Triply responsive coumarin-based microgels with remarkably large photo-switchable swelling

Swellable crosslinked polymer colloid particles (microgels) which have a swollen size that can be reversibly changed using light are rare. We report the synthesis of four different triply responsive microgels with as-made diameters ranging from 25 to 124 nm. Each MG system is pH-, temperature- and photo-responsive. These poly(MEO₂MA-co-MAA-co-CMA) microgels were prepared using precipitation copolymerisation of 2-(2-methoxyethoxy)ethyl methacrylate (MEO₂MA), methacrylic acid (MAA) and 7-(2-methacryloyloxyethoxy)-4-methylcoumarin (CMA). This one-step synthesis method is scalable. The microgel systems were strongly pH- and temperature-responsive. They were also photo-switchable between highly crosslinked and de-crosslinked states using UV-irradiation. The effective crosslinker density was easily controlled by irradiation at wavelengths of 365 nm (photo-crosslinked state) and 254 nm (photo-de-crosslinked state). Interestingly, the particle volume ratio increase upon photo-de-crosslinking was found to be at least twice that reported for other nanoparticle-based systems. The strong photo-switching behaviour of the present microgels was shown to be due to a uniform incorporation of CMA within the particles. Unexpectedly, it was found that the extent of photo-induced dimerisation increased with a decrease in the as-made microgel size. The photo-induced swelling of the microgel was used to accelerate the release of the anti-cancer drug doxorubicin to demonstrate potential for future application for on-demand drug release.