*3.1. Characterization of MFNS*

#### 3.1.1. Morphology Analysis *3.1. Characterization of MFNS*

The morphology and particle size of Fe3O<sup>4</sup> MNPs and Fe3O4@ZnS@MPS core-shell nanocomposites were investigated using SEM and TEM. 3.1.1. Morphology Analysis The morphology and particle size of Fe3O<sup>4</sup> MNPs and Fe3O4@ZnS@MPS core-shell

Figure 1a,c show the SEM and TEM images of Fe3O<sup>4</sup> MNPs, from the commercial Fe3O<sup>4</sup> with a diameter distribution from 150 to 250 nm, which reveal that Fe3O<sup>4</sup> MNPs have a regular and uniform distribution. In addition, the SEM image of Fe3O4@ZnS@MPS is shown in Figure 1b. Compared with Figure 1a,b, it can be seen that ZnS particles are accumulated on the surface of Fe3O<sup>4</sup> after the modification of the thiol group, and the surface of Fe3O<sup>4</sup> MNPs becomes irregular and rough. In addition, TEM images (Figure 1d) of Fe3O4@ZnS@MPS with a diameter distribution from 200 to 300 nm were obtained, which show that ZnS particles modified with sulfhydryl groups have been deposited on the surface of Fe3O<sup>4</sup> MNPs [23]. nanocomposites were investigated using SEM and TEM. Figure 1a,c show the SEM and TEM images of Fe3O<sup>4</sup> MNPs, from the commercial Fe3O<sup>4</sup> with a diameter distribution from 150 to 250 nm, which reveal that Fe3O<sup>4</sup> MNPs have a regular and uniform distribution. In addition, the SEM image of Fe3O4@ZnS@MPS is shown in Figure 1b. Compared with Figure 1a,b, it can be seen that ZnS particles are accumulated on the surface of Fe3O<sup>4</sup> after the modification of the thiol group, and the surface of Fe3O<sup>4</sup> MNPs becomes irregular and rough. In addition, TEM images (Figure 1d) of Fe3O4@ZnS@MPS with a diameter distribution from 200 to 300 nm were obtained, which show that ZnS particles modified with sulfhydryl groups have been deposited on the surface of Fe3O<sup>4</sup> MNPs [23].

**Figure 1. Figure 1.** SEM and TEM images of Fe SEM and TEM images of Fe3O <sup>3</sup>O<sup>4</sup> ( 4 **a** (,**ca**), Fe ,**c**), Fe3O <sup>3</sup><sup>4</sup>O @ZnS@MPS ( <sup>4</sup>@ZnS@MPS ( **b**,**d b** ).,**d**).

#### 3.1.2. XRD Analysis 3.1.2. XRD Analysis

Figure 2 shows the XRD patterns of Fe3O4, Fe3O4@ZnS, and MFNPs. In the XRD pattern, several diffraction peaks around 2θ = 30.1◦ , 35.6◦ , 43.1◦ , 53.7◦ , 57.1◦ , and 62.8◦ were observed for Fe3O4@ZnS@MPS, which could be assigned to the (220), (311), (400), (422), (511), and (440) planes of the cubic spinel crystal structure of Fe3O4, respectively. There were also three new diffraction peaks at around 2θ = 28.9◦ , 47.7◦ , and 57.0◦ , which could confirm the existence of ZnS crystals in the composites [24]. The position of the diffraction peak did not change when the quantum dots and sulfhydryl groups were modified, indicating that the Fe3O<sup>4</sup> magnetic core did not undergo chemical or structural changes during the coating process. Figure 2 clearly shows that the spectral peaks are basically the same before and after functionalization, indicating that the crystal structure of Fe3O<sup>4</sup> does not change during different functionalization processes [25,26]. Figure 2 shows the XRD patterns of Fe3O4, Fe3O4@ZnS, and MFNPs. In the XRD pattern, several diffraction peaks around 2θ = 30.1°, 35.6°, 43.1°, 53.7°, 57.1°, and 62.8° were observed for Fe3O4@ZnS@MPS, which could be assigned to the (220), (311), (400), (422), (511), and (440) planes of the cubic spinel crystal structure of Fe3O4, respectively. There were also three new diffraction peaks at around 2θ = 28.9°, 47.7°, and 57.0°, which could confirm the existence of ZnS crystals in the composites [24]. The position of the diffraction peak did not change when the quantum dots and sulfhydryl groups were modified, indicating that the Fe3O<sup>4</sup> magnetic core did not undergo chemical or structural changes during the coating process. Figure 2 clearly shows that the spectral peaks are basically the same before and after functionalization, indicating that the crystal structure of Fe3O<sup>4</sup> does not change during different functionalization processes [25,26].

**Figure 2.** XRD patterns of (**a**) Fe3O4, (**b**) Fe3O4@ZnS, and (**c**) Fe3O4@ZnS@MPS nanocomposites. **Figure 2.** XRD patterns of (**a**) Fe3O<sup>4</sup> , (**b**) Fe3O4@ZnS, and (**c**) Fe3O4@ZnS@MPS nanocomposites.

#### 3.1.3. FT−IR Analysis 3.1.3. FT−IR Analysis

Figure 3 shows the infrared (IR) spectra of Fe3O4, Fe3O4@ZnS, Fe3O4@ZnS@MPS, and MPS. It can be seen from the infrared spectra of the magnetic microspheres that the strong absorption peak at 584 cm−1 is related to the stretching vibration of the Fe–O bond. For Fe3O<sup>4</sup> @ZnS magnetic fluorescent nanoparticles, the peaks corresponding to the stretching vibration of Fe–O and Zn–S appeared at 580 cm−1 and 627 cm−1. The stretching vibration of -SH in MPS corresponds to a peak at 2555 cm−1. For MFNP nanoparticles, the Zn–S stretching vibration peak was obviously masked after sulfhydryl modification at 1018 cm−1, and a new C–H peak appeared at 2922 cm−1 [27]. The corresponding peak of the new S–O stretching vibration was 1042 cm−1, while the peak at 1178 cm−1 was assigned to the O=S=O symmetric stretching vibration. This can prove that the sulfhydryl group is bound to the surface of the microspheres and the ligand is successfully modified on the surface of the microspheres [28– 31]. Figure 3 shows the infrared (IR) spectra of Fe3O4, Fe3O4@ZnS, Fe3O4@ZnS@MPS, and MPS. It can be seen from the infrared spectra of the magnetic microspheres that the strong absorption peak at 584 cm−<sup>1</sup> is related to the stretching vibration of the Fe–O bond. For Fe3O<sup>4</sup> @ZnS magnetic fluorescent nanoparticles, the peaks corresponding to the stretching vibration of Fe–O and Zn–S appeared at 580 cm−<sup>1</sup> and 627 cm−<sup>1</sup> . The stretching vibration of -SH in MPS corresponds to a peak at 2555 cm−<sup>1</sup> . For MFNP nanoparticles, the Zn–S stretching vibration peak was obviously masked after sulfhydryl modification at 1018 cm−<sup>1</sup> , and a new C–H peak appeared at 2922 cm−<sup>1</sup> [27]. The corresponding peak of the new S–O stretching vibration was 1042 cm−<sup>1</sup> , while the peak at 1178 cm−<sup>1</sup> was assigned to the O=S=O symmetric stretching vibration. This can prove that the sulfhydryl group is bound to the surface of the microspheres and the ligand is successfully modified on the surface of the microspheres [28–31].

**Figure 3.** FT−IR spectra of commercial Fe3O4, Fe3O4@ZnS, Fe3O4@ZnS@MPS, and MPS. **Figure 3.** FT−IR spectra of commercial Fe3O<sup>4</sup> , Fe3O4@ZnS, Fe3O4@ZnS@MPS, and MPS.

#### 3.1.4. XPS Analysis 3.1.4. XPS Analysis

The elemental composition of the Fe3O4@ZnS@MPS nanoparticle was explored by XPS analysis (Figure 4), The five peaks at 1019.2 eV, 709.8 eV, 529.6 eV, 285.6 eV, and 159.2 eV are composed of Zn 2p, Fe 2p, O 1s, C 1s, and S 2p, respectively, confirming the successful synthesis of Fe3O4@ZnS@MPS nanoparticles. Figure 4b shows that the binding energies of Fe3+ 2p3/2 and Fe3+ 2p1/2 are 710.8 and 725.0 eV, respectively, and the bimodal fitting of Fe 2p can obtain the binding energies of Fe2+ 2p3/2 and Fe2+ 2p1/2 at 708.7 and 721.6 eV, respectively. The results indicate that Fe3O<sup>4</sup> exists in the nanoparticle. According to Figure 4c, the difference in binding energy between Zn 2p3/2 and Zn 2p1/2 is 22.5 eV, indicating that metallic Zn supported by the MFNPs mainly exists in the Zn2+ valence state. Peaks at 159.5 eV and 160.7 eV (Figure 4d) are attributed to metal sulfides S2− (2p3/2 and S 2p1/2) from ZnS, respectively [32,33]. The elemental composition of the Fe3O4@ZnS@MPS nanoparticle was explored by XPS analysis (Figure 4), The five peaks at 1019.2 eV, 709.8 eV, 529.6 eV, 285.6 eV, and 159.2 eV are composed of Zn 2p, Fe 2p, O 1s, C 1s, and S 2p, respectively, confirming the successful synthesis of Fe3O4@ZnS@MPS nanoparticles. Figure 4b shows that the binding energies of Fe3+ 2p3/2 and Fe3+ 2p1/2 are 710.8 and 725.0 eV, respectively, and the bimodal fitting of Fe 2p can obtain the binding energies of Fe2+ 2p3/2 and Fe2+ 2p1/2 at 708.7 and 721.6 eV, respectively. The results indicate that Fe3O<sup>4</sup> exists in the nanoparticle. According to Figure 4c, the difference in binding energy between Zn 2p3/2 and Zn 2p1/2 is 22.5 eV, indicating that metallic Zn supported by the MFNPs mainly exists in the Zn2+ valence state. Peaks at 159.5 eV and 160.7 eV (Figure 4d) are attributed to metal sulfides S2<sup>−</sup> (2p3/2 and S 2p1/2) from ZnS, respectively [32,33].

#### 3.1.5. Hysteresis Curve

3.1.5. Hysteresis Curve The magnetic properties of the Fe3O<sup>4</sup> and Fe3O4@ZnS@MPS MFNPs were studied using a VSM in an external magnetic field from −20,000 to +20,000 Oe, and results are shown in Figure 5. The saturation magnetization of Fe3O<sup>4</sup> MNPs was 64.52 emu/g, and that of Fe3O4@ZnS@MPS nanocomposites was 47.09 emu/g. The magnetization in the nanocomposites is reduced due to the diamagnetic effect of the thick sulfhydryl-modified ZnS layer around the Fe3O<sup>4</sup> MNPs. Ion redistribution of Zn2+ may also have contributed to the decrease in the saturation magnetization of the composite. However, it still possesses typical superparamagnetism, which can meet the experimental requirements, and still shows excellent The magnetic properties of the Fe3O<sup>4</sup> and Fe3O4@ZnS@MPS MFNPs were studied using a VSM in an external magnetic field from −20,000 to +20,000 Oe, and results are shown in Figure 5. The saturation magnetization of Fe3O<sup>4</sup> MNPs was 64.52 emu/g, and that of Fe3O4@ZnS@MPS nanocomposites was 47.09 emu/g. The magnetization in the nanocomposites is reduced due to the diamagnetic effect of the thick sulfhydryl-modified ZnS layer around the Fe3O<sup>4</sup> MNPs. Ion redistribution of Zn2+ may also have contributed to the decrease in the saturation magnetization of the composite. However, it still possesses typical superparamagnetism, which can meet the experimental requirements, and still shows excellent magnetic properties after removal of Ag<sup>+</sup> (inset figure) [11,34].

magnetic properties after removal of Ag<sup>+</sup> (inset figure) [11,34].

**Figure 4.** XPS analysis of Fe3O4@ZnS@MPS (**a**), high-resolution XPS spectra of Fe 2p (**b**), Zn 2p (**c**), S 2p (**d**) of Fe3O4@ZnS@MPS. **Figure 4.** XPS analysis of Fe3O4@ZnS@MPS (**a**), high-resolution XPS spectra of Fe 2p (**b**), Zn 2p (**c**), S 2p (**d**) of Fe3O4@ZnS@MPS. **Figure 4.** XPS analysis of Fe3O4@ZnS@MPS (**a**), high-resolution XPS spectra of Fe 2p (**b**), Zn 2p (**c**), S 2p (**d**) of Fe3O4@ZnS@MPS.

**Figure 5.** VSM measurements of Fe3O<sup>4</sup> and Fe3O4@ZnS@MPS. Inset: The response of Fe3O4@ZnS@MPS to external magnetic field. **Figure 5.** VSM measurements of Fe3O<sup>4</sup> and Fe3O4@ZnS@MPS. Inset: The response of Fe3O4@ZnS@MPS to external magnetic field. **Figure 5.** VSM measurements of Fe3O<sup>4</sup> and Fe3O4@ZnS@MPS. Inset: The response of Fe3O4@ZnS@MPS to external magnetic field.

#### 3.1.6. Thermogravimetric Analysis 3.1.6. Thermogravimetric Analysis

In order to study the thermal properties of the samples, a thermal weight loss analyzer was used to analyze the samples. The thermal properties constitute an important index used to evaluate the hybrid materials, and differ with different synthesis methods. The TGA curves of MFNFs were measured at a heating rate of 10 ◦C/min under a nitrogen atmosphere, and the results are shown in Figure 6. As shown in Figure 6, the overall weight loss rate of the products is not very large, and the weight loss rate of Fe3O<sup>4</sup> is only about 8%. After the modification of ZnS, its weight loss is reduced to 15%, and the extra weight loss is the water contained in ZnS. The weight loss rate reaches 18% after the modification of MPS, indicating that the polymer in the sample was removed. Therefore, the TGA analysis shows that the sulfhydryl group was modified on the surface of Fe3O<sup>4</sup> magnetic microspheres. In order to study the thermal properties of the samples, a thermal weight loss analyzer was used to analyze the samples. The thermal properties constitute an important index used to evaluate the hybrid materials, and differ with different synthesis methods. The TGA curves of MFNFs were measured at a heating rate of 10 °C/min under a nitrogen atmosphere, and the results are shown in Figure 6. As shown in Figure 6, the overall weight loss rate of the products is not very large, and the weight loss rate of Fe3O<sup>4</sup> is only about 8%. After the modification of ZnS, its weight loss is reduced to 15%, and the extra weight loss is the water contained in ZnS. The weight loss rate reaches 18% after the modification of MPS, indicating that the polymer in the sample was removed. Therefore, the TGA analysis shows that the sulfhydryl group was modified on the surface of Fe3O<sup>4</sup> magnetic microspheres.

**Figure 6.** TGA curves of Fe3O4, Fe3O4@ZnS, and Fe3O4@ZnS@MPS. **Figure 6.** TGA curves of Fe3O<sup>4</sup> , Fe3O4@ZnS, and Fe3O4@ZnS@MPS.

Combined with the above data, it can be proven that the Fe3O4@ZnS@MPS magnetic fluorescent nanosensors (MFNPs) have been successfully synthesized. Combined with the above data, it can be proven that the Fe3O4@ZnS@MPS magnetic fluorescent nanosensors (MFNPs) have been successfully synthesized.

#### *3.2. Detection Performance Study of MFNPs*

*3.2. Detection Performance Study of MFNPs* 3.2.1. Performance Analysis of Magnetic Fluorescence Nanosensor MFNPs

3.2.1. Performance Analysis of Magnetic Fluorescence Nanosensor MFNPs Figure 7 shows the comparison of fluorescence characteristic spectra before and after the addition of Ag<sup>+</sup> to MFNPs. Figure 7 shows that the addition of Ag<sup>+</sup> can significantly quench the fluorescence intensity of MFNPs, and the inset shows the TEM images of MFNPs before and after the addition of Ag<sup>+</sup> . It can be clearly seen that Ag<sup>+</sup> has been loaded and dispersed on the surface of the MFNPs. MFNPs have good dispersibility and are almost spherical. The complexation of Ag<sup>+</sup> with sodium 3-sulfhydryl-1-propane sulfonate on the Figure 7 shows the comparison of fluorescence characteristic spectra before and after the addition of Ag<sup>+</sup> to MFNPs. Figure 7 shows that the addition of Ag<sup>+</sup> can significantly quench the fluorescence intensity of MFNPs, and the inset shows the TEM images of MFNPs before and after the addition of Ag<sup>+</sup> . It can be clearly seen that Ag<sup>+</sup> has been loaded and dispersed on the surface of the MFNPs. MFNPs have good dispersibility and are almost spherical. The complexation of Ag<sup>+</sup> with sodium 3-sulfhydryl-1-propane sulfonate on the surface of MFNPs leads to Ag<sup>+</sup> aggregation and obvious fluorescence quenching.

surface of MFNPs leads to Ag<sup>+</sup> aggregation and obvious fluorescence quenching.

**Figure 7.** Comparison of fluorescence spectra of MFNPs before and after Ag<sup>+</sup> addition. Inset: TEM images of MFNPs before and after Ag<sup>+</sup> addition. **Figure 7.** Comparison of fluorescence spectra of MFNPs before and after Ag<sup>+</sup> addition. Inset: TEM images of MFNPs before and after Ag<sup>+</sup> addition. **Figure 7.** Comparison of fluorescence spectra of MFNPs before and after Ag<sup>+</sup> addition. Inset: TEM images of MFNPs before and after Ag<sup>+</sup> addition.

#### 3.2.2. The Effect of pH on the Fluorescence of MFNPs 3.2.2. The Effect of pH on the Fluorescence of MFNPs 3.2.2. The Effect of pH on the Fluorescence of MFNPs

One of the crucial factors determining the sensor's capacity for detection is the pH level of the solution. As can be seen from Figure 8, the fluorescence intensity of the probe itself does not change greatly within the range of 4.8–9.0, and the degree of fluorescence intensity quenched by the addition of silver ions was not affected by the pH value. Therefore, we chose the common neutral liquid pH value of 7.0 as the experimental standard. One of the crucial factors determining the sensor's capacity for detection is the pH level of the solution. As can be seen from Figure 8, the fluorescence intensity of the probe itself does not change greatly within the range of 4.8–9.0, and the degree of fluorescence intensity quenched by the addition of silver ions was not affected by the pH value. Therefore, we chose the common neutral liquid pH value of 7.0 as the experimental standard. One of the crucial factors determining the sensor's capacity for detection is the pH level of the solution. As can be seen from Figure 8, the fluorescence intensity of the probe itself does not change greatly within the range of 4.8–9.0, and the degree of fluorescence intensity quenched by the addition of silver ions was not affected by the pH value. Therefore, we chose the common neutral liquid pH value of 7.0 as the experimental standard.

**Figure 8.** Fluorescence intensity of MFNPs at 425 nm in the absence and presence of Ag<sup>+</sup> at different pH. **Figure 8.** Fluorescence intensity of MFNPs at 425 nm in the absence and presence of Ag<sup>+</sup> at different pH. **Figure 8.** Fluorescence intensity of MFNPs at 425 nm in the absence and presence of Ag<sup>+</sup> at different pH.

3.2.3. The Response Range and Relation Curve of MFNPs to Ag<sup>+</sup> 3.2.3. The Response Range and Relation Curve of MFNPs to Ag<sup>+</sup>

With the increase in the concentration of Ag<sup>+</sup> , the fluorescence intensity gradually weakens in the range of 0–100 µM (Figure 9). There was a linear relationship between the concentration of Ag<sup>+</sup> and the fluorescence intensity I/I<sup>0</sup> <sup>=</sup> <sup>−</sup>0.00426x + 1.00934 (R <sup>2</sup> = 0.9967), where x is the concentrations of Ag<sup>+</sup> (µM), with the LOD of 7.04 µM. Therefore, the sensor of the presented invention can more accurately determine the content of Ag<sup>+</sup> . Based on the data of the linear range and the detection limit, it can be seen that the probe showed good performance (Table 1). With the increase in the concentration of Ag<sup>+</sup> , the fluorescence intensity gradually weakens in the range of 0–100 µM (Figure 9). There was a linear relationship between the concentration of Ag<sup>+</sup> and the fluorescence intensity I/I<sup>0</sup> = −0.00426x + 1.00934 (R<sup>2</sup> = 0.9967), where x is the concentrations of Ag<sup>+</sup> (µM), with the LOD of 7.04 µM. Therefore, the sensor of the presented invention can more accurately determine the content of Ag<sup>+</sup> . Based on the data of the linear range and the detection limit, it can be seen that the probe showed good performance (Table 1).

**Figure 9.** Emission spectra of MFNPs in the presence of increasing amounts of Ag+ at room temperature. Inset: The curve of fluorescence intensity at 425 nm; the concentrations of Ag<sup>+</sup> are 0, 4, 8, 12, 20, 32, 40, 60, 80, 100 µM, respectively. **Figure 9.** Emission spectra of MFNPs in the presence of increasing amounts of Ag<sup>+</sup> at room temperature. Inset: The curve of fluorescence intensity at 425 nm; the concentrations of Ag<sup>+</sup> are 0, 4, 8, 12, 20, 32, 40, 60, 80, 100 µM, respectively.

**Table 1.** Comparison of the present nanoprobe with other reported sensors for Ag<sup>+</sup> detection. **Table 1.** Comparison of the present nanoprobe with other reported sensors for Ag<sup>+</sup> detection.


Fe3O4@ZnS@MPS 0–100 µM 7.04 µM This work 3.2.4. Particle Selectivity and Interference Ion Determination

> 3.2.4. Particle Selectivity and Interference Ion Determination High selectivity is a necessary condition for the sensor. Therefore, under the same conditions, the Ag<sup>+</sup> selectivity of the prepared magnetic fluorescent sensor was investigated by detection of the reaction of the relevant analytes. The results show that the fluorescence quenching effect of Ag<sup>+</sup> is the best. Although the other ions will experience weak quenching, High selectivity is a necessary condition for the sensor. Therefore, under the same conditions, the Ag<sup>+</sup> selectivity of the prepared magnetic fluorescent sensor was investigated by detection of the reaction of the relevant analytes. The results show that the fluorescence quenching effect of Ag<sup>+</sup> is the best. Although the other ions will experience weak quenching, this is obviously negligible compared with that of silver ions (Figure 10a). To further investigate the ability of the magnetic fluorescent sensor to recognize silver ions in the

presence of other metal ions, the anti-interference ability of the sensor was also investigated. When an equal amount of silver ions was added to an equal amount of other metal ions (400 µM Ag<sup>+</sup> , Co2+, Ni2+, Al3+, Cu2+, Zn2+, Cd2+, Fe3+, Fe2+, K<sup>+</sup> , Ca2+, Na<sup>+</sup> , Pb2+, Hg2+), the other ions did not affect the detection of silver ions by the magnetic fluorescent sensor. The bar graph clearly proves this point (Figure 10b); it is obvious that the common metal ions in the environment do not interfere with the qualitative and quantitative detection of silver ions by the particles. of other metal ions, the anti-interference ability of the sensor was also investigated. When an equal amount of silver ions was added to an equal amount of other metal ions (400 µM Ag<sup>+</sup> , Co2+, Ni2+, Al3+, Cu2+, Zn2+, Cd2+, Fe3+, Fe2+, K<sup>+</sup> , Ca2+, Na<sup>+</sup> , Pb2+, Hg2+), the other ions did not affect the detection of silver ions by the magnetic fluorescent sensor. The bar graph clearly proves this point (Figure 10b); it is obvious that the common metal ions in the environment do not interfere with the qualitative and quantitative detection of silver ions by the particles.

this is obviously negligible compared with that of silver ions (Figure 10a). To further investigate the ability of the magnetic fluorescent sensor to recognize silver ions in the presence

*Coatings* **2023**, *13*, x FOR PEER REVIEW 11 of 15

**Figure 10.** (**a**) Bar graph represents the ratio of fluorescence quenching of MFNPs in the presence of different metal ions (1, Co2+; 2, Pb2+; 3, Ni2+; 4, Hg2+; 5, Al3+; 6, Cu2+; 7, Zn2+; 8, Cd2+; 9, Fe3+; 10, Fe2+; 11, K<sup>+</sup> ; 12, Ca2+; 13, Na<sup>+</sup> ; 14, Ag<sup>+</sup> ); (**b**) bar graph represents the ratio of fluorescence quenching of MFNPs upon the addition of Ag<sup>+</sup> (400 µM) to the solution containing other metal ions (400 µM, 1, Co2+; 2, Pb2+; 3, Ni2+; 4, Hg2+; 5, Al3+; 6, Cu2+; 7, Zn2+; 8, Cd2+; 9, Fe3+; 10, Fe2+; 11, K<sup>+</sup> ; 12, Ca2+; 13, Na<sup>+</sup> ;14, Blank). 3.2.5. Removal of Ag<sup>+</sup> **Figure 10.** (**a**) Bar graph represents the ratio of fluorescence quenching of MFNPs in the presence of different metal ions (1, Co2+; 2, Pb2+; 3, Ni2+; 4, Hg2+; 5, Al3+; 6, Cu2+; 7, Zn2+; 8, Cd2+; 9, Fe3+; 10, Fe2+; 11, K<sup>+</sup> ; 12, Ca2+; 13, Na<sup>+</sup> ; 14, Ag<sup>+</sup> ); (**b**) bar graph represents the ratio of fluorescence quenching of MFNPs upon the addition of Ag<sup>+</sup> (400 µM) to the solution containing other metal ions (400 µM, 1, Co2+; 2, Pb2+; 3, Ni2+; 4, Hg2+; 5, Al3+; 6, Cu2+; 7, Zn2+; 8, Cd2+; 9, Fe3+; 10, Fe2+; 11, K<sup>+</sup> ; 12, Ca2+; 13, Na<sup>+</sup> ;14, Blank).

#### Adsorption capacity is considered to be one of the most important properties of nano-3.2.5. Removal of Ag<sup>+</sup>

materials, so adsorption tests were performed in conical flasks (100 mL) to determine the adsorption amount of Ag<sup>+</sup> . The effect of the initial Ag<sup>+</sup> concentration on the adsorption efficiency is shown in Figure 11. The adsorption analysis showed that with the increase in the initial Ag<sup>+</sup> concentration, the adsorption capacity of MFNPs also gradually increased. This was due to the increase in the initial concentration, which improved the adsorption driving force and ion mass transfer rate, and the binding site on the adsorbents' surface was gradually occupied by Ag<sup>+</sup> ; thus, the adsorption capacity increased. The removal rate of Ag<sup>+</sup> in the solution gradually decreased after 100 µM, reaching 99.62%, which is because the amount of adsorbent in the solution is finite, and the adsorption site provided for Ag<sup>+</sup> is also finite. When the concentration of heavy metals is low, almost all Ag<sup>+</sup> can be combined with the adsorption site to achieve a higher removal rate, and then the adsorption gradually reaches equilibrium. Therefore, the adsorption effect of the adsorbent on the free metal ions in the solution is reduced, resulting in a lower removal rate. When the Ag<sup>+</sup> concentration changed from 300 µM to 600 µM, the removal rate decreased from 99.54% to 93.54%, and the adsorption capacity increased from 322.496 mg/g to 606.112 mg/g. In general, the optimal Ag<sup>+</sup> concentration at the intersection point of 400 µM could be obtained. Compared with other materials, the adsorption and transfer rates of this material are relatively high (Table 2). Adsorption capacity is considered to be one of the most important properties of nanomaterials, so adsorption tests were performed in conical flasks (100 mL) to determine the adsorption amount of Ag<sup>+</sup> . The effect of the initial Ag<sup>+</sup> concentration on the adsorption efficiency is shown in Figure 11. The adsorption analysis showed that with the increase in the initial Ag<sup>+</sup> concentration, the adsorption capacity of MFNPs also gradually increased. This was due to the increase in the initial concentration, which improved the adsorption driving force and ion mass transfer rate, and the binding site on the adsorbents' surface was gradually occupied by Ag<sup>+</sup> ; thus, the adsorption capacity increased. The removal rate of Ag<sup>+</sup> in the solution gradually decreased after 100 µM, reaching 99.62%, which is because the amount of adsorbent in the solution is finite, and the adsorption site provided for Ag<sup>+</sup> is also finite. When the concentration of heavy metals is low, almost all Ag<sup>+</sup> can be combined with the adsorption site to achieve a higher removal rate, and then the adsorption gradually reaches equilibrium. Therefore, the adsorption effect of the adsorbent on the free metal ions in the solution is reduced, resulting in a lower removal rate. When the Ag<sup>+</sup> concentration changed from 300 µM to 600 µM, the removal rate decreased from 99.54% to 93.54%, and the adsorption capacity increased from 322.496 mg/g to 606.112 mg/g. In general, the optimal Ag<sup>+</sup> concentration at the intersection point of 400 µM could be obtained. Compared with other materials, the adsorption and transfer rates of this material are relatively high (Table 2).

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L-PRL(o -phenanthrolinebased polymer)

**Figure 11.** Effect of the initial concentration of Ag<sup>+</sup> on the adsorption capacity and removal efficiency of magnetic fluorescent nanoparticles. **Figure 11.** Effect of the initial concentration of Ag<sup>+</sup> on the adsorption capacity and removal efficiency of magnetic fluorescent nanoparticles.



*3.3. Infrared Spectroscopy Analysis before and after Complexation of MFNPs with Ag<sup>+</sup>*

325.8 Chinese Chemical Letters 34 (2023) 107,485 [43] MFNPs 395.79 This work *3.3. Infrared Spectroscopy Analysis before and after Complexation of MFNPs with Ag<sup>+</sup>* Infrared spectra of MFNPs before and after the addition of Ag<sup>+</sup> were analyzed using FT-IR, and the FT-IR spectra of MFNPs and MFNPs + Ag<sup>+</sup> complexes are shown in Figure 12. From the infrared spectra of MFNPs, the corresponding peaks of S = O telescopic vibration at 1042 cm−<sup>1</sup> were observed. *Coatings* **2023**, *13*, x FOR PEER REVIEW 13 of 15

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laced electrons. Thus, it was speculated that the oxygen atom of the S–O and O=S=O group donated electrons to the Ag<sup>+</sup> , which led to the transfer of charge and resulted in changes in **Figure 12.** Infrared absorption spectra of MFNPs and inclusion complex MFNPs + Ag<sup>+</sup> . **Figure 12.** Infrared absorption spectra of MFNPs and inclusion complex MFNPs + Ag<sup>+</sup> .

MFNPs had great potential for the detection of Ag+ in real water samples.

*in Actual Samples*

The practicability of the MFNPs for the detection of Ag<sup>+</sup> was demonstrated by analyz-

. Certain volumes of the supernatant and Ag<sup>+</sup> so-

these water samples using centrifugal separation. Then, the supernatant was analyzed ac-

lutions were added to 200 µL of the Fe3O4@ZnS@MPS suspension, and the final volume was tuned to 2 mL by PBS buffer solution. As shown in Table 3, Ag<sup>+</sup> was not detected in the real water samples. The recovery of Ag<sup>+</sup> detection was in the range of 85.20%–105.30%, with the RSD (relative standard deviation) in the range of 3.18%–5.32%. The result indicated that the

**Sample Added (μM) Measured (μM) Recovery (%) RSD (%)**

In summary, we present the design, synthesis, and performance of a novel magnetic fluorescent nanoprobe (Fe3O4@ZnS@MPS(MFNPs)) modified with 3-sulfhydryl-1-propane sodium for simultaneous detection and removal of Ag<sup>+</sup> from water solutions. Our proposed Fe3O4@ZnS@MPS sensor exhibits significant quenching fluorescence intensity ability and

the sensor. In addition, it could be applied over a wide pH range. The above results confirmed that this method is an inexpensive, simple, convenient, and extremely sensitive method that enables highly specific recognition for the simultaneous detection and removal of Ag<sup>+</sup> from aqueous solutions. This study provided a new idea for the determination and

1 0.926 92.60 3.18 5.0 4.859 97.18 4.25 10.0 10.321 103.21 3.86

1 1.053 105.30 4.34 5.0 4.987 99.74 3.89 10.0 9.842 98.42 4.45

1 0.852 85.20 5.32 5.0 4.320 86.40 4.96 10.0 8.620 86.20 4.85

. It was shown that the Fe3O4@ZnS@MPS fluorescent sensor was a

, due to the aggregation and magnetic separation of

**Table 3.** The performance of the MFNPs in real water samples.

the infrared spectrum [43].

Tap water

River water

Electrolysis waste water

**4. Conclusions**

high selectivity for Ag<sup>+</sup>

good adsorbent for the removal of Ag<sup>+</sup>

removal of Ag<sup>+</sup> and other heavy metals.

*3.4. The Fluorescent Detection of Ag<sup>+</sup>*

cording to the previous procedure of Ag<sup>+</sup>

The peak shape of the absorption peaks at 1045.31 cm−<sup>1</sup> , 1198.70 cm−<sup>1</sup> , and 1407.66 cm−<sup>1</sup> changed, indicating that S–O, O=S=O, and C–O might play a key role in the synergistic effect with Ag<sup>+</sup> . The result can be clearly observed in Figure 8; when the Fe3O4@ZnS-MPS interacted with Ag+, the tensile peak of methoxy (S–O) was changed from 1045.31 cm−<sup>1</sup> to 1014.02 cm−<sup>1</sup> , while the characteristic absorption peak of O=S=O moved from 1198.70 cm−<sup>1</sup> to 1159.20 cm−<sup>1</sup> and the characteristic absorption peak of C–O moved from 1407.66 cm−<sup>1</sup> to 1382.18 cm−<sup>1</sup> . The reason for the change in the infrared absorption peak of the sulfonic acid group might be that the oxygen atoms in the sulfonic acid group cooperated with Ag<sup>+</sup> to form a synergistic complexation. The oxygen atoms of the sulfonic acid group had a negative charge, indicating that the S–O and O=S=O group could adsorb Ag<sup>+</sup> by electrostatic interaction as Ag<sup>+</sup> laced electrons. Thus, it was speculated that the oxygen atom of the S–O and O=S=O group donated electrons to the Ag<sup>+</sup> , which led to the transfer of charge and resulted in changes in the infrared spectrum [43].

#### *3.4. The Fluorescent Detection of Ag<sup>+</sup> in Actual Samples*

The practicability of the MFNPs for the detection of Ag<sup>+</sup> was demonstrated by analyzing real water samples. The real samples were selected from tap water, water from Songhua River in Jilin, and electrolysis waste water. First, suspended particles were removed from these water samples using centrifugal separation. Then, the supernatant was analyzed according to the previous procedure of Ag<sup>+</sup> . Certain volumes of the supernatant and Ag<sup>+</sup> solutions were added to 200 µL of the Fe3O4@ZnS@MPS suspension, and the final volume was tuned to 2 mL by PBS buffer solution. As shown in Table 3, Ag<sup>+</sup> was not detected in the real water samples. The recovery of Ag<sup>+</sup> detection was in the range of 85.20%–105.30%, with the RSD (relative standard deviation) in the range of 3.18%–5.32%. The result indicated that the MFNPs had great potential for the detection of Ag+ in real water samples.


**Table 3.** The performance of the MFNPs in real water samples.

#### **4. Conclusions**

In summary, we present the design, synthesis, and performance of a novel magnetic fluorescent nanoprobe (Fe3O4@ZnS@MPS(MFNPs)) modified with 3-sulfhydryl-1-propane sodium for simultaneous detection and removal of Ag<sup>+</sup> from water solutions. Our proposed Fe3O4@ZnS@MPS sensor exhibits significant quenching fluorescence intensity ability and high selectivity for Ag<sup>+</sup> . It was shown that the Fe3O4@ZnS@MPS fluorescent sensor was a good adsorbent for the removal of Ag<sup>+</sup> , due to the aggregation and magnetic separation of the sensor. In addition, it could be applied over a wide pH range. The above results confirmed that this method is an inexpensive, simple, convenient, and extremely sensitive method that enables highly specific recognition for the simultaneous detection and removal of Ag<sup>+</sup> from aqueous solutions. This study provided a new idea for the determination and removal of Ag<sup>+</sup> and other heavy metals.

**Author Contributions:** Conceptualization, Y.G. and X.C.; methodology, X.Z.; software, X.C.; validation, Z.L., S.Y. and J.C.; formal analysis, P.X.; investigation, X.Z.; resources, Z.L.; data curation, X.C.; writing—original draft preparation, X.C.; writing—review and editing, X.Z.; visualization, X.Z.; supervision, X.Z.; project administration, X.Z.; funding acquisition, Z.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by X.Z. grant number (20220203020SF) And The APC was funded by Jilin Provincial Department of Science and Technology. This research was funded by J.C. grant number (51902125) And The APC was funded by National Natural Science Foundation of China. This research was funded by S.Y. grant number (22106051) And The APC was funded by National Natural Science Foundation of China.

**Data Availability Statement:** Z.X. and G.Y. designed the work. C.X. performed the experiment and data analysis.

**Acknowledgments:** We gratefully acknowledge financial support from Jilin Provincial Department of Science and Technology (20220203020SF), the National Natural Science Foundation of China (51902125, 22106051).

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