Optical Chemosensors based on Spiropyran-Doped Polymer Nanoparticles for Sensing pH of Aqueous Media

Photochromic polymers, which are prepared by the incorporation of photochromic compounds into polymer matrices, show fluorescence emission along with color change under UV light irradiation. Polymer nanoparticles yield high chromic properties at low chromophore loadings, as they have a large surface area to absorb a high level of light irradiation. Particle size is a significant parameter to control optical properties, where the decrease of particle size results in a high light absorption and efficiency of photochromism and fluorescence emission. Reverse atom transfer radical polymerization was used to synthesize methyl methacrylate homopolymer and its copolymers with different comonomers to yield polymers with a narrow molecular weight distribution. Spiropyran was doped to the polymeric nanoparticles during nanoprecipitation to yield photochromic polymer nanoparticles. Particle size below 100 nm for the photochromic nanoparticles was shown by dynamic light scattering. Morphology investigation with microscopic analysis showed spherical morphology for nanoparticles. The photochromic properties of the polymer nanocarriers were studied in both acidic and alkaline media. The results indicated that the pH of the media as well as the copolymer composition significantly affect the optical properties. Therefore, the photochromic polymer nanoparticles could have potential applications as optical pH chemosensors by colorimetric and fluorometric detection mechanisms. The nanoparticles with hydroxyl- or amine-functional groups were shown to be highly efficient for pH chemosensor applications. Finally, photochromic cellulosic papers prepared from the photochromic polymer nanoparticles were highly applicable in the detection of acid vapors.