Embodiments include chemical sensors, devices and systems including the same, and related methods. In an embodiment, a medical device is provided. The medical device can include a graphene varactor, including a graphene layer and a self-assembled monolayer disposed on an outer surface of the graphene layer through electrostatic interactions between a partial positive charge on hydrogen atoms of one or more hydrocarbons of the self-assembled monolayer and a π-electron system of graphene. The self-assembled monolayer can include one or more substituted porphyrins or substituted metalloporphyrins, or derivatives thereof. Other embodiments are also included herein.