Multifunctional yellow/green fluorescent sulfur quantum dots with aggregation-induced red-shifted emission and multiple applications.

Yellow and green fluorescent sulfur quantum dots (SQDs) were successfully synthesized using cysteamine as a passivator via a one-pot hydrothermal approach. The as-obtained SQDs have uniform particle size and their average particle diameters are 19 nm and 17 nm, respectively. Dynamic light scattering (DLS) tests further validate the concentration-dependent aggregation behavior of the two types of SQDs. Optical property investigations demonstrate that the as-prepared Y-SQDs and G-SQDs exhibit prominent aggregation-induced red-shift emission and reversible fluorescence emission switching behavior regulated by concentration variation. Fluorescence lifetime decay curves are well fitted with a bi-exponential model, and the results reveal that Y-SQDs possess more surface states and defect sites than G-SQDs. The elevation of SQDs concentration aggravates nanoparticle aggregation and shortens the average fluorescence lifetime. These results demonstrate that the fluorescence emission behaviors of Y-SQDs and G-SQDs can be precisely and controllably modulated via concentration regulation. Benefiting from the unique concentration-dependent aggregation-induced red-shift emission characteristics, the obtained SQDs are successfully applied to the preparation of fluorescent inks, fluorescent composite films, and light-emitting diode (LED) devices. Furthermore, application exploration verifies that Y-SQDs and G-SQDs possess excellent application prospects in colorimetric detection, fluorescence sensing, and biological imaging fields.