New environmentally responsive fluorescent N-isopropylacrylamide copolymer and its application to DNA sensing

We report two novel multifunctional copolymers consisting of a temperature-responsive poly(N-isopropylacrylamide) (PNIPAA) segment and a fluorescent fluorene-containing acrylic polymer segment with pH responsiveness and/or DNA-sensing ability. The functional acrylic monomer with a fluorene dimer side group substituted with amino units was synthesized first. Then, it was copolymerized with N-isopropylacrylamide to result in a new water-soluble, fluorescent PNIPAA copolymer (P1). The fluorescent properties of PI under neutral and acidic conditions did not change with the temperature. However, significant variation was observed under basic conditions. The protonation of the amino moiety at a low pH improved the solubility and prevented aggregation for fluorescence quenching, but not under the basic conditions. Although aggregation of the fluorene units was significant at room temperature under basic conditions, the aggregation was resolved at a temperature above the lower critical solution temperature. These findings indicated the pH- and temperature-responsive characteristics of P1. Moreover, after the amino groups were quaternized, the obtained polymer could be used as a biosensor because the fluorescence intensity was quenched with the addition of DNA. This study demonstrates that multifunctional materials with pH- and temperature-sensing characteristics and biological molecules could be realized by the incorporation of a functional fluorene-containing moiety with PNIPAA.