Hyperbranched Polysiloxane With Aggregation-induced Emission Behavior: Functionalization And Emission Mechanism Exploration

Hyperbranched polysiloxane（HBPSi）,a novel unconventional fluorescent polymer,does not contain any aromatic structures,and the length and bond angle of silicon-oxygen bond is larger than that of silicon-carbon bond and carbon-carbon double bond.HBPSi has the advantages of easy preparation,low cost,and good biocompatibility,which renders it highly suitable for applications in clinical diagnosis,cellular imaging,biosensors and genetic analysis.However,compared with traditional organic emitters,HBPSi bears low fluorescence intensity and quantum yield,debated emission mechanism,at the same time,the exploration of its applications in biological field is lacked.Herein,in order to enhance the fluorescence intensity and quantum yield of HBPSi,as well as explore its emission mechanism and potential applications in biological field,two kind of water-soluble HBPSi were designed and synthesized,after that HBPSi was modified with water-soluble L-glutamic acid,hydrophobic oleic acid,and amphipathicβ-cyclodextrin,respectively,obtaining three functionalized HBPSi with different fluorescence properties and applications.Furthermore,a kind of HBPSi modified with carbon dots（HBPSi-CDs）was prepared through hydrothermal method from a kind of HBPSi.Their relationship between fluorescence properties and molecular structures,emission mechanism,and applications in ions detection,drug delivery and cell imaging were investigated systematically.The main research contents of this thesis are as follows:（1）A kind of water soluble HBPSi simultaneously containing primary amines,hydroxyls and ether groups was synthesized from（3-Aminopropyl）triethoxysilane and diethylene glycol through transesterification reaction.After that,different contents of L-glutamic acid were grafted onto HBPSi via amidation reaction（HBPSi-GA）.The fluorescence of HBPSi and HBPSi-GA are enhanced with the addition of poor solvent and increase of concentration,showing typical aggregation-induced emission（AIE）and concentration-enhanced emission（CEE）characteristics.Compared with HBPSi,the fluorescence intensity and absolute quantum yield of HBPSi-GA are enhanced with the increase of L-glutamic acid.The quantum yield of HBPSi is 8.76%,while the quantum yield of HBPSi-GA reaches to 11.78%.The density functional theory results demonstrate that L-glutamic acid increases the inter/intramolecular hydrogen bonds among HBPSi-GA and promotes the self-assembly of HBPSi-GA,which results into the clustering of electron-rich atoms and generation of through-space conjugation,leading to the enhancement of fluorescence.Interestingly,HBPSi-GA has four excitation peaks at 238,262,300 and 360 nm,and two emission peaks at 380 and 450 nm,which is different from the reported unconventional fluorescent polymer.Results shows that the multi-excitation and dual emission is caused by the energy transfer from the free hydroxyls,amines,carboxyls et al.to the different through-space conjugation in the heterogeneous clusters,and mechanism named resonance energy transfer induced multi excitation and enhanced emission is proposed.On the same time,L-glutamic acid increases the biocompatibility of HBPSi significantly.Moreover,HBPSi-GA exhibits dual stimuli-responsive fluorescence to temperature and Fe³⁺,and shows potential application in cell imaging.（2）To further enhance the quantum yield and fluorescence intensity of HBPSi,a high molecular weight HBPSi containing primary amines,hydroxyls and ether groups was prepared by increasing the reaction temperature and adjusting the ratio of（3-Aminopropyl）triethoxysilane and diethylene glycol.Then,different amount of oleic acid was introduced into HBPSi（HBPSi-OA）through amidation reaction.Results show that the quantum yields of HBPSi and HBPSi-OA reach to 23.35 and 28.57%,separately.Moreover,oleic acid enhances the fluorescence intensity of HBPSi-OA significantly.HBPSi and HBPSi-OA also show aggregation-induced emission（AIE）and concentration-enhanced emission（CEE）phenomenon.Results show that the fluorescence of HBPSi-OA is generated from the polymer clusters in the self-assembly of HBPSi-OA.Namely,the electron clouds of electron-rich atoms in the polymer clusters are overlapped and form through-space conjugation,and thus light is generated.HBPSi and HBPSi-OA also show multi-excitation and milti-emission property,which results from the energy adsorbed by the free hydroxyls,amines and amide bond et al.transfer to different through-space conjugation through resonance energy transfer.The fluorescence of HBPSi-OA is sensitive to Fe³⁺,showing potential applications in detecting Fe³⁺.Furthermore,the self-assembly of HBPSi-OA has high ibuprofen loading capacity,the loading amount reaches to 480 mg g^-1.HBPSi-OA shows pH-responsive drug release performance,the release rate and cumulative ibuprofen release amount are higher in pH 6.4（Phosphate Buffered Saline）PBS,while the release rate and cumulative ibuprofen release are lower in pH 7.4 PBS.（3）To endow the derivatives of HBPSi with water solubility and drug delivery ability,HBPSi containing primary amines,hydroxyls and tertiary amines was synthesized from N-methyldiethanolamine and（3-Aminopropyl）triethoxysilane through transesterification reaction.Then,different contents ofβ-cyclodextrin were grafted onto HBPSi,and thusβ-cyclodextrin modified HBPSi（HBPSi-CD）was obtained.HBPSi and HBPSi-CD show typical aggregation-induced emission（AIE）and concentration-enhanced emission（CEE）characteristics.Results show that the quantum yields of HBPSi and HBPSi-CD reach to 12.24 and 18.72%,respectively,moreover,β-cyclodextrin enhances the fluorescence of HBPSi obviously.Density functional theory calculation results illustrate thatβ-cyclodextrin enhances the inter/intramolecular hydrogen bond and self-assembly of HBPSi-CD,and promotes the generation of through-space conjugation,which makes the excited state more likely to return to the ground state in the form of radiation,resulting into the enhancement of fluorescence.HBPSi and HBPSi-CD have multi excitation and multi emission,which is caused by the energy adsorbed by the free hydroxyls and amines transfer to the different through-space conjugation through resonance energy transfer.The introduction ofβ-cyclodextrin increases the biocompatibility of HBPSi-CD significantly,moreover,HBPSi-CD can penetrate the cell membrane of mouse osteoblast cells and light up the cells,showing potential application in cell imaging.Furthermore,HBPSi-CD has high drug loading capacity(160 mg g^-1).The cumulative ibuprofen release amount of HBPSi-CD reaches to 78%under mild acidic environment（pH 6.4）,whereas only 35%of ibuprofen is released at near-neutral pH（7.4）after 24 h,showing pH-responsive drug release performance.（4）To improve the quantum yield and fluorescence intensity of HBPSi,as well as endow HBPSi with anti-background light interference ability,HBPSi modified by carbon dots（HBPSi-CDs）was prepared from multi-excitation and dual-emission HBPSi through one-pot hydrothermal reaction.Results show that the fluorescence intensity of HBPSi-CDs is enhanced with the increase of reaction time and temperature,and the fluorescence reaches to the highest under reaction pH of 10.99.HBPSi-CDs possesses three excitations at 240,300 and 360 nm,while only one emission at 440 nm.Structural and optical investigation demonstrates that such multi-excitation and single emission is caused by the energy transfer from sp²carbon core and free fluorescent groups on the surface to the aggregated fluorescent groups through resonance energy transfer.HBPSi-CDs can emit blue light under excitation from 210 to 380 nm UV light,thus,HBPSi-CDs possesses good anti-background light interference ability.Cell uptake experiments show that HBPSi-CDs can be internalized into live cells with obvious accumulation in cell nuclei.Furthermore,HBPSi-CDs shows high sensitivity,selectivity and anti-interference in detecting Fe³⁺.