Wavelength dependent pH optical sensor using the layer-by-layer technique

In this work, the design and characteristics of a wavelength-dependent pH optical sensor have been studied. To create the sensor itself, brilliant yellow (BY) as a pH indicator and poly(allylamine hydrochloride) [PAH] as a cross-linker have been deposited on the end of a bare silica core of an optical fibre by use of a ‘layer-by-layer’ technique. In the experiments carried out to characterize the sensor, it was observed that the value of pKa (the dissociation constant) of the thin film is dependent both on the outer layer and the number of bilayers. A heat treatment process was applied to the sensor to reduce the effect on the deposited layers during the testing of the probe. As a result of these series of experiments, it could be concluded that the probe design on which were deposited structured layers comprising six double layers of (PAH/BY) showed the best sensitivity for a pH range from 6.80 to 9.00 (with an accuracy of ±0.20) and showing an average wavelength shift of 4.65 nm per 0.2 pH units, while the concentration of the BY and the PAH solutions was maintained as 0.25 mM and 2.5 mM respectively.