Europium(III) Complex-Functionalized SiO₂@mTiO₂ Nanospheres for Al³⁺-Modulated Multicolor Emission

Abstract

A europium(III) hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ (Htta = 2-thenoyltrifluoroacetone, H₂bpdc = 2,2′-bipyridine-3,3′-dicarboxylic acid) was successfully designed and synthesized by the covalent grafting complex Eu(tta)₃bpdc to SiO₂@mTiO₂ core–shell nanosphere. The FT-IR, PXRD, XPS, TEM, HRTEM, SAED, TGA and PL were performed to characterize these materials. The results indicate that core–shell nanosphere structure and anatase crystallites of SiO₂@mTiO₂ are retained well after grafting the europium complex. Hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ displays uniform nanosphere structure, bright red color and long lifetime, which can serve as a multicolor emission material modulated by using Al³⁺ ions via the cation exchange approach under a single-wavelength excitation. To the best of our knowledge, this work is the first multicolor emissive sensor for Al³⁺ ions based on the lanthanide hybrid material.

1. Introduction

Lanthanide-based luminescent hybrid materials have sharp emission bands, high color purity, long luminescent lifetime and high quantum efficiency. They can also act as excellent candidates for multicolor luminescent materials, and further are applied in many fields, such as bio-imaging and bioanalytical detection [1,2], luminescent probes and sensors [3,4,5], tunable luminescence [6,7,8], and optoelectronic devices and displays [9,10,11]. In the past years, color-tunable lanthanide-based materials usually control the stoichiometric ratios of doped ions in the same material to realize multicolor emission (containing white-light emission) [12,13,14,15]. Recently, Li et al. have achieved some progress in the field of single-component lanthanide-based hybrid materials with multicolor emission [16,17]. They declared that full-color emission, particularly white emission colors of europium-based hybrid materials, can be facilely tuned by using only a kind of cation or anion due to the cation exchange process or interaction between ligands and anions. However, to date, single-component lanthanide hybrid materials with multicolor emission by using the cation exchange approach remain a challenge.

Aluminum is the third most abundant metal in the Earth’s crust [18], it has been widely used in the fields of aluminum containers, cooking tools, food additives, water treatment, and aluminum-based pharmaceuticals. According to the standard of WHO, the average human intake of aluminum is about 3.0~10.0 mg/day and weekly dietary intake is 7.0 mg/kg body weight [19,20,21]. The excessive intake of aluminum can lead to the production of neurotoxic issues in humans such as Parkinson’s or Alzheimer’s disease [18,22]. Therefore, the selective detection and control of Al³⁺ ions in biological systems are essential. Fluorescence-based probes as a kind of simple and rapid responsive method have drawn a lot of interest for sensing applications due to these methods being low-cost, very sensitive, selective and visualizable while offering rapid response times and real-time monitoring. To date, several fluorescent Al³⁺ sensors such as organic compounds (Schiff base [23], phenylhydrazine carboxamide [24], coumarin-rhodamine derivative [25], amino derivative [26]), Cd-MOF [27] and europium complexes [28,29] have been reported. Noticeably, lanthanide-based hybrid materials have received considerable attention as a sensor for detecting Zn²⁺ [16] and Cu²⁺ [30], due to their novel and unique properties, such as photostability, adjustable pore size, large surface area, as well as easy recyclability.

On the basis of above consideration, we designed and fabricated a europium(III) hybrid material based on SiO₂@mTiO₂ core–shell nanosphere (namely, Eu(tta)₃bpdc-SiO₂@mTiO₂) with covalent grafting complex Eu(tta)₃bpdc (Htta = 2-thenoyltrifluoroacetone, H₂bpdc = 2,2′-bipyridine-3,3′-dicarboxylic acid). The resulting core–shell hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ shows multicolor emission based on sensing of Al³⁺ ions by the cation exchange process. The emission color changed from red to pink and blue based on luminescence intensity changes by modulating only the concentration of Al³⁺ ions in an ethanol solution of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂. To the best of our knowledge, this work is the first multicolor emissive sensor for Al³⁺ ions based on the lanthanide hybrid material.

2. Experiment Section

2.1. Materials

1,10-Phenanthroline monohydrate (phen, AR, Aladdin, Shanghai, China), 2-thenoyltrifluoroacetone (Htta, AR, J&K, Beijing, China), tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄, AR, J&K, Beijing, China), hexadecylamine (HDA, AR, J&K, Beijing, China), titanium(IV) isopropoxide (Tip, AR, J&K, Beijing, China) were purchased from a commercial source without any further purification. Hydrated EuCl₃ was prepared by dissolving europium(III) oxide (99.99%) in concentrated hydrochloric acid.

2.2. Preparation of H₂bpdc Ligand

To deionized water (350 mL), phen (0.04 mol, 8.00 g), NaOH (0.08 mol, 3.20 g) and KMnO₄ (0.12 mol, 19.09 g) were added. The mixture was heated to 100~105 °C and stirred for 5 h, and then cooled to room temperature. The filtrate was obtained by filtration and then concentrated to 100 mL and adjusted the pH of the filtrate to 2.0 by HCl (aq, conc.). The resulting white precipitate was filtered off, recrystallized with methanol and dried in vacuo to give colorless needle-like crystals, yield: 8.2 g, 85%. Elemental Anal. Calc. for C₁₂H₈N₂O₄ (244.05): C, 59.02; H, 3.30; N, 11.47%. Found: C, 59.12; H, 3.15; N, 11.54%.

2.3. Preparation of Eu(tta)₃(H₂O)₂

The complex Eu(tta)₃(H₂O)₂ was synthesized according to the reported literature [31]. An appropriate amount of 1.0 M sodium hydroxide solution was added to an Htta ethanol solution to adjust pH = 7.0 under stirring at room temperature. Subsequently, a hydrated EuCl₃ ethanol solution was dropwise added into the above mixture and kept the molar ratio of Eu^III/Htta equal to 1/3. An appropriate amount of deionized water was added to the mixture and heated at 65 °C for 5 h, then cooled to room temperature. The yellow precipitates were obtained by filtration, then washed with cold ethanol and deionized water three times.

2.4. Preparation of SiO₂@mTiO₂ Core–Shell Nanospheres

The SiO₂@mTiO₂ core–shell nanospheres were synthesized according to the reported literature [32,33]. (i) SiO₂ nanospheres were prepared by a modified Stöber method. Deionized water (5 mL) and 25% NH₃·H₂O (3 mL) were added to an ethanol solution (35 mL), and stirred continuously for 0.5 h at 25 °C. Then, TEOS (5 mL) was added dropwise to the above mixture and stirring continued for 24 h. The resultant SiO₂ nanospheres were collected by centrifugation, then washed with deionized water and ethanol and dried at 60 °C for 24 h under vacuum. (ii) The SiO₂@mTiO₂ core–shell nanospheres were prepared by a modified cooperative assembly-directed method. Firstly, SiO₂ nanospheres (150 mg) were dispersed in ethanol (15 mL) by ultrasonication, then 25% NH₃·H₂O (0.3 mL) and HDA (150 mg) were added under stirring at room temperature. Secondly, Tip (0.3 mL) was quickly added to the above mixture and continued to stir for 1 h. The resultant core–shell SiO₂@TiO₂/HDA nanospheres were collected by centrifugation, then washed with deionized water and ethanol. Thirdly, SiO₂@TiO₂/HDA nanospheres were dispersed in a mixture of deionized water (10 mL) and ethanol (20 mL) by ultrasonication, then transferred to a 100 mL Teflon-lined autoclave and heated to 160 °C for 36 h to completely remove the HDA. Finally, the resultant SiO₂@mTiO₂ core–shell nanospheres were collected by centrifugation, then washed with deionized water and ethanol, dried at 60 °C for 24 h under vacuum.

2.5. Preparation of SiO₂@mTiO₂-bpdc

To DMF (20 mL), SiO₂@mTiO₂ (0.10 g) and H₂bpdc (0.30 g) were dispersed by ultrasonication, followed by refluxed at 160 °C for 6 h under stirring. The resultant SiO₂@mTiO₂-bpdc was collected by centrifugation, then washed with DMF and ethanol, dried at 60 °C for 24 h under vacuum.

2.6. Preparation of Hybrid Material Eu(tta)₃bpdc-SiO₂@mTiO₂

To ethanol (20 mL), SiO₂@mTiO₂-bpdc (0.10 g) and Eu(tta)₃(H₂O)₂ (0.5 mmol) were dispersed by ultrasonication, followed by reflux at 80 °C for 5 h under stirring. The resultant Eu(tta)₃bpdc-SiO₂@mTiO₂ was collected by centrifugation and excess unbound Eu(tta)₃(H₂O)₂ was washed with ethanol until no red emission of supernatant was observed under a 254 nm UV lamp, dried at 60 °C for 24 h under vacuum.

2.7. Luminescent Sensing Experiment

Hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ (3 mg) was dispersed in 3 mL 0.01 M different cations ethanol solutions (K⁺, Na⁺, Li⁺, Zn²⁺, Ca²⁺, Cd²⁺, Pb²⁺, Mg²⁺, Mn²⁺, Cr³⁺ and Al³⁺) at room temperature. Then, hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ suspensions were obtained by sonicating for 30 min for luminescent measurements.

2.8. Characterization

Fourier transform infrared (FT-IR) spectra were measured with Bruker Tensor 27 FT-IR spectrometer (Bruker physik-AG, Karlsruhe, Germany) by using KBr pellets in the range of 400–4000 cm⁻¹. Powder X-ray diffraction (PXRD) patterns were performed with Bruker D8 ADVANCE diffractometer (Bruker physik-AG) (Cu-K_α, 1.5418 Å) at room temperature. Scanning electron microscopy (SEM) was recorded on SU8010 (Hitachi Limited, Tokyo, Japan). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and the energy-dispersive X-ray spectroscopy (EDS) were recorded on Talos F200X (Thermo Fisher Scientific, Waltham, MA, USA). Thermogravimetric analyses (TGA) were carried out Universal V4.5A instrument (Universal Instrument, Conklin, NY, USA). Photoluminescence spectra (PL) were measured on the Hitachi F-2700 spectrophotometer (Hitachi Limited) at room temperature. The lifetimes of the samples were recorded on an Edinburgh FLSp920 fluorescence spectrometer (Edinburgh Instruments, Livingston, UK). X-ray photoelectron spectra (XPS) were measured on PHI5000 VersaProbeIII XPS (ULVAC Co. Ltd, Kanagawa, Japan) with an Al-K_α achromatic X-ray source. Inductively coupled plasma mass spectroscopy (ICP-MS) was performed on an Agilent 7900 ICP-MS (Agilent Technologies, Santa Clara, CA, USA).

3. Results and Discussion

The preparation procedure and predicted structure of core–shell hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ are exhibited in Scheme 1. The choice of H₂bpdc ligand is very important during the reaction process. The –COOH groups of H₂bpdc ligand can covalently anchor onto the surface of SiO₂@mTiO₂ core–shell nanosphere through condensation reaction between hydroxyl groups on the surface of SiO₂@mTiO₂ and carboxylic groups. Meanwhile, two pyridyl N atoms of H₂bpdc can coordinate to Eu^III ions in the first coordination sphere of Eu(tta)₃(H₂O)₂ complex by ligand-exchange reaction and then sensitize luminescence of Eu^III ions with the primary ligand Htta via the “antenna effect”. Finally, complex Eu(tta)₃bpdc can be successfully formed and covalently bonded to SiO₂@mTiO₂ core–shell nanospheres, then the hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ was successfully obtained.

3.1. FT-IR Spectra

In order to confirm complex Eu(tta)₃ successfully graft on the surface of SiO₂@mTiO₂ core–shell nanospheres, FT-IR spectra of H₂bpdc ligand, SiO₂@mTiO₂, SiO₂@mTiO₂-bpdc and hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ were measured and shown in Figure 1 and Figure S1 and Table S1. In the spectra of SiO₂@mTiO₂ (Figure 1a), SiO₂@mTiO₂-bpdc (Figure 1b) and hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ (Figure 1c), the characteristic bands of Ti–O network can be observed in the range of 450~750 cm⁻¹. In addition, the vibrational bands of Si–O asymmetric stretching vibration and symmetric stretching vibration appear at 1109 and 794 cm⁻¹, respectively. While the peak at 953 cm⁻¹ is ascribed to stretching vibration of the Si–OH surface groups. As shown in Figure 1d, H₂bpdc ligand displays a strong band at 1716 cm⁻¹ because of the stretching of carboxylic groups. The stretching of carboxylic groups is not found in the spectra of SiO₂@mTiO₂-bpdc (Figure 1b) and hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ (Figure 1c), indicating that the carboxylic groups of H₂bpdc in SiO₂@mTiO₂-bpdc and Eu(tta)₃bpdc-SiO₂@mTiO₂ were completely deprotonated. Meanwhile, the ν_asym(COO⁻) and ν_sym (COO⁻) stretching vibration of carboxylate appear at 1554 and 1380 cm⁻¹ and the difference (Δ = ν_asym − ν_sym = 174 cm⁻¹) of carboxylate stretching frequencies can be used to identify the bonding mode, further indicating that carboxylate groups form bidentate mode with the titania in SiO₂@mTiO₂ nanospheres [34,35,36,37,38]. In addition, the position of stretching vibration of C=N of H₂bpdc ligand (1611 cm⁻¹) in Eu(tta)₃bpdc-SiO₂@mTiO₂ has shifted compared with that in SiO₂@mTiO₂-bpdc (1598 cm⁻¹), indicating that a possible coordination bond of Eu–N may be formed in hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂. The results above confirm that complex Eu(tta)₃ successfully graft on the surface of SiO₂@mTiO₂ core–shell nanospheres by a carboxylic functionalized bipyridyl ligand.

3.2. PXRD

The PXRD patterns of SiO₂@TiO₂/HDA, SiO₂@mTiO₂, SiO₂@mTiO₂-bpdc and Eu(tta)₃bpdc-SiO₂@mTiO₂ are shown in Figure 2. The pattern of SiO₂@TiO₂/HDA is completely amorphous, while that of SiO₂@mTiO₂ exhibits an anatase phase (JCPDS card No. 21-1272), indicating that the solvothermal method can not only remove the surfactant (HDA) in material but also change the crystal phase of TiO₂ in the shell. According to the Scherrer formula, the average crystallite size of TiO₂ in core–shell material can be estimated to be about 22 nm. The characteristic diffraction peaks of SiO₂@mTiO₂ appear at 25.27, 37.84, 48.03, 54.23, 55.04, 62.64, 69.32, 71.29 and 75.63° and correspond to the (101), (004), (200), (105), (211), (204), (116), (220) and (215) diffractions of anatase phase (JCPDS card No. 21-1272), respectively. In addition, characteristic diffraction peaks of SiO₂@mTiO₂-bpdc and Eu(tta)₃bpdc-SiO₂@mTiO₂ (Table S2) are nearly unchanged compared with those of SiO₂@mTiO₂, indicating that the anatase phase was maintained well after complex Eu(tta)₃bpdc was introduced.

3.3. SEM Images

The SEM images of SiO₂, SiO₂@TiO₂/HDA, SiO₂@mTiO₂, SiO₂@mTiO₂-bpdc and Eu(tta)₃bpdc-SiO₂@mTiO₂ are displayed in Figure 3. It can be observed that SiO₂, SiO₂@TiO₂/HDA, SiO₂@mTiO₂, SiO₂@mTiO₂-bpdc and Eu(tta)₃bpdc-SiO₂@mTiO₂ show monodisperse spherical structures. The average diameter of SiO₂ is about 350 nm, and that of SiO₂@TiO₂/HDA, SiO₂@mTiO₂, SiO₂@mTiO₂-bpdc and Eu(tta)₃bpdc-SiO₂@mTiO₂ is about 430 nm, the thickness of the shell in the core–shell structure is about 40 nm. The surfactant (HDA) in SiO₂@TiO₂/HDA materials were removed by solvothermal method, then SiO₂@mTiO₂ core–shell nanospheres were obtained. Meanwhile, it is found that the shell of SiO₂@mTiO₂ becomes rough and consists of many small nanoparticles. The surface morphologies and sizes of SiO₂@mTiO₂-bpdc and Eu(tta)₃bpdc-SiO₂@mTiO₂ are nearly unchanged in comparison with those of SiO₂@mTiO₂, suggesting that the load of the europium complex does not influence the morphology or size of SiO₂@mTiO₂.

3.4. TEM Images

The TEM, HRTEM and SAED images of SiO₂@mTiO₂ and the TEM image of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ are shown in Figure 4. SiO₂@mTiO₂ (Figure 4a) and Eu(tta)₃bpdc-SiO₂@mTiO₂ (Figure 4d) exhibit monodisperse spherical core–shell structures with an average diameter of about 430 nm. The shell of SiO₂@mTiO₂ and Eu(tta)₃bpdc-SiO₂@mTiO₂ is comprised of many small-ordered nanoparticles (Figure S2). The HRTEM can be used to identify the nature of nanocrystalline titania in SiO₂@mTiO₂ (Figure 4b). The fringe spacing is about 3.52 Å, corresponding to the (101) plane of anatase titania, further confirming that the shell of the nanosphere consists of single anatase nano-crystallites. As shown in Figure 4c, the SAED pattern clearly shows several continuous rings, corresponding to the (101), (004), (200), (105/211), (204), (116/220) and (215) diffractions of the anatase phase. In addition, the morphology and size of the hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ are nearly unchanged in comparison with those of SiO₂@mTiO₂, indicating that the introduction of europium complex does not influence the morphology and size of the SiO₂@mTiO₂ matrices. The composition and distribution of the elements in SiO₂@mTiO₂ and Eu(tta)₃bpdc-SiO₂@mTiO₂ were analyzed by EDS mapping (Figure S3 and Figure 5). As displayed in Figure S3, Si and Ti elements are uniformly distributed in the core and shell, suggesting that SiO₂@mTiO₂ is a core–shell structure of silica coated by titania. Furthermore, for hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ (Figure 5), a uniform dispersity of N, Eu, S and F elements indicates the existence of complex Eu(tta)₃bpdc in core–shell nanosphere, which further suggests the hybrid material was successfully obtained.

3.5. Luminescent Properties

The luminescent behavior of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ in solid state and ethanol solution was investigated at room temperature. The excitation spectra of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ are obtained by monitoring the most intense ⁵D₀ → ⁷F₂ emission. As presented in Figure 6, the excitation spectrum shows a broad band in the range of 300~400 nm, which was attributed to the π → π* electron transition of H₂bpdc ligand. Upon the maximum excitation (λ_ex = 366 nm), the emission spectrum of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ in a solid state presents characteristic ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu^III ions in the range of 550~710 nm, indicating that the effective energy transfer is occurred from the ligands (H₂bpdc and Htta) to the central Eu^III ion, further implying that core–shell SiO₂@mTiO₂ nanospheres can act as an efficient host to sensitize the luminescence of Eu^III ion. The ⁵D₀ → ⁷F₂ emission intensity of Eu^III is strongly dependent on the environment because of its electric-dipole character, while the intensity of the ⁵D₀ → ⁷F₁ transition is independent of the environment due to its magnetic dipole character. The intensity ratios I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁) will provide valuable information about the surrounding environment changes around Eu^III ion. The ratio in complex Eu(tta)₃(H₂O)₂ is about 7.7, while that in hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ is about 3.9, the decrease in the ratio demonstrates that the symmetry of the coordination environment around the Eu^III ions has changed during the formation of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, which further indicates that the tripyridyl group replaces the coordination water molecules in the first coordination sphere of Eu^III ions in a complex by ligand-exchange reaction, and the coordination bonds of Eu–N are formed. The ⁵D₀ luminescent decay curve was recorded at 613 nm under the maximum excitation (λ_ex = 366 nm) and is exhibited in Figure S4. The curve can be fitted by biexponential functions, the average lifetime (τ) can be calculated using the following equation:

$$\tau =\frac{{\displaystyle \sum A_i{\tau }_i^2}}{{\displaystyle \sum A_i{\tau }_i}},$$

(1)

where τ_i is the component decay times and A_i is the preexponential factors related to the statistical weights of each exponential. For hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, the obtained ⁵D₀ lifetime value was 0.52 ms.

As shown in Figure S5, hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ in an ethanol solution also shows five characteristic ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) emission peaks of Eu^III ions and shows luminous red color emission. At the same time, Eu(tta)₃bpdc-SiO₂@mTiO₂ in an ethanol solution possesses excellent luminescent stability at room temperature. In addition, hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ also has a nanoscale nature, so it can act as a suitable luminescent sensor in environmental and biological systems.

3.6. Sensing of Al³⁺ Cations

In order to investigate the potential application of core–shell hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ for the sensing of metal ions, Eu(tta)₃bpdc-SiO₂@mTiO₂ powder samples were dispersed in ethanol solutions of different metal ions (KCl, NaCl, LiCl, ZnCl₂, CaCl₂, CdCl₂, PbCl₂, MgCl₂, MnCl₂, CrCl₃ and AlCl₃) for luminescent measurements. The luminescent emission of samples was recorded and presented in Figure 7a. Most of these cations influence the emission intensities of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, especially for Mn²⁺ and Cr³⁺ ions show a significant quenching effect on the luminescent intensity. These phenomena are caused by the different electron configuration of metal ions: K⁺, Na⁺, Li⁺, Zn²⁺, Ca²⁺, Cd²⁺, Pb²⁺ and Mg²⁺ with a closed-shell electron configuration have much weaker effects on luminescent intensity, whereas other ions (such as Mn²⁺ and Cr³⁺) have different electron configurations and produce varying degrees of quenching of luminescent intensity [39]. However, only Al³⁺ ions have a significant impact on the emission spectrum of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, characteristic emission of Eu^III ions disappears, a broad band of H₂bpdc ligand appears at 396 nm, and the luminescent color has clearly changed from red to blue under UV-light irradiation (Figure 7b). The reason for the selective recognition of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ toward Al³⁺ ions may be that there existed stronger coordination interaction between bpdc²⁻ ligand and Al³⁺ ions than bpdc²⁻ ligand and Eu^III ions [40].

A luminescent titration experiment was carried out to evaluate the luminescent response of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ to Al³⁺ ions by the addition of Al³⁺ ions to an Eu(tta)₃bpdc-SiO₂@mTiO₂ ethanol solution. As shown in Figure 8a, with the increase in Al³⁺ ion concentration, the luminescent intensity of Eu^III ions at 613 nm gradually weakens; meanwhile, a broad band of H₂bpdc ligand at 396 nm appears and gradually enhances, leading to emission color change from red to pink, purplish pink, purplish blue, and finally blue under the 365 nm UV lamp (Figure 8c). The distinguishable colors at different concentrations of Al³⁺ ions are attributed to the changeable rate of the luminescent relative intensity of H₂bpdc ligand-centered emissions and the characteristic emission of Eu^III ions. Hence, the naked-eye recognition of Al³⁺ ions is feasible according to the color change of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ ethanol solution. As a result, by changing the concentration of Al³⁺ ions in hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ ethanol solution, the relative emission intensities of two constituent colors (red and blue) can be precisely manipulated, resulting in a tunable multicolor output. The emission color CIE coordinates of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ ethanol solution with various concentrations of Al³⁺ ions excited at 367 nm in the CIE 1931 chromaticity diagram (Figure 8d) and listed in Table S3. When Al³⁺ ion concentration increases to 243 μM, the red emission from Eu^III ions almost completely quenches. Noticeably, a linear relationship is observed at low concentrations between the emission intensity ratio of 396 nm/613 nm and the concentration of Al³⁺ ions, followed the simple linear equation of y = 0.0439x + 0.0011 (R² = 0.983) (Figure 8b inset). The limit of detection (LOD) for Al³⁺ ions was determined as 1.8 × 10⁻⁴ M according to the equation LOD = 3σ/k. The result indicates that hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ can be acted as a highly sensitive and selective probe for the quantitative luminescent detection of Al³⁺ ions in an ethanol solution.

To assess the selectivity of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, the competitive tests of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ with Al³⁺ and different metal ions were carried out in an ethanol solution. As shown in Figure S6, after adding other ions in Al³⁺ ethanol solution, the luminescent intensity of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ at 396 nm increases compared with that in Al³⁺ ethanol solution. The result suggests that the detection of Al³⁺ ions is not affected by the other ions.

According to the reported literature [16,30], the possible luminescent sensing mechanism for the detection of Al³⁺ ions originates from two aspects: (i) the cation exchange of central lanthanide ions in lanthanide hybrid materials with targeted cations, (ii) the interaction between organic ligands and Al³⁺ ions. Therefore, in order to verify this hypothesis, ICP-MS, XPS, PXRD, TEM and EDS mapping of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ after immersed in an Al³⁺ ion ethanol solution (namely, Eu(tta)₃bpdc-SiO₂@mTiO₂ + Al³⁺) were carried out. As exhibited in Figure 9, as the content of added Al³⁺ ions in the filtrate increases, the concentration of Eu^III ions in the filtrate gradually increases. As a result, Al³⁺ ions may gradually replace Eu^III ions in hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, which corresponds to luminescence responses of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ toward Al³⁺ ions in ethanol solutions with varying concentrations of Al³⁺ ions, indicating that the luminescent emission changes are attributed to the cation exchange. As shown in Figure 10 and Figure S7 and Table S4, it is clearly observed that Eu 3p peak (~1138.4 eV) disappears and Al 2p peak (~74.8 eV) appears in Eu(tta)₃bpdc-SiO₂@mTiO₂ + Al³⁺ when compared with those peaks in hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, indicating that Eu^III ions in hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ were gradually replaced by Al³⁺ ions in an ethanol solution, further suggesting that there existed a cation exchange when hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ was immersed in an ethanol solution of Al³⁺ ions. In addition, the O 1s peak [41] in the spectrum of Eu(tta)₃bpdc-SiO₂@mTiO₂ + Al³⁺ almost have no shift compared with that in the spectrum of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ (Figure 10b), to some content, which excludes the possibility of interaction between oxygen atoms in coordinated carboxylate group of bpdc²⁻ ligand and Al³⁺ ions. As displayed in Figure S8 and Table S2, characteristic diffraction peaks of Eu(tta)₃bpdc-SiO₂@mTiO₂ + Al³⁺ materials are nearly unchanged compared with those of hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, indicating that the anatase phase was maintained well after hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ was immersed in an ethanol solution of Al³⁺ ions. Additionally, it can be observed that Eu(tta)₃bpdc-SiO₂@mTiO₂ + Al³⁺ shows the core–shell structure (Figure S9) and the size of the core–shell nanosphere of Eu(tta)₃bpdc-SiO₂@mTiO₂ + Al³⁺ almost have no change compared with that of Eu(tta)₃bpdc-SiO₂@mTiO₂. This suggests that the subsequent addition of Al³⁺ does not nearly change the morphology or size of the hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂. As shown in Table S5, the Al element could be seen besides Si, O, Ti, N, F, S, and mass ratio of Eu element decreases and approaches zero compared with the mass ratio of the Eu element in hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂, which suggests that Eu^III ions in Eu(tta)₃bpdc-SiO₂@mTiO₂ were replaced by Al³⁺ ions, and then excludes interaction between the ligands of Eu(tta)₃bpdc-SiO₂@mTiO₂ and Al³⁺ ions. Furthermore, Si, O, Ti, N, F, S and Al elements are homogeneously distributed in the hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ + Al³⁺ (Figure S10). Based on the above results, the possible luminescence sensing mechanism for the detection of Al³⁺ ions originates from the cation exchange between Eu^III ions in hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ and Al³⁺ ions.

4. Conclusions

A europium(III) hybrid material based on a SiO₂@mTiO₂ core–shell nanosphere by covalent grafting complex Eu(tta)₃bpdc was successfully designed and synthesized. H₂bpdc ligand can act as a bifunctional linker to graft Eu(tta)₃ to the surface of the core–shell SiO₂@mTiO₂ nanosphere. The results indicate that core–shell structure and anatase crystallites are retained after grafting to the europium complex. Hybrid material Eu(tta)₃bpdc-SiO₂@mTiO₂ displays uniform nanosphere structure, bright red color emission and long lifetime, which can sever as a multicolor emission material to modulate by using Al³⁺ ions via an ion exchange approach under a single-wavelength excitation.