Abstract: A robust fiber optic sensor and sensing method for reliable, long-term measurement of analytes is disclosed. The sensor comprises an optical interrogation region comprising an indicator dye confined at a distal end of an optical fiber and an analyte permeable membrane enclosing the interrogation region at the distal fiber end. In a preferred embodiment, a reservoir member is provided with excess dye for continuous replenishment of the interrogation region with dye over the lifetime of the sensor. In another preferred embodiment, the reservoir member comprises an indicator support for containment of excess dye. In one preferred embodiment, a ratiometric dye is used for monitoring optical signal-to-noise and signal drift caused by sensor aging. The sensor may be configured with a variety of alternative indicator dyes and membrane materials as a specific ion sensor for analyzing dissolved analytes such as gases, cations, and anions.