Highly selective luminescent sensing of Cu2+ in aqueous solution based on a Eu(III)-centered periodic mesoporous organosilicas hybrid

Materials and Design 172 (2019) 107712

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Highly selective luminescent sensing of Cu2+ in aqueous solution based on a Eu(III)-centered periodic mesoporous organosilicas hybrid Heng Li, Yajuan Li ⁎, Zheng Zhang, Xuelei Pang, Xudong Yu College of Science, Hebei University of Science and Technology, Yuhua Road 70, Shijiazhuang 050080, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• A terpyridine based europium centered periodic mesoporous hybrid has been synthesized. • The hybrid retained the well-ordered mesoporous structures and showed good thermal stability. • The hybrid possesses excellent luminescent property both in solid-state and aqueous solution. • The hybrid exhibited highly sensitive and selective sensing of Cu2+ ions in aqueous solution.

a r t i c l e

i n f o

Article history: Received 18 December 2018 Received in revised form 19 March 2019 Accepted 19 March 2019 Available online 21 March 2019 Keywords: Europium ion Fluorescent probe Luminescent Mesoporous materials Organic-inorganic hybrid material Terpyridine

a b s t r a c t Using Pluronic P123 surfactant as template, a periodic mesoporous organosilica (PMO) functionalized with 4′-(4carboxy-methyleneoxy phenyl)-2,2′:6′,2″-terpyridine (L-COOH) was successfully prepared via co-condensation using 1,2-bis(triethoxysilyl)ethane (BTESE) and the modified L-COOH (L-COOH-NH2). This terpyridine moiety forms interesting chelates to europium ions and enhances the luminescence of Eu3+ ions. Thus, a novel luminescent hybrid material was developed via linking Eu3+ complex Eu(NTA)3(H2O)2 to L-COOH-functionalized PMO material (L-COOH-PMO) using ligand exchange reactions. Investigation of photoluminescent properties shows that the prepared mesoporous hybrid Eu(NTA)3L-COOH-PMO exhibited red emission, long lifetime and high absolute quantum efficiency Φ in solid-state and aqueous solution. More interestingly, it demonstrates greatly sensitive and selective detection of Cu2+ ions in aqueous medium among test ions. The sensing mechanism was also discussed in this report. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Copper is an important and essential transition metal in the human body, and performs key roles in various physiological processes, such as ⁎ Corresponding author. E-mail address: [email protected] (Y. Li).

iron absorption, hemoglobin biosynthesis and nerve function regulation [1–3]. However, both excess and deficiency that deviate from the normal permissible limits can cause serious disorders. Excess amounts of Cu2+ ion in living systems can cause various diseases including Parkinson's disease and Alzheimer's disease [4–6]. Deficiency of Cu2+ ion can lead to brain diseases and human neurological disorders. In addition, Cu2+ ion is regarded as a significant ecological pollutant because

https://doi.org/10.1016/j.matdes.2019.107712 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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H. Li et al. / Materials and Design 172 (2019) 107712

of its widespread applications in industry [7,8]. Thus, it is of greatly importance to develop highly sensitive and selective sensors for the detection of Cu2+ in aqueous solutions. To date, many analytical methods have been applied for the detection of Cu2+, including colorimetry, inductively coupled plasma atomic emission spectroscopy, UV–vis spectrophotometry, and fluorescence spectrometry. Among these methods, fluorescent probe technology is an excellent tool because of its high sensitivity and selectivity, noninvasive detection, and facile operation. Recently, as a new type of fluorescent probes, rare earth complexes, especially Eu(III) complexes, have been extensively studied [9–11]. This is mainly because that they possess bright red emission, sharp emission profile, large Stokes shift and long luminescence lifetime, which enable them to be highly preferred fluorescent probes [12–14]. However, pure europium complexes are not suitable for applications in luminescent sensor devices mainly because of their drawbacks such as low thermal and photostability and poor mechanical properties. Therefore, several researches have been focusing on the design and preparation of organic-inorganic hybrid materials via covalent immoblization of lanthanide complexes onto host matrices, such as ionic liquid [15,16], polymers [17–19] and silicabased material [20–23]. Mesoporous silica nanoparticles (MSNs) have been widely applied as a host matrix for lanthanide complexes owing to their easily modified inner/outer surfaces, modifiable pore sizes and large surface areas [24–29]. Compared with conventional mesoporous silica nanoparticles, periodic mesoporous organosilica (PMO) display unique properties such as markedly higher mechanical and hydrothermal stabilities, as well as a homogeneous distribution of inorganic and organic components over the whole framework [30,31]. As a result, in recent years, increasing studies have begun to investigate the preparation and luminscent properties of lanthanide hybrid materials that use PMO as a solid support material [32–35]. However, such hybrid materials used as a florescent sensor for metal ion have rarely been explored to date. This is probably because most examples of lanthanide hybrid materials in aqueous solutions exhibited poor luminescent property and low solubility. Therefore, the water-soluble lanthanide PMO hybrid that display sufficient luminescence are greatly appealing for metal ion sensing field, the design and construction for this type of material still remains a huge challenge. Based on the above considerations, herein we synthesized a dualfunctional ligand 4′-(4-carboxy-methyleneoxyphenyl)-2,2′:6′,2″terpyridine (denoted as L-COOH). On one hand, the carboxyl group of L could be modified with silane coupling agent 1, 2-bis(triethoxysilyl)ethane (BTESE) and further grafted onto the frame of PMO host matrix to form the hybrid material L-COOH-PMO. On the other hand, its pyridine nitrogen atoms can chelate with Eu3+ ion and enhance the luminescence of Eu3+ via “antenna effect”. Then, a new organic-inorganic mesoporous hybrid Eu(NTA)3L-COOH-PMO was obtained via introduction of Eu (NTA)3(H2O)2 into the L-COOH-PMO material by ligand exchange reactions. The resulting hybrid material displays many advantages, as they integrate outstanding properties of the guest europium complexes with the porosity and high thermal stability of host matrices. The solution of Eu (NTA)3L-COOH-PMO also exhibits excellent luminescent property. In addition, the study of sensing properties illustrates that the material exhibits high-sensitivity sensing effect for Cu2+ in aqueous solution, which can be distinguished by visualization with the help of a small portable UV light.

were obtained from Sinopharm Chemical Reagent Corporation (China) and were used with no further purification. 2.2. Synthesis procedures 2.2.1. Synthesis of precursor L-COOH-NH2 L-COOH (1 mmol, 0.383 g) was transformed to acyl chloride with heating in excess SOCl2 under reflux and N2 atmosphere over approximately 5 h. The resulting acyl chloride was then isolated and directly subjected to reaction with 3-(methylpropyl)-triethoxysilane (APTS) (1 mmol, 0.221 g) in ethyl ether with the presence of triethylamine. Next, the mixture was subjected to heating in a covered flask under N2 atmosphere at 65 °C for around 12 h. Following isolation and purification, the L-COOH-NH2 sample was obtained as a yellow oil. 1H NMR (500 MHz, CDCl3, δ): 0.63–0.66 (t, 2H, J = 8 Hz), 1.21–1.23 (t, 11H, J = 7 Hz), 1.69–1.72 (m, 2H, J = 8 Hz), 3.37–3.40 (m, 2H), 3.70–3.74 (m, 2H), 3.49–3.84 (m, 4H), 4,57 (s, 2H), 7.05–7.06 (d, 2H, J = 8.5 Hz), 7.35–7.37 (m, 2H), 7.87–7.91(m, 4H), 8.67–8.73(m, 6H). 2.2.2. Synthesis of L-COOH functionalized PMO material (L-COOH-PMO) Using BTESE and L-COOH-NH2 as bridging organic groups for the silicon sources, L-COOH-PMO was prepared according to literature procedures [34]. First, 1.0 g P123 was dissolved with 30 g HCl solution (2 M) supplemented with 7.5 mL deionized water at 35 °C. To the above solution was then added a mixture of BTESE and L-COOH-NH2, resulting in a molar composition ratio of 0.0172 P123: (1-X) BTESE: X L-COOH-NH2: 6 HCl: 208.33 H2O, where, where X = 0, 0.02, 0.04, and 0.06. After stirring for 24 h, the resulting reaction material was transferred to an autoclave, and heated for 2 days at 100 °C. After cooling down and filtration, the solid material was rinsed repeatedly with deionized water, followed by drying at 65 °C. Copolymer surfactant P123 was removed by Soxhlet extraction over two days using ethanol under reflux. The resulting material L-COOH(X)-PMO (X) (X = 0, 0.02, 0.04, and 0.06) was dried overnight under vacuum at 65 °C, where X is the phen-Si: (TEOS + phen-Si) molar ratio. 2.2.3. Synthesis of L-COOH-PMO material grafted with Eu(NTA)3 complex (denoted as Eu(NTA)3L-COOH-PMO) For preparation of the Eu(NTA)3L-COOH-PMO material, the complex Eu(NTA)3(H2O)2 was first synthesized following a literature procedure [37]. Eu(NTA)3(H2O)2 was dissolved with ethanol followed by addition of L-COOH-PMO to the resulting solution. The mixture was refluxed and stirred for 12 h. The molar ratio of Eu(NTA)3(H2O)2: L-COOH-PMO was 1:1. The precipitate was collected by filtration and thoroughly rinsed using ethanol to ensure complete removal of physisorbed Eu(NTA)3 (H2O)2 complex. In the end, the resulting material was dried overnight under vacuum at 70 °C. The synthetic procedure and predicted structure of Eu(NTA)3L-COOH-PMO was obtained as outlined in Scheme 1. 2.3. Luminescence-sensing experiment 3 mg of mesoporous hybrid Eu(MSN-TTA)2L was dissolved in 3 mL aqueous solutions of M(NO3)z (2 × 10−4 mol·L−1) at room temperature (Mz+ = Zn2+, Cd2+, Cu2+, Fe3+, Mn2+, Mg2+, Ni2+, Ca2+, Na+, K+, Cr3+, Co3+, Ag+, Pb2+, Hg2+). Then, metal ion-incorporated suspensions for luminescent measurements were obtained by sonicating the mixtures for 10 min.

2. Experimental section 2.4. Characterizations 2.1. Materials Triblock copolymer EO 20 PO 70EO20 (Pluronic P123), 3aminopropyltriethoxy silane (APTS) and 1, 2-bis(triethoxysilyl) ethane (BTESE) were purchased from Sigma-Aldrich. 4′-(4-carboxymethyleneoxyphenyl)-2,2′:6′,2″-terpyridine (L-COOH) was synthesized according to the literature procedure [36]. The remaining reagents

For characterization purposes, 1H and 13C NMR spectra were collected at 298 K on a Bruker AVANCE-500 spectrometer using tetramethylsilane (TMS) as an internal reference. Using the KBr pellet technique, Fourier Transform Infrared (FTIR) spectra were collected with the Nicolet 6700 spectrophotometer in the wavenumber range of 4000–400 cm−1. Measurements of powder X-ray diffraction (PXRD) patterns were performed

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Scheme 1. Synthetic procedure and predicted structure of the resulting europium-centered luminescent mesoporous hybrid material Eu(NTA)3L-COOH-PMO.

using the Rigaku D/max2500VB+/PC diffractometer with copper Kα radiation (λ = 1.54 Å). Nitrogen adsorption-desorption isotherms were obtained with the Nova 2000 analyzer. Prior to measurements, each sample was degassed at 120 °C for 4 h under nitrogen flow. N2 adsorptiondesorption information were collected at liquid nitrogen temperature. The Brunauer-Emmett-Teller (BET) equation was used to calculate surface areas, and the distribution of pore sizes was determined based on the desorption branches of N2 isotherms with the Barrett-JoynerHalenda (BJH) method. High-resolution transmission electron microscopy (HRTEM) observation was performed using the JEOL JEM2011 electron microscope operating at 200 kV. High Resolution Scanning Electronic Microscopy (HR-SEM) micrographs were obtained using a ZEISS Merlin scanning electron microscope, operating under low vacuum at 10 kV. Elemental analysis was performed by Energy Dispersive SpectroscopyOxford Instruments. The X-ray photoelectron spectroscope (XPS) were carried out in a Thermo Fisher Scientific K-Alpha spectrometer, equipped with a mono-chromatic Al K α x-ray source (1486.6 eV). The samples are scraped in ultra-high vacuum (UHV: 2 × 10−9 mbar) at room temperature. The thermal stability was determined with the LABSYS Evo thermogravimetric analyzer (TGA-DTA). 3. Results and discussion

for the preparation of PMO hybrid material covalently bonded with the europium complex Eu(NTA)3L-COOH-PMO. FTIR spectra were used to verify the presence of precursors and the mesoporous hybrid material. FTIR spectra for organic ligand L-COOH (a), percursor L-COOH-NH2 (b) and mesoprous hybrid material LCOOH-PMO (c) are shown in Fig. 1. The sharp bands at 1593 and 1517 cm−1 are attributed to the stretching vibrations of C_C and C_N of the pyridine rings, respectively, and the band at 778 cm−1 is assigned as the C\\C bond between the pyridine rings [38]. In Fig. 1b, the sharp bands at 2923, 2878 and 2851 cm−1 are ascribed to stretching vibrations of \\CH2\\ groups in the coupling agent. Moreover, stretching vibration of Si\\C at 1180 cm−1 indicates the presence of siloxane bonds. Furthermore, disappearance of the band at 3426 cm−1 resulted from O\\H stretching of the carboxyl group, and appearance of the adsorption bands at 3436 and 1542 cm−1 corresponding to the , stretching and bending vibrations for NH group, respectively, further prove that 3-aminopropyltriethoxy silane (APTS) was covalently grafted to the organic ligand L-COOH. Construction of the Si\\O\\Si framework was confirmed based on the spectra of mesoporous hybrid material (Fig. 1c). The strongest absorption band associated with the silica host structure appears at 1080 cm−1, and corresponds to the Si\\O asymmetric stretching vibrations, while the peak of 793 cm−1 is ascribed to the Si\\O symmetric stretching vibration [39]. While the

3.1. Characterization of mesoporous hybrid material Eu(NTA)3L-COOHPMO Mesoporous silica containing L-COOH-NH 2 organic ligand was synthesized as described in the Experimental Section. During the synthesis, a series of L-COOH-NH2 -functionalized periodic mesoporous organosilica (PMO) samples with different concentrations of LCOOH-NH2 in the initial mixture were obtained. Fig. S1 exhibits the HRTEM images for the resulting L-COOH(X)-PMO (X = 0, 0.02, 0.04, 0.06). From the figure, it is can be clearly seen the samples with L-COOH-NH2 : (L-COOH-NH2 + BTESE) molar ratios below 0.04 exhibit highly ordered two-dimensional hexagonal symmetry. However, with the concentration of L-COOH-NH2 further increasing, the material shows worm-like and mesoscopic order of the materials decrease. These are in agree with the XRD results (Fig. S2). The reason for the decrease of mesoscopic order is possibly that the molecular size and the increasing of loadings of L-COOH-NH2 disrupt the ability for P123 to form micelles and corresponding mesophases. Therefore, the optimum molar ratio X L-COOH-NH2: (L-COOH-NH2 + BTESE) = 0.04 is employed to synthesize L-COOH-NH2 functionalized PMO material since it possesses both a high loading of chelated organic content and a good mesostructure, which was further served as the parent material

Fig. 1. FTIR spectra of the organic ligand L-COOH (a), precursor L-COOH-NH2 (b), and mesoporous hybrid material L-COOH-PMO (c).

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H. Li et al. / Materials and Design 172 (2019) 107712 Table 1 Structural parametersa. Sample

d100 (nm)

a0 (nm)

SBET (m2/g)

V (cm3/g)

D (nm)

t (nm)

PMO L-COOH-PMO Eu(NTA)3L-COOH-PMO

10.47 10.21 10.08

12.09 11.79 11.64

924 678 503

1.12 0.96 0.85

9.65 8.34 6.97

2.44 3.45 4.85

a d100 is the d(100) spacing, a0 the cell parameter (a0 = 2 d100/√3), SBET the BET surface area, V the total pore volume, D the average pore diameter, and t the wall thickness, calculated by a0-D.

Fig. 2. Powder X-ray diffraction patterns of PMO (a), L-COOH-PMO (b) and Eu(NTA)3LCOOH-PMO (c).

bands at 963 cm−1 are attributed to stretching vibrations of the Si-OH surface groups, the signal for Si\\O\\Si bending vibration appears at 464 cm −1 . Moreover, absorption bands in the range of 2983–2870 cm −1 , which are merely the absorption bands for \\CH2\\, confirm that L-COOH-NH2 is successfully grafted on to the wall of PMO. The powder X-ray diffraction analysis results for PMO, L-COOH-PMO and Eu(NTA)3L-COOH-PMO are presented in Fig. 2. All the materials show a strong reflection peak (100) at low angle 2θ and two short second-order reflection peaks (110) and (200), characteristic of

hexagonal arrangement of uniform pores in materials. In contrast to X-ray diffraction patterns for the parent material (L-COOH-PMO), the d100-spacing value of Eu(NTA)3L-COOH-PMO was almost unchanged, suggesting the ordered hexagonal mesostructure was conserved following introduction of the Eu3+ compound. Nevertheless, it should be noted that the Bragg peak intensities of Eu(NTA)3L-COOH-PMO are reduced to the extent in comparison with L-COOH-PMO, which can be assigned to the decline of local order likely resulting from the decrease of scattering contrast between the covalently bonded europium complexes and channel walls of the matrices in the mesoporous materials [40]. Moreover, The ordered mesostructures of the pure periodic mesoporous silica PMO, L-COOH-PMO and Eu(NTA)3L-COOH-PMO were confirmed by HRTEM. As shown in Fig. 3, all the materials exhibited an regular hexagonal array of uniform channels, demonstrating that Eu (NTA)3L-COOH-PMO maintains its mesoporous structure well after the organic modification and complexation. It is worth noting that the Eu-containing periodic mesoporous hybrid material Eu(NTA)3LCOOH-PMO exhibits smaller d-spacing compared with pure periodic mesoporous silica, which also further confirms the presence of anchored Eu(NTA)3L-COOH moieties in the PMO channels. In addition,

Fig. 3. HRTEM images of pure periodic mesoporous silica PMO (a, b), L-COOH-PMO (c, d) and Eu(NTA)3L-COOH-PMO (e, f). (a, c and e) imaged along the [110] zone axis and (b, d and f) imaged along the [100] zone axis, respectively.

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Fig. 4. N2 adsorption-desorption isotherms of PMO (a), L-COOH-PMO (b) and Eu(NTA)3LCOOH-PMO (c).

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the d-spacing values measured from HRTEM images of the three materials are in good agreement with the values calculated based on the relevant XRD data (Table 1). The N2 adsorption measurement, an efficient and widely applied method to analyze highly ordered host-guest nanostructured hybrid materials and mesoporous materials, was used to confirm the pore structures of mesoporous materials. The nitrogen adsorption-desorption isotherms of PMO, L-COOH-PMO and Eu(NTA)3L-COOH-PMO are shown in Fig. 4. Similar to other mesoporous materials containing uniform mesopores, all samples display type IV isotherms with H1-type hysteresis loops at relative pressures within the range of 0.5–0.9. The structural data of the materials (total pore volume, pore size and BET surface area, etc.) are tabulated in Table 1. Following functionalization with L-COOH, L-COOH-PMO displays smaller surface area, as well as slightly smaller pore volume and pore size. The SBET, pore size and pore volume are reduced from 924 m2/g, 1.12 cm3/g and 9.65 nm for the pure PMO to 678 m2/g, 0.96 cm3/g and 8.34 nm, respectively. However, pore wall thickness (t) shows a slight increase, possibly because of the volume occupied by the surface organic groups. Furthermore, it can be observed

Fig. 5. SEM images of L-COOH-PMO (a, b), Eu(NTA)3L-COOH-PMO (c, d), and Eu(NTA)3L-COOH-PMO + Cu2+ (e, f), inset: the corresponding EDS curves.

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that the surface area, pore diameter and pore volume decrease with linkage of Eu complexes to the parent material L-COOH-PMO, further confirming that the Eu complexes are grafted within the mesopores. Fig. 5 shows SEM images of L-COOH-PMO (a), Eu(NTA)3L-COOHPMO (b), and Eu(NTA)3L-COOH-PMO + Cu2+ (c). It can be seen that the sample L-COOH-PMO showed rod-like structures or larger clusters of aggregated rods, and the rod is consist of many small particles. Eu (NTA)3L-COOH-PMO and Eu(NTA)3L-COOH-PMO + Cu2+ exhibited similar surface morphology, although the length and width of rods were somewhat different. This suggests that the load of europium complexes and subsequent addition of Cu2+ do not nearly change the morphology of mesoporous hybrid material. The TEM images of L-COOH-PMO (a), Eu(NTA)3L-COOH-PMO (b), and Eu(NTA)3L-COOH-PMO + Cu2+ (c) were also given in Fig. S3. The rod-like structures of the three materials were further confirmed by TEM images. For further confirmation, the elemental composition and distribution of L-COOH-PMO (a), Eu(NTA)3L-COOH-PMO (b), and Eu(NTA)3L-COOHPMO + Cu2+ (c) were investigated by EDS and mapping, respectively. As shown in Fig. 5 (d) inset, the peaks of Eu could be found besides the peaks of C, N, O, Si, which confirmed that Eu complexes were successfully covalently bonded to the skeleton of the periodic mesoporous hybrid. In addition, after addition of Cu2+, the peaks of Cu could be observed, and mass ratio of Eu element decrease (Fig. 5(f) inset), which suggested that there existed interaction between the ligands of Eu(NTA)3L-COOHPMO and Cu2+, and Eu3+ in Eu(NTA)3L-COOH-PMO wasn't replaced by Cu2+. Table S1 listed he mass ratio of each element in L-COOH-PMO, Eu (NTA)3L-COOH-PMO, and Eu(NTA)3L-COOH-PMO + Cu2+. Fig. S4 demonstrates that all the elements are homogeneously distributed in the hybrid material Eu(NTA)3L-COOH-PMO. 3.2. Luminescence properties of Eu(NTA)3L-COOH-PMO The solid-state luminescent behaviors of Eu(NTA)3L-COOH-PMO are investigated in detail at room temperature (RT), and the emission and excitation spectra of the title hybrid material are shown in Fig. 6. The excitation spectrum was recorded by monitoring the strongest emission at 618 nm, which exhibited a weak band at 294 nm and a strong broad band at 360 nm, corresponding to the absorption of the organic ligand L and NTA, respectively. The emission spectrum obtained at excitation wavelength of 360 nm exhibited five distinct characteristic peaks at 581, 594, 618, 656 and 704 nm within the range of 550–750 nm. These signals can be attributed to the 4f-4f transitions to the low-lying [7]FJ (J = 1, 2, 3, 4 and 5) levels from the [5]D0 excited state of the Eu3+ ions. Under this excitation, emission peaks of ligands were not observed, indicating highly efficient energy transfer to the Eu3+ ion center from the ligands in the Euhybrid. In addition, an outstanding feature of these spectra is the great intensity of the [5]D0 → 7F2 transition at 618 nm. It has been established that the [5]D0 → 7F1 transition is a parity-allowed magnetic dipole transition that is not sensitive to local structural environments, while the [5] D0 → 7F2 transition is a typical electric dipole transition that is sensitive to the coordination environment of the europium ion. As interactions between rare earth materials and their local environment strengthen, the material becomes increasingly nonsymmetrical, accompanied by enhancement of electric-dipolar transitions. In consequence, the integration intensity ratio (I2/I1) of the [5]D0 → 7F2 to the [5]D0 → 7F1 transition is widely applied as an indicator of site symmetry for europium ions. The value of the intensity ratio (I2/I1) for the Eu-hybrid is 14.17. Such high value can only be reasonably achieved if the Eu3+ ions do not occupy sites with inversion symmetry. Under UV irradiation, Eu(NTA)3LCOOH-PMO exhibited bright red emissions (inset of Fig. 6), which were readily observed with the naked eye, further confirming the presence of antenna effect. To further examine the solid-state luminescence properties of the Eu-centered hybrid material, the [5]D0 excited state luminescence decay curves were measured at RT by observing the most intense emission line ([5]D0 → 7F2) of the Eu3+ center at 618 nm with 360 nm

Fig. 6. Excitation (EX) and emission (EM) spectra of the Eu-centered mesoporous hybrid Eu(NTA)3L-COOH-PMO. The inset is the corresponding luminescence picture under UVlight irradiation of 365 nm.

excitation from a xenon lamp. The profile for luminescence decays was fitted with single exponential functions to determine the RT lifetime (τ) of the [5]D0 level. The Eu3+ hybrid exhibited very long lifetimes (0.488 ms), which was ascribed to the effective energy transfer to Eu3+ from the ligands. Fig. S5 shows the luminescence time decay curves for Eu(NTA) 3 L-COOH-PMO. The absolute quantum efficiency Φ of Eu(NTA)3L-COOH-PMO was 23.4%, which is determined using the integrated sphere coated with BaSO4. The luminescence properties of Eu(NTA)3L-COOH-PMO in aqueous solution have been studied as well. As shown in Fig. S6, the mesoporous hybrid material Eu(NTA)3L-COOH-PMO in aqueous solution also showed five characteristic emission peaks of europium ions. It is worthy noting that in comparison with the solid-state luminescent spectra of Eu (NTA)3L-COOH-PMO, the ligand-centered emission of Eu(NTA)3LCOOH-PMO in aqueous solution (concentration = 1 mg/mL) cannot be completely suppressed by the luminescence of Eu3+. This is mainly caused by the quenching effect of water molecules. In addition, Eu (NTA)3L-COOH-PMO exhibits high quantum yields (19.2%) and reasonably long lifetimes (0.433 ms). Furthermore, the fluorescence stability of Eu(NTA)3L-COOH-PMO in aqueous solution at room temperature were investigated. As shown in Fig. S7, as time progressed, the luminescence intensity of Eu(TTA-MSN)2L in aqueous solution showed a limited reduction, which indicates that Eu(NTA)3L-COOH-PMO in aqueous

Fig. 7. Responses of the fluorescence of Eu(NTA)3L-COOH-PMO (1 mg/mL) towards various metal cations in aqueous solution (2 × 10−4 M). The emission spectra were recorded at an excitation wavelength of 333 nm.

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Fig. 8. Normalized [5]D0 → 7F2 transition intensities of Eu(NTA)3L-COOH-PMO (1 mg/mL) upon the addition of Cu2+ (2 × 10−4 M) with a background of metal cations (2 × 10−4) in aqueous solution. λex = 333 nm; λem = 616 nm.

solution has excellent fluorescence stability. The excellent luminescence properties in aqueous solution, high stability, and the nanoscale nature of Eu(NTA)3L-COOH-PMO suggest that it possess great potential as a fluorescence material for sensing purposes. 3.3. Sensing of Cu2+ cations The fascinating luminescent properties of Eu(NTA)3L-COOH-PMO made it an excellent candidate for potential applications such as luminescent sensors. Encouraged by the high water-stability and bright luminescence of the Eu(NTA)3L-COOH-PMO material, its luminescent properties for sensing metal ions were explored. The as-synthesized materials were immersed in aqueous solutions of 1 mM M(NO3)x (Mz+ = Zn2+, Cd2+, Cu2+, Fe3+, Mn2+, Mg2+, Ni2+, Ca2+, Na+, K+, Cr3+, Co3+, Ag+, Pb2+, Hg2+) for the luminescent studies. The corresponding photoluminescence properties are collected and compared in Fig. 7. The data showed that most suspensions containing a variety of metal ions display the characteristic red light emission according to the Eu3+ transitions, while only Cu2+ shows a significant luminescence quenching effect, reflecting the high selectivity of Eu(NTA)3L-COOH-PMO for the detection and specific recognition of Cu2+ in aqueous solution. For practical applications, selectivity is a crucial indicator for evaluating the performance of sensing material to detect specific analytes without being affected by other components of the environment. In order to probe the selectivity of Eu(NTA)3L-COOH-PMO for metal sensing, Cu2+

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was subsequently added to each of the above suspensions, and the concentration of Cu2+ in suspension is 2 × 10−4 M. As can be observed from Fig. 8, after the addition of Cu2+ to the mixture of Eu(NTA)3L-COOHPMO and other metal cations, significant fluorescence quenching was observed. The luminescent quenching effects were comparable to that of the materials immersed in 1 mM solution of Cu2+. This indicates that these metal cations had negligible interfering effect on Eu (NTA)3L-COOH-PMO for Cu2+ ion, further proving the high selective of Eu(NTA)3L-COOH-PMO for Cu2+ detection and potential for practical use. To quantitatively investigate the luminescence response behaviors of Eu(NTA)3L-COOH-PMO towards Cu2+ ions, luminescence titration experiments was further performed. As presented in Fig. 9, the emission intensity of Eu(NTA)3L-COOH-PMO decreased sharply as the Cu2+ concentration increased; when Cu2+ concentration reached 2 × 10−4 M, fluorescence emission of the Eu hybrid solution was nearly completely quenched. Thus, sharp and distinguishable changes could be identified with UV light excitation by the naked eye (inset of Fig. 9a). The quenching effect of Eu(NTA)3L-COOH-PMO was examined as a function of Cu(NO3)2 concentration within 0–2 × 10−5 M. Quantitatively, the above changes can be rationalized using the Stern-Volmer equation: I0 =I ¼ 1 þ K sv ½M where I0 and I represent luminescence intensities of the Eu(NTA)3LCOOH-PMO suspension in the absence and presence of added Cu2+, respectively; Ksv is the Stern-Volmer constant and [M] is the molar concentration of Cu2+. The experimental data of the Ksv curve for Eu (NTA)3L-COOH-PMO with Cu2+ is illustrated in Fig. 9b. The linear correlation coefficient (R) is 0.99877, indicating that the quenching effect of Cu 2+ for luminescence of Eu(NTA) 3L-COOH-PMO is highly consistent with the Stern-Volmer equation. The Ksv value is calculated to be 2.5 × 106 M, suggesting a strong inhibitory effect on the luminescence of Eu(NTA)3L-COOH-PMO. The detection limit, calculated according to the reported method [41], is 4.6 × 10−8 M, and is better than or comparable to several previously reported fluorescence sensors of Cu2+ ions [42], suggesting that Eu(NTA)3L-COOHPMO can be used as an exceedingly sensitive probe for quantitative luminescence detection of Cu2+ ions in aqueous media. Possible sensing mechanisms for the luminescence response to Cu2+ ions was further discussed. According to the available literature, the luminescence quenching effect of lanthanide complexes exerted by metal cations follows two mechanisms: (1) cation exchange of central lanthanide ions of the lanthanide complexes with the targeted cations [43,44]; (2) interaction between organic ligands and the metal cations [45–47].

Fig. 9. (a) The PL spectra of Eu(NTA)3L-COOH-PMO in Cu(NO3)2 aqueous solution at different concentrations (excited at 333 nm). The inset shows changes in luminescence following the addition of Cu2+ ions (2 × 10−4 M) to the Eu(NTA)3L-COOH-PMO suspension under UV light (365 nm); (b) Stern-Volmer plot for Eu(NTA)3L-COOH-PMO sensing of Cu2+ at the range of 0–2 × 10−5 M in aqueous solution.

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H. Li et al. / Materials and Design 172 (2019) 107712

Fig. 10. (a) XPS for Eu(NTA)3L-COOH-PMO and Eu(NTA)3L-COOH-PMO + Cu2+. (b) O 1 s XPS for Eu(NTA)3L-COOH-PMO and Eu(NTA)3L-COOH-PMO + Cu2+. (c) N 1 s XPS for Eu(NTA)3LCOOH-PMO and Eu(NTA)3L-COOH-PMO + Cu2+.

We collected the filtrates originating from Eu(NTA)3L-COOH-PMO aqueous solution with addition of various concentration of Cu2+, and performed ICP-MS measurements. The results show that Eu3+ concentration of the filtrate with different Cu2+ concentrations has little variation, thus excluding the possibility of cation exchange. Therefore, it is probably possible that the mechanism of quenching by Cu2+ ion to be related to the interactions between the Cu2+ ion and the phenyl oxygen atom and amide nitrogen atom, consistent with the observations in relevant literature [48]. To confirm this hypothesis, X-ray photoelectron spectra (XPS) was formed. As depicted in Fig. 10, the peak of 2p 1 for Cu 2+ appeared, and the Eu 3d 5 peak can be also observed in sample Eu (NTA)3L-COOH-PMO + Cu 2+. This confirmed the simultaneous exist of Eu3+ and Cu2+ in hybrid Eu(NTA)3L-COOH-PMO + Cu2+, which excluded the possibility of cation exchange. Furthermore, the O 1 s peak from phenyl oxygen atom at 532.2 eV is shifted to 532.6 eV, and N 1 s peak from amide nitrogen atom at 401.3 eV is shifted to 401.6 eV on the addition of Cu2+, which indicates that there may be a weak coordination binding between N, O atoms and Cu2+. Such coordination of Cu2+ can enance various non-radiative activations and increase energy transfer efficiency, and quenches Eu red light emission. 3.4. Thermal stability of Eu(NTA)3L-COOH-PMO The thermal stability of Eu(NTA)3L-COOH-PMO was investigated by thermogravimetric analyses (TGA) measurements. Shown in Fig. 11 are thermogravimetric weight loss curve (TG) and derivative weight loss curve (DTG) for Eu(NTA)3L-COOH-PMO. The DTG curve shows three

major weight loss peaks. The first weight loss (about 6%) peak at 52 °C is attributed to physically adsorbed water, with no decomposition of the chemical bonds. The second weight loss (approximately 13%) peak appears at 370 °C and corresponds to decomposition of the residual surfactants [49]. The third weight loss (approximately 11%) peak at 535 °C is ascribed to decomposition of the organic components and collapse of the coordination structures. It can be concluded based on the TGA and DTG curves of this mesoporous hybrid system that introduction of mesoporous matrix enhances thermal stability in comparison with the general europium complexes, the thermal decomposition of which frequently occurs at around 200–300 °C [50]. 4. Conclusion Taken together, A novel Eu3+ complexex-functionalized periodic mesoporous organosilicas hybrid material Eu(NTA)3L-COOH-PMO was designed and synthesized as a highly sensitive and selective fluorescence sensor for detecting Cu2+ ions in aqueous solution. The hybrid Eu(NTA) 3L-COOH-PMO retains the ordered mesoporous structures. Both the precipitate and its corresponding aqueous solutions show bright emission when illuminated with a UV lamp. The novel hybrid luminescent composite Eu(NTA)3L-COOH-PMO was targeted for the highly sensitive and selective sensing of Cu2+ ions in aqueous solution. These results indicate that such highly sensitive and selective materials for sensing ions in aqueous solution will have potential as luminescent sensors in treatment of waste water and ecological pollutants at various locations in the future. CRediT authorship contribution statement Heng Li: Conceptualization, Data curation, Supervision, Resources. Yajuan Li: Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization, Funding acquisition. Zheng Zhang: Conceptualization, Validation, Resources. Xuelei Pang: Resources, Writing review & editing, Funding acquisition. Xudong Yu: Writing - review & editing, Funding acquisition. Acknowledgement This work was funded by the National Natural Science Foundation of China (21771051 and 21401040), the Natural Science Foundation of Hebei Province (B2018208186 and B2018208112), the Hebei Province Department of Education Fund (QN2017072), the High Level Talent Funding Project in Hebei (2016002014), and the Young Talent Support Plan of Hebei province. Appendix A. Supplementary data

Fig. 11. Thermogravimetry analysis traces (TG) and derivative weight loss curve (DTG) of Eu(NTA)3L-COOH-PMO.

Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2019.107712.

H. Li et al. / Materials and Design 172 (2019) 107712

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