Fluorescent Nanoparticles Assembled From A Poly (ionic Liquid) Via Ionic Self-assembly Strategy

Fluorescent nanoparticles have received much attention for their comprehensive applicationsin many fields such as materials, biological medicine, pharmacy, environmental monitoring, and soon. Varieties of fluorescent nanoparticles have been developed so far, including inorganicsemi-conductive quantum dots, inorganic nanoparticles or polymer particles modified or blendedby fluorescent dyes, and all sorts of fluorescent polymer particles obtained by covalent conjunction.However, some disadvantages stand out in these fluorescent nanoparticles. For example, inorganicnanoparticles have low biological compatibility and poor dispersive ability in aqueous systems.And fluorescent polymer particles obtained by covalent conjunction are always with complicatedmulti-step synthesis process, difficulty in separation and purification and low yield. Most of all,the serious drawback of those fluorescent nanoparticles above is that they all have poorphoto-stability with very quick photobleaching. Therefore designing and developing fluorescentnanoparticles with excellent properties to meet the application demands in different fields is stillan important research theme.Ionic self-assembly is a strategy mainly using structural units with converse electric chargeto assembly through the electrostatic interaction and form supermolecules possessing advancedarchitectures and functionalities. The greatest advantage of ionic self-assembly is that thestructural units are optional to prepare nanoparticles with different single functionality ormulti-functionalities according to demands of applications. In this thesis we use ionicself-assembly strategy to synthesis seriers of fluorescent nanoparticles and also study their structures, photo properties and morphology, paving a new avenue for designing and preparationof novel highly functional fluorescent nanoparticles.First of all, using ionic self-assembly strategy we design and synthesis a novel fluorescentnanoparticles which are obtained via electrostatic interaction and hydrophobic/hydrophilicinteraction between poly （ionic liquid）, fluorescent molecules and the second anion which containsazobenzene structure. We use FTIR,1H NMR, DSC, TGA, DLS, SEM, TEM, UV-Vis and FL tostudy amply the structures, fluorescence properties and morphology of fluorescent nanoparticles,proving the formation of the nanoparticles. It is found that when the second anion, compound withazobenzene structure, has a higher hydrophobic substituent group （trifluoromethyl） or a higherconcentration in the assembly system, fluorescent nanoparticles with higher fluorescence emissionintensity are obtained. Meanwhile, the fluorescence emission intensity is sensitive to pH value, andreaches the maximum and changes little when pH>9. The critical concentration of AHBTAwrapped into the core of fluorescent nanoparticles is confirmed by studying the changes of thefluorescence excitation spectra. The fluorescent nanoparticles own higher photostability which isproved by the residual80%fluorescence intensity after exposure to UV light for60min.The fluorescent nanoparticles own both fluorescence enhancement effect and goodphotostability, but the second anion with azobenzene structure has little universality, so we checkthe universality of the strategy using ionic self-assembly to produce photostable fluorescentnanoparticles. We keep the same structures of polymer chain and fluorescent molecules, and thenchoose four different molecules without azobenzene structure as second anions. We study thestructures, fluorescence properties and morphology of fluorescent nanoparticles with the samemeasurements mentionsed above. The results show that the second anions with different structurescan still form fluorescent nanoparticles with poly （ionic liquid） and fluorescent molecules, and themore hydrophobic or high concentrate second anions are beneficial to enhancing the fluorescenceemission intensity of fluorescent nanoparticles. Under the same feed ratio of three self-assemblyunits, the fluorescent nanoparticles could be formed under a lower concentration of the secondanions with the more hydrophobic substituent group by wrapping the AHBTA into the core ofnanoparticles. The pH responsibility of nanoparticles here is similar to the above. Moreover, thefluorescent nanoparticles produced by the second anions without azobenzene structure also have high photostability. Eventually, we give the probable mechanism of the formation of fluorescentnanoparticles through ionic self-assembly.We try to apply the photostable fluorescent nanoparticles to ionic sensor, finding out thefluorescent nanoparticles have high sensibility and selectivity to Cu2+, of which the detection limitcan be low to3.0μM. After S2-added into the fluorescent nanoparticles whose fluorescence isquenched by Cu2+, it is found that S2-can recovery the fluorescence and it can repeated severaltimes. So the fluorescent nanoparticles whose fluorescence is quenched by Cu2+can be used as S2-sensor. We also stuty the detection of the fluorescent nanoparticles to a series of anions and findthat fluorescent nanoparticles only show high sensibility and selectivity to Cr（VI） anion. TEMimages indicate that the morphology of fluorescent nanoparticles doesn’t change when they arequenched by Cu2+and recovered by S2-either. So, the fluorescent nanoparticles obtained throughionic self-assembly can be applied as different ions sensor based on different mechanisms such ascoordination, redox and so on.We also introduce thermal-sensitive group （NIPAM） into the poly （ionic liquid） through freeradical polymerization and produce thermal-sensitive fluorescent nanoparticles via ionicself-assembly with fluorescent molecules. We study their structures and morphology via1H NMR,DSC, TGA, and TEM, and study the LCST and sizes of nanoparticles at different temperature. Weinvestigate the thermal-induced fluorescence enhancement tendency of fluorescent nanoparticles atpH=7and9, respectively. Under condition of pH=7, the fluorescence intensity increases withoutclearly sudden change near LCST with increasing temperature. Under pH=9, the fluorescenceemission intensity only suddenly increases for several times near LCST. It is shown that thethermal-sensitive fluorescent nanoparticles are also sensitive to pH value because there is noobvious fluorescence emission intensity change when it reaches the maximum after pH>9. Inaddition, the thermal fluorescent nanoparticles are repeatly reversible and pretty photostable.We also develop fluorescent nanoparticles without the presence of fluorescent moleculesonly based on ionic self-assembly of poly （ionic liquid） and compounds containing azobenzenegroup which have no fluorescence emission in solutions at room temperature. In H2O/DMF system,high H2O volume content proportion is advantageous for azobenzene-based compounds containinghydrophobic substituent group to form nanoparticles with poly （ionic liquid）, thus making azobenzene-based compounds themselves aggregate together and consequently getting thefluorescence enhancement. In pure H-2O system, fluorescent nanoparticles formed byazobenzene-based compounds containing hydrophobic substituent group （e.g.-CF₃、-MeO、-CN）have high initial fluorescence but experience a fluorescence quenching process after UVirradiation destroys the nanoparticles structures. To fluorescent nanoparticles formed byazobenzene-based compounds containing hydrophilic substituent group （-NMe₂）, UV irradiationrender fluorescence emission intensity quench at first but increase later, and the fluorescenceemission peak shifts from420nm to450nm. It is supposed that UV irradiation destroys themicelles formed in water followed by azobenzene aggregation caused by longer UV irradiationresulting in fluorescence enhancement. TEM images support the above mechanism suppositionabove.