Photoresponsive Nanogels Based on Photocontrollable Cross-Links

Abstract

We present a new and general strategy for preparing photoresponsive nanogels. It is based on using light to reversibly change the cross-linking density of nanogel particles to control their swelling degree in aqueous solution. This control mechanism allows for gradual volume change of nanogel particles by light. For proof of concept, diblock copolymers composed of poly(ethylene oxide) and poly[2-(2-methoxyethoxy)ethyl methacrylate-co-4-methyl-[7-(methacryloyl)oxyethyloxy]coumarin] (PEO-b-P(MEOMA-co-CMA)) were synthesized; nanogels could easily be prepared by first photo-cross-linking the micellar aggregates at T > LCST of the P(MEOMA-co-CMA) block through dimerization of coumarin side groups upon absorption of λ > 310 nm UV light and then cooling the solution to T < LCST to obtain cross-linked water-soluble polymer nanoparticles. Under λ < 260 nm UV light, the reverse photocleavage of cyclobutane rings could be used to reduce the cross-linking density, leading to the swelling of nanogel particles with a volume increase by about 90%. The reversibility of the photoinduced volume change, the effects of the molecular weight of P(MEOMA-co-CMA) block and the content of coumarin groups on the photoresponsive behavior, and the use of nanogel particles for photocontrolled release were investigated.