Comment pubs.acs.org/ac
Reply to Comment on “Hypersensitive Luminescence of Eu3+ in Dimethyl Sulfoxide As a New Probing for Water Measurement” Wei Chen* and Mingzhen Yao Department of Physics, The University of Texas at Arlington, Arlington, Texas 76019-0059
Anal. Chem. 2011, 83, 1879−1882. DOI: 10.1021/ac200072s for practical applications, particularly field applications, using the peak intensity is always easier and more convenient. In our preliminary studies, we use the intensity ratios of the two peaks at 613 and 617 nm for water probing. It does not exclude the possibilities of other emission peaks and their ratios that can be used for detection as pointed out in the comment by Pekarkova et al.9 However, we are not only trying to detect the amount of water; we are working on using these new and smart materials to recognize harmful molecules (i.e., threat detection). In this case, the spectra should have some fingerprints, like the two emissions at 613 and 617 nm, for identification. Sensing based on the relative intensity changes of two or more peaks is a new developing technology.10−12 The advantages of these dual-emissions based sensors are that they are more reliable, sensitive and may provide forensic signals. For example, we found that in CaF2:Eu,Mn and MgF2:Eu,Mn phosphors, the luminescence from Eu2+ decrease while the emission from Mn2+ increase in intensity as the X-ray dose is increased.11,12 Therefore, on the basis of the ratio of the two emission intensities, radiation dose can be measured. However, this cannot be done easily by measuring their decay lifetime changes because both of their decay lifetimes become shorter as a result of defects produced by the radiation. In this respect, sensing based on the relative intensity changes of two or more peaks has its advantages and deserves more attention. We appreciate some valuable suggestions and comments from Pekarkova et al.,9 but we cannot agree with them that our intensity ratio method can be fully replaced by a method using the same reagents that is based on measurement of Eu(III) luminescence lifetimes. First, the lifetime measurement cannot provide the fingerprints we need for threat detection. Second, lifetime measurement is not so easy and convenient as the intensity-based measurement. Third, lifetime measurement is more costly. Even though it was not accepted by the authors of the comment, it is true that lifetime measurement is much more expensive than simple spectra or intensity measurement. For example, let us look at the device for luminescence lifetime measurements of europium(III) ion based on a nitrogen-dye laser system cited in the comment by Pekarkova et al.9 The current cost for the system is not less than $100 000. In the United States, even devices for security applications, the maximum cost allowed for one unit is not higher than $30 000. The authors of the comment pointed out that a xenon flash lamp can be used for lifetime measurement. (Unfortunately, in ref 3 of Taborsky et al.,13 pointed out by the authors, a xenon lamp was not used for lifetime measurement; pulse lasers were
L
uminescence of rare earth ions in dimethyl sulfoxide (DMSO) has been recently observed and reported by our group.1,2 Potentially, luminescence of rare earth/DMSO compounds has many applications including the probing of trace water in organic solvents like DMSO. The measurement based on the relative intensity changes in two peaks is more reliable and selective than using the intensity change of only one emission peak. Both measurements based on intensity and lifetime changes have advantages and disadvantages. Techniques based on luminescence intensity changes are easy, fast, convenient, and cost-effective. Decay lifetime-based measurement is complimentary to intensity-based detection; however, it will likely never replace intensity-based techniques for practical applications. It has been reported previously that no luminescence could be observed for rare earth ions (RE) dissolved in dimethyl sulfoxide (DMSO).3−6 However, intense luminescence was recently observed and reported by our group by dissolving RE in DMSO at high temperatures.1,2 Luminescent RE/DMSO compounds may find applications in lighting, sensing, labeling, and imaging. The detection of trace water in organic solvents such as DMSO is an interesting project that has been studied extensively.7,8 We observed that the emissions of Eu3+ in DMSO are very strong and very sensitive to water. The emission band from the 5D0 → 7F2 transition has two peaks at 613 and 617 nm, and these two peaks change in opposite ways when water is added to the DMSO. Using the relative changes of the two peaks for measurement, as reported in our paper, is more reliable than using the change of only one peak. However, we did point out that the measurement based on the two intensity changes is only valid in a certain range of water concentration in DMSO; it is not applicable for the entire concentration range. We did explain that the excitation and emission spectra were measured on a commercially available Shimadzu RF-5301PC fluorometer. Standard procedure was followed for the spectral measurement, and spectra measured with a standard fluorometer are not like the spectra measured using homemade systems that need special calibrations. For a Shimadzu RF-5301PC fluorometer, the light source is a 150 W xenon lamp and the wavelength scale is from 220 to 990 nm. In the emission spectral measurement, the bandwidth is set at 1.5 nm and the measurement wavelength accuracy is ±1.5 nm. The data and spectra reported in our paper are valid, reliable, accurate, and repeatable. We prefer to use the emission peak intensity rather the emission area for the measurement because the measurement of the peak intensity is easier and more accurate because the two peaks are overlapped and a deconvolution of them might induce errors due to possible artificial elements. Furthermore, © 2012 American Chemical Society
Published: September 10, 2012 8429
dx.doi.org/10.1021/ac302266v | Anal. Chem. 2012, 84, 8429−8430
Analytical Chemistry
Comment
used.) It is true that a xenon lamp can be used for lifetime measurement using the frequency-domain method.14 However, the frequency-domain method has some limitations. For example, it only works well on samples with strong luminescence.15 The frequency-domain method cannot detect multiple lifetime components directly.16 It was also found that the frequency-domain method had a higher uncertainty in the estimate of luminescent lifetime than the time domain sequences that included a dark period and the uncertainty is 4 times higher than that of the time-domain lifetime estimate.15 On the basis of these studies, it is still a question whether the cost of a lifetime measurement system can be reduced while maintaining precision and accuracy. On the other hand, one intensity-ratio based detection system can be easily built or assembled with costs less than $5 000. On the basis of the above discussion, we conclude that both of the measurements based on intensity and lifetime changes have advantages and disadvantages. Techniques based on luminescence intensity changes are easy, fast, convenient and cost-effective. Decay lifetime-based measurement is complementary to intensity-based detection; however, it will likely never replace intensity-based techniques for practical applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to acknowledge the support from the startup funds from UTA, the NSF and DHS joint ARI program (Grants 2011-DN-077-ARI053-3 and CBET-1039068), DOD Grant DTRA08-005, and the U.S. Army Medical Research Acquisition Activity (USAMRAA) under Contracts W81XWH10-1-0279 and W81XWH-10-1-0234.
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REFERENCES
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