Design,Preparation,and Dynamic Fluorescence Properties Of Fluorescent Polymer Nanoparticles Based On FRET And AIE Effects

Polychromatic fluorescent polymer nanoparticles find critical applications in areas as diverse as biological imaging,anti-counterfeiting,information encryption and intelligent textiles due to their desirable overall characteristics including good water dispersibility,cell compatibility,and unique polychromatic emission behavior.Existing methods to achieve such polychromatic fluorescent property relies mostly on the dosage regulation between multiple fluorescent components or incorporating stimuli responsive fluorescent dyes.However,these methods suffer from several stark limitations including cumbersome material synthetic routes or sophisticated molecular structures designed to develop responsive dyes,giving rise to foreseeable issues and complications during their practical applications.Herein,instead of focusing on the dyes,we propose a new concept of a polymeric matrix-regulated fluorescence strategy.In this study,aggregation-induced emission(AIE)luminogens(AIEgens)(tetrastyryl,TPE-)and photochromic dye(spiropyran)were designed to prepare FRET based fluorescent polymer nanoparticles where the AIE property of TPE units and the isomerization process of SP(?)MC could manipulate by the flexibility and polarity of the polymer matrix,realizing dynamic color-tunable and intensity of fluorescence.Whilst both solid spherical and core-shell particle structures were designed,the FRET system could be modulated synergistically or independently.The specific research content and results are as follows:Firstly,the solid spherical fluorescent polymer nanoparticles(FRET-PNPs)with a dynamic variation of fluorescence properties via synergistic modulation of AIE property and FRET process were prepared by the copolymerization of conventional monomers,AIE monomer(TPE-PBE),and spiropyran monomer(SPEA)in a facile one-pot miniemulsion polymerization process.The photoisomerization characteristics of spiropyran were proved to be well preserved in the polymer matrix and the UV absorption spectrum of ring opening spiropyran(MC)overlaps with the fluorescence emission spectrum of the TPE unit,exhibiting FRET based dynamic color-tunable fluorescence.The emission behavior of TPE and photoisomerization of SP(?)MC were presented to be well related to the properties of polymer matrix(flexibility and polarity).More specifically,the improvement of the flexibility of the polymer matrix will reduce the restriction of the intramolecular movement of TPE units and facilitate the isomerization of the SP units,whilst the stability of MC units declined with the reducing polarity of polymer matrix.Therefore,the FRET efficiency between TPE units and MC units varied in different polymer matrices,leading to different color change process of color of FRET-PNPs from TPE blue fluorescence toward MC red fluorescence for a vast of innovative applications including warning labels,dynamic decorative painting,and multiple information encryption.Subsequently,in order to optimize the fluorescence control strategy of the polymer matrix,the core-shell fluorescent polymer nanoparticles were synthesized wherein the TPE units and spiropyran are combined with the core and shell layer of particles respectively,realizing the independent regulation of spiropyran isomerization by shell polymer,thereby regulating the FRET process between TPE units and MC units.First,the seed emulsion containing TPE units was obtained by miniemulsion polymerization,followed by the addition of the monomer solution containing SPEA to form the core-shell fluorescent polymer nanoparticles(CSFRET-PNPs).The resultant CSFRET-PNPs presented good colloidal stability and homogeneous polymer particle size and the DLS and the TEM images proved the core-shell structure of CS FRET-PNPs.Whilst the reversible photoisomerization process of SP in the shell polymer matrix proceeded smoothly under UV irradiation,CS FRET-PNPs demonstrated reversible dynamic color changing property with a rich color gradient.In addition,the shell polymer matrices of CS FRET-PNPs can be facilely tuned,realizing the independent regulation of spiropyran photoisomerization behavior by shell polymer without influencing the AIE property in core.Conclusively,AIEgens and photochromic dye were used to construct a luminescent system with dynamic fluorescence characteristics based on FRET effect through miniemulsion polymerization.The strategy of " polymeric matrix-regulated fluorescence " was proposed and the effect of polymer matrix on the photolisomerization behavior of spiropyran and the luminescence behavior of AIEgens was systematically studied.Based on this,the polymeric matrix-regulated fluorescence strategy was optimized by constructing core-shell structure to realize the independent regulation of the SP isomerization by shell polymer matrix.