Photon Factory Activity Report 2017 #35 (2018)

BL-27B/2016G101

Structural Study of Selective Complexation of Sr(II) in Micellar Solution by XAFS
MethodKazuhiro Akutsu
Comprehensive Research Organization for Science and Society (CROSS), Neutron R&D
Division,162-1 Shirakata, Tokai, Ibaraki, 319-1106, Japan

1 Introduction
Strontium-90 (90Sr) having long radiological half-life isradioactive in nature and it falls under the category of
most hazardous of radioactive fallout [1]. Investigation of
the effects of 90Sr on environment and human body are
one of the most important issues for the management ofradioactive elements. Fluorescent probes are sufficiently
sensitive to detect a tiny amount of metal ions such as
strontium ion (Sr(II)) because of its exquisite sensitivity
and selectivity. Therefore, the study of Sr(II) fluorescentprobes have been carried out [2, 3].
Recently, we synthesized a Sr(II) selective fluorescentprobe, N-(2-hydroxy-3-(1H-benzimidazol-2-yl)-phenylmethyl)-1-aza-18-crown-6-ether: BIC), and studied its
fluorescence properties and complexation behavior withSr(II). We revealed that the stability constant of Sr(II)BIC complex was 10 times higher than that in aqueoussolution. In addition, its detection limit value was also
improved up to 300 times by this system. However, themechanisms of these phenomena have remained obscure.
In present study, the local structure of the Sr(II)-BIC
complex in micelle solutions is determined using EXAFS
analyses to reveal the species of Sr(II) complex which
formed in the solution.
2 Experiment
BIC was synthesized according to a proceduredescribed previously [4]. An aqueous solution of SrCl2(0.001 M), BIC (0.001 M), and SDS (100 mM) wereprepared. A solution of 0.01 M of SrCl2 and Sr(18-crown6 ether) complex were prepared as reference sample. Allof XAFS spectra were measured in fluorescence mode atthe BL-27B station in the KEK-PF. The Sr K-edge XAFS
spectra were recorded within 15.77 – 17.05 keV. Eachsample was measured for 1 h. The EXAFS spectra wereanalyzed using the WinXAS 3.1 program [5]. Theoreticalphase shifts and backscattering amplitude functions weregenerated using the FEFF8.00 code. The amplitude of thereduction factor, S02, was fixed at 1.0. All of XAFS
measurements were performed at room temperature.
3 Results and Discussion
We measured the XAFS spectra of SrCl2 and Sr(18crown-6 ether) complex as reference sample for theXAFS analysis. Fig. 1 shows the k3-weighted Sr K-edge
EXAFS oscillation of Sr(II) complexes in various
solutions, and the corresponding magnitude of theFourier-transformed data. Because O/N and Cl canbehave as ligands for Sr(II) in this system, both Sr-O/Nand Sr-Cl models were used to fit the EXAFS data. The
Fourier-transformed spectra exhibited two peaks at 2.0
and 3.0 Å in the data of Sr(18-crown-6 ether) andSr(BIC) complexes. The larger peak at 2.0 Å can beclearly assigned to a Sr-O/N interaction [6].

Fig. 1: (a) k3-weighted Sr K-edge EXAFS oscillation ofSr(II) complexes in solution, and (b) the corresponding
magnitude of the Fourier-transformed data. Solid linesand dashed lines indicate the experimental data andtheoretical fit, respectively. The phase shift in theFourier-transformed data was not corrected.
The obtained EXAFS spectra and their fitting results
indicated that Sr(II) and BIC formed a Sr(18-crown-6ether)-like complex in aqueous micellar solution. TheEXAFS results also indicated that the hydrophilic headgroup of surfactant molecule was directly coordinatedwith Sr(II). Therefore, we concluded that Sr(II), BIC, andsurfactant molecule formed a ternary complexes inaqueous micellar solution, and at least, it is clear that theimprovement of the stability constant in micellar solution
is attributed to the result of the formation ofSr(BIC)(surfactant) complex.
To understand Sr(II) recognition mechanism in detail,more study is in progress.

Photon Factory Activity Report 2017 #35 (2018)

Acknowledgement
Sample preparation was conducted at the UserExperiment Preparation Lab III that is provided byComprehensive Research Organization for Science andSociety (CROSS). This work was supported by JSPS
KAKENHI Grant Number 16K21615.
References[1] S. Kaur, et al., Org. Biomol. Chem. 12, 8230 (2014).
[2] H.-F. Ji, et al., Org. Biomol. Chem. 4, 770 (2006).
[3] K.-H. Leung, et al., RSC Advances 2, 8273 (2012).
[4] K. Akutsu, et al., Abstract of the 95th CSJ Annual
Meeting, Funabashi, Japan, 4F6-08 (2015).
[5] T. Ressler, J. Synchrotron Radiat. 5, 118 (1998).
[6] M. P. Jensen, J. Am. Chem. Soc., 124, 10664 (2002).
* k_akutsu@cross.or.jp