  Topologically protected plasmonic modes located inside topological bandgaps
are attracting increasing attention, chiefly due to their robustness against
disorder-induced backscattering. Here, we introduce a bilayer graphene
metasurface that possesses plasmonic topological valley interface modes when
the mirror symmetry of the metasurface is broken by horizontally shifting the
lattice of holes of the top layer of the two freestanding graphene layers in
opposite directions. In this configuration, light propagation along the
domain-wall interface of the bilayer graphene metasurface shows unidirectional
features. Moreover, we have designed a molecular sensor based on the
topological properties of this metasurface using the fact that the Fermi energy
of graphene varies upon chemical doping. This effect induces strong variation
of the transmission of the topological guided modes, which can be employed as
the underlying working principle of gas sensing devices. Our work opens up new
ways of developing robust integrated plasmonic devices for molecular sensing.
