Sensing Uranyl(VI) Ions by Coordination and Energy Transfer to a Luminescent Europium(III) Complex

Abstract The release of uranyl(VI) is a hazardous environmental issue, with limited ways to monitor accumulation in situ. Here, we present a method for the detection of uranyl(VI) ions through the utilization of a unique fluorescence energy transfer process to europium(III). Our system displays the first example of a “turn‐on” europium(III) emission process with a small, water‐soluble lanthanide complex triggered by uranyl(VI) ions.

Abstract: 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 abioremediation tool. [1a] Thef ew luminescence-based detection systems reported to date [3] have failed to exploit the intrinsic photophysical 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 forbitals. [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).
[  (Table S4 and Figure S8). Thedecrease in ligand-excitation efficiency can be explained by the strong competing absorption associated with the increasing presence of UO 2 2+ species ( Figure S1). Thea ppearance of two new excitation bands (l em = 613 nm) at 430 and 320 nm was observed upon addition of UO 2 2+ (Figures 1and S2). Such bands are characteristic of the presence of UO 2 2+ complexes in solution and could only realistically be attributed to UO 2 2+ transitions from the Laporte forbidden O-to-U ligand-to-metal charge transfer (LMCT) transition and from the LMCT from the equatorial ligands,r espectively. [9] Thee xcitation spectra clearly suggest that the energy absorbed by UO 2 2+ ,o rb yi ts corresponding hydrolysis species,i st ransferred to Eu 3+ .M oreover,ared shift (ca. 14 nm) in the band at 330 nm is observed upon further addition of uranyl(VI) nitrate,providing evidence for the alteration of the equatorial coordination environment of the UO 2 2+ ion from 0.5 to 1equivalents.Such energy transfer from UO 2 2+ ,a nd other actinides,t ot he 5 D 0 excited state of Eu 3+ is known to be efficient in solid matrixes,polymers and glasses; [10] however,i th as previously only been seen in aqueous solution with highly concentrated mixtures. [11] Further titrations were carried out upon excitation into the uranyl-based LMCT transitions at 320 nm ( Figure 1) and 420 nm ( Figure S3). As expected, exciting into the UO 2

2+
LMCT bands led to Eu 3+ emission from the 5 D 0 excited state.S ignificant variations are observed in the emission intensity of Eu 3+ ,p ointing to the formation of several UO 2 -Eu coordination species in solution ( Figure 1). Thea ddition of UO 2 2+ nitrate is first characterised by as trong increase of the overall Eu 3+ emission intensity with, at maximum, a6 .4fold increase obtained at 613 nm in the presence of 0.5 equivalents of UO 2

2+
.T his observation clearly suggests the formation of a2:1 EuL/uranyl(VI) species.A fter 0.5 equiv-alents,t he Eu 3+ emission intensity decreases,p ointing to the formation of additional species in solution. Broad bands at around 530 nm, corresponding to UO 2 2+ emission, only became significant in the presence of an excess of 1equivalent of UO 2 2+ ( Figure S2). Linear regression analysis of the initial addition of UO 2 2+ to [EuL] provided limit of detection (LOD) values down to 12 mm (8.2 10 À5 m [EuL], l exc = 320 nm). It however should be noted that here,n either the complex nor the titrations were desgined to maximise the LOD.
Thespectral variations were analysed using the nonlinear regression analysis provided by SPECFIT (see the Supporting Information). [12] Theanalysis confirmed the formation of two new species and the titrations were modelled, with the fitting procedure converging towards logarithmic values of 4.3 AE 0.1 and 7.4 AE 0.1 for b 11 and b 21 ,respectively,corresponding to the formation of [(EuL)UO 2 ]a nd [(EuL) 2 UO 2 ]s pecies.E xcitation into the LMCT UO 2 2+ transition, at 420 nm ( Figure S3), revealed as imilar evolution.
During the titration, the intensity decays of Eu 3+ (l em = 613 nm) were monitored with excitation at 280 nm and 340 nm (Table S1). In the absence of UO 2 2+ ,t he excited state lifetime of [EuL] in TRIS buffer upon ligand excitation was 589 ms, in excellent agreement with previously reported data. [7] For[ U]/[Eu] < 0.5, ab i-exponential decay was obtained with lifetimes of t 1 = 340 msa nd t 2 = 688 ms, in almost equal proportions.T his behavior is likely due to the formation of an asymmetric 2:1Eu 3+ /UO 2 2+ species and points to the presence of two distinct coordination environments around Eu 3+ .F rom 0.6 equivalents and beyond, as hort component corresponding to the 1:1E u 3+ /UO 2 2+ species is observed with alifetime of 180 ms.
Detailed examination of the contribution of each lifetime between 0a nd 0.7 equivalents (l exc = 280 nm) shows that the 589 msc omponent reflects the disappearance of [EuL] according to the species distribution diagram in Figure S4 and the gradual increase of the 340 msc omponent (from 4% to 54 %) corresponds well with the formation of the 2:1 species.
Significant changes were also observed by monitoring the UO 2 2+ lifetime (l exc = 303 nm, l em = 520 nm). Thet imeresolved emission decay of UO 2 (NO 3 ) 2 was initially recorded in the same conditions and am ono-exponential decay was observed with aluminescent lifetime of 1.9 ms, as expected for aqueous UO 2 2+ ions. [4] Ab i-exponential decay was clearly observed for an EuL/U ratio of 1:0.25, showing am ajor component with t 1 = 379 ns (92 %) and am inor component t 2 = 54 ns (8 %). Theobvious shortening of the lifetime of the UO 2 2+ fluorescence corroborates the depopulation of the UO 2 2+ excited states due to an intramolecular energy transfer. Ther elative populations of the two species are in strong agreement with the species distribution postulated. Fort he EuL/U ratio of 1:0.75, bi-exponential decay was also observed, with am ajor component (t 1 = 379 ns,7 9%), accounting for the (EuL) 2 UO 2 species,a nd am inor component (t 2 = 36 ns,2 1%), which can be attributed to the formation of the 1:1c omplex. At at wofold excess of UO 2 2+ ab ioexponential decay is observed, with the predominant species (t 1 = 1.9 ms, 98 %) being related to the presence of uncomplexed UO 2

2+
.O nt he basis of these observations,t he   [13] and other theoretical studies. [14] TheE u-U distances are 4.12 for the Eu 3+ complex coordinated through the Oatom of the UO 2 2+ and 5.62 for the unbound uranyl oxygen atom. DFT calculations were also performed on the dinuclear [(EuHL)(UO 2 )(H 2 O) 3 ] 2À species ( Figure S6, Table S3). The UO 2 2+ group is coordinated to the Eu 3+ center with aE u-O distance of 2.57 and aEu-U distance of 4.02 .T wo oxygen atoms of phosphonate groups coordinate to the UO 2 2+ ion (U-O = 2.16, 2.20 ), while three water molecules complete the equatorial coordination positions with relativley long U-Od istances in the range 2.49-2.54 . [15] Our DFT calculations should be taken with some care in view of the complexity of the systems under study,a nd the fact that our simplified model did not include explicit water molecules (bulk solvent effects were included using ap olarizable continuum model). Nevertheless,they suggest that the polynuclear species formed upon UO 2 2+ addition are related to the coordination of phosphonate groups to the equatorial positions of UO 2 2+ ,l ikely resulting in two different Eu 3+ environments.S uch coordination is in excellent agreement with the luminescence lifetimes measured for the heterotrinuclear species.T he two distinct lifetimes observed (340/ 690 ms) could correlate perfectly to two species with different hydration states as suggested by the calculations.O ne Eu 3+ species is heptacoordinated by the ligand and fulfils its coordination by awater molecule,asisobserved for the [EuL] complex itself, [7] while coordination of the apical Oa tom of UO 2 2+ to the second Eu 3+ centre likely prevents water coordination, resulting in an increased lifetime (t = 690 ms) compared to the [EuL] complex (t = 590 ms). Although Raman spectroscopy was attempted to characterise these interactions further,o verlapping bands and weak signals precluded any definitive conclusion by this technique.
Thes olution assembly process was studied by 1 HNMR spectroscopy in D 2 O( Figure S7). To avoid the complexity associated with paramagnetic contributions,t he association behaviour of UO 2 (NO 3 ) 2 with the diamagnetic surrogate complex, [YL],w as studied. [16] Thep attern and chemical shifts were similar to those observed for the previously studied lanthanum complex, pointing to ac omplex with C 2v symmetry and ac oordination around Y III in which the nitrogen atoms and two phosphonate functions form aq uasi-planar pentadentate chelating arrangement;t he two remaining phosphonate moieties are coordinated on the upper and lower hemisphere of the complex, with the in-plane and out-of-plane phosphonate functions in rapid exchange on the NMR timescale. [17] Addition of UO 2 2+ results in ap rogressive decrease in intensity of the [YL] signals as an ew set of peaks emerges. Then ew signals present significant downfield shifts with respect to the parent complex (Dd %+ 0.5 to + 0.7 ppm), except for the aromatic methylene bridges,w hich show asignificant shift to higher fields (Dd %À0.6 ppm). In contrast to the UV/Vis and emission spectroscopy titration, 1 HN MR spectroscopy did not provide evidence for the formation of different heteronuclear species,s uggesting they are in fast exchange under the conditions applied, even at lower temperatures (5 8 8C, data not shown). Ther elatively broad peaks of the new resonances compared with those of the [YL] complex are in line with this hypothesis.Additionally,the observation of up to five broad signals in the aliphatic region ( Figure S7) suggests that the overall symmetry around the Y III ion is decreased to C 2 ,p ointing to ar igidification of the structure upon UO 2 2+ interaction and slower in/out-of-plane exchange of the phosphonate functions.
Despite their widespread use as cation sensors,t hrough both luminescence [8,18] and/or magnetic resonance responses, [19] to the best of our knowledge there have been no reports of am olecular lanthanide long-lived emissive complex that is responsive to UO 2 2+ .This example adds to the scope of recent examples of energy transfer in molecular lanthanide(III) complexes, [20] expanding applications into lanthanide-actinide interactions.U pon addition of UO 2 2+ to [EuL],o ur data indicate the formation of heteronuclear adducts in solution, accompanied by an appearance of characteristic UO 2 2+ transitions at 320 and 430 nm in the Eu 3+ excitation spectra. Such transitions can only be due to resonant energy transfer from the UO 2 2+ ion to Eu 3+ ,w ith energy transfer efficiencies up to 97 %. Multiplex sensing may also be feasible through resonance fluorescence measurements. [21] While the unoptimised LOD presented here (ca. 12 mm)i sh igher than some previously reported (destructive) fluorescence sensors, [22] it is significantly lower than commonly used X-ray absorption techniques (ca. mm or ppm); [23] further studies and ligand design should lead to lower detection limits for such phosphorescent sensors.
Thec omplex used in this study was not designed to selectively bind UO 2 2+ and so,w hile other cations cannot cause the energy transfer presented, competing metal ions (e.g. Mg 2+ ) [7] may displace UO 2 2+ and lower the detection limit in actual environmental samples.H owever,u pt o 200 equivalents of environmentally ubiquitous Ca 2+ ions have been shown not to significantly interact with [GdL]. [7] Higher specificity,ina ddition to the potential for time-gated luminescence,s hould likely preclude interference from environmental chromophores,s uch as humic acid. Future incorporation of this strategy with asmall-molecule Eu 3+ complex specifically designed with ah igh UO 2 2+ binding constant would result in apowerful and relatively inexpensive tool that could be developed to selectively detect environmental UO 2 2+ in situ in contaminated groundwater sites.