cannot endow the SQD with enough stability. The prepared SQDs generally exhibit poor optical stability, limiting their down-to-earth applicability. For example, the fluorescent intensity of PEG-capped SQDs declined by 48% after exposure to UV lamp for 10 min. [4] In this context, it is very important and urgent to develop a simple and effective approach to enhance the optical stability of SQDs.As a kind of essential trace ion in organism, Fe 3+ plays an important role in numerous metabolic processes, such as enzyme catalysis, oxygen exchange, and transport. [13] Excessive or deficiency of Fe 3+ may result in a series of diseases (e.g., heart failure and anemia), and thus monitoring of Fe 3+ is quite meaningful for early clinical diagnosis. [14] On the other hand, temperature is a critical physical parameter that can affect the behavior of chemical and biological systems. Hence, the development of temperature sensor to monitor temperature in diverse environments especially in biosystem is highly needed. Among various technologies for monitoring of Fe 3+ or temperature, much attention has been paid on fluorescence detection because of its advantages of facile sample treatment process, superior sensitivity, rapid response, noninvasiveness, and low cost. [15,16] In recent years, a variety of fluorescent materials such as semiconductor quantum dots, silicon nanocrystals, metal nanoclusters, organic dyes, conjugated polymers, and carbon dots have been developed to detect Fe 3+ or temperature. [17][18][19][20][21][22][23][24] Unfortunately, most of them are either toxic or suffer from poor stability and high cost, which seriously hinders their practical applications. In addition, many fluorescent materials exhibit poor aqueous solubility or dispersibility, so their ability to detect Fe 3+ or temperature in biological environment (e.g., cells) is greatly limited. Therefore, the rational design and preparation of desired fluorescent probes to detect Fe 3+ or temperature in biological environment remain a challenge and is valuable.In this contribution, we attempt to utilize water-soluble and biocompatible polyvinyl alcohol (PVA) as ligand for fabricating fluorescent SQDs (Figure 1). PVA was carefully chosen because each PVA macromolecule contains a number of lateral hydroxyl groups, which can provide strong affinity interaction with the surface of formed SQDs to enhance the stability of SQDs. On the other hand, there is a strong complex interaction between As a new class of metal-free fluorescent dots, sulfur quantum dots (SQDs) have attracted more and more attention because of their unique properties and promising applications in many fields. However, synthesizing SQDs with high optical stability and stimuli-responsive fluorescence is still in its infancy. Herein, a simple one-pot strategy is reported for preparing polyvinyl alcohol (PVA)-capped SQDs from cheap elemental sulfur by employing PVA as ligand. The as-prepared SQDs with size in range of 1.5-5 nm show fine water dispersibility and stability, bright fluorescence,...
