo-alkanethiols on gold followed by their reaction with POCl3.38 They reported on the appearance of a peak at 1072 cm-1 and attributed it, with some degree of uncertainty, to the strong absorption of the P-O-C group at this region. Their findings seem to support our conclusion that the appearance of the peak at 1104 cm-1 is an indication that P-O-C groups were successfully formed and phosphate derivatives are present on the surface. Nevertheless, an appearance of a second P-O-C peak at a lower frequency, 1038 cm-1, was not observed by them. The third new peak at 1282 cm-1 is assigned to the PdO group, and its wavenumber provides excellent indication that the phosphoryl terminal groups are anchored to the surface by a bridging reaction with either three or two neighboring hydroxyls.38,42 Yet, all these data provide neither an estimation of the yield of the phosphorylation stage nor the surface coverage. Further support for the modification of the surface was obtained by capacity measurements. Specifically, the interfacial (39) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350. (40) Balzer, F.; Gerlach, R.; Polanski, G.; Rubahn, H.-G. Chem. Phys. Lett. 1997, 274, 145–151. (41) Spori, D. M.; Venkataraman, N. V.; Tosatti, S. G. P.; Durmaz, F.; Spencer, M. D.; Zu ¨ rcher, S. Langmuir 2007, 23, 8053–8060. (42) Thomas, L. C.; Chittenden, R. A. Spectrochim. Acta 1964, 20, 467–487.

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Figure 3. Square wave voltammetry of an Au electrode after treatment with 2-mercaptoethanol, followed by modification with (A) POCl3 and (B) POBr3 in 1 µM UO22+ (pH 5.0) and as a function of preconcentration time.

capacity of the bare gold electrode was reduced from ∼70 to 8 µF cm-2 upon modification with ME and further decreased to 6.5 µF cm-2 after the reaction with POCl3. The attachment of a phosphate group was further confirmed by XPS (Figure 2A). The P 2p XPS spectrum of the SAM after treatment with POCl3 reveals a clear peak at 134.3 eV, which is in agreement with previous studies.41,43 Although the energy binding of P 2p can vary between 130-140 eV, it is fairly insensitive to changes if it is bound to four highly electronegative elements, such as O or Cl. Evidently, no signal of P was observed prior to the modification. Eventually, the affinity of the phosphate modified electrode toward uranyl ions has been studied. Figure 2B shows the U 4f XPS spectrum of uranyl after the electrode was immersed in a 1 mM solution of UO22+ for 1 h. It is needless to add that no signal of U was found prior to the immersion step. The two peaks of U assigned to the 4f7/2 and 4f5/2 at 381 and 392 eV, respectively, are clearly detected. These values are in accordance with previous reports44,45 which studied the sorption of uranyl on a lanthanum phosphate surface. The best method for determining the extent of extraction of uranyl onto a gold modified electrode is by electrochemical means. We have recently studied the electrochemistry of uranyl on bare and calix[6]arene-modified gold surfaces.28 The electrochemical reduction of UO22+ into UO2+ results in a distinct cathodic wave at ∼-0.10 V. Hence, we studied the extraction of uranyl from slightly acidic solution by our phosphate modified gold electrode. Figure 3A shows the square wave voltammetry (SWV) of an Au electrode modified with POCl3 after immersing in a 1 µM UO22+ solution at pH 5.0 for different durations. A cathodic wave at ∼-0.29 V develops as a function of the immersion duration. Notably, the cathodic wave shifts toward more negative potentials by ∼190 mV with respect to the peak of the uranylcalix[6]arene modified electrode.28 This might indicate that the formed complex between the uranyl ion and the phosphate moiety is of higher stability than the uranyl-calix[6]arene complex. The peak reaches an asymptotic value as would be expected for an adsorption process. This peak is, yet, quite broad and unsatisfactory for analytical purposes. (43) Fabianowski, W.; Coyle, L. C.; Weber, B. A.; Granata, R. D.; Castner, D. G.; Sadownik, A.; Regen, S. L. Langmuir 1989, 5, 35–41. (44) Chadwick, D. Chem. Phys. Lett. 1973, 21, 291–294. (45) Ordon ˜ez-Regil, E.; Drot, R.; Simoni, E.; Ehrhardt, J. J. Langmuir 2002, 18, 7977–7984.

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Figure 4. Calibration curve for uranyl obtained by extraction under open circuit potential for 20 min.

Employing POBr3 instead of POCl3 quite significantly improved the shape of the peak as well as decreased the equilibration time (Figure 3B). It is evident that maximum adsorption is obtained after approximately 60 min and that the peaks attain a more Gaussian shape, which is typical for a surface confined process. The difference in the response of the two modified electrodes (Figure 3) might be due to the higher reactivity of POBr3, which is likely to increase the density of phosphate sites on the electrode surface. At this stage it is unclear how many phosphate moieties associate with a uranyl ion. It is worth mentioning that only two phosphates participate in the well-known tributylphosphate-uranyl complex.46 In order to demonstrate the performance of the modified electrode as a quantitative analytical tool, the extraction experiments were repeated upon increasing the concentration of uranyl using the same modified electrode. Figure 4 shows a basic calibration curve comprised of voltammetric data recorded after immersion of the electrode in the uranyl solutions for duration of 20 min. It is evident that the electrode responses linearly to uranyl concentration in solution, and therefore can be regarded as a good sensor. The electrode was regenerated by electrocycling it several times between -0.1 and -0.4 V. It should be noted that other metal ions, e.g., Fe3+, might interact and interfere with uranyl determination using these SAMs. Yet, at pH 5.0 that was used for extracting uranyl ions, Fe3+ exists as Fe(OH)3 and therefore we do not expect that most (46) Den Auwer, C.; Lecouteux, C.; Charbonnel, M. C.; Madic, C.; Guillaumont, R. Polyhedron 1997, 16, 2233–2238.

of the hard metals will interfere under these conditions. Nevertheless, this subject requires further examination. In conclusion, the formation and characterization of a phosphate modified gold electrode has been accomplished and successfully used for the extraction and electrochemical determination of uranyl ions. The kinetics of uranyl binding with this electrode is faster than that of our former calixarene modified electrode. However, the sensitivity of this electrode needs to be further improved, which might be achieved by better understanding the uranyl-monolayer interaction.

ACKNOWLEDGMENT This project is supported by The Council for Higher EducationThe Planning & Budgeting Committee. The Harvey M. Krueger Family Center for Nanoscience Nanotechnology of the Hebrew University is acknowledged.

Received for review May 19, 2009. Accepted August 27, 2009. AC901092T

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