Covalently cross-linked multilayer thin films composed of diazoresin and brilliant yellow for an optical pH sensor

The multilayered thin films composed diazoresin (DR) and brilliant yellow (BY) were fabricated by a layer-by-layer (LbL) deposition technique, and the ionic bonds between diazonium ion in DR and sulfonate residues in BY were converted to covalent bonds by UV light irradiation. The cross-linking between BY and DR prevented BY from desorption even in a pH 13 solution. The cross-linked (DR/BY)10DR film exhibited pH-dependent absorption spectra in pH 9–13, and the response was repeatable and quick. The pH response time of the films was within a few seconds upon change from pH 9.0 to 13.0, and ca. 40 s upon change from pH 13.0 to 9.0.