Synthesis of Poly(N-phenylmaleimide) and Characterization of Fluorescence Properties for Biomedical Applications

Abstract

We synthesized poly(N-phenylmaleimide) by anionic ring opening polymerization method that have light emitting property for biomedical applications, especially sensors. Many polymer based sensors for biomolecules are based on turn-off fluorescence under target substance exposure. Due to the low sensitivity of turn-off type sensors to human naked eye, several attempts have been done to develop turn-on type sensors. Not like turn-off sensors, turn-on type sensor has advantage, easy to find the sensing target material with low concentration. So, for biomolecule sensing, we need to develop new polymeric material suitable for turn-on type sensor. Series of poly(N-phenylmaleimide) have been synthesized by varying initiators through anionic polymerization method. The synthesized polymer shows different light emitting properties according to its initiators. Polymers synthesized by alkali-metal tert-butoxide type of initiator emit yellow color. On the other hand, using n-butyllithium as initiator case, green emission was observed. For understanding different fluorescence properties of synthesized polymers, we carried out 1H and 13C NMR analysis for studying tacticity of polymers. Through the analysis we can assume that different portion of threo-disyndiotactic and threo-diisotactic causes the differences in wavelength of emitted wavelength. At the same time, poly(N-phenylmaleimide) shows strong pH-dependent fluorescence properties. Under acid condition, the polymer shows quenched fluorescence properties. The fluorescence from acidified polymer solution became recovered when the surroundings became neutral. To understand the stimuli-responsive behavior of poly(N-phenlmaleimide), we suggest the mechanism of fluorescence quenching. The peak disappearance under acidic condition, we assume the changes in the state of amines in N-phenylmaleimide, repeating unit. Forming and collapse of quaternary amines causes such an fluorescence changing of poly(N-phenylmaleimide). For biomedical application of poly(N-phenylmaleimide), human creatine kinase M (CKM) and interferon gamma (IFN-γ) were used. Those proteins are strongly related to common diseases, sensing these proteins can help early diagnosis. The principles of sensing proteins by using poly(N-phenylmaleimide) is accommodation of protons according their PI values. Under acidic condition, proteins accommodate protons and their surface charges became positive and in here different PI values causes different sensing performance. In the presence of few nanomole of proteins, pre-quenched fluorescence by acid became recovered. The limit concentration of proteins for sensing is different according to their PI values, 6.8 for CKM and 11 for IFN-γ.