Comment pubs.acs.org/ac
Comment on “Hypersensitive Luminiscence of Eu3+ in Dimethyl Sulfoxide As a New Probing for Water Measurement” Pavla Pekarkova,† Premysl Lubal,†,‡ and Petr Taborsky*,†,‡ †
Department of Chemistry, Faculty of Science, Masaryk University Central European Institute of Technology, CEITEC MU, Masaryk University
‡
Anal. Chem. 2011, 83, 1879−1882. DOI: 10.1021/ac200072s S Supporting Information *
around 594 nm (related to the 5D0 → 7F1 transition) may be used for analysis as well (see Figure 3 in the original paper). Such an approach would certainly be more sensitive to water content even if it required to use an exact amount of the Eu(III) probe. Another possibility lies in determination of the I616/I594 ratio between the intensity (or the peak area) of the 5 D0 → 7F1 transition (about 592 nm) and the intensity (or the peak area) of the 5D0 → 7F2 transition related to the concentration of DMSO. Although the authors do not mention it the ratio changes as well. The I616/I594 ratio of europium(III) probe in water is 0.393 whereas in DMSO it is about 1.65.1 Although this method does not seem to be better at first glance, the resolution of the bands at 616 and 594 nm is much better resulting in the peaks to be completely separated. We strongly recommend another luminescence method for determination of water concentration in DMSO, which is in the original article mentioned only marginally in the last paragraph. It is based on experiments published originally by Lis and Choppin in 1991.4 Incidentally, Yao and Chen use the authors first names, Stefan and Gregory, when referring to the article. Lis and Choppin have suggested europium(III) luminescence lifetime to be used for trace water quantification in DMSO. The dependence of lifetimes on water concentration was assumed to be linear for low concentrations of water. The method could be extended also to a region where water predominates (determination of trace DMSO in H2O) (Figure 1). It was found that a second degree polynomial better fits the experimental curve (R2 = 0.9976) than linear fitting especially when working in the 0−100% region (v/v water in DMSO). The luminescence lifetime of Eu(III) perchlorate dissolved in pure water is 0.114 ms and in DMSO is 1.520 ms. Although the calibration curve is not linear, the method is believed to be more accurate and valid in a significantly wider concentration range. Eventually, the authors claim a lifetime measurement to be expensive and more complicated, which is at least misleading because a common xenon flash lamp3,5 is perfectly sufficient in the range of milliseconds and no instrumentation equipped by a pulse laser is necessary.6 Additionally, the lifetime utilizing methods are not only suitable for a H2O/DMSO system but may be as well used when two “ligands” coordinating Eu3+ are present.7 The first class of potential samples include different donor solvents such as DMF, ethanol, methanol, THF, acetone, dimethylacetamide,
Yao and Chen1 have shown a method for determination of water traces in dimethyl sulfoxide (DMSO). The described method is interesting, but in our opinion, it can be entirely replaced by other methods using the same reagents and based on measurement of Eu(III) luminescence lifetimes. Moreover, there is quite a lot of inaccuracy in the article which we would like to comment. Finally, we want to briefly show a few other possible applications of the method described in this comment. The published method is based on different splitting of a 5D0 → 7F2 transition of Eu(III) in various mixtures of H2O/DMSO as a result of J-mixing in the crystal field expansion and the spin−orbit interaction. In other words, the changes are induced by transformation of the Eu3+ environment and symmetry of the complex when ligand is added to the system.2 However, as can be seen in Figure 3 of Yao and Chen's work, the splitting of the 5D0 → 7F2 band results in poor peak separation and the resolution is very low. Unfortunately, authors do not mention much about the instrumental and experimental setup, such as a light source, scan rate, and step size, if spectra are corrected or not, etc. At least wavelength accuracy should be mentioned in the experimental part because the difference between wavelengths of measured intensities is only 4 nm. Deconvolution which may slightly improve separation of the peaks is not described in the article and was probably not done. From Figure 5 is evident that the luminescence intensity at 617 and 613 nm is almost constant in a wide region (0−15%), and according to our measurements it can be barely used for successful quantification of H2O. Moreover, the difference between intensity ratio (Figure 6) at 0% and 40% (the highest and the lowest measurable concentration) is only about 14% of the original intensity ratio value. The authors also state that at concentrations higher than 50% (v/v water) luminescence emission at 617 nm disappears, thus the method is not further applicable. From Figure 6 is quite clear that the obtained dependence is not linear; therefore, any linear calibration fitting is pointless and increases the overall error of determination. In our honest opinion, the method suggested in this way is applicable only for a rough water content estimation in a very narrow region (15−40%) without being tested on other samples but model ones. It is not clear why the authors do not consider the quantification of water in DMSO by simple integration of “double” peak area (related to 5D0 → 7F2 transition), when they themselves admit that the band intensity decreased 5 times after the addition of water. Optionally, a nonsplitting peak © 2012 American Chemical Society
Published: September 10, 2012 8427
dx.doi.org/10.1021/ac301934d | Anal. Chem. 2012, 84, 8427−8428
Analytical Chemistry
Comment
which can be successfully used for determination of water traces in ionic liquids. It was first demonstrated by Billard et al.10 on 1-methyl-3-butylimidazolium bis(trifluoromethanesulfonyl)imide. Figure 3 shows a dependence of Eu(III) luminescence lifetime on a different content of H2O in another ionic liquid (1-butyl-3-methylimidazolium) chloride measured in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Address: Petr Taborsky, Department of Chemistry, Faculty of Science, Masaryk University, Kotlarska 2, 611 37, Brno, Czech Republic.
Figure 1. Dependence of Eu(III) luminescence lifetimes on different ratio of H2O/DMSO.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Project “CEITEC-Central European Institute of Technology” (Grant CZ.1.05/1.1.00/ 02.0068) from the European Regional Development Fund and the Ministry of Education of the Czech Republic (KONTAKT ME09065 project).
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REFERENCES
(1) Yao, M. Z.; Chen, W. Anal. Chem. 2011, 83 (6), 1879−1882. (2) Reisfeld, R.; Zigansky, E.; Gaft, M. Mol. Phys. 2004, 102 (11−12), 1319−1330. Dickins, R. S.; Parker, D.; Bruce, J. I.; Tozer, D. J. Dalton Trans. 2003, 7, 1264−1271. (3) Taborsky, P.; Svobodova, I.; Hnatejko, Z.; Lubal, P.; Lis, S.; Forsterova, M.; Hermann, P.; Lukes, I.; Havel, J. J. Fluoresc. 2005, 15 (4), 507−512. (4) Lis, S.; Choppin, G. R. Anal. Chem. 1991, 63 (21), 2542−2543. (5) Taborsky, P.; Svobodova, I.; Lubal, P.; Hnatejko, Z.; Lis, S.; Hermann, P. Polyhedron 2007, 26 (15), 4119−4130. Pekarkova, P.; Spichal, Z.; Taborsky, P.; Necas, M. Luminescence 2011, 26 (6), 650− 655. (6) Stryla, Z.; Lis, S.; Elbanowski, M. Opt. Appl. 1993, 23 (2−3), 163−170. (7) Tanaka, F.; Kawasaki, Y.; Yamashita, S. J. Chem. Soc.-Faraday Trans. I 1988, 84, 1083−1090. (8) Dissanayake, P.; Mei, Y. J.; Allen, M. J. ACS Catal. 2011, 1 (10), 1203−1212. Kimura, T.; Nagaishi, R.; Kato, Y.; Yoshida, Z. J. Alloys Compd. 2001, 323, 164−168. (9) Lis, S. J. Alloys Compd. 2002, 341 (1−2), 45−50. (10) Billard, I.; Mekki, S.; Gaillard, C.; Hesemann, P.; Moutiers, G.; Mariet, C.; Labet, A.; Bunzli, J. C. G. Eur. J. Inorg. Chem. 2004, 6, 1190−1197.
Figure 2. Dependence of Eu(III) luminescence lifetimes on different ratios of D2O/H2O.
Figure 3. Dependence of Eu(III) luminescence lifetimes on different concentrations of H2O in 1-butyl-3-methylimidazolium chloride (ionic liquid).
hexamethyl phosphoramide, N-methylformamide, pyridine, acetonitrile,8 etc. The second class of analytes can be a mixture of water molecules which contains various hydrogen isotopes such as systems with D2O/H2O (Figure 2), T2O/H2O, etc. Replacement of the O−H oscillators by O−D or O−T ones leads to less efficient quenching of the Eu(III) excited state,9 8428
dx.doi.org/10.1021/ac301934d | Anal. Chem. 2012, 84, 8427−8428
