Fiber Optic pH Sensor Using Optimized Layer-by-Layer Coating Approach

In this paper, the design and characterization of a high resolution wavelength dependent pH optical sensor fabricated using the layer-by-layer technique was undertaken. In this approach, brilliant yellow (BY) as a pH indicator and poly (allylamine hydrochloride) (PAH) as a cross-linker have been deposited on an uncladded silica fiber. A number of key parameters such as the number of bilayers, the concentration of the BY solution, the shape of the fiber, as well as its core diameter have been varied with a view to optimizing the design and performance of the pH sensor. The results obtained from a series of evaluations show that the sensitivity was enhanced by reducing the concentration of the indicator solution used and by designing a U-bend configuration sensor probe with a sharply bent fiber. However, when making an overall comparison, the straight (unbent) fiber probe resulted in a more sensitive probe when compared to the use of a high radius bend. Further, using a small core diameter of the fiber allows a wide pH range to be measured and with high sensitivity. In addition, the performance was shown to be improved for measurements over a narrower range of pH, using a fiber with a larger core diameter. Considering the effect of the number of layers, work carried out has shown that probes with 5-6 bilayers presented the best performance. The sensitivity has been shown to diminish when bilayers were used and the sensing range shifts toward higher pH values. When monitored, the value of pKa (the dissociation constant) of the thin film showed the smallest change of any of the design factors considered. In summary, using a larger core diameter, employing a larger curve radius, a higher number of bilayers, a higher concentration of the indicator solution, and applying PAH as an outer layer, all cause a higher pKa value and consequently the probe sensitivity moves toward alkaline region.