An extended-range fibre optic pH sensor

The present work is concerned with the development of an optical-fibre pH sensor, realized with acid-base indicators immobilized on controlled-pore glasses, that is able to measure gastric pH. Several acid-base indicators have been covalently bound on controlled-pore glasses by a silylation process. A shift of the absorption peak, broadening of the absorption bands and in some cases a noticeable broadening of the pH range are observed. This last characteristic must not be undervalued, because it allows pH detection with optical fibres in an extended range (at least four pH units) compared with the typical pH range of an indicator in solution (1.5-2.OpH units). Particular care is also devoted to the presence of hysteresis and the ionic-strength effect, which can greatly decrease the lifetime of the probe and often prevents the use of the sensor for biomedical applications. A very compact and easy transportable sensor has been developed, equipped with an internal microprocessor that processes the signals and gives the results on a liquid-crystal display. The optical-fibre probe is constituted by a stainless-steel capillary drilled on the lateral surface with 7O?m holes and filled with treated controlled-pore glasses. A sensitivity of 0.007 pH units in the range 2.00-4.00 and of 0.015 in the extended range 1.2-5.2 is reached.