English

• 1.   Introduction

  With rapid development of international globalization and industrialization, heavy-metal pollution has attracted much more attention in the field of environment and human health.^{[1, 2]} Due to the non-biodegradation nature of mercury ion (Hg²⁺), it can only be diluted but not to be destroyed.^[3] Long term accumulation of this metal in organisms can destroy DNA, inhibit ligand-receptor interactions, disable normal functions of liver and kidney, disrupt the homeostasis of immune system, and even lead to death.^[4~8] Consequently, the search for an effective detection method of Hg²⁺ has attracted increasing attention in the environmental and medical field. Ferric ion (Fe³⁺), as an essential trace element in plants and animal body, plays important roles in cell metabolism, enzymatic catalysis, oxygen transport in hemoglobin, and acts as a cofactor in enzyme-based reactions.^[9~13] But abnormal levels of ferric iron are signs of diseases, such as anemia, hemochromatosis, Parkinson's diease, arthritis, diabetes, hepatitis and cancer.^{[14, 15]} The Fe³⁺ is also one of important sources of the environmental pollution.^{[16, 17]} Therefore, many studies have tried to develop a simple, sensitive and reliable method to detect and distinguish Hg²⁺ and Fe³⁺.

  Multifunctional fluorescent sensors^[18~23] have attracted significant interest in recent years because of their simplicity, sensitivity, low-cost and real-time performance compared to traditional methods.^[24~35] It has been reported that many fluorescent sensors are used for monitoring Hg²⁺ and Fe³⁺, respectively.^[36~39] However, a single probe capable of detecting and absorbing Hg²⁺and Fe³⁺ is still rare. Therefore, search of a fluorescent sensor with high selectivity and sensitivity, rapid response, recyclable and absorption performance is a very challenging issue.

  Barbituric acid (BA) as an important compound precursor has been utilized for organic functional materials. BA derivatives can be readily obtained from reaction of BA with chromophore substrates through Knoevenagel reactions under mild conditions.^{[2, 40, 41]} A few studies have reported two thymine-like imide-nitrogen atoms in the structure of BA, which have the ability to coordinate with Hg²⁺ via the formation of a mercury(II)-BA coordination complex.^[42] However, BA-based fluorescent sensors that can be used for monitoring and separating Hg²⁺ and Fe³⁺ have not been reported so far.

  Based on the above considerations and as part of our ongoing interests in fluorescent sensors, ^[43~47] BA was chosen as a receptor moiety for designing fluorescent sensor QS, which has simple structure and can be readily synthesized by Knoevenagel reaction of BA with 5- (4-nitrophenyl)-furan-2-carbaldehyde (Scheme 1). Interestingly, sensor QS showed highly sensitivity and selectivity for the "individual recognition" of Hg²⁺ and Fe³⁺ with different fluorescence performance. Upon addition of Hg²⁺ and Fe³⁺ into the QS solution, respectively, Hg²⁺ induced the fluorescence of QS to increase and change to orange fluorescence, Fe³⁺ induced the fluorescence quenching. The limits of detection of QS for Hg²⁺ and Fe³⁺ were 3.25×10^－8 and 4.0×10^－8 mol•L^－1, respectively. It is worth mentioning that the properties of sensor QS based on BA derivatives for Fe³⁺ detection were first reported. Moreover, the fluorescence of QS-Fe recovered upon addition of H₂PO₄^- into the solution, which indicate that QS can be used for recyclable detection test for Fe³⁺. More importantly, sensor QS also has good absorption capacity for Hg²⁺ and Fe³⁺ (absorption percentages were up to 92.0% and 91.8%, respectively). Hence, sensor QS shows potential application prospect in ion detection and separation of Hg²⁺ and Fe³⁺.

  Scheme 1

  

  Scheme 1.  Synthesis of the sensor QS

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  2.   Results and discussion

  To investigate the real-time recognition ability of sensor QS towards metal ions (such as Fe³⁺, Hg²⁺, Ni²⁺, Zn²⁺, Ca²⁺, Cu²⁺, Cr³⁺, Ba²⁺, Co²⁺, Ag⁺, Pb²⁺, Cd²⁺, Al³⁺ and Mg²⁺), changes in fluorescence spectra were measured immediately by adding above metal ions to the solution of QS. Sensor QS exhibited green fluorescence (λ_ex＝355 nm). After addition of Hg²⁺ and Fe³⁺, an obvious fluorescent color changes from green to orange and dim could be observed, respectively. As depicted in Figure 1, after addition of various metal ions (Fe³⁺, Hg²⁺, Ni²⁺, Zn²⁺, Ca²⁺, Cu²⁺, Cr³⁺, Ba²⁺, Co²⁺, Ag⁺, Pb²⁺, Cd²⁺, Al³⁺ and Mg²⁺) into QS, only Hg²⁺ remarkably enhanced fluorescence and accompanied by red shifted from 498 to 526 nm, meanwhile only Fe³⁺ caused significantly quenching at 498 nm. These results indicated that sensor QS had excellent selectivity for Hg²⁺ and Fe³⁺.

  Figure 1

  

  Figure 1.  Fluorescence spectra of QS (1.0×10^－4 mol•L^－1) in DMSO with metal ions (Fe³⁺, Hg²⁺, Ni²⁺, Zn²⁺, Ca²⁺, Cu²⁺, Cr³⁺, Ba²⁺, Co²⁺, Ag⁺, Pb²⁺, Hg²⁺, Al³⁺ and Mg²⁺) in response to Hg²⁺ and Fe³⁺ (10 equiv.) upon excitation at 355 nm

  Inset: color change of QS with metal ions

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  To further explore the practical applicability of sensor QS for Hg²⁺ and Fe³⁺, respectively, competitive experiments were carried out toward other metal ions (Fe³⁺, Ni²⁺, Zn²⁺, Ca²⁺, Cu²⁺, Cr³⁺, Ba²⁺, Co²⁺, Ag⁺, Pb²⁺, Hg²⁺, Al³⁺ and Mg²⁺). As a result, the fluorescence emission spectra of sensor QS with Hg²⁺ was not be influenced by other metal ions (Figure 2a). Similarly, other metal ions did not induce any similar fluorescence changes towards QS-Fe solution. Therefore, the sensor QS has good selectivity for the detection of Hg²⁺ and Fe³⁺ with different fluorescence performance.

  Figure 2

  

  Figure 2.  Fluorescence emission data for QS (1.0×10^－4 mol• L^－1) with different perchlorate salts of cations (10 equiv.) and the previous mixture with (a) Hg²⁺ (10 equiv.) and (b) Fe³⁺ (10 equiv.) in DMSO solutions at room temperature (excitation wavelength＝355 nm)

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  The fluorescent titration experiments were further carried out to investigate the fluorescent sensitivity of QS toward Hg²⁺ and Fe³⁺, respectively. As shown in Figures 3a and 3b, upon addition of Hg²⁺ (0~11 equiv.) to solution of QS, the QS exhibited an emission enhancement at λ_em＝498 nm and lightly red-shifted from 498 to 526 nm, which directly led to the change of fluorescence (changed from green to light orange). However, the addition of Fe³⁺ (0~5.5 equiv.) induced an emission decreasing at λ_em＝498 nm. The above process of fluorescent changing may contribute to the intramolecular charge transfer (ICT) effect induced by Hg²⁺ and Fe³⁺. Furthermore, the association constants for Hg²⁺ and Fe³⁺ were determined to be 1.42×10⁵ and 2.25×10⁵ L•mol^－1, respectively. The limits of detection of fluorescence spectral change for Hg²⁺ and Fe³⁺ on the basis of 3δ/s method^[48] are 3.25×10^－8 and 4.0×10^－8 mol•L^－1, respectively, which indicated that QS has high sensitivity for Hg²⁺ and Fe³⁺.

  Figure 3

  

  Figure 3.  Fluorescence emission spectra of QS (0.1 mmol•L^－1) excited at 355 nm with addition of increasing concentrations of (a) Hg²⁺ (0~11.0 equiv.) and (b) Fe³⁺ (0~5.5 equiv.) in DMSO solution

  Inset: a plot of fluorescence intensity depending on the concentration of (a) Hg²⁺ in the range from 0 to 11 equiv. and (b) Fe³⁺ in the range from 0 to 5.5 equiv.

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  Interestingly, the fluorescence of QS-Fe reversed to its initial optical state after adding H₂PO₄^- (Figure 4). Because Fe³⁺ has binding ability with H₂PO₄^- to form a complex, these performances make the QS act as Fe³⁺ and H₂PO₄^- controlled "OFF-ON-OFF" fluorescent switch. The switch can be reversibly operated for at least three cycles with a slight fluorescent efficiency loss.

  Figure 4

  

  Figure 4.  Fluorescence emission spectra of QS-Fe (0.1 mmol• L^－1) excited at 355 nm with addition of increasing concentrations of H₂PO₄^- in DMSO solution

  Inset: fluorescence changes under 365 nm UV lamp

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  Moreover, ingestion capacities of the powder QS in aqueous solution for Hg²⁺/Fe³⁺ were assessed by inductively coupled plasma (ICP) analysis. The powder QS (0.4 mmol) was added into the aqueous solution of Hg²⁺ and Fe³⁺ (0.5 equiv.), respectively, and the mixture as stirred at room temperature in water for 30 min, the precipitates were separated by centrifugation (1000 r/min, 10 min) and the supernatant retained. The absorption percentages of powder QS for Hg²⁺ and Fe³⁺ were up to 92.0% and 91.8%, respectively. It indicated that the powder QS had nice absorption ability for Hg²⁺ and Fe³⁺ in aqueous solution.

  The possible mechanism of QS for Hg²⁺ and Fe³⁺ was carefully investigated by Job's method, FT-IR, MS and scanning electron microscope (SEM). The Job's method demonstrated that QS was combinded with Hg²⁺ and Fe³⁺ by 1:1 stoichiometry, respectively. The formation of a 1:1 complex was further confirmed by the appeareance of peaks at m/z＝561.2607 and 415.9336, corresponding to [QS+Hg－2H+Cl]^－ and [QS+Fe－2H+Cl]. In FT-IR spectra (Figure 5), the typical stretching vibrations of amides (N—H) at ca. 3198 cm^－1, which correspond to stretching vibrations in the spectra of QS, ^[49~51] weredisappeared significantly in the spectrum of QS-Hg and QS-Fe, respectively. This indicates that the amides (N—H) are involved in the coordination with Hg²⁺ and Fe³⁺, respectively. In addition, the stretching vibrations of the C＝O peaks in the amides [at ca. 1730 (ν_{C＝O, NHCONH}), 1691 (ν_{C＝O, NHCOC}) cm^－1] were obviously weakened in the spectrum of QS-Hg, while the corresponding peaks show blue-shift and red-shift respectively [at ca. 1759 (ν_{C＝O, NHCONH}), 1649 (ν_{C＝O, NHCOC}) cm^－1] in the spectrum of QS-Fe. This further provides evidence for the binding between QS and Hg²⁺, Fe³⁺ via M—N and M—O bonds (M＝Hg²⁺, Fe³⁺). Moreover, the addition of H₂PO₄^- into QS-Fe solution caused the stretching vibrations of the C＝O peaks in the amides recovered [at ca. 1730 (ν_{C＝O, NHCONH}), 1658 (ν_{C＝O, NHCOC})] cm^－1, respectively. These results indicate that the fluorescent changes were caused by the strong binding affinity of Hg²⁺, Fe³⁺ with N, O atom in BA ring, ^{[42, 49]} respectively. Hence, the enhanced fluorescence caused by Hg²⁺ can be attributed to the strong binding affinity of Hg²⁺ with N atom in BA moiety and the subsequent enhancement of the intramolecular charger transfer (ICT) process.^[49] The quenched fluorescence of QS likely was induced by the formation of paramagnetic complex between Fe³⁺ with QS, ^{[18, 21]} which subsequently enhanced the intramolecular charger transfer (ICT) process.^{[18, 21, 49]} Moreover, scanning electron microscope (SEM) was used to study this conjecture. The regular rod-like structure of powder QS transformed into an irregular block structure of QS-Hg, which further proves the existing of coordination interaction between QS and Hg²⁺. The result is in agreement with previous report.^[49] According to above-mentioned evidences, the possible binding modes of QS with Hg²⁺ and Fe³⁺ were shown in Scheme 2.

  Figure 5

  

  Figure 5.  FT-IR spectra of powder QS and QS-Hg

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  Scheme 2

  

  Scheme 2.  Proposed combination of QS with Hg²⁺ and Fe³⁺

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  3.   Conclusions

  A novel fluorescent sensor based on 5-(3-nitrophenyl)- furan-2-carbaldehyde functionalized barbituric acid (QS) has been successfully constructed. Sensor QS showed high sensitivity and selectivity for monitor Hg²⁺ and Fe³⁺, accompanied by different fluorescent performance, respectively. The limits of detection for Hg²⁺ and Fe³⁺ were determined to be 3.25×10^－8 and 4.0×10^－8 mol•L^－1, respectively. Moreover, the quenched fluorescence of QS-Fe can be recovered by addition of H₂PO₄^- which could be performed reversibly for Fe³⁺ detection. More importantly, sensor QS showed excellent absorption capacity for Hg²⁺ and Fe³⁺ (absorption percentages were up to 92.0% and 91.8%, respectively). In short, sensor QS has the advantages of recoverability, cost effective, high sensitivity, convenient synthesis and would be highly desirable from the view point of practical application.

  4.   Experimental section

  4.1   Synthesis of sensor 5-((5-(3-nitrophenyl)furan-2- yl)methylene)pyrimidine-2, 4, 6(1H, 3H, 5H)-trione (QS)

  5-(3-Nitrophenyl)-furan-2-carbaldehyde (1 mmol, 0.215 g), barbituric acid (1 mmol, 0.128 g) and acetic acid (0.1 mL, as a catalyst) were added into ethanol (10 mL). The mixture was stirred at 78 ℃ for 8 h. After cooling to room temperature, precipitate of QS was obtained as a crude product. A yellow solid of QS (0.268 g, yield 82%) was got after washed with ethanol for three times. m.p. > 300 ℃; ¹H NMR (DMSO-d₆, 600 MHz) δ: 11.36 (s, 1H), 11.28 (s, 1H), 8.49 (d, J＝3.9 Hz, 1H), 8.32 (d, J＝8.8 Hz, 2H), 8.17 (d, J＝8.8 Hz, 2H), 8.09 (s, 1H), 7.66 (d, J＝3.9 Hz, 1H); ¹³C NMR (DMSO-d₆, 101 MHz) δ: 163.18, 162.01, 156.90, 151.15, 150.20, 147.41, 135.77, 134.05, 28.46, 126.02, 124.52, 114.42, 114.05; ESI-MS calcd for C₁₅H₁₀N₃O₆ [M+H]⁺ 328.2600, found 328.2576.

  Supporting Information Characterization of compounds, the fluorescence response of the sensor, FT-IR spectrum, SEM, absorption percentage and so on. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

  
  
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• 

  Scheme 1  Synthesis of the sensor QS

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  Figure 1  Fluorescence spectra of QS (1.0×10^－4 mol•L^－1) in DMSO with metal ions (Fe³⁺, Hg²⁺, Ni²⁺, Zn²⁺, Ca²⁺, Cu²⁺, Cr³⁺, Ba²⁺, Co²⁺, Ag⁺, Pb²⁺, Hg²⁺, Al³⁺ and Mg²⁺) in response to Hg²⁺ and Fe³⁺ (10 equiv.) upon excitation at 355 nm

  Inset: color change of QS with metal ions

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  Figure 2  Fluorescence emission data for QS (1.0×10^－4 mol• L^－1) with different perchlorate salts of cations (10 equiv.) and the previous mixture with (a) Hg²⁺ (10 equiv.) and (b) Fe³⁺ (10 equiv.) in DMSO solutions at room temperature (excitation wavelength＝355 nm)

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  Figure 3  Fluorescence emission spectra of QS (0.1 mmol•L^－1) excited at 355 nm with addition of increasing concentrations of (a) Hg²⁺ (0~11.0 equiv.) and (b) Fe³⁺ (0~5.5 equiv.) in DMSO solution

  Inset: a plot of fluorescence intensity depending on the concentration of (a) Hg²⁺ in the range from 0 to 11 equiv. and (b) Fe³⁺ in the range from 0 to 5.5 equiv.

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  Figure 4  Fluorescence emission spectra of QS-Fe (0.1 mmol• L^－1) excited at 355 nm with addition of increasing concentrations of H₂PO₄^- in DMSO solution

  Inset: fluorescence changes under 365 nm UV lamp

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  Figure 5  FT-IR spectra of powder QS and QS-Hg

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  Scheme 2  Proposed combination of QS with Hg²⁺ and Fe³⁺

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