Multi-functional ratiometric fluorescent chemosensors of poly(N-isopropylacrylamide) containing rhodamine 6G and 1,8-naphthalimide moieties

We have synthesized a multi-functional copolymer poly(N-isopropylacrylamide) (PNIPAM) containing rhodamine 6G and 1,8-naphthalimide moieties via reversible addition-fragmentation chain transfer (RAFT) radical polymerization. In a neutral condition, only an emission band with a peak at 520 nm is observed in the copolymer, which originates from 1,8-naphthalimide. The 520-nm photoluminescence (PL) intensity is markedly increased with decreasing the pH value of Britton-Robinson buffers, but changed instinctively by the concentration of Fe3+. The spirolactam ring-opening of the rhodamine 6G moiety is responsible for the PL changes of the copolymer in acidic conditions and Fe3+ aqueous solution, correspondingly, the 555-nm PL band of rhodamine 6G can be found for excitation at 400 nm. The PL intensity at 555 nm relative to that at 520 nm (I555/I520) is dependent on the resonance energy transfer from 1,8-naphthalimide to rhodamine 6G, which can be controlled by the chain conformations of the themoresponsive copolymer. The good linear relationships between the PL changes and the detection objects mean that the copolymer can be served as ratiometric chemosensors to probe pH, Fe3+ ions and temperature.