Single-layer molybdenite (MoS2) is a direct-bandgap semiconductor. Extremely thin flakes of the material can be produced using the same liquid exfoliation methods used to obtain graphene and the flakes produced are just 1–2 nm thick and 10–30 nm wide.

Recently, researchers found that 2D MoS2 emits light when optically excited. The material photoluminesces thanks to the hybridization between the pz orbitals of S atoms and the d orbitals of Mo atoms in MoS2 when the crystal thickness is reduced. As the thickness reduces to a single layer, the material goes from being an indirect bandgap semiconductor to a direct bandgap one – something that enhances the amount of light it emits even further.

Monitoring photoluminescence change

A team of scientists in the US and Australia has now exploited the photoluminescence of 2D MoS2 to turn it into a biosensor. The researchers have found that when hydrogen and potassium ions are released from cells and enzymes, they insert (or intercalate) between the individual MoS2 flakes and so modulate the photoluminescence (PL) of the material. “By monitoring this PL change, we are able to use the material as a fluorophore to optically identify ion exchanges in yeast cells, for example, and detect glucose in solution,” explained team member Jian Zhen Ou.

The researchers showed that the PL of a 2D MoS2-glucose oxidase system quenches as the concentration of glucose in its surrounding environment increases. In the same way, the PL in a 2D MoS2-live yeast cell system quenches as potassium ions are released by the yeast cell membranes.

A better understanding of biological processes

“This ion-driven PL change could come in very useful for monitoring ion exchanges in living cells and so help us better understand a number of fundamental biological processes,” team leader Kourosh Kalantar-Zadeh told Ions such as H+, Li+, Na+ and K+ are vital in bioprocesses such as nerve transmission, regulating body fluids, heart activity and certain metabolic functions.

The team says that it now plans to focus on integrating micro/nanofluidic technologies with 2D MoS2 to make practical and easy to use systems for biosensing.

The current work is detailed in Nano Letters DOI: 10.1021/nl4042356.

Further reading

First light for MoS2 (Apr 2013)
MoS2 in new mobility record Mar 2013)