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• Uranium is a toxic and radioactive nuclide, which is widely applied in civil and military nuclear fields [1, 2]. Human exposure to uranium has reported to result in a series of health problems, such as cancer, renal system ailments and genetic diseases [3, 4]. As the growing utilization of nuclear energy, especially the occurrence of undesirable nuclear accidents, uranium contamination is widely found in natural waters, such as river [5], groundwater [6] and ocean [7]. The pollution of uranium in environment water has become a social concern. Thus, it is of great significance to detect the concentration of uranium in aqueous environment.

  Uranium is usually associated with oxygen, forming various oxidation states (i.e. +2, +3, +4, +5, +6), and uranyl ion (UO₂²⁺) is the most stable chemical form in aqueous environment. Until now, routine methods for the trace level uranyl detection mainly include inductively coupled plasma mass spectrometry (ICP-MS) [8], inductively coupled plasma optical emission spectrometry (ICP-OES) [9], atomic emission spectroscopy (AES) [10] and surface-enhanced Raman scattering (SERS) [11]. Although these techniques are sensitive and reliable, they often require large and expensive equipments, complicated processing procedures, strict operations and relatively long analysis time, which restrict the on-site rapid detection of UO₂²⁺. Fluorescence analysis, with the advantages of easy operation, simple apparatus, excellent sensitivity and real-time monitoring, has been received increasing attention [12]. In recent years, some organic fluorescence sensors for detecting UO₂²⁺ have been reported [13, 14]. However, most of these sensors show low solubility in water and would suffer from suppressed emission or even complete fluorescence quenching in high water fractions, which limited their practical applications.

  Among the fluorescence analysis techniques, fluorescence organic molecules with aggregation-induced emission (AIE) property have been shown to be particularly valuable, as first reported by Zhang et al. [15]. Tetraphenylenthene (TPE) moiety has been reported as an excellent fluorophore for designing of AIE sensors. To date, a variety of TPE-based fluorescent sensors have been developed and utilized, such as for Th⁴⁺ [16], Cu²⁺ [15] and Hg²⁺ [17]. Wen et al. [18] and Lin et al. [19] developed two TPE-based fluorescent sensors, TPE-T and TPE-BSA, respectively. The two sensors monitored UO₂²⁺ with high water fractions, whereas the detection limit could not reach the levels of UO₂²⁺ in natural waters (0.31 µg/L for global rivers [20], and 3.3 µg/L for ocean [7]), approximatively nanomolar level. Therefore, to further improve the sensitivity is the key to achieve the determination of trace uranium in the environment.

  Amidoxime group (–C(NH₂)=N-OH), which has been reported to be used for uranium extraction from seawater, is one of the best groups to adsorb UO₂²⁺ from aqueous solution and reveals strong affinity for UO₂²⁺ [13, 21, 22]. Herein, a novel and sensitive fluorescence sensor (TPE-A) is developed through a synthesis of tetraphenylenthene fluorophore with amidoxime group. It is demonstrated that this sensor system could greatly improve the fluorescence detection sensitivity reaching 4.7 × 10⁻⁹ mol/L, attributed to the promoted adsorption ability of amidoxime groups to UO₂²⁺. In addition, analytical application of the method for determination of UO₂²⁺ in FuJiang River samples was successfully conducted, which showed that TPE-A could be a sensitive and convenient fluorescent sensor for trace UO₂²⁺ detection.

  Details of TPE-A synthesis, characterization methods, spectroscopic and adsorption measurements are described in Supporting information.

  The AIE fluorescence characteristic of TPE-A was studied in H₂O/THF (from 0 to 1, v/v) with various water volume fractions (f_w). Fig. 1a shows the emission of TPE-A in various H₂O/THF mixtures. When the water fraction increases from 0% to 80% (Fig. S5 in Supporting information), the fluorescence intensity at 444 nm remains almost unchanged. While the water content reaches 85%, the fluorescence intensity of TPE-A increases significantly, and gets to its climax when the water content reaches 90%. The fluorescence intensity of the mixture (f_w = 90%) increases by 147-fold compared to the TPE-A in mixture (f_w = 0%). TPE-A aggregates in aqueous solutions with high water fractions since it is soluble in THF and poorly soluble in water. In dispersed solution, the fluorophores emit almost no light caused by their rotation or twisting motions against each other with the solvent media transforms the photonic energy to thermal energy. Upon aggregate formation, the emission of TPE-A is induced or rejuvenated by the restriction of intramolecular motions (RIM) [23, 24]. When f_w increases from 90% to 100%, the fluorescence intensity decreases, because TPE-A may agglomerate rapidly to form amorphous materials with decreased emission [25].

  Figure 1

  

  Figure 1.  (a) Fluorescence spectra of TPE-A (10 µmol/L) in H₂O/THF mixtures with f_w ranging from 0% to 100%. Dynamic light scattering results of (b) TPE-A (10 µmol/L) in H₂O/THF (90/10, v/v) mixture, (c) TPE-A (10 µmol/L) in H₂O/THF (85/15, v/v) mixture, and (d) TPE-A (10 µmol/L) with UO₂²⁺ (0.5 µmol/L) in the H₂O/THF (85/15, v/v) mixture.

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  To confirm the presence of aggregates, particle size distribution analyses (Figs. 1b–d) were performed on dynamic light scattering (DLS) experiments. As shown in Figs. 1b and c, nanoparticles with an average size of ca. 1822.1 nm (f_w = 90%) and 137.6 nm (f_w = 85%) are observed, respectively, which suggests the formation of fluorescent aggregates of TPE-A. Moreover, the stability test of the fluorescent sensor was conducted to evaluate the performance. It is observed that the fluorescence intensity of the mixture (f_w = 85%) at 444 nm stays almost constant in 600 s, while the fluorescence intensity of the mixture (f_w = 90%) fluctuates continually and enhances dramatically at 323 s (Fig. S6 in Supporting information), indicating the mixture (f_w = 90%) is unstable. Then, flocculent precipitation is observed after 10 min (Fig. S7 in Supporting information). Considering the stability, the water fraction (f_w = 85%) would be chosen as the optimum condition for detecting experiments of UO₂²⁺.

  The fluorescence properties of the sensor are affected by the pH. As shown in Fig. 2a, the effect of pH value was studied in the range of 4.0 to 9.0. TPE-A exhibits strong fluorescence intensity in the absence of UO₂²⁺ at different pH values (from 4.0 to 9.0). After addition of 0.5 µmol/L UO₂²⁺, the emission of TPE-A (10 µmol/L) could be quenched remarkably under acidic and neutral conditions, while quenched only 56.3% at pH 8.0 and 23.6% at pH 9.0 (Table S1 in Supporting information), which may be due to some uranyl exists in the form of anions in alkaline conditions (Fig. S8a in Supporting information). Considering the rigorous operation under acidic condition, neutral condition (pH 7.0) is chosen as the optimal experimental condition.

  Figure 2

  

  Figure 2.  (a) Fluorescence intensity of TPE-A (10 µmol/L) and TPE-A (10 µmol/L) + UO₂²⁺ (0.5 µmol/L) at 444 nm in H₂O/THF (85/15, v/v) solution at different pH. (b) The response time curve of TPE-A (10 µmol/L) to uranyl ions (0.5 µmol/L) at 444 nm in H₂O/THF (85/15, v/v) mixture at pH 7.0. (c) Fluorescence spectra of TPE-A (10 µmol/L) in the presence of different concentrations (0-0.5 µmol/L) in the H₂O/THF (85/15, v/v) solution at pH 7.0. (d) The linear plot and relationship of (F₀-F)/F₀ versus the concentrations of UO₂²⁺ (0.05-0.45 µmol/L).

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  In order to study the response time of TPE-A to UO₂²⁺, the fluorescence intensity within 8 min was recorded (Fig. 2b). It can be seen that the fluorescence intensity at 444 nm stays almost constant after 4 min. The short reaction time of the TPE-A toward UO₂²⁺ might be related to the strong coordination of amidoxime group.

  In order to estimate the potential of TPE-A as a sensor for the UO₂²⁺ detection, UO₂²⁺ was gradually added to H₂O/THF (f_w = 85%) mixture in titration experiment. Fluorescence responses of TPE-A to UO₂²⁺ were then recorded. Fig. 2c shows the emission intensity of TPE-A in the presence of UO₂²⁺. Notably, fluorescence intensity of the mixture decreases gradually with the concentration of UO₂²⁺ increases. Fig. S9 (Supporting information) shows the relationship between fluorescence intensity at 444 nm and the concentration of UO₂²⁺. Moreover, the fluorescence quenching efficiency ((F₀-F)/F₀, F and F₀ are the emission intensities at 444 nm in the presence and absence of the UO₂²⁺, respectively) exhibits a good linear relationship with the concentration of UO₂²⁺ in the range of 0.05 µmol/L to 0.45 µmol/L (Fig. 2d). The linear equation is Y = 1.784X-0.00826 (R² = 0.9988) with Y as the fluorescence quenching efficiency, X as the UO₂²⁺ concentration. The minimum detection limit is 4.7 × 10⁻⁹ mol/L calculated according to LOD = 3σ/k, where σ is the standard deviation of the blank measurement, and k is the slope of linear equation. The results show that the LOD is below the limit for uranium in drinking water (30 µg/L, ca. 1.1 × 10⁻⁷ mol/L) defined by the WHO, suggesting that TPE-A could be used as a novel fluorescence sensor for the determination of trace UO₂²⁺ in environmental samples.

  The existence of UO₂²⁺ results in a gradual quenching of TPE-A emission. The Stern-Volmer plot (F₀/F = 1 + K_svQ) was determined to evaluate the fluorescence quenching efficiency of TPE-A upon UO₂²⁺, where F₀ and F are the fluorescence intensities in the absence and presence of UO₂²⁺, Q is the concentration of UO₂²⁺, and K_sv is the Stern-Volmer quenching constant, and the bigger K_sv, the better quenching efficiency. As shown in Fig. S10 (Supporting information), the plot curves upward with the increase of UO₂²⁺ concentration, indicating that TPE-A has a high quenching efficiency. The inset in Fig. S10 shows that a linear Stern-Volmer plot is obtained with a large K_sv of 2.76 × 10⁶ L/mol at low concentration of UO₂²⁺, indicating that the TPE-A has high sensitivity for uranyl ion.

  Metal ions in environmental waters could interfere with the detection of UO₂²⁺. Various common metal ions (Be²⁺, Ca²⁺, Fe³⁺, K⁺, Li⁺, Mg²⁺, Na⁺, W⁶⁺, Zn²⁺) were selected as potential interference substances. Fluorescence quenching degrees (1-F/F₀) were performed in this study. As illustrated in Fig. 3a, most metal ions only cause ignorable fluorescence changes of TPE-A (by about 5%). It is noticed that Mg²⁺ quenches the fluorescence by about 9%. On the contrast, the addition of UO₂²⁺ results in almost complete fluorescence quenching of TPE-A (> 99%), much higher than those of common metal ions (Table S2 in Supporting information). To further evaluate the influence of metal ions to the detection of UO₂²⁺, competition experiments were also performed. As showed in Fig. 3b and Table S2 (Supporting information), the fluorescence intensity in the presence of UO₂²⁺ mixed with common ions is almost same with that of UO₂²⁺ alone, suggesting that the sensor has strong binding capacity among the tested metal ions. The interference and competition experiments of anions and humic acid (HA) as natural organic matter were also conducted, respectively. Fig. S11 and Table S2 (Supporting information) show the fluorescence intensity in the presence of UO₂²⁺ and HCO₃⁻ is higher than that of UO₂²⁺ alone, indicating that HCO₃⁻ may complex with uranium [26, 27]. Fig. S8b (Supporting information) shows the speciation of U(Ⅵ) in the presence of bicarbonate. In alkaline conditions, HCO₃⁻ could easily bind with U(Ⅵ) to form UO₂(CO₃)₂²⁻ and UO₂(CO₃)₃⁴⁻, which may impede the reaction between TPE-A and UO₂²⁺ by competing for the reactive sites.

  Figure 3

  

  Figure 3.  (a) Fluorescence intensity of TPE-A (10 µmol/L) in H₂O/THF (85/15, v/v) solution in the presence of different metal ions (10 µmol/L). (b) A competitive binding assay with TPE-A (10 µmol/L) for UO₂²⁺ (10 µmol/L) over another metal ion (10 µmol/L). F₀: fluorescence intensity of [TPE-A+U]; F: fluorescence intensity of [TPE-A+U+another metal ion].

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  To verify the practical application of the sensor, a standard addition method was used to analyze the detection of UO₂²⁺ in FuJiang River water samples. The concentration of UO₂²⁺ was determined by fluorescence spectrophotometry and ICP-MS. Table S3 (Supporting information) shows that the TPE-A sensor could be used to reliably determine the concentration of UO₂²⁺ with satisfactory recoveries (98.7%-104.0%) and relative standard deviation (RSD) values (< 5%). There is good agreement between the TPE-A sensor and ICP-MS measurements. The performance of the proposed sensor was compared with some reported sensors (Table S4 in Supporting information). It was found that TPE-A could detect UO₂²⁺ in a relatively wider pH range and established a bigger quenching constant K_sv. More importantly, the detection limit of TPE-A could reach nanomolar level, which could be applied in practical samples in the environment.

  In view of the fluorescence quenching, FTIR analyses were carried out to confirm whether the coordination between TPE-A and UO₂²⁺ occurred. Fig. S12 shows the FTIR spectra of TPE-A and U-laden TPE-A, the bands at 1641 and 906 cm⁻¹ in TPE-A are indicative of the –C=N– and =N–O– of amidoxime group, respectively. The adsorption bands at 1641 and 906 cm⁻¹ shift to 1646 and 927 cm⁻¹ after adsorption, respectively, which indicates the coordination of TPE-A with UO₂²⁺. The possible fluorescence quenching mechanisms are as follows: (1) after the coordination of TPE-A with UO₂²⁺, the aggregates may dissociate, and the solubility of the complex in water may be better, leading to the decrease of fluorescence; (2) the heavy atom effect of UO₂²⁺. To determine whether the aggregates dissociated or not after coordination with uranyl ion, particle size analysis using DLS was performed (Fig. 1d). Particles are still observed in the presence of 0.05 equiv. of UO₂²⁺ and the particles increase from ca. 137.6 nm to 294.2 nm. The size increase implies that the aggregates of TPE-A do not dissociate after coordination with uranyl ion.

  To further understand the photophysical properties of TPE-A and [TPE-A+UO₂²⁺], density functional theory (DFT) calculations were carried out using Gaussian 16 at the level of B3LYP/6-31+G(d), SDD-MWB60. Amidoxime group can coordinate with UO₂²⁺ via its electrons of N or O, leading to a strong chelating reaction for uranyl ions. Researchers have proposed several binding motifs [28-30]; while DFT results exhibit that η² binding motif with the N-O bond is the most stable form, confirmed by single-crystal X-ray diffraction of UO₂²⁺ complexes with acetamidoxime and benzamidoxime anions [31]. The possible coordination patterns between the sensor TPE-A and UO₂²⁺ were explored, as shown in Fig. S13 (Supporting information). The binding energy of the combination [2TPE-A+UO₂²⁺] (−135.92 kcal/mol) is much lower than that of [TPE-A+UO₂²⁺] (−109.27 kcal/mol), indicating that two amidoxime groups lean to bind one UO₂²⁺. Previous studies have proved that HOMO and LUMO orbitals can describe the sites where molecules tend to lose or gain electrons in the reaction process [32, 33]. The calculated energy levels and HOMO-LUMO energy gaps for the TPE-A and [2TPE-A+UO₂²⁺] are illustrated in Fig. 4. The LUMO energy level (−2.0249 eV) in the [2TPE-A+UO₂²⁺] is much lower than that of the TPE-A (−1.4582 eV). Before and after TPE-A combines with UO₂²⁺, the HOMO-LUMO energy gap decreased from 3.8685 eV to 3.3383 eV. In addition, the combination of TPE-A and UO₂²⁺ seems to have strong influence on the HOMO and LUMO properties of the complexes. In the free TPE-A, the electron density of HOMO mainly locates on the TPE and amidoxime moieties and the LUMO mainly distributes on the TPE moiety. When UO₂²⁺ is coordinated to the TPE-A ligand, the electrons density on the HOMO of [2TPE-A+UO₂²⁺] mainly locates on the TPE and amidoxime moieties, while the LUMO mainly disperses on the UO₂²⁺ moiety. Thus, the intramolecular charge transfer from TPE-A to UO₂²⁺ is allowed, leading to the fluorescence quenching. These results indicate that the fluorescence quenching after the addition of UO₂²⁺ is mainly due to the heavy atom effect of UO₂²⁺.

  Figure 4

  

  Figure 4.  Molecular orbitals of the free TPE-A ligand and [2TPE-A+ UO₂²⁺].

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  In summary, a simple and sensitive fluorescence AIE-active sensor TPE-A has been successfully designed and synthesized for rapid uranyl ion detection in aqueous solutions. The fluorescence property of TPE-A and its application were investigated. The proposed sensor is effective with a limit of detection as low as 4.7 × 10⁻⁹ mol/L at pH 4.0-7.0. This sensor showed good accuracy in the determination of UO₂²⁺ concentrations in river water and may have potential applications for the detection of trace UO₂²⁺ in environmental systems.

  Declaration of competing interest

  The authors declare no financial interests/personal relationships which may be considered as potential competing interests.

  Acknowledgments

  We appreciate the financial support received from the National Natural Science Foundation of China (No. 21702193) and China Academy of Engineering Physics (Nos. TP03201601, TP02201711 and JMJJ20190101).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.08.005.

  
  
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  Figure 1  (a) Fluorescence spectra of TPE-A (10 µmol/L) in H₂O/THF mixtures with f_w ranging from 0% to 100%. Dynamic light scattering results of (b) TPE-A (10 µmol/L) in H₂O/THF (90/10, v/v) mixture, (c) TPE-A (10 µmol/L) in H₂O/THF (85/15, v/v) mixture, and (d) TPE-A (10 µmol/L) with UO₂²⁺ (0.5 µmol/L) in the H₂O/THF (85/15, v/v) mixture.

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  Figure 2  (a) Fluorescence intensity of TPE-A (10 µmol/L) and TPE-A (10 µmol/L) + UO₂²⁺ (0.5 µmol/L) at 444 nm in H₂O/THF (85/15, v/v) solution at different pH. (b) The response time curve of TPE-A (10 µmol/L) to uranyl ions (0.5 µmol/L) at 444 nm in H₂O/THF (85/15, v/v) mixture at pH 7.0. (c) Fluorescence spectra of TPE-A (10 µmol/L) in the presence of different concentrations (0-0.5 µmol/L) in the H₂O/THF (85/15, v/v) solution at pH 7.0. (d) The linear plot and relationship of (F₀-F)/F₀ versus the concentrations of UO₂²⁺ (0.05-0.45 µmol/L).

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  Figure 3  (a) Fluorescence intensity of TPE-A (10 µmol/L) in H₂O/THF (85/15, v/v) solution in the presence of different metal ions (10 µmol/L). (b) A competitive binding assay with TPE-A (10 µmol/L) for UO₂²⁺ (10 µmol/L) over another metal ion (10 µmol/L). F₀: fluorescence intensity of [TPE-A+U]; F: fluorescence intensity of [TPE-A+U+another metal ion].

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  Figure 4  Molecular orbitals of the free TPE-A ligand and [2TPE-A+ UO₂²⁺].

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