An optical fiber pH sensor based on the electrostatic self-assembly method

A new type of optical fiber pH sensors has been fabricated using the electrostatic self-assembly (ESA) method. The device is based on the co-deposition of the polymer poly(allylamine hydrochloride) and the colorimetric pH indicator neutral red as the cationic monolayers, and the polymer polyfacrylic acid) that conforms the anionic monolayers. The coatings are built up monolayer by monolayer on the ends of standard communications optical fibers with a thickness of 9 nanometers per bilayer. The final structure of the coating is a homogeneous film that forms a nanoFabry-Perot interferometer. The sensing mechanism of the sensor relies on changes in the spectral response of the coating with the variations of pH. The range of measurement goes from pH 5 to 8.