  The Hall resistance obtained in liquid gated Hall effect measurement of
graphene demonstrates a higher sensitivity than the sheet resistance and the
gate-source current for L-histidine of different concentrations in the pM
range. This indicates that the extra information offered by the liquid gated
Hall measurement of graphene can improve the sensitivity of the
transistor-based potentiometric biosensors, and it could also be a
supplementary method to the amperometric techniques for electrochemically
inactive molecules. Further analysis of the system suggests that the asymmetry
of the electron-hole mobility induced by the ions in the liquid serves as the
sensing mechanism. The calculation on the capacitance values shows that the
quantum capacitance is only dominant near the "Dirac" point in our system. This
conclusion is useful for many applications involving graphene-electrolyte
systems, such as bio-sensing, energy storage, neural stimulation, and so on.
