Polyamidoamine Dendrimers Functionalized Water-Stable Metal–Organic Frameworks for Sensitive Fluorescent Detection of Heavy Metal Ions in Aqueous Solution

In this work, polyamidoamine (PAMAM)-functionalized water-stable Al-based metal–organic frameworks (MIL-53(Al)-NH2) were proposed with enhanced fluorescence intensity, and used for the sensitive detection of heavy metal ions in aqueous solution. The size and morphology of MIL-53(Al)-NH2 were effectively optimized by regulating the component of the reaction solvents. PAMAM dendrimers were subsequently grafted onto the surface with glutaraldehyde as a cross-linking agent. It was found that the size and morphology of MIL-53(Al)-NH2 have great influence on their fluorescence properties, and PAMAM grafting could distinctly further improve their fluorescence intensity. With higher fluorescence intensity, the PAMAM-grafted MIL-53(Al)-NH2 showed good linearity (R2 = 0.9925–0.9990) and satisfactory sensitivity (LOD = 1.1–8.6 μmol) in heavy metal ions determination. Fluorescence enhancement and heavy metal ions detection mechanisms were discussed following the experimental results. Furthermore, analogous water-stable Materials of Institute Lavoisier (MIL) metal–organic frameworks such as MIL-53(Fe)-NH2 were also proved to have similar fluorescence enhancement performance after PAMAM modification, which demonstrates the universality of the method and the great application prospects in the design of PAMAM-functionalized high-sensitivity fluorescence sensors.


Introduction
With the rapid development of industrialization, heavy metal contamination has prompted great concern all over the world. Compared to organic pollution, toxic heavy metal ions cannot be biodegraded and thus accumulate in surface water, soil environment and food. Over the past few decades, humans have been negatively affected by heavy metal ions due to their continuous enrichment [1]. Therefore, trace detection of heavy metal ions is of great significance.
Up to now, various technologies have been validated for the sensitive detection of heavy metal ions, including inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectroscopy (ICP-OES), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and atomic absorption spectroscopy (AAS) [2][3][4][5]. However, these analytical technologies have many limitations such as being time consuming, complex procedures, requiring bulky instrumentation and professional expertise, which fail to meet the requirements of rapid and on-line detection [6,7]. Whereas, with the distinct merits of high sensitivity, economy, simplicity and real-time monitoring,  [29]. Firstly, 10 mL ethylenediamine and 55.6 g methyl acrylate were dissolved in 100 mL methanol solvent, and the Michael addition reaction was carried out at room temperature for 24 h. A half-generation PAMAM dendrimer (0.5G PAMAM) was obtained by subsequent rotary evaporation purification under the condition of 300 mbar and 60 • C. Subsequently, 54.0 g 0.5G PAMAM and 150 mL methanol solvent were placed into a flask, excess ethylenediamine was added and stirred overnight to obtain 1.0G PAMAM. Obviously, PAMAM dendrimers in different generations could be obtained by repeating the steps above. (Element analysis results are shown in Table S1.)

Synthesis of MIL MOFs
The MIL MOFs materials were prepared by solvent thermal method, which links the organic metal center with the NH 2 -BDC regent [30]. Take MIL-53(Al)-NH 2 for example: 0.76 g AlCl 3 ·6H 2 O and 0.56 g NH 2 -BDC were dissolved in 30 mL solvents. The mixture was placed in a 50 mL Teflon-lined autoclave and kept at 150 • C for 24 h. The resulting products were separated from the reaction mixture by centrifugation and washed by DMF and methanol successively. Finally, the obtained products were dried at 70 ºC in vacuum overnight. To obtain MIL-53(Al)-NH 2 in various morphologies, deionized water was added into DMF solvents in the proportion of 0%, 25%, 50% and 75%, respectively. Other MIL MOFs (MIL-53(Fe)-NH 2 ) were synthesized by the same method [31], which is specified in the Supplementary Materials.

Preparation of PAMAM-Grafted MOFs Materials
As for the preparation of PAMAM-grafted MOFs materials, 300 mg MIL MOFs were dispersed in 100 mL methanol solvent containing 5% glutaraldehyde, and the reaction was carried out at room temperature for 3 h. The resulting glutaraldehyde-grafted MOFs were washed three times with the reaction solvent. Afterward, the obtained products were treated with 1 mL PAMAM dendrimers in 30 mL methanol solvent. After carrying out the reaction for 2 h at room temperature, the resulting PAMAM-grafted MOFs materials were fully washed with methanol and dried in vacuum overnight. To explore the effect of PAMAM generations on the fluorescence properties of MOFs materials, 1.0G PAMAM and 2.0G PAMAM were utilized for the modification of MIL-53(Al)-NH 2

Characterization
The sizes and morphologies of the MOFs materials were studied using scanning elec tron microscopy (Zeiss Sigma 300, Oberkochen, Germany). The X-ray diffraction (XRD

Characterization
The sizes and morphologies of the MOFs materials were studied using scanning electron microscopy (Zeiss Sigma 300, Oberkochen, Germany). The X-ray diffraction (XRD) patterns were obtained by a diffractometer (Ultima IV, Tokyo, Japan). Fourier transform infrared (FTIR) spectra were collected on an IR spectrophotometer (ThermoFisher iN 10, Waltham, MA, USA). The valence states of contained elements and surface metal ions of the MOFs were affirmed with an X-ray photoelectron spectroscopy (XPS) instrument (Thermo Scientific K-Alpha, Waltham, MA, USA). EDS-mapping analysis of the quenched MOFs was evaluated by a transmission electron microscope (EOL JEM 2100F, Tokyo, Japan). All fluorescence tests were performed on a fluorescence spectrophotometer (Hitachi F-7100, Tokyo, Japan).

Luminescent Measurements
In order to investigate the influence of morphology, PAMAM dendrimers, pH and concentrations on the fluorescence properties of the PAMAM-grafted MOFs materials, fluorescent determination experiments were conducted at room temperature with 370 nm as the excitation wavelength. The experiment was carried out by controlling a single variable method, concentrations of the synthesized materials varied from 0.01 mg mL −1 to 1 mg mL −1 , and the pH of the solutions was adjusted from 4 to 10.
To evaluate the sensitivity and selectivity of the PAMAM-grafted MOFs materials to metal ions, selected ions (Pb 2+ , Cu 2+ , Fe 3+ , Co 2+ , Zn 2+ , K + , Na + , Mg 2+ ) were added to 1 mg mL −1 MIL-53(Al)-2.0G PAMAM solutions. The pH of solutions was adjusted to 7 by 0.1 mM HCl and 0.1 mM NaOH. After incubation for 5 min, fluorescence intensities for the mixture were measured under the excitation wavelength of 370 nm.

Morphology and Structure of MIL-53(Al)-NH 2
It is well known that the composition of a solvent has a big impact on the solvothermal reaction. In this work, four different MIL-53(Al)-NH 2 materials were prepared in mixed solvents with different DMF and deionized water ratios. The morphologies of the obtained products were investigated by scanning electron microscopy (SEM). According to the results in Figure 2, MIL-53(Al)-NH 2 prepared without deionized water (0% H 2 O) consisted of aggregated granular particles (Figure 2A,E). With the increasing proportion of water, particle size decreased gradually and the morphology became irregular ( Figure 2B-D,F-H), indicating that the water proportion has a great impact on the morphology and particle size of the MIL-53(Al)-NH 2 . These results could be interpreted as different metal coordination capacities between DMF and the water solvent [32].
Polymers 2023, 15, x FOR PEER REVIEW 5 of 13 and particle size of the MIL-53(Al)-NH2. These results could be interpreted as different metal coordination capacities between DMF and the water solvent [32]. In addition to morphology characteristics, the crystal structure of MIL-53(Al)-NH2 products were characterized by powder X-ray diffraction (PXRD) analysis. The XRD patterns of the materials obtained in different solvents are shown in Figure 3A. The results showed that the crystal structures of the MIL-53(Al)-NH2 products were identical despite the different reaction solvents. Therefore, we can assume that the solvent composition has little impact on the crystal structure of the products. Furthermore, the XRD spectra were In addition to morphology characteristics, the crystal structure of MIL-53(Al)-NH 2 products were characterized by powder X-ray diffraction (PXRD) analysis. The XRD  Figure 3A. The results showed that the crystal structures of the MIL-53(Al)-NH 2 products were identical despite the different reaction solvents. Therefore, we can assume that the solvent composition has little impact on the crystal structure of the products. Furthermore, the XRD spectra were in good agreement with previous research [33], which implies that the MIL-53(Al)-NH 2 MOFs were successfully synthesized. In addition to morphology characteristics, the crystal structure of MIL-53(Al)-NH2 products were characterized by powder X-ray diffraction (PXRD) analysis. The XRD patterns of the materials obtained in different solvents are shown in Figure 3A. The results showed that the crystal structures of the MIL-53(Al)-NH2 products were identical despite the different reaction solvents. Therefore, we can assume that the solvent composition has little impact on the crystal structure of the products. Furthermore, the XRD spectra were in good agreement with previous research [33], which implies that the MIL-53(Al)-NH2 MOFs were successfully synthesized. Finally, FITR spectra of the MIL-53(Al)-NH2 materials were collected by an FTIR spectroscope from 4000-400 cm −1 ( Figure 3B). As the results show in Figure 3B, two obvious peaks at 3482 and 3376 cm −1 corresponded to symmetric and asymmetric stretching vibrations of the N-H bond, evidence that the -NH2 had been well introduced after the coordination reaction between BDC-NH2 and Al 3+ , which contributed to the solubility and reactivity of MIL-53(Al)-NH2. The adsorption band at 1669 cm −1 resulted from the vibration of C = O and the band at 1260 cm −1 was attributed to the adsorption of the C-N bond, indicating that NH2-BDC was linked into the framework of the NH2-MIL-53(Al) nanoplates. The wide peak at 1120-1000 cm −1 was related to Al-O, and further confirmed that the O atoms within NH2-BDC had been linked to Al 3+ , and the MIL-53(Al)-NH2 framework had been established [7]. In addition, the spectra of the MOFs materials synthesized in four different solvents were identical, from which it could be concluded that the reaction solvent has little effect on the composition of a MOFs material. Finally, FITR spectra of the MIL-53(Al)-NH 2 materials were collected by an FTIR spectroscope from 4000-400 cm −1 ( Figure 3B). As the results show in Figure 3B, two obvious peaks at 3482 and 3376 cm −1 corresponded to symmetric and asymmetric stretching vibrations of the N-H bond, evidence that the -NH 2 had been well introduced after the coordination reaction between BDC-NH 2 and Al 3+ , which contributed to the solubility and reactivity of MIL-53(Al)-NH 2 . The adsorption band at 1669 cm −1 resulted from the vibration of C=O and the band at 1260 cm −1 was attributed to the adsorption of the C-N bond, indicating that NH 2 -BDC was linked into the framework of the NH 2 -MIL-53(Al) nanoplates. The wide peak at 1120-1000 cm −1 was related to Al-O, and further confirmed that the O atoms within NH 2 -BDC had been linked to Al 3+ , and the MIL-53(Al)-NH 2 framework had been established [7]. In addition, the spectra of the MOFs materials synthesized in four different solvents were identical, from which it could be concluded that the reaction solvent has little effect on the composition of a MOFs material.
According to the above characterization results, the composition of the reaction solvent has a great influence on the size and morphology of MIL-53(Al)-NH 2 materials, but does not affect their crystal structure and surface functional groups.

Characterization of PAMAM-Grafted MOFs Materials
In this work, PAMAM dendrimers were utilized for the surface modification of the prepared MIL-53(Al)-NH 2 materials. Functionalization with PAMAM dendrimers greatly changed the surface groups of MIL-53(Al)-NH 2 materials, which was directly reflected in the XPS analysis. N1s spectra of MIL-53(Al)-NH 2 and MIL-53(Al)-2.0G PAMAM are shown in Figure 4. The observed 399.3 eV peak in raw MIL-53(Al)-NH 2 materials ( Figure 4A) was attributed to free amine groups [34]. After grafting with PAMAM dendrimers, two additional peaks at a higher binding energy (400.8 eV) and lower energy (398.6 eV) were observed in MIL-53(Al)-2.0G PAMAM ( Figure 4B), which could be assigned to amide bond and Schiff base structure [35,36] in PAMAM, respectively. The obtained results indicated that PAMAM dendrimers have been grafted onto MIL-53(Al)-NH 2 materials successfully.

Fluorescence Properties
The fluorescence properties of raw MIL-53(Al)-NH 2  changed the surface groups of MIL-53(Al)-NH2 materials, which was directly reflected in the XPS analysis. N1s spectra of MIL-53(Al)-NH2 and MIL-53(Al)-2.0G PAMAM are shown in Figure 4. The observed 399.3 eV peak in raw MIL-53(Al)-NH2 materials ( Figure  4A) was attributed to free amine groups [34]. After grafting with PAMAM dendrimers, two additional peaks at a higher binding energy (400.8 eV) and lower energy (398.6 eV) were observed in MIL-53(Al)-2.0G PAMAM ( Figure 4B), which could be assigned to amide bond and Schiff base structure [35,36] in PAMAM, respectively. The obtained results indicated that PAMAM dendrimers have been grafted onto MIL-53(Al)-NH2 materials successfully.

Fluorescence Properties
The fluorescence properties of raw MIL-53(Al)-NH2 in different morphologies and of MIL-53(Al)-NH2 grafted with PAMAM in different generations (MIL-53(Al)-1.0G PA-MAM and MIL-53(Al)-2.0G PAMAM) were determined. All of the materials have an obvious fluorescence emission around 440 nm under excitation at 370 nm. In order to obtain the best fluorescence detection conditions, the influences of pH and concentration were also investigated.

Influence of Morphology
According to the above discussion, MIL-53(Al)-NH2 in different morphologies were synthesized, due to the different metal coordination capacities between DMF and the water solvent. To investigate the effect of morphology on fluorescence properties, emission spectra of the MIL-53(Al)-NH2 prepared in different solvents (0% H2O, 25% H2O, 50% H2O, 75% H2O) were characterized under the same condition. As the results show in Figure 5A, the fluorescence intensity of different materials exhibited the same trend with the increase of excitation wavelength, but the intensity was different. The similar emission spectrum could be considered as part of the same mechanism that resulted in electric transitions relating to the surface states. Different fluorescence intensities suggested that the fluorescence intensities of MIL-53(Al)-NH2 MOFs were affected by their reaction solvents in the synthesis process, probably due to the difference in size and morphology of the obtained products [37]. Because of the maximum fluorescence intensity, the MIL-53(Al)-NH2 synthesized in pure DMF (0% H2O) was selected for further modification.  Figure 5A, the fluorescence intensity of different materials exhibited the same trend with the increase of excitation wavelength, but the intensity was different. The similar emission spectrum could be considered as part of the same mechanism that resulted in electric transitions relating to the surface states. Different fluorescence intensities suggested that the fluorescence intensities of MIL-53(Al)-NH 2 MOFs were affected by their reaction solvents in the synthesis process, probably due to the difference in size and morphology of the obtained products [37]. Because of the maximum fluorescence intensity, the MIL-53(Al)-NH 2 synthesized in pure DMF (0% H 2 O) was selected for further modification.

Influence of PAMAM Dendrimers
To investigate the effect of PAMAM dendrimers, the fluorescence properties of MIL-53(Al)-NH 2 , MIL-53(Al)-1.0G PAMAM and MIL-53(Al)-2.0G PAMAM were determined at the same time, and the result is shown in Figure 5B. Compared to MIL-53(Al)-NH 2 , the fluorescence intensity of MIL-53(Al)-1.0G PAMAM and MIL-53(Al)-2.0G PAMAM increased dramatically, accompanied by a blue shift. Meanwhile, the fluorescence enhancement increased with higher PAMAM generations. Generally, PAMAM grafting transformed the surface amino groups of MIL-53(Al)-NH 2 into Schiff base, which should have resulted in the red shift of emission spectrum. However, more terminal amino groups in PAMAM dendrimers caused blue shift at the same time. Considering the comprehensive influence of the two factors, the emission peak of PAMAM-grafted MIL-53(Al)-NH 2 exhibited slight blue shift. As for the enhancement of fluorescence intensity by PAMAM, high-branched structure and large electron clouds of PAMAM dendrimers could lead to efficient intramolecular charge transfer (ICT) and thus enhance the fluorescence intensity of MOFs materials [38,39].

Influence of pH and Concentration
As fluorescence properties of LMOFs would be affected by pH and concentration, fluorescence intensities of PAMAM-grafted MIL-53(Al)-NH 2 were tested in aqueous solutions with different pHs (pH = 4-10) and concentrations (0.01-1 mg L −1 ). As indicated in Figure 5C, the fluorescence intensity of MIL-53(Al)-1.0G PAMAM materials were relatively pH-independent in the range of 4 to 10. This property being identical to raw MIL-53(Al)-NH 2 materials implied that PAMAM grafting did not change the acid-base stability of MIL-53(Al)-NH 2 [40]. However, according to the results in Figure 5D, the fluorescence intensity gradually increased with increasing concentration of MIL-53(Al)-1.0G PAMAM solutions. When the concentration was 0.25 mg mL −1 , the fluorescence intensity reached a maximum. Subsequently, the fluorescence intensity decreased with increasing concentration, which could be interpreted as self-quenching behavior [41].

Possible Application for Metal Ions Detection
On the basis of their fluorescence properties, PAMAM-grafted MIL-53(Al)-NH 2 were applied for metal ions detection. Various kinds of metal ions (1 mmol L −1 ) were added to a certain concentration of MIL-53(Al)-2.0G PAMAM solution (pH = 7). The fluorescence quenching efficiency (QE) was calculated by the following equation: where F 0 and F are the fluorescence intensities of the MOFs solutions before and after metal ion quenching, respectively. As displayed in Figure S1, common metal ions have little response to MIL-53(Al)-2.0G PAMAM, while heavy metal ions exhibited different effects on the fluorescence intensity. In particular, Pb 2+ , Cu 2+ , Fe 3+ and Co 2+ quenched the fluorescence dramatically. Such results implied that MIL-53(Al)-2.0G PAMAM materials have great potential for high-sensitivity detection of heavy metal ions in aqueous solution.

Detection of Heavy Metal Ions
The fluorescence responses of MIL-53(Al)-2.0G PAMAM toward Pb 2+ , Cu 2+ , Fe 3+ and Co 2+ were investigated at different concentrations. Relationships between metal ions concentrations and fluorescence quenching efficiency were shown in Figures 6 and S2. The limit of detection (LOD) was calculated according to three times the F 0 standard deviation [7]. Good linear relationships were obtained with linear correlation coefficients (R 2 ) ranging from 0.9925 to 0.9990. The LOD of Pb 2+ , Cu 2+ , Fe 3+ and Co 2+ were 1.1 µmol, 1.1 µmol, 2.9 µmol and 8.6 µmol, respectively. Comparing to the reported MIL LMOFs, PAMAM grafting improved the detection sensitivity effectively (Table 1). centrations and fluorescence quenching efficiency were shown in Figures 6 and S2. The limit of detection (LOD) was calculated according to three times the F0 standard deviation [7]. Good linear relationships were obtained with linear correlation coefficients (R 2 ) ranging from 0.9925 to 0.9990. The LOD of Pb 2+ , Cu 2+ , Fe 3+ and Co 2+ were 1.1 µmol, 1.1 µmol, 2.9 µmol and 8.6 µmol, respectively. Comparing to the reported MIL LMOFs, PAMAM grafting improved the detection sensitivity effectively (Table 1).   To evaluate the practicability of MIL-53(Al)-2.0G PAMAM fluorescence sensors, 0.05 mM Pb 2+ and Fe 3+ were added to tap water, drinking water and tea samples and detected. Recovery rates of the added ions are shown in Table 2. Satisfactory results were obtained in tap water (88.2-95.4%) and drinking water (94.0-95.0%). However, the recovery rates in tea samples were lower than that in tap water and drinking water; the complex ingredients in tea samples always display strong background UV absorption and fluorescence, and weaken the response signal of MIL-53(Al)-2.0G PAMAM [42]. Therefore, the proposed MIL-53(Al)-2.0G PAMAM fluorescence sensor has great feasibility for the determination of heavy metal ions. But the selectivity needs to be improved further.

Sensing Mechanism
In the detection of heavy metal ions, the fluorescent responses of LMOFs are mainly reflected in fluorescent changes. As the functional groups of MIL-53(Al)-2.0G PAMAM Polymers 2023, 15, 3444 9 of 12 could provide abundant coordination sites for heavy metal ions, the distance between the heavy metal ions and PAMAM-grafted MOFs greatly decreased, which significantly encouraged electron transfer and quenched the fluorescence.
In order to verify the assumed mechanism, MIL-53(Al)-2.0G PAMAM materials were collected and characterized by XPS after being quenched by Pb 2+ . The states of the contained elements are shown in Figure 7. Peaks in 531. 41, 399.20, 284.80 and 74.03 eV corresponded to O 1s, N 1s, C 1s and Al 2p in MIL-53(Al)-2.0G PAMAM, respectively ( Figure 7A). After being quenched by Pb 2+ , the peaks of Pb appeared distinctly ( Figure 7B). At the same time, EDS-mapping analysis was performed on the quenched MIL-53(Al)-2.0G PAMAM materials. According to the result, the quenched fluorescent materials contained Al and Pb elements ( Figure 7C-E). Al exists in the material as the skeleton structure, while Pb is the metal ion adsorbed during fluorescence quenching, which verifies the coordination of metal ions and the electron transfer mechanism.

Fluorescence Enhancement Property for Other MIL MOFs
The results of the current study showed that the fluorescence intensity of MIL-53(Al)-NH2 could be further enhanced by PAMAM grafting, providing higher sensitivity in detecting heavy metal ions. To explore the general applicability of this approach, other water-stable MIL MOFs MIL-53(Fe)-NH2) were synthesized and subsequently grafted with 2.0G PAMAM dendrimers. Fluorescence intensities of the obtained materials were investigated and compared with that of raw MOFs. According to the results in Figure S3, PA-MAM grafting could also increase the fluorescence intensity of MIL-53(Fe)-NH2. Therefore, it could be assumed that the proposed method in this study is universal for analogous water-stable MIL LMOFs.

Conclusions
In this work, water-stable MOFs named MIL-53(Al)-NH2 were synthesized and functionalized to have high fluorescence intensities via morphology control and PAMAM grafting. It was found that MIL-53(Al)-NH2 materials synthesized with different solvents had different morphologies, sizes and fluorescence properties. Due to ICT mechanism, PAMAM dendrimers further improved the fluorescence intensity of MIL-53(Al)-NH2. With the enhanced fluorescence intensity, the proposed materials showed good linearity (R 2 = 0.9925-0.9990) and satisfactory sensitivity (LOD = 1.1-8.6 µmol) in heavy metal ions detection. Moreover, such a fluorescence enhancement approach was proved to be universal for analogous MIL MOFs, which is of great significance for the preparation of highly sensitive fluorescent MOFs probes in the future.

Fluorescence Enhancement Property for Other MIL MOFs
The results of the current study showed that the fluorescence intensity of MIL-53(Al)-NH 2 could be further enhanced by PAMAM grafting, providing higher sensitivity in detecting heavy metal ions. To explore the general applicability of this approach, other water-stable MIL MOFs MIL-53(Fe)-NH 2 ) were synthesized and subsequently grafted with 2.0G PAMAM dendrimers. Fluorescence intensities of the obtained materials were investigated and compared with that of raw MOFs. According to the results in Figure S3, PAMAM grafting could also increase the fluorescence intensity of MIL-53(Fe)-NH 2 . Therefore, it could be assumed that the proposed method in this study is universal for analogous water-stable MIL LMOFs.

Conclusions
In this work, water-stable MOFs named MIL-53(Al)-NH 2 were synthesized and functionalized to have high fluorescence intensities via morphology control and PAMAM grafting. It was found that MIL-53(Al)-NH 2 materials synthesized with different solvents had different morphologies, sizes and fluorescence properties. Due to ICT mechanism, PAMAM dendrimers further improved the fluorescence intensity of MIL-53(Al)-NH 2 . With the enhanced fluorescence intensity, the proposed materials showed good linearity (R 2 = 0.9925-0.9990) and satisfactory sensitivity (LOD = 1.1-8.6 µmol) in heavy metal ions detection. Moreover, such a fluorescence enhancement approach was proved to be universal for analogous MIL MOFs, which is of great significance for the preparation of highly sensitive fluorescent MOFs probes in the future.