pH Dependent Optical Switching and Fluorescence Modulation of Molybdenum Sulfide Quantum Dots

Investigations on fluorescent molecules with fluorescence switching behavior as function of pH, temperature, ion concentration, etc. is highly desirable in the field of sensor device fabrication. In this study, the authors report an easy and eco-friendly hydrothermal route for the synthesis of pH dependent fluorescent MoS2 quantum dots (MoS2-QDs). The average size of the synthesized QDs is found to be approximate to 7 nm which was confirmed with transmission electron microscopy and atomic force microscopy. These MoS2-QDs show the pH dependent fluorescence switching behavior. Under pH approximate to 1 the fluorescence intensity of the MoS2-QDs is quenched while it shows approximate to 200 times enhancement under pH approximate to 13. This florescence ON/OFF switching is mainly due to the surface adsorbed functional groups (NH2, SO42-, OH-, etc.) and is a combined effect viz. protonation-deprotonation process, acid etching, quantum confined Stark effect and particle agglomeration. A plausible mechanism for this pH dependent ON/OFF switching behavior is also discussed in this study. The band gap calculation under different pH environment is also in good agreement to the hypothesis.