▎ 摘　　要

A challenge for pH sensing is decreasing the minimum measurable pH per unit bandwidth in an economical fashion. Minimizing noise to reach the inherent limit imposed by charge fluctuation remains an obstacle. We demonstrate here graphene-based ion-sensing field-effect transistors that saturate the physical limit of sensitivity, defined here as the change in electrical response with respect to pH, and achieve a precision limited by charge-fluctuation noise at the sensing layer. We present a model outlining the necessity for maximizing the device carrier mobility, active sensing area, and capacitive coupling in order to minimize noise. We encapsulate large-area graphene with an ultrathin layer of parylene, a hydrophobic polymer, and deposit an ultrathin, stoichiometric pH-sensing layer of either aluminum oxide or tantalum pentoxide. With these structures, we achieve gate capacitances similar to 0.6 mu F/cm(2), approaching the quantum-capacitance limit inherent to graphene, along with a near-Nernstian pH response of similar to 55 +/- 2 mV/pH. We observe field- effect mobilities as high as 7000 cm(2) V-1 s(-1) with minimal hysteresis as a result of the parylene encapsulation. A detection limit of 0.1 mpH in a 60-Hz electrical bandwidth is observed in optimized graphene transistors.