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

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.


Introduction
Manganese has two primary valence and oxidation states, namely, Mn 2+ and Mn 7+ , which have different effects in practice [1,2]. Mn 7+ 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, Mn 7+ 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, Mn 7+ has attracted a lot of attention among pollutants in recent years, and the detection of Mn 7+ 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 Mn 7+ . Monitoring Mn 7+ 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 Mn 7+ at low concentration is complicated and requires pretreatment steps [10]. Therefore, it is necessary 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. igure 1. Schematic illustration of TiO2 nanoparticles modified with Eu(TTA)3Phen preparation and sensing mechanism f manganese ion concentration.
As shown in Figure 2, TiO2, TiO2 modified with Eu 3+ (TiO2-Eu 3+ ), 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.  3 Phen preparation and sensing mechanism of manganese ion concentration.
As shown in Figure 2, TiO 2 , TiO 2 modified with Eu 3+ (TiO 2 -Eu 3+ ), and TiO 2 modified with Eu (TTA) 3 Phen (ETP) (TiO 2 -ETP) were prepared using the solvothermal method. Tetrabutyl titanate (TBT) was used as a precursor. Ethanol (CH 3 CH 2 OH) and acetic acid (CH 3 COOH) 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, EuCl 3 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 TiO 2 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, Mn 7+ 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. The quenching experiments using metal ions were performed by adding TiO 2 -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, Mn 7+ 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.

Morphological Structures
The additions of Eu 3+ and ETP into TiO 2 change the shape and size of TiO 2 nanoparticles. Figure 3 shows typical transmission electron microscopy (TEM) images of TiO 2 nanoparticles. TiO 2 nanoparticles with spherical morphology can be seen in TEM images ( Figure 3a). The morphology of TiO 2 nanoparticles with Eu 3+ varies from spherical to ellipsoidal shapes (Figure 3b). The TiO 2 nanoparticles modified with ETP have a cuboid shape (Figure 3c). These changes are similar to the previous report [35]. Eu 3+ and ETPdoped TiO 2 cause different shapes of TiO 2 -Eu 3+ and TiO 2 -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 TiO 2 -Eu 3+ is between 10 and 40 nm. Compared to TiO 2 , the average size of TiO 2 -ETP nanoparticles decreases, which suggests that the inclusion of ETP largely suppresses the growth of TiO 2 nanoparticles. This size change of TiO 2 -Eu 3+ and TiO 2 -ETP nanoparticles can also relate to crystalline structures described later, based on X-ray diffraction analyses [37]. The growth of TiO 2 -Eu 3+ crystals is hindered by the formation of Eu-O-Ti bond in the crystal void of TiO 2 -Eu 3+ nanoparticles. The decrease of particle size of ETP-modified TiO 2 is mainly caused by ETP entering the lattice and binding with oxygen. Due to internal stress in the crystal lattice, the diffusion of Ti 4+ and O 2and the obstacle of crystal migration, the crystal growth at the boundary is retarded [38].

Morphological Structures
The additions of Eu 3+ 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 Eu 3+ 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]. Eu 3+ and ETPdoped TiO2 cause different shapes of TiO2-Eu 3+ 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-Eu 3+ 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-Eu 3+ and TiO2-ETP nanoparticles can also relate to crystalline structures described later, based on X-ray diffraction analyses [37]. The growth of TiO2-Eu 3+ crystals is hindered by the formation of Eu-O-Ti bond in the crystal void of TiO2-Eu 3+ 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 Ti 4+ and O 2-and the obstacle of crystal migration, the crystal growth at the boundary is retarded [38].

Crystalline Structure
Modification with Eu 3+ can effectively change the crystal structure and inhibit grain growth of TiO 2 nanoparticles. This effect is more pronounced when the organic complex (ETP) is used. Figure 4 shows the diffraction patterns of TiO 2 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 TiO 2 phase [39]. The XRD shows that TiO 2 -Eu 3+ and TiO 2 -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 TiO 2 (JCPDS NO. . 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 TiO 2 -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 TiO 2 -ETP compared to the TiO 2 and TiO 2 -Eu 3+ nanoparticles, indicating that the crystallinity decreased [41]. When ETP is added to TiO 2 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 TiO 2 -ETP has a smaller particle size than TiO 2 and TiO 2 -Eu 3+ . 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), 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.

Crystalline Structure
Modification with Eu 3+ 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-Eu 3+ 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. . 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-Eu 3+ 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-Eu 3+ . 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) where 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 1.5406Å), β is the full diffraction width at half maximum, measured in radians at 2θ Peak.    Figure 5 shows the Raman spectra of the obtained TiO 2 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 TiO 2 nanoparticles. The inclusion of ETP in TiO 2 -ETP nanoparticles changes features of the crystal structure of TiO 2 , 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 TiO 2 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 TiO 2 -Eu 3+ [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 TiO 2 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 TiO 2 -ETP nanoparticles have local lattice defects, the Raman peak becomes weaker and broader, which means that the crystallinity of synthesized nanoparticles decreases.   Figure 5 shows the Raman spectra of the obtained TiO2 nanoparticles. The Raman peaks at 143, 395, 514, and 639 cm −1 correspond to Eg, B1g, A1g, or B1g, and Eg of the anatase phase, respectively [42]. The most dominant Eg 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 Eg 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-Eu 3+ [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 Eg 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.

Confirmation of Eu 3+ in TiO 2
X-ray photoelectron spectroscopy (XPS) was used for elemental analysis of ETPmodified titanium dioxide nanoparticles. Figure 6A(a-c) shows the survey XPS spectra of TiO 2 , TiO 2 -Eu 3+ , and TiO 2 -ETP, respectively. The XPS spectra in Figure 6B clearly shows the changes of the binding energy of the Ti2p electron orbital in TiO 2 , TiO 2 -Eu 3+ , and TiO 2 -ETP in which the binding energies in TiO 2 , TiO 2 -Eu 3+ and TiO 2 -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 Eu 3+ and ETP inserting between crystal planes. The much larger decrease of binding energy in TiO 2 -ETP is due to the larger TTA and Phen ligands carried by Eu 3+ . Figure 6C shows spectra of Eu3d with significantly higher intensity for TiO 2 -ETP than for TiO 2 -Eu 3+ , indicating that the TTA and Phen ligands tightly bind the Eu. At the same time, the binding energy of Eu3d in TiO 2 -ETP is slightly lower than that in TiO 2 -Eu 3+ , which is also due to the stronger interaction of ligands with the Eu3d electron orbital [46]. Figure 6D Figure 6F)

Confirmation of Eu 3+ 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-Eu 3+ , 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-Eu 3+ , and TiO2-ETP in which the binding energies in TiO2, TiO2-Eu 3+ 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 Eu 3+ 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 Eu 3+ . Figure 6C shows spectra of Eu3d with significantly higher intensity for TiO2-ETP than for TiO2-Eu 3+ , 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-Eu 3+ , 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-Eu 3+ , and TiO2-ETP, showing that Ti-O and Eu-O have almost the same binding energies in TiO2-Eu 3+ and TiO2-ETP. The binding energy of Ti-O in TiO2 is higher ( Figure  6D) than the corresponding binding energy in the TiO2-Eu 3+ ( Figure 6E) and TiO2-ETP ( Figure 6F (Figure 7a), the curves of TiO 2 -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 TiO 2 absorbs ultraviolet light and releases it into the conduction band and defect state energy level of TiO 2 . Because the excited state of Eu 3+ is lower than the conduction band and defect state, the energy is transferred to Eu 3+ [50]. UV-visible spectra shown in Figure 7b show that modification with ETP shifted the TiO 2 absorption edge from the UV to the visible region. This means Eu 3+ and ETP doping produce defects in the TiO 2 host crystal, and thus these defects result in band gap decrease [51][52][53].

UV Absorption and Bandgap of TiO2
Figure 7a,c show the UV-visible absorption curves of TiO2-Eu 3+ and TiO2-ETP. Compared with the curves of TiO2-Eu 3+ (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 Eu 3+ is lower than the conduction band and defect state, the energy is transferred to Eu 3+ [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 Eu 3+ and ETP doping produce defects in the TiO2 host crystal, and thus these defects result in band gap decrease [51][52][53].   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 TiO 2 -Eu 3+ are 3.08 eV, 3.05 eV, 3.06, and 3.09 eV for TiO 2 modified by 2, 4, 6, and 8 mol% Eu 3+ , respectively. The bandgap energies for TiO 2 -ETP are 2.72 eV, 2.40 eV, 2.30 eV, and 2.26 eV for TiO 2 modified by 2, 4, 6, and 8 mol% ETP, respectively. Compared with the band gap of 3.2 eV of TiO 2 , the band gaps of TiO 2 -Eu 3+ and TiO 2 -ETP are decreased. The bandgap decrease is caused by interactions between TiO 2 host and dopants, either Eu 3+ or ETP in TiO 2 -Eu 3+ and TiO 2 -ETP, respectively. Based on the results of XRD to indicating the (101) crystal plane distance extension, and the XPS to confirm the interactions between Eu 3+ -O and the binding energy changes of Eu3d and Ti2p in Eu 3+ and ETP, we conclude that Eu 3+ and ETP as dopants have interacted with TiO 2 in different ways. Thus, these changes can be attributed to the "solubility limit" of Eu 3+ and ETP in TiO 2 host. The former Eu 3+ is from EuCl 3 ·6H 2 O in which both the Cl − counter ion and the bound H 2 O molecules affect the solubility of Eu 3+ . 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 TiO 2 host to change the electron transition bandgap [54]. These interactions also affect the fluorescence behaviors as shown in Figure 8.

Photoluminescence Analysis
The luminescence mechanism of Eu 3+ -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 Eu 3+ , which can subsequently emit a photon when the 5 D 0 transitions to the 7 F J configurations. The luminescence of TiO 2 is due to the electron transition between the valence band and the conduction band. Figure 8a shows the excitation spectra of TiO 2 -Eu 3+ and TiO 2 -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 Eu 3+ from 7 F 0 . The excitation spectrum consists of sharp lines at 384, 394, 418, and 464 nm, assigned to the 7 F 0 → 5 L 7 , 7 F 0 → 5 L 6 , 7 F 0 → 5 D 3 , and 7 F 0 → 5 D 2 transitions of Eu 3+ [55]. Strong peaks at 394 nm and 464 nm correspond to the 7 F 0 → 5 L 6 and 7 F 0 → 5 D 2 Eu 3+ transitions. The intensity of the excitation spectrum of TiO 2 -ETP is higher than that of TiO 2 -Eu 3+ . The organic ligands in ETP help absorb more ultraviolet light. Figure 8b shows the emission spectra of TiO 2 -Eu 3+ and TiO 2 -ETP. When excited at a wavelength of 394 nm, the emission spectrum consists of 5 D 0 → 7 F J (J = 0, 1, 2, 3, 4) (578, 592, 612, 652, and 703 nm) Eu 3+ transitions. Due to the allowable electric dipole of the 5 D 0 → 7 F 2 transition, the strongest emission is produced at 612 nm, which is red. Figure 8c shows the emission spectra of TiO 2 -Eu 3+ , prepared with different concentrations of Eu 3+ (2, 4, 6, and 8 mol%). The influence of concentration on PL intensity is shown in Figure 8e. The optimal concentration of Eu 3+ is 4% [39]. When the concentration exceeds 4%, the fluorescence of TiO 2 -Eu 3+ nanoparticles decreases. This suggests that 4% Eu 3+ concentration is the upper solubility limit in the TiO 2 host. However, the fluorescence intensity of TiO 2 -ETP increases with increasing concentrations of ETP. This indicates that the organic ligands in ETP improve the solubility of ETP in the TiO 2 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, Eu 3+ and ETP were successfully incorporated into TiO 2 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 TiO 2 nanoparticles, and the energy enters the defect state. Energy is then transferred to the Eu in the ETP, realizing the energy transfer process from TiO 2 to Eu. Because the energy level of the emission state of Eu 3+ is lower than the energy level of the defect in TiO 2 nanoparticles, the energy is transferred from the defect state of TiO 2 to the crystal field state of Eu 3+ 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 Eu 3+ . Figure 9 shows the fluorescence lifetime diagram of TiO 2 -Eu 3+ and TiO 2 -ETP. The fluorescence attenuation of TiO 2 -ETP is slower than that of TiO 2 -Eu 3+ , and the quantum yield of TiO 2 -ETP is higher than that of TiO 2 -Eu 3+ . The fluorescence lifetime of TiO 2 -ETP and TiO 2 -Eu 3+ were 0.51 ms and 0.39 ms, and the quantum yields of TiO 2 -ETP and TiO 2 -Eu 3+ were 10% and 5%.

Photoluminescence Analysis
The luminescence mechanism of Eu 3+ -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 transi-

Fluorescence Spectra of TiO2-ETP in the Presence of Metal Ions
Eu 3+ 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 Zn 2+ , Mn 3+ , K + , Mn 7+ , Fe 2+ , Mg 2+ , Ca 2+ , and Co 2+ 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 Eu 3+ complex in different hosts, showing that the tested organic molecules exhibit no quenching effect [40,62].

Fluorescence Spectra of TiO 2 -ETP in the Presence of Metal Ions
Eu 3+ 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 Zn 2+ , Mn 3+ , K + , Mn 7+ , Fe 2+ , Mg 2+ , Ca 2+ , and Co 2+ are selected to determine whether these metal ions will affect the fluorescence properties of TiO 2 -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 Eu 3+ complex in different hosts, showing that the tested organic molecules exhibit no quenching effect [40,62].
As shown in Figure 10a, TiO 2 -ETP shows a strong fluorescence peak located at 464 and 616 nm with excitation at λ ex = 394 nm. The fluorescence of TiO 2 -ETP is influenced by the addition of Mn 7+ , where a significant quenching effect can be observed. The fluorescence intensity of ETP-modified TiO 2 nanoparticles decreases with the increase of Mn 7+ concentration in the solution. Figure 10b shows the ratio (I/I 0 ) of the fluorescence intensity of TiO 2 -ETP in an aqueous solution containing no metal ions and a solution containing a single metal ion. I 0 is the fluorescence intensity of TiO 2 -ETP in the absence of metal ions at 464 nm and 616 nm, and I is the fluorescence intensity of TiO 2 -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 Mn 7+ will not significantly reduce the fluorescence intensity of TiO 2 -ETP. The aqueous solution containing Mn 7+ will cause fluorescence quenching of TiO 2 -ETP. The decrease in fluorescence intensity can also be detected by adding TiO 2 -ETP to an aqueous solution containing a small amount of Mn 7+ . Experiments show that when other ions are present, only manganese will quench the Eu 3+ fluorescence. The possible mechanism of quenching can be either the absorption of photons by Mn 7+ , or the Mn 7+ excimer formation by interaction with the excited state of with ETP, preventing energy transfer to Eu 3+ . The detection of Mn 7+ at the micromolar level can be achieved. Based on the different responses of TiO 2 -ETP in the presence of Mn 7+ and other metal ions, a method is proposed for determining the concentration of Mn 7+ .  As shown in Figure 10a, TiO2-ETP shows a strong fluorescence peak located at 464 and 616 nm with excitation at λex = 394 nm. The fluorescence of TiO2-ETP is influenced by the addition of Mn 7+ , where a significant quenching effect can be observed. The fluorescence intensity of ETP-modified TiO2 nanoparticles decreases with the increase of Mn 7+ 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 Mn 7+ will not significantly reduce the fluorescence intensity of TiO2-ETP. The aqueous solution containing Mn 7+ 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 Mn 7+ . Experiments show that when other ions are present, only manganese will quench the Eu 3+ fluorescence. The possible mechanism of quenching can be either the absorption of photons by Mn 7+ , or the Mn 7+ excimer formation by interaction with the excited state of with ETP, preventing energy transfer to Eu 3+ . The detection of Mn 7+ at the micromolar level can be achieved. Based on  Figure 10c,e shows the relationship between the fluorescence intensity of TiO 2 -ETP and the concentration of Mn 7+ in an aqueous solution. For semiconductor TiO 2 -ETP, fluorescence quenching is explained by the efficient electron transition process through annihilation of nonradiative electron-hole recombination. The quenching normally is from the Mn 7+ acceptance of energy from the excited states of TiO 2 -ETP. Because there are two excited states corresponding to TiO 2 and ETP, the emissions of TiO 2 and ETP will be quenched by Mn 7+ . The Stern-Volmer diagram used to determine the sensitivity of Mn 7+ to TiO 2 -ETP is shown in Figure 10d,f. The Mn 7+ concentration is linearly related to the fluorescence intensity. As the concentration of Mn 7+ increases from 0 µmol/L to 1 mmol/L, the position of the fluorescence emission peak does not move, and the fluorescence intensity of TiO 2 -ETP gradually decreases. This linear relationship means that the charge transfer mechanism between Mn 7+ and TiO 2 -ETP is caused by a dynamic mechanism. Figure 10d shows a graph of the variation of the radiation intensity (I/I 0 ) of TiO 2 at 464 nm as a function of the concentration of Mn 7+ . A linear regression equation is obtained: I/I 0 = 20.7C + 23,570.9 with a correlation coefficient R 2 equal to 0.99 (n = 14), where I 0 is the TiO 2 -ETP radiation intensity at 464 nm, I is the intensity of TiO 2 -ETP with different concentration of Mn 7+ , and C is the concentration of Mn 7+ . Likewise, Figure 10f shows a graph of the variation of the emission intensity (I/I 0 ) of TiO 2 -ETP at 616 nm as a function of the Mn 7+ concentration. The linear regression equation for Mn 7+ is I/I 0 = 13.8C + 2882.3 (R 2 = 0.98, n = 14).

Conclusions
In this study, we have synthesized TiO 2 -ETP nanoparticles using a simple solvothermal process. XRD patterns, Raman spectra, and XPS spectra show that ETP is successfully incorporated into TiO 2 nanoparticles. TiO 2 -ETP nanoparticles exhibit a higher PL intensity than TiO 2 -Eu 3+ nanoparticles upon excitation at a wavelength of 394 nm. With the increase of Eu 3+ concentration, the fluorescence intensity of TiO 2 -Eu 3+ at 550-750 nm increases, and the optimal concentration is 4.0 mol%. When the concentration of Eu 3+ exceeds 4.0 mol%, the fluorescence decreases, indicating that a solubility limit has been reached. TiO 2 -ETP overcomes the solubility limit, and realizes a fluorescence increase with increasing ETP concentration. Exploiting the quenching effect of Mn 7+ on the fluorescence intensity of TiO 2 -ETP, a simple and efficient Mn 7+ fluorescence sensor was proposed. Unlike the previously reported detection using Eu (TTA) 3 Phen or TiO 2 , the detection range of the TiO 2 -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.  Data Availability Statement: All data, models, and code generated or used during the study appear in the submitted article.

Conflicts of Interest:
The authors declare no conflict of interest.