Tb-MOF: a naked-eye and regenerable fluorescent probe for selective and quantitative detection of Fe³⁺ and Al³⁺ ions

Abstract

Sensitivity and selection for metal ion detection are of crucial importance in protecting human health. Using 4,4′,4′′-tricarboxytriphenylamine (H₃TCA) as the ligand, a unique terbium based metal–organic framework (Tb-MOF), [Tb₃(TCA)₂(DMA)_0.5(OH)₃(H₂O)_0.5]·3H₂O (1), has been prepared, structurally identified and further employed as a fluorescent probe for selective and quantitative detection of metal ions. Compound 1 selectively detects Fe³⁺ and Al³⁺ through fluorescence quenching effects at 549 nm and fluorescence enhancement at 463 nm, respectively, from among 19 types of metal ions without interference. Most notably, compound 1 exhibits a clearly visible color change in the presence of Fe³⁺ from yellow to deep brown, with a detection limit of 8 × 10⁻⁶ M. As a fluorescent probe for Al³⁺, 1 can be simply and quickly regenerated. Its detection limit of 7 × 10⁻⁷ M is significantly lower than the highest limit of Al³⁺ in drinking water recommended by the WHO (7.41 μM), representing a rare example in reported fluorescent sensors for Al³⁺. Tb-MOF 1 is an example of a fluorescent probe that features naked-eye detection, regeneration, high selectivity and quantitative detection of Fe³⁺ and Al³⁺ ions.

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Introduction

Iron and aluminum ions are very important to biological and environmental systems.¹ Iron is known to participate in oxygen uptake, oxygen metabolism and electron transfer.² Aluminium and its alloys are widely employed in food packaging, cookware and drinking water supplies.³ However, abnormal concentrations of Fe³⁺ and Al³⁺ may result in numerous diseases such as bone softening and encephalopathy.⁴ Thus, it is of considerable importance to detect Fe³⁺ and Al³⁺ with high selectivity and sensitivity, both in the environment and in organisms.⁵ Conventional analytical techniques, including inductively coupled plasma-mass spectrometry (ICP-MS), liquid chromatography and atomic absorption spectrometry, are most often utilized for Fe³⁺/Al³⁺ analysis.⁶ Advantages of these techniques are short response times and high sensitivity, whereas disadvantages include multistep processing and low selectivity. A fluorescent sensor technique would bring to this field high selectivity and easy manipulation, in addition to high sensitivity.⁷

In recent years, fluorescent metal–organic frameworks (MOFs) as a new type of sensing material have attracted increasing attention for use in fluorescent probes and assays.⁸ In particular, lanthanide metal–organic frameworks (Ln-MOFs) have been regarded as very promising fluorescent sensing materials because they possess high color purity, large Stokes' shifts and long fluorescence lifetimes.⁹ Fluorescent Ln-MOFs for sensing metal ions, such as Pb²⁺, Cu²⁺, K⁺, Fe²⁺, and Fe³⁺, anions and organic molecules have been successfully developed.^9,10 However, only a few examples were reported for sensing Fe³⁺ and Al³⁺ ions. In 2013,¹¹ Sun and co-workers first reported a Eu-MOF as a fluorescent probe for the detection of Fe³⁺ and Al³⁺ at 617 nm. Obviously, the interference of Fe³⁺ and Al³⁺ with each other at the same wavelength cannot be neglected. To avoid such interference, Zang and co-workers¹² synthesized a Tb-MOF that could selectively sense Fe³⁺ and Al³⁺ ions through fluorescence quenching and enhancement at 541 nm and 452 nm, respectively. To date, it remains a challenge to regenerate any Ln-MOF fluorescence sensor, particularly for selective and quantitative detection of Fe³⁺ and Al³⁺ ions.

In this contribution, we selected terbium as the metal center and tricarboxytriphenylamine (H₃TCA) as the ligand to construct a metal–organic framework (Tb-MOF) for Fe³⁺ and Al³⁺ detection mainly based on the following considerations: (1) the majority of Ln-MOFs have unique structures and excellent photofluorescence properties that provide a suitable platform to identify Fe(III) ions due to their fluorescence quenching effect¹³ and (2) H₃TCA possesses large conjugated groups that can absorb ultraviolet light and effectively transfer the energy to the central lanthanide ion via f–f transitions in an “antenna effect”.¹⁴ A unique MOF, formulated as {[Tb₃(TCA)₂(DMA)_0.5(OH)₃(H₂O)_0.5]·3H₂O}_n (1; H₃TCA = tricarboxytriphenylamine), was successfully obtained and structurally characterized. Fluorescence studies showed selective sensing by 1 of Fe³⁺ and Al³⁺ through fluorescence quenching effects at 549 nm and fluorescence enhancement at 463 nm, respectively, among 19 types of metal ions without interference. Importantly, the effective concentration range was quite favorable from 1 × 10⁻⁵ to 4 × 10⁻⁴ M for Fe³⁺ with a detection limit of 8 × 10⁻⁶ M and 10⁻⁶ to 10⁻³ M for Al³⁺ with a detection limit of 7 × 10⁻⁷ M. Most significantly, Tb-MOF 1 is an example of a fluorescent probe that features naked-eye detection, regeneration, high selectivity and quantitative detection of Fe³⁺ and Al³⁺ ions.

Experimental

Materials and instrumentation

All chemicals and reagents were purchased from commercial sources and were used without further purification. The hydrothermal reaction was performed in a 10 mL Teflon-lined stainless steel autoclave under autogenous pressure. Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Elemental analyses (C, H and N) were performed on an Elementar Vario EL III analyzer. IR spectra were obtained on a Tensor 27 spectrometer (Bruker Optics, Ettlingen, Germany) with pressed KBr pellets in the range of 4000–400 cm⁻¹. Optical diffuse reflectance was carried out with a Shimadzu UV-2450 spectrophotometer. The solid-state photofluorescence analyses were performed using an Edinburgh FLSP920 fluorescence spectrometer. Fluorescence spectra for the suspension samples were acquired on a Hitachi F-4500 fluorescence spectrophotometer. The thermogravimetric analyses (TGA) data for compound 1 were collected using a NETZSCH STA 449F3 simultaneous TG-DSC instrument with a heating rate of 10 °C min⁻¹ from 30 to 900 °C in a N₂ atmosphere.

Synthesis of [Tb₃(TCA)₂(DMA)_0.5(OH)₃(H₂O)_0.5]·3H₂O (1)

A mixture of H₃TCA (19 mg, 0.05 mmol) and TbCl₃·6H₂O (75 mg, 0.05 mmol) was dissolved in a solvent mixture of DMA/H₂O (4 mL/2 mL) in a 10 mL Teflon-lined autoclave and heated at 140 °C for three days, then cooled at 5 °C h⁻¹ to room temperature. Yellow block-shaped crystals were collected in 31% yield based on TbCl₃·6H₂O. Elemental analysis (%): calcd for C₄₄H_32.5O₁₆N_2.5Tb₃: C, 39.75; H, 2.45; N, 2.63; found: C, 39.72; H, 2.47; N, 2.61. IR data (KBr, cm⁻¹, Fig. S1†): 3380 (s), 3066 (m), 2470 (w), 1935 (w), 1593 (s), 1535 (s), 1409 (s), 1280 (s), 1176 (m), 849 (m), 788 (m), 563 (w).

X-ray crystallographic determination

Single-crystal X-ray diffraction data for [Tb₃(TCA)₂(DMA)_0.5(OH)₃(H₂O)_0.5]·3H₂O was measured on a Bruker SMART APEXII CCD diffractometer, equipped with a graphite-monochromatized Mo Kα radiation source (λ = 0.71073 Å) using ω and φ scan modes at room temperature. The structure was solved by direct methods and refined with full-matrix least-squares based on F² using SHELXS-97 and SHELXL-97 programs.¹⁵ Hydrogen atoms were placed in geometrically calculated positions. All non-hydrogen atoms were refined anisotropically. Crystal data and structure refinement results for compound 1 are summarized in Table S1.† Selected bond distances and angles are listed in Table S2.†

Results and discussion

Description of the structure

X-ray crystallography showed that compound 1 crystallized in the monoclinic system with space group C2/c and features a porous MOF structure. The asymmetry unit of compound 1 contains three Tb(III) centers, two L³⁻ ligands, three hydroxyl anions, a coordinated H₂O and a coordinated DMA. Tb1 is nine-coordinated by six oxygen atoms from four carboxylate groups and three hydroxyl anions. Tb2 is coordinated by four oxygen atoms from four carboxylate groups of L³⁻, three hydroxyl anions and a DMA molecule. Tb3 is coordinated by seven oxygen atoms from four carboxylate groups of L³⁻ and three hydroxyl anions. The three Tb³⁺ ions (Tb1, Tb2 and Tb3) are centrosymmetrically-related with three other Tb³⁺ ions (Tb1A, Tb2A and Tb3A) to form a {Tb₆} core, which contains three Tb₃ triangular units sharing an edge. The Tb(III) centers form an infinite ribbon in the sequence of Tb1–Tb2–Tb3–Tb1–Tb2 with a side-sharing structure (Fig. 1b up). The terbium ribbons are bridged via the multi-dentate TCA³⁻ ligands (Fig. S2†) into a 3D channel-like network along the c-axis (Fig. 1b down). The Tb–O–Tb bond angles are in the range of 92°–108°. The Tb⋯Tb distances are Tb1⋯Tb2 = 3.664 Å, Tb1⋯Tb3 = 3.863 Å, and Tb2⋯Tb3 = 3.915 Å.

Fig. 1 (a) Coordination environment of compound 1 cluster. Symmetry code: A, 0.5 + x, −0.5 + y, z; B, 1.5 − x, −0.5 + y, 0.5 − z; C, 1.5 − x, 0.5 − y, −z. (b) The upper image is of dimers linked by the carboxylate groups of L³⁻ and hydroxyl anions to shape an infinite 1D chain. The lower image is a polyhedral view of the 3D framework. (c) 3D nest net of compound 1.

PXRD patterns and thermal properties

After a series of purification steps, we obtained a pure product. First, the experimental conditions were selected, including the ratio of metal to ligand, solvent, reaction temperature, time and pH. Under the optimal conditions, a high purity sample was obtained. Second, the sample was washed with water and methanol several times to remove residual of metal and organic precursors. Third, the product was observed under a microscope and purified with a fine wire one by one to select crystals and eliminate any non-crystalline material. At last, we can obtain high purity of the samples through the above operation.

PXRD of 1 was in good agreement with its simulated pattern (Fig. S3†), indicating that 1 was prepared in high purity. The difference in reflection intensities between the simulated and experimental patterns was due to variation in preferred orientation of the powder samples.

To examine the thermal stability of 1, TG and DSC analyses were performed under a N₂ atmosphere with heating rate of 10 °C min⁻¹ from ambient temperature up to 900 °C. TGA and DSC curves (Fig. S4†) indicated that the thermal behavior of 1 can be divided into three obvious decomposition processes. First, a 3.99% weight loss occurred at 80–118 °C, which matches well with the theoretical value (3.88%) for loss of three guest water molecules per formula unit. This was followed by a second weight loss of 3.30% at 118–252 °C, corresponding to loss of coordinated H₂O and DMA molecules (calcd: 3.29%). Finally, thermo-collapse of 1 with a weight loss of 23.95% occurred at a temperature ranging from 252 to 710 °C.

Sensing of metal ions

The ultraviolet-visible light (UV/vis) absorption spectra of the free H₃TCA ligand and compound 1 were obtained in CH₃OH solution (c = 1 × 10⁻⁵ M) at room temperature. Both displayed strong absorption bands in the UV spectral region from 300 nm to 400 nm, which are attributed to the π–π* electronic transitions of the aromatic rings. A similar trend in absorption spectra indicates that coordination does not have an obvious effect on the singlet excited state of the ligand. Compared with the ligand, a slight red shift was discernible in the maxima absorbance of 1, which could be ascribed to coordination perturbation of Tb³⁺ ions (Fig. S5†).

The excitation and emission spectra for 1 at room temperature are shown in Fig. S6.† The excitation peaks at 395 nm can be assigned to the π–π* electron transition of the ligand.¹⁶ The emission bands arise from ⁵D₄ → ⁷F_j (J = 6, 5, 4, 3) transitions typical of Tb³⁺.¹⁷ The emission spectrum of 1 (excitation wavelength 395 nm) indicates the well-resolved magnified fluorescence of the f–f transitions, which result from energy transferred from L³⁻ ligand to Tb³⁺ ions.^14a,b The evident characteristic transitions of the Tb³⁺ ions are observed at 497, 545, 587 and 620 nm corresponding to the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, ⁵D₄ → ⁷F₃ transitions, respectively. The more intense transition centered at 545 nm correspond to the transition of ⁵D₄ → ⁷F₅.

Powder XRD patterns of the as-synthesized and water-treated samples are completely superimposed (Fig. S7†), indicating retention of the framework structure after the water treatment. Moreover, as-synthesized Tb-MOF was immersed in an aqueous solution at room temperature and its luminescence was measured every day. As shown in Fig. S8,† no obvious change in intensity was found after 10 days, implying that the proposed sensor is of acceptable stability. This can probably be ascribed to sufficient protection of Tb³⁺ provided by the MOF scaffold.

A fluorescence investigation was carried out to explore the influence of various metal cations (Fig. 2). A sample 1 (2 mg) was dispersed in 10 mL of a mixed solvent solution (water : methanol = 4 : 1, Fig. S10 and S11†) containing 10⁻³ M of M(NO₃)_x (Mⁿ⁺ = Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Al³⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Hg²⁺, As³⁺ Cd²⁺, Ag⁺, and Pb²⁺) to form a 1–Mⁿ⁺ suspension by ultrasound methods. The resultant suspension was monitored using a fluorescence spectrophotometer. It was found that most of the metal ions (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Hg²⁺, As³⁺ Cd²⁺, Ag⁺ or Pb²⁺) have negligible effects on the fluorescence of 1. Cu²⁺ had a minor quenching effect on the luminescence intensity, whereas Fe³⁺ completely quenched the emission at 549 nm (Fig. S12†). Conversely, Al³⁺ significantly enhanced the fluorescence at 463 nm (Fig. S13†). The fluorescence studies clearly demonstrated that the Tb-MOF probe can selectively sense Fe³⁺ through a fluorescence quenching effect at 549 nm and Al³⁺ through fluorescence enhancement at 463 nm, respectively, which is quite rare in previous reports of luminescent MOFs. The visual changes on addition of Al³⁺ and Fe³⁺ under the fluorescent lamp and the laboratory UV light are shown in Fig. S15.† The remarkable quenching and enhancing effects can be further confirmed by the images under UV-light irradiation. While Fe³⁺ completely quenched the emission of compound 1, resulting in dark emission color under UV light, the color changes to blue after Al³⁺ addition.

Fig. 2 Comparison of the luminescence intensity of compound 1 in the presence of various metal cations (10⁻³ M).

To testify the selectivity of compound 1 for the detection for Fe³⁺ and Al³⁺, the following control experiments were performed. 1 + Al³⁺ and 1 + Fe³⁺ were immersed in mixed solvent solutions (water : methanol = 4 : 1) containing potentially interfering metal ions (Mⁿ⁺ = Na⁺, K⁺, Mg²⁺, Gd²⁺, As³⁺, Ca²⁺, Sr²⁺, Ba²⁺, Fe²⁺, Co²⁺, Hg²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ag⁺, Pb²⁺, Al³⁺ or Fe³⁺) for 12 h. As expected, there were no apparent changes in fluorescence intensity when the metal ions above are involved (Fig. S16 and S17†), indicating high selectivity of 1 for Fe³⁺ and Al³⁺ among 19 types of metal ions without interference.

As reported in literatures,^10k,12,18 Cu²⁺ might interfere with the detection of Fe³⁺. In order to understand this potential interruption, we investigated the concentration-dependence for the luminescence measurement of Cu²⁺. The results show that the quenching efficiency of Cu²⁺ is only one fifth that of the blank when the concentration is 0.1 mM. On comparing Fig. S18 with S19,† it is observed that the quenching intensity of Fe³⁺ is greater than Cu²⁺ under UV-light irradiation at 365 nm.

Fig. S20 and S21† shows the low-(right) and high-magnification (left) TEM and SEM image of compound 1, 1 + Fe³⁺ and 1 + Al³⁺, respectively. Compared with compound 1, the morphology of TEM of 1 + Fe³⁺ has changed a lot. 1 + Fe³⁺ is of black materials covered with a lot of Fe³⁺.

Quantitative analysis of the proposed sensor was carried out by taking Fe³⁺ and Al³⁺ concentration-dependant fluorescence measurements. Compound 1 was immersed in different concentrations of Fe³⁺ and Al³⁺, and the fluorescence spectra were obtained (Fig. 3a and c, respectively). The fluorescence intensity of the Tb-MOF was steadily quenched by increasing the concentrations of Fe³⁺ from 4 × 10⁻⁴ to 10⁻⁵ M at 549 nm (Fig. 3a) with a detection limit of 8 × 10⁻⁶ M. In Fig. 3b, the results were linearly fitted into I₀/I = 37.14c + 0.7334 (where I₀ is initial fluorescence intensity, I is intensity at concentration c, and I₀/I is the relative fluorescence intensity), which is close to the Stern–Volmer equation: I₀/I = 1 + K_sv[Fe³⁺] (where [M] is the concentration of Fe³⁺ and K_sv is the quenching rate constant). Therefore, the quantitative detection of Fe³⁺ can be measured using the I₀/I of the system. Moreover, the K_sv value was calculated to be 3.714 × 10⁻⁴ L mol⁻¹, indicating the high quenching efficiency of compound 1 for Fe³⁺. At the same time, the fluorescence intensity was gradually enhanced with increasing Al³⁺ concentration from 10⁻⁶ to 10⁻³ M at 463 nm (Fig. 3c). As seen from Fig. 3d, the calibration plot displays a good linear relationship between the fluorescence intensity and the analyte concentrations with an equation of I = 1.458 × 10⁻³c + 1684 (mM). The detection limit of 7 × 10⁻⁷ M is significantly lower than the highest limit of the WHO recommendation for Al³⁺ in drinking water (7.41 μM),¹⁹ representing a rare example among reported fluorescent sensors for Al³⁺.

Fig. 3 (a) Fluorescence intensity of 1 + Fe³⁺ with various Fe³⁺ ion concentrations. (b) The Stern–Volmer plot of compound 1 quenched by Fe³⁺. (c) The liquid fluorescence spectra of 1 under different concentrations of Al³⁺ aqueous solution. (d) The fluorescence intensity (λ_em = 463 nm) vs. Al³⁺ concentration plot.

To check the time-effect, 1 was treated with 1 × 10⁻³ M of Fe³⁺ at room temperature and then fluorescence measurements were taken at different reaction times (5 min, 10 min, 15 min, 1 h, 2 h, 6 h, 12 h). In general, the fluorescence intensity of 1 + Fe³⁺ at 549 nm was decreased with increasing time, as show in Fig. 4a. It can be noted that the intensity decreased sharply to 40% of the initial response after about 5 min and was almost completely quenched after about 12 h. These results clearly demonstrate the highly effective quenching process. Furthermore, immediately after addition of Fe³⁺, the color of 1 obviously changed from light yellow to deep brown (Fig. 4b), an example of the naked-eye detection feature for Fe³⁺.

Fig. 4 (a) Variation of fluorescence intensity of 1 + Fe³⁺ solid sample at 549 nm with immersion time in Fe³⁺ solution of 1 × 10⁻³ M. (b) Images of compound 1 and 1 + Fe³⁺. (c) Four runs of Al³⁺ detection of 1 (λ_em = 463 nm). (d) Fluorescence changes of 1 before and after addition of Al³⁺ under a portable UV lamp at 365 nm.

The ability to regenerate a fluorescence probe is a very important factor in its use. To investigate its regeneration, compound 1 was immersed in a mixed solution of Al³⁺ (10⁻³ M) for 12 h; 1 + Al³⁺ was formed, accompanied by a color change to blue under UV light. Significantly, upon repeated rinses with methanol and water, a second color change to green under UV light was observed (Fig. 4d). The fluorescence intensity and PXRD of the regenerated 1 was consistent with those of as-prepared 1, and four cycles were performed (Fig. 4c). The result shows that 1 is stable to regeneration in a fast and simple method.

In previous studies, cation-caused fluorescence change is often ascribed to collapse of the host framework, resulting from interaction with metal cations.^9b,10b,12,20 In order to understand Fe³⁺/Al³⁺ effects on the luminescence of compound 1, PXRD was first employed to investigate the structures of 1 + Fe³⁺ and 1 + Al³⁺. Both are consistent with the original sample (Fig. S3†), suggesting that the framework of 1 is stable in a mixed solution even after the addition of the metal ions. In other words, addition of Fe³⁺/Al³⁺ does not change the host framework of 1. In future studies, the qualitative and quantitative interaction between Fe³⁺/Al³⁺ with 1 will be investigated. The detection mechanism for Fe³⁺/Al³⁺ ions by 1 on a molecule level is being explored at our lab.

Conclusions

In summary, a Tb-MOF was prepared and employed as a fluorescent sensing material for highly selective and sensitive detection of Fe³⁺ and Al³⁺ through emission quenching and fluorescence enhancing, respectively. Importantly, the prepared Tb-MOF presents quantitative detection of Fe³⁺ and Al³⁺ ions among 19 types of metal ions without interference from any. More interestingly, compound 1 acts as a fluorescent probe for Fe³⁺ through naked-eye detection and can be quickly and simply regenerated after the detection of Al³⁺. This study demonstrates the potential application of fluorescent Tb-MOFs as multiple response probes for selective and quantitative detection of Fe³⁺ and Al³⁺.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant no. 21673180, 21373162, 21473135 and 21173168), the Natural Science Foundation of Shanxi Province (No. 2016Q2018) and the 59^th China Postdoctoral Science Foundation Funded Project (No. 2016M590966).

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Footnotes

† Electronic supplementary information (ESI) available: The CIF files give crystallographic data for compound 1. Tables S1, S2 and Fig. S1–S24. CCDC 1477841. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra20359k

‡ These authors contributed equally to this work.

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