We report the development of environmentally responsive fluorescent
polymers. The reversible temperature-induced phase transition of copolymers composed of
N-isopropylacrylamide and a fluorescent monomer based on the
fluorescein (FL),
coumarin (CO),
rhodamine (RH), or
dansyl (DA) skeleton was used as a molecular switch to control the fluorescence intensity. The
poly(N-isopropylacrylamide) (
PNIPAAm) chain showed an expanded coil conformation below the lower critical
solution temperature (LCST) due to hydration, but it changed to a globular form above the LCST due to
dehydration. Through the combination of a polarity-sensitive fluorophore with
PNIPAAm, the synthetic fluorescent
polymer displayed a response to external temperature, with the fluorescence strength dramatically changing close to the LCST. Additionally, the P(NIPAAm-co-FL) and P(NIPAAm-co-CO)
polymers, containing
fluorescein and
coumarin groups, respectively, exhibited pH responsiveness. The environmental responsiveness of the reported
polymers is derived directly from the
PNIPAAm and fluorophore structures, thus allowing for the cellular uptake of the fluorescence copolymer by RAW264.7 cells to be temperature-controlled. Cellular uptake was suppressed below the LCST but enhanced above the LCST. Furthermore, the cellular uptake of both P(NIPAAm-co-CO) and P(NIPAAm-co-RH) conjugated with a fusogenic
lipid, namely, l-α-
phosphatidylethanolamine, dioleoyl (DOPE), was enhanced. Such
lipid-conjugated fluorescence probes are expected to be useful as physiological indicators for intracellular imaging.