An optical fiber acid-sensor based on protonation of a porphyrin solution within a single-hole structured optical fiber is proposed and demonstrated. The liquid-core fiber acts as a microcell reactor, and changes in the spectral signature with acidification are detected. Challenges and limitations in the practical deployment of such sensors are evaluated. An effective chemical sensor is demonstrated, but issues such as diffusion limit its full utilization. Some solutions are discussed. (c) 2010 Optical Society of America