A water-stable terbium metal–organic framework as a highly sensitive fluorescent sensor for nitrite

Abstract

A novel water-stable Tb-based metal–organic framework (Tb-MOF), namely {[Tb(CA)(OA)_0.5(H₂O)₂]·H₂O}_n (H₂CA = chelidonic acid; H₂OA = oxalic acid), was synthesized via a solvothermal method. Tb-MOF was studied for its highly sensitive detection towards nitrite, which is a hazardous chemical in water and food. As a luminescent sensor, Tb-MOF displays a linear detection range between 0 and 15.6 μM, with a limit of detection of 28.25 nM. Dynamic quenching process was observed in this system and was studied in detail. A mechanism study revealed that the fluorescence response of Tb-MOF towards nitrite was due to the energy transfer from the sensitized Tb³⁺ ion to the nitrite.

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1. Introduction

Recently, metal–organic frameworks (MOFs) have evoked intensive interests owing to their intrinsic features such as varied structures, permanent porosity and adjustable functionality.^1–3 Inclusive applications in gas adsorption and separation,^4,5 catalysis,^6,7 sensors,^8–11 magnetic materials,¹² drug release^13,14 and energy-related areas^15–17 have been explored. For the MOFs with sensing-based applications,^18–20 lanthanide MOFs (Ln-MOFs) have displayed excellent luminescence properties that can be taken into consideration.^21,22 Through the reasonable adjustment of the host–guest interactions, Ln-MOFs have been applied in the chemical sensing of ions,^23–25 organic small molecules,^26,27 volatile organic compounds (VOCs)²⁸ and gases,^29–31 nitro explosives,^32–34 biomolecules^35,36 and so on.

Nitrite (NO₂⁻) ion is widely used in pickled products but it has a great potential threat to human health because of the generation of carcinogenic N-nitrosamines in vivo from the ingested nitrite.³⁷ Medical issues such as oesophageal cancer and birth defects have been reported due to the excessive consumption of nitrite ions.³⁸ According to the World Health Organization guidelines, the maximum contaminant level (MCL) of nitrite ions in drinking water is 3 mg L⁻¹ (65 μM).³⁹ Traditional nitrite ion detection was performed using toxic naphthyl ethylenediamine via colorimetry assay.⁴⁰ To avoid the usage of toxic materials, other methods including chromatography,⁴¹ electrochemical techniques⁴² and fluorescent technology^43–45 have been developed. However, these methods need complicated instruments, tedious detection procedures, and are time-consuming. Among these methods, the fluorescence sensing technology include the advantages of rapid response, low cost and easy-operation.⁴⁶

In this study, based on the fluorescence sensing mechanism and our experience on MOF-based sensing materials,^47–56 a new water-stable Tb-based MOF (Tb-MOF), {[Tb(CA)(OA)_0.5(H₂O)₂]·H₂O}_n (H₂CA = chelidonic acid; H₂OA = oxalic acid) was synthesized for highly sensitive nitrite detection. An isomorphic Gd-MOF was also synthesized for the mechanism studies. The dynamic quenching mechanism of Tb-MOF with nitrite provides a large quenching rate constant (k_q) of 1.16 × 10⁹ M⁻¹ s⁻¹. Both the high k_q and the low limit of detection (LOD) of 28.25 nM indicate a high sensitivity of Tb-MOF towards nitrite ion.

2. Experimental

2.1 Materials and methods

All purchased chemicals were of reagent grade and were used directly. Fourier transform infrared (FT-IR) spectra were carried out on a Bruker ALPHA spectrophotometer. Elemental analyses (EA) for C and H were executed on a PerkinElmer 240 CHN elemental analyzer. Ultraviolet visible (UV–vis) spectra were acquired using a Shimadzu UV-2600 spectrophotometer. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Smartlab SE X-ray diffractometer. Thermogravimetric analysis (TGA) was performed on a Labsys NETZSCH TG209 Setaram apparatus in nitrogen atmosphere (at the heating rate of 10 °C min⁻¹). Luminescence-related spectra were recorded on a FS-5 fluorescence spectrophotometer.

2.2 Synthesis

The synthesis of {[Tb(CA)(OA)_0.5(H₂O)₂]·H₂O}_n: Terbium nitrate hexahydrate (0.0453 g, 0.1 mmol), chelidonic acid (0.0272 g, 0.15 mmol) and CH₃CN/H₂O (v : v = 3 : 1; 8 mL in total) were successively placed in a 25 mL Teflon-lined stainless steel reactor. The reactor was heated at 95 °C for three days with an additional one day cooling to 30 °C. Light yellow crystals were selected and washed with water and acetonitrile with 28.9% yield based on terbium nitrate hexahydrate. EA calculated: C, 21.01%; H, 2.19%. Found: C, 21.42%; H, 2.12%. IR data (cm⁻¹): 3603 (s), 3175 (br), 1717 (s), 1618 (d), 1363 (t), 1212 (s), 1122 (s), 925 (d), 792 (s), 716 (s), 537 (m), 442(s).

Gd-MOF was obtained via the same method used for Tb-MOF by replacing terbium nitrate with gadolinium nitrate.

2.3 X-ray crystallography

Using an Agilent Technologies SuperNova single-crystal diffractometer equipped with graphite-monochromatic Mo-Kα radiation (λ = 0.71073 Å), the crystal data of Tb-MOF was collected at 120 K. The structure was solved by SHELXS (direct methods) and refined by SHELXL (full matrix least-squares techniques) in the Olex2 package.^57,58 The parameters, data collection, and refinements of the single crystal are summarized in Table S1.† CCDC 2011050† contains the details of the crystal structure.

2.4 Luminescence measurements

Tb-MOF (30 mg) was finely ground. This powder was then scattered in distilled water (100 mL). An aqueous suspension of Tb-MOF was obtained after ultrasonicating it for 10 min. After the suspension was stabilized, luminescence tests were performed using 3 mL of supernatant. The luminescence spectra of Tb-MOF suspension were measured in situ with freshly prepared 0.4 mM analyte solutions.

3. Results and discussion

3.1 Structural description and characterizations

Crystallized in the P1̄ space group, the asymmetric unit of Tb-MOF contains one Tb³⁺ ion, two coordinated water molecules, one solvent water molecule, one CA²⁻ ligand and a half OA²⁻ ligand. Eight oxygen atoms coordinate to the Tb³⁺ ion, completing a triangular dodecahedron configuration (Fig. 1a and b). Four oxygen atoms are from two CA²⁻, including three carboxylate oxygen atoms (Tb–O bond lengths ranging from 2.304(10) Å to 2.428(10) Å) and a carbonyl oxygen atom (the bond length of Tb–O is 2.366(9) Å). Two oxygen atoms are from two coordinated water molecules (Tb–O bond lengths of 2.380(10) Å and 2.407(10) Å). Rest of the two oxygen atoms are from the carboxylate oxygen atoms of the oxalates derived from the partial decomposition of H₂CA (Tb–O bond lengths of 2.399(9) Å and 2.402(9) Å). Tb³⁺ ions are connected by CA²⁻ ions in chains (Fig. 1c) and these chains are connected into a layer by CA²⁻ (Fig. 1d). The layers are further connected by OA²⁻ into a three-dimensional framework (Fig. 1e). Taking OA²⁻, CA²⁻ and Tb³⁺ ions as two, four and five-connected nodes, respectively, the framework is simplified as a (2,4,5)-connected trinodal net with a short (Schläfli) vertex symbol of {4⁴·6²·8⁴}²{4⁴·6²}²{8} (Fig. S1†).

Fig. 1 Coordination environment (a) and coordination geometry (b) of Tb³⁺ ion; (c) the connection mode of TbO₈; (d) 2D layer of Tb-MOF along the a axis; (e) 3D framework of Tb-MOF. Atom codes: Tb (green), O (red), C (gray). Hydrogen atoms are removed for clarity.

PXRD showed that the diffraction peaks of the as-synthesized Tb-MOF are accorded with the calculated ones from the crystal data (Fig. S2†). This result confirms the high phase purity of the as-synthesized Tb-MOF. In addition, the diffraction peaks of Tb-MOF after immersion in water for 36 h agree well with that of the primitive data. The results also indicate the high water-stability of Tb-MOF, which benefits from the suitable coordination models and rigid framework of Tb-MOF. The TGA of Tb-MOF displays a weight loss of 11.96% below 195 °C, which corresponds to the release of three water molecules (Fig. S3,† calc. 11.81%). Further heating causes a dramatic weight loss, indicating the disintegration of Tb-MOF.

3.2 Luminescence study and detection of nitrite

The luminescence spectrum is shown in Fig. 2. The excitation spectrum was recorded at a wavelength of 544 nm, which exhibited a broad band with a peak at 295 nm. Excited by 295 nm ultraviolet light, Tb-MOF displayed four well-resolved characteristic emissions at 489, 545, 585 and 621 nm, corresponding to the ⁵D₄ → ⁷F_J (J = 6, 5, 4, and 3) transitions of Tb³⁺ ions, respectively. The schematic energy transfer process within Tb-MOF is shown in Fig. S4.† The singlet and triplet levels of the ligands were calculated through the UV-Vis spectrum (Fig. S5a†) and phosphorescence spectrum of the isomorphic Gd-MOF at 77 K (Fig. S5b†). The singlet level of H₂OA is observed at 39 582 cm⁻¹ (252 nm), in accordance with our previous study,⁵² while the singlet level of H₂CA is at 33 630 cm⁻¹ (273 nm). The triplet levels of OA²⁻ and CA²⁻ are 24 390 cm⁻¹ (410 nm) and 22 831 cm⁻¹ (438 nm), respectively. The energy gaps (ΔE) between the singlet-state and triplet-state energy levels of the ligands are 12 647 cm⁻¹ and 10 799 cm⁻¹, respectively. The values of ΔE are much higher than 5000 cm⁻¹, indicating effective intersystem crossing processes using the Reinhold's empirical rule.^59,60 Hence, the gaps of the triplet level of the ligands and the ⁵D₄ energy level of the Tb³⁺ ion (20 500 cm⁻¹) are 3890 cm⁻¹ (H₂OA) and 2331 cm⁻¹ (H₂CA) in the range of the most capable values (2500–4500 cm⁻¹),⁶¹ which indicates an effective energy transfer process from CA²⁻/OA²⁻ to Tb³⁺ ion, giving rise to a bright green emission when exposed to 254 nm UV light (Fig. 2, inset).

Fig. 2 The luminescence spectra of Tb-MOF. The inset picture is Tb-MOF excited under a 254 nm ultraviolet lamp.

For studying the selectivity of Tb-MOF in water, the quenching effects of different salts were studied. The luminescence intensities of Tb-MOF with other salts have almost no change except that with the nitrite salt (Fig. 3a). A certain degree of fluorescence quenching of Tb-MOF occurs in the presence of Fe³⁺, Cu²⁺ and PO₄³⁻. However, when nitrite was added, a clear quenching effect occurred, indicating selectivity of Tb-MOF towards nitrite ions. The quenching efficiencies were calculated by (I₀ − I)/I₀, as shown in Fig. 3b, where I₀ and I are intensities at 544 nm of Tb-MOF dispersions without or with the addition of different salts, respectively. In particular, nitrite ions exhibited a quenching efficiency of 82.3%. Compared with Fe³⁺, Cu²⁺ and PO₄³⁻, the much higher quenching efficiency of NaNO₂ indicated the excellent selectivity of Tb-MOF towards nitrite ions. The results indicated that Tb-MOF can be used for the specific detection of nitrite.

Fig. 3 (a) Fluorescence intensities of Tb-MOF without or with different species. (b) the quenching efficiency of different salts.

To improve the accuracy for the detection of nitrite ions, three different fluorescence titration experiments were performed by adding different volumes of nitrite ions (3 μL, 5 μL, and 10 μL). Considering the first one for example, the luminescence intensities of Tb-MOF with successive additions of 3 μL nitrite are shown in Fig. S6.† The fluorescence quenching result of Tb-MOF is quantitatively mustered using the Stern–Volmer (S–V) eqn (1):

I₀/I = 1 + K_SV[C] (1)

where K_SV is the S–V quenching constant (M⁻¹), [C] is the nitrite ion concentration, and I₀ and I are fluorescence intensities without and with adding nitrite ions in dispersions, respectively. In Fig. S7,† the linear equation is well fitted with I₀/I − 1 = 0.474[C] + 0.016 in the range of 0–6 μM with a R² of 0.999. The K_SV value of Tb-MOF is 4.74 × 10⁵ M⁻¹. The K_SV value is higher than that of most MOF-based ion sensing materials (usually 10³–10⁴ M⁻¹), indicating a high sensitivity of Tb-MOF towards nitrite.^23–25 The results of the three sets of fluorescence titrations (Fig. S6–S11†) were combined in Fig. 4. The linear correlation was further modified as I₀/I − 1 = 0.482[C] + 0.002 with a R² of 0.999 in the range of 0–15.6 μM, with the K_SV value of 4.82 × 10⁵ M⁻¹.

Fig. 4 Summary of S–V plots at 544 nm of Tb-MOF towards nitrite ions with different volumes, 3 μL (green plots), 5 μL (red plots), 10 μL (blue plots). Inset is the picture of Tb-MOF suspension excited by 254 nm ultraviolet light before and after adding 20 μL nitrite aqueous solution (4 mM).

The LOD was calculated using the 3σ IUPAC criteria by eqn (2) and (3):

LOD = 3S_d/slope (3)

where S_d is the standard deviation of the replicate fluorescence intensities of Tb-MOF dispersions performed twenty times (Fig. S12†). I_i is the fluorescence intensity at 544 nm without nitrite, and is the average of . The slope value was calculated from eqn (1) and the LOD was calculated to be 28.25 nM, which is far below the criterion of the World Health Organization for drinking water.

To investigate the practical application, Tb-MOF was employed to measure the nitrite concentration in tap water. The related results of spike-and-recovery experiences are shown in Table S2.† The recoveries were found to be 97.7–100.9%. Moreover, the anti-interference of Tb-MOF was also evaluated (Fig. S13†). With coexisting anions and cations, the degree of the quenched fluorescence of Tb-MOF stayed almost the same after the addition of nitrite, indicating that Tb-MOF displayed a considerable anti-interference ability. The recycling performance of Tb-MOF was also studied (Fig. 5). The suspensions with nitrite ions were centrifuged and washed with water and acetonitrile for several times after the sensing experiments. It was found that the luminescence intensity can be recycled at least 5 times. All these results indicate that Tb-MOF has potential applications in the nitrite ion detection in polluted water and food preservatives.

Fig. 5 Reproducibility of Tb-MOF with five continuous quenching cycles.

3.3 Sensing mechanism

To the best of our knowledge, this sensing material provides the lowest LOD (28.25 nM) for the nitrite ion detection amongst the reported MOF-based fluorescence sensing materials (Table S3†). To figure out the quenching behaviour towards nitrite ions, the sensing mechanism was investigated. First, the PXRD of Tb-MOF immersed in 4 mM nitrite ions for 4 h showed similar results to that obtained with the calculated data (Fig. S14†), indicating the structural stability of Tb-MOF with nitrite ions. As shown in Fig. 4, the S–V plots show a linear relationship at the completed concentration range, which is different from that of the concentration controlled by a self-absorption process.^51–55 The luminescence lifetimes of Tb-MOF were measured at 544 nm with different concentrations of the nitrite ions (Fig. S15 and Table S4†). The lifetimes decrease with the increase in the concentration of nitrite ions, and the lifetime-based S–V curve provides a same K_SV of 4.82 × 10⁵ M⁻¹ (Fig. S16†) with that of the S–V curve, indicating a dynamic quenching progress. The quenching rate constant (k_q) can be calculated using eqn (4):

k_q = K_SV/τ₀ (4)

where K_SV is lifetime-based S–V quenching constant (M⁻¹), and τ₀ is the luminescence lifetime (s⁻¹) of Tb-MOF without nitrite ions. The k_q value was 1.16 × 10⁹ M⁻¹ s⁻¹, which is 10 times lower than that of the purely diffusion-controlled quenching process (about 10¹⁰ M⁻¹ s⁻¹)⁶² possibly due to the steric hindrance of Tb-MOF, limiting the number of collisional quenching occurring.^23,63 The decrease in lifetimes indicates the presence of other relaxation pathway that can depopulate the excited state. To further understand the specific energy transfer between Tb-MOF and nitrite ions, the singlet and triplet energy levels of NaNO₂ were obtained as 48 077 cm⁻¹ (208 nm) and 19 084 cm⁻¹ (524 nm) (Fig. S17 and S18†), which are similar to the results reported by Fujio.^64,65 The triplet level of NaNO₂ is slightly lower than the ⁵D₄ energy level of the Tb³⁺ ion (20 500 cm⁻¹) with a gap of 1416 cm⁻¹, which is lower than 1850 cm⁻¹. This result indicates that energy transfer between Tb-MOF and NaNO₂ is possible.^61,66 In addition, no obvious overlap was observed between the UV-Vis spectra of the ligands and NaNO₂ (Fig. S5a and S17†), which excludes the possibility of the competitive absorption mechanism.

Based on the above analysis, the mechanism of the sensing function is shown in Fig. 6. Excited at 295 nm, the Tb³⁺ ion is sensitized by antenna effect and displays characteristic emissions; when nitrite ion was added, dynamic quenching behaviour between Tb-MOF and nitrite ion happened. In this process, energy transfer between Tb-MOF and nitrite ion occurred, leading to the decrease in the emission intensity.

Fig. 6 The schematic energy transfer process of Tb-MOF in the presence of nitrite.

4. Conclusions

In summary, a water-stable Tb-based MOF was synthesized as a sensitive and selective nitrite ion sensor. The sensing mechanism studies show that in the presence of nitrite ions, a part of the energy for the emission of Tb-MOF is transferred to nitrite ion, hence decreasing the emission intensity. To the best of our knowledge, the sensor provides the lowest LOD (28.25 nM) for the nitrite ion detection compared with the recently reported MOF-based fluorescence sensing materials for nitrite. The satisfactory recyclability, sensitivity, selectivity, and stability indicate a promising utilization of Tb-MOF for nitrite detection in water.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant numbers 21931004 and 21861130354), the Natural Science Foundation of Tianjin (18JCJQJC47200), the Fundamental Research Funds for the Central Universities, Nankai University (63201016 and 63201043) and the Ministry of Education of China (grant number B12015). W. S. acknowledges the receipt of a Newton Advanced Fellowship from Royal Society.

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data of Tb-MOF, PXRD, TGA and other figures and tables. CCDC 2011050. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qi00780c

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