Europium Activated Aluminum Organic Frameworks for Highly Selective and Sensitive Detection of Fe3+ and Cr(Ⅵ ) in Aqueous Solution①

ZHANG Zhen FANG Qi-Hui ZHUANG Zan-Yong HAN Yang LI Ling-Yun YU Yan

(Key Laboratory of Eco-materials Advanced Technology, College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China)

ABSTRACT The aluminum based MOFs compound, MIL-96(Al), has been studied as a luminescence sensor by incorporating of Eu3+ ions, which renders the compound strong red-light emission. The as-synthesized MIL-96(Al):Eu3+ nanocrystals exhibit water dispersibility and environmental stability, which are general requirement for an ideal sensing material. The MIL-96(Al):Eu3+ nanocrystals show excellent selective detection ability on Fe3+ ions in aqueous solution with value of low detection limit 20 μM. Meanwhile, it also exhibits excellent selective detection ability on CrO42- and Cr2O72- in aqueous solution with value of low detection limit 10 and 22 μM, respectively. The results of this study show that MIL-96(Al):Eu3+ nanocrystals could be used as a multi-responsive luminescent senor for Fe3+ and Cr(VI) species in aqueous solution. The possible luminescence quenching mechanism has also been discussed.

Keywords: luminescent sensor, selective detection, quenching mechanism;

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 electro- plating, 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-benzenetricar- boxylic 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 applica- tion 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−1using the KBr pressed technique. Thermogravimetric analysis (TGA) was carried out on a Netzsch STA449-F5 system at a heating rate of 5 K·min−1from 30 to 800 °C under a nitrogen atmosphere in Al2O3crucibles. 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 °C to form solution A. In the meantime, 0.21 g of H3BTC nanocrystals was dissolved into 8 mL of DMF at 80 °C 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 °C 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 °C for 4 h and activating in the oven at 150 °C 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 spectro- meter 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−1indicates that H3BTC is linked into the framework of MIL-96(Al):Eu3+. The strong vibration band at ～3398 cm−1is attributed to the stretching vibration of -OH of adsorbed water. The characteristic stretching vibrations of the coor- dinating carboxylate groups, νas(-COO-) at 1500～1750 and νs(-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 °C due to the loss of adsorbed solvent and decomposition of the compound.

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

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 to5D0→7FJ(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 the5D0→7F2transition 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.

Fig.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)

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.

Fig.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)

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 I0and 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 × 103and 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 immer- sed 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 lumine- scence quenching.

Fig.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)

3. 4 Sensing for Cr(VI)

The influence of different potassium salts water solutions of KyX (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 CrO42–and Cr2O72-solutions with different concentrations, respectively. The linear correlation coefficients of CrO42-and Cr2O72-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, KSV, is calculated as 4.43 × 103and 9.08 × 103M–1for Cr2O72–and CrO42–, respectively. The detection limits of MIL-96(Al):Eu3+for CrO42–and Cr2O72–are determined as 10 and 22 μM, which are comparable to or better than some previously reported fluorescence sensors for CrO42–and Cr2O72–[24–26].

Fig.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 Cr2O72- and CrO42-, respectively (c) and (e), Ksv curve of MIL-96(Al):Eu3+ in aqueous solutions in the presence of various concentrations of Cr2O72- and CrO42- (d) and (f)

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 lumine- scence quenching of MIL-96(Al):Eu3+nanocrystals in aqueous solution containing Cr(VI) anions.

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 × 103and 20 μM for Fe3+ion, 4.43 × 103and 22 μM for Cr2O72−ion, 9.08 × 103and 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.

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