Preparation of Eu0.075Tb0.925-Metal Organic Framework as a Fluorescent Probe and Application in the Detection of Fe3+ and Cr2O72−

Luminescent Ln-MOFs (Eu0.075Tb0.925-MOF) were successfully synthesised through the solvothermal reaction of Tb(NO3)3·6H2O, Eu(NO3)3·6H2O, and the ligand pyromellitic acid. The product was characterised by X-ray diffraction (XRD), TG analysis, EM, X-ray photoelectron spectroscopy (XPS), and luminescence properties, and results show that the synthesised material Eu0.075Tb0.925-MOF has a selective ratio-based fluorescence response to Fe3+ or Cr2O72−. On the basis of the internal filtering effect, the fluorescence detection experiment shows that as the concentration of Fe3+ or Cr2O72− increases, the intensity of the characteristic emission peak at 544 nm of Tb3+ decreases, and the intensity of the characteristic emission peak at 653 nm of Eu3+ increases in Eu0.075Tb0.925-MOF. The fluorescence intensity ratio (I653/I544) has a good linear relationship with the target concentration. The detection linear range for Fe3+ or Cr2O72− is 10–100 μM/L, and the detection limits are 2.71 × 10−7 and 8.72 × 10−7 M, respectively. Compared with the sensor material with a single fluorescence emission, the synthesised material has a higher anti-interference ability. The synthesised Eu0.075Tb0.925-MOF can be used as a highly selective and recyclable sensing material for Fe3+ or Cr2O72−. This material should be an excellent candidate for multifunctional sensors.


Introduction
Heavy metals and inorganic anion pollutants in water pose hidden dangers to human health [1]. The United Nations Sustainable Development Goals set in September 2015 indicated that countries are expected to greatly improve human water quality by 2030. Thus, the detection of pollutants in water has become increasingly important. Fe 3+ is one of the basic trace elements in humans. The lack or excess of this element can cause many physiological disorders, such as nausea, abdominal pain, anaemia, liver cirrhosis, and organ failure [2][3][4]. Salonen et al. [2] confirmed that elevated iron content is an important risk factor for acute myocardial infarction, Bijeh et al. [3] confirmed that the increased risk of cardiovascular disease is related to elevated iron content, and Jehn et al. [4] confirmed that elevated iron could lead to abnormal baseline metabolism. In addition, Cr 2 O 7 2− is an important oxidant in laboratories and industry [5], and it is highly carcinogenic in the environment and harmful to the ecology, environment, and biological system [6][7][8][9]. Mansi et al. [6] confirmed that it is the second most abundant inorganic groundwater pollutant due to its wide application in many industrial fields, such as electroplating chrome, dyes, and leather tanning. Costa [7] confirmed that it is mutagenic and carcinogenic to organisms' sexual function. Therefore, the selective sensing of Fe 3+ and Cr 2 O 7 2− in water quality has attracted growing attention from scholars. Many methods are used for the determination of Fe 3+ and Cr 2 O 7 2− , such as atomic emission spectrometry, atomic absorption spectrometry, inductively coupled plasma mass spectrometry, electrochemical methods, and ion chromatography. However, these methods are complicated to operate, and heated in a 120 °C vacuum drying oven for 48 h and gradually cooled to ambient temperature. After the autoclave was opened, the product was collected after centrifugation, washed thoroughly with DMF and ethanol, paralleled three times, and dried. Thus, the target product Eu0.075Tb0.925-MOF was obtained.

XRD Characterisation
Eu-MOF, Tb-MOF, and Eu0.075Tb0.925-MOF combined with lanthanide nitrate and pyromellitic acid were prepared by the solvothermal method. Figure 1 shows the XRD patterns of Ln-MOFs. As shown in the figure, the 2θ diffraction angle peak positions of the simulated XRD pattern and the synthesised samples Eu-MOF, Tb-MOF, and Eu0.075Tb0.925-MOF are the same, and there are sharp peaks at the diffraction angles from 9 to 10. At the same time, the diffraction peaks 9 to 10 of the crystal synthesised by Silva et al. [36] are basically the same, indicating that the synthesised Eu0.075Tb0.925-MOF has high purity and good crystallinity [36][37][38][39].  Figure 2 shows the TG analysis results of Ln-MOFs. The weight loss of Ln-MOFs is mainly divided into two stages. Before 340 °C, Eu0.075Tb0.925-MOF has good thermal stability.   Figure 2 shows the TG analysis results of Ln-MOFs. The weight loss of Ln-MOFs is mainly divided into two stages. Before 340 • C, Eu 0.075 Tb 0.925 -MOF has good thermal stability. and heated in a 120 °C vacuum drying oven for 48 h and gradually cooled to ambient temperature. After the autoclave was opened, the product was collected after centrifugation, washed thoroughly with DMF and ethanol, paralleled three times, and dried. Thus, the target product Eu0.075Tb0.925-MOF was obtained.

XRD Characterisation
Eu-MOF, Tb-MOF, and Eu0.075Tb0.925-MOF combined with lanthanide nitrate and pyromellitic acid were prepared by the solvothermal method. Figure 1 shows the XRD patterns of Ln-MOFs. As shown in the figure, the 2θ diffraction angle peak positions of the simulated XRD pattern and the synthesised samples Eu-MOF, Tb-MOF, and Eu0.075Tb0.925-MOF are the same, and there are sharp peaks at the diffraction angles from 9 to 10. At the same time, the diffraction peaks 9 to 10 of the crystal synthesised by Silva et al. [36] are basically the same, indicating that the synthesised Eu0.075Tb0.925-MOF has high purity and good crystallinity [36][37][38][39].  Figure 2 shows the TG analysis results of Ln-MOFs. The weight loss of Ln-MOFs is mainly divided into two stages. Before 340 °C, Eu0.075Tb0.925-MOF has good thermal stability.   Figure 3 shows the FTIR spectrum of Ln-MOFs. Compared with the FTIR spectrum of pyromellitic acid, the main characteristic peaks in the FTIR spectrum of Eu 0.075 Tb 0.925 -MOF are similar to those of pyromellitic acid, but the C=O stretching vibration peak disappeared at 1720 cm −1 in the original pyromellitic acid spectrum (significantly weakened), thereby indicating that the carboxyl oxygen is coordinated with Tb and Eu atoms in the ligand. of pyromellitic acid, the main characteristic peaks in the FTIR spectrum of Eu0.075 MOF are similar to those of pyromellitic acid, but the C=O stretching vibration pea appeared at 1720 cm −1 in the original pyromellitic acid spectrum (significantly weak thereby indicating that the carboxyl oxygen is coordinated with Tb and Eu atoms ligand.

Elemental Analysis and XPS
A comparison of elemental (Table 1) and XPS ( Figure 4) analyses shows that, sponding to the content of the element, the distribution ratio of Eu to Tb in Eu0.075 MOF is 0.075:0.925. The specific loadings of Tb(NO3)3 and Eu(NO3)3 are 42.74% and respectively, and the cooling rate is 0.017 K/s.

Elemental Analysis and XPS
A comparison of elemental (Table 1) and XPS ( Figure 4) analyses shows that, corresponding to the content of the element, the distribution ratio of Eu to Tb in Eu 0.075 Tb 0.925 -MOF is 0.075:0.925. The specific loadings of Tb(NO 3 ) 3 and Eu(NO 3 ) 3 are 42.74% and 3.46% respectively, and the cooling rate is 0.017 K/s.  Figure 3 shows the FTIR spectrum of Ln-MOFs. Compared with the FTIR spectrum of pyromellitic acid, the main characteristic peaks in the FTIR spectrum of Eu0.075Tb0.925-MOF are similar to those of pyromellitic acid, but the C=O stretching vibration peak disappeared at 1720 cm −1 in the original pyromellitic acid spectrum (significantly weakened), thereby indicating that the carboxyl oxygen is coordinated with Tb and Eu atoms in the ligand.

Elemental Analysis and XPS
A comparison of elemental (Table 1) and XPS ( Figure 4) analyses shows that, corresponding to the content of the element, the distribution ratio of Eu to Tb in Eu0.075Tb0.925-MOF is 0.075:0.925. The specific loadings of Tb(NO3)3 and Eu(NO3)3 are 42.74% and 3.46% respectively, and the cooling rate is 0.017 K/s.     Figure 5 shows the TEM and SEM images of Eu 0.075 Tb 0.925 -MOF, which indicate that the prepared Eu 0.075 Tb 0.925 -MOF has a regular external morphology, a nanocolumn shape, and a diameter of about 500 nm.  Figure 5 shows the TEM and SEM images of Eu0.075Tb0.925-MOF, which indicate that the prepared Eu0.075Tb0.925-MOF has a regular external morphology, a nanocolumn shape, and a diameter of about 500 nm.  Figure 6 shows the N2 adsorption desorption isotherms of Eu0.075Tb0.925-MOF. The adsorption capacity increases slowly with the increase of pressure at the middle-high-pressure stage, indicating that Eu0.075Tb0.925-MOF is a porous material with an average pore size of 3.38 nm, a BJH average pore diameter of 20.99 nm, and a BET specific surface area of 12.9542 m 2 /g.  Figure 7a shows the fluorescence emission spectrum of Eu0.075Tb0.925-MOF measured at ambient temperature. The figure shows that Eu0.075Tb0.925-MOF exhibits characteristic transitions of Tb 3+ and Eu 3+ under the excitation of 310 nm light, located at 544 and 653 nm respectively, showing the same intensity of fluorescence emission. This finding indicates that the ligand can effectively transfer energy to Tb 3+ and Eu 3+ at the same time [40][41][42][43][44].

Photoluminescence Characteristics
As shown in the CIE diagram in Figure 7c Figure 5 shows the TEM and SEM images of Eu0.075Tb0.925-MOF, which indicate tha the prepared Eu0.075Tb0.925-MOF has a regular external morphology, a nanocolumn shape and a diameter of about 500 nm. 3.6. Adsorption Characteristics of Eu0.075Tb0.925-MOF Figure 6 shows the N2 adsorption desorption isotherms of Eu0.075Tb0.925-MOF. The adsorption capacity increases slowly with the increase of pressure at the middle-high-pressure stage, indicating that Eu0.075Tb0.925-MOF is a porous material with an average pore size of 3.38 nm, a BJH average pore diameter of 20.99 nm, and a BET specific surface area o 12.9542 m 2 /g.  Figure 7a shows the fluorescence emission spectrum of Eu0.075Tb0.925-MOF measured at ambient temperature. The figure shows that Eu0.075Tb0.925-MOF exhibits characteristic transitions of Tb 3+ and Eu 3+ under the excitation of 310 nm light, located at 544 and 653 nm respectively, showing the same intensity of fluorescence emission. This finding indicates that the ligand can effectively transfer energy to Tb 3+ and Eu 3+ at the same time [40][41][42][43][44].

Photoluminescence Characteristics
As shown in the CIE diagram in Figure 7c Figure 7a shows the fluorescence emission spectrum of Eu 0.075 Tb 0.925 -MOF measured at ambient temperature. The figure shows that Eu 0.075 Tb 0.925 -MOF exhibits characteristic transitions of Tb 3+ and Eu 3+ under the excitation of 310 nm light, located at 544 and 653 nm respectively, showing the same intensity of fluorescence emission. This finding indicates that the ligand can effectively transfer energy to Tb 3+ and Eu 3+ at the same time [40][41][42][43][44].

Photoluminescence Characteristics
As shown in the CIE diagram in Figure 7c,d, Eu-MOF shows red fluorescence, and Tb-MOF shows green fluorescence. When Eu 3+ and Tb 3+ synthesise Eu 0.075 Tb 0.925 -MOF at a ratio of 0.075:0.925, Eu 0.075 Tb 0.925 -MOF shows the intermediate colour of the two, which is a yellow-green fluorescence sensitive to the human eye. This material has potential application as a luminescent material and a light-sensitive material for naked-eye detection [45]. a yellow-green fluorescence sensitive to the human eye. This material has potential application as a luminescent material and a light-sensitive material for naked-eye detection [45].

Fluorescence Sensing of Fe 3+
Gao and Ma [46,47] prepared Tb-MOF and used it for sensitive fluorescence sensing of Fe 3+ and Cr2O7 2− . On this basis, this paper designs a ratio fluorescent probe, Eu0.075Tb0.925-MOF, for the fluorescence sensing of Fe 3+ and Cr2O7 2− to improve the measurement accuracy and expand the linear range of the test. To determine the fluorescence performance of Eu0.075Tb0.925-MOF to Fe 3+ , the fluorescence response of Eu0.075Tb0.925-MOF to Fe 3+ was investigated, and the results are shown in Figure 8. Figure 8a shows that with the increase of the Fe 3+ concentration, the characteristic emission peak intensity of Tb 3+ decreases at 544 nm, and the characteristic emission peak intensity of Eu 3+ increases at 653 nm. The intensity at IEu = 653 nm and ITb = 544 nm is used to calculate the intensity change I0/I, where I0 (IEu0/ITb0) is the initial fluorescence intensity before fluorescence, and I (IEu/ITb) is the fluorescence intensity in the presence of Fe 3+ . Figure 8b shows that I0/I and Fe 3+ present a linear relationship in the concentration range of 10-100 μM/L, and the linear regression equation is: The limit of detection (LOD) of Fe 3+ is evaluated by the equation 3Sb/S, where Sb is the standard deviation of repeated detection of the original solution, and S is the slope of the linear fit. The LOD is calculated as 2.71 × 10 −7 M. Figure 8c shows that the colour change trend of Eu0.075Tb0.925-MOF is yellow green-yellow-orange-red with the increase in Fe 3+ concentration. This material is expected to achieve naked-eye detection of Fe 3+ .

Fluorescence Sensing of Fe 3+
Gao and Ma [46,47] prepared Tb-MOF and used it for sensitive fluorescence sensing of Fe 3+ and Cr 2 O 7 2− . On this basis, this paper designs a ratio fluorescent probe, Eu 0.075 Tb 0.925 -MOF, for the fluorescence sensing of Fe 3+ and Cr 2 O 7 2− to improve the measurement accuracy and expand the linear range of the test. To determine the fluorescence performance of Eu 0.075 Tb 0.925 -MOF to Fe 3+ , the fluorescence response of Eu 0.075 Tb 0.925 -MOF to Fe 3+ was investigated, and the results are shown in Figure 8. Figure 8a shows that with the increase of the Fe 3+ concentration, the characteristic emission peak intensity of Tb 3+ decreases at 544 nm, and the characteristic emission peak intensity of Eu 3+ increases at 653 nm. The intensity at I Eu = 653 nm and I Tb = 544 nm is used to calculate the intensity change I 0 /I, where I 0 (I Eu0 /I Tb0 ) is the initial fluorescence intensity before fluorescence, and I (I Eu /I Tb ) is the fluorescence intensity in the presence of Fe 3+ . Figure 8b shows that I 0 /I and Fe 3+ present a linear relationship in the concentration range of 10-100 µM/L, and the linear regression equation is: The limit of detection (LOD) of Fe 3+ is evaluated by the equation 3S b /S, where S b is the standard deviation of repeated detection of the original solution, and S is the slope of the linear fit. The LOD is calculated as 2.71 × 10 −7 M. Figure 8c shows that the colour change trend of Eu 0.075 Tb 0.925 -MOF is yellow green-yellow-orange-red with the increase in Fe 3+ concentration. This material is expected to achieve naked-eye detection of Fe 3+ .
The prepared Eu 0.075 Tb 0.925 -MOF was subjected to a cyclic application experiment, and KNO 3 solution was used to wash the used materials. Figure 8d,e shows that the ratio of the luminous intensity of the material and the XRD did not change considerably, even after five cycles. Eu 0.075 Tb 0.925 -MOF is very stable in the sensing experiment.
yellow-orange-red, which is expected to realise the naked-eye detection of Cr2O7 2− .
A cyclic application experiment was performed on Eu0.075Tb0.925-MOF. Figure 9d shows that the luminous intensity ratio of Eu0.075Tb0.925-MOF does not change much after five cycles. Eu0.075Tb0.925-MOF was very stable in the sensing experiment.

Comparison with Other Sensors That Detect Fe 3+ and Cr2O7 2− Ions
Compared with the Fe 3+ and Cr2O7 2− detection methods used in other studies, as shown in Table 2, the prepared Eu0.075Tb0.925-MOF can reduce the effects of interference caused by excitation light, the environment, and probe concentration changes, and it has improved the detection accuracy relative to other methods.   Figure 9a shows that with the increase in Cr 2 O 7 2− concentration, the characteristic emission peak intensity of Tb 3+ decreases at 544 nm, and the characteristic emission peak and the LOD was 8.72 × 10 −7 M. The CIE diagram in Figure 9c shows that with the increase in Cr 2 O 7 2− concentration, the colour change trend of Eu 0.075 Tb 0.925 -MOF is yellow green-yellow-orange-red, which is expected to realise the naked-eye detection of Cr 2 O 7 2− . A cyclic application experiment was performed on Eu 0.075 Tb 0.925 -MOF. Figure 9d shows that the luminous intensity ratio of Eu 0.075 Tb 0.925 -MOF does not change much after five cycles. Eu 0.075 Tb 0.925 -MOF was very stable in the sensing experiment.
Similarly in an aqueous solution.

Comparison with Other Sensors That Detect Fe 3+ and Cr 2 O 7 2− Ions
Compared with the Fe 3+ and Cr 2 O 7 2− detection methods used in other studies, as shown in Table 2, the prepared Eu 0.075 Tb 0.925 -MOF can reduce the effects of interference caused by excitation light, the environment, and probe concentration changes, and it has improved the detection accuracy relative to other methods.

Mechanism Study
The mechanism of Fe 3+ and Cr 2 O 7 2− on Eu 0.075 Tb 0.925 -MOF fluorescence sensing is examined. Figure 10a shows that the UV absorption spectrum of Fe 3+ overlaps with the excitation spectrum of Eu 0.075 Tb 0.925 -MOF, which indicates that Fe 3+ and Eu 0.075 Tb 0.925 -MOF are competitively adsorbed. At the same time, Figure 4 shows that Fe 3+ is attached to the surface of Eu 0.075 Tb 0.925 -MOF and that the interaction between Fe 3+ and the uncoordinated O atom in the ligand is weak. Eu 0.075 Tb 0.925 -MOF reduces the energy transfer from the ligand to Tb 3+ , and Tb 3+ is quenched. As a result, the energy transfer from the ligand to Eu 3+ is increased, and the characteristic red fluorescence of Eu 3+ is displayed. Figure 10b shows that the UV absorption spectrum of Cr 2 O 7 2− overlaps with the excitation spectrum of Eu 0.075 Tb 0.925 -MOF, which indicates that Cr 2 O 7 2− and Eu 0.075 Tb 0.925 -MOF are competitively adsorbed. It will also cause the energy transfer from the ligand to Eu 3+ to increase and show the characteristic red fluorescence of Eu 3+ . biphenyl R 2 = 0.9927) Cr2O7 2− 1.01 × 10 −5

Mechanism Study
The mechanism of Fe 3+ and Cr2O7 2− on Eu0.075Tb0.925-MOF fluorescence sensing is examined. Figure 10a shows that the UV absorption spectrum of Fe 3+ overlaps with the excitation spectrum of Eu0.075Tb0.925-MOF, which indicates that Fe 3+ and Eu0.075Tb0.925-MOF are competitively adsorbed. At the same time, Figure 4 shows that Fe 3+ is attached to the surface of Eu0.075Tb0.925-MOF and that the interaction between Fe 3+ and the uncoordinated O atom in the ligand is weak. Eu0.075Tb0.925-MOF reduces the energy transfer from the ligand to Tb 3+ , and Tb 3+ is quenched. As a result, the energy transfer from the ligand to Eu 3+ is increased, and the characteristic red fluorescence of Eu 3+ is displayed. Figure 10b shows that the UV absorption spectrum of Cr2O7 2− overlaps with the excitation spectrum of Eu0.075Tb0.925-MOF, which indicates that Cr2O7 2− and Eu0.075Tb0.925-MOF are competitively adsorbed. It will also cause the energy transfer from the ligand to Eu 3+ to increase and show the characteristic red fluorescence of Eu 3+ .

Application in Actual Water Sample Analysis
The ratio fluorescent probe Eu0.075Tb0.925-MOF was used for Fe 3+ and Cr2O7 2− in tap water. The results are shown in Table 3. The sample recovery rate is 101-114%, thereby

Application in Actual Water Sample Analysis
The ratio fluorescent probe Eu 0.075 Tb 0.925 -MOF was used for Fe 3+ and Cr 2 O 7 2− in tap water. The results are shown in Table 3. The sample recovery rate is 101-114%, thereby showing that the established method has high accuracy and precision for the determination of Fe 3+ and Cr 2 O 7 2− content in actual samples.

Conclusions
The ratio fluorescent probe Eu 0.075 Tb 0.925 -MOF was synthesised in this experiment by using the solvothermal method and was used for Fe 3+ and Cr 2 O 7 2− determination. Mainly on the basis of the internal filtering effect, the characteristic fluorescence emission peak intensity of Tb 3+ decreased, and the characteristic emission peak intensity of Eu 3+ increased on Eu 0.075 Tb 0.925 -MOF as the concentration of Fe 3+ and Cr 2 O 7 2− increased. The ratio of the emission fluorescence intensity at the two wavelengths has a linear relationship with the target concentration, which realises the selective detection of Fe 3+ and Cr 2 O 7 2− . The linear detection range was 10-100 µM, and the LOD was 2.71 × 10 −7 and 8.72 × 10 −7 M, respectively. The synthesised material was used as a ratio fluorescent probe, which can effectively eliminate background fluorescence interference in the detection process and improve the detection accuracy. The trend of the fluorescence colour change of the synthesised material during the detection process indicates that the material is expected to realise naked-eye detection of Fe 3+