  Recently, modulation of the energy bandgap of graphene when gas molecules are
adsorbed to its surface has been proved to be possible. Motivated by this,
based on numerical calculations, we investigate the effect of the associated
bandgap opening in graphene's spectrum on the sensing properties of the Faraday
rotation (FR) of linearly polarized electromagnetic modes in a 1D photonic
graphene-based sensor with a microcavity defect channel covered by two graphene
layers. Our proposed model introduces an optical contact-free mechanism for the
detection of low-concentration molecules attached to graphene's surface. We
also show that FR angles could reveal reversal signs for a different amount of
surface transfer doping.
