The construction of Tb³⁺ doped Sr–BDC metal–organic framework materials and their high-sensitivity fluorescent detection of Fe³⁺

Abstract

This study presents the synthesis of a strontium-based metal–organic framework (Sr–BDC) through a solvothermal method, employing strontium chloride and terephthalic acid as primary precursors. The as-prepared Sr–BDC was subsequently functionalized with terbium ions (Tb³⁺) to yield a Tb³⁺@Sr–BDC composite. Extensive structural characterization, including X-ray diffraction (XRD), thermogravimetric analysis (TG), and the scanning electron microscope (SEM), confirmed that the Tb³⁺ incorporation preserved the integrity of the Sr-MOF framework without inducing structural degradation. Photoluminescence analysis demonstrated that Tb³⁺@Sr–BDC exhibits distinct Tb³⁺ emission peaks at 545 nm upon excitation at 294 nm, showcasing exceptional selectivity and sensitivity toward Fe³⁺ ions. Stern–Volmer quenching analysis revealed a remarkably low detection limit of 7.3 × 10⁻⁶ mol L⁻¹ for Fe³⁺, with a linear response range spanning from 5 × 10⁻⁶ to 1 × 10⁻⁴ mol L⁻¹. The potential mechanisms responsible for Fe³⁺-induced fluorescence quenching in Tb³⁺@Sr–BDC was also analysised in the study. These results underscore the potential of Tb³⁺@Sr–BDC as a highly efficient fluorescent probe for applications in environmental monitoring and biomedical sensing.

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1 Introduction

With the rapid advancements in analytical chemistry and materials science, there is an increasing demand for fluorescent sensing materials with high sensitivity and selectivity.^1,2 This is particularly critical in environmental monitoring and biomedical applications,^3,4 where the detection of metal ions such as iron (Fe³⁺) is of paramount importance.^5,6 Iron ions not only play a vital role in biological systems but are also frequently detected in industrial wastewater and environmental pollutants. Rare-earth metal–organic frameworks (Ln-MOFs) have emerged as a research hotspot in the field of fluorescent sensing^7,8 due to their unique porous structures and tunable chemical functionalities, offering broad application prospects for the detection of cations, anions, small organic molecules, and biomolecules.^9–15

Metal–organic frameworks (MOFs) possess tunable porosity and abundant active sites, and have demonstrated great potential in electronics,¹⁶ photocatalysis,^17,18 and other fields. Post-synthetic modification (PSM) strategies^19–25 further extend their application in highly sensitive detection. Embedding photo-active Ln³⁺ ions into MOF lattices offers significant advantages: the intrinsic porosity of the MOF is preserved, while characteristic lanthanide emission—long fluorescence lifetimes, narrow emission bands, and large Stokes shifts—is introduced. Compared with conventional luminescent probes, this approach markedly improves the signal-to-noise ratio and thus detection sensitivity, especially in biological systems with endogenous organic fluorescence. However, the high and variable coordination numbers of rare-earth ions²⁶ render the direct synthesis of targeted Ln-MOFs challenging. Terbium(III), with its unique electronic configuration and excellent luminescence properties, has been widely used to construct high-performance fluorescent probes. In this work, we employed a post-synthetic modification strategy to embed Tb³⁺ into the Sr–BDC framework, yielding a novel fluorescent probe for the efficient detection of Fe³⁺.

Among rare-earth ions, terbium (Tb³⁺) is widely utilized for constructing high-performance fluorescent probes due to its unique electronic configuration and luminescent properties. In this study, we employed a post-synthetic modification strategy to incorporate Tb³⁺ into the Sr–BDC framework, successfully developing a novel fluorescent probe for the highly efficient detection of Fe³⁺.

2 Materials and methods

2.1. Reagents and instruments

Strontium chloride hexahydrate (SrCl₂·6H₂O), terephthalic acid (H₂BDC), N,N-dimethylacetamide (DMA), terbium(III) nitrate hexahydrate (Tb(NO₃)₃·6H₂O), copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O), chromium(III) nitrate nonahydrate (Cr(NO₃)₃·9H₂O), nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O), potassium nitrate (KNO₃), sodium chloride (NaCl), cobalt(II) nitrate (Co(NO₃)₂), cadmium(II) nitrate (Cd(NO₃)₂), silver nitrate (AgNO₃), zinc(II) nitrate hexahydrate (Zn(NO₃)₂·6H₂O), lead(II) nitrate (Pb(NO₃)₂), and absolute ethanol were purchased as analytical-grade reagents and further purified prior to use. All the reagents above were purchased from Macklin.

Thermogravimetric analysis (TGA) was performed using a Netzsch STA 449F3 instrument under a nitrogen atmosphere, with a temperature range of 30–700 °C and a heating rate of 10 °C min⁻¹. X-ray diffraction (XRD) patterns were collected at room temperature using a Rigaku Miniflex 600 diffractometer with Cu Kα radiation (λ = 1.5418 Å). Fluorescence spectra were recorded at room temperature using a Horiba-HR320 fluorescence spectrophotometer.

The microstructural characterization of the specimens was performed using a Hitachi SU-5000 field-emission scanning electron microscope (FE-SEM, Hitachi High-Tech, Japan) operated at an acceleration voltage of 5–10 kV under high-vacuum conditions. Elemental distribution analysis was conducted via energy-dispersive spectroscopy (EDS) utilizing a Bruker Quantax system (Bruker, Germany).

2.2. Synthesis of Sr-BDC and Tb³⁺@Sr-BDC

The systhesis methods and technique were similar with that provided in the previous studies.²⁷ A mixture of strontium chloride hexahydrate (SrCl₂·6H₂O, 0.1 mmol, 26.6 mg) and terephthalic acid (H₂BDC, 0.1 mmol, 16.6 mg) was dissolved in 3 mL of N,N-dimethylacetamide (DMA). The solution was transferred into a stainless steel autoclave lined with a polytetrafluoroethylene (PTFE) inner tube and heated at 80 °C for 72 h. After the reaction, the autoclave was allowed to cool naturally to room temperature. The resulting colorless block crystals were collected by filtration, washed thoroughly, and dried in air. The final product was weighed for further use.

To prepare Tb³⁺@Sr-BDC, the as-synthesized Sr–BDC powder (100 mg) was immersed in 50 mL of an ethanolic solution of terbium(III) nitrate hexahydrate (Tb(NO₃)₃·6H₂O), with a Tb(NO₃)₃ concentration of 10⁻² mol L⁻¹, for 24 h. The powder was then separated by centrifugation, washed three times with ethanol, and dried in air for 24 h.

3 Results and discussion

3.1. The property and structure of the samples

The X-ray powder diffraction (XRD) patterns of Sr–BDC and Tb³⁺@Sr–BDC were recorded at room temperature (Fig. 1). The experimental diffraction peaks of Sr–BDC align well with the simulated pattern (CCDC: 1551141), confirming the synthesis of the Sr–BDC framework. Notably, the diffraction peak positions remain unchanged after Tb³⁺ doping, suggesting that Tb³⁺ ions partially substitute Sr²⁺ sites within the original framework while preserving its structural integrity. This observation indicates that Tb³⁺@Sr–BDC retains the same crystallographic structure as the parent Sr–BDC.

Fig. 1 X-ray powder diffraction (XRD) patterns of Sr–BDC and Tb³⁺@Sr–BDC.

Thermogravimetric analysis (TGA) of Tb³⁺@Sr–BDC was conducted in the temperature range of 30–700 °C (Fig. 2). The initial weight loss (150–320 °C) corresponds to the removal of free and coordinated DMA solvent molecules. The Sr–BDC complex (chemical formula: C₁₂H₁₅NO₆Sr) exhibited an experimental weight loss of 24.54% within the temperature range of 150–320 °C, demonstrating close agreement with the theoretical mass percentage (24.41%) calculated for the coordinated dimethylacetamide molecule (C₄H₉NO) in its crystal structure. At approximately 505 °C, a structural collapse of the Sr–BDC framework occurs.

Fig. 2 Thermogravimetric analysis (TGA) curve of Tb³⁺@Sr–BDC.

Microstructural characterization revealed that Tb³⁺@Sr–BDC crystals (Fig. 3c and d) exhibited well-faceted short-rod or plate-like morphologies, in contrast to the irregular granular particles observed in undoped Sr–BDC (Fig. 3a and b). Energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of trace Tb (0.90 wt%, Fig. 3d) in Tb³⁺@Sr–BDC, while no detectable terbium signal was observed in Sr–BDC. These results demonstrate successful Tb³⁺ incorporation into the crystal lattice, which significantly modified the crystallization kinetics and consequently altered the morphological evolution of the crystals.

Fig. 3 The SEM images and EDS of the Sr–BDC and Tb³⁺@Sr–BDC (SEM images: (a) Sr–BDC; (c) Tb³⁺@Sr–BDC. EDS (mapping): (b) Sr–BDC; (d) Tb³⁺@Sr–BDC).

3.2. Luminescence properties of Tb³⁺@Sr–BDC

As illustrated in Fig. 4, the Tb³⁺@Sr–BDC material exhibits characteristic emission peaks of Tb³⁺ ions at 489 nm, 545 nm, and 583 nm under 294 nm excitation. These peaks are attributed to the electronic transitions of ⁵D₄ → ⁷FJ (J = 6, 5, 4) within Tb³⁺ ions. Notably, the intense green emission at 545 nm suggests that this material can serve as an efficient luminescent sensor.

Fig. 4 Excitation (black line) and emission (red line) spectra of Tb³⁺@Sr–BDC.

To evaluate the fluorescence sensing capability of Tb³⁺@Sr–BDC, its luminescent properties were investigated in the presence of various metal cations. The Tb³⁺@Sr–BDC powder was finely ground and dispersed in ethanol to form a 5 mg mL⁻¹ suspension. Subsequently, 200 μL of the suspension was uniformly mixed with 3.8 mL of ethanol solutions containing 0.001 mol L⁻¹ of M(NO₃)_x (M = Pb²⁺, Zn²⁺, Cd²⁺, Na⁺, K⁺, Ni²⁺, Ag⁺, Co²⁺, and Fe³⁺). The luminescence spectra were recorded and are presented in Fig. 5. The results reveal that the luminescence intensity of Tb³⁺ ions is significantly influenced by the presence of different metal ions. The ratio (I/I₀) of the fluorescence intensity at 545 nm for the sample after metal ion introduction (I) to the Tb³⁺@Sr–BDC sample (I₀) was shown in Fig. 5b. The results suggests that Fe³⁺ induces pronounced luminescence quenching of Tb³⁺@Sr–BDC, indicating its selective response to Fe³⁺.

Fig. 5 (a) Emission spectra of Tb³⁺@Sr–BDC in the presence of various metal ions (λ_ex = 294 nm) and (b) intensity of the ⁵D₄ → ⁷F₅ transition at 545 nm for Tb³⁺@Sr–BDC in the presence of blank (I₀) and the different metal ions solution (I).

To further investigate the selectivity of Tb³⁺@Sr–BDC for Fe³⁺, the material was dispersed in ethanol solutions containing Fe³⁺ alongside other metal ions (Pb²⁺, Zn²⁺, Cd²⁺, Na⁺, K⁺, Ni²⁺, Ag⁺, and Co²⁺). Under 294 nm excitation, the luminescence intensity of Tb³⁺@Sr–BDC in the presence of individual metal ions (blue bars in Fig. 6) differs significantly from that in mixed-ion solutions containing Fe³⁺ (green bars in Fig. 6). When Tb³⁺@Sr–BDC is immersed in ethanol solutions containing a mixture of 1 × 10⁻³ mol L⁻¹ of other metal ions and 1 × 10⁻³ mol L⁻¹ of Fe³⁺, complete luminescence quenching is observed, demonstrating the material's selective detection capability for Fe³⁺ in complex environments.

Fig. 6 Intensity of the ⁵D₄ → ⁷FJ transitions for Tb³⁺@Sr–BDC upon addition of different metal ions (1 × 10⁻³ M, blue bars) and subsequent introduction of Fe³⁺ (1 × 10⁻³ M, green bars) (λ_ex = 294 nm).

To quantify the Fe³⁺ detection performance, a concentration gradient of Fe³⁺ solutions was prepared (1 × 10⁻⁴ mol L⁻¹, 2 × 10⁻⁵ mol L⁻¹, 2.5 × 10⁻⁵ mol L⁻¹, 5 × 10⁻⁵ mol L⁻¹, 8 × 10⁻⁵ mol L⁻¹, and 5 × 10⁻⁶ mol L⁻¹). A 0.2 mL aliquot of the 5 mg per mL Tb³⁺@Sr–BDC–DMA suspension was mixed with 3.8 mL of each Fe³⁺ solution, and the luminescence intensity was measured using a fluorescence spectrophotometer (Fig. 7). The results demonstrate a well-defined linear relationship between Fe³⁺ concentration and the luminescence intensity of Tb@Sr-BDC, with the emission intensity decreasing progressively as Fe³⁺ concentration increases from 5 × 10⁻⁶ to 1 × 10⁻⁴ mol L⁻¹. Stern–Volmer analysis of the Tb@Sr-BDC + Fe³⁺ system reveals a strong quenching effect, exhibiting a linear correlation coefficient (R) of 0.9947. The limit of detection (LOD = 3δ/S, δ represents the blank solution was measured ten times, and S stands for the slope of the calibration curve) was about 7.3 × 10⁻⁶ mol L⁻¹.²⁸ Compared with the reported literature (Table 1), the Tb³⁺@Sr-BDC sensor exhibits a markedly lower detection limit. These findings confirm that Tb@Sr-BDC not only enables qualitative identification but also facilitates quantitative detection of Fe³⁺ ions.

Fig. 7 (a) Emission spectra of Tb³⁺@Sr–BDC in ethanol solutions with varying Fe³⁺ concentrations (λ_ex = 294 nm) and (b) linear relationship between the luminescence intensity of Tb³⁺ and Fe³⁺ concentration in the range of 1 × 10⁻⁴ to 5 × 10⁻⁶ mol L⁻¹.

Table 1 Comparison of Fe³⁺ ion detection based on different MOF materials

MOF	LOD/(mol L^−1)	Ref.
[Tb(BTB)(DMF)]	10 × 10^−6	1
[Zn2(tpt)(tda)2]·H2O	4.72 × 10^−6	29
Eu^3+@UiO-66	12.5 × 10^−6	30
Zn-MOF-74	1.04 × 10^−6	31
Tb(3+)@Zn-MOF	7.5 × 10^−6	32
Tb^3+@Sr-BDC	7.3 × 10^−6	This work

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3.3. Quenching mechanism

The luminescence quenching mechanisms of cations can be attributed to four primary factors:^33,34 (1) interactions between target metal ions and MOFs; (2) exchange between lanthanide ions in MOFs and central metal ions; (3) collapse of the crystalline structure; (4) energy competition between linkers and cations. To elucidate the potential sensing mechanism of Fe³⁺-induced quenching in Tb³⁺@Sr–BDC, comprehensive characterization was performed using XRD, SEM, and EDS before and after Fe³⁺ treatment. XRD patterns (Fig. 8) revealed distinct differences between Fe³⁺-treated and untreated Tb³⁺@Sr–BDC, indicating framework modification. SEM showed significant morphological alterations in the powdered samples (Fig. 3 and 9). EDS data (Table 2) confirmed the replacement of Tb³⁺ by Fe³⁺ in the framework. The quenching effect is likely caused by displacement of luminescent Tb³⁺ ions by Fe³⁺ and the concurrent framework collapse. This aligns with known cation-MOF interaction paradigms, where heavy metal ions disrupt lanthanide-centered emission through structural and electronic perturbations.

Fig. 8 XRD patterns of Tb³⁺@Sr–BDC before and after treatment with iron ions.

Fig. 9 SEM images of Tb³⁺@Sr–BDC after treatment with Fe³⁺ at the microscopic scale.

Table 2 Elemental composition analysis (wt%) of Tb³⁺@Sr–BDC before and after Fe³⁺ coordination (EDS)

Elements	Sr^2+	Tb^3+	Fe^3+
Tb^3+@Sr–BDC	33.19	0.90	0
Tb^3+@Sr–BDC + Fe^3+	32.47	0	3.47

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4 Conclusions

A strontium-based metal–organic framework (Sr-MOF) was synthesized via a solvothermal reaction using terephthalic acid (H₂BDC) and strontium chloride (SrCl₂) as precursors. The as-prepared Sr-BDC was further functionalized with terbium ions (Tb³⁺) to yield Tb³⁺@Sr–BDC, which exhibited excellent luminescent properties. Fluorescence sensing studies revealed that Tb³⁺@Sr–BDC demonstrates high sensitivity and selectivity toward Fe³⁺ ions, with a distinct linear correlation (R² > 0.99) between fluorescence intensity and Fe³⁺ concentration. The detection limit was determined to be approximately 7.3 × 10⁻⁶ mol L⁻¹. These findings highlight the potential of rare-earth-functionalized Sr–BDC materials as efficient fluorescent probes for the detection and quantification of Fe³⁺ in environmental and biomedical applications.

Author contributions

Drafting and the manuscript, H. Y.; investigation, H. Y. and Y. Y.; analysis and interpretation of the data, C. X. and Y. W.; formal analysis, B. Q.; writing-review and editing, C. Z. and S. L.; conception and planning of the work, C. L.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data presented in this study are available on request.

Acknowledgements

The Guangdong Chaoshan Institute of Higher Education and Technology are acknowledged. This research was supported by the Scientific Research Project of Hanshan Normal University (XPN202106 and XSDBF2025205), the Special Project in Key Fields of General Universities in Guangdong Province (2024ZDZX3030 and 2024ZDZX3029), the Scientific Research Project of the Department of Education of Guangdong Province (2022KQNCX046), the Mentorship Support Program of Hanshan Normal University (XWT2025101); the Science and Technology Planning Project of Guangdong Province (2017B090921002), the Chaozhou Branch of Chemistry and Chemical Engineering Guangdong Laboratory (HJL202202A009).

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Footnote

† These authors contributed equally to this work.

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