Multifunctional Ln-MOF luminescent probe displaying superior capabilities for highly selective sensing of Fe3+ and Al3+ ions and nitrotoluene

Journal Pre-proof Multifunctional Ln-MOF luminescent probe displaying superior capabilities for highly selective sensing of Fe3+ and Al3+ ions and nitrotoluene Hao Guo, Ning Wu, Rui Xue, Hui Liu, Li Li, Ming-yue Wang, Wen-qin Yao, Qi Li, Wu Yang

PII:

S0927-7757(19)31086-6

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124094

Reference:

COLSUA 124094

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

9 August 2019

Revised Date:

26 September 2019

Accepted Date:

8 October 2019

Please cite this article as: Guo H, Wu N, Xue R, Liu H, Li L, Wang M-yue, Yao W-qin, Li Q, Yang W, Multifunctional Ln-MOF luminescent probe displaying superior capabilities for highly selective sensing of Fe3+ and Al3+ ions and nitrotoluene, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124094

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Multifunctional Ln-MOF luminescent probe displaying superior capabilities for highly selective sensing of Fe3+ and Al3+ ions and nitrotoluene

Hao Guo*a, Ning Wua, Rui Xueb, Hui Liua, Li Lia, Ming-yue Wanga, Wen-qin Yaoa, Qi Lia, Wu Yang*a aKey

Lab of Eco-Environments Related Polymer Materials of MOE, Key Lab of

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Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, P R China. bCollege

of Chemistry and Chemical Engineering, Provincical Key Laboratory of Gansu Higher

P R China. 1

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Education for City Environmental Pollution Control, Lanzhou City University, Lanzhou 730070,

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[*] Correspondence authors. Email: [email protected](W. Yang),[email protected](H. Guo).

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Graphical abstract

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Abstract：A pillar-like 3D lanthanide-organic framework (Eu-MOF) with opposite

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chirality was assembled according to previous report and fully characterized. The Eu-MOF exhibits excellent luminescence and structural stability in the aqueous solution or other organic solvents and good pH-independent luminescence stability in

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the pH range of 3-12. It is important to point out that the as-synthesized Eu-MOF can be developed as a highly selective and sensitive luminescent probe to rapidly detect

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Fe3+ (detection limit, 0.39 μM) and Al3+ (detection limit, 0.084 μM) from mixed metal ions, which can be explained by different response mechanisms and the color change

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are visible to the naked eyes under UV-lamp. In addition, it also exhibits highly sensitive for nitrobenzene (NB) (detection limit, 0.013 μM). Considering the obvious spectral response and low limit detection, the Eu-MOF can be employed as a promising multi-functional luminescent sensor for sensing metal ions and small organic molecules in the future.

Keywords: Pillar-like; Ln-MOF; Luminescent probe; Highly selective and sensitive; 2

Low limit detection.

1. Introduction Metal-organic frameworks (MOFs) are composed by self-assembly of inorganic metals and organic ligands, also known as coordination polymers (CPs)[1,2], a very promising family of highly ordered porous materials, which have been received great attention during the last two decades because of their adjustable pore size, structural design ability[3-6], and extensive applications for gas separation[7,8], catalysis[9,10], drug delivery[11], luminescent sensors[12]. Recently, luminescent MOFs have

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attracted more and more interest[13-17]. Among the family members of MOFs[18], lanthanide MOFs (Ln-MOFs) have received growing attention in luminescent probes due to their significant advantages and characteristics, including high color purity,

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large stokes shift, visible color with the naked eye and relatively long luminescence

lifetimes, which arises from the 4f electronic configuration of lanthanides[19,20]. On

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the other hand, large chromophores generated between lanthanide ions and organic ligands as "antennas" can enhance fluorescence intensity of lanthanide ions through

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energy transfer[21-23]. It is the reason why Ln-MOF have good luminescence efficiency compared with other materials[24,25]. Hence, Ln-MOF have been extensively developed as an excellent candidate for the detection of metal ions,

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organic molecules and anions[26-28].

Metal elements play an important role in pharmacy, environment and industrial

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production[29,30]. Among various metal ions, iron (III) is not only considered as an industrial pollutant but also a ubiquitous metal element in biochemical systems, such

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as metabolism and oxygen transport processes of the human body[31,32]. But every coin has two sides, excess and deficiency of iron may result in serious diseases[33]. Aluminum ion is the most abundant metal element in the earth's crust[34]. Moreover, on the one hand, with the massive use of aluminum products, more and more Al3+ ions are exposed to the air and enter the human body through food and drinking[35]. It might raise Parkinson’s and Alzheimer’s diseases, and even cancer[36,37]. Its maximum intake is estimated to be 7 mg/kg body weight per week[38]. Hence, 3

developing an effective method for the selective detection of iron and aluminum ion is of great importance to human health. At the same time, the rapid development of industry is accompanied by the release of toxic organic small molecules, which will cause many environmental and health problems. Therefore, detection of toxic small molecules is also crucial for the environment and the health of human beings[39-41]. According to previous report[42], a pillar-like 3D MOF was assembled from 3,3’,4,4’-biphenyltetracarboxylic acid (H4L) and lanthanide(III) ions through in situ hydrothermal reactions(Scheme 1). In the literature, the excellent performances of

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Eu-MOF in magnetism and fluorescence have been described, but its practical application was not reported. Here we further explore the potential application of Eu-MOF in fluorescence sensing. The Eu-MOF has been fully characterized, which

exhibited excellent luminescence and structural stability in the aqueous solution and

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chemical stabilities in a wide pH range (3-11). It could be developed as a luminescent probe to rapidly detect Fe3+, Al3+ and nitrobenzene. The possible mechanisms have

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also been discussed in detail. Considering the particular spectral response and low limit detection (0.39 μM for Fe3+, 0.084μM for Al3+ and 0.013 μM for nitrobenzene

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(NB), respectively), Eu-MOF can be developed as a multi-functional luminescence

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sensor with high selectivity.

Scheme 1. Schematic illustration of the synthetic process of Eu-MOFs.

2. Experimental section 2.1. Materials All of the chemicals were of analytical grade, and used without further purification. 3,3’,4,4’-biphenyltetracarboxylic acid (H4L) was purchased from 4

Aladdin Chemistry Co., Ltd. (Shanghai). Metal nitrates (K+, Na+, Ag+, Ba2+, Cd2+, Mg2+, Mn2+, Ni2+, Ca2+, Pd2+, Zn2+, Hg2+, Cu2+, Fe3+ and Al3+) were supplied from Yantai Shuangshuang Chemical Co., Ltd, China. Methanol (MeOH), ethanol (EtOH), N,N-dimethylformamide (DMF), 1,4-dioxane, tetrahydrofuran (THF), acetonitrile (CH3CN), trichloromethane (CHCl3), toluene, acetone, tetrahydrofuran (THF), formaldehyde (HCHO), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMA), nitrobenzene (NB), NaAc·3H2O and Eu(NO3)3·6H2O were purchased from Aladdin Industrial Corporation, China.

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2.2. Preparation of Eu-MOF

The Eu-MOF was successfully prepared from a mixture of H4L (0.1 mmol, 30 mg), Eu(NO3)3·6H2O (0.1mmol, 44.6 mg), NaAc·3H2O (0.15 mmol,20mg,), HAc-H2O(1 :

5, v/v, 10 mL) according to the literature. The mixture was stirred about 5 min and

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placed into a 25 mL Teflon-lined stainless steel autoclave, and then heated at 190 °C

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with a heating rate of 5 °C/min for 48 h. The colorless needle-like crystals were washed three times with absolute ethanol, and then dried in air.

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2.3. Materials characterizations

Powder diffraction (PXRD) was recorded on a D8 Advance X-ray Diffractometer with Cu Kα radiation (λ=1.5406Å). Fourier transform infrared spectra (FT-IR) of the

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sample was measured on a Digilab FTS-3000 IR spectrometer (with KBr pellet) in the range of 4000~400cm-1. Thermogravimetric analyses (TGA) was examined on a

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Perkin-Elmer SSC-5200 TG-DTA analyzer with a heating rate of 10 °C /min from room temperature to 800 °C under flowing nitrogen atmosphere. Scanning electron

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microscopy (SEM) and energy dispersive spectrum (EDS) were measured on a Zeiss Ultra Plus field emission scanning electron microscope (Germany) for viewing the morphologies and composition of the sample. UV-vis diffuse reflectance spectra were taken with T6 New Century. The fluorescent spectra were recorded on a FluoroMax-4spectrophotometer. Inductively coupled plasma spectroscopy (ICP) was performed on a ThermoScientific iCAP RQ ICP-MS. 2.3. Luminescence sensing experiment 5

The solid-state fluorescence properties of Eu-MOF and H4L were investigated at room temperature. We carried out the fluorescent detection of metal ions in aqueous solution. The corresponding Eu-MOF/Mm+ was prepared by introducing Eu-MOF powder (2 mg) into 10 mL aqueous solution of MNOx (Mm+ = K+, Na+, Ag+, Ba2+, Cd2+, Mg2+, Mn2+, Ni2+, Ca2+, Pd2+, Zn2+, Hg2+, Cu2+, Fe3+, Al3+, 1×10-2 M), which was dispersed by ultrasound for ten minutes. For sensing the fluorescence intensity of organic small molecules, the Eu-MOF-solvent emulsions were prepared by introducing the as-prepared Eu-MOF (2 mg) into various 2 mL organic solvents, for

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instance, 1,4-dioxane, H2O, DMA, DMF, EtOH, DMSO, CHCl3, formaldehyde, MeOH, THF, CH3CN, toluene, acetone and NB.

3. Results and discussion

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3.1Characterization of Eu-MOF

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Relative Intensity/a.u.

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Fig. 1 Coordination mode for the H4L ligand in Eu-MOF.

as-prepared Eu-MOF

simulated Eu-MOF

10

20

30 2/degree

40

50

Fig. 2 The XRD patterns of as-prepared Eu-MOF and the simulated Eu-MOF.

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The Eu-MOF was synthesized by the combination of H4L, Eu(NO3)3·6H2O and NaAc·3H2O in HAc/H2O solution as reported, the asymmetric unit of Eu-MOF consisted of one Eu(III) ion, one H4L ligand, and one coordinated water molecule. The Eu(III) ion was coordinated with nine oxygen atoms from seven carboxylate groups of six H4L ligands and one water molecule (Fig. 1), which adopted a distorted tricapped trigonal prism. The XRD pattern of as-prepared Eu-MOF was closely matched with that of the simulated one (as shown in Fig. 2), which indicated that the Eu-MOF has been synthesized successfully. A pillar-like structure of Eu-MOF with a

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size of 3-200 μm could be clearly observed according to the scanning electron microscopy (SEM) image (Fig. S1). The FTIR spectra of the H4L were similar to that

of Eu-MOF (Fig. S2). It was noteworthy that the peaks at 3164 cm-1 and 2797 cm-1 belonged to the stretching vibration for O-H group of H4L, the characteristic peak at

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1715 cm-1 referred to the stretching vibration of C=O group and 813 cm-1 belonged to the vibration absorption peak of 1,2,4-bit benzene ring substitution type of H4L.

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Compared with the characteristic peaks of H4L, the IR spectra of Eu-MOF showed that the stretching vibration of C=O at 1715 cm-1was disappear. We speculated that it

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might be attributed to the coordination between Eu3+ and carboxylate of ligand (H4L), which suggested that the successful preparation of Eu-MOF. The thermal stability of

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Eu-MOF was investigated by the thermogravimetric analysis (TGA) from room temperature to 800 °C (Fig. S3). At first, a small weight loss occurred in the range of 35 - 345 °C, which was due to the loss of coordinated H2O molecules and the release

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of CO2, and then the sharp weight loss above 345 °C was attributed to the collapse of the framework, the residual after 600 °C can be ascribed to Eu2O3. These results

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indicated the prepared Eu-MOF possessed a relatively high thermal stability. 3.2 Luminescence properties of Eu-MOF

7

D0 → 7F2

Ex(em = 617 nm)

D0 → 7F4 5

5

5

D0 → 7F0

D0 → 7F3

5

D0 → 7F1

5

Relative intensity/a.u.

m(ex = 320 nm)

200

300

400

500

600

700

800

Wavelength / nm

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Fig. 3 Excitation (black line) and emission (red line) spectra of Eu-MOF. The inset is the corresponding luminescence picture under UV-light irradiation of 365nm.

The luminescence excitation and emission spectra of the H4L ligand in the solid

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state were investigated at room temperature. As shown in Fig. S4, it exhibited an emission peak at 384 nm (λex = 350 nm), arising from ligand centered π* → n

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electronic transitions. The solid-state fluorescence properties of Eu-MOF were determined at room temperature and the excitation and emission spectra were shown

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in Fig. 3. As expected, after coordination with Eu3+, Eu-MOF exhibited a weak fluorescence emission at 435 nm besides Eu3+ characteristic peaks upon excitation at

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320 nm. There was a red shift of about 51 nm compared to the emission of free H4L ligand, which may belong to the ligand-to-metal charge transfer (LMCT). Furthermore, Eu-MOF displayed five characteristic emission of Eu3+ at 580, 591, 617,

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650 and 695 nm, which could be ascribed to the transitions 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3 and 5D0 → 7F4, respectively (at the maximum excitation wavelength

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of 320 nm). Among these emission peaks, the red luminescence peak at 617 nm was the hypersensitive from the typical dipole transition of 5D0 → 7F2, which was a typical characteristic of Eu3+. The corresponding CIE chromaticity diagram (X = 0.522, Y = 0.3086) has been depicted in Fig. S5, and a strong red output can be readily observed by naked eyes under UV-light irradiation at 365 nm, as shown in the inset of Fig. 3. Considering the practical applications, we further studied the fluorescence characteristics of the as-synthesized Eu-MOF material in aqueous solution, as shown 8

in Fig. S6. The similarity both the aqueous and solid fluorescence of Eu-MOF was the characteristic luminescence emissions of Eu3+ excited at 320 nm. In addition, the fluorescence and structural stability of Eu-MOF were further studied to test whether it could be used as a luminescent probe in aqueous media. Firstly, the powder material was uniformly dispersed in 373 K water for 24 hours, and positions of all the diffraction peaks unchanged compared with the characteristic XRD peaks of Eu-MOF in the solid state (Fig. S7a). This results indicated that the excellent structural stability of Eu-MOF in aqueous solution. The relative intensity of Eu3+ at 617 nm showed tiny

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changes as the time increased, which further illustrated its great day-to-day luminescence stability(Fig. S7b). Finally, we further dispersed Eu-MOF into water in

the range of pH from 3-12, as displayed in Fig. S7c, it demonstrated that Eu-MOF exhibited good pH-independent luminescence stability.

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3.3 Sensing of metal ions

Considering that the as-synthesized Eu-MOF exhibited intense red fluorescence,

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excellent structure and fluorescence stability, we speculated it can be developed into a promising luminescent probe in aqueous media. In order to investigate the potential of

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Eu-MOF for sensing of metal ions in aqueous solution at room temperature. 2 mg of Eu-MOF was introduced into aqueous solvent of MNOx (M= K+, Na+, Ag+, Ba2+, Cd2+,

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Mg2+, Mn2+, Ni2+, Ca2+, Pd2+, Zn2+, Hg2+, Cu2+, Fe3+ and Al3+) (10 mL, 1×10-2 M) and a series of luminescent response were recorded in Fig. 4a. By comparing the luminescence intensity of Eu-MOF at 617 nm (Fig. 4b), we could clearly find that the

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luminescence intensity of various metal ions showed varying degrees of quenching effect. When Eu-MOF samples were immersed in the aqueous solution containing

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Hg2+, Cu2+, Al3+ and Fe3+ ion , which exhibited the relatively significant quenching effect on luminescence of Eu-MOF. Compared with Hg2+ and Cu2+ ion, only addition of Fe3+ ion to Eu-MOF aqueous solution could completely quench the emission of Eu3+, which also lead the dark under UV-light. In addition, Al3+ could markedly quench the emission of Eu3+, but it could enhance the emission of the ligand at 435 nm (Fig. 4c), and the color of fluorescent emission could vary from red to bright blue under the UV lamp (As shown inset of Fig. 4b). The corresponding CIE chromaticity 9

diagram (X = 0.1744, Y = 0.1596) has been depicted in Fig. S5. Therefore, Eu-MOF

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can selectively sense Fe3+ and Al3+ions in aqueous solution.

(c) 80000

(b) 200000

60000

40000

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100000

0

+

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H

2+

0

A 3 l H + g2 C + d 2+ Pd 2 M + g 2+ Ba

20000

K+ N a+ A g+ Ba 2+ C d2 M + g2 M + n 2+ N 2 i + C a 2+ Pd 2+ Zn 2+ C o 2+ H g 2+ C u 2+ A 3 l + Fe 3

50000

K H + 2O N a+ N 2 i C + a2 M + n 2+ A g Zn + 2 C + o2 C + u 2+ Fe 3

Intensity/a.u.

Al3+ H2O Fe3+

2O

Intensity/a.u.

150000

+

435 nm

D0 → 7F2

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Fig. 4 (a)Photoluminescence spectra and (b) (Bottom) the 5D0 → 7F2 transition intensities of the Eu-MOF introduced into different cation aqueous solutions of 10−2 M (at ex = 320 nm).(Top) Photographs showing color changes after adding Al3+ and Fe3+ under UV- light irradiation at 365

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nm. (c) The intensities of Eu-MOF at 435 nm when dispersed in various metal ions aqueous under

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excitation at 320 nm.

As shown in Fig.S8a and Fig. S8b, the anti-interference capability of Eu-MOF

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was further studied by mixing Fe3+or Al3+ with various coexisting ions. The results demonstrated that other ions had no or minor effect on the detection of Fe3+or Al3+, it also proved that Eu-MOF exhibited high selectivity for the luminescent sensing of Fe3+ and Al3+ ions in aqueous solution. Both Fe3+ and Al3+ ions could quench the characteristic emission of Eu3+, so we further explored whether selective detection can be achieved when Fe3+ and Al3+ ion co-existed simultaneously. We found that Eu-MOF could be dissolved when Al3+ was added alone, the suspension became clear, 10

and the luminescence intensity of Eu3+ decreased while the emission intensity of ligand increased (Fig. S9). However, when only Fe3+ ion was added to Eu-MOF aqueous solution, the MOF did not dissolve and remained suspended, and the emission intensity of both ligand and Eu3+ were quenched. When Fe3+ and Al3+ ions were added simultaneously, the solution gradually became clear. Compared with Fe3+ and Al3+ ions separately, the fluorescence intensity of ligands was in an intermediate state, which was neither completely quenched nor enhanced significantly. Therefore, it can be judged whether Fe3+ and Al3+ existed in solution simultaneously based on the

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above basis. If both existed, selective detection can be achieved by adding masking agent.

In order to further evaluate the high selectivity of Eu-MOF toward Fe3+ and Al3+, a series of luminescence spectra of Eu-MOF suspensions in water with varying

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concentrations of Fe3+ or Al3+ were recorded. It could clearly be found that the fluorescence intensity decreased with the increasing concentration of Fe3+ (Fig. 5a).

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The quenching effect on the luminescence of Eu3+ was close to 98% when the concentration of Fe3+ reached 500 μM (Fig. S10). The quenching efficiency could be

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approximately calculated by the equation of Stern-Völmer equation: I0/I = 1+Ksv[M], where Ksv is the quenching constant (M-1), [M] is the concentration of Fe3+, the I0 and

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I are the luminescent intensity of Eu-MOF suspension in absence and presence of Fe3+ ions, respectively. The Stern-Völmer curve shown that there was a good linear relationship between fluorescence intensity and the concentration of Fe3+ in the range

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of 0.01-220 μM (R2 = 0.9918), (as shown in Fig. 5b) and Ksv was calculated as 1.78104 M-1. Based on the equation DL = 3Sb/K, the fluorescence detection limit

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(DL) of Eu-MOF for Fe3+was 0.39 μM (where Sb is the standard deviation for fluorescence measurements of blank solution; K is the slope of the fitting line). The SV curves deviated from the linear correlation with the increase of concentration (Fig. 5b) illustrated the existence of both static and dynamic quenching processes[43-45]. As demonstrated in Fig. 6a, the luminescence intensity of Eu3+ decreased slowly in the Al3+concentration region from 0 to 500 μM, while the emission intensity of the ligand was gradually enhanced and possessed a good linear relationship between 11

emission intensity of the ligand and Al3+ concentration(R2 = 0.9930) (Fig. 6b). From the slope of the fitting line (K) and the standard deviation (Sb), the detection limit (3Sb/K) of Eu-MOF towards Al3+ was calculated to be 0.084μM. The characteristic emission of the Eu3+ will disappear when the concentration of Al3+ reaches 10 mM. In addition, in order to test whether the fluorescence of this Eu-MOF could be reversible, we added EDTA (10 mM) into the suspension of Fe3+@Eu-MOF and Al3+@Eu-MOF. The Eu-MOF fluorescence probe returned to its initial value with the addition of

0.005 μM 0.01 μM 6 μM 8 μM 10 μM 15 μM 20 μM 50 μM 80 μM

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100 μM 150 μM 200 μM 220 μM 250 μM 300 μM 400 μM 450 μM

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Intensity/a.u.

(a) 120000

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EDTA (Fig. S11), and the red color appeared again under the UV-lamp.

0 500 600 Wavelength/nm

700

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400

(b) 25

(c)

Ksv=1.78×104

4.5

R2=0.9918

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20

5.0 4.0 3.5

I0/I

I0/I

15 10

3.0 2.5 2.0

0

100 200 300 400 concentration/Fe3+(mol)

1.5 1.0 0.5

500

0

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50 100 150 200 concentration/Fe3+(mol)

250

Fig.5 (a) Emission spectra and (b) SV curve of Eu-MOF in aqueous solution (excited at 320 nm)

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with different concentrations of Fe3+ ions (0-450 μM), (c) the Stern-Volmer plot of Fe3+ ion at low concentration (0.05-220 μM).

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(c) 40000

(a) 200000 0 mM 0.1 mM 0.2 mM 0.3 mM 0.4 mM 0.5 mM 1.0 mM 2.0 mM 5.0 mM 10.0 mM

100000 Al3+

Eu

R2=0.993 Intensity/a.u.

Intensity/a.u.

150000

Ksv=5.11×106 3+

30000

20000

50000 10000

0 400

500

600

0.0

700

0.1

0.2

0.3

0.4

0.5

3+

concentration/Al (mM)

Wavelength/nm

Fig. 6 (a) Emission spectra and (b) the Stern-Volmer plot of Al3+ ion at low concentration (0-0.5

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mM).

So far, the quenching mechanisms of metal ions for MOFs are still not very clear. Generally speaking, it can be mainly attributed to the following factors according to

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literature reports: 1) The collapse of the frameworks structures; 2) the interactions

between ions and organic ligands; 3) cation exchange between the central cations of

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the frameworks and the targeted cations; 4) the competition in energy absorption[46-52]. The XRD patterns (Fig. S12) of Eu-MOF after immersing in

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various metal ion aqueous solutions were well consistent with the original solid powder, which indicated that Eu-MOF was relatively stable and the collapse of the framework was not the main cause of luminescence quenching effect. On the other

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hand, in order to further confirm the fluorescence quenching caused by cation exchange, 4 mg of Eu-MOF was dispersed in 10 mL of 0.01 M Fe3+ solution. After

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immersion for 2 h, the concentrations of Fe3+ detected by ICP were shown in Table S1. It was found that when the MOF was immersed to Fe3+ solution for 2 h, Fe3+

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concentration of the solution remarkably decreased. Therefore, the quenching mechanism caused by cation exchange between the central cations of the frameworks and the targeted cations can explain why Fe3+ can reduce the luminescent intensity of Eu-MOF in a degree. The SV curve resulted from the linear correlation with the increase of concentration indicated that the existence of both static and dynamic quenching processes, but which process played a dominant role need be distinguished by the value of Ksv at different temperatures. 13

The values of KSV at different temperatures could be obtained based on Stern-Volmerequation (1)[53,54]: 𝐹0 𝐹

= 1 + 𝐾𝑆𝑉 [Q] (1)

Where, the F0 and F are the luminescent intensity of Eu-MOF suspension in the absence and presence of Fe3+ ions, respectively. [Q] is the concentration of Fe3+. KSV is the Stern-Volmer constant, which indicates the quenching efficiency between the Eu-MOF and Fe3+. The calculated KSV values are summarized in Table S2. It can be found that dynamic quenching constants increase with the increasing temperature, it

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also illustrate that the number of collision between Fe3+ and the Eu-MOF increases. Therefore, dynamic quenching is dominant.

The number of binding sites of Fe3+ can be determined according to the following

𝑙𝑔

𝐹0 −𝐹 𝐹

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equation (2): = 𝑙𝑔𝐾𝐴 + 𝑛lg[𝑄](2)

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Where, the F0 and F are the same meaning as equation (1), [Q] is the concentration of Fe3+. KA and n are the binding constant and the binding sites,

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respectively. The fitting graph showed that there was a good linear relationship between lg[(F0-F)/F] versus lg[Q], we could obtain the values of n and K by the slope and intercept. As shown in Fig. S13, after calculation, the value of n was almost equal

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to 1, which showed that there was one binding site for Fe3+ with Eu-MOF. Antenna effect can be interpreted as the appropriate ligand absorbs energy to

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generate excitation, and then transfers energy to the luminescent central lanthanide ions[55]. As shown in Fig. S14, when Al3+ was added to the aqueous solution of the

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ligand (H4L), the emission intensity of H4L increased significantly. However, the addition of Eu3+ obviously weakened the emission intensity of H4L and the characteristic peak of Eu3+ was enhanced significantly, which could be attributed to the antenna effect. Furthermore, when Al3+ and Eu3+ were added to the aqueous solution of H4L simultaneously, the characteristic peak intensity of Eu3+ increased by the same amount compared with the only addition of Eu3+, and the emission intensity of H4L was significantly reduced compared with only Al3+ addition, but almost similar 14

to Eu3+ addition alone. When Al3+ was added to the suspension of Eu-MOF, the suspension became clear gradually and the emission intensity of H4L was enhanced at the same time. In addition, the characteristic emission intensity of Eu3+ would disappear when the concentration of Al3+ increased to some extent, and the emission spectrum of Eu-MOF was similar to that of the both addition of Al3+ and Eu3+ in H4L aqueous solution. These results indicated that Al3+ would destroy the structure of Eu-MOF, and further form H4L and Eu3+separately in the aqueous solution, which made the emission intensity of H4L enhanced and achieved the determination of Al3+.

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An analytical performance comparison of the proposed method with reported results was listed Table S3, it was found that the developed process here possessed lower detection limits for Fe3+ and Al3+ determination (Table S3). 3.4 Sensing for organic small molecule.

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In order to examine the potential of Eu-MOF for the sensing of organic small

molecules, several small organic solvent molecules were selected for detection. As

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shown in Fig. 7a, the luminescent intensity of the 5D0 → 7F2 transition for the Eu3+ ions significantly depended on the solvent molecules (1,4-dioxane > H2O >DMA >

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DMF >EtOH> DMSO > CHCl3> HCHO >MeOH> THF > CH3CN > acetone > toluene > NB). Interestingly, NB has a striking quenching effect on the Eu3+, and the

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photograph under UV light could further confirm the significant quenching effect as shown in the inset of Fig. 7b. To evaluate the relationship between the luminescence intensity of Eu-MOF and the concentration of NB in more detail, Eu-MOF was firstly

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immersed into ethanol as the standard suspension, and the fluorescence responses were recorded with the NB content increased. Fig. 8a showed that the luminescence

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intensity of Eu-MOF decreased gradually and the emitted visible red light of Eu3+ fades with the increase of NB content. When the concentration of NB reached 30 μM, subsequently, the luminescence intensity of Eu-MOF was almost quenched, and the fluorescence intensity was about 0.7% of the original fluorescence intensity (Fig. S15).

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(b)

D 0  7 F2

H2O NB

1. 4-

di

ox

an e H D2O M D A M Et F O D H M S C O H H Cl C 3 C HO H 3O H T C HF H ac 3 CN e To t on lu e en e N B

Relative Intensity/a.u.

5

Fig. 7 (a) Photoluminescence spectra of the Eu-MOF introduced into different organic solvents (2

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mg/3 mL) (at ex = 320 nm). (b) (Bottom) the 5D0 → 7F2 transition intensities of Eu-MOF when dispersed in different organic solvents, (Top) photographs showing color changes after adding NB under UV- light irradiation at 365 nm.

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Through the analysis of the obtained data, there were two sections of linearity in

the range of low concentration (0-100 μM). The results of Fig. 8b and Fig. 8c showed

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that there was good linear correlations between I0/I and the content of NB in the low concentration range of 0-8 μM (R2 = 0.9950) and 10-100 μM (R2 = 0.9942). The Ksv

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for NB was calculated to be 6.63×105 μM-1and 5.34×106 μM-1 by using the Stern-Völmer equation. Combining two linear correlations, based on the slope of the fitting line and the standard error, the detection limit was calculated to be 0.013 μM

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(S/N =3).

0.5 μM 1 μM 2 μM 4 μM 6 μM 8 μM 10 μM 15 μM 20 μM

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Intensity / a.u.

(a)

550

600

650 Wavelength / nm

30 μM 40 μM 50 μM 70 μM 80 μM 100 μM 200 μM 300 μM

700

16

(c)

7

(c) 500

KSV=6.63×105

Ksv=5.34×106

R2=0.9950

5

400

4

300

R2=0.9942

I0/I

I0/I

6

3

200

2

100

1

0 0

2

4

6

8

0

20

Concentration/NT(M)

40

60

80

100

Concentration/NT(M)

Fig. 8(a) Emission spectra of Eu-MOF dispersed in EtOH solution with different concentrations of

Stern-Völmer plot of NB at low concentration (10-100 μM).

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NB (0-300 μM), (b) the Stern-Völmer plot of NB at low concentration (0-8 μM) and (c) the

The UV-vis spectra of various organic small molecules in EtOH solution were

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recorded (Fig. S14). NB has a higher absorbance than other reagents at 320 nm. In

this process, the NB competes with the ligand for absorption of light energy, and the

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energy of the excitation light will be strongly absorbed and the ligand absorption is weakened, so that the efficiency of energy transfer to the luminescent center ion was

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reduced, leading to Eu-MOF fluorescence quenching. As shown in Fig. S17, the absorption intensity increases with the increasing concentration of NB, which can further illustrate the NB competition with the ligand for absorption of light energy,

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such phenomenon has been reported in the literatures[56,57].

4. Conclusion

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In summary, a pillar-like structure luminescent Eu-MOF has been prepared under facile and mild conditions according to previous literatures. Study of the luminescent

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properties exhibited that Eu-MOF could be developed into a highly sensitive and selective probe for Fe3+, Al3+ and NB by different response mechanisms. Considering the obvious spectral responses and low detection limits (0.39μM for Fe3+, 0.084μM for Al3+ and 0.013μM for NB), Eu-MOF displayed superior capabilities for highly selective sensing of Fe3+ and Al3+ ions and nitrotoluene. The proposed method had a higher sensitivity for Fe3+ and Al3+ ion determination than many reported procedures recently. 17

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests

Acknowledgements

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Authors are very grateful to National Natural Science Foundation of China (21665024), Northwest Normal University Young Teachers Research Capacity Promotion Plan (NWNU-LKQN-18-23), the Key Lab of Eco-Environments Related Polymer Materials of MOE, Key Lab of Polymer Materials of Gansu Province for

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their financially supports.

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