Smart Mn⁷⁺ Sensing via Quenching on Dual Fluorescence of Eu³⁺ Complex-Modified TiO₂ Nanoparticles

Abstract

In this work, titania (TiO₂) nanoparticles modified by Eu(TTA)₃Phen complexes (ETP) were prepared by a simple solvothermal method developing a fluorescence Mn⁷⁺ pollutant sensing system. The characterization results indicate that the ETP cause structural deformation and redshifts of the UV-visible light absorptions of host TiO₂ nanoparticles. The ETP also reduce the crystallinity and crystallite size of TiO₂ nanoparticles. Compared with TiO₂ nanoparticles modified with Eu³⁺ (TiO₂-Eu³⁺), TiO₂ nanoparticles modified with ETP (TiO₂-ETP) exhibit significantly stronger photoluminescence under the excitation of 394 nm. Under UV excitation, TiO₂-ETP nanoparticles showed blue and red emission corresponding to TiO₂ and Eu³⁺. In addition, as the concentration of ETP in TiO₂ nanoparticles increases, the PL intensity at 612 nm also increases. When ETP-modified TiO₂ nanoparticles are added to an aqueous solution containing Mn⁷⁺, the fluorescence intensity of both TiO₂ and ETP decreases. The evolution of the fluorescence intensity ratio (I₁/I₂) of TiO₂ and ETP is linearly related to the concentration of Mn⁷⁺. The sensitivity of fluorescence intensity to Mn⁷⁺ concentration enables the design of dual fluorescence ratio solid particle sensors. The method proposed here is simple, accurate, efficient, and not affected by the environmental conditions.

1. Introduction

Manganese has two primary valence and oxidation states, namely, Mn²⁺ and Mn⁷⁺, which have different effects in practice [1,2]. Mn⁷⁺ has been widely used as a strong disinfectant, but its strong oxidation property and its heavy metal characteristic make it a toxic and carcinogenic species in water recycling systems and in human health [3]. In industry, Mn⁷⁺ has been used as a strong oxidation agent, generating large amounts of toxic waste water [4]. Manganese ions have contributed to serious pollution, causing toxic drinking water and damage to plants [5]. Thus, Mn⁷⁺ has attracted a lot of attention among pollutants in recent years, and the detection of Mn⁷⁺ is very important for environmental protection. Ion chromatography (IC) [6], atomic absorption spectroscopy (AAS) [7], inductively coupled plasma mass spectroscopy (ICP-MS) [8] and spectrophotometry [9] can be used to detect Mn⁷⁺. Monitoring Mn⁷⁺ in water samples requires complex methods such as atomic spectrometry. However, due to the low efficiency of this method and the interference of impurities present in the real samples, the detection of Mn⁷⁺ at low concentration is complicated and requires pretreatment steps [10]. Therefore, it is necessary to explore a simple, accurate, efficient, and low-interference method to detect Mn⁷⁺ in complex samples.

Even with the use of highly sensitive metal ion detectors, the content level of Mn⁷⁺ in environmental samples is usually low or close to the detection limit, so the samples require a unique separation. When high concentrations of interfering particles are present in the matrix, efficient extraction of Mn⁷⁺ is required. Qian et al. proposed a FAAS method that uses crosslinked chitosan to separate Mn²⁺ and Mn⁷⁺. This method is simple and sensitive and can be used for environmental sample detection [11]. A potential problem limiting the application of this method for actual samples is that the Mn²⁺ is determined by its oxidation to Mn⁷⁺, and then the total Mn concentration is determined. Therefore, this method requires that the analyte species are only Mn²⁺ and Mn⁷⁺. This motivates the development of new Mn sensors.

Europium complexes constitute an important class of optical probes, with applications ranging from sensing of bioactive species, high throughput assays and screening protocols in vitro, to time-resolved imaging studies in cellulo or in vivo [12]. Eu complexes might also provide an opportunity for Mn⁷⁺ sensing. In this work, we design a strategy to combine the advantages of Eu³⁺ fluorescence and TiO₂ dispersion in water. Our fluorescent nanomaterials can detect in water or other liquids, with minimal interference and low cost. In recent years, fluorescence spectrometry has been used to detect and quantify metal ions [13]. A new method for the determination of copper, manganese, nickel, and lead in diesel oil has been developed, which combines liquid–liquid reversed-phase eddy microextraction and energy dispersive X-ray fluorescence spectroscopy [14]. Fluorescence sensors [15] have received widespread attention due to their high sensitivity, selectivity, and simple operation; however, a single wavelength fluorescence sensor is still affected by sample concentration, environmental factors, and excitation intensity [16]. Dual fluorescence sensors can measure emission peaks at two different wavelengths, and use the ratio of the two peak intensities to solve the above problems [17,18,19,20], increasing sensitivity and selectivity [21,22]. Nanomaterial fluorescence sensors are a new type of sensors, which have large specific surface area, controllable size, predictable nanostructure [23,24,25], and polychromatic and adjustable radiation characteristics [26]. In general, photoluminescence is achieved by organic lanthanide complexes that absorb UV light and emit photons due to f-f or f-d electronic transitions in the lanthanide ion [27,28]. Among all the lanthanide ions, europium ions have been widely used because of their prominent emission peak and long fluorescence lifetime [29,30,31]. The advantages of their complexes are stable characteristic emission peaks and increased fluorescence intensity. TiO₂ nanoparticles have excellent optical properties, catalytic properties, chemical stability, and biocompatibility [32]. Therefore, TiO₂-ETP nanoparticles combine the advantages and fluorescence properties of europium complexes and TiO₂ nanoparticles.

In this work, we synthesized europium complex-modified TiO₂ nanoparticles (TiO₂-ETP) by a solvothermal method, and we investigated the structure, properties, and application of TiO₂-ETP nanoparticles. The resulting nanoparticles have outstanding luminescence characteristics, indicating the possibility of using TiO₂ nanoparticles to improve the effective luminescent properties of rare earth complexes. TiO₂-ETP nanoparticles exhibit significantly stronger photoluminescence (PL) than TiO₂-Eu³⁺ nanoparticles. Thus, TiO₂-ETP nanoparticles have the potential to be used as new semiconductor luminescent materials. In our study, the fluorescence intensity of TiO₂-ETP was sensitive to the change of Mn⁷⁺ concentration. In addition, the high specific surface area of TiO₂ nanoparticles can increase the contact area between the sensor and Mn⁷⁺, which can improve the sensitivity of the sensor. In the fluorescence spectrum, excited with 394 nm light, TiO₂-ETP has emission peaks at 454 nm and 616 nm for titania and the ETP, respectively. The fluorescence of ETP and TiO₂ both decreases in the presence of Mn⁷⁺, but with different characteristic sensitivity to Mn⁷⁺. The Mn⁷⁺ dual fluorescence sensor shows a wide detection range and high sensitivity, and the effectiveness of the sensor has been verified through experiments. In this research, we propose an intelligent dual fluorescence sensor, which is low-cost and easy to operate. It has high sensitivity and high efficiency. Compared with previous reports [33,34], our method is simple and practical, reduces the need for pretreatment, and has a larger detection range. The preparation and detection mechanism of the sensor is shown in Figure 1.

2. Experimental Details

Ethanol (AR, 99.7%), acetic acid (AR, 99.7%), and tetrabutyl titanate (AR) were purchased from Macklin (Shanghai, China). Europium chloride hexahydrate (EuCl₃·6H₂O, 99.9%), 1,10-phenanthroline monohydrate (Phen, AR, 98%) and 2-thenoyltrifluoroacetone (TTA, 98%) were purchased from Aladdin (Shanghai, China).

As shown in Figure 2, TiO₂, TiO₂ modified with Eu³⁺ (TiO₂-Eu³⁺), and TiO₂ modified with Eu (TTA)₃Phen (ETP) (TiO₂-ETP) were prepared using the solvothermal method. Tetrabutyl titanate (TBT) was used as a precursor. Ethanol (CH₃CH₂OH) and acetic acid (CH₃COOH) were used as solvents and hydrolysis inhibitors. Before the final synthesis, two solutions were prepared (solution A and solution B). Solution A was prepared by adding acetic acid and TBT in ethanol. In solution B, EuCl₃ was dissolved in ethanol by stirring. Then 1,10-phenanthroline monohydrate and methyl 1H-benzotriazole, dissolved in absolute ethanol were added to solution B, and the mixed solution was stirred with a magnetic stirrer for 1 h at room temperature. Solution A was added to solution B. The mixture became cloudy with continuous stirring. The mixture was heated in an autoclave to 150 °C for 24 h. After the reaction, the resulting material was cooled to room temperature. The synthesized material was centrifuged and washed with ethanol and distilled water several times to remove impurities. The resulting white solid was collected and dried in an oven at 60 °C. For the synthesis of unmodified TiO₂ nanoparticles, the same conditions are used, without the addition of ETP.

The quenching experiments using metal ions were performed by adding TiO₂-ETP (0.1 mol/L) into different metal ion analyte solutions with the concentrations of 1 mM/L, and the mixtures were stirred for 2 h. To determine the quenching behavior, Mn⁷⁺ concentrations in the range of 1 µM/L to 1000 µM/L were used.

A Thermo Scientific F200i (Thermo, Waltham, MA, USA) transmission electron microscope was used to obtain transmission electron microscopy (TEM) images at an accelerating voltage of 200 kV. X-ray powder diffraction (XRD) measurements were performed using a Bruker D8 Advance diffractometer (Bruker, Karlsruhe, Germany), which was operated at a generator voltage of 40 keV and a current of 30 mA. The X-ray source is CuKα radiation (λ = 0.154 nm). The diffraction pattern was collected at a scanning speed of 1°/min within a 2θ scanning range of 20° to 80°. Measurements of Raman spectra were performed on a Thermo Scientific DXR 2xi (Thermo, Waltham, MA, USA) Raman Spectrometer under a backscattering geometry. The valence states of Eu, O, and Ti atoms were measured by X-ray photoelectron spectroscopy (XPS) on a Thermo Scientific ESCALAB 250 (Thermo, Waltham, MA, USA) spectrometer. The XPS experiment was performed under vacuum using AlKα (1486.6 eV) radiation. The ultraviolet absorption spectrum was obtained using PerkinElmer Lambda 750s (PerkinElmer, Shanghai, China) with a solid sample frame, on which the powder samples were flattened when the powder samples were used. The PL spectrum is an important tool for determining the luminescent properties of materials. An Edinburgh Instrument Fluorescence Spectrometer FLS 1000 (Livingston, Edinburgh, UK) was used to record the excitation and emission spectra of each sample, on which the data of excitation spectra, emission spectra, fluorescence lifetimes were collected. A 450W xenon arc lamp capable of emitting a continuous spectrum with greater intensity was used as the light source. The excitation monochromator was used to select the specified spectrum with the excitation wavelength of 394 nm. Fluorescence analyzer calibration was performed in accordance with the instrument operating procedures using standard sample, sample preparation and processing, resulting in excellent calibration curves.

3. Results and Discussion

3.1. Morphological Structures

The additions of Eu³⁺ and ETP into TiO₂ change the shape and size of TiO₂ nanoparticles. Figure 3 shows typical transmission electron microscopy (TEM) images of TiO₂ nanoparticles. TiO₂ nanoparticles with spherical morphology can be seen in TEM images (Figure 3a). The morphology of TiO₂ nanoparticles with Eu³⁺ varies from spherical to ellipsoidal shapes (Figure 3b). The TiO₂ nanoparticles modified with ETP have a cuboid shape (Figure 3c). These changes are similar to the previous report [35]. Eu³⁺ and ETP-doped TiO₂ cause different shapes of TiO₂-Eu³⁺ and TiO₂-ETP nanoparticles [36]. The corresponding histograms of the diameter distributions and the changes of the average sizes are shown in Figure 3 in which the average nanoparticle sizes can be found to be 15 ± 0.09 nm, 12.3 ± 0.08 nm, and 9 ± 0.1 nm in diameter. The length of TiO₂-Eu³⁺ is between 10 and 40 nm. Compared to TiO₂, the average size of TiO₂-ETP nanoparticles decreases, which suggests that the inclusion of ETP largely suppresses the growth of TiO₂ nanoparticles. This size change of TiO₂-Eu³⁺ and TiO₂-ETP nanoparticles can also relate to crystalline structures described later, based on X-ray diffraction analyses [37]. The growth of TiO₂-Eu³⁺ crystals is hindered by the formation of Eu-O-Ti bond in the crystal void of TiO₂-Eu³⁺ nanoparticles. The decrease of particle size of ETP-modified TiO₂ is mainly caused by ETP entering the lattice and binding with oxygen. Due to internal stress in the crystal lattice, the diffusion of Ti⁴⁺ and O^2- and the obstacle of crystal migration, the crystal growth at the boundary is retarded [38].

3.2. Crystalline Structure

Modification with Eu³⁺ can effectively change the crystal structure and inhibit grain growth of TiO₂ nanoparticles. This effect is more pronounced when the organic complex (ETP) is used. Figure 4 shows the diffraction patterns of TiO₂ nanoparticles obtained by the solvothermal method. The presence of diffraction peaks corresponding to (101), (004), (200), (105), (211), and (204) planes indicate the formation of the anatase TiO₂ phase [39]. The XRD shows that TiO₂-Eu³⁺ and TiO₂-ETP nanoparticles have peaks at 2θ = 25.3°, 38.1°, 47.9°, 54.1°, 55.2°, and 62.6°, which correspond to peaks of anatase TiO₂ (JCPDS NO.21-1272). No additional peaks of any other phases or impurities were found, which indicates the high purity of the nanoparticles. Figure 4 shows that the XRD peaks of the (101) crystal plane in TiO₂-ETP are slightly shifted towards a smaller diffraction angle from 25.3° to 25.1°, while other diffraction peaks have almost no observable shift. This is likely due to the addition of ETP [40]. Because the smaller diffraction angle relates to the larger gaps between crystal planes, this shift means that the distance of the (101) crystal plane slightly increases upon ETP addition [40]. The relative intensity of the peak at 2θ = 25.3° is significantly decreased in TiO₂-ETP compared to the TiO₂ and TiO₂-Eu³⁺ nanoparticles, indicating that the crystallinity decreased [41]. When ETP is added to TiO₂ nanoparticles, deformation is induced in the system, leading to a change in the periodicity of the lattice and a decrease in the crystal symmetry. From the full width at half maximum, one can judge that TiO₂-ETP has a smaller particle size than TiO₂ and TiO₂-Eu³⁺. The characteristic peaks of the (101) (004), and (200) crystal planes from the XRD image were selected, and the Scherrer formula (Equation (1)) was used to calculate the average size of the modified and unmodified nanoparticles (

Table 1. XRD results with parameters.

Sample	hkl	2θ (deg)	D (Å)	FWHM (deg)	Mean Grain Size (nm)	Crystal Structure
TiO2	101	25.34	3.51	0.671	15.1	tetragonal
	004	37.84	2.37	0.846	15.0	tetragonal
	200	48.07	1.89	0.710	15.1	tetragonal
TiO2-Eu3+	101	25.31	3.50	1.115	12.2	tetragonal
	004	37.66	2.38	0.786	12.4	tetragonal
	200	47.88	1.89	0.825	12.1	tetragonal
TiO2-ETP	101	25.06	3.55	0.966	9.0	tetragonal
	004	37.48	2.39	0.847	9.1	tetragonal
	200	47.76	1.90	0.971	8.9	tetragonal

),

$$L_{hkl}=\frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }},$$

(1)

where $L_{hkl}$ is the size of the particle crystallites, K is the shape constant, usually taken as 0.9, λ is the wavelength of X-rays (CuKα is 1.5406 Å), β is the full diffraction width at half maximum, measured in radians at 2θ Peak.

Figure 5 shows the Raman spectra of the obtained TiO₂ nanoparticles. The Raman peaks at 143, 395, 514, and 639 cm⁻¹ correspond to E_g, B_1g, A_1g, or B_1g, and E_g of the anatase phase, respectively [42]. The most dominant E_g mode appears due to the external vibration of the anatase structure at 143 cm⁻¹. This indicates that the anatase phase is formed in the prepared europium complex-modified TiO₂ nanoparticles. The inclusion of ETP in TiO₂-ETP nanoparticles changes features of the crystal structure of TiO₂, so the Raman spectrum was slightly shifted. It can be seen from the Raman spectrum that, especially in the E_g mode near 144 cm⁻¹, the TiO₂ nanoparticles modified with ETP move to a higher wavenumber direction, and their intensity drops sharply. The observation can be explained by a decrease in the particle size in TiO₂-Eu³⁺ [41,43,44]. When the grain size decreases, it will significantly affect the Raman spectrum of titanium dioxide nanoparticles. Generally speaking, dimensional changes will produce pressure, and volume shrinkage will occur in TiO₂ nanoparticles. The reason for the increase in pressure is the decrease in the distance between atoms. The sudden drop in the intensity of the Raman spectrum, especially the drop in the scattering intensity of the E_g mode, is related to the destruction of the atomic symmetry of the crystal, which is caused by the defects modified with ETP. Because TiO₂-ETP nanoparticles have local lattice defects, the Raman peak becomes weaker and broader, which means that the crystallinity of synthesized nanoparticles decreases.

3.3. Confirmation of Eu³⁺ in TiO₂

X-ray photoelectron spectroscopy (XPS) was used for elemental analysis of ETP-modified titanium dioxide nanoparticles. Figure 6A(a–c) shows the survey XPS spectra of TiO₂, TiO₂-Eu³⁺, and TiO₂-ETP, respectively. The XPS spectra in Figure 6B clearly shows the changes of the binding energy of the Ti2p electron orbital in TiO₂, TiO₂-Eu³⁺, and TiO₂-ETP in which the binding energies in TiO₂, TiO₂-Eu³⁺ and TiO₂-ETP, are gradually decreased at 458.72, 458.62, and 458.57 eV. This phenomenon is similar to a previous report [45]. The binding energy decreases are caused by Eu³⁺ and ETP inserting between crystal planes. The much larger decrease of binding energy in TiO₂-ETP is due to the larger TTA and Phen ligands carried by Eu³⁺. Figure 6C shows spectra of Eu3d with significantly higher intensity for TiO₂-ETP than for TiO₂-Eu³⁺, indicating that the TTA and Phen ligands tightly bind the Eu. At the same time, the binding energy of Eu3d in TiO₂-ETP is slightly lower than that in TiO₂-Eu³⁺, which is also due to the stronger interaction of ligands with the Eu3d electron orbital [46]. Figure 6D–F show the binding energy changes of O1s in TiO₂, TiO₂-Eu³⁺, and TiO₂-ETP, showing that Ti-O and Eu-O have almost the same binding energies in TiO₂-Eu³⁺ and TiO₂-ETP. The binding energy of Ti-O in TiO₂ is higher (Figure 6D) than the corresponding binding energy in the TiO₂-Eu³⁺ (Figure 6E) and TiO₂-ETP (Figure 6F), indicating the influence of Eu³⁺ insertion between crystal planes of TiO₂. The binding energies corresponding to Ti-O, O-H, Eu-O, and C=O in TiO₂-ETP (Figure 6F) are located at 529.8, 530.8, 531.4, and 532.3 eV. Compared to TiO₂, the formation of an Eu-O bond indicates that Eu has reacted with TiO₂. The C=O bond belongs to TTA in ETP, which indicate that ETP is interacting with TiO₂ [47,48,49].

3.4. UV Absorption and Bandgap of TiO₂

Figure 7a,c show the UV-visible absorption curves of TiO₂-Eu³⁺ and TiO₂-ETP. Compared with the curves of TiO₂-Eu³⁺ (Figure 7a), the curves of TiO₂-ETP have significant redshift (Figure 7c). As the ETP concentration increases, the absorption edge moves to the right, and the energy required to generate electron-hole pairs gradually decreases. The valence band of TiO₂ absorbs ultraviolet light and releases it into the conduction band and defect state energy level of TiO₂. Because the excited state of Eu³⁺ is lower than the conduction band and defect state, the energy is transferred to Eu³⁺ [50]. UV–visible spectra shown in Figure 7b show that modification with ETP shifted the TiO₂ absorption edge from the UV to the visible region. This means Eu³⁺ and ETP doping produce defects in the TiO₂ host crystal, and thus these defects result in band gap decrease [51,52,53].

The absorption spectra in the UV and visible regions of TiO₂-Eu³⁺ and TiO₂-ETP nanoparticles are used to estimate the bandgap. [F(R) × hν] ^1/2 of hν in the vicinity of the absorption edge are plotted for all samples in Figure 7c,d, where F(R) is the Kubelka–Munk function, defined as F(R) = (1 − R)²/2R, hν is the photon energy, and R is the reflection coefficient converted to absorption intensity. By extrapolating the linear part of the curve to the intersection with the x-axis, the bandgap energies can be estimated. The bandgap energies for TiO₂-Eu³⁺ are 3.08 eV, 3.05 eV, 3.06, and 3.09 eV for TiO₂ modified by 2, 4, 6, and 8 mol% Eu³⁺, respectively. The bandgap energies for TiO₂-ETP are 2.72 eV, 2.40 eV, 2.30 eV, and 2.26 eV for TiO₂ modified by 2, 4, 6, and 8 mol% ETP, respectively. Compared with the band gap of 3.2 eV of TiO₂, the band gaps of TiO₂-Eu³⁺ and TiO₂-ETP are decreased. The bandgap decrease is caused by interactions between TiO₂ host and dopants, either Eu³⁺ or ETP in TiO₂-Eu³⁺ and TiO₂-ETP, respectively. Based on the results of XRD to indicating the (101) crystal plane distance extension, and the XPS to confirm the interactions between Eu³⁺-O and the binding energy changes of Eu3d and Ti2p in Eu³⁺ and ETP, we conclude that Eu³⁺ and ETP as dopants have interacted with TiO₂ in different ways. Thus, these changes can be attributed to the “solubility limit” of Eu³⁺ and ETP in TiO₂ host. The former Eu³⁺ is from EuCl₃·6H₂O in which both the Cl⁻ counter ion and the bound H₂O molecules affect the solubility of Eu³⁺. However, ETP is a complex with the organic ligand molecules (Phen and TTA), which modify the solubility of ETP. Solubilized ions have efficient interaction with the TiO₂ host to change the electron transition bandgap [54]. These interactions also affect the fluorescence behaviors as shown in Figure 8.

3.5. Photoluminescence Analysis

The luminescence mechanism of Eu³⁺-complexes is generally described as follows: the organic ligand absorbs incident photons, transitioning from the ground state to the excited singlet state. Normally, the excited electron will experience an intersystem transition from the singlet state to the triplet state. The triplet excited state transfers energy to the S1 excited state of Eu³⁺, which can subsequently emit a photon when the ⁵D₀ transitions to the ⁷F_J configurations. The luminescence of TiO₂ is due to the electron transition between the valence band and the conduction band. Figure 8a shows the excitation spectra of TiO₂-Eu³⁺ and TiO₂-ETP nanoparticles. The excitation spectra are measured by the emission wavelength of the nanoparticles at 612 nm. The characteristic excitation peak is related to the 4f-4f transition of Eu³⁺ from ⁷F₀. The excitation spectrum consists of sharp lines at 384, 394, 418, and 464 nm, assigned to the ⁷F₀→⁵L₇, ⁷F₀→⁵L₆, ⁷F₀→⁵D₃, and ⁷F₀→⁵D₂ transitions of Eu³⁺ [55]. Strong peaks at 394 nm and 464 nm correspond to the ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ Eu³⁺ transitions. The intensity of the excitation spectrum of TiO₂-ETP is higher than that of TiO₂-Eu³⁺. The organic ligands in ETP help absorb more ultraviolet light. Figure 8b shows the emission spectra of TiO₂-Eu³⁺ and TiO₂-ETP. When excited at a wavelength of 394 nm, the emission spectrum consists of ⁵D₀→⁷F_J (J = 0, 1, 2, 3, 4) (578, 592, 612, 652, and 703 nm) Eu³⁺ transitions. Due to the allowable electric dipole of the ⁵D₀→⁷F₂ transition, the strongest emission is produced at 612 nm, which is red. Figure 8c shows the emission spectra of TiO₂-Eu³⁺, prepared with different concentrations of Eu³⁺ (2, 4, 6, and 8 mol%). The influence of concentration on PL intensity is shown in Figure 8e. The optimal concentration of Eu³⁺ is 4% [39]. When the concentration exceeds 4%, the fluorescence of TiO₂-Eu³⁺ nanoparticles decreases. This suggests that 4% Eu³⁺ concentration is the upper solubility limit in the TiO₂ host. However, the fluorescence intensity of TiO₂-ETP increases with increasing concentrations of ETP. This indicates that the organic ligands in ETP improve the solubility of ETP in the TiO₂ host, which provide a more effective “antenna effect” of organic ligands [56,57,58].

Figure 8f shows a diagram of energy levels. Based on XRD, Raman, and XPS analysis, Eu³⁺ and ETP were successfully incorporated into TiO₂ nanoparticles. In Figure 8f, the phrase “defect state” is representative of a variety of defects. Europium ions and ETP will produce point defects in the crystal lattice and combine with oxygen atoms to form Eu-O bonds [59], and the multiple defect energy levels are marked as multiple lines. This indicates that the external ultraviolet rays are absorbed by the TiO₂ nanoparticles, and the energy enters the defect state. Energy is then transferred to the Eu in the ETP, realizing the energy transfer process from TiO₂ to Eu. Because the energy level of the emission state of Eu³⁺ is lower than the energy level of the defect in TiO₂ nanoparticles, the energy is transferred from the defect state of TiO₂ to the crystal field state of Eu³⁺ ions, which leads to effective photoluminescence of the nanoparticles. Due to the small size and a large number of nanoparticles, there are many surface states available for transferring energy to the states of the crystal field of Eu³⁺. Figure 9 shows the fluorescence lifetime diagram of TiO₂-Eu³⁺ and TiO₂-ETP. The fluorescence attenuation of TiO₂-ETP is slower than that of TiO₂-Eu³⁺, and the quantum yield of TiO₂-ETP is higher than that of TiO₂-Eu³⁺. The fluorescence lifetime of TiO₂-ETP and TiO₂-Eu³⁺ were 0.51 ms and 0.39 ms, and the quantum yields of TiO₂-ETP and TiO₂-Eu³⁺ were 10% and 5%.

3.6. Fluorescence Spectra of TiO₂-ETP in the Presence of Metal Ions

Eu³⁺ can be complexed with organic ligands containing oxygen or nitrogen groups, such as methyl 1H-benzotriazole and 1,10-phenanthroline monohydrate [60,61]. Therefore, when the europium complex is in contact with metal ions, the fluorescence properties will change. In this paper, common metal cations such as Zn²⁺, Mn³⁺, K⁺, Mn⁷⁺, Fe²⁺, Mg²⁺, Ca²⁺, and Co²⁺ are selected to determine whether these metal ions will affect the fluorescence properties of TiO₂-ETP. These experimental analyses prove that these common impurities will not affect the sensitivity of the sensor. The results of these experiments are shown in Figure 10. We have also previously reported of the effects of organic molecules, such as carbohydrates, cholesterol, and amino acids, on the emission of Eu³⁺ complex in different hosts, showing that the tested organic molecules exhibit no quenching effect [40,62].

As shown in Figure 10a, TiO₂-ETP shows a strong fluorescence peak located at 464 and 616 nm with excitation at λ_ex = 394 nm. The fluorescence of TiO₂-ETP is influenced by the addition of Mn⁷⁺, where a significant quenching effect can be observed. The fluorescence intensity of ETP-modified TiO₂ nanoparticles decreases with the increase of Mn⁷⁺ concentration in the solution. Figure 10b shows the ratio (I/I₀) of the fluorescence intensity of TiO₂-ETP in an aqueous solution containing no metal ions and a solution containing a single metal ion. I₀ is the fluorescence intensity of TiO₂-ETP in the absence of metal ions at 464 nm and 616 nm, and I is the fluorescence intensity of TiO₂-ETP at 464 nm and 616 nm in the presence of a single metal ion. It can be seen from Figure 10b that the addition of other metal ions besides Mn⁷⁺ will not significantly reduce the fluorescence intensity of TiO₂-ETP. The aqueous solution containing Mn⁷⁺ will cause fluorescence quenching of TiO₂-ETP. The decrease in fluorescence intensity can also be detected by adding TiO₂-ETP to an aqueous solution containing a small amount of Mn⁷⁺. Experiments show that when other ions are present, only manganese will quench the Eu³⁺ fluorescence. The possible mechanism of quenching can be either the absorption of photons by Mn⁷⁺, or the Mn⁷⁺ excimer formation by interaction with the excited state of with ETP, preventing energy transfer to Eu³⁺. The detection of Mn⁷⁺ at the micromolar level can be achieved. Based on the different responses of TiO₂-ETP in the presence of Mn⁷⁺ and other metal ions, a method is proposed for determining the concentration of Mn⁷⁺.

Figure 10c,e shows the relationship between the fluorescence intensity of TiO₂-ETP and the concentration of Mn⁷⁺ in an aqueous solution. For semiconductor TiO₂-ETP, fluorescence quenching is explained by the efficient electron transition process through annihilation of nonradiative electron-hole recombination. The quenching normally is from the Mn⁷⁺ acceptance of energy from the excited states of TiO₂-ETP. Because there are two excited states corresponding to TiO₂ and ETP, the emissions of TiO₂ and ETP will be quenched by Mn⁷⁺. The Stern–Volmer diagram used to determine the sensitivity of Mn⁷⁺ to TiO₂-ETP is shown in Figure 10d,f. The Mn⁷⁺ concentration is linearly related to the fluorescence intensity. As the concentration of Mn⁷⁺ increases from 0 μmol/L to 1 mmol/L, the position of the fluorescence emission peak does not move, and the fluorescence intensity of TiO₂-ETP gradually decreases. This linear relationship means that the charge transfer mechanism between Mn⁷⁺ and TiO₂-ETP is caused by a dynamic mechanism. Figure 10d shows a graph of the variation of the radiation intensity (I/I₀) of TiO₂ at 464 nm as a function of the concentration of Mn⁷⁺. A linear regression equation is obtained: I/I₀ = 20.7C + 23,570.9 with a correlation coefficient R² equal to 0.99 (n = 14), where I₀ is the TiO₂-ETP radiation intensity at 464 nm, I is the intensity of TiO₂-ETP with different concentration of Mn⁷⁺, and C is the concentration of Mn⁷⁺. Likewise, Figure 10f shows a graph of the variation of the emission intensity (I/I₀) of TiO₂-ETP at 616 nm as a function of the Mn⁷⁺ concentration. The linear regression equation for Mn⁷⁺ is I/I₀ = 13.8C + 2882.3 (R² = 0.98, n = 14).

4. Conclusions

In this study, we have synthesized TiO₂-ETP nanoparticles using a simple solvothermal process. XRD patterns, Raman spectra, and XPS spectra show that ETP is successfully incorporated into TiO₂ nanoparticles. TiO₂-ETP nanoparticles exhibit a higher PL intensity than TiO₂-Eu³⁺ nanoparticles upon excitation at a wavelength of 394 nm. With the increase of Eu³⁺ concentration, the fluorescence intensity of TiO₂-Eu³⁺ at 550–750 nm increases, and the optimal concentration is 4.0 mol%. When the concentration of Eu³⁺ exceeds 4.0 mol%, the fluorescence decreases, indicating that a solubility limit has been reached. TiO₂-ETP overcomes the solubility limit, and realizes a fluorescence increase with increasing ETP concentration. Exploiting the quenching effect of Mn⁷⁺ on the fluorescence intensity of TiO₂-ETP, a simple and efficient Mn⁷⁺ fluorescence sensor was proposed. Unlike the previously reported detection using Eu (TTA)₃Phen or TiO₂, the detection range of the TiO₂-ETP nanomaterial is larger, and the detection accuracy and sensitivity are higher. Experimental results show that the proposed new sensor is practical, can be used to detect real samples, does not exhibit interference with common metal ions, can be used for detection in complex environments, is simple to operate, and has excellent potential for application.