  Despite the great promise of carbon nanotube field effect transistors (CNT
FETs) for applications in chemical and biochemical detection, a quantitative
understanding of sensor responses is lacking. To explore the role of
electrostatics in sensor transduction, experiments were conducted with a set of
highly similar compounds designed to adsorb onto the CNT FET via a pyrene
linker group and take on a set of known charge states under ambient conditions.
Acidic and basic species were observed to induce threshold voltage shifts of
opposite sign, consistent with gating of the CNT FET by local charges due to
protonation or deprotonation of pyrene compounds by interfacial water. The
magnitude of the gate voltage shift was controlled by the distance between the
charged group and the CNT. Additionally, functionalization with an un-charged
pyrene compound showed a threshold shift ascribed to its molecular dipole
moment. This work illustrates a method to produce CNT FETs with controlled
values of the turnoff gate voltage, and more generally, these results will
inform the development of quantitative models for the response of CNT FET
chemical and biochemical sensors.
