Engineering of new spiropyran photochromic fluorescent polymeric nanoparticles of narrow size distribution by emulsion polymerization process

Abstract Spiropyran derivatized vinylic monomer (MSP) was prepared by coupling a typical spiropyran dye, 1′-(2-hydroxyethyl)-3′,3′-dimethyl-6-nitrospiro(2H-1-benzo-pyran-2,2′-indoline), to methacryloyl chloride. Poly(styrene-butylmethacrylate-divinylbenzene-MSP), P(S-BMA-DVB-MSP), photochromic fluorescent nanoparticles (NPs) of narrow size distribution were prepared by emulsion polymerization of the vinylic monomers: S, BMA, DVB and MSP. The influence of various polymerization parameters (e.g., concentration of the monomers, initiator and surfactant and pH) on the diameter, diameter distribution and photochromic fluorescence properties of the particles was elucidated. Spiropyran type molecules as well as MSP are colorless and non-fluorescent at the Spiro conformation. These compounds at the Merocyanine conformation are non-fluorescent in water or in non-polar organic solvents. However, when the Merocyanine conformation is entrapped within hydrophobic polymer particles, the particles exhibit blue color and strong red fluorescence. Absorbance and fluorescence measurements reveal that increasing the MSP or BMA relative concentration leads to a higher absorbance and fluorescence intensity of the formed P(S-BMA-DVB-MSP) NPs. Photoisomerization switching of the P(S-BMA-DVB-MSP) NPs was studied by exposing the NPs aqueous dispersion to alternating UV and visible light cycles. This study indicates 100% reversibility after five cycles of photoswitching.