Synthesis And Properties Of Fluorescent Polymer Gel Probes Based On Azobenzene Reduction Responsiveness

In addition to photoisomerization,azobenzene show special reduction responsiveness.The N=N bond of azobenzene can be reduced by chemical reductants(such as Na2S2O4,hydrazine,etc.)and specific azo-reductases existed in the colon.Therefore,azobenzene polymers have been widely used in the field of stimuli-responsive smart materials.In the past decades,azobenzene biomaterials based on azo reductase responsivenesshave attracted much attention.Polymer gels,as widely used reactive materials have been the focus of research,especially the biocompatible hydrogels.Due to its good performance,polymer fluorescent probes have shown important application prospects in the fields of bio-detection,bio-imaging and biosensors.Tetraprene(TPE)is a typical fluorescence molecule with the properties of aggregation induced emission(AIE).Azobenzene and TPE produce a fluorescence energy resonance transfer(FRET)effect under certain conditions,which causes fluorescence quenching;however,once the N=N bond of azobenzene is interrupted,the FRET effect will disappear,and TPE fluorescence will be restored again.Therefore,the introduction of azobenzene and TPE into the same molecule can impart polymer material a fluorescent "ON-OFF" property responsive to the reduction.In this paper,a novel gel-fluorescent probe with reducing responsiveness was designed and prepared by using the FRET effect and the reduction of azobenzene.Due to the FRET effect between azobenzene and TPE,the gel is not fluorescently emitted.the fluorescence of gel is quenched.However,under the action of a reducing agent(such as Na2S2O4),the azo bond is interrupted,which leads to gel degradation,accompanied with the fluorescence recovery.The research work of this paper mainly includes the following aspects:(1)Preparation of polymer gel P(OEGMA-co-TPEMA)fluorescent probe.Raw materials required for the preparation of the synthetic gel:a hydrophilic monomer OEGMA,a hydrophobic monomer TPEMA which containing a tetraphenylethylene and a ketal-protected diol structure,and a diphenylboronic acid crosslinking agent having an azobenzene structure.The amphiphilic random copolymer P(OEGMA-co-TPEMA)was obtained by reversible addition fragmentation chain transfer(RAFT)polymerization.The structures of the synthesized monomers,crosslinkers and polymers were characterized by nuclear magnetic resonance spectroscopy('H NMR)and fourier transform infraredspectroscopy(FT-IR).The molecular weight and molecular weight distribution of the polymers were determined by GPC.The random copolymer is subjected to a one-step aminolysis/michael addition reaction,and after deprotection,polymers with diol hydroxyl groups suspended on the side chains were obtained.The polymers were crosslinked with the crosslinker Azo-2BOH which has phenylboronic acid attached to both ends of the azo group to obtain a polymer gel P(OEGMA-co-TPEMA).(2)Firstly,the structure of polymer gel P(OEGMA-co-TPEMA)was characterized by SEM and FT-IR.Subsequently,the rheological behavior and pH responsiveness properties of the gel were studied.,The gel with the cross-linking agent p-phenylenediboric acid which is free of azobenzene was prepared as control group and tested by Uv/vis and fluorescence spectrometer and the FRET effect and its reduction response mechanism were confirmed,and a reasonable explanation was given.The degradation and reduction responsive fluorescence "on-off" of the gel under sodium hydrite(Na2S2O4)were investigated.(3)Preparation of fluorescent polymer hydrogel P(PEGMA-co-AAc)probe.The crosslinked copolymer P(PEGMA-co-AAc)was prepared using polyethylene glycol methacrylate(PEGMA)and acrylic acid(AAc)as monomers and azo diisobutyronitrile(AIBN)as initiators.Then the polymer hydrogel P(PEGMA-co-AAc)was obtained by soxhlet extraction,drying and swelling in water.(4)Study on the performance of polymer hydrogel P(PEGMA-co-AAc).Na2S2O4 was used to simulate azo reductase in simulated human colon environment(PBS buffer,pH=7.4).The gel was reduced and degraded in vitro at a temperature of 37?,and the fluorescence spectrometer was used to detect the recovery of fluorescence with degradation.The loading and release of simulated drug BSA on hydrogels were studied.The release amount of drug BSA was detected by ultraviolet/visible spectrometer and the fluorescence change was detected by fluorescence spectrometer.The results show that the drug carrier gel was degraded gradually in the presence of Na2S2O4,and the release of drugs and the synchronous enhancement of fluorescence in solution could be realized.To some extent,the release of drugs could be monitored by monitoring the fluorescence intensity.