Rapid and green synthesis of highly luminescent MoS2 quantum dots via microwave exfoliation of MoS2 powder and its application as a fluorescence probe for cortisol detection in human saliva

Abstract

We have introduced a simple, straightforward, green, and fast microwave exfoliation of MoS2 powder to prepare highly luminescent MoS2 quantum dots (QDs). The microwave irradiation of MoS2 powder was evaluated in the different solvents, including EtOH 30%, MeOH 30%, NaOH 1 M, and NH4OH 1 M in both open and closed microwave system setups. The results indicated that the highest luminescent MoS2 QDs is obtained when MoS2 powder was dispersed in 1 M ammonium solution and exposed to 850 W microwave radiation in a closed microwave system at 30 bars for 30 min. Different microscopic and spectroscopic techniques were used to characterize as-prepared MoS2 QDs. The MoS2 QDs have shown the maximum emission at 413 nm with excitation at 310 nm. The Quantum yield of as-prepared QDs was 1.98%. Cu2+, Pb2+, Cd2+, Ag+, Fe3+, and Al3+ quenched the luminescence intensity of MoS2 QDs. The luminescence of MoS2 QDs-Ag+ and Cu2+ mixtures were recovered on addition of cortisol with the higher emission recovery for Cu2+ ion. Therefore, proof-of-concept for cortisol sensing in human saliva was evaluated and discussed using off-on protocol. A linear cortisol calibration curve was obtained in the range of 100 ng mL−1 to 500 ng mL−1, with a detection limit of 16 ng mL−1 (S / N = 3). The method was successful in determining the cortisol content of saliva.