  Much effort has been made to modify the properties of transition metal
dichalcogenide layers via their environment as a route to new
functionalization. However, it remains a challenge to induce large electronic
changes without chemically altering the layer or compromising its
two-dimensionality. Here, a non-invasive technique is used to shift the
chemical potential of monolayer MoS$_2$ through p- and n-type doping of
graphene (Gr), which remains a well-decoupled 2D substrate. With the
intercalation of oxygen (O) under Gr, a nearly rigid Fermi level shift of 0.45
eV in MoS$_2$ is demonstrated, whereas the intercalation of europium (Eu)
induces a metal-insulator transition in MoS$_2$, accompanied by a giant band
gap reduction of 0.67 eV. Additionally, the effect of the substrate charge on
1D states within MoS$_2$ mirror-twin boundaries (MTBs) is explored. It is found
that the 1D nature of the MTB states is not compromised, even when MoS$_2$ is
made metallic. Furthermore, with the periodicity of the 1D states dependent on
substrate-induced charging and depletion, the boundaries serve as chemical
potential sensors functional up to room temperature.
