Reply to Comment on “Hypersensitive Luminescence of Eu³⁺ in Dimethyl Sulfoxide As a New Probing for Water Measurement”

Abstract

Luminescence 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 Eu 3+ in DMSO are very strong and very sensitive to water. The emission band from the 5 D 0 → 7 F 2 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, 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 ...