Highly effective chemosensor of a luminescent silica@lanthanide complex@MOF heterostructured composite for metal ion sensing

Abstract

Heterostructured chemosensors have excellent chemical stability, low contamination and excellent recyclability, and are regarded as a promising new generation of sensors with critical properties related to their sensing sensitivity and selectivity. Our preparation strategy for a heterostructured sensor is to construct it using a luminescent lanthanide complex and metal organic framework (MOF). A heterogeneous sensor is systematically produced using EuTTA and ZIF-8 to cover silica, and thin particles are obtained with small size and uniform morphology. Then, the particles were used as a highly selective and sensitive sensor to detect Cu²⁺ in aqueous solution. Eventually, we find that the well-structured silica@EuTTA@ZIF-8 microspheres have a good fluorescence stability, low detection limit and broad linear range in aqueous environments. These excellent properties make it a potential chemosensor for sensing of Cu²⁺ in environmental or biological solution systems.

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Introduction

Photoluminescent lanthanide coordination compounds have been fascinating researchers for many years due to the effective energy transfer process from the organic ligands to the lanthanide ions.^1,2 Unfortunately, lanthanide complexes themselves are still restricted from being utilized in practical applications due to their poor thermal and mechanical stability. Therefore, lanthanide complexes can be linked to inorganic or organic polymeric hosts through physical doping or chemical bonding approaches,^3,4 resulting in all kinds of lanthanide hybrid materials.^5,6 These hybrids can display favourable luminescence behaviours such as long lifetimes and high internal quantum efficiencies, which can be expected to have potential applications in optical devices.⁷

To date, host units for luminescent lanthanide hybrid materials involve sol–gel derived silica, mesoporous silica and microporous zeolite, and organic polymers.^8–11 Besides, to overcome the shortcomings of non-crystalline hybrids, inorganic crystalline units are introduced to further improve their physical and chemical properties. For example, all kinds of inorganic semiconductor nanocrystals or polyoxometalate species can be assembled with lanthanide complexes in the same hybrid system.^12,13 Some hybrid systems can be further realized by tuning the luminescence and integrating.¹⁴

Metal–organic frameworks (MOFs) are crystalline hybrids formed by coordination reactions of metal ions and organic ligands,¹⁵ whose large specific surface area, ordered crystalline structure, and highly regular pores give them many potential applications such as gas storage, separation, ion exchange, catalysis and sensing etc.¹⁶ Zeolitic imidazolate frameworks (ZIFs) are a kind of MOF composed of tetrahedrally-coordinated transition metal ions connected by organic imidazole ligands which are topologically isomorphic with zeolites.¹⁷ Among these, ZIF-8 (Zn(MeIM)₂, MeIM = 2-methylimidazole) is a chemically robust and thermally stable material and possesses a sodalite zeolite-type structure with large cavities and small pore apertures.¹⁸ ZIF-8 is a suitable host to compose all kinds of nanoparticles due to its intersecting three-dimensional structure, high thermal stability, large pore size and surface area.¹⁹

Considering versatile chemical modifications, different host units can be further assembled together. In fact, some studies have tried to compose MOFs and other host units.²⁰ On the other hand, MOFs can be utilized to construct nanocomposites or nanostructures.²¹ For ZIF-8, we have tried its assembly with other functional units to form hybrid systems, involving upconversion nanocrystals, polyoxometalates and complex species.²² The different luminescent centers in these hybrids produce multi-color luminescence and white luminescence under selected excitation. It is worthwhile pointing out that coordination polymers can be immobilized onto carboxylate-terminated silica particles, resulting in the formation of novel silica@coordination polymer heterostructures.²³ This methodology provides a path for the in situ immobilization of organic molecules to form coordination polymers and the further preparation of heterostructure composites with silica, which can be expected to be utilized in many applications.

In this paper, a new composite with a silica@lanthanide complex@MOF heterostructure is designed and prepared and the synthesis process is shown in Scheme 1. Detailed physical characterization, in particular the luminescence performance and sensing properties, was conducted.

Scheme 1 Schematic representation of the synthesis process of SiO₂@EuTTA@ZIF-8 and the fluorescence quenching phenomenon of Cu²⁺ to SiO₂@EuTTA@ZIF-8.

Experimental

Chemicals

All reagents were obtained from commercial sources (Sinoreagent, Sigma Aldrich, Aladdin) and were used without further purification unless otherwise stated. All of the metal ion aqueous solutions (Cd²⁺, Ca²⁺, Ni²⁺, Fe³⁺, Hg²⁺, Pb²⁺, Co²⁺, Fe²⁺ and Cu²⁺) were prepared from their nitrate salts. Europium chlorides were prepared by dissolving the corresponding oxides (Eu₂O₃) in excess hydrochloric acid (37%) followed by evaporation and crystallization.

Surface carboxyl-modified SiO₂ microspheres

An equal molar ratio of 3-triethoxysilylpropylamine (APTES) and succinic anhydride was dispersed uniformly in a certain amount of DMF with magnetic stirring for 3 h under room temperature (25 °C). Then, a DMF suspension of silica after ultrasonic dispersion (20 mL) and deionized water (2 mL) were dropped into the above solution, and magnetic stirring continued for 5 h at the same temperature. The nano-silica was isolated using an ultra-high speed centrifuge, then washed with alcohol several times, and the carboxylate-modified SiO₂ spheres obtained.

Preparation of luminescent SiO₂@EuTTA microspheres

Solution A: EuCl₃·6H₂O (0.3662 g) was mixed with carboxylic acid-terminated silica (1.0 g) in 5 mL of water. Solution B: 2-thenoyltrifluoroacetone (HTTA, 0.6664 g, 3 mmol) was dissolved in 10 mL of ethanol and NaOH aqueous solution (5% w/v) was used to adjust the pH of this solution to 6.0. The protonated solution B was added dropwise to the aqueous solution A of Eu³⁺ and silica. After addition of 100 mL of water, the mixture was vigorously stirred for 120 min at 60 °C, and luminescent SiO₂@EuTTA microspheres were generated. The microspheres were isolated by cooling the reaction mixture to room temperature, collecting the precipitate through centrifugation, and washing the precipitate several times with deionized water, then dried in a desiccator at room temperature.

Synthesis of luminescent SiO₂@EuTTA@ZIF-8

Seeding process. An “in situ” seeding process was developed as follows. 0.1 g of SiO₂@EuTTA spheres was added to a solution of 0.195 g Zn(NO₃)₂·6H₂O and 1.3 g H₂O and sonicated for 5 min. Another 0.1 g of SiO₂@EuTTA was added into a solution of 3.78 g of 2-methylimidazole and 13.3 g of H₂O and sonicated for the same time. Then the Zn²⁺ solution was dropped into the 2-methylimidazole solution with stirring for 5 min. The product was collected by centrifugation, washed 3 times with deionized water and dried overnight.

Regrowth of seeded SiO₂@EuTTA. 0.47 g of Zn(NO₃)₂·6H₂O was dissolved in 20 mL of MeOH and water (1 : 1 v/v). In addition, 1 g of 2-methylimidazole was dissolved in 10 mL of MeOH with 0.14 g of “in situ” seeded SiO₂@EuTTA. The two solutions were sonicated for 5 min and then the Zn²⁺ solution was dropped into the 2-methylimidazole solution with strong stirring for 2 h. After that, the final product, the SiO₂@EuTTA@ZIF-8 heterostructure composite, was collected by centrifugation with deionized water and MeOH several times, then dispersed in MeOH to separate the microspheres and dried overnight.

Luminescence measurements

The photoluminescence phenomena of SiO₂@EuTTA@ZIF-8 with different metal ions were studied at room temperature. For the sensing properties of various metal ions, emulsions of SiO₂@EuTTA@ZIF-8/Mⁿ⁺ were obtained by diffusing a certain amount of SiO₂@EuTTA@ZIF-8 powder (3.0 mg) into a deionized water solution (3.0 mL) of M(NO₃)_n (Mⁿ⁺ = Cd²⁺, Ca²⁺, Ni²⁺, Fe³⁺, Hg²⁺, Pb²⁺, Co²⁺, Fe²⁺ and Cu²⁺) under concentrations of 10⁻² M. Then the solution mixtures were sonicated for 30 min to form various metal ion-incorporated suspensions for luminescence measurements. Every sample had the same excitation slit width and the emission slit excitation wavelength of 396 nm.

Physical measurements

Scanning electronic microscopy (SEM) images were recorded with a Hitachi S-4800 with a cold field emission gun operating at 2 kV and 10 μA. Transmission electron microscopy (TEM) was conducted with a JEOL JEM-2010F electron microscope and operated at 200 kV. The crystalline phases of the products were determined using powder X-ray diffraction (PXRD) patterns which were recorded at room temperature under ambient conditions with a Bruker D8 VANDANCE X-ray diffractometer with CuKa radiation under 40 kV and 40 mA, and the data were collected within the 2θ range of 5–60°. Thermal gravimetric analysis (TGA) was carried out on a STA 449C (Netzsch) system with a heating rate of 5 K min⁻¹ from 40 °C to 800 °C under a nitrogen atmosphere in Al₂O₃ crucibles. Fourier transform infrared (FTIR) spectra were measured within the range 4000–400 cm⁻¹ on a Nexus 912 AO446 spectrophotometer using KBr pellets. Luminescence excitation and emission spectra of the samples were obtained on an Edinburgh FLS920 spectrophotometer. Lifetime measurements were performed on an Edinburgh FLS920 phosphorimeter using a 450 W xenon lamp as the excitation source. The lifetime data was obtained from fitting the experiment luminescence decay.

Results and discussion

Characterization

The carboxyl-modified silica microspheres and the fabricated SiO₂@EuTTA@ZIF-8 heterostructure microspheres are characterized using FTIR (Fig. 1). The appearance of –COOH characteristic bands at 3500–2500 cm⁻¹ and –CO–NH– at 1559 cm⁻¹ demonstrates the successful carboxyl modification of the silica spheres.²⁴ The increase in the peak intensity at 1107 cm⁻¹ for Si–O–Si also confirms the condensation of (3-aminopropyl)triethoxysilane (APTES) with Si–OH.²⁵ The appearance of characteristic bands of ZIF-8 for the SiO₂@EuTTA@ZIF-8 heterostructured composite microspheres indicates growth of the ZIF-8 nanocrystals on the surface of the silica spheres. Moreover, the characteristic bands of silica at 800 and 472 cm⁻¹ for the carboxylate-terminated SiO₂, SiO₂@EuTTA and SiO₂@EuTTA@ZIF-8 also reveal that carboxyl modification, EuTTA and ZIF-8 growth do not affect the structure and composition of the core.

Fig. 1 FTIR spectra of the SiO₂ spheres, carboxylate terminated SiO₂ (SiO₂–COOH), SiO₂@EuTTA, and SiO₂@EuTTA@ZIF-8 heterostructured microspheres.

PXRD patterns are also used to evaluate the composition of SiO₂@EuTTA@ZIF-8 (Fig. 2). The existence of the characteristic signal at 2θ = 20–25° for SiO₂ and the characteristic peaks of ZIF-8 confirm the successful formation of the ZIF-8 layer on the SiO₂@EuTTA microspheres. In the meantime, SiO₂@EuTTA@ZIF-8 blankets the peaks of SiO₂@EuTTA, which means that most of the SiO₂@EuTTA was dispersed into the inner of the layer.

Fig. 2 PXRD patterns of SiO₂, SiO₂@EuTTA, SiO₂@EuTTA@ZIF-8 and simulated ZIF-8.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to monitor the fabrication of the monodispersed SiO₂@EuTTA@ZIF-8 heterostructured composite microspheres. Fig. 3a and b show the SEM and TEM images of bare SiO₂ microspheres, and the diameter of the particles was in the range of 100 to 110 nanometer. Selected SEM images and their corresponding elemental mappings of SiO₂@ETTA and SiO₂@ETTA@ZIF-8 are shown in Fig. S1 and S2.† Comparison of the TEM images of SiO₂@EuTTA and SiO₂@EuTTA@ZIF-8 further demonstrates the ZIF-8 layer on the SiO₂@EuTTA core. ZIF-8 nanocrystals are observed on the surface of the SiO₂@ETTA@ZIF-8 microspheres by controlling the ZIF-8 growth (Fig. 3d). The diameter of the SiO₂@EuTTA@ZIF-8 heterostructured composite microspheres increases from 130 mm for the SiO₂@ETTA microspheres to 170 nm, thus indicating that a 20 nm thickness of ZIF-8 is grown on the SiO₂@ETTA microspheres.

Fig. 3 SEM images: (a) SiO₂; TEM images: (b) SiO₂, (c) SiO₂@EuTTA, and (d) SiO₂@EuTTA@ZIF-8.

The thermal stability of the compounds is examined using thermal gravimetric analysis (TGA) measurements which were performed in air with a heating rate of 5 K min⁻¹ (Fig. S3†). TGA data reveal that the lanthanide complex starts to release its coordinated organic molecules as the temperature rises from 25 °C. SiO₂@EuTTA@ZIF-8 is stable up to 380 °C, which is much higher than the synthesized SiO₂@EuTTA heterostructure microspheres (stable up to 200 °C). ZIF-8 also has the effect of protecting the core (SiO₂@EuTTA).

Luminescence properties

The photoluminescence of SiO₂@EuTTA@ZIF-8 is studied at room temperature. The fluorescence excitation and emission spectra of SiO₂@EuTTA@ZIF-8 are shown in Fig. 4. The excitation spectra is monitored using a sharp absorption band located in the near ultraviolet region at 396 nm. The excitation spectra can indicate that the organic ligand can absorb the energy of near ultraviolet light efficiently and then sensitize the emission of Eu³⁺ by the antenna effect. When the excited energy level of the organic ligand matches the resonant excited levels of Eu³⁺ with a subsequent energy transfer, then SiO₂@EuTTA@ZIF-8 can have a final strong luminescence emission. Moreover, the emission spectrum of SiO₂@EuTTA@ZIF-8 displays the characteristic emission peaks of the Eu³⁺ ion using an appropriate wavelength as the excitation source. The emission lines of SiO₂@EuTTA@ZIF-8 were assigned to ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions and the corresponding peaks are located at about 579, 591, 614, 651 and 701 nm, respectively. The emission peaks of SiO₂@EuTTA@ZIF-8 with different metal ions (Cd²⁺, Ca²⁺, Ni²⁺, Fe³⁺, Hg²⁺, Pb²⁺, Co²⁺, Fe²⁺ and Cu²⁺) are similar to those of the original which has a characteristic emission peak at about 614 nm under an excitation wavelength of 396 nm.

Fig. 4 Excitation and emission spectra of SiO₂@EuTTA@ZIF-8.

pH stability studies are used to test the fluorescence stability of the samples with changing pH value induced by various metal ions in solution. The XRD patterns of SiO₂@EuTTA@ZIF-8 are almost unaltered in these aqueous solutions, indicating that SiO₂@EuTTA@ZIF-8 can retain its crystallinity and is stable in a wide pH range (Fig. S4†).²⁶ The fluorescence intensity of SiO₂@EuTTA@ZIF-8 does not show obvious fluctuation when dispersed in different pH solutions (Fig. S5†), and this result indicates that SiO₂@EuTTA@ZIF-8 has excellent fluorescence stability and can be developed as a luminescent sensor for metal ions.

Sensing properties

The potential sensing of SiO₂@EuTTA@ZIF-8 for metal ions is examined as follows. A certain amount of SiO₂@EuTTA@ZIF-8 is dispersed into aqueous solutions containing different Cd²⁺, Ca²⁺, Ni²⁺, Fe³⁺, Hg²⁺, Pb²⁺, Co²⁺, Fe²⁺ and Cu²⁺ ions. The luminescence of the SiO₂@EuTTA@ZIF-8 solutions with the metal ions is investigated. The photoluminescence intensity of SiO₂@EuTTA@ZIF-8 was relevant as the metal ion was in aqueous solution (Fig. 5). When Cu²⁺ is incorporated, the luminescence intensity of SiO₂@EuTTA@ZIF-8 is decreased compared with the others; the other metal ions (Cd²⁺, Ca²⁺, Ni²⁺, Fe³⁺, Hg²⁺, Pb²⁺, Co²⁺ and Fe²⁺) have a negligible effect on the luminescence of Eu³⁺ in aqueous solution. Only Cu²⁺ can quench the red light emission and can be clearly seen under UV light excitation; the addition of other metal ions can not induce a visible change, as shown in Fig. 5b. The microspheres have a high selectivity for the specific recognition and sensing of Cu²⁺ in aqueous solutions. Table S1† shows the response of the luminescence lifetime of SiO₂@EuTTA@ZIF-8 to various metal ions in aqueous solutions. The results indicate that most metal ions have a negligible effect on the emission lifetime of SiO₂@EuTTA@ZIF-8. But the Cu²⁺-incorporated SiO₂@EuTTA@ZIF-8 has a shorter emission lifetime than the original, decreasing from 590 μs to 12 μs, which is in agreement with the decrease in the luminescence intensity.

Fig. 5 (a) Suspension-state photoluminescence spectra and (b) the relative intensities of ⁵D₀–⁷F₂ at 613 nm for SiO₂@EuTTA@ZIF-8 dispersed in various metal ion aqueous solutions upon excitation at 396 nm. Inset are the corresponding photographs under UV-light irradiation.

In Fig. 6, SiO₂@EuTTA@ZIF-8 is immersed in different concentrations of Cu²⁺ for further luminescence studies. The luminescence intensity of SiO₂@EuTTA@ZIF-8 incorporated with Cu²⁺ shows a strong dependence on the concentration. The luminescence intensity decreases proportionally with the concentration of Cu²⁺. The emission intensity of the SiO₂@EuTTA@ZIF-8 solution decreases sharply as the Cu²⁺ concentration increases from 0 to 500 μM. Quantitatively, this decrease can be rationalized using the Stern–Volmer equation:

I₀/I = 1 + K_sv[M]

Fig. 6 Emission spectra of SiO₂@EuTTA@ZIF-8 in aqueous solutions in the presence of various concentrations of Cu²⁺ under excitation at 396 nm. Inset is the K_sv curve (linear relationship of the emission intensity) of SiO₂@EuTTA@ZIF-8 decreased by Cu²⁺ ions.

The values of I₀ and I are the luminescence intensities of SiO₂@EuTTA@ZIF-8 and SiO₂@EuTTA@ZIF-8/Cu²⁺, respectively. K_sv is the decrease constant, and [M] is the concentration of Cu²⁺. The experimental data of the K_sv curve of SiO₂@EuTTA@ZIF-8 with Cu²⁺ is illustrated in the inset of Fig. 6, and the linear correlation coefficient (R) is 0.99218, which suggest that the decrease effect of Cu²⁺ on the luminescence of SiO₂@EuTTA@ZIF-8 agrees well with the Stern–Volmer mode. The K_sv value is calculated to be 3.144 × 10⁴ M⁻¹, which reveals a strong decrease effect on the SiO₂@EuTTA@ZIF-8 luminescence.

To examine the possible mechanism for the luminescence quenching effect by the metal ions, O 1s X-ray photoelectron spectroscopy studies are carried out on SiO₂@EuTTA@ZIF-8 and SiO₂@EuTTA@ZIF-8/Cd²⁺ (Fig. 7). The O 1s peak is shifted to 530.4 eV upon the addition of Cu²⁺ compared with SiO₂@EuTTA@ZIF-8 at 530.0 eV. There may be a weak coordination binding of uncoordinated carbonyl to Cu²⁺ in SiO₂@EuTTA@ZIF-8/Cd²⁺. Such coordination of Cu²⁺ ions can enhance various non-radiative activations and increase energy loss. This reduces the intraligand ligand-to-Eu energy transfer efficiency, and quenches Eu red light emission. Time-resolved fluorescence emission measurements are also used to further demonstrate the luminescence quenching effect of the Cu²⁺ion, and the response of the luminescence lifetime is shown in Table S1.† It is found that Cu²⁺-incorporated SiO₂@EuTTA@ZIF-8 has a shorter emission lifetime than SiO₂@EuTTA@ZIF-8, which is in agreement with the decrease in the photoluminescence intensity.

Fig. 7 O 1s XPS spectra of SiO₂@EuTTA@ZIF-8 (black) and Cu²⁺-incorporated SiO₂@EuTTA@ZIF-8/Cu²⁺ (red) dispersed in 10⁻² M H₂O solution of Cu(NO₃)₂.

Finally, we examine the recyclability of the SiO₂@EuTTA@ZIF-8 microspheres for sensing processes. SiO₂@EuTTA@ZIF-8 microspheres can be reused several times by simply washing the chemosensors with fresh MeCN after the sensing process (Fig. 8).

Fig. 8 (a) On–off cycles of SiO₂@EuTTA@ZIF-8 by alternating treatment of Cu²⁺ and MeCN at room temperature; (b) photograph of the SiO₂@EuTTA@ZIF-8 microspheres recycled 1 time for Cu²⁺ sensing.

Conclusions

In summary, heterostructured sensors are constructed using a luminescent lanthanide complex (Eu–TTA) and metal organic framework (ZIF-8) to cover the silica core. The thin heterostructured composite silica@EuTTA@ZIF-8 microsphere particles are used as a highly selective and sensitive sensor to detect Cu²⁺ in aqueous solution, showing a good fluorescence stability, low detection limit and broad linear range in an aqueous environment. This makes it a potential chemosensor for environmental sensing of Cu²⁺ due to its excellent chemical stability, low contamination and excellent recyclability ,which are regarded as necessary criteria for a new generation of sensors.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21571142) and Developing Science Funds of Tongji University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19973e

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