We constructed a type of sensor by depositing a solgel layer within the interior holes of a silica-structured fiber and, subsequently, coating this with an acid-responsive porphyrin. Protonation of the porphyrin by an acidic gas (HCl in this case), is detected by a large change in the visible spectrum. Compared to previous work on a liquid-core sensor in a structured optical fiber, the signal-to-noise ratio of this gas sensor shows a reduced signal strength, but the detection rate is increased about fortyfold.