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Terpyridine-based complex nanofibers with Eu3+ as a highly selective chemical probes for UO22+
Journal of Hazardous Materials 378 (2019) 120713
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Terpyridine-based complex nanofibers with Eu3+ as a highly selective chemical probes for UO22+
T
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Seonae Leea, 1, Ka Young Kima, 1, Na Young Lima, Jin Hwan Junga, Ji Ha Leeb, , ⁎ ⁎ Myong Yong Choia, , Jong Hwa Junga, a b
Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju, 52828, Republic of Korea Department of Chemistry and Biochemistry, The University of Kitakyushu, Hibikino, Kitakyushu, 808-0135, Japan
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Europium(III) complex Uranyl ion sensor Fluorescence probe Benzenetricarboxyamide Electrospun film
Uranyl is a radioactive, toxic pollutant commonly found in the waste remaining after nuclear fuel reprocessing, and it poses several types of risks to human health; therefore, developing absorbents and chemical probes for this compound is crucial to overcoming these issues. This study examined the sensing abilities of terpyridine-appended benzenetricarboxyamide (T-BTA) as a chromogenic probe for detecting uranyl ions (UO22+). The complex with Eu3+ (1–Eu) spontaneously formed nanostructured fibers in H2O owing to the triamide groups of T-BTA, which induced intermolecular hydrogen-bonding interactions. The strong blue emission of these nanofibers in H2O was quenched upon adding UO22+ but not upon adding any other metal ion. This high selectivity was probably because of the interactions between the nitrigen atoms of the terpyridine moieties of 1 and UO22+. Furthermore, the 1–Eu nanofibers assumed spherical morphologies when UO22+ was added. To develop a convenient UO22+ sensor, an electrospun film incorporating 1–Eu (ESF–1–Eu) was manufactured, and it exhibited high selectivity for UO22+ over a variety of rival metal ions. The plot for luminescence change of ESF–1–Eu vs UO22+ concentrations in seawater samples showed a good linearty. Thus, the ESF–1–Eu shows potential as a useful sensor for detecting and removing UO22+ in H2O.
1. Introduction Uranium is a natural radionuclide widely used to produce nuclear energy and nuclear weapons [1–3]. It has various oxidation states
(namely, +2, +3, +4, +5, and +6) but exists mostly in the hexavalent form uranium(VI)). In nature, uranium generally combines with oxygen to form uranyl ions (UO22+), which pose risks to both human health and the environment [4]. Thus, sensing UO22+ in aqueous
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Corresponding authors. E-mail addresses: [email protected] (J.H. Lee), [email protected] (M.Y. Choi), [email protected] (J.H. Jung). 1 Seonae Lee and Ka Young Kim contributed equally. https://doi.org/10.1016/j.jhazmat.2019.05.106 Received 14 November 2018; Received in revised form 22 May 2019; Accepted 30 May 2019 Available online 31 May 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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solutions is of great importance in the medical and environmental fields [5–7]. Various instrumental methods have been used for this purpose, including atomic absorption spectroscopy (AAS) [8], inductively coupled plasma–mass spectroscopy (ICP–MS) [9], and surface-enhanced Raman spectroscopy (SERS) [10]. However, although these spectroscopic methods are highly selective for and sensitive to UO22+, most of them are expensive, time-consuming, and destructive [11,12]. Recently, several organic and inorganic substances have been used to fabricate chemical sensors for detecting UO22+ based on various sensing techniques and principles [13–20]. In theory, inorganic-based sensing materials, including pyrophosphate and phosphate salts [21], have low stability and selectivity because they tend to crystallize below room temperature, and other metal ions can easily interfere with their sensing ability. By contrast, organic-based fluorescence sensing is an easy, rapid, highly selective, and low-cost method for detecting metal ions [22]. Nevertheless, only a few studies have been reported on the detection of UO22+ using chromogenic probes. For example, X. Wang’s research group used tetraphenylethene-modified 2-(4,5-dihydrothiazol2-yl)phenol as a fluorescence “turn-off” sensing agent for detecting UO22+ [23]. In addition, X. Chen et al. investigated a chemical probe called 4-pethoxycarboxyl salicylaldehyde azine (PCSA), which exhibited the aggregation-induced emission (AIE) effect and showed excellent sensitivity to not only UO22+ [24] but also pH. However, while PCSA should have worked at a pH of 10.3, the uranyl ions could precipitate as UO2(OH)2 at this pH owing to the low constant of precipitation (1.22 × 10–22), which could lead to inaccurate results. Therefore, the development of highly selective, sensitive chemical probes for UO22+ that are also applicable over a wide pH range remains challenging. Also, L. Charbonniere et al. reported a new method for through the utilization of the unique fluorescence energy transfer process to Eu3+, which was the first example of a “turn-on” europium (III) emission process with a small, water-soluble lanthanide complex triggered by uranyl-(VI) ions [15]. Furthermore, using chemical probes that exhibit the AIE effect [25–29] may represent an efficient method for colorimetrically detecting UO22+ owing to their unique photophysical properties. However, our concept is based on the AIE effect of terpyridine-appended benzenetricarboxyamide (T-BTA) complex with Eu3+ as a selective chemoprobe for UO22+ in aqueous solution, which was different from the reported methods previously in the literatures [15]. In this study, the selective detection of UO22+ was used by exchange reaction between complex Eu3+ with blue emission and UO22+. This is different from the fluorescent energy transfer they reported [15]. It is because the formation of the nanostructure by the intermolecular interaction such as the intermolecular hydrogen-bonds and π-π stacking. In addition, one of the important point of our system is that a portable kit for UO22+ detection is available. The film type of the complex with Eu3+ can be easily prepared by electro spin. In this study, we report a novel nanostructured terpyridine-appended benzenetricarboxyamide (T-BTA) complex with Eu3+ (hereafter referred to as 1–Eu) as a fluorescence turn-off sensor for UO22+ because of its AIE characteristics in aqueous media. Furthermore, we demonstrate that an electrospun film incorporating 1–Eu (ESF–1–Eu) is a convenient, efficient UO22+-detecting device.
and an acceleration voltage of 0.1–30 kV. Elemental analysis was performed with a Perkin Elmer 2400 series II Fourier transform infrared (FTIR) spectrophotometer, and the presence of synthetic products was confirmed with a Shimadzu FTIR 8400S spectrophotometer. A JEOL JMS-700 mass spectrometer was used to measure the mass spectra. 2.2. Synthesis of compounds 1 and 2 Compounds 1 and 2 were prepared according to a method previously reported in the literature [30], which is described together with the characterization of 1 and 2 in the supplementary information. 2.3. Preparation of 1–Eu To prepare the 1–Eu complex, compound 1 (17.2 mg, 0.016 mmol) was dissolved in tetrahydrofuran (THF)/water (1:1 v/v%) Then, Eu (NO3)3 (5.4 mg, 0.016 mmol, 1.0 equiv.) was added to the compound 1 solution. The 1–Eu complex was characterized by UV–Vis, fluorescence spectroscopy, and elemental analysis (Figs. 1 and S1). 2.4. Preparation of electrospun ESF–1–Eu films 1–Eu was dissolved homogeneously in dimethylformamide (DMF) and mixed with polymethyl methacrylate (PMMA) (MW: 350 000 g mol−1) for several hours at room temperature. The resulting mixture contained 0.5 wt% 1–Eu and 5.0 wt% PMMA. To electrospin the films, the homogeneously mixed solution was loaded into a syringe (5 mL) that was straight fixed inside a KD Scientific 200 syringe pump, and an electrode was linked to the tip of the syringe needle with a working distance of 15 cm. Electrospinning was performed at 25 °C with a flow rate of 16.7 mL min–1 and voltage of 15 kV [31]. 2.5. UO22+ detection methods using ESF–1–Eu After electrospinning the ESF–1–Eu film and trimming it to dimensions of 2 × 2 cm2, the materials were immersed slowly into separate vials containing aqueous UO22+ solutions with concentrations of 0.5, 1.0, 1.5, 2.0, and 2.5 μM at pH 7.4. To confirm the films’ selectivity for UO22+, they were immersed into vials containing 1.5 μM aqueous metal solutions at pH 7.4, including Na+, Cs+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+, Cu2+, Fe3+, Zn2+, La3+, Tb3+ and UO22+, for 30 min. After removing the films from their respective solutions, the films were dried in air for 24 h. In addition, the luminescence intensity of the prepared samples was measured by solid-state luminescence spectroscopy. 2.6. Measurement of the association constant and detection limit The supplementary information describes these methods in detail [32]. 3. Results and discussion 3.1. Ability of 1–Eu to detect UO22+
2. Experimental section To prepare the 1-Eu complex in THF/water (1:1, v/v%), Eu(NO3)3 was added to the compound 1 solution, and the solution was characterized by UV-vis and fluorescence spectroscopy (Figs. S1 and 1). A detailed examination of the UV–Vis spectrum of 1 upon titration of 0-10 equiv. of Eu(NO3)3, the absorption band for the π → π* transition of the terpyridine moiety appeared at 280 nm with a shoulder at 325 nm (Fig. S1A). The broad luminescence spectrum showed a strong emission at 410 nm, corresponding to the terpyridine moieties of 1–Eu (Fig. 1). This strong blue emission originated from the tilted π–π stacking between the uncomplexed terpyridine and the uncomplexed terpyridine moieties
2.1. Reagents and instruments All reagents were purchased from TCI, Sigma-Aldrich, or Alfa Aesar. All solvents were provided by Sigma-Aldrich or Samchun Pure Chemical Co., Ltd. and used with additional purification. The 1H and 13C spectra were recorded using a Bruker DRX 300 spectrophotometer and the absorption spectrum of the solution with an ultraviolet–visible (UV–Vis) absorption spectrophotometer (U-4100). Images were recorded using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7610 F) with an emission current of 100 nA 2
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Fig. 1. (A) Luminescence spectra of the terpyridine-appended benzenetricarboxyamide (1) (8.0 × 10−5 M) without and with various equivalents (0–10) of Eu3+ in THF/water (1:1, v/v%, ex =325 nm). (B) Luminescence intensity of 1 at 410 nm as a function of the Eu3+ amount.
determine its applicability as a chemical probe for UO22+ in a THF/ water solution. These findings suggest that excess UO22+ binds to the free terpyridine moiety of 1-Eu after exchange reaction between Eu3+ of 1-Eu and UO22+, which continually resulted in the quenching effect. We monitored its value at 410 nm to quantify UO22+. The overall spectrum of 1–Eu decreased gradually with increasing UO22+ and was virtually quenched when 1 equiv. UO22+ was added to the 1–Eu solution. In addition, the plot of the UO22+ concentration vs. luminescence intensity exhibited good linearity (Fig. 2B). This quenching effect was due to the dissociation of Eu3+ from the 1–Eu complex caused by the addition of UO22+; then, 1 re-complexed with UO22+ due to the exchange reaction (Scheme 1). In the previous study [15], they claimed the fluorescence energy transfer to Eu3+ was main force to detection of UO22+. Fortunately, the pyridine-based ligand showed two new excitation bands at 430 nm and 320 nm upon addition of UO22+. These two bands originated from the Laporte forbidden O-to-U ligand-tometal charge transfer (LMCT) transition. Exciting to this LMCT band led to Eu3+ emission from the 5D0 excited state [38]. Although the previous study looks similar to ours, there was no luminescence band of the range of 480–540 nm in our system. This is because UO22+ is not bounded to Eu3+ directly but to terpyridine. In contrast, when other metal ions such Na+, Cs+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+, Cu2+, Fe2+, Fe3+, Zn2+, La3+ and Tb3+ were added, no remarkable changes in the luminescence spectra were observed at 410 nm (Figs. 3 and S4), suggesting that 1–Eu selectively bound to UO22+ in THF/water. This high selectivity was probably due to the interactions between the nitrogen atoms of the terpyridine moieties of 1 and the uranyl ions, according to the hard and soft acids and bases (HSAB) theory [15]. Furthermore, the luminescence intensity was plotted as a function of the UO22+ concentration in the presence of 1.0–5.0 equiv. of UO22+, and the slope of the plot was smaller than that of 0–1.0 equiv. of UO22+ (Figs. 2B and S5). These findings suggest that excess UO22+ binds to the free terpyridine moiety of 1–Eu after exchange reaction between Eu3+ of 1–Eu and UO22+, which continually resulted in the quenching effect. When ligand 1 (80 μM) was dissolved in a 1:1 (v/v%) THF/water mixture, the 1–Eu solution was not transparent both with and without UO22+ (1 equiv.), and the aggregated morphologies of 1–Eu in both cases were observed by SEM. Fig. 4A shows that 1–Eu without UO22+ consists of fibrillar structures with widths of 50–100 nm and a micrometer-level lengths. These fibrillar structures, or nanofibers, could be ascribed to the self-aggregation of 1–Eu via intermolecular interactions such as hydrogen bonding and π–π stacking; these nanofibers produced blue emission owing to the AIE effect. In contrast, 1–Eu aggregates with UO22+ exhibited spherical morphologies with a diameter of approximately 30–50 nm (Fig. 4B). This morphological change was due to the
of 1. Furthermore, AIE effect of 1 was confirmed in the presence of Eu3+ (1.0 equiv.) in the different composition of H2O and THF. The fluorescence intensity of 1 in the presence of Eu3+ was no observed in THF. In contrast, the fluorescence enhancement of 1 with Eu3+ (1.0 equiv.) was observed at ca. 410 nm in a mixed H2O and THF (1:1 v/v) which was a typical evidence for AIE effect (Fig. S2) [33]. In addition, temperature-dependent fluorescence spectra of 1 in the presence of Eu3+ was observed in a mixed H2O and THF (1:1 v/v). The strong fluorescence intensity of 1 with Eu3+ decreased at high temperature (80 °C), indicating that the self-assembled 1-Eu was changed to monomer 1 (Fig. S3). The nanofiber structure of 1-Eu in SEM observation was also indirect evidence for AIE effect. The plateaus of the absorption and emission spectra observed for 1 equiv. of Eu(NO3)3 suggests that Eu3+ and 1 complexed (i.e., 1–Eu) at a 1:1 stoichiometry (Fig. S1B and Scheme 1), indicating that the two terpyridine moieties of 1 did not form the complex with Eu3+. Furthermore, typical emission bands of the Eu3+ complex at 579, 593, 616, 650, and 696 nm did not appear because three H2O molecules were coordinated to Eu3+ [34,35]. It is known that interaction with water (both in the inner and outer coordination sphere of the Ln3+ ion) leads to a severe quenching of the metal luminescence via OeH vibrations. In our previous study [34], the complex 1 with Tb3+ showed a strong emission at the range of 550–700 nm in methanol whereas the complex 1 with Tb3+ showed non-emission in methanol and water (1:99 v/v). To demonstrate quenching effect of the complex 1 with Tb3+ in a mixed methanol and water, the coordination number of H2O in the complex 1 with Tb3+ was measured by Horrocks method [36]. The emission lifetime (τ) for H2O was calculated to be 0.16 ms while that of D2O was 0.21 ms. These findings indicate that three molecules would coordinate to Tb3+. The coordination of water molecules caused the quenching effect in the range of 550˜700 nm. Thus, we believe that the quenching effect of 1Eu in the range of 570˜700 nm was due to bound to water molecules as the complex 1 with Tb3+. Based on UV–vis and luminescence observations as well as reported results previously in the literatures [34], the proposed structure of the complex 1 with Eu3+ in THF/water (1:1, v/v%) was represented (Scheme 1). Since Eu3+ ion has nine coordination sites available, one terpyridine moiety, three water molecules, and three nitrate anions would coordinate to a single Eu3+ as shown in Scheme 1. On the other hand, two terpyridine moieties of 1 were empty in the absence of metal ions, which caused the AIE effect based blue emission owing to the tilted π–π stacking. 1–Eu exhibited strong emission associated with the photoelectron transfer from terpyridine to Eu3+ in THF/water. However, upon adding guest ions, specifically, UO22+, quenching was observed due to the exchange reaction, i.e., a “turn-off” response (Fig. 2). Thus, the change in the luminescence of 1–Eu after adding UO22+ was evaluated to
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Scheme 1. Luminescence mechanism of the terpyridine-appended benzenetricarboxyamide (T-BTA; 1) complex with Eu3+ (1–Eu) for sensing UO22+; (A) compound 1, (B) 1–Eu complexation between 1 and Eu3+, and (C) re-complexation between 1 and UO22+ via the exchange reaction.
Fig. 2. (A) Luminescence spectra of the terpyridine-appended benzenetricarboxyamide–Eu3+ complex (1–Eu) (8.0 × 10−5 M) without and with various equivalents (0–10) of UO22+ in THF/water (1:1, v/v%, ex =325 nm). (B) Luminescence intensity changes of 1–Eu as a function of the UO22+ amount. 4
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Fig. 3. (A) Photographs, (B) luminescence spectra, and (C) luminescence intensity changes at 410 nm of the terpyridine-appended benzenetricarboxyamide–Eu3+ complex (1–Eu) (8.0 × 10–5 M) with a variety of metal ions (1 equiv.) in THF/water (1:1, v/v%, ex =325 nm).
S9A). To confirm the exchange reaction between Eu3+ of 1–Eu and UO22+, Ka between 1 and Eu3+ was also determined under the same conditions as those used for UO22+. The association constant for 1 with Eu3+ was 2.4 × 103 M−1 (Fig. S9B), which was 1.14-fold smaller than that of UO22+. These findings suggest that Eu3+ dissociated from 1–Eu upon adding UO22+, as shown in Scheme 1, due to the relatively weak complexation. Furthermore, the luminescence lifetimes for 1–Eu without and with UO22+ were 0.964 ns and 0.657 ns (Fig. S10), respectively. The luminescence lifetime for 1–Eu decreased in the presence of UO22+ owing to the quenching effect between the terpyridine and the terpyridine moieties of 1 with the complexation of 1-UO22+ via an exchange reaction. We observed the excitation spectrum of 1-Eu before and after addition of UO22+ ion. As a result, the absorption at 295 nm was decreased. This is because the formation of 1 + UO22+ complex with stronger association constant (Fig. S11).
geometrical change of ligand 1 when complexing with UO22+. The luminescence spectral changes of 1–Eu in HEPES buffer (0.01 M, pH 7.4) were investigated in a binary system (Fig. S6). As expected, significant intensity changes were observed upon adding UO22+ (1 equiv.) with other metal ions, and the blue emission was quenched. To confirm the quantitative response of 1–Eu to UO22+, its luminescence intensity was plotted as a function of the increasing UO22+ concentration in HEPES buffer at pH 7.4. Fig. S7 shows that the luminescence intensity of 1–Eu decreased linearly with the increasing UO22+ concentrations (from 0 to 1 equiv.), confirming that UO22+ interacted with 1–Eu. In addition, the nanostructured 1–Eu exhibited excellent sensitivity to UO22+, as evidenced by a limit of detection (LOD) of 2.17 ppm, which is sufficiently sensitive to detect UO22+. To determine the stoichiometric ratio between 1–Eu and UO22+, a Job plot was used. Maximum absorption was attained at a molar ratio (UO22+/ (1–Eu + UO22+)) of approximately 0.5, indicating a 1:1 stoichiometric ratio between 1–Eu and UO22+ (Fig. S8). Moreover, the association constant (Ka) between 1 and UO22+ calculated for this stoichiometry based on the Benesi–Hildebrand method was 2.75 × 103 M−1 (Fig.
3.2. Detection ability of ESF–1–Eu To make a convenient chemical probe, a 1–Eu-incorporated PMMA
Fig. 4. SEM images of the morphologies of 1–Eu (80 μM) aggregates (A) without and (B) with UO22+ (1 equiv.) in a THF/water mixture (1:1, v/v%). 5
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Fig. 5. (A) Photograph and (B) SEM image of the electrospun film (10 cm × 10 cm) incorporating the 1–Eu complex.
Fig. 6. Photographs of ion selective for UO22+ in (A) single and (B) binary systems. (C) Histogram of the luminescence intensity of ESF–1–Eu in (C) single and (D) binary systems.
film, i.e., ESF–1–Eu, was prepared via electrospinning. SEM images of the indiscriminately oriented electrospun PMMA and 1–Eu ensemble in ESF–1–Eu (Fig. 5) revealed fibers with a relatively uniform diameter of ca. 100 nm. Furthermore, energy dispersive spectroscopy mapping confirmed that ESF–1–Eu contained evenly distributed carbon, oxygen, nitrogen, and europium (Fig. S12), indicating that the 1–Eu complex was regularly embedded in the fibers. To evaluate the detection ability of ESF–1–Eu, it was first immersed in the cation solution of interest and then dried in air, and the luminescence intensity changes were analyzed by solid-state luminescence spectroscopy. Similar to the behavior of 1–Eu in THF/water (1:1, v/v %), when ESF–1–Eu was immersed in solutions containing increasing UO22+ concentrations, its strong blue emission was quenched within a few minutes (Fig. S13). Thus, ESF–1–Eu became increasingly less emissive with the gradually increasing UO22+ concentration. By contrast, immersion in other metal ion solutions did not significantly affect the luminescence spectrum of ESF–1–Eu (Fig. 6C). These results indicate that ESF–1–Eu can detect UO22+ similarly to litmus paper for measuring pH The calculated LOD of ESF–1–Eu was 0.292 ppm, which is lower than that of the nanostructured 1–Eu in a bulk solution (Fig. S14), and it exhibited a higher reactivity due to its wider surface area. To further assess the usefulness of ESF–1–Eu as an ion-selective chemical sensor for UO22+, its luminescence changes in aqueous solutions were investigated in binary systems. ESF–1–Eu was also immersed in a solution containing various biologically and environmentally important metal ions (Fig. 6B and D). In addition to UO22+, various metal ions including Na+, Cs+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+, Cu2+, Fe3+,
Zn2+, La3+, and Tb3+ were tested. The strong blue emission of ESF–1–Eu was again quenched when the film was immersed in thesolution. As expected, the luminescence quenching observed in the solution containing UO22+ due to the 1–UO22+ complex in ESF–1–Eu remained unaffected in the presence of these metal ions, indicating that ESF-1 is a selective chemical sensor for UO22+, even in a mixture of these metal ions. However, with some other metal ions, the quenching effect of ESF–1–Eu was smaller than that in the abovementioned binary systems. Specifically, when ESF–1–Eu was immersed in a UO22+ solution containing Ni2+, this ion interfered slightly in the detection process of UO22+, thus decreasing the quenching effect. Biological and environmental chromogenic sensors must be both effective and stable over a wide pH range. Therefore, the effect of pH on ESF–1–Eu was evaluated in the absence of UO22+ (Fig. S15), and no emission change was observed in the pH range of 3–11. Conversely, the luminescence significantly changed upon adding UO22+ in the same pH range, indicating that ESF–1–Eu can be used as a chemical sensor for UO22+ at pH > 3. To demonstrate the exchange reaction between Eu3+ of the 1–Eu complex and UO22+, we measured the Eu3+ concentration of ESF–1–Eu before and after immersion in the UO22+ solution (1.5 μM) by EDS technique. The Eu3+ concentration of ESF–1–Eu was 0.85 wt % before immersion of UO22+ solution (Fig. S12). In contrast, the Eu3+ concentration in ESF–1–Eu decreased drastically after immersion of UO22+ solution (Fig. S16). Furthermore, the uropium peak in EDS spectrum of ESF–1–Eu after immersion of UO22+ solution disappeared. These results also indicated an exchange reaction between Eu3+ of the 1-
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Eucomplex in ESF-1-Eu and UO22+. To demonstrate the exchange reaction between Eu3+ of the 1–Eu complex and other metal ions (Ni2+ and Co2+), we measured the Eu3+ concentration of ESF–1–Eu before and after immersion in the Ni2+ and Co2+ solutions (1.5 μM) by EDS technique. The results showed no significant changes before and after immersion in the Ni2+ and Co2+solutions. This indicates that the Eu3+ of ESF–1–Eu was not exchanged to Ni2+ and Co2+ (Fig. S17). To prove the selective luminescence change of ESF–1–Eu in the presence of a practical sample, we measured various UO22+ concentrations in seawater (standard addition method) using EFS–1–Eu. The luminescence intensity of EFS–1–Eu was plotted against the UO22+ concentration, showing good linearity (Fig. S18). The UO22+ concentrations obtained by EFS–1–Eu were approximately 92–97% of those determined by ICP–MS measurement. These findings suggest that EFS–1–Eu is useful as a chemical probe for detecting UO22+.
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Zhu, B.Z. Tang, Aggregation-induced emission of 1-methyl-1,2,3,4,5-penta-phenylsilole, Chem. Commun. (2001) 1740–1741. [26] Y. Hong, J.W.Y. Lam, B.Z. Tang, Aggregation-induced emission, Chem. Soc. Rev. 40 (2011) 5361–5388. [27] H. Wang, E. Zhao, J.W.Y. Lam, B.Z. Tang, AIE luminogens: emission brightened by aggregation, Mater. Today 18 (2015) 365-337. [28] J. Wen, L. Dong, S. Hu, W. Li, S. Li, X.L. Wang, Fluorogenic thorium sensors based on 2,6pyridinedicarboxylic acid-substituted tetraphenylethenes with aggregation-induced emission characteristics, Chem. Asian J. 11 (2016) 49–53. [29] T. Sanji, M. Nakamura, M. Tanaka, Fluorescence ‘turn-on’ detection of Cu2+ ions with aggregation-induced emission-active tetraphenylethene based on click chemistry, Tetrahedron Lett. 52 (2011) 3283–3286. [30] S.H. Jung, J. Jeon, H. Kim, J. Jaworski, J.H. Jung, Chiral arrangement of achiral Au nanoparticles by supramolecular assembly of helical nanofiber templates, J. Am. Chem. Soc. 136 (2014) 6446–6452. [31] G. Mun, S.H. Jung, A. Ahn, S.S. Lee, M.Y. Choi, D.H. Kim, J.Y. Kim, J.H. Jung, Fluorescence imaging for Fe3+ in arabidopsis by using simple naphthalene-based ligands, RSC Adv. 6 (2016) 53912. [32] G.L. Long, J.D. Winefordner, Limit of detection a closer look at the IUPAC definition, Anal. Chem. 55 (1983) 712–724. [33] S.H. Jung, K.Y. Kwon, J.H. Jung, A turn-on fluorogenic Zn(II) chemoprobe based on a terpyridine derivative with aggregation-induced emission (AIE) effects through nanofiber aggregation into spherical aggregates, Chem. Commun. 51 (2015) 952–955. [34] S.H. Jung, K.Y. Kim, J.H. Lee, C.J. Moon, N.S. Han, S.J. Park, D. Kang, J.K. Song, S.S. Lee, M.Y. Choi, J. Jaworski, J.H. Jung, Self-assembled Tb3+ complex probe for quantitative analysis of ATP during its enzymatic hydrolysis via time-resolved luminescence in vitro and in civo, ACS Appl. Mater. Interfaces 9 (2017) 722–729. [35] J.C. Bunzli, C. Piguet, Taking advantage of luminescent lanthanide ions, Chem. Soc. Rev. 34 (2005) 1048–1077. [36] W.D. Horrocks, D.R. Sudnick, Lanthanide ion luminescence probes of the structure of biological macromolecules, Acc. Chem. Res. 14 (1981) 384–392. [38] K.P. Zhuravlev, V.I. Tsaryuk, A.V. Vologzhanina, P.P. Gawryszewska, V.A. Kudryashova, Z.S. Klemenkova, Crystal structures of new lanthanide hydroxybenzoates and different roles of LMCT state in the excitation energy transfer to Eu3+ ions, ChemistrySelect 1 (2016) 3428–3437.
4. Conclusion A simple, sensitive, selective fluorescence turn-off method for detecting UO22+ in aqueous solutions was developed based on a TBTA–Eu3+ complex. Ligand 1 aggregates exhibited a nanofiber structure with diameters of approximately 50–100 nm, which showed strong blue emission with an AIE effect. Interestingly, in the presence of UO22+, this morphology became spheres, with diameters of approximately 30–50 nm. The strong luminescence intensity of 1–Eu was largely quenched upon adding UO22+ owing to an exchange reaction. The nanostructured 1–Eu complex detected UO22+ with a linear range of 1–30 ppm UO22+ and a LOD of 2.17 ppm at pH 7.4. By contrast, the luminescence quenching did not significantly change when other metal ions were added. Then, we fabricated a large film embedded 1–Eu via electrospinning, which also exhibited a selective quenching effect in the presence of UO22+; in addition, its LOD (0.292 ppm) for UO22+ was lower than that of 1–Eu in bulk solutions, which can be attributed to its wider surface area. Furthermore, ESF-1-Eu exhibited good linearity between the luminescence intensity and UO22+ concentration in seawater samples. All these results demonstrate that the proposed method has potential usefulness for detecting uranyl contamination related to the nuclear industry, wastewater treatment, and biological systems. Acknowledgments This work was supported by a grant from NRF (2017R1A4A1014595 and 2018R1A2B2003637 for J. H. Jung and 2017M2B2A9A02049940 for M. Y. Choi). In addition, this work was partially supported by a grant from the Next-Generation BioGreen 21 Program (SSAC, grant no. PJ013186052019), Rural development Administration, Korea. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.05.106. References [1] M. Luo, B. Hu, X. Zhang, D. Peng, H. Chen, L. Zhang, Y. Huan, Extractive electrospray ionization mass spectrometry for sensitive detection of uranyl species in natural water samples, Anal. Chem. 82 (2010) 282–289. [2] A.M. Macfarlane, The overlooked back end of the nuclear fuel cycle, Science 333 (2011) 1225–1226. [3] D.J. Hill, Nuclear energy for the future, Nat. Mater. 7 (2008) 680–682. [4] R. Gwak, H. Kim, S.M. Yoo, S.Y. Lee, G.J. Lee, M.K. Lee, C.K. Rhee, T. Kang, B. Kim, Precisely determining ultralow level UO22+ in natural water with plasmonic nanowire interstice sensor, Sci. Rep. 6 (2016) 19646. [5] L. Newsome, K. Morris, J. Lloyd, The biogeochemistry and bioremediation of uranium and other priority radionuclides, Chem. Geol. 363 (2014) 164–184. [6] E.J. Bouwer, J.W. McKlveen, W.J. McDowell, Solvent extraction-liquid scintillation method for assay of uranium and thorium in phosphate-containing material, Nucl.
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Terpyridine-based complex nanofibers with Eu3+ as a highly selective chemical probes for UO22+
T
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Seonae Leea, 1, Ka Young Kima, 1, Na Young Lima, Jin Hwan Junga, Ji Ha Leeb, , ⁎ ⁎ Myong Yong Choia, , Jong Hwa Junga, a b
Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju, 52828, Republic of Korea Department of Chemistry and Biochemistry, The University of Kitakyushu, Hibikino, Kitakyushu, 808-0135, Japan
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Europium(III) complex Uranyl ion sensor Fluorescence probe Benzenetricarboxyamide Electrospun film
Uranyl is a radioactive, toxic pollutant commonly found in the waste remaining after nuclear fuel reprocessing, and it poses several types of risks to human health; therefore, developing absorbents and chemical probes for this compound is crucial to overcoming these issues. This study examined the sensing abilities of terpyridine-appended benzenetricarboxyamide (T-BTA) as a chromogenic probe for detecting uranyl ions (UO22+). The complex with Eu3+ (1–Eu) spontaneously formed nanostructured fibers in H2O owing to the triamide groups of T-BTA, which induced intermolecular hydrogen-bonding interactions. The strong blue emission of these nanofibers in H2O was quenched upon adding UO22+ but not upon adding any other metal ion. This high selectivity was probably because of the interactions between the nitrigen atoms of the terpyridine moieties of 1 and UO22+. Furthermore, the 1–Eu nanofibers assumed spherical morphologies when UO22+ was added. To develop a convenient UO22+ sensor, an electrospun film incorporating 1–Eu (ESF–1–Eu) was manufactured, and it exhibited high selectivity for UO22+ over a variety of rival metal ions. The plot for luminescence change of ESF–1–Eu vs UO22+ concentrations in seawater samples showed a good linearty. Thus, the ESF–1–Eu shows potential as a useful sensor for detecting and removing UO22+ in H2O.
1. Introduction Uranium is a natural radionuclide widely used to produce nuclear energy and nuclear weapons [1–3]. It has various oxidation states
(namely, +2, +3, +4, +5, and +6) but exists mostly in the hexavalent form uranium(VI)). In nature, uranium generally combines with oxygen to form uranyl ions (UO22+), which pose risks to both human health and the environment [4]. Thus, sensing UO22+ in aqueous
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Corresponding authors. E-mail addresses: [email protected] (J.H. Lee), [email protected] (M.Y. Choi), [email protected] (J.H. Jung). 1 Seonae Lee and Ka Young Kim contributed equally. https://doi.org/10.1016/j.jhazmat.2019.05.106 Received 14 November 2018; Received in revised form 22 May 2019; Accepted 30 May 2019 Available online 31 May 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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solutions is of great importance in the medical and environmental fields [5–7]. Various instrumental methods have been used for this purpose, including atomic absorption spectroscopy (AAS) [8], inductively coupled plasma–mass spectroscopy (ICP–MS) [9], and surface-enhanced Raman spectroscopy (SERS) [10]. However, although these spectroscopic methods are highly selective for and sensitive to UO22+, most of them are expensive, time-consuming, and destructive [11,12]. Recently, several organic and inorganic substances have been used to fabricate chemical sensors for detecting UO22+ based on various sensing techniques and principles [13–20]. In theory, inorganic-based sensing materials, including pyrophosphate and phosphate salts [21], have low stability and selectivity because they tend to crystallize below room temperature, and other metal ions can easily interfere with their sensing ability. By contrast, organic-based fluorescence sensing is an easy, rapid, highly selective, and low-cost method for detecting metal ions [22]. Nevertheless, only a few studies have been reported on the detection of UO22+ using chromogenic probes. For example, X. Wang’s research group used tetraphenylethene-modified 2-(4,5-dihydrothiazol2-yl)phenol as a fluorescence “turn-off” sensing agent for detecting UO22+ [23]. In addition, X. Chen et al. investigated a chemical probe called 4-pethoxycarboxyl salicylaldehyde azine (PCSA), which exhibited the aggregation-induced emission (AIE) effect and showed excellent sensitivity to not only UO22+ [24] but also pH. However, while PCSA should have worked at a pH of 10.3, the uranyl ions could precipitate as UO2(OH)2 at this pH owing to the low constant of precipitation (1.22 × 10–22), which could lead to inaccurate results. Therefore, the development of highly selective, sensitive chemical probes for UO22+ that are also applicable over a wide pH range remains challenging. Also, L. Charbonniere et al. reported a new method for through the utilization of the unique fluorescence energy transfer process to Eu3+, which was the first example of a “turn-on” europium (III) emission process with a small, water-soluble lanthanide complex triggered by uranyl-(VI) ions [15]. Furthermore, using chemical probes that exhibit the AIE effect [25–29] may represent an efficient method for colorimetrically detecting UO22+ owing to their unique photophysical properties. However, our concept is based on the AIE effect of terpyridine-appended benzenetricarboxyamide (T-BTA) complex with Eu3+ as a selective chemoprobe for UO22+ in aqueous solution, which was different from the reported methods previously in the literatures [15]. In this study, the selective detection of UO22+ was used by exchange reaction between complex Eu3+ with blue emission and UO22+. This is different from the fluorescent energy transfer they reported [15]. It is because the formation of the nanostructure by the intermolecular interaction such as the intermolecular hydrogen-bonds and π-π stacking. In addition, one of the important point of our system is that a portable kit for UO22+ detection is available. The film type of the complex with Eu3+ can be easily prepared by electro spin. In this study, we report a novel nanostructured terpyridine-appended benzenetricarboxyamide (T-BTA) complex with Eu3+ (hereafter referred to as 1–Eu) as a fluorescence turn-off sensor for UO22+ because of its AIE characteristics in aqueous media. Furthermore, we demonstrate that an electrospun film incorporating 1–Eu (ESF–1–Eu) is a convenient, efficient UO22+-detecting device.
and an acceleration voltage of 0.1–30 kV. Elemental analysis was performed with a Perkin Elmer 2400 series II Fourier transform infrared (FTIR) spectrophotometer, and the presence of synthetic products was confirmed with a Shimadzu FTIR 8400S spectrophotometer. A JEOL JMS-700 mass spectrometer was used to measure the mass spectra. 2.2. Synthesis of compounds 1 and 2 Compounds 1 and 2 were prepared according to a method previously reported in the literature [30], which is described together with the characterization of 1 and 2 in the supplementary information. 2.3. Preparation of 1–Eu To prepare the 1–Eu complex, compound 1 (17.2 mg, 0.016 mmol) was dissolved in tetrahydrofuran (THF)/water (1:1 v/v%) Then, Eu (NO3)3 (5.4 mg, 0.016 mmol, 1.0 equiv.) was added to the compound 1 solution. The 1–Eu complex was characterized by UV–Vis, fluorescence spectroscopy, and elemental analysis (Figs. 1 and S1). 2.4. Preparation of electrospun ESF–1–Eu films 1–Eu was dissolved homogeneously in dimethylformamide (DMF) and mixed with polymethyl methacrylate (PMMA) (MW: 350 000 g mol−1) for several hours at room temperature. The resulting mixture contained 0.5 wt% 1–Eu and 5.0 wt% PMMA. To electrospin the films, the homogeneously mixed solution was loaded into a syringe (5 mL) that was straight fixed inside a KD Scientific 200 syringe pump, and an electrode was linked to the tip of the syringe needle with a working distance of 15 cm. Electrospinning was performed at 25 °C with a flow rate of 16.7 mL min–1 and voltage of 15 kV [31]. 2.5. UO22+ detection methods using ESF–1–Eu After electrospinning the ESF–1–Eu film and trimming it to dimensions of 2 × 2 cm2, the materials were immersed slowly into separate vials containing aqueous UO22+ solutions with concentrations of 0.5, 1.0, 1.5, 2.0, and 2.5 μM at pH 7.4. To confirm the films’ selectivity for UO22+, they were immersed into vials containing 1.5 μM aqueous metal solutions at pH 7.4, including Na+, Cs+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+, Cu2+, Fe3+, Zn2+, La3+, Tb3+ and UO22+, for 30 min. After removing the films from their respective solutions, the films were dried in air for 24 h. In addition, the luminescence intensity of the prepared samples was measured by solid-state luminescence spectroscopy. 2.6. Measurement of the association constant and detection limit The supplementary information describes these methods in detail [32]. 3. Results and discussion 3.1. Ability of 1–Eu to detect UO22+
2. Experimental section To prepare the 1-Eu complex in THF/water (1:1, v/v%), Eu(NO3)3 was added to the compound 1 solution, and the solution was characterized by UV-vis and fluorescence spectroscopy (Figs. S1 and 1). A detailed examination of the UV–Vis spectrum of 1 upon titration of 0-10 equiv. of Eu(NO3)3, the absorption band for the π → π* transition of the terpyridine moiety appeared at 280 nm with a shoulder at 325 nm (Fig. S1A). The broad luminescence spectrum showed a strong emission at 410 nm, corresponding to the terpyridine moieties of 1–Eu (Fig. 1). This strong blue emission originated from the tilted π–π stacking between the uncomplexed terpyridine and the uncomplexed terpyridine moieties
2.1. Reagents and instruments All reagents were purchased from TCI, Sigma-Aldrich, or Alfa Aesar. All solvents were provided by Sigma-Aldrich or Samchun Pure Chemical Co., Ltd. and used with additional purification. The 1H and 13C spectra were recorded using a Bruker DRX 300 spectrophotometer and the absorption spectrum of the solution with an ultraviolet–visible (UV–Vis) absorption spectrophotometer (U-4100). Images were recorded using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7610 F) with an emission current of 100 nA 2
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Fig. 1. (A) Luminescence spectra of the terpyridine-appended benzenetricarboxyamide (1) (8.0 × 10−5 M) without and with various equivalents (0–10) of Eu3+ in THF/water (1:1, v/v%, ex =325 nm). (B) Luminescence intensity of 1 at 410 nm as a function of the Eu3+ amount.
determine its applicability as a chemical probe for UO22+ in a THF/ water solution. These findings suggest that excess UO22+ binds to the free terpyridine moiety of 1-Eu after exchange reaction between Eu3+ of 1-Eu and UO22+, which continually resulted in the quenching effect. We monitored its value at 410 nm to quantify UO22+. The overall spectrum of 1–Eu decreased gradually with increasing UO22+ and was virtually quenched when 1 equiv. UO22+ was added to the 1–Eu solution. In addition, the plot of the UO22+ concentration vs. luminescence intensity exhibited good linearity (Fig. 2B). This quenching effect was due to the dissociation of Eu3+ from the 1–Eu complex caused by the addition of UO22+; then, 1 re-complexed with UO22+ due to the exchange reaction (Scheme 1). In the previous study [15], they claimed the fluorescence energy transfer to Eu3+ was main force to detection of UO22+. Fortunately, the pyridine-based ligand showed two new excitation bands at 430 nm and 320 nm upon addition of UO22+. These two bands originated from the Laporte forbidden O-to-U ligand-tometal charge transfer (LMCT) transition. Exciting to this LMCT band led to Eu3+ emission from the 5D0 excited state [38]. Although the previous study looks similar to ours, there was no luminescence band of the range of 480–540 nm in our system. This is because UO22+ is not bounded to Eu3+ directly but to terpyridine. In contrast, when other metal ions such Na+, Cs+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+, Cu2+, Fe2+, Fe3+, Zn2+, La3+ and Tb3+ were added, no remarkable changes in the luminescence spectra were observed at 410 nm (Figs. 3 and S4), suggesting that 1–Eu selectively bound to UO22+ in THF/water. This high selectivity was probably due to the interactions between the nitrogen atoms of the terpyridine moieties of 1 and the uranyl ions, according to the hard and soft acids and bases (HSAB) theory [15]. Furthermore, the luminescence intensity was plotted as a function of the UO22+ concentration in the presence of 1.0–5.0 equiv. of UO22+, and the slope of the plot was smaller than that of 0–1.0 equiv. of UO22+ (Figs. 2B and S5). These findings suggest that excess UO22+ binds to the free terpyridine moiety of 1–Eu after exchange reaction between Eu3+ of 1–Eu and UO22+, which continually resulted in the quenching effect. When ligand 1 (80 μM) was dissolved in a 1:1 (v/v%) THF/water mixture, the 1–Eu solution was not transparent both with and without UO22+ (1 equiv.), and the aggregated morphologies of 1–Eu in both cases were observed by SEM. Fig. 4A shows that 1–Eu without UO22+ consists of fibrillar structures with widths of 50–100 nm and a micrometer-level lengths. These fibrillar structures, or nanofibers, could be ascribed to the self-aggregation of 1–Eu via intermolecular interactions such as hydrogen bonding and π–π stacking; these nanofibers produced blue emission owing to the AIE effect. In contrast, 1–Eu aggregates with UO22+ exhibited spherical morphologies with a diameter of approximately 30–50 nm (Fig. 4B). This morphological change was due to the
of 1. Furthermore, AIE effect of 1 was confirmed in the presence of Eu3+ (1.0 equiv.) in the different composition of H2O and THF. The fluorescence intensity of 1 in the presence of Eu3+ was no observed in THF. In contrast, the fluorescence enhancement of 1 with Eu3+ (1.0 equiv.) was observed at ca. 410 nm in a mixed H2O and THF (1:1 v/v) which was a typical evidence for AIE effect (Fig. S2) [33]. In addition, temperature-dependent fluorescence spectra of 1 in the presence of Eu3+ was observed in a mixed H2O and THF (1:1 v/v). The strong fluorescence intensity of 1 with Eu3+ decreased at high temperature (80 °C), indicating that the self-assembled 1-Eu was changed to monomer 1 (Fig. S3). The nanofiber structure of 1-Eu in SEM observation was also indirect evidence for AIE effect. The plateaus of the absorption and emission spectra observed for 1 equiv. of Eu(NO3)3 suggests that Eu3+ and 1 complexed (i.e., 1–Eu) at a 1:1 stoichiometry (Fig. S1B and Scheme 1), indicating that the two terpyridine moieties of 1 did not form the complex with Eu3+. Furthermore, typical emission bands of the Eu3+ complex at 579, 593, 616, 650, and 696 nm did not appear because three H2O molecules were coordinated to Eu3+ [34,35]. It is known that interaction with water (both in the inner and outer coordination sphere of the Ln3+ ion) leads to a severe quenching of the metal luminescence via OeH vibrations. In our previous study [34], the complex 1 with Tb3+ showed a strong emission at the range of 550–700 nm in methanol whereas the complex 1 with Tb3+ showed non-emission in methanol and water (1:99 v/v). To demonstrate quenching effect of the complex 1 with Tb3+ in a mixed methanol and water, the coordination number of H2O in the complex 1 with Tb3+ was measured by Horrocks method [36]. The emission lifetime (τ) for H2O was calculated to be 0.16 ms while that of D2O was 0.21 ms. These findings indicate that three molecules would coordinate to Tb3+. The coordination of water molecules caused the quenching effect in the range of 550˜700 nm. Thus, we believe that the quenching effect of 1Eu in the range of 570˜700 nm was due to bound to water molecules as the complex 1 with Tb3+. Based on UV–vis and luminescence observations as well as reported results previously in the literatures [34], the proposed structure of the complex 1 with Eu3+ in THF/water (1:1, v/v%) was represented (Scheme 1). Since Eu3+ ion has nine coordination sites available, one terpyridine moiety, three water molecules, and three nitrate anions would coordinate to a single Eu3+ as shown in Scheme 1. On the other hand, two terpyridine moieties of 1 were empty in the absence of metal ions, which caused the AIE effect based blue emission owing to the tilted π–π stacking. 1–Eu exhibited strong emission associated with the photoelectron transfer from terpyridine to Eu3+ in THF/water. However, upon adding guest ions, specifically, UO22+, quenching was observed due to the exchange reaction, i.e., a “turn-off” response (Fig. 2). Thus, the change in the luminescence of 1–Eu after adding UO22+ was evaluated to
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Scheme 1. Luminescence mechanism of the terpyridine-appended benzenetricarboxyamide (T-BTA; 1) complex with Eu3+ (1–Eu) for sensing UO22+; (A) compound 1, (B) 1–Eu complexation between 1 and Eu3+, and (C) re-complexation between 1 and UO22+ via the exchange reaction.
Fig. 2. (A) Luminescence spectra of the terpyridine-appended benzenetricarboxyamide–Eu3+ complex (1–Eu) (8.0 × 10−5 M) without and with various equivalents (0–10) of UO22+ in THF/water (1:1, v/v%, ex =325 nm). (B) Luminescence intensity changes of 1–Eu as a function of the UO22+ amount. 4
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Fig. 3. (A) Photographs, (B) luminescence spectra, and (C) luminescence intensity changes at 410 nm of the terpyridine-appended benzenetricarboxyamide–Eu3+ complex (1–Eu) (8.0 × 10–5 M) with a variety of metal ions (1 equiv.) in THF/water (1:1, v/v%, ex =325 nm).
S9A). To confirm the exchange reaction between Eu3+ of 1–Eu and UO22+, Ka between 1 and Eu3+ was also determined under the same conditions as those used for UO22+. The association constant for 1 with Eu3+ was 2.4 × 103 M−1 (Fig. S9B), which was 1.14-fold smaller than that of UO22+. These findings suggest that Eu3+ dissociated from 1–Eu upon adding UO22+, as shown in Scheme 1, due to the relatively weak complexation. Furthermore, the luminescence lifetimes for 1–Eu without and with UO22+ were 0.964 ns and 0.657 ns (Fig. S10), respectively. The luminescence lifetime for 1–Eu decreased in the presence of UO22+ owing to the quenching effect between the terpyridine and the terpyridine moieties of 1 with the complexation of 1-UO22+ via an exchange reaction. We observed the excitation spectrum of 1-Eu before and after addition of UO22+ ion. As a result, the absorption at 295 nm was decreased. This is because the formation of 1 + UO22+ complex with stronger association constant (Fig. S11).
geometrical change of ligand 1 when complexing with UO22+. The luminescence spectral changes of 1–Eu in HEPES buffer (0.01 M, pH 7.4) were investigated in a binary system (Fig. S6). As expected, significant intensity changes were observed upon adding UO22+ (1 equiv.) with other metal ions, and the blue emission was quenched. To confirm the quantitative response of 1–Eu to UO22+, its luminescence intensity was plotted as a function of the increasing UO22+ concentration in HEPES buffer at pH 7.4. Fig. S7 shows that the luminescence intensity of 1–Eu decreased linearly with the increasing UO22+ concentrations (from 0 to 1 equiv.), confirming that UO22+ interacted with 1–Eu. In addition, the nanostructured 1–Eu exhibited excellent sensitivity to UO22+, as evidenced by a limit of detection (LOD) of 2.17 ppm, which is sufficiently sensitive to detect UO22+. To determine the stoichiometric ratio between 1–Eu and UO22+, a Job plot was used. Maximum absorption was attained at a molar ratio (UO22+/ (1–Eu + UO22+)) of approximately 0.5, indicating a 1:1 stoichiometric ratio between 1–Eu and UO22+ (Fig. S8). Moreover, the association constant (Ka) between 1 and UO22+ calculated for this stoichiometry based on the Benesi–Hildebrand method was 2.75 × 103 M−1 (Fig.
3.2. Detection ability of ESF–1–Eu To make a convenient chemical probe, a 1–Eu-incorporated PMMA
Fig. 4. SEM images of the morphologies of 1–Eu (80 μM) aggregates (A) without and (B) with UO22+ (1 equiv.) in a THF/water mixture (1:1, v/v%). 5
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Fig. 5. (A) Photograph and (B) SEM image of the electrospun film (10 cm × 10 cm) incorporating the 1–Eu complex.
Fig. 6. Photographs of ion selective for UO22+ in (A) single and (B) binary systems. (C) Histogram of the luminescence intensity of ESF–1–Eu in (C) single and (D) binary systems.
film, i.e., ESF–1–Eu, was prepared via electrospinning. SEM images of the indiscriminately oriented electrospun PMMA and 1–Eu ensemble in ESF–1–Eu (Fig. 5) revealed fibers with a relatively uniform diameter of ca. 100 nm. Furthermore, energy dispersive spectroscopy mapping confirmed that ESF–1–Eu contained evenly distributed carbon, oxygen, nitrogen, and europium (Fig. S12), indicating that the 1–Eu complex was regularly embedded in the fibers. To evaluate the detection ability of ESF–1–Eu, it was first immersed in the cation solution of interest and then dried in air, and the luminescence intensity changes were analyzed by solid-state luminescence spectroscopy. Similar to the behavior of 1–Eu in THF/water (1:1, v/v %), when ESF–1–Eu was immersed in solutions containing increasing UO22+ concentrations, its strong blue emission was quenched within a few minutes (Fig. S13). Thus, ESF–1–Eu became increasingly less emissive with the gradually increasing UO22+ concentration. By contrast, immersion in other metal ion solutions did not significantly affect the luminescence spectrum of ESF–1–Eu (Fig. 6C). These results indicate that ESF–1–Eu can detect UO22+ similarly to litmus paper for measuring pH The calculated LOD of ESF–1–Eu was 0.292 ppm, which is lower than that of the nanostructured 1–Eu in a bulk solution (Fig. S14), and it exhibited a higher reactivity due to its wider surface area. To further assess the usefulness of ESF–1–Eu as an ion-selective chemical sensor for UO22+, its luminescence changes in aqueous solutions were investigated in binary systems. ESF–1–Eu was also immersed in a solution containing various biologically and environmentally important metal ions (Fig. 6B and D). In addition to UO22+, various metal ions including Na+, Cs+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+, Cu2+, Fe3+,
Zn2+, La3+, and Tb3+ were tested. The strong blue emission of ESF–1–Eu was again quenched when the film was immersed in thesolution. As expected, the luminescence quenching observed in the solution containing UO22+ due to the 1–UO22+ complex in ESF–1–Eu remained unaffected in the presence of these metal ions, indicating that ESF-1 is a selective chemical sensor for UO22+, even in a mixture of these metal ions. However, with some other metal ions, the quenching effect of ESF–1–Eu was smaller than that in the abovementioned binary systems. Specifically, when ESF–1–Eu was immersed in a UO22+ solution containing Ni2+, this ion interfered slightly in the detection process of UO22+, thus decreasing the quenching effect. Biological and environmental chromogenic sensors must be both effective and stable over a wide pH range. Therefore, the effect of pH on ESF–1–Eu was evaluated in the absence of UO22+ (Fig. S15), and no emission change was observed in the pH range of 3–11. Conversely, the luminescence significantly changed upon adding UO22+ in the same pH range, indicating that ESF–1–Eu can be used as a chemical sensor for UO22+ at pH > 3. To demonstrate the exchange reaction between Eu3+ of the 1–Eu complex and UO22+, we measured the Eu3+ concentration of ESF–1–Eu before and after immersion in the UO22+ solution (1.5 μM) by EDS technique. The Eu3+ concentration of ESF–1–Eu was 0.85 wt % before immersion of UO22+ solution (Fig. S12). In contrast, the Eu3+ concentration in ESF–1–Eu decreased drastically after immersion of UO22+ solution (Fig. S16). Furthermore, the uropium peak in EDS spectrum of ESF–1–Eu after immersion of UO22+ solution disappeared. These results also indicated an exchange reaction between Eu3+ of the 1-
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Eucomplex in ESF-1-Eu and UO22+. To demonstrate the exchange reaction between Eu3+ of the 1–Eu complex and other metal ions (Ni2+ and Co2+), we measured the Eu3+ concentration of ESF–1–Eu before and after immersion in the Ni2+ and Co2+ solutions (1.5 μM) by EDS technique. The results showed no significant changes before and after immersion in the Ni2+ and Co2+solutions. This indicates that the Eu3+ of ESF–1–Eu was not exchanged to Ni2+ and Co2+ (Fig. S17). To prove the selective luminescence change of ESF–1–Eu in the presence of a practical sample, we measured various UO22+ concentrations in seawater (standard addition method) using EFS–1–Eu. The luminescence intensity of EFS–1–Eu was plotted against the UO22+ concentration, showing good linearity (Fig. S18). The UO22+ concentrations obtained by EFS–1–Eu were approximately 92–97% of those determined by ICP–MS measurement. These findings suggest that EFS–1–Eu is useful as a chemical probe for detecting UO22+.
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4. Conclusion A simple, sensitive, selective fluorescence turn-off method for detecting UO22+ in aqueous solutions was developed based on a TBTA–Eu3+ complex. Ligand 1 aggregates exhibited a nanofiber structure with diameters of approximately 50–100 nm, which showed strong blue emission with an AIE effect. Interestingly, in the presence of UO22+, this morphology became spheres, with diameters of approximately 30–50 nm. The strong luminescence intensity of 1–Eu was largely quenched upon adding UO22+ owing to an exchange reaction. The nanostructured 1–Eu complex detected UO22+ with a linear range of 1–30 ppm UO22+ and a LOD of 2.17 ppm at pH 7.4. By contrast, the luminescence quenching did not significantly change when other metal ions were added. Then, we fabricated a large film embedded 1–Eu via electrospinning, which also exhibited a selective quenching effect in the presence of UO22+; in addition, its LOD (0.292 ppm) for UO22+ was lower than that of 1–Eu in bulk solutions, which can be attributed to its wider surface area. Furthermore, ESF-1-Eu exhibited good linearity between the luminescence intensity and UO22+ concentration in seawater samples. All these results demonstrate that the proposed method has potential usefulness for detecting uranyl contamination related to the nuclear industry, wastewater treatment, and biological systems. Acknowledgments This work was supported by a grant from NRF (2017R1A4A1014595 and 2018R1A2B2003637 for J. H. Jung and 2017M2B2A9A02049940 for M. Y. Choi). In addition, this work was partially supported by a grant from the Next-Generation BioGreen 21 Program (SSAC, grant no. PJ013186052019), Rural development Administration, Korea. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.05.106. References [1] M. Luo, B. Hu, X. Zhang, D. Peng, H. Chen, L. Zhang, Y. Huan, Extractive electrospray ionization mass spectrometry for sensitive detection of uranyl species in natural water samples, Anal. Chem. 82 (2010) 282–289. [2] A.M. Macfarlane, The overlooked back end of the nuclear fuel cycle, Science 333 (2011) 1225–1226. [3] D.J. Hill, Nuclear energy for the future, Nat. Mater. 7 (2008) 680–682. [4] R. Gwak, H. Kim, S.M. Yoo, S.Y. Lee, G.J. Lee, M.K. Lee, C.K. Rhee, T. Kang, B. Kim, Precisely determining ultralow level UO22+ in natural water with plasmonic nanowire interstice sensor, Sci. Rep. 6 (2016) 19646. [5] L. Newsome, K. Morris, J. Lloyd, The biogeochemistry and bioremediation of uranium and other priority radionuclides, Chem. Geol. 363 (2014) 164–184. [6] E.J. Bouwer, J.W. McKlveen, W.J. McDowell, Solvent extraction-liquid scintillation method for assay of uranium and thorium in phosphate-containing material, Nucl.
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