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

Abstract

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 MoS 2 quantum dots (MoS 2 ‐QDs). The average size of the synthesized QDs is found to be ≈7 nm which was confirmed with transmission electron microscopy and atomic force microscopy. These MoS 2 ‐QDs show the pH dependent fluorescence switching behavior. Under pH ≈ 1 the fluorescence intensity of the MoS 2 ‐QDs is quenched while it shows ≈200 times enhancement under pH ≈ 13. This florescence ON/OFF switching is mainly due to the surface adsorbed functional groups (NH 2 , SO 4 2− , 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.