Organic field-effect transistors (OFETs) have shown great promise for use as chemical sensors for applications that range from the monitoring of food spoilage to the determination of air quality and the diagnosis of disease. However, for these devices to be truly useful, they must deliver reliable and stable low-voltage operation over extended timescales. An important element to address this challenge is the development of a high-capacitance gate dielectric that delivers excellent insulation with robust chemical resistance against the solution processing of organic semiconductors (OSC). The development of a bilayer gate dielectric containing a high-k fluoropolymer relaxor ferroelectric layer modified at the OSC/dielectric interface with a photo-crosslinked chemically resistant low-k methacrylate-based copolymer buffer layer is reported. Bottom-gate OFET chemical sensors using this bilayer dielectric operate at low-voltage with exceptional operational stability. They deliver reliable sensing performance over multiple cycles of ammonia exposure (2 to 50 ppm) with an estimated limit-of-detection below 1 ppm.