Study On Tetraphenylethylene Based Aggregation Induced Emission Regulation And Hydrogel Construction Via A Host-guest Recognition

Fluorescence imaging is a promising visualization tool and closely related to our daily life.It possesses the advantages in situ response and facile operation.Therefore,more and more fluorescent probes have been developed to applied in biochemistry and luminescent materials,such as in diagnosis,light-harvesting and nanocarrier tracking.Especially with the aggregation induced emission（AIE）has been reported by Tang et al in 2001,which is not limited by aggregation-caused quenching（ACQ）,the luminescent materials based AIE have attracted much attention and enormous potential applications in various fields because of the strong fluorescence signal,large stoke shift,excellent light stability and low signal-to-noise ratio.It should be emphasized that the precise regulation of the fluorescence properties is the basis for AIE-based materials in application.Host-guest recognition,as one of the noncovalent interactions,is dynamic and reversible.The reversible process of assembly and disassembly through host-guest interactions has emerged as a powerful and flexible strategy to regulate molecular aggregation/disaggregation.Meanwhile,host-guest recognition has unique advantages in optical sensing and imaging.Hence,we choose the most studied AIE molecules,tetraphenylethylene（TPE）as guest,and γ-Cyclodextrin（γ-CD）as host to form TPE/γ-CD host-guest complex in this thesis.Based on TPE/γ-CD host-guest complex and the host guest supramolecular chemistry regulation,we construct luminescent materials with precise regulation in the solution state and gel state respectively.The system can be used as a biosensor for α-amylase.There are four contents in the system work.An amphiphilic tetraphenylethylene（TPE）derivative TPE-PEG was prepared via the simple amidation reaction and chosen as an AIE based guest polymer.The aggregated state of TPE-PEG was altered and the fluorescence of TPE-PEG could be turned off in the presence of γ-cyclodextrin（γ-CD）due to the formation of host-guest complex between TPE units andγ-CDs.TPE units in the cavities of γ-CDs were released free and an aggregated state of TPEPEG restored via the guest competition epistasis of bile acid.Meanwhile,α-amylase could hydrolyze the host molecules of γ-CDs,resulting in an aggregated state of TPE-PEG again and turning the fluorescence on subtly.In which the multi-regulation of fluorescence behavior based on host-guest recognition accomplishes the reversible cycle.Tetraphenylethylene（TPE）as a typical AIE molecule has been incorporated into the main chains of the guest polyurethane via a step condensation between poly（ethylene glycol）（PEG）-based dicarbonate and TPE-diamine along with the cleavable disulfide bonds.γ-Cyclodextrins（γ-CDs）can selectively recognize the TPE units at the polyurethane chains to afford a PPR.Hydrophilic PEG segments and γ-CD molecules in the PPR could promote the disaggregation of TPE units,suppressing the fluorescence emission of TPE.To restore the aggregated state and fluorescence of TPE units,tris（2-carboxyethyl）phosphine（TCEP）andα-amylase are sequentially introduced to cleave the disulfide bonds and cut α-1,4 glycosidic bonds of γ-CD,reactivating the AIE behavior of PPR tandemly and accomplishing the reversible cycle of tuning the fluorescence of TPE.The present study provides a tandem way to switch the AIE behavior of polymeric materials reversibly.A fluorescent self-healing hydrogel with dual-network was designed,in which host-guest recognition of tetraphenylethylene（TPE）modified poly（L-glutamic acid）（PLGA-TPE）with acrylate γ-cyclodextrin（Ac-γ-CD）was strengthened by the second chemical crosslinking of Ac-γ-CD with acrylamide.Taking advantage of host-guest recognition,the fluorescence of TPE moieties has been suppressed in hydrogel,and then the dual-network architecture of the hydrogel can be degraded by α-amylase cutting α-1,4 glycosidic bonds of Ac-γ-CD,lighting up the hydrogel.The fluorescence of the hydrogel increases gradually and mechanical properties decrease,simultaneously,indicating the relationship between the degree of hydrolysis of Ac-γ-CD and the disruption of the stable network architecture of the hydrogel.Such dual-network hydrogel possesses the fast self-healing ability and recovery ratio reaches nearly 100% in 3 min due to the dynamic nature of host-guest recognition.Furthermore,by altering the temperature of the system,the fluorescence intensity of hydrogel changes reversibly.In order to further improve the mechanical properties of solid materials and simplify the synthesis steps,we design and synthesis polyurethane by classical step condensation between poly（ethylene glycol）（PEG-2000）and TPE-OH along with the hexamethylene diisocyanate（HDI）,TPE units has been incorporated into the main chains of the guest polyurethane.Acrylate γ-cyclodextrins（γ-CDs）can selectively recognize the TPE units by host-guest interaction at the polyurethane chains to afford a PPR.Ac-γ-CD was strengthened by the second chemical crosslinking of Ac-γ-CD with acrylamide.Taking advantage of host-guest recognition,γ-CD can slide on the main chains under strain.Compared with traditional covalent crosslinked hydrogels,the slide-ring hydrogel has stronger tensile strength and greater mechanical properties such as elongation at break.In addition to the controllable AIE fluorescence properties,such hydrogels have great applications in wearable and flexible devices.