Pushing the Boundaries of Interfacial Sensitivity in Graphene FET Sensors: Polyelectrolyte Multilayers Strongly Increase the Debye Screening Length

Nanomaterial-based FET sensors represent an attractive platform for ultrasensitive, real-time, and label-free detection of chemical and biological species. Nevertheless, because their response is screened by mobile ions, it remains a challenge to use them to sense in physiological ionic strength solutions. In this work, it is demonstrated, both experimentally and theoretically, that polyelectrolyte multilayers are capable of increasing the sensing range of graphene-based FETs. Potential shifts at graphene surfaces and film thickness are recorded upon the construction of PDADMAC/PSS polyelectrolyte multilayer (PEM) films. By correlation of the potential shift with the film thickness, the electrostatic screening length and the concentration of mobile ion inside the films have been deduced. Across the polymer interface the Debye length is increased more than 1 order of magnitude. The fundamentals of this strategy are described by a conceptually simple thermodynamic model, which accounts for the entropy loss of ion confinement and incorporates the effect of ions finite volume. Interestingly, the electrostatic screening inside the film strongly depends on the polymer density and the ionic strength of the solution. Of particular interest in physiological condition sensing, the PEM interfaces can extend the Debye length from 0.8 to 10 nm.