Monitoring gastric pH for long periods, usually 24 h, may be essential in analyzing the physiological pattern of acidity,
in obtaining information on changes in activity during peptic ulcer disease, and in assessing the effect of antisecretory
drugs. Gastro-esophageal reflux, which causes a pH decrease in the esophagus content from pH 7 even down to pH 2,
can determine esophagitis with possible strictures and Barrett's esophagus. One of the difficulties of the optical
measurement of pH in the gastro-esophageal apparatus lies in the required extended working range from 1 to 8 pH units.
The present paper deals with a novel optical pH sensor, using methyl red as optical pH indicator. Contrary to all acidbase
indicators characterized by working ranges limited to 2-3 pH units, methyl red, after its covalent immobilization on
controlled pore glass (CPG), is characterized by a wide working range which fits with the clinical requirements. The
novel probe design here described is suitable for gastro-esophageal applications and allows the optimization of the
performances of the CPG with the immobilised indicator. This leads to a very simple configuration characterized by a
very fast response time.