*Article* **A Novel Magnetic Fluorescent Fe3O4@ZnS@MPS Nanosensor for Highly Sensitive Determination and Removal of Ag<sup>+</sup>**

**Yan Gao <sup>1</sup> , Xin Chen <sup>2</sup> , Ping Xu <sup>2</sup> , Jie Chen <sup>1</sup> , Shihua Yu <sup>2</sup> , Zhigang Liu 1,\* and Xiaodan Zeng 1,\***


**\*** Correspondence: lzg@jlict.edu.cn (Z.L.); jiangzxd@jlict.edu.cn (X.Z.)

**Abstract:** A novel magnetic fluorescent nanoprobe (Fe3O4@ZnS@MPS(MFNPs)) was synthesized, which recognized and cooperated with Ag<sup>+</sup> ions, and a rapid method for detecting Ag<sup>+</sup> was established in solution. It was found by fluorescence spectroscopy analysis that the MFNPs could detect Ag<sup>+</sup> in PBS solution and, upon addition of Ag<sup>+</sup> ions, the fluorescence (FL) of MFNPs could be quenched significantly. The sensor has a low limit of detection (LOD) of 7.04 µM for Ag<sup>+</sup> . The results showed that MFNPs were extremely specific and sensitive for the quantitative detection of Ag<sup>+</sup> over a wide pH range. Then, the recognition mechanism between MFNPs and guest Ag<sup>+</sup> was explored via measures of infrared spectroscopy and electron microscopy. It was speculated that the oxygen atoms in the sulfonic acid group cooperated with Ag<sup>+</sup> to form a synergistic complexation. The assay was successfully used to determine the content of Ag+ in real samples.

**Keywords:** magnetic fluorescent nanosensor; selective recognition; silver ion; 3-sulfhydryl-1-propane sodium (MPS)

### **1. Introduction**

Silver has always attracted attention due to its unique chemical properties of strong corrosion resistance and high antioxidant capacity. Because of its rarity and high gloss, silver is widely used in the production of daily necessities [1,2]. A large number of widespread uses have a relatively significant impact on the environment. Therefore, it is necessary to adopt appropriate detection methods to analyze it. Excessive human exposure to silver is likely to cause silver poisoning, growth retardation, etc., and too much silver hurts the eyes and skin [3,4]. Thus, it is of great significance to investigate an effective method for detection of Ag<sup>+</sup> .

Silver ions can be detected by a variety of methods, such as fast but unstable electrochemical detection and accurate but expensive flame atomic absorption spectrometry [5–7]. In addition to these techniques, the extraction method using molecular receptors or chelating ligands is also widely used in the detection of Ag(I). Compared with conventional assays, fluorescence detection has greater advantages, such as rapid reaction, simple operation, low cost, and high sensitivity [8–10].

However, these fluorescent probes also cause further pollution to the environment when they are used to detect Ag<sup>+</sup> . Compared with these fluorescent probes, ZnS QDs have unique advantages, such as a narrow and symmetric emission spectrum, excellent light stability, and resistance to photobleaching [11]. Therefore, the development of multifunctional fluorescence chemosensors for sensing of heavy metals, that are friendly to the environment, is increasingly attracting attention [12].

In recent years, magnetic nanomaterials have attracted significant attention in the academic field due to their excellent magnetic reaction ability, high biocompatibility, and good stability [13–15]. Among these, superparamagnetic iron oxide (Fe3O4) is particularly

**Citation:** Gao, Y.; Chen, X.; Xu, P.; Chen, J.; Yu, S.; Liu, Z.; Zeng, X. A Novel Magnetic Fluorescent Fe3O4@ZnS@MPS Nanosensor for Highly Sensitive Determination and Removal of Ag<sup>+</sup> . *Coatings* **2023**, *13*, 1557. https://doi.org/10.3390/ coatings13091557

Academic Editor: M. Shaheer Akhtar

Received: 24 July 2023 Revised: 23 August 2023 Accepted: 29 August 2023 Published: 6 September 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

versatile due to its unique high coercivity, easily functional modification, excellent controllable magnetic responsiveness, which can be manipulated by external magnetic fields, and controllable size and surface [16–18]. This enables researchers in various fields, such as chemistry, biology, medicine, and materials science, to use MFNPs to construct multifunctional nanoprobes and conduct significant research in sewage treatment, biological imaging, drug delivery, and other fields [19,20]. magnetic responsiveness, which can be manipulated by external magnetic fields, and controllable size and surface [16–18]. This enables researchers in various fields, such as chemistry, biology, medicine, and materials science, to use MFNPs to construct multifunctional nanoprobes and conduct significant research in sewage treatment, biological imaging, drug delivery, and other fields [19,20]. However, due to the strong magnetic dipole attraction between the particles, Fe3O<sup>4</sup> na-

stability [13–15]. Among these, superparamagnetic iron oxide (Fe3O4) is particularly versatile due to its unique high coercivity, easily functional modification, excellent controllable

*Coatings* **2023**, *13*, x FOR PEER REVIEW 2 of 15

However, due to the strong magnetic dipole attraction between the particles, Fe3O<sup>4</sup> nanoparticles tend to aggregate. Therefore, stabilizers such as organic matter, semiconductors, and oxides with specific functional groups are often applied to the nanoparticles' surface in order to improve stability. Using a variety of biocompatible polymers to functionalize and modify the surface of Fe3O<sup>4</sup> nanoparticles to provide new functions is the focus of current research. When the stability of Fe3O<sup>4</sup> nanoparticles is guaranteed, the introduction of fluorescent substances is conducive to the separation and transfer of detection substances, especially heavy metals in water, thus expanding the application field of fluorescent probes. Aggregation-induced sedimentation technology can efficiently and rapidly remove Ag<sup>+</sup> from water samples and can avoid secondary pollution [21]. noparticles tend to aggregate. Therefore, stabilizers such as organic matter, semiconductors, and oxides with specific functional groups are often applied to the nanoparticles' surface in order to improve stability. Using a variety of biocompatible polymers to functionalize and modify the surface of Fe3O<sup>4</sup> nanoparticles to provide new functions is the focus of current research. When the stability of Fe3O<sup>4</sup> nanoparticles is guaranteed, the introduction of fluorescent substances is conducive to the separation and transfer of detection substances, especially heavy metals in water, thus expanding the application field of fluorescent probes. Aggregation-induced sedimentation technology can efficiently and rapidly remove Ag<sup>+</sup> from water samples and can avoid secondary pollution [21]. In this work, a novel environmentally friendly magnetic fluorescent nanosensor

In this work, a novel environmentally friendly magnetic fluorescent nanosensor (Fe3O4@ZnS@MPS(MFNPs)) modified with 3-sulfhydryl-1-propane sodium for coinstantaneous detection and removal of Ag<sup>+</sup> from water samples is reported (Scheme 1). The results showed that MFNPs achieved highly specific recognition and were extremely sensitive for the quantitative detection of Ag<sup>+</sup> over a wide pH range. The sensor has a low limit of detection (LOD) of 7.04 µM for Ag<sup>+</sup> . The optimal adsorption capacity of MFNPs was calculated to be about 395.79 mg/g and the optimal adsorption rate capacity of MFNPs was calculated to be about 98%. This work provides a new method for the synthesis of magnetic fluorescent nanosensors, exploiting their application not only for the detection of Ag<sup>+</sup> , but also other heavy metal ions, with the multiple functions of enrichment, detection, and separation. (Fe3O4@ZnS@MPS(MFNPs)) modified with 3-sulfhydryl-1-propane sodium for coinstantaneous detection and removal of Ag<sup>+</sup> from water samples is reported (Scheme 1). The results showed that MFNPs achieved highly specific recognition and were extremely sensitive for the quantitative detection of Ag<sup>+</sup> over a wide pH range. The sensor has a low limit of detection (LOD) of 7.04 µM for Ag<sup>+</sup> . The optimal adsorption capacity of MFNPs was calculated to be about 395.79 mg/g and the optimal adsorption rate capacity of MFNPs was calculated to be about 98%. This work provides a new method for the synthesis of magnetic fluorescent nanosensors, exploiting their application not only for the detection of Ag<sup>+</sup> , but also other heavy metal ions, with the multiple functions of enrichment, detection, and separation.

**Scheme 1.** Fabrication of MFNPs. **Scheme 1.** Fabrication of MFNPs.

#### **2. Materials and Methods**

#### **2. Materials and Methods** *2.1. Preparation of Fe3O<sup>4</sup> Magnetic Nanoparticle*

*2.1. Preparation of Fe3O<sup>4</sup> Magnetic Nanoparticle* The magnetic Fe3O<sup>4</sup> nanoparticle was synthesized via the solvothermal method and prepared as follows: FeCl3·6H2O (2.7 g) was dissolved in 60 mL ethylene glycol in a water bath and stirred until fully dissolved. Sodium acetate (7.2 g) and surfactant (0.5 g) were added and stirred for 30 min. The solution was moved to a Polytetrafluoroethylene kettle, which was sealed tightly, and the hydrothermal reaction was carried out at high temperature. The reaction was then allowed to end and to cool naturally. The solution was moved to a Teflon kettle, which was sealed, followed by a hydrothermal reaction at a high temperature and natural cooling at the end of the reaction. The black precipitate was collected after The magnetic Fe3O<sup>4</sup> nanoparticle was synthesized via the solvothermal method and prepared as follows: FeCl3·6H2O (2.7 g) was dissolved in 60 mL ethylene glycol in a water bath and stirred until fully dissolved. Sodium acetate (7.2 g) and surfactant (0.5 g) were added and stirred for 30 min. The solution was moved to a Polytetrafluoroethylene kettle, which was sealed tightly, and the hydrothermal reaction was carried out at high temperature. The reaction was then allowed to end and to cool naturally. The solution was moved to a Teflon kettle, which was sealed, followed by a hydrothermal reaction at a high temperature and natural cooling at the end of the reaction. The black precipitate was collected after washing several times, and the dispersive solution was prepared by adding water for further modification.
