Optical sensor for pH monitoring using a layer-by-layer deposition technique emphasizing enhanced stability and re-usability

Stable and reliable operation of an optical sensor for pH monitoring is important for many industrial applications of these types of devices. The layer-by-layer deposition technique is a simple and versatile method used to deposit a sensitive thin film on such an optical fibre-based device but creating a coating which can often be destroyed in use in highly acid or alkali solutions i.e. with very low or very high pH. It is thus important to create stable and durable sensors to meet the needs of users for operation under these extreme environments. The main aim of this study has been to prepare a number of such sensors and compare the performance of three different stabilization approaches used for the development of an effective wavelength-dependent pH-sensitive optical sensor. Techniques such as employing heat treatment, the deposition of two layers of a PAH/SiO2 (poly(allylamine hydrochloride)/Silica nanoparticle) thin film and the deposition of two layers of APTMS/SiO2 (3-aminopropyl-trimethoxy silane/Silica nanoparticle) as topping layers have been studied to determine the optimum approach to creating a stable and reliable sensor-one yielding the same value of peak wavelength for a measurement of a known value of pH and to do so repeatedly. An improvement in performance and in shelf-life, stability and re-usability of the sensor has been achieved by the addition of two bilayers of APTMS/SiO2 in the work carried out and the results of the investigation undertaken are reported.