Analytica Chimica Acta 847 (2014) 55–60

Contents lists available at ScienceDirect

Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Trace analysis of uranyl ion (UO22+) in aqueous solution by fluorescence turn-on detection via aggregation induced emission enhancement effect Xiaotong Chen *, Linfeng He, Yang Wang, Bing Liu, Yaping Tang Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

 A fluorescence turn-on method for trace amounts of UO22+ has been developed.  The method was based on aggregation induced emission enhancement (AIEE) mechanism.  The AIEE was linearly related to 1–25 ppb UO22+, with LOD of 0.2 ppb.  The method could be utilized in quantifying UO22+ in fuel processing wastewaters.

A R T I C L E I N F O

A B S T R A C T

Article history: Received 12 May 2014 Received in revised form 19 June 2014 Accepted 15 July 2014 Available online 4 August 2014

A sensitive fluorescence turn-on method for trace amounts of uranyl ion (UO22+) in solution has been developed in this study, based on aggregation induced emission enhancement (AIEE) characteristics of 4pethoxycarboxyl salicylaldehyde azine (PCSA) induced by complex interaction between UO22+ and PCSA. Under optimized conditions, a fluorescence enhancement at 540 nm could be observed, which was linearly related to the concentration of UO22+ in the range of 1–25 ppb (part per billion). Analytical data showed that a detection limit of 0.2 ppb was achieved with the relative standard deviation (R.S.D.) 1.3% (n = 5). The proposed method was successfully utilized in quantifying UO22+ in fuel processing wastewaters. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Uranyl ion detection Schiff base Aggregation induced emission enhancement Fluorescence turn-on detection

1. Introduction Uranium is an important nuclear material with isotopic abundance of 99.3% U238 and 0.7% U235 in nature and is a toxic and radioactive metal [1]. It is of great commercial interest because of its use in the production of nuclear energy. Unfortunately,

* Corresponding author. Tel.: +86 10 89796097; fax: 86 10 69771464. E-mail addresses: [email protected] (X. Chen), [email protected] (L. He), [email protected] (Y. Wang), [email protected] (B. Liu), [email protected] (Y. Tang). http://dx.doi.org/10.1016/j.aca.2014.07.016 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.

human activities involving mining and milling, nuclear fuel fabrication, power reactors operation and waste reprocessing have caused widespread environmental contamination [2–5]. Therefore, the analysis of uranium is of great importance in every step of nuclear industry. Uranium naturally presents various oxidation states (namely +2, +3, +4, +5 and +6), but uranium appears mostly in its hexavalent form. Usually in nature, uranium is associated with oxygen, forming the uranyl ion (UO22+). Up to now, a number of instrumental methods are proposed to determine UO22+, such as spectrophotometry [6,7], surface enhanced Raman spectroscopy [8,9], atomic emission spectrometry [10], inductively coupled plasma-mass spectrometry

56

X. Chen et al. / Analytica Chimica Acta 847 (2014) 55–60

[11] and fluorescence spectrometry [12]. Since UO22+ itself has the capability of fluorescent luminescence in 450–600 nm [13], fluorescence analysis of UO22+ concentration is a traditional method, with the advantages of easy operation, simple apparatus and small volume [9,14–16]. Also, the compact fluorescence instrument allows on-site analysis and can be placed in the glove box such that radioactive harm to analysts can be reduced. However, the UO22+ fluorescence is susceptible to environmental impact. Therefore, the development of fluorescence sensor of UO22+, which might greatly increase the sensitivity and selectivity of detecting method, attracts much attentions. Compared with inorganic UO22+ sensors, organic ones theoretically possess better selectivity and higher stability [17–21], because inorganic ones, such as phosphate/pyrophosphate salts, usually tend to crystallize below room temperature and ease to be interfered by other metal ions [22–24]. Notably, a difficulty in designing sensors for UO22+ is that the fluorescence intensity of sensors might be quenched by solubilized UO22+, due to the energy- or electrontransfer between UO22+ and ligands [13], which would seriously quench the fluorescence and limit effective detection. Therefore, to the best of our knowledge, compared with other metal ions, much fewer examples of fluorescent chemosensors for the evaluation of UO22+ have been reported so far [25–27]. In recent years, the phenomenon of aggregation induced emission enhancement (AIEE) has been discovered with gradually expanding applications, molecules of which would emit efficiently in aggregate state [28–30]. Our group has recently reported a series of salicylaldehyde azine (SA) derivatives linked by N N single bond with AIEE characteristics [31]. Owing to their facile preparations and efficient AIEE fluorescence at aggregated/solid state which would mitigate the problem of fluorescence quenching, there is great potential for these compounds applied in UO22+ anaylzing system. In this study, we synthesized a new SA derivative, 4-pethoxycarboxyl salicylaldehyde azine (PCSA), and explored its utility as a UO22+ sensor based on its AIEE characteristic. PCSA was proved to be a highly sensitive and selective fluorescence turn-on sensor for UO22+ detection in aqueous media (Scheme 1). 2. Experimental 2.1. Reagents Unless otherwise noted, all reagents and solvents used in this paper were analytical grade without further purification. UO22+

stock solution (UO2(NO3)2, 500 mg g 1) was purchased from CNNC Beijing Research Institute of Uranium Geology, and diluted to targeting concentrations when needed. Other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. China. Milli-Q water was used throughout the experiment. 1 mM stock solution of PCSA was prepared by dissolving appropriate products in ethanol. 2.2. Synthesis of PCSA Synthesis of PCSA began with the substitution of ethyl 5bromopentanoate of 4-hydroxylbenzaldehyde [32]. Above synthetic routes afforded intermediate products with yields of 23% after purification by flash chromatography (petroleum ether: ethyl acetate = 1:1.5). Intermediate products were then hydrolyzed in the mixed solution of 1 M NaOH and ethanol (1:1, v/v) at room temperature for 4 h, and then 1 M HCl was added to modulate pH to 4.0. After that, the resulting precipitations were filtered and washed by 15.0 mL water for 3 times. After drying in vacuum, the salicylaldehyde derivative with a carboxylic carbon chain at 4-position was obtained, which was then reacted with hydrazine at molar ratio of 2:1 in ethanol at room temperature overnight to precipitate [33,34]. The resulting precipitations were filtered and washed by 15.0 mL cold ethanol/water (1:1, v/v) for 3 times. After drying in vacuum and recrystallization in DMSO/ ethanol (1:1, v/v), PCSA was obtained with high yields of about 80%. Products were characterized by 1H, 13C NMR, ESI-MS and elemental analysis. ESI mass spectrometry for 4-pentoxycarboxyl salicylaldehyde: m/z 251.17 ([M H] ); M calculated 251.10. 1H NMR (DMSO-d6), d (ppm): 1.43 (m, 2H), 1.56 (m, 2H), 1.73 (m, 2H), 2.30 (t, 2H, J = 7.23 Hz), 4.06 (t, 2H, J = 6.54 Hz), 5.19 (s, 1H), 6.73 (m, 2H), 7.61 (d, 1H, J = 6.82 Hz) 9.83 (s, 1H), 12.09 (s, 1H). 13C NMR (DMSO-d6) d (ppm): 24.7, 25.3, 29.3, 34.1, 68.3, 103.2, 107.9, 112.8, 134.2, 163.6, 166.2, 178.8, 194.0. Elemental analysis: C 61.82%, H 6.49%, O 31.69%, calculated for C13H16O5: C 61.90%, H 6.39%, O 31.71%. ESI mass spectrometry for PCSA: m/z 499.18 ([M H] ); M calculated 499.22. 1H NMR (DMSO-d6), d (ppm): 1.43 (m, 4H), 1.56 (m, 4H), 1.73 (m, 4H), 2.30 (t, 4H, J = 7.23 Hz), 4.06 (t, 4H, J = 6.54 Hz), 5.23 (s, 2H), 6.41 (dd, 2H, J1 = 1.78 Hz, J2 = 0.48 Hz), 6.65 (dd, 2H, J1 = 7.64 Hz, J2 = 1.78 Hz), 7.76 (dd, 2H, J1 = 7.64 Hz, J2 = 0.48 Hz), 8.48 (s, 2H), 11.25 (s, 2H). 13C NMR (DMSO-d6) d (ppm): 24.2, 25.7, 29.2, 34.3, 68.4, 103.5, 107.3, 110.8, 133.6, 157.9, 161.5, 163.8, 178.2.

Scheme 1. The design rationale of the fluorescence turn-on detection of UO22+ based on aggregation induced emission enhancement (AIEE) characteristics of PCSA.

X. Chen et al. / Analytica Chimica Acta 847 (2014) 55–60

57

2.3. Analytical procedures For AIEE spectral measurement, 4.95 mL ethanol/water (0–1, v/ v) was added to a 5.0 mL glass tube, and 50 mL stock solution of PCSA was added. After well mixed and left at room temperature for 10 min, 3.0 mL of the solution was transferred out for AIEE spectra measurement. For UO22+ detection, PCSA and UO22+ were subsequently added to a 5.0 mL glass tube, and then diluted to the mark with water. After well mixed and left at room temperature for 30 min, 3.0 mL of each solution was transferred out for fluorescence measurement by excitation/emission at 370/540 nm. 2.4. Apparatus and spectroscopic measurements Absorption spectra were carried on JASCO V-650 UV–vis spectrophotometer. Fluorescence spectra were recorded on JASCO FP-8300 spectrofluorimeter equipped with a xenon discharge lamp, 1 cm quartz cell. All pH measurements were performed with a pHS-3C pH meter (Shanghai Precision & Scientific instrument Co., Ltd., Shanghai, China). Elemental analyses were operated on an Elementar Vario EL (Germany). 1H and 13C NMR spectra were measured using a JOEL JNM-ECA300 spectrometer operated at 300 MHz. ESI-MS spectra were operated on an HP 1100 LC–MS spectrometer. Dynamic light scattering (DLS) experiments were constructed with MIORO-PLUS (50 nm–1500 mm) light scattering instrument. Unless otherwise mentioned, all measurements were operated at room temperature of about 293 K.

Fig. 2. Dynamic lighter scattering (DLS) results. Blue triangles: PCSA (30 mM) at pH 7.2; red dots: PCSA (30 mM) with the addition of 10 ppb UO22+ at pH 10.3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.2. The pH effect on AIEE characteristics of PCSA

The AIEE characteristics of PCSA was studied in water/ethanol (from 0 to 1, v/v) at pH 7.2. The quantum yields for PCSA in solution (’s) and aggregation (’a) states were 0.001 and 0.21, respectively [35]. As shown in Fig. 1, PCSA exhibited obvious AIEE effect after water fraction was above 0.8, which correlated well with the fact that more aggregates were formed in poorer solvents. The formation of fluorescent aggregates of PCSA in poor solvents was further supported by DLS (blue triangles in Fig. 2): (1) no particle (>5 nm) could be detected for PCSA when water fraction was 0 (non-fluorescent); (2) particles of micrometer sizes of PCSA (fluorescent) could be observed in aqueous solution at pH 7.2 with a narrow distribution.

As discussed above, PCSA displayed AIEE fluorescence when aggregated in poor solvent. Notably, the dispersion of PCSA in aqueous solution could be modified as pH value changing due to the protonation–deprotonation process of its groups. Fig. 3 shows the absorption spectra of PCSA at various pH. As shown in the inset, the level-off tail, which are commonly observed in nanoaggregates suspensions [36], obviously demonstrated the formation of aggregation when pH was low. As pH increased, the deprotonation of carboxylic groups occurred, and PCSA2 became well dispersed in solution, with the increasing of absorbance at 330 nm and disappearing of level off tails. If pH further increased, the hydroxyl group began to deprotonate to form PCSA4 with the increasing of 410 nm, and the solution color turned from colorless to yellow. From an analysis of pH titration curves obtained from the absorbance measurements at 330 nm and 404 nm (Fig. S1), acid dissociation constants (pKa) of PCSA were estimated as 8.1 and 10.7 (Scheme 2). Increasing pH led to an abrupt decrease of AIEE fluorescence in the range 9.9–10.3, with a nearly elimination when pH reached 10.4 (Fig. 4). This phenomenon was reasonably attributed to the formation of non-fluorescent monomers of PCSA2 in deprotonation process, in which the negatively charged ions released from fluorescent aggregates.

Fig. 1. Water fraction effect on fluorescence of PCSA. c(PCSA) = 30 mM, pH 7.2. lex = 370 nm, lem = 540 nm.

Fig. 3. Absorbance spectra of PCSA at pH 8.2, 10.2 and 10.9. Inset was enlarged range of level-off tails.

3. Results and discussion 3.1. The AIEE characteristics of PCSA

58

X. Chen et al. / Analytica Chimica Acta 847 (2014) 55–60

Scheme 2. The deprotonation process of PCSA.

3.3. The fluorescence turn-on detection of UO22+ Fluorescence responses of PCSA to UO22+ in aqueous solution were then recorded at lex/lem = 370/540 nm. As demonstrated above, PCSA (30 mM) was deprotonated at carboxylic group at pH 10.3 to form dispersed PCSA2 , showing almost no characteristic fluorescence emission in the absence of UO22+ (the bottom line in Fig. 5). After the addition of UO22+, significant fluorescence enhancement at 540 nm could be observed, which increased over 100-fold with 30 ppb UO22+ (Fig. 5a). Since the analogous peak shape and wavelength for aggregated PCSA (Fig. S2) and the above fluorescence enhancement, this phenomenon could possibly induced by the aggregation of PCSA via coordination of its carboxylate and Schiff-base groups with UO22+, which restricted the intramolecular rotations of PCSA to prevent non-radiative deactivation from fluorescence quenching [20,37–39]. The linear relationship of the fluorescence enhancement upon UO22+ concentration was also shown in Fig. 5b. 3.4. Absorbance response of PCSA to UO22+ To verify this coordination mechanism, the UV–vis absorption spectra of PCSA was recorded. Fig. 6 shows absorption spectra of PCSA (30 mM) upon addition of 0 and 0.01 equiv. of UO22+. In the presence of UO22+, the absorption intensity in the range of 350– 450 nm decreased and blueshifted, with a tail above 500 nm increased, indicating that PCSA is binding with UO22+ through coordination. 3.5. DLS and SEM results Fig. 5. (a) Fluorescence spectra (lex = 370 nm) of PCSA (30 mM at pH 10.3) in the presence of different amounts of UO22+ (from 0 to 30 ppb). (b) Linear relationship of the PCSA (30 mM, pH 10.3) with the addition of different amounts of UO22+. lex = 370 nm, lem = 540 nm.

There are two possible coordination modes for PCSA and UO22+ according to the molecular structure of PCSA. One is the intramolecular coordination of complex units in PCSA (Schiff-base and carboxylate groups) with UO22+, the other is the intermolecular coordination of neighboring molecules with UO22+, which might lead to coordination oligomers or even polymers. To verify the existing of intermolecular coordination modes, DLS measurements were performed. According to DLS results (red dots in Fig. 2),

the PCSA aggregates DLS signal corresponds to size of 1000 nm, and aggregates of 300–1300 nm could also be observed in the presence of 0.01 equiv. of UO22+, which should be accorded with the aggreagtion of the PCSA-UO22+ coordination oligomers or

Fig. 4. Effect of pH on the fluorescence intensity (peaks in fluorescence spectra) of PCSA (30 mM) at various pH. lex = 370 nm, lem = 540 nm.

Fig. 6. Absorption spectra of PCSA (30 mM) upon addition of 0 and 0.01 equiv. of UO22+ at pH 10.3.

X. Chen et al. / Analytica Chimica Acta 847 (2014) 55–60

59

Table 1 Interference of other metal ions. Foreign substances Th(IV) Fe(III) Cr(III) Pb(II) Co(II) Cu(II) Mn(II) Ni(II)

Concentration coexisting (ppb)

< < < < < < <

Trace analysis of uranyl ion (UO2(2+)) in aqueous solution by fluorescence turn-on detection via aggregation induced emission enhancement effect.

A sensitive fluorescence turn-on method for trace amounts of uranyl ion (UO2(2+)) in solution has been developed in this study, based on aggregation i...
2MB Sizes 2 Downloads 3 Views