  The rapidly increasing use of sensors throughout different research
disciplines and the demand for more efficient devices with less power
consumption depends critically on the emergence of new sensor materials and
novel sensor concepts. Atomically thin transition metal dichalcogenides have a
huge potential for sensor development within a wide range of applications.
Their optimal surface-to-volume ratio combined with strong light-matter
interaction results in a high sensitivity to changes in their surrounding.
Here, we present a novel, highly efficient sensing mechanism to detect
molecules based on dark excitons in these materials. We show that the presence
of molecules with a dipole moment transforms dark states into bright excitons
resulting in an additional pronounced peak in easy accessible optical spectra.
This effect exhibits a huge potential for sensor applications, since it offers
an unambiguous optical finger print for the detection of molecules - in
contrast to common sensing schemes relying on small peak shifts and intensity
changes.
