A highly sensitive MOF fluorescence probe for discriminative detection of aliphatic and aromatic amines

Abstract

The highly sensitive detection of organic amines is of great significance, as even low concentrations of amines can pose environmental and human health risks. Current methods to effectively sense amines in aqueous solution remain scarce, particularly in discriminative detection of aliphatic and aromatic amines. Herein, we have strategically designed and synthesized a multifunctional metal–organic framework (MOF) fluorescence probe that exhibits “turn-on” fluorescence for aliphatic amines and “turn-off” fluorescence for aromatic amines. Remarkably, based on the donor–acceptor electron transfer and competitive absorption mechanisms, ultralow detection limits are achieved with 0.22 μmol L⁻¹ for diethylamine (DEA) and 0.33 μmol L⁻¹ for p-phenylenediamine (PPD), respectively. This achievement underscores the potential of our MOF fluorescence probe in enhancing the precision and selectivity of amine detection in aqueous environments.

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Introduction

Organic amines, as important chemical materials, have been extensively used in dyeing, plastics, pesticides, the pharmaceutical industry, etc.^1–4 However, the inherent toxic nature of amines may seriously damage the environment and human health, even at low concentration. Through diffusing into air and dissolving in water, amines can cause various health problems in humans, affecting skin, eyes and the respiratory system.^5–7 Some conventional analytical techniques have been adopted for detecting amines, including gas chromatography, high-performance liquid chromatography, gas chromatography-mass spectrometry and Raman scattering.^8–10 But these methods suffer from different limitations of expensive instruments, professional operation, high cost and long testing time.^11–17 Recently, various fluorescence probes have been developed for the detection of amines due to the advantages of high sensitivity, fast responsiveness, simple operation and naked-eye detection.^18,19 Unfortunately, owning to the unique basicity and chemical structures of amines, the performance of probes in aqueous solution lags behind that in organic solvents. Therefore, there is urgent need to develop a stable, sensitive, selective, and efficient fluorescence probe for the detection of organic amines in aqueous solution.

Luminescent metal–organic frameworks (MOFs) have attracted much attention due to their high stability, large specific surface area, tunable compositions and structures.^20–24 Generally, MOFs are organic–inorganic hybrid materials consisting of metals or metal clusters and polydentate organic linkers. Thus, the diversity of MOFs is almost infinite, offering various designable structure features and tunable functionalities. For the detection of amines, the careful design of organic linkers with appropriate geometry, functional groups and energy levels is crucial. A nitrogen-containing heterocyclic carboxylic acid ligand (pyridine-2,6-dicarboxylic acid, DPA) is adopted to construct the MOF fluorescence probe, primarily considering three key aspects: (1) DPA provides numerous coordination sites to interact with metal ions through coordination and hydrogen bonds; (2) electron-deficient nitrogen endows MOFs with sensitive and selective response to electron-rich amines; (3) the rigid conjugated structure guarantees effective emission and suitable energy levels conducive to energy transfer in fluorescence sensing.^25–28 Simultaneously, the lanthanide Eu³⁺ ion is a judicious choice because of the unique 4f electronic composition and antenna effect.²⁹ The Eu-DPA is predicted to possess excellent stability in aqueous solution. And the fluorescence of it would undergo dramatic changes with surrounding environments involved with complex energy transfer processes, showing great potential in amine sensing. Different from recent reports on amine detection,^30–35 we have developed an attractive MOF with “turn-on” behavior for aliphatic amines and “turn-off” behavior for aromatic amines. This approach holds significant meaning for the discriminative detection of various organic amines with the same functional groups.

In this work, aiming to detect organic amines in aqueous solution, we designed and synthesized a novel MOF fluorescence probe with DPA as linkers and Eu³⁺ ions as the metal node. The MOF, denoted as Eu-DPA, exhibits red emission in water due to the antenna effect. Fluorescence sensing behaviors of Eu-DPA were systematically studied toward various amines. Owing to the competitive absorption effect, aromatic amines quench the fluorescence of the probe with “turn-off” sensing behavior. Interestingly, the fluorescence intensities present “turn-on” sensing behavior upon the addition of aliphatic amines, which can be ascribed to the donor–acceptor electron transfer mechanism. More importantly, this multifunctional fluorescence probe not only achieves rapid and discriminative detection of aliphatic and aromatic amines, but also demonstrates high sensitivities with a low detection limit of 220 nM for diethylamine (DEA) and 0.33 μM for p-phenylenediamine (PPD).

Experimental

General information

All reagents and solvents used in this study were procured from Sinopharm Chemical Reagent Co. Ltd or Shanghai Macklin Biochemical Co. Ltd without further purification. The details of characterization, including powder X-ray diffraction (PXRD) patterns, density functional theory calculations, and fluorescence-sensing experimental methods, can be found in the ESI.†

Synthesis of Eu-DPA

Eu-DPA was synthesized by the simpler and faster synthesis method with europium chloride hexahydrate (EuCl₃·6H₂O) (0.25 mmol, 91.5 mg) and dipicolinic acid (DPA) (0.5 mmol, 83.5 mg) in the mixed solvents of ethanol (15 mL), ultrapure water (5 mL) and trimethylamine (101.2 mg) as reported in previous work.³⁶ After sonication and being fully dissolved, the mixture was stirred at 40 °C for 2 hours (h) in a round bottom flask. Then, the reaction slowly cooled to room temperature. The crude product was separated by centrifugation to eliminate solvents, and washed alternately with ethanol and ultrapure water three times. A white powder of Eu-DPA was obtained after drying at 40 °C for 12 h.

Results and discussion

Characterization of Eu-DPA

The scanning electron microscope (SEM) images were collected to reveal the surface morphology of the prepared probe material. As shown in Fig. 1a–c, Eu-DPA displays a uniform, irregular grainy morphology with particle sizes ranging from 20 to 30 μm. To further confirm the crystalline structure, powder X-ray diffraction (PXRD) patterns were obtained for Eu-DPA (Fig. S1, ESI†). The XRD spectrum shows prominent peaks at 2θ values of 8.2°, 10.5°, 12.1°, 16.6° and 19.1°, which match well with the corresponding simulated peaks.³⁷ The porous nature and surface area of the probe were investigated by carrying out nitrogen adsorption/desorption measurements at a temperature of −195.85 °C (Fig. S2, ESI†). The Brunauer–Emmett–Teller (BET) surface area of Eu-DPA was estimated to be 40.2 m² g⁻¹. The results of SEM and PXRD demonstrate that the fluorescence materials have relatively poor crystallinity and high purity. Moreover, the energy dispersive X-ray spectrum (EDX) mappings and XPS survey spectra (Fig. S3, ESI†) also prove that the C, N, O and Eu elements are uniformly distributed in Eu-DPA (Fig. 1d–h). These characterization results collectively signify the successful synthesis of fluorescence probe materials.

Fig. 1 The SEM images (a–c) and EDX mappings (d–h) of Eu-DPA.

Furthermore, the Fourier transform infrared (FT-IR) spectra of DPA and Eu-DPA were recorded using the ATR direct method in the range of 4000–400 cm⁻¹ (Fig. S4, ESI†). After the ligand coordinated with lanthanide metal ions, the free carboxyl stretching vibration peak at 1690 cm⁻¹ in DPA disappears. And two characteristic peaks appear at 1580 cm⁻¹ and 1400 cm⁻¹, which can be assigned to the asymmetric and symmetric stretching vibrations of the coordinated carboxyl groups.^38,39 Meanwhile, the broad band of 2500–3000 cm⁻¹ vanishes from Eu-DPA, indicating the complete deprotonation of the ligands. Besides, the surface deformation vibration peak at 910 cm⁻¹ fades in the pyridine ring, which is associated with the coordination of a nitrogen atom.⁴⁰

Stability test

The stability plays a very crucial role in sensing applications of MOF materials. Thermal gravimetric analysis (TGA) was performed to examine the thermal stability of the fluorescence probe (Fig. S5, ESI†). Eu-DPA exhibits a weight loss of ∼11.5% from 25 °C to 131 °C, which could be ascribed to the loss of residuary ethanol and coordination water. Subsequently, the ligands begin to decompose and the MOF framework collapses at temperatures exceeding 407 °C. The good thermal stability can be attributed to the strong coordination interaction between the DPA groups and Eu³⁺ ions. The water and chemical stabilities of Eu-DPA were further investigated by PXRD measurements. After treatment in aqueous solution for 7 days, it retains the as-synthesized PXRD peaks (Fig. S6, ESI†). Meanwhile, Eu-DPA shows good fluorescence stability in aqueous solution (Fig. S7, ESI†). Even in acidic (pH = 2) and basic (pH = 12) aqueous solutions (Fig. S8, ESI†) and amine solvents (Fig. S9, ESI†), the PXRD patterns show no obvious differences compared with that of pristine Eu-DPA, demonstrating good water and chemical stabilities.

Photophysical properties of Eu-DPA

The photophysical properties of Eu-DPA were investigated by excitation spectra and photoluminescence (PL) spectra (Fig. S10, ESI†). For comparison, the ligand of DPA and metal Eu³⁺ demonstrate absorption peaks at 272 nm and 207 nm, respectively (Fig. S11, ESI†). The stronger absorption of DPA can be attributed to the π–π* transition from conjugated aromatic heterocycles. Upon the formation of Eu-DPA, the absorption intensity is enhanced for UV light with a slight red-shift peak at 275 nm. This enhancement suggests that the appropriate ligand of DPA can effectively improve the UV absorption capability of the prepared MOFs. Under excitation at 275 nm, the PL spectra of Eu-DPA exhibit characteristic red emission of typical Eu³⁺ with peaks at 592, 614, 649 and 691 nm, corresponding to ⁵D₀–⁷F₁, ⁵D₀–⁷F₂, ⁵D₀–⁷F₃ and ⁵D₀–⁷F₄ transitions, respectively.⁴¹ Owning to the matched energy levels between the ligands and lanthanide ions, the Eu³⁺ ions are fully sensitized by DPA through the antenna effect (Fig. S12, ESI†).

Fluorescence detection of amines

To explore the detection performance of Eu-DPA, PL spectra were recorded in aqueous solution by adding various amines, including diethylamine (DEA), ethylamine (EA), triethylamine (TEA), ethylenediamine (EDA), aniline (AN), 1,2-diaminobenzene (OPD), m-phenylenediamine (MPD) and p-phenylenediamine (PPD). In fluorescence sensing experiments, the concentration of the MOF suspension is 0.5 mg ml⁻¹, the amine concentrations are set at 1 × 10⁻³ mol L⁻¹ (M), and the pH is around 7 (Fig. S13, ESI†). As shown in Fig. 2, the fluorescence is significantly enhanced upon the addition of aliphatic amines. Specifically, for sensing DEA, the emission intensity of Eu-DPA increases by 327%. Such rare “turn on’’ sensing behavior is desirable for devices due to the great anti-interference ability against background signals. In contrast, the aromatic amines can effectively quench the fluorescence with “turn off’’ sensing behavior in spite of the same functional group (Fig. S14, ESI†). The emission intensity is reduced by 83% after the addition of PPD. The response times of Eu-DPA toward DEA and PPD are within 3 seconds, displaying a very fast detection (Fig. S15, ESI†). The development of a multifunctional fluorescence probe for discriminative sensing of aliphatic and aromatic amines holds significant meaning.

Fig. 2 (a) PL spectra and (b) relative luminescence intensities peaking at 614 nm of Eu-DPA dispersed in aqueous solutions with various amines.

To further examine the detection sensitivities of Eu-DPA, fluorescence titration experiments were carried out through gradually increasing the concentration of amines (Fig. S16, ESI†). As illustrated in Fig. 3a, the fluorescence intensity improves to 256% as the concentration of DEA increased from 0 to 5.2 × 10⁻⁴ M. The corresponding linear calibration curve between fluorescence intensities (at 614 nm) and concentrations of DEA is presented in Fig. 3b. Thus, the limit of detection (LOD) can be calculated by the following equation:⁴²

LOD = 3σ/m (1)

where σ represents the standard deviation of fluorescence intensity before adding amines (Fig. S17 and Table S1, ESI†), and m is the slope of concentration versus intensity curve. The estimated LOD of the fluorescence probe for the detection of DEA is 220 nM, which is one of the lowest values reported for sensing DEA (Table S2, ESI†).⁴³

Fig. 3 PL spectra of Eu-DPA after the addition of (a) DEA and (c) PPD with different concentrations; corresponding linear relationships between concentration and intensity (at 614 nm) of (b) DEA and (d) PPD.

Conversely, the PL intensity exhibits an obvious decrease and reaches a quenching rate of 51% as the concentration of PPD increased from 0 to 2.0 × 10⁻⁴ M. According to the Stern–Volmer equation,⁴⁴ the fluorescence static quenching coefficient K_sv can be calculated to evaluate the sensitivity:

I₀/I = 1 + K_sv[Q] (2)

where I₀ represents the original fluorescence intensity of Eu-DPA in aqueous solvent, I is the fluorescence intensity after adding PPD at different concentrations, and Q is the concentration of PPD. The K_sv of Eu-DPA is calculated to be 4957 M⁻¹, and the LOD of Eu-DPA for sensing PPD is calculated to be 0.33 μM. The recyclability experiments indicate that the fluorescence intensity and sensing performance of Eu-DPA remain almost unchanged after five recycling cycles (Fig. S18, ESI†). In addition, luminescence test strips were prepared to explore the potential applications of the probe. Upon uniform application of organic amines (10 mmol L⁻¹) to the test strips, significant fluorescence enhancement was observed for aliphatic amines, while aromatic amines induced fluorescence quenching (Fig. S19, ESI†). In a word, the novel fluorescence probe of Eu-DPA not only discriminately detects aliphatic and aromatic amines, but also shows ultra-high detection sensitivities.

Sensing mechanism

To explore the fluorescence quenching mechanism, a series of spectroscopy experiments were performed. As shown in Fig. 4, a large overlap is observed between the absorption spectra of aromatic amines and the excitation spectrum of Eu-DPA. In comparison, there is almost no overlap between the absorption spectra of aliphatic amines and the excitation spectrum of Eu-DPA. Hence, competitive absorption emerges as one of the possible mechanisms leading to fluorescence quenching.^45,46 On the other hand, the emission of Eu-DPA lacks overlap with the absorption of aromatic amines, which can rule out the fluorescence resonance energy transfer mechanism. In addition, the energy levels and frontier orbital distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) for DPA, aliphatic and aromatic amines were simulated by the density functional theory (DFT) method at the level of B3LYP/6-31G(d,p) (Fig. 5 and Fig. S20, ESI†). The HOMO of ligand DPA is lower than that of aromatic amines, resulting in electron transition from analyte to fluorescence probe. To verify the photoinduced electron transfer (PET) mechanism, transient photoluminescence spectra of Eu-DPA were collected before and after the addition of aromatic and aliphatic amines. The PET process decreases the number of excited states and generally results in a shorter fluorescence lifetime. Therefore, the reduced lifetimes for sensing aromatic amines suggest that the PET is another conceivable mechanism for fluorescence quenching.^47,48

Fig. 4 UV-vis spectra of (a) aromatic amines and (b) aliphatic amines compared with excitation spectra of Eu-DPA, fluorescence lifetimes of Eu-DPA before and after sensing (c) aromatic amines and (d) aliphatic amines.

Fig. 5 The HOMO and LUMO energy levels of aromatic amines, DPA and aliphatic amines.

In contrast, the electron transfer from the LUMO of aliphatic amines to the LUMO of Eu-DPA enhances the photoluminescence quantum efficiency, which is assigned to the donor–acceptor electron transfer mechanism.⁴⁹ The extent of electron transfer depends on the energy gaps of the LUMOs between aliphatic amines and DPA. According to the theoretical simulation in Fig. 5, the enhancements of fluorescence are expected to follow this order: TEA > DEA > EA > EDA, which agrees well with the trend in PL spectra of Fig. 2b, except for DEA. Indeed, the dipole–dipole interaction plays an important role in the electron transfer process from aliphatic amines to Eu-DPA. The dipole moments are calculated to be 0.86 D and 0.52 D for DEA and TEA, respectively. The larger dipole moment accelerates electron transfer for the detection of DEA. Due to the introduction of charge transfer (CT) state characteristics, the fluorescence lifetimes of Eu-DPA increase upon detection of aliphatic amines. These two opposing trends in lifetime indicate that the probe undergoes two distinct processes for sensing aromatic and aliphatic amines, corresponding to the PET and donor–acceptor electron transfer mechanisms, respectively.

Conclusions

In summary, a novel MOF fluorescence probe of Eu-DPA was designed and synthesized for the detection of organic amines in aqueous solution. Remarkably, the Eu-DPA exhibits discriminative detection of aromatic amines through “turn-off” behavior and aliphatic amines through “turn-on” behavior, accompanied by competitive detection limits. To our knowledge, it stands as a rare example of a sensor capable of discriminately detecting organic amines through opposite fluorescence behavior with both high selectivity and sensitivity. Photophysical characterization combined with theoretical calculations demonstrate that the competitive absorption and donor–acceptor electron transfer mechanism are responsible for the quenching effect and fluorescence enhancement, respectively. This work not only presents a promising material with potential application in detecting organic amines, but also provides a new design strategy for developing multifunctional MOF sensing materials.

Author contributions

Changjiang Zhou: design, synthesis, characterization, data analysis, writing – original draft, financial support of experiments. Yafei Liu: synthesis, characterization, graphing, data analysis. Zimeng Liu: characterization, data analysis. Zhichao Qiu: characterization. Zhuangzhuang Sun: data analysis. Minrui Chen: data analysis, financial support of experiments. Bo Xie: data analysis. Hui-min Wen: data analysis, financial support of experiments. Jun Hu: Design and supervise the whole experiments, data analysis, writing – review & editing, financial support of experiments.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (52303255, 52103309 and 92163110), the Key Research and Development Program of Zhejiang Province (2021C01182), and the Zhejiang Provincial Natural Science Foundation of China (LY22E030011, LQ22E010008).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc03702b

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