Anionic metal–organic framework for high-efficiency pollutant removal and selective sensing of Fe(III) ions

Abstract

An anionic metal–organic framework, (DMA)₂[Y₉(μ₃-OH)₈(μ₂-OH)₃BTB₆]_n·(solv)_x (DMA = dimethylamine cation and BTB = 1,3,5-benzene(tris)benzoate), (gea-MOF-1), is a novel rare earth (RE) nonanuclear MOF with an unusual gea topology. It could be used as an adsorbent material for efficiently removing the cationic dye MB⁺. This MOF can not only emit strong fluroescence in MeOH suspension, but also serves as a fluorescent probe for selectively detecting Fe³⁺ among other metal ions through luminescence emission quenching.

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With the development of chemical industry, poisonous chemicals, like toxic heavy metal ions and organic small molecules, are increasingly released from industrial and other human activities, which have a negative impact on human health and environment.^1–3 Particularly, organic dyes have both acute and chronic toxicity effects on human health, which may lead to transgeneration, cancer, endocrine disruption and other serious diseases.⁴ As indispensable industrial products, organic dyes have generally been used in paper-making, textile industry, etc.⁵ Therefore, it is extremely important to resolve the environmental contamination caused by organic dyes. Up to now, several approaches have been presented to solve the problems, such as advanced oxidation, adsorption, photocatalysis, membrane filtration and coagulation.⁶ Among them, adsorption is well known to be one of the strategies because of its low cost, high efficiency and environmental friendliness.⁷ For this matter, many common adsorbents, such as zeolites, active carbon and polymeric materials, have been extensively applied to remove organic dye contaminants.⁸ Although these common adsorbents can easily adsorb multifarious mixed organic dyes, they are usually inferior to selectively separating objective organic dye wastes.⁹ Therefore, it is desirable to prepare novel materials which could separate the target selectively from mixed dye waste. In addition, iron is essential for the human body. An excess or lack of Fe³⁺ in body fluids may cause a potentially deadly or hereditary disease such as hemochromatosis and iron-deficiency anemia. Therefore, the detection of Fe³⁺ using a selective and accurate manner is crucial.

Porous metal–organic frameworks (MOFs) have emerged with attractive applications in areas of gas storage, separation, luminescence, drug delivery and heterogeneous catalysis owing to their diverse structural topologies and functional sites.¹⁰ So far, ionic MOFs can be applied in adsorption and separation of organic dyes.^2,11 For example, Zhao et al. constructed a series of mesoporous positive MOFs, which serves as a platform for the anion exchange-based separation process to capture anionic organic dyes.^2e Our group also synthesized anionic In^III-MOFs for adsorption and separation of methylene blue, which act as the immobile phase of a chromatographic column to separate smaller sized cationic organic dyes.^2f Most of the reported MOFs are utilized to separate smaller sized organic dyes based on their size exclusion, which is excellent for highly selective separation of organic dyes.^2f Meanwhile, RE(III)-based MOFs are outstanding candidates as fluorescence probe materials for heavy metal ion sensing.¹²

The porous material (DMA)₂[Y₉(μ₃-OH)₈(μ₂-OH)₃BTB₆]_n·(solv)_x (DMA = dimethyl amine cation and BTB = 1,3,5-benzene(tris)benzoate) gea-MOF-1 was synthesized according to the previously reported lietrature¹³ and confirmed by powder X-ray diffraction (PXRD) patterns (Fig. S1, ESI†). This porous MOF material exhibits the potential for gas storage and heterogeneous catalysis applications. In this work, we found that anionic gea-MOF-1 in dimethyl formamide (DMF) solution can rapidly adsorb the cationic dye methylene blue (MB⁺) with a smaller size based on cation exchange, but hardly adsorbs anionic methyl orange (MO⁻), neutral Sudan I (SD⁰) and the cationic dye methylene violet (MV⁺) (Scheme 1) with larger sizes. Surprisingly, gea-MOF-1 can preferentially adsorb MB⁺ from the mixed organic dyes in DMF solution. Moreover, gea-MOF-1 also displays excellent luminescence properties, and can act as a luminescence probe for Fe³⁺.

Scheme 1 Chemical structures of MB⁺, MO⁻, SD⁰ and MV⁺.

As shown in Fig. 1, gea-MOF-1 displays a pillared hexagonal (hxl) layer structure along the c axis, creating one-dimensional channels with window sizes of 12.834 Å × 9.386 Å. DMA⁺ cations reside in the hxl channels.

Fig. 1 Illustration of the pillaring of hxl layers in the gea-MOF-1, creating one-dimensional channels.

According to the related ref. 2e and 14, the charge characteristics and size effects of the organic dye molecules play crucial roles during the process of dye adsorption or separation.^2e Due to the highly porous features and anionic characteristics of gea-MOF-1, we investigate its potential capacity for adsorption and separation of organic dye molecules from DMF solution. In the adsorption study, three organic dyes with different charges were chosen and dissolved in DMF (5 × 10⁻⁵ M), namely MB⁺, MO⁻ and SD⁰, all of which have a similar relative molecular mass and size but different charges (Table S1, ESI†). The as-synthesized gea-MOF-1 was soaked in fresh as-prepared DMF solutions of MB⁺, MO⁻ and SD⁰. The capacity of gea-MOF-1 for capturing dyes from the DMF solution was detected by UV-vis absorption spectroscopy at certain time intervals. As shown in Fig. 2a, the absorbance peak value of the supernatant declined gradually until 210 min, suggesting that almost all of the MB⁺ in the supernatants was removed by the gea-MOF-1. On the contrary, the absorbance peak values of MO⁻ (Fig. S2, ESI†) and SD⁰ (Fig. S3, ESI†) solutions were unchanged. Moreover, further exploration of the selective separation of MB⁺ from mixtures of MB⁺/MO⁻ and MB⁺/SD⁰ in solution were implemented. As demonstrated in Fig. 2b and c, the results were as anticipated. Only the MB⁺ in the mixture solutions was removed by the gea-MOF-1 and the solutions subsequently showed the colors of MO⁻ and SD⁰. Spectroscopic evaluations of the supernatants revealed that gea-MOF-1 can highly efficiently capture organic cationic dyes, whereas organic anionic and neutral dyes can not be adsorbed by gea-MOF-1. Finally, the color of the gea-MOF-1 changed from transparent to blue. This phenomenon was ascribed to the anionic characteristics of the gea-MOF-1 framework, in which free DMA⁺ present in the channels can be exchanged with the organic cationic dye MB⁺. Therefore, gea-MOF-1 can remove organic cationic dyes among neutral and positively charged organic dyes through ion-exchange processes. The organic dye molecules with similar sizes but different charges could be selectively separated by gea-MOF-1 through ion-exchange processes.

Fig. 2 UV-vis spectra of DMF solutions of equimolar dyes in the presence of gea-MOF-1 monitored with time. (a) MB⁺, (b) MB⁺/MO⁻, (c) MB⁺/SD⁰, and (d) MB⁺/MV⁺. The photographs show the colors of the dye solutions and the crystalline samples of gea-MOF-1, before and after ion-exchange for 210 min.

We also investigate the effect of size on the absorption of cationic dyes. Two organic cationic dyes with different molecular sizes were chosen as candidates: MB⁺ and MV⁺. They have identical charge characteristics but different molecular sizes: MV⁺ (4.00 Å × 16.32 Å) is larger than MB⁺ (4.00 Å × 7.93 Å) along the x and y directions (Table S1, ESI†). Typically, the fresh as-synthesized samples of gea-MOF-1 were immersed in DMF solutions of MB⁺ and MV⁺. As shown in Fig. 2a and Fig. S4, ESI,† the concentration of smaller dyes declined sharply. However, almost no change was observed in the concentration of the larger ones. The size selective effect was also tested for the mixed solution of MB⁺/MV⁺. As shown in Fig. 2d, only the characteristic peak of MB⁺ decreased significantly, meanwhile, the solution changed from blue-violet to purple. The above results show that only MB⁺ can be exchanged by the gea-MOF-1 and MV⁺ cannot finish the cation-exchange process with gea-MOF-1 owing to its larger size. From the above experiments, gea-MOF-1 might be an admirable adsorbent for the efficient and selective removal of smaller cationic dyes.

Within the dye adsorption and release process, the stability and reversibility of the crystal are also crucial. The experiments of MB⁺ release are also implemented. The MB⁺@gea-MOF-1 sample was immersed into the saturated NaNO₃ DMF solution and the concentration change of MB⁺ in the supernatant was monitored using UV-visible spectra. As shown in Fig. S5, ESI,† the absorbance peak value of the supernatant increased gradually. This phenomenon is ascribed to the cation exchange process that Na⁺ entered into the pore of gea-MOF-1 replacing MB⁺. The PXRD of gea-MOF-1 after investigation of release confirmed that the host skeleton of the gea-MOF-1 material did not change (Fig. S6 and S7, ESI†).

With a view to the potential applications of RE^III-MOFs as luminescence sensor materials in biological systems, we also studied the solid state photoluminescence properties of H₃BTB and gea-MOF-1, as shown in Fig. S8 and S9, ESI.† Excited at 284 nm and 290 nm at room temperature, H₃BTB and gea-MOF-1 reveal emission peaks at 385 nm and 375 nm, respectively, which can be tentatively attributed to the π–π* transition of the intraligand.¹⁵ The test of gea-MOF-1 for recognizing metal ions was also performed. M(NO₃)_n (Mⁿ⁺ = Ag⁺, Li⁺, Cd²⁺, Al³⁺, Pb²⁺, Fe³⁺ Cr³⁺, Zn²⁺, Mg²⁺, Ca²⁺, Co²⁺, Cu²⁺, In³⁺, Ni²⁺ or K⁺) was added into the methanol suspension of gea-MOF-1 at room temperature. As shown in Fig. 3, the luminescence intensity of the methanol suspension of gea-MOF-1 has the strongest quenching effect with added Fe³⁺ ion. However, with Ag⁺, Al³⁺, Ni²⁺ and Cr³⁺, the emission intensity of gea-MOF-1 was weakened to a small degree, and other metal ions had inappreciable effect on the photoluminescence intensity of gea-MOF-1, indicating that gea-MOF-1 could selectively detect Fe³⁺ ions through luminescence quenching. Furthermore, the luminescence emission intensity of gea-MOF-1 declined gradually upon the addition of 0.1–2.0 equiv. of Fe³⁺ ions (Fig. 4) until the photoluminescence was completely quenched when 2.0 equiv. of Fe³⁺ ions were introduced into the methanol suspension of gea-MOF-1. Meanwhile, according to previous references,¹⁶ we investigated the selectivity to the Fe³⁺ ion among other metal ions (Fig. S10 and S11, ESI†) as demonstrated by the introduction of Li⁺, Cd²⁺, Pb²⁺, Zn²⁺, Mg²⁺, Ca²⁺, Co²⁺, Cu²⁺, In³⁺ and K⁺ into the system. As a result, it indicated that gea-MOF-1 can selectively probe Fe³⁺ among other metal ions.

Fig. 3 Room-temperature luminescence intensity of gea-MOF-1 at 375 nm in methanol suspension, upon the addition of various metal ions (λ_ex = 290 nm).

Fig. 4 Emission spectra of the gea-MOF-1 in methanol at room temperature in the presence of different equiv. of Fe³⁺ ions, with respect to gea-MOF-1 (λ_ex = 290 nm). Inset: photograph showing the change in the original fluorescence of the methanol suspension of gea-MOF-1 (left) and the decreased luminescence upon the addition of Fe³⁺ ions (right).

In order to better understand the mechanism of luminescence quenching upon Fe³⁺ addition, the PXRD of the sample after Fe³⁺ sensing was measured. The pattern indicated that this quenching phenomenon has no relation with the crystallographic alteration, and that the detection mechanism of gea-MOF-1 in methanol solution would not induce framework collapse (Fig. S12, ESI†). Furthermore, inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements were also carried out to monitor the amount of change in Y and Fe elements during the sensing process. The ICP-OES results showed that the amount of the Fe element was much less than that of Y (Table S2, ESI†), indicating that the luminescence quenching effect should not arise from a cation-exchange process.¹⁷ Lastly, according to the UV-vis absorption spectrum (see Fig. S13, ESI†), the strong absorption of the Fe³⁺ methanol solution was in the range of 200–500 nm, while the other metal ions in methanol solution did not overlap this absorption range. Thus, the following two possible mechanisms may account for the origin of the high selectivity and sensitivity towards Fe(III). Firstly, there exists the competition of the absorption excitation wavelength (290 nm) energy between the Fe³⁺ ions methanol solution and gea-MOF-1. Such competitive adsorption will significantly decrease the transfer of excitation energy. Secondly, there exists a complete overlap between the absorption spectrum of the Fe³⁺ methanol solution and the broad emission band at 375 nm of gea-MOF-1. By increasing the Fe³⁺ ion concentration, the color of the gea-MOF-1 suspension would gradually deepen and the luminescence intensity of gea-MOF-1 suspension would decline gradually. Based on these pieces of evidence, we then suggest that a competitive absorption mechanism may account for the selective quenching response of gea-MOF-1 towards Fe³⁺.¹⁸

In summary, anionic gea-MOF-1 containing 1D channels exhibits excellent size-selective adsorption of organic cationic dyes MB⁺, MO⁻, SD⁰ and MV⁺ via a cation exchange process. Furthermore, the gea-MOF-1 could be utilized as a potential luminescence probe for Fe³⁺, which shows significant quenching effect for Fe³⁺ among other metal cations. Therefore, RE-based MOFs have great potential applications to develop MOF-based multifunctional materials.

Acknowledgements

This work was granted financial support from the National Natural Science Foundation of China (21271096).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, PXRD, UV-vis absorption spectra, luminescence spectra. See DOI: 10.1039/c6ra08500h

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