  Liquid-based applications of biomolecule-decorated field-effect transistors
(FETs) range from biosensors to in vivo implants. A critical scientific
challenge is to develop a quantitative understanding of the gating effect of
charged biomolecules in ionic solution and how this influences the readout of
the FETs. To address this issue, we fabricated protein-decorated graphene FETs
and measured their electrical properties, specifically the shift in Dirac
voltage, in solutions of varying ionic strength. We found excellent
quantitative agreement with a model that accounts for both the graphene
polarization charge and ionic screening of ions adsorbed on the graphene as
well as charged amino acids associated with the immobilized protein. The
technique and analysis presented here directly couple the charging status of
bound biomolecules to readout of liquid-phase FETs fabricated with graphene or
other two-dimensional materials.
