An apparatus comprises: a sensing element formed on a buried oxide layer of a substrate and providing communication between a source region and a drain region; a gate dielectric layer on the sensing element, the gate dielectric layer defining a sensing surface on the sensing element; a passive surface surrounding the sensing surface; and a compound bound to the sensing surface and not bound to the passive surface, the compound having a ligand specifically configured to preferentially bind a target molecule to be sensed. An electrolyte solution in contact with the sensing surface and the passive surface forms a top gate of the apparatus.