Response time enhancement of pH sensing films by means of hydrophilic nanostructured coatings

This work describes the improvement in response time of thin film optical fluorescent sensors for pH detection based on the 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) pH-sensitive molecule. Two different kinds of structures were fabricated to demonstrate these concepts, using the electrostatic layer-by-layer (LbL) assembly method. These structures were built up on both glass slides and tapered ends of standard communications optical fiber substrates. The basic structures are based on a sensitive coating formed by the alternate deposition of the polymer poly(allylamine hydrochloride) (PAH) as the cationic monolayers and the polymer poly(acrylic acid) (PAA) and the fluorescent pH indicator HPTS as the anionic monolayers. These sensitive coatings showed a long response time when they were immersed in different pHs. In order to improve the performance of basic devices one approach was followed, which consisted of the deposition of a highly hydrophilic block before the sensitive coating. These enhanced devices were proved to decrease the response time dramatically. AFM and water contact angle measurements showed that properties of these structures including the hydrophilic block had higher roughness and lower contact angle (around 10°) whereas the ones without the hydrophilic block were smoother and had higher contact angles (around 50°). The response time of both devices was also measured while the pH was changed several times resulting in a remarkable reduction with the new devices. The most significant reduction was in rise time response between pH 3 and pH 7 which was five times minimized.