Hollow terbium metal–organic-framework spheres: preparation and their performance in Fe³⁺ detection

Abstract

Hollow metal–organic framework (MOF) micro/nanostructures have been attracting a great amount of research interest in recent years. However, the synthesis of hollow metal–organic frameworks (MOFs) is a great challenge. In this paper, by using 1,3,5-benzenetricarboxylic acid (H₃BTC) as the organic ligand and 2,5-thiophenedicarboxylic acid (H₂TDC) as the competitive ligand and protective agent, hollow terbium MOFs (Tb-MOFs) spheres were synthesized by a one-pot solvothermal method. By comparing the morphology of Tb-MOFs in the presence and absence of H₂TDC, it is found that H₂TDC plays a key role in the formation of the hollow spherical structure. Single crystal analyses and element analysis confirm that H₂TDC is not involved in the coordination with Tb³⁺. Interestingly, Tb-MOFs can be used as the luminescent probes for Fe³⁺ recognition in aqueous and N,N-dimethylformamide (DMF) solutions. In aqueous solution, the quenching constant (K_SV) is 5.8 × 10⁻⁴ M⁻¹, and the limit of detection (LOD) is 2.05 μM. In DMF, the K_SV and LOD are 9.5 × 10⁻⁴ M⁻¹ and 0.80 μM, respectively. The sensing mechanism is that the excitation energy absorption of Fe³⁺ ions reduces the energy transfer efficiency from the ligand to Tb³⁺ ions.

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1. Introduction

Due to their enhanced surface-to-volume ratio, micro-reactor environment, high loading capacities, excellent designability and flexibility, hollow metal–organic frameworks (MOFs) have shown fascinating physicochemical properties and wide applications, especially in molecule adsorption, filtration, diffusion and transfer^..^1–3 Recently, several strategies have been developed to synthesize hollow MOFs, such as template-mediated assembly, interfacial synthesis, stepped dissolution-regrowth, ion-exchange reactions, chemical etching, etc.^4–6 Using carboxylate modified polystyrene spheres as a hard template, hollow zeolitic imidazolate framework-8 (ZIF-8) microspheres were fabricated by electrostatic interaction.⁷ Choe et al. prepared single-shell and double shelled MOFs using metal organic polyhedra as a self-template and building block.⁸ Wang et al. synthesized a hollow zirconium-porphyrinic MOF with single atoms embedded using benzoic acid as a competitive reagent.⁹ By adding carbon dioxide bubbles into continuous ionic liquid to form a liquid–gas interface, hollow Zn-BTC was successfully prepared.¹⁰ Hollow MOFs can also be obtained through liquid–liquid interface and solid–liquid interface methods.^11–13 Although great achievements have been achieved, the synthesis of hollow MOFs with controllable structure is still a complicated and difficult task; it is urgently needed to synthesize hollow MOFs with a simple and easy method.^3,4

The Fe³⁺ ion is one of the essential metal ions in industrial production and biological systems; a deficiency of Fe³⁺ ions in the human body will cause physical problems such as anemia, liver injury, diabetes, and heart failure. But excessive Fe³⁺ will cause serious environmental issues.^14–16 Therefore, the simple, rapid and selective detection of Fe³⁺ ions is meaningful to research. Due to the advantages of high sensitivity, good stability, simple operation and real-time detection, a luminescent quenching-type sensor is often considered to be an ideal choice for high-sensitivity and high-precision Fe³⁺ ion detection.^17,18 Lanthanide metal–organic frameworks have high luminous efficiency, high color purity, large Stokes-shift, long luminescence life, and have been widely used in the selective detection of metal ions and small molecules.^18–20 Thus, in this work, novel hollow spherical Tb-MOFs were prepared easily by the solvothermal method based on 1,3,5-benzenetricarboxylic acid (H₃BTC) as the ligand, and 2,5-thiophenedicarboxylic acid (H₂TDC) as the competitive and protective agent. The crystal structure, morphology, elemental information, thermal stability and luminescent properties of the Tb-MOFs were characterized and analyzed, and the sensing performance of Fe³⁺ ions is evaluated and discussed in detail.

2. Experimental section

2.1 Reagents

H₃BTC (98%), H₂TDC (98%) and terbium nitrate hexahydrate (Tb(NO₃)₃·6H₂O, 99.9%) were purchased from Macklin Inc. Ferric nitrate nonaqueous (Fe(NO₃)₃·9H₂O, A.R.) and N,N-dimethylformamide (DMF, A.R.) were obtained from Aladdin. All reagents were not further purified before use.

2.2 Instruments

The single crystal diffraction data were recorded by Bruker D8 Advance X-ray diffractometer with Ga Kα radiation. Powder X-ray diffraction (XRD) patterns were measured on a PANalytical Empyrean Series 2 diffractometer (Cu K_α) in the 5–50° angular range with a step of 0.026° and a scanning speed of 7° per minute. Thermogravimetric analysis (TGA) was performed under a flow of N₂ in the temperature range 10–700 °C at a heating rate of 10 °C min⁻¹ on a TGA Q-50 thermogravimetric analyzer. The UV-Vis absorption and luminescence spectra were determined on a UV 3600 spectrometer (Shimadzu, Japan) and an F-4600 fluorescence spectrometer (Shimadzu, Japan), respectively. The emission decay processes and quantum yields were recorded on an FLS980 spectrofluorometer system (Edinburgh, UK). Morphology analysis was obtained using a field emission scanning electron microscope (FE-SEM, JSM-7800F, Japan, LaB6, ZEISS, Germany). The X-ray photoelectron spectroscopy (XPS) was collected on an XPS-Theta probe instrument (Escalab250Xi, Thermo Fisher, USA). The high-resolution transmission electron microscope (HRTEM) with high-angle annular dark field (HAADF, ThermoFisher Scientific Talos F200S, Czech Republic) was used to analyze the element distribution and composition of the Tb-MOFs. Fourier-transform infrared spectroscopy (FTIR) measurements were recorded using an FTIR spectrometer (iS50, Nicolet, USA).

2.3 Synthesis of Tb-MOFs

1 mmol (0.453 g) Tb(NO₃)₃·6H₂O, a certain proportion of H₃BTC and H₂TDC (Table S4, ESI†) were dissolved in 10 ml DMF, respectively. The above three solutions were fully mixed and transferred to a Teflon-lined reactor and heated at 150 °C for 72 h. The reaction products were centrifuged and cleaned with DMF, and the Tb-MOFs were obtained after drying in a vacuum drying oven at 60 °C.

3. Results and discussion

3.1 Structure of Tb-MOFs

The single-crystal X-ray diffraction analysis results of Tb-MOFs are shown in Tables S1–S3, ESI.† Tb-MOFs are assigned to the monoclinic space group C2/c with the lattice parameters of a = 18.6318(8) Å, b = 18.6318(8) Å, c = 19.7386(8) Å. Both H₃BTC and DMF are participated in the coordination, forming a three-dimensional MOFs structure with Tb³⁺ as the center, while H₂TDC does not coordinate with Tb³⁺.

The synthesis flow chart of Tb-MOFs is shown in Fig. S1, ESI.† Hollow sphere Tb-MOFs are synthesized using H₃BTC and Tb³⁺ in the DMF system under the action of H₂TDC. In this process, both H₃BTC and DMF participated in the synthesis of Tb-MOFs structure. The coordination environment of Tb³⁺ is shown in Fig. 1a, Tb01 is 8-coordinated environment consisting of 6 oxygen atoms from H₃BTC (H₃BTC1, H₃BTC2, H₃BTC3) anion and 2 oxygen atoms from DMF (DMF1, DMF2, DMF3). As shown in Fig. S2, ESI,† O002 and O005 from H₃BTC1 are coordinated with Tb³⁺ by asymmetric chelation, the Tb–O bond lengths are 2.443(4) Å and 2.399(4) Å, respectively, and the bond angle of Tb–O–Tb is 54.30(12)°. O009 and O006 in H₃BTC2 are coordinated with Tb³⁺ by symmetric chelation, the Tb–O bond lengths are 2.413(4) Å and 2.415(4) Å, respectively, and the bond angle of Tb–O–Tb is 53.61(14)°. O003 and O004 in H₃BTC3 and H₃BTC4 coordinate with Tb³⁺ by bridging, and the Tb–O bond lengths are 2.330(4) Å and 2.283(4) Å, respectively. O007 and O008 monodentate coordination Tb³⁺ from DMF1 and DMF2, the bond lengths of Tb–O are 2.379(5) Å and 2.354(4) Å, respectively. In the whole coordination structure, the bond angle of O–Tb–O ranges from 53.61(14)° to 156.56(14)°.

Fig. 1 (a) Coordination environment of Tb atom. (b) Single crystal structure of Tb-MOFs. (c) Two-dimensional frame diagram of Tb-MOFs. (d) Three-dimensional frame diagram of Tb-MOFs.

So each local asymmetric unit in the cell contains one Tb³⁺, four H₃BTC anions and two DMF molecules (Fig. 1b). Each structural unit cell contains two local asymmetric units, each local asymmetric unit contains 1 Tb³⁺, four H₃BTC anions and two DMF molecules. Each of the two asymmetric units forms a Tb binuclear structure. The chain connection between the two dual-core structures forms a two-dimensional planar structure (Fig. 1c). The two-dimensional plane structure forms a stable three-dimensional frame structure with micropores through the cross-layer connection form (Fig. 1d).

3.2 XRD, TG, SEM, FTIR and element analyses

The XRD pattern of Tb-MOFs (CCDC 2082489, Fig. S3, ESI†) shows that Tb-MOFs have good crystallinity, and the TG curve (Fig. S4, ESI†) indicates that Tb-MOFs have good thermal stability. The comparative morphologies of Tb-MOFs with and without H₂TDC are shown in Fig. 2a and b. It is found that Tb-MOFs without H₂TDC are self-assembled into stable rod bundles, while the addition of H₂TDC leads to forming the hollow sphere structure. The hollow structure is very stable and ultrasonic treatment for 30 minutes can't damage the hollow structure. SEM morphology of Tb-MOFs prepared with different H₃BTC/H₂TDC ratios (Fig. S5, ESI†) indicates that the hollow sphere structure is not changed by different H₃BTC/H₂TDC ratios. These results indicate that H₂TDC is not only a competitive reagent but also a protective agent. High-angle annular darkfield (HAADF) and element mappings (C, O, N and Tb) analysis of Tb-MOFs are respectively shown in Fig. 2c–g, which indicate that these elements are distributed uniformly throughout the whole sphere. The Tb-MOFs are consistent with that of the single crystal. The shadow in the sphere center of Tb element indicates that Tb-MOFs are hollow spheres (Fig. 2g). The line scanning profile of Tb element distribution (Fig. S6, ESI†) indicates that there are fewer elements in the center of the sphere, which further confirms that it is a hollow spherical structure.

Fig. 2 (a) SEM image of Tb-MOFs without H₂TDC. (b) SEM image of Tb-MOFs with H₂TDC. (c) HAADF image of Tb-MOF with H₂TDC. (d)–(g) mapping images of C, O, N and Tb element, respectively.

As shown in Fig. 3, Tables S5 and S6,† no sulfur can be detected in all samples during the preparation process and the final sample, which confirms that H₂TDC was not involved in the coordination with Tb³⁺. So the formation mechanism of hollow spherical Tb-MOF is deduced as follows. In the initial stage, H₂TDC serves as the coordination competitor. Due to the coordination ability between H₃BTC and Tb³⁺ being stronger than that of H₂TDC, the surface-energy-driven mechanism may be responsible for the crystallization process,²¹ forming solid spheres stacked by Tb-BTC nanoblocks (1–2 h in Fig. 3a). In the second stage, many defects exist in the inner region of the solid spheres, such as incomplete coordination metal nodes and grain boundaries, which makes the solid spheres unstable. In addition, the concentration of H₃BTC decreases due to the participation in coordination, while the concentration of H₂TDC does not change. The rich carboxyl groups on H₂TDC surface can provide protons and participate in competitive chelation with Tb³⁺, partially destroying the formed coordination bond. With the dissolution of the original unstable coordination and the formation of new coordination, the solid sphere structure collapsed (3 h in Fig. 3a) to form the hollow structure (4–6 h in Fig. 3a). This deduction is proved by the elemental data in Table S5, ESI.† In the third stage, the protons released by H₂TDC not only penetrate the hollow core but also cover the surface of the spheres, shielding the formed coordination bonds. Finally, a stable hollow spherical structure is formed (Fig. S5†). Therefore, H₂TDC serves as a competitive ligand and protective agent during the synthesis of Tb-MOF.

Fig. 3 (a) SEM images for 1 h, 1.5 h, 2 h, 3 h, 4 h, 6 h during Tb-MOFs preparation. The synthesis process of Tb-MOFs hollow spheres. (b) FTIR spectra of Tb-MOFs and Fe³⁺@Tb-MOFs.

The FTIR spectrum of the Tb-MOFs is shown in Fig. 3b. The characteristic band 650–900 cm⁻¹ was due to the out-plane bending vibration of C–H in benzene ring of Tb-MOFs. The peak at 918 cm⁻¹ can be attributed to the out-plane bending vibration of hydroxyl group. Peaks at 1626 cm⁻¹ and 1434 cm⁻¹ (or 1568 cm⁻¹ and 1375 cm⁻¹) are attributed to the asymmetric and symmetric stretch vibration of COO⁻ group (ν_as(COO⁻) and ν_s(COO⁻)), respectively. These results indicate that the carboxylic groups of H₃BTC are dehydrogenated and then coordinated with Tb³⁺ ions in Tb-MOF.²² The difference between ν_as(COO⁻) and ν_s(COO⁻) value is 192 cm⁻¹ (or 193 cm⁻¹), which implies that three coordinate modes, monodentate, chelation and bridging exist in the synthesized Tb-MOF.²²

3.3 Luminescence properties

The emission spectra of Tb-MOFs with different ratios of H₃BTC and H₂TDC using the excitation wavelength of 320 nm are shown in Fig. S7, ESI.† In the luminescence process, H₃BTC serves a sensitizer to enhance the luminescence of Tb-MOFs. When H₃BTC/H₂TDC ratio is 1 : 1, the synthesized Tb-MOFs have fewer coordination defects, H₃BTC can transfer the absorbed energy to Tb-MOFs easily which promotes the luminescence of Tb-MOFs. While H₃BTC/H₂TDC ratios are other values, some uncoordinated Tb³⁺ and carboxyl groups hinder the energy transfer from H₃BTC to Tb³⁺. Thus, Tb-MOFs with the ratio of H₃BTC/H₃TDC of 1 : 1 have the strongest luminescence intensity. Four emission peaks at 488 nm, 545 nm, 584 nm and 621 nm are corresponding to the ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions of Tb³⁺, respectively. Therefore, Tb-MOFs with the H₃BTC/H₂TDC ratio of 1 : 1 are synthesized for further exploration.

3.4 Sensing performance of Tb-MOFs to Fe³⁺

2 mg Tb-MOFs was, respectively, mixed evenly in 1 × 10⁻³ mol L⁻¹ M(NO₃)_n (M = Ca²⁺, Cr³⁺, NH₄⁺, Ba²⁺, Zn²⁺, Mn²⁺, Co²⁺, Bi³⁺, Ag⁺, Cu²⁺, Fe²⁺, Fe³⁺) aqueous solutions, and then the suspended solutions of different metal ions were obtained. Before photoluminescence (PL) performance measurements, the suspensions were treated with ultrasonication for 10 minutes. The PL properties are recorded and compared in Fig. 4a. Obviously, expected for Fe³⁺ solution, other cations display a little influence on the emission spectra of Tb-MOFs, while the emission intensities decreased rapidly in Fe³⁺ solution. Then, when multiple metal ions exist in aqueous solution at the same time, the selectivity to Fe³⁺ has been investigated (Fig. S9, ESI†). When there are 1, 3, 5, 7, and 9 metal ions in an aqueous solution at the same time, it still has good selectivity to Fe³⁺. Therefore, Tb-MOFs can be used as a luminescent sensor for Fe³⁺ ions detection.

Fig. 4 (a) Histogram of Tb-MOFs luminescence intensities in aqueous solutions with different metal ions (at 545 nm), and the inset is emission spectra. (b) Emission spectra of Tb-MOFs in various concentrations of Fe³⁺ solutions, and the inset is emission spectra near 545 nm. (c) Fitting curves of luminescence intensities of Tb-MOFs versus Fe³⁺ concentrations (0–150 μM), and the inset is luminescence intensities versus low Fe³⁺ concentration (0–10 μM). (d) Stern–Volmer fitting curve (Fe³⁺ concentration: 0–150 μM), and the inset is Stern–Volmer fitting curve at low (Fe³⁺ concentration: 0–10 μM).

To clarify the quenching behaviors of Tb-MOFs to Fe³⁺ ions in detail, the concentration-dependent emission measurements were carried out (Fig. 4b). With the increasing concentration of Fe³⁺ ions, the luminescence intensities of Tb-MOFs gradually decreased, when the concentration of Fe³⁺ ions is 150 μM, the luminescence is almost completely quenched.

The fitting curve between the luminescence intensity of Tb-MOFs at 545 nm versus the concentration of Fe³⁺ is shown in Fig. 4c. With the increase of Fe³⁺ concentration in the range of 0–150 μM, the luminescence intensities decrease exponentially which satisfies the functional relationship y = 526.8 exp(−x/6.3) + 1014.5 exp(−x/89.9) − 176.4, and the fitting parameter R² = 0.9981. The inset shows that there is a good linear relationship between the luminescence intensities and the Fe³⁺ concentrations (0–10 μM), which satisfies y = 1335.7 − 55.8x with the linearity of R² = 0.9974. The limit of detection (LOD) of Tb-MOFs for Fe³⁺ can be calculated by,²³

where N is the number of blank groups measured without Fe³⁺ (N = 20), F₀ is the luminous intensity of the sample in aqueous solution, F is the average value of F₀, and S_b (S_b = 49.7) is the standard deviation of 20 blank groups, S is the slope of the curve in the Fe³⁺ concentration range of 0–10 μM in the inset of Fig. 4c (S = 55.8), substituting the above values to calculate the detection limit of Fe³⁺ for Tb-MOFs samples in aqueous solution is about 2.05 μM. Further, the quenching efficiency of Tb-MOFs to Fe³⁺ can be evaluated by the Stern–Volmer²⁴ equation,where I₀ and I are the relative PL intensities before and after incorporation of the analyte, [M] is the molar concentration of the Fe³⁺, and K_SV is the Stern–Volmer quenching constant. As illustrated in inserted Fig. 4d, the Stern–Volmer quenching curve for Fe³⁺ shows a good linear relationship y = 0.98 + 0.07x at low concentration (0–10 μM) with a linear fit correlation coefficient of 0.9887. From the inset of Fig. 4d, the K_SV (5.8 × 10⁻⁴ M⁻¹) value can be obtained, suggesting a strong quenching effect of Fe³⁺ on Tb-MOFs. However, the increase in concentration (10–150 μM) leads to the curve deviation from the linear relation (Fig. 4d), and a good exponential relationship y = 0.11 exp(−x/24.61)+1.49 can be obtained with the fitting parameter of R² = 0.9967.

In addition, the sensing properties of Tb-MOFs to different metal ions in DMF solvents were also evaluated. The research method is the same as those in aqueous case. Tb-MOFs still have good selectivity to Fe³⁺ in the DMF system (Fig. S9, ESI†). Continue to explore the sensitivity test of Tb-MOFs to Fe³⁺ in DMF (Fig. S10, ESI†). Interestingly, Tb-MOFs also show a good specific recognition ability to Fe³⁺. The LOD is about 0.80 μM, and the Stern–Volmer quenching constant K_SV is 9.5 × 10⁻⁴ M⁻¹. These results indicate that Tb-MOFs can act as an excellent luminescent sensing material for Fe³⁺ ion detection.

In general, the luminescence quenching mechanism of the MOFs can be mainly summarized as three kinds: ion exchange, structural collapse or change, photon absorption competition between adsorbed ions and MOFs,²⁵ In order to fully explain the luminescent quenching mechanism of Fe³⁺ to Tb-MOFs, the following four measurements are carried out. First, the XRD patterns of Tb-MOFs and Fe³⁺@Tb-MOFs measurement results are shown in Fig. S3, ESI,† the Tb-MOFs and Fe³⁺@Tb-MOFs display similar XRD pattern characteristics, which excludes the possibility of MOFs skeleton collapse or change. Second, before and after Fe³⁺ adsorption, XPS curves of the Tb-MOFs and Fe³⁺@Tb-MOFs (Fig. 5a) show that no obvious variation occurs in the positions of the element binding energy, only a weak characteristic peak of Fe³⁺ (710.8 eV) appears in the XPS curve of Fe³⁺@Tb-MOFs. The binding energy positions of Tb 3d remain unchanged (Fig. 5b), which indicates that the coordination environment of Tb atom does not change before and after adsorption of Fe³⁺, and there is no ion exchange between Fe³⁺ and Tb³⁺. Broadening and splitting of carboxyl characteristic peaks in FTIR spectrum of Fe³⁺@Tb-MOF (Fig. 3b) proves that Fe³⁺ affected the coordination environment of the carboxyl group. Third, the UV-Vis absorption spectrum of Fe³⁺ and the excitation spectrum of Tb-MOFs are shown in Fig. 5c, the major degree overlap demonstrates that the excitation energy can be effectively absorbed by Fe³⁺, and the energy transfer efficiency from organic ligands to Tb³⁺ ions will be reduced and the luminescence quenching occurs.^26,27 In Fig. 5d, the luminescence lifetime of Tb-MOFs is about 0.65 ms, and that of Fe³⁺@Tb-MOFs is about 0.49 ms, the reduced lifetime further confirms the existence of the energy transfer in Fe³⁺@Tb-MOFs.^28,29 These results indicate that the co-existence of absorption competition and energy transfer should be responsible for the quenching mechanism.

Fig. 5 (a) XPS spectra of Tb-MOFs and Fe³⁺@Tb-MOFs. (b) Tb 3d XPS curves of Tb-MOFs and Tb–Fe³⁺@Tb-MOFs. (c) UV-Vis spectrum of Fe³⁺ and excitation spectrum of Tb-MOFs, (d) Luminescence lifetime of Tb-MOFs and Fe³⁺@Tb-MOFs.

Due to the hollow spherical structure, the large specific surface area of Tb-MOFs, the adsorption performance of Tb-MOFs to Fe³⁺ are also evaluated. 500 μM Fe³⁺ aqueous solution was added to the aqueous suspension of Tb-MOFs, and the results are shown in Fig. 6a. After Fe³⁺ was added to Tb-MOFs suspension, the luminescence of Tb-MOFs was quenched, the original yellow Fe³⁺ solution turned into a colorless transparent solution (Fig. 6b), and the original white Tb-MOFs powder changed into yellow (Fig. 6c). Combined with the above quenching mechanism analysis, it can be concluded that the hollow Tb-MOFs can not only adsorb Fe³⁺, but also can capture it, which provides a new platform for the adsorption and removal of Fe³⁺ in aqueous and DMF solutions.

Fig. 6 (a) Photographs of Tb-MOFs suspension (left) and Fe³⁺ (right) under 365 nm illumination. (b) Photographs of Fe³⁺ before (left) and after (right) addition of Tb-MOFs. (c) Photographs of Tb-MOFs powder before (left) and after (right) adsorption of Fe³⁺.

For a clearer comparison between this work and related researches, the Fe³⁺ sensing performance of Tb³⁺ based MOFs is shown in Table 1. The results indicate that the Fe³⁺ sensing performance of the Tb-MOFs in this work is better than most related reports.^30–34

Table 1 Comparisons of the sensing parameters with other Tb³⁺ based MOF for Fe³⁺ sensing

MOF	KSV (M^−1)	LOD (μM)	Detection range (μM)	Ref.
[Tb(cpia)(H2O)2]n·nH2O	1.2 × 10^4	2.06	0–120	30
Tb^3+ with H4ddb	7.64 × 10^4	0.936	33–330	31
[Tb(C5H7O4)(C5H6O4)(H2O)2]n	8.07 × 10^4	—	0–100	32
H3O[Tb-(L-N)(H2O)2]·2H2O	7.93 × 10^3	—	0–250	33
[Tb3(L-)2(HCOO)(H2O)4]·6H2O	1.39 × 10^4	—	0–400
[Tb(tftba)1.5(phen)(H2O)]n	4.043 × 10^4	12.7	0–800	34
Tb-MOF	5.8 × 10^4	2.05 in water	0–150	This work
	9.5 × 10^4	0.8 in DMF	0–150

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4. Conclusion

Hollow spherical Tb-MOFs were synthesized under one-pot solvothermal method by using H₂TDC as a competitive and protective agent. Tb-MOFs can be used to detect Fe³⁺ by the luminescence quenching effect with excellent selectivity and sensitivity. The LOD and K_SV quenching constant of Tb-MOF for Fe³⁺ in aqueous solution are 2.05 μM and 5.8 × 10⁻⁴ M⁻¹, respectively. In DMF, the K_SV and LOD are 9.5 × 10⁻⁴ M⁻¹ and 0.80 μM, respectively. More importantly, this work extends the synthetic strategy and methodic versatility for further development of hollow MOFs, and this method has the advantages of simple operation, mild conditions, high yield, uniform morphology and good stability, and can be applied to trace Fe³⁺ detection.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51574054), Chongqing Municipal Education Commission (KJZD-M201901102), Chongqing Science and Technology Bureau (cstc2021jcyj-msxmX0493, CSTCCXLJRC201905), the Venture & Innovation Support Program for Chongqing Overseas Returnees (Cx2019092), and Guangzhou Science and Technology Project (202002030053).

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Footnote

† Electronic supplementary information (ESI) available: Crystal data, powder X-ray diffraction patterns (PXRD), TGA curves, SEM images, emission spectra and sensing performance (PDF). CCDC 2082489. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ra08088a

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