High resolution potassium sensing with large-area graphene field-effect transistors

We demonstrate large-area graphene based ion sensitive field effect transistors (ISFETs) with potassium ionophore sensing layers. Graphene ISFETs encapsulated with an ultra-thin hydrophobic layer, parylene C, and active areas similar to 0.4 cm(2) are characterized by an rms current noise as low as 5 nA in a 60 Hz electrical bandwidth, field effect mobilities up to 5000 cm(2) V-1 s(-1) and quantum capacitance limited coupling. Real-time sensing of K t was achieved with a detection limit of 10(-9) M K+, equivalent to 39 ng/L, and a resolution of similar to 2 x 10(-3)log [K+]. The ISFETs exhibit reversibility under spiking experiments and long term stability with limited drift over a course of five months. The cross-sensitivity has been measured to be 2.5 mV/decade for Na+, 4.2 mV/decade for Ca2+, 1.5 mV/decade for Mg2+ and 9.0 mV/decade for NH4+. Experiments with a variety of specimens, including beverages and blood, confirm the suitability of graphene ISFETs for K+ sensing in fluids with multiple solutes. The graphene ISFET design can be extended to sensing of other ionic species by substitution of alternative ionophores within the sensing membrane.