Point-functionalized carbon nanotube !eld-e”ect transistors can serve as highly sensitive detectors for biomolecules. With a probemolecule covalently bound to a defect in the nanotube sidewall, two-level random telegraph noise (RTN) in the conductance of the device is observed as a result of a charged target biomolecule binding and unbinding at the defect site. Charge in proximity to the defect modulates the potential (and transmission) of the conductance-limiting barrier created by the defect. In this Letter, we study how these single molecule electronic sensors are a”ected by ionic screening. Both charge in proximity to the defect site and bu”er concentration are found to a”ect RTN amplitude in a manner that follows from simple Debye length considerations. RTN amplitude is also dependent on the potential of the electrolyte gate as applied to the reference electrode; at high enough gate potentials, the target DNA is completely repelled and RTN is suppressed.