**1. Introduction**

Fluorescent quantum dots (QDs) have been widely used in the fields of biosensing, cell imaging and biomedicine due to their unique size and superior optical properties [1–9]. However, heavy metal quantum dots such as CdS and CdTe are limited in practical applications due to their potential cytotoxicity and environmental hazard [10–13]. As a kind of pure elemental quantum dot, sulfur quantum dots (SQDs) have attracted much attention in recent years because of their low toxicity, good water solubility, stable optical properties and abundant raw materials [14–19]. So far, there have been reports on the application of SQDs in biosensing, cell imaging and antibacterial [20–24]. However, the complex and time-consuming synthesis process and the use of environmentally harmful substances make the synthesis of SQDs still a challenge, which limits its practical applications [25–29]. Therefore, it is necessary to develop green and time-saving methods to synthesize SQDs.

Alkaline phosphatase (ALP), as a membrane-bound enzyme widely exists in a variety of organisms, participating in the process of dephosphorylation in cells and hydrolyzing the phosphate groups of various substrates [30,31]. The normal level of ALP in adult blood is 40 to 190 U/L. Its abnormal expression is closely related to many diseases, such as bone diseases, liver dysfunction and various cancers. Serum ALP levels can be used as a reference for clinical diagnosis [32,33]. Therefore, it is of grea<sup>t</sup> significance to develop

**Citation:** Ma, F.; Zhou, Q.; Yang, M.; Zhang, J.; Chen, X. Microwave-Assisted Synthesis of Sulfur Quantum Dots for Detection of Alkaline Phosphatase Activity. *Nanomaterials* **2022**, *12*, 2787. https:// doi.org/10.3390/nano12162787

Academic Editor: Simone Morais

Received: 14 June 2022 Accepted: 10 August 2022 Published: 14 August 2022

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methods for effective detection of ALP. To date, surface-enhanced Raman scattering (SERS), electrochemical analysis, colorimetric detection, fluorescence analysis and other methods have been developed for the detection of ALP [34–38]. Among them, fluorescence detection technology has attracted significant attention because of its convenient operation, high throughput and high sensitivity.

Several fluorescent material-based chemical sensors have been developed for the detection and analysis of ALP [39]. For example, Li and co-workers designed a smartphonebased sensing strategy for ALP analysis using amino-functionalized copper (II)-based metal-organic frameworks (NH2-Cu-MOFs), which have both oxidase mimetic properties and fluorescence properties. The catalytic activity of NH2-Cu-MOFs was greatly inhibited due to the binding ability of Cu2+ with pyrophosphate (PPi). After the addition of ALP, the catalytic activity of NH2-Cu-MOFs was recovered due to the hydrolysis of PPi into orthophosphate by ALP, and then o-phenylenediamine was further catalyzed to form 2,3-diaminophenazine, which constituted a ratiometric fluorescent probe for the detection of ALP. The method has been successfully applied to the determination of ALP in serum samples. Zhang et al. constructed a novel near infrared ratiometric fluorescent probe (APT), which can achieve a rapid response to ALP (Within 10 min) [40]. After adding ALP, the fluorescence spectrum showed a shift (from 580 to 650 nm), and the near-infrared fluorescence emission (650 nm) made it more suitable for biological detection. The method has been successfully applied to the determination of ALP in serum and the detection and imaging of endogenous ALP in cells. In addition, nanocomposites have also been used to construct fluorescent sensors to detect ALP. Li's group developed a simple hydrothermal method to construct "three-in-one" nanocomposites (Fef NCs) for the detection of ALP [41]. Fef NCs consist of three components, in which MnO2 nanosheets (NSs) are assembled on Fe3O4 nanoparticles (NPs), and then CeO2 NPs are modified. The nanometer material has various catalytic activity, and can realize label-free, ultrasensitive and selective detection of ALP by utilizing that characteristic. Duan et al. utilized WS2 quantum dots and MnO2 nanosheets to form a nanocomposite system to detect ALP [42]. MnO2 nanosheets can quench the blue fluorescence of tungsten disulfide quantum dots (WS2 QDs). However, in the presence of ALP and amifostine, their hydrolysis products triggered the decomposition of MnO2 nanosheets. This results in the restoration of fluorescence. Based on this discovery, the researchers successfully used the switch principle to detect ALP and used it in the analysis of actual samples. However, the above methods have limitations such as use of inorganic substances which limit their application and are harmful to the environment, and cumbersome synthesis or construction processes. Therefore, it is necessary to develop green, simple methods for efficient and sensitive detection of ALP.

In this work, SQDs with stable optical properties were prepared by microwave-assisted heating using sublimed sulfur as a precursor and PEG-400 as a stabilizer. The synthesized SQDs was utilized for detecting ALP through a fluorescence "off-on" mechanism, as shown in Scheme 1. As the emission band of SQDs and the absorption band of Cr (VI) are well matched, there is a strong internal filtering effect (IFE) between them, so the addition of Cr (VI) can well quench the fluorescence of SQDs. However, ascorbic acid (AA) can reduce Cr (VI) to Cr (III), so the addition of AA can restore the fluorescence of the quenched SQDs. In addition, ALP can hydrolyze 2-phospho-L-ascorbic acid (AAP) to AA and phosphate ions; therefore, the activity of ALP can be detected via the recovery of the fluorescence of SQDs. Based on the above principle, the relationship between the fluorescence characteristics of sulfur quantum dots, Cr (VI), AA as well as ALP was explored, and the performance of SQDs for detection of ALP was studied in detail. The microwave synthesis of SQDs greatly shortens the synthesis time of SQDs and simplifies the operation steps, which have grea<sup>t</sup> reference significance for the exploration of the synthesis of SQDs. At the same time, the obtained SQDs are successfully used in the detection of ALP, which broadens the application of SQDs in biosensors.

**Scheme 1.** Synthesis of SQDs and schematic diagram for determination of ALP.
