High-stability pH sensing with a few-layer MoS₂ field-effect transistor

Recently, molybdenum disulfide (MoS₂), an emerging 2D material, has become an alternative candidate for ultra-sensitive biosensors due to its semiconducting behavior and the unique layer-by-layer atomic structure. Here, we report on highly stable and repeatable real-time pH sensing with few-layer MoS₂ field-effect transistor (FET) biosensors, fabricated with both HfO₂ and Al₂O₃/HfO₂ gate dielectrics on top of MoS₂ flakes exfoliated from natural crystals onto SiO₂/Si samples. Both types of sensors demonstrate a highly linear, stable and repeatable response over a wide pH range with near-ideal pH sensitivity close to the theoretical limit of 59.6 mV pH^-1. Ascribing from a different device operation regime in the pH sensing test-subthreshold regime for a sensor with an Al₂O₃/HfO₂ dielectric and linear regime for a sensor with HfO₂-a sensor with Al₂O₃/HfO₂ shows significantly higher current sensitivity (∼105-fold) and relatively better linearity than a sensor with HfO₂, while the latter shows relatively higher stability and higher repeatability. An Al₂O₃/HfO₂-coated MoS₂ FET reveals a high sensitivity or low detection limit of 0.01 pH.