English

• 1. INTRODUCTION

  Fe³⁺ ion is an essential element in humans or other living organisms. Fe³⁺ deficiency is the most common cause for hypochromic microcytic anemia. However, excessive Fe³⁺ ion is toxic to humans since it may lead to serious iron metabolism disorders such as Alzheimer's disease, Parkinson's syndrome and other neurodegenerative diseases^[1]. Therefore, it is necessary to monitor and detect Fe³⁺ in our daily life. Hexavalent chromium Cr(VI), mainly coming from electroplating, printing and dyeing, metal processing and other industries, is one of the metallic contaminations that cause serious environmental issues. The strong oxidation state of Cr(VI) can damage DNA, interfere with DNA repair^[2], and destroy the skeleton of liver cells^[3], leading to a significant raise of cancers^[4]. Therefore, exploring methods for efficient and accurate identification of Cr(VI) is of great significance for human health, environmental management, and clinical medicine.

  Because of structural diversity and optical spectroscopic adjustability, luminescent metal-organic frameworks (MOFs) have shown great application potential in detection of small molecules^[5], toxic cations^[6], pollutant anions^[7], gases^[8] and explosives^[9]. However, some luminescent sensing MOFs could not work in a real environment, since they are unstable in aqueous solutions^[10-13]. Therefore, it is very urgent and necessary to look for a MOFs compound that is stable in real environment. T. Loiseau firstly reported MIL-96(Al) in 2006^[14]. This compound consists of Al³⁺, 1, 3, 5-benzenetricarboxylic acid (H₃BTC), aluminum-centered octahedral units bridged through the H₃BTC ligand and μ₃-oxo-centered trinuclear building blocks assembled with a two-dimensional network of hexagonal 18-ring. It is reported that MIL-96 (Al) is a material with high practical potential due to using of inexpensive metal of aluminum^[15], high hydrothermal stability^[16], flexible structure^[17], and high porosity^[14]. What's more, MIL-96 (Al) has excellent stability in neutral-to-acidic aqueous solutions^[18], which attracts us to look into its application feasibility as a luminescent sensing material.

  In the current work, we prepared luminescent MIL-96(Al) nanocrystals by introducing Eu³⁺ into MIL-96(Al) lattice to form MIL-96(Al): Eu³⁺ compound. The substitution of Eu³⁺ for Al³⁺ makes it become a luminescent material with intense red light emission. The optical spectroscopy and detection ability of MIL-96(Al): Eu³⁺ nanocrystals on Fe³⁺ and Cr(VI) ions in aqueous systems has been studied. The mechanism of luminescence response of MIL-96(Al): Eu³⁺ nanocrystals upon the target ion is also discussed.

  2. EXPERIMENTAL

  2.1 Materials and instruments

  The chemicals of Al(NO₃)₃·9H₂O (98%), Eu(NO₃)₃·6H₂O (98%), H₃BTC (98%) and DMF were obtained from commercial vendors and used without further purification. XRD patterns were recorded using a Rigaku MiniFlex 600 X-ray diffractometer operated at 40 kV and 15 mA with CuKa radiation. The data were collected within the 2θ range of 3~40°. Scanning electronic microscope (SEM) images were recorded on Zeiss Supra 55. Fourier transform infrared spectra (FTIR) were recorded with a Nicolet 5700 infrared spectrum radiometer within the wavenumber range of 4000~400 cm⁻¹ using the KBr pressed technique. Thermogravimetric analysis (TGA) was carried out on a Netzsch STA449-F5 system at a heating rate of 5 K·min⁻¹ from 30 to 800 ℃ under a nitrogen atmosphere in Al₂O₃ crucibles. The inductively coupled plasma atomic emission spectroscopy (ICP-AES, iCAP™ 7400 ICP-OES, Thermo Fisher Scientific) was employed to analyze the contents of various metal ions.

  2.2 Synthesize of MIL-96(Al): Eu³⁺

  MIL-96(Al): Eu³⁺ nanocrystals were synthesized according to the method reported by A. Knebel et al.^[19] with slight modification. At the beginning, 0.3954 g of Al(NO₃)₃·9H₂O and 0.03 g of Eu(NO₃)₃·6H₂O were dissolved in 8 mL of deionized water at 80 ℃ to form solution A. In the meantime, 0.21 g of H₃BTC nanocrystals was dissolved into 8 mL of DMF at 80 ℃ to form solution B. The clear solutions A and B were mixed together and put into a Teflon-lined autoclave, then 0.5 mL of 10 mM acetic acid solution was added. The sealed autoclave was put in an oven to perform solvothermal reaction at 210 ℃ for 2 h. After cooling down in the oven, the solvothermal product was separated from solvent by washing with deionized water and anhydrous ethanol. After drying in vacuum at 60 ℃ for 4 h and activating in the oven at 150 ℃ for 2 h, white MIL-96(Al): Eu³⁺ nanocrystals were obtained.

  2.3 Optical spectroscopic properties and environmental stability of MIL-96(Al): Eu³⁺

  The luminescent spectra were all recorded at room temperature using a HORIBA FM-4 fluorescence spectrometer with xenon flash lamp as light source. The wavelength of excited light is 393 nm. The luminescent properties of the hydrothermal products were investigated in solid state and suspension formed by adding 5 mg of MIL-96(Al): Eu³⁺ nanocrystals into 5 mL of deionized water. The suspension was usually treated by sonication before the measurement of luminescence spectrum. To check the stability of MIL-96(Al): Eu³⁺ nanocrystals in aqueous solution, we recorded the day-to-day luminescence and pH-dependent luminescence spectra of MIL-96(Al): Eu³⁺ suspensions. Meanwhile, the nanocrystals were filtrated again from the suspensions to collect XRD pattern to recheck their phase stability after luminescence measurement.

  2.4 Luminescent sensing experiment

  Luminescence responses of MIL-96(Al): Eu³⁺ nanocrystals toward the aqueous solution containing various metal cations and anions were studied at room temperature. 5 mg of MIL-96(Al): Eu³⁺ nanocrystals was dispersed into 5 mL aqueous solutions containing different metal cations and anions (2 mM) to form a stable suspension by sonication. To ensure MIL-96(Al): Eu³⁺ nanocrystals react fully with target ions, the suspension was treat by sonication for 5 min before the collection of luminescence data.

  3. RESULTS AND DISCUSSION

  3.1 Characterization of MIL-96(Al): Eu³⁺

  The XRD pattern of MIL-96(Al): Eu³⁺ sample is in good agreement with the simulated one, showing the successful preparation of MIL-96(Al): Eu³⁺ nanocrystals (Fig. 1a). The as-synthesized MIL-96(Al): Eu³⁺ particle is monodispersing with size about 500 nm. As demonstrated in FTIR spectra, the disappearance of a broad band in the region of ~3000 cm⁻¹ indicates that H₃BTC is linked into the framework of MIL-96(Al): Eu³⁺. The strong vibration band at ~3398 cm⁻¹ is attributed to the stretching vibration of -OH of adsorbed water. The characteristic stretching vibrations of the coordinating carboxylate groups, ν_as(-COO-) at 1500~1750 and ν_s(-COO-) at 1350~1490 cm^–1, can also be figured out^[20]. Another clear change is that two new bands assigned to Al–O appear in the range of 1000~1120 cm^–1, confirming the coordination between Al and O atoms^[21]. As revealed by the TGA data, there are several weigh loss processes between room temperature and 650 ℃ due to the loss of adsorbed solvent and decomposition of the compound.

  Figure 1

  

  Figure 1.  XRD pattern (a), FTIR spectra (b), SEM image (c) and TGA curve (d) of MIL-96(Al): Eu³⁺ nanocrystals

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  3.2 Luminescent properties of MIL-96(Al): Eu³⁺

  As shown in Fig. 2a, MIL-96(Al): Eu³⁺ gives intense optical emission when excited by 393 nm light. Five emission peaks belonging to ⁵D₀ → ⁷F_J (J = 0~4) transitions of Eu³⁺ with center at 576, 589, 618, 650, and 697 nm appear in the luminescence spectrum^[22]. One can find that the peak originated from the ⁵D₀ → ⁷F₂ transition at 618 nm is the strongest one. The MIL-96(Al): Eu³⁺ nanocrystals display good environmental stability in aqueous solution. As seen from Fig. 2b, the luminescence intensity of MIL-96(Al): Eu³⁺ suspension is nearly unchanged after 5 days. Furthermore, the luminescence intensity of suspension keeps near constant with the pH value varying from 3 to 11. The luminescence stability of MIL-96(Al): Eu³⁺ suspension originates from the phase stability of MIL-96(Al): Eu³⁺ nanocrystals in aqueous solution, since the XRD patterns of MIL-96(Al): Eu³⁺ nanocrystals after pH dependent luminescence measurement still match well with the standard one. This result indicates that the framework and phase of luminescent nanocrystals are not affected by the evolution of pH value of aqueous solution and it has application potential in practical environment.

  Figure 2

  

  Figure 2.  Excitation and emission spectra (a), day-to-day luminescence spectra (b), pH-dependent luminescence intensity (c) and XRD patterns of MIL-96(Al): Eu³⁺ nanocrystals after pH-dependent luminescence measurement (d)

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  3.3 Sensing for Fe³⁺ ions

  The responses of the luminescence of MIL-96(Al): Eu³⁺ toward K⁺, Na ⁺, Ca²⁺, Mg²⁺, Cu²⁺, Zn²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Cr³⁺, and Al³⁺ aqueous solution were investigated. As shown in Fig. 3a and 3b, the luminescence of MIL-96(Al): Eu³⁺ significantly quenches in Fe³⁺ aqueous solution. Indeed, the quenching of MIL-96(Al): Eu³⁺ luminescence is not interfered by other cations, demonstrating that MIL-96(Al): Eu³⁺ nanocrystals have excellent selective detection ability on Fe³⁺ ions.

  Figure 3

  

  Figure 3.  Luminescence spectra of MIL-96(Al): Eu³⁺ suspension with the presence of various cations (2 mM) (a), relative intensities of ⁵D₀-⁷F₂ at 618 nm of MIL-96(Al): Eu³⁺ suspension containing different cations (2 mM) (b), luminescence spectra of MIL-96(Al): Eu³⁺ suspension with the presence of various concentrations of Fe³⁺ (c), and Ksv curve of MIL-96(Al): Eu³⁺ in aqueous solutions with the presence of various concentrations of Fe³⁺ (d)

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  The detection ability of MIL-96(Al): Eu³⁺ nanocrystals on Fe³⁺ ions is studied quantitatively by Stern-Volmer equation: I₀/I = 1 + Ksv×[Q]^[23]. The values of I₀ and I stand for the luminescent intensities of MIL-96(Al): Eu³⁺ with the absence and presence of Fe³⁺ ion, respectively. [Q] is the concentration of Fe³⁺ ion, and Ksv is the Stern-Volmer quenching constant. As shown in Fig. 3c, the luminescence of MIL-96(Al): Eu³⁺ gradually quenches with the addition of Fe³⁺ ions. A good linear relation with a linear correlation coefficient value (R) of 0.9996 obtains according to the fitting by Stern-Volmer model. The corresponding quenching constant (Ksv) is 4.75 × 10³ and the detection limit is calculated to be 20 μM (3δ/slope, where δ is the standard deviation calculated by measuring the luminescent intensity of a blank solution^[23]).

  We investigated the phase of luminescent nanocrystals after luminescence sensing experiment. As shown in Fig. 4a to 4b, the XRD patterns of luminescent nanocrystals reacted with various cations and anions match well with the simulated pattern of MIL-96(Al): Eu³⁺, suggesting that the basic frameworks remain unchanged after the luminescence sensing experiment. Meanwhile, we studied the dynamic evolution of luminescence intensity of suspension after MIL-96(Al): Eu³⁺ nanocrystals immersed into aqueous solution. As seen from Fig. 4c, the luminescence quenches gradually in 20 min, then remains nearly unchanged. Furthermore, we found that Al³⁺ and Eu³⁺ appear when MIL-96(Al): Eu³⁺ nanocrystals immersed into aqueous solution, and the concentration of Al³⁺ and Eu³⁺ gradually increases in this procedure, indicating that cation exchange between M³⁺ and Fe³⁺ ion takes place during the luminescence probing experiments and lead to luminescence quenching.

  Figure 4

  

  Figure 4.  XRD patterns of MIL-96(Al): Eu³⁺ nanocrystals reacting with various cations (a) and anions (b), and the dynamic concentration evolution of M³⁺ (M³⁺ = Al³⁺, Eu³⁺) in the test aqueous solution and dynamic luminescence intensity evolution of the test MIL-96(Al): Eu³⁺ (The initial Fe³⁺ concentration of the test solution is 300 μM) (c)

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  3.4 Sensing for Cr(VI)

  The influence of different potassium salts water solutions of K_yX (X = F⁻, Cl⁻, Br⁻, I⁻, SO₄²⁻, CO₃²⁻, NO₃⁻, PO₄³⁻, CrO₄²⁻, Cr₂O₇²⁻) on the MIL-96(Al): Eu³⁺ was also investigated. The MIL-96(Al): Eu³⁺ nanocrystals exhibit excellent selective detection ability on Cr(VI) anion. As shown in Fig. 5a and 5b, only Cr(VI) anions strongly quench the luminescence of MIL-96(Al): Eu³⁺, which is not interfered by the other anions. Fig. 5c and 5e exhibit the emission spectra of MIL-96(Al): Eu³⁺ dispersed into CrO₄^2– and Cr₂O₇^2- solutions with different concentrations, respectively. The linear correlation coefficients of CrO₄^2- and Cr₂O₇^2- are fitted to be 0.9994 and 0.9966, indicating the quenching effect of Cr(VI) species on MIL-96(Al): Eu³⁺ matches the Stern-Volmer mode well. The Stern-Volmer constant, K_SV, is calculated as 4.43 × 10³ and 9.08 × 10³ M^–1 for Cr₂O₇^2– and CrO₄^2–, respectively. The detection limits of MIL-96(Al): Eu³⁺ for CrO₄^2– and Cr₂O₇^2– are determined as 10 and 22 μM, which are comparable to or better than some previously reported fluorescence sensors for CrO₄^2– and Cr₂O₇^{2–[24–26]}.

  Figure 5

  

  Figure 5.  Luminescence spectra of MIL-96(Al): Eu³⁺ suspension with the presence of various anions (2 mM) (a), relative intensities of ⁵D₀-⁷F₂ at 618 nm of MIL-96(Al): Eu³⁺ suspension containing different cations (2 mM) (b), emission spectra of MIL-96(Al): Eu³⁺ in aqueous solutions in the presence of various concentrations of Cr₂O₇^2- and CrO₄^2-, respectively (c) and (e), Ksv curve of MIL-96(Al): Eu³⁺ in aqueous solutions in the presence of various concentrations of Cr₂O₇²-and CrO₄^2- (d) and (f)

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  The zeta potentials (ζ) of the MIL-96(Al): Eu³⁺ solution ranges from 21.6 to 9.7 when the pH value of solution varies from 3 to 10, as shown in Fig. 6a. The positive charged surface of MIL-96(Al): Eu³⁺ nanocrystals is feasible for adsorbing negative ions such as CrO₄²⁻ and Cr₂O₇²⁻, which is confirmed by the optical absorption spectra. It can be found from Fig. 6b that the intrinsic UV-Vis optical absorption bands of Cr(VI) anions coincide with that of MIL-96(Al): Eu³⁺ nanocrystals, indicating that the Eu³⁺ centers in MIL-96(Al) nanocrystals could be shielded to some extend from excitation light with wavelength of 393 nm by the Cr(VI) anions adsorbed on the surface. In a word, the competitive absorption of excitation light is the most possible reason for luminescence quenching of MIL-96(Al): Eu³⁺ nanocrystals in aqueous solution containing Cr(VI) anions.

  Figure 6

  

  Figure 6.  pH dependent zeta potentials (ζ) of the MIL-96(Al): Eu³⁺ solution (a) and UV-Vis optical absorption of different anions (b)

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  4. CONCLUSION

  In summary, MIL-96(Al): Eu³⁺ nanocrystals with high stability in aqueous solution have been successfully prepared by solvothermal method. The as-obtained luminescent MIL-96(Al): Eu³⁺ nanocrystals show excellent selective detection ability on Fe³⁺ ion and Cr(Ⅵ) anions in aqueous system. The quenching constant (Ksv) and the detection limit are determined: 4.75 × 10³ and 20 μM for Fe³⁺ ion, 4.43 × 10³ and 22 μM for Cr₂O₇²⁻ ion, 9.08 × 10³ and 10 μM for CrO₄²⁻ ion. The luminescence quenching of MIL-96(Al): Eu³⁺ upon Fe³⁺ ion seems to be caused by ion exchange between Fe³⁺ and M³⁺ (Al³⁺, Eu³⁺), while the possible reason for luminescence quenching of MIL-96(Al): Eu³⁺ upon Cr(Ⅵ) anions is competitive absorption of excitation light. The result shows MIL-96(Al): Eu³⁺ nanocrystals prepared in this work have high application potential in practical environment.

  
  
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• 

  Figure 1  XRD pattern (a), FTIR spectra (b), SEM image (c) and TGA curve (d) of MIL-96(Al): Eu³⁺ nanocrystals

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  Figure 2  Excitation and emission spectra (a), day-to-day luminescence spectra (b), pH-dependent luminescence intensity (c) and XRD patterns of MIL-96(Al): Eu³⁺ nanocrystals after pH-dependent luminescence measurement (d)

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  Figure 3  Luminescence spectra of MIL-96(Al): Eu³⁺ suspension with the presence of various cations (2 mM) (a), relative intensities of ⁵D₀-⁷F₂ at 618 nm of MIL-96(Al): Eu³⁺ suspension containing different cations (2 mM) (b), luminescence spectra of MIL-96(Al): Eu³⁺ suspension with the presence of various concentrations of Fe³⁺ (c), and Ksv curve of MIL-96(Al): Eu³⁺ in aqueous solutions with the presence of various concentrations of Fe³⁺ (d)

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  Figure 4  XRD patterns of MIL-96(Al): Eu³⁺ nanocrystals reacting with various cations (a) and anions (b), and the dynamic concentration evolution of M³⁺ (M³⁺ = Al³⁺, Eu³⁺) in the test aqueous solution and dynamic luminescence intensity evolution of the test MIL-96(Al): Eu³⁺ (The initial Fe³⁺ concentration of the test solution is 300 μM) (c)

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  Figure 5  Luminescence spectra of MIL-96(Al): Eu³⁺ suspension with the presence of various anions (2 mM) (a), relative intensities of ⁵D₀-⁷F₂ at 618 nm of MIL-96(Al): Eu³⁺ suspension containing different cations (2 mM) (b), emission spectra of MIL-96(Al): Eu³⁺ in aqueous solutions in the presence of various concentrations of Cr₂O₇^2- and CrO₄^2-, respectively (c) and (e), Ksv curve of MIL-96(Al): Eu³⁺ in aqueous solutions in the presence of various concentrations of Cr₂O₇²-and CrO₄^2- (d) and (f)

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  Figure 6  pH dependent zeta potentials (ζ) of the MIL-96(Al): Eu³⁺ solution (a) and UV-Vis optical absorption of different anions (b)

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