**1. Introduction**

As one of the essential trace elements for humans, Cu2+ plays an important role in many physiological processes, such as hemoglobin regulation, bone formation, and cell metabolism [1]. However, high concentrations of Cu2+ can affect the function of the kidney and liver and may cause brain diseases, such as Alzheimer's disease and Parkinson's disease [2]. Even short-term exposure to high concentrations of Cu2+ can cause stomach and intestinal discomfort [3]. Cu2+ is taken up by humans through contaminated water and food and accumulates in the human body. To this end, the maximum residue levels of Cu2+ in water are clearly defined. The United States Environmental Protection Agency (USEPA) stipulated that the maximum allowable amounts of Cu2+ in drinking water and in industrial effluents are 20 μM [4] and 1.3 mg/L [5], respectively. Therefore, detecting Cu2+ in water is of grea<sup>t</sup> significance for preventing Cu2+ from entering the human body and ensuring human health.

There are many methods for detecting Cu2+, such as atomic absorption spectrometry [6], inductively coupled plasma optical emission spectrometry (ICP-OES) [7], inductively coupled plasma mass spectrometry (ICP-MS) [8], electrochemical methods [9], and fluorescence methods [10]. Atomic absorption spectrometry, ICP-OES, and ICP-MS are highly accurate for the detection of Cu2+; however, the disadvantages of expensive equipment, cumbersome operation, and time consumption [11,12] make them unable to detect mobile Cu2+ in real time. Electrochemical methods have the advantages of low cost, simple operation, and portability; however, the electrodes have to be maintained due to the

**Citation:** Xu, Z.; Wang, Y.; Zhang, J.; Shi, C.; Yang, X. A Highly Sensitive and Selective Fluorescent Probe Using MPA-InP/ZnS QDs for Detection of Trace Amounts of Cu2+ in Water. *Foods* **2021**, *10*, 2777. https://doi.org/10.3390/foods10112777

Academic Editors: Hong Wu and Hui Zhang

Received: 13 October 2021 Accepted: 8 November 2021 Published: 11 November 2021

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polarization effect [13]. Fluorescence methods have attracted increasing attention in the field of Cu2+ detection in recent years due to their high sensitivity, anti-interference ability, and fast response. In particular, with the development of fluorescent nanomaterials, Cu2+ fluorescent strategies with simple and environmentally friendly designs have attracted increasing interest [14,15].

Quantum dots (QDs) are widely used in the detection of Cu2+ in water due to their advantages of simple preparation, stable optical performance, adjustable emission, and surface modification [16]. Sadeghi et al. [17] developed a fluorescent probe based on CdSe QDs capped with deep eutectic solvent (DES-CdSe QDs) to determine the levels of Cu2+ in various drinks based on aggregation-induced emission (AIE), but CdSe QDs are poisonous and environmentally unfriendly, which limits the application of the method. Carbon dots (CDs) are some of most popular QDs in the detection of Cu2+ [18]. The fluorescence of CDs can be quenched in the presence of Cu2+ based on the complexation reaction of Cu2+ with the oxygen-containing functional groups on the surface of the CDs. Although CDs have the advantages of simple synthesis, low price, and nontoxicity, some metal ions that could be present in the water, such as iron ions and mercury ions, can also quench the fluorescence of the CDs [19,20], which can interfere with the detection of Cu2+. Therefore, in real environmental samples, eliminating the influence of potential interfering substances to detect Cu2+, even in trace amounts and with highly sensitive methods, is one of the main challenges.

In this work, we synthesized 3-mercaptopropionic acid (MPA)-capped InP/ZnS QDs (MPA-capped InP/ZnS QDs) for the first time and developed a fluorescent probe based on MPA-InP/ZnS QDs for the detection of trace Cu2+ in water. With proper capping of MPA, the MPA-InP/ZnS QDs exhibit excellent monodispersity in aqueous media and are suitable for the highly selective detection of trace Cu2+. In the presence of Cu2+, the fluorescence of MPA-InP/ZnS QDs could be significantly quenched, the sensing performance was studied, and the possible mechanism was proposed. Furthermore, the probe was applied to the detection of Cu2+ in environmental water and drink samples. To the best of our knowledge, there are few reports on the use of MPA-InP/ZnS QDs to detect Cu2+ in water.
