Sulfur quantum dots for fluorescence sensing and beyond: Advances in synthesis and multifunctional applications

Sulfur quantum dots (SQDs) have attracted increasing attention as an emerging class of metal-free nanomaterials due to their tunable photoluminescence, low toxicity, aqueous processability, elemental abundance, and versatile surface chemistry. These attributes have positioned SQDs as promising candidates for a wide range of optoelectronic, environmental, and biomedical applications. This review presents a comprehensive overview of recent progress in the synthesis, characterization, and functional applications of SQDs, with particular focus on the relationships between preparation methods, physicochemical features, and resulting performance. Diverse synthetic approaches, including top-down, bottom-up, hydrothermal, microwave-assisted, and green methodologies, are critically discussed to highlight the influence of key reaction parameters such as precursor composition, temperature, reducing agents, solvents, and surface passivation on particle size, defect structure, quantum yield, colloidal stability, and emission characteristics. The role of advanced characterization tools in elucidating morphology, crystallinity, surface functionalities, and electronic structure is also examined to provide insight into the origin of their optical and catalytic properties. Furthermore, recent developments in the application of SQDs are surveyed across fluorescence sensing, bioimaging, antimicrobial and antioxidant systems, photocatalysis, and emerging nanozyme platforms. Finally, current challenges associated with scalable production, mechanistic understanding, long-term stability, and practical implementation are outlined, together with future perspectives for the rational development of next-generation SQD-based nanomaterials.