Sensing UranylI on Detection Sensing Uranyl(VI) Ions by Coordination and Energy Transfer to aL uminescent Europium(III) Complex Communications
Résumé
The release of uranyl(VI) is ah azardous environmental issue,w ith limited ways to monitor accumulation in situ. Here,wepresent amethod for the detection of uranyl(VI) ions through the utilization of au nique fluorescence energy transfer process to europium(III). Our system displays the first example of a" turn-on" europium(III) emission process with asmall, water-soluble lanthanide complex triggered by uranyl-(VI) ions. The development of nuclear technologies has led to many cases of accidental and intentional release of radionuclides, with accumulation of significant levels of uranium in the environment. [1] Of particular concern is the uranyl(VI) cation, UO 2 2+ .T his species,apotent nephrotoxin, [2] is highly mobile in groundwater and biological systems,l eading to possible problematic spread of radiotoxic material following containment breaches. To date,there has been limited development of probes for UO 2 2+ detection, with scintillation counting and X-ray based methods generally preferred. [1a] While these allow determination of total uranium content they,i mportantly,c annot distinguish between different oxidation states and, compared to fluorescence-based techniques,a re limited in their in situ application. This limitation hinders the real-time and remote monitoring of remediation strategies,s uch as the biotic reduction of UO 2 2+ to more immobile U IV-containing minerals ,astrategy currently under development as abioremedia-tion tool. [1a] Thef ew luminescence-based detection systems reported to date [3] have failed to exploit the intrinsic photo-physical properties of UO 2 2+ ,w hich allow distinct identification over other oxidation states and, with the correct design, afford an opportune and selective handle with which to monitor local concentration fluctuations of this environmentally hazardous species. Theintrinsic photophysical properties of the UO 2 2+ cation arise from formally forbidden charge transfer transitions from oxo-based molecular orbitals to nonbonding,u noccupied f-orbitals. [4] While direct interpretation of these transitions can be complicated by speciation and spectral overlap with optical transitions from biological media, [5] they do provide ameans for indirect UO 2 2+ detection via energy transfer to other longer wavelength (and longer-lived) emissive species.O f particular interest here is the spectral overlap of the UO 2 2+ emission (ca. 520 nm) and the europium(III) excitation bands (principally 5 D 1 ! 7 F 0,1), [6] which enable efficient energy transfer to occur from the former to the latter (Scheme 1). Here,wereport the first exampleofUO 2 2+ to lanthanide energy transfer in aw ater-soluble,m olecular europium(III) complex, [EuL]. [7] We suggest thatthis energy transfercould provide ahighly selective method of UO 2 2+ detection, due to the unique photophysical properties of UO 2 2+ that allow this process to occur. Initial spectrophotometric titrations were performed by following the absorption, steady-state emission and excitation spectra of [EuL] as afunction of addedUO 2 2+ (Figures 1, S1 and S2). In the absence of UO 2 2+ ,t he emission spectrum of [EuL] uponl igande xcitation(280 nm) is typical of Eu 3+ emission with the narrow emission bands corresponding to the 5 D 0 ! 7 F J transitions (578, 595, 613, 654 and 702 nm for J = 0t o4 ,r espectively). [7, 8] Addition of uranyl(VI) nitrate (0-2equivalents) at pH 7.4 led to adecrease of the overall Eu 3+ Scheme 1. "Turn-on" emission of [EuL] at selected excitation wavelengths due to energy transfer from UO 2 2+. Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.
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