Ion-Selective Membrane-Coated Graphene-Hexagonal Boron Nitride Heterostructures for Field-Effect Ion Sensing

An intrinsic ion sensitivity exceeding the Nernst-Boltzmann limit and an sp -hybridized carbon structure make graphene a promising channel material for realizing ion-sensitive field-effect transistors with a stable solid-liquid interface under biased conditions in buffered salt solutions. Here, we examine the performance of graphene field-effect transistors coated with ion-selective membranes as a tool to selectively detect changes in concentrations of Ca²⁺, K⁺, and Na⁺ in individual salt solutions as well as in buffered Locke's solution. Both the shift in the Dirac point and transconductance could be measured as a function of ion concentration with repeatability exceeding 99.5% and reproducibility exceeding 98% over 60 days. However, an enhancement of selectivity, by about an order magnitude or more, was observed using transconductance as the indicator when compared to Dirac voltage, which is the only factor reported to date. Fabricating a hexagonal boron nitride multilayer between graphene and oxide further increased the ion sensitivity and selectivity of transconductance. These findings incite investigating ion sensitivity of transconductance in alternative architectures as well as urge the exploration of graphene transistor arrays for biomedical applications.