Quantum capacitance and electrolyte screening effects in graphene biosensors

Ultra-sensitive detection of bio-chemical molecules is a topic of immense recent research interest. Accordingly, there has been several reports of detection of DNA and proteins using Graphene field effect transistors (GFET). In spite of these exciting results, surprisingly, the theoretical understanding of GFET biosensors is still not fully developed to the extent that new experiments could be designed and optimized. In this regard, here we provide an analytical model for the response of GFET biosensors by considering relevant aspects like kinetics of bio-molecule conjugation, the quantum capacitance of graphene and the electrolyte screening effects. Through this, we identify the functional dependence of the charge neutrality point or Dirac point of GFET on the various parameters like target molecule concentration, ionic charge density, etc. Our modeling results compare well with experimental data and are aimed towards providing a much needed predictive capability to the research on GFET biosensors.