Preparation And Photophysical Properties Of Fluorescence And Room-temperature Phosphorescence Boron Nitride Nanodots

Zero-dimensional boron nitride nanodots,also known as boron nitride quantum dots,are a new type of boron nitride nanomaterials with fluorescent properties,which have many advantages,such as low toxicity,good photobleaching resistance,excellent biocompatibility,nonmetal elements,environmental friendliness,chemical inertness and so on and boron nitride nanodots have shown great potential in fluorescence sensing,cell imaging,catalysis and other fields.Although the photophysical properties and applications of boron nitride nanodots have been explored,there are still many scientific questions to be solved.In the aspect of synthesis,the conventional "top-down"preparation method using bulk boron nitride as raw material suffered from the poor control of particle size and structure of boron nitride nanodots and the results of synthesizing boron nitride nanodots with excellent photophysical properties from small molecules are still scarce.As for the mechanism investigation,the luminescence mechanism of boron nitride nanodots can only be postulated from the experimental results at present,because of the difficulty in accurately characterize the structure of boron nanodots.Besides,in the aspect of photophysical properties,there are very few reports on the long-lifetime room-temperature phosphorescence of boron nitride nanodots.Room temperature phosphorescence has broad application prospects in information encryption,biological imaging and many other fields.It is of great significance to explore the room temperature phosphorescence properties of boron nitride nanodots.Therefore,in this dissertation,the preparation methods and photophysical properties of boron nitride nanodots with fluorescence and room-temperature phosphorescence were studied.The controllable preparation of ultralong lifetime room-temperature phosphorescence boron nitride nanodots and room temperature phosphorescence emission in solution environment were realized,and their applications were explored.The main contents of this dissertation are as follows:(1)Hydrothermal synthesis and application of fluorescent boron nitride nanodots.Boron nitride nanodots were synthesized by a facile one-step hydrothermal method using small molecule as raw materials.The as-prepared boron nitride nanodots exhibit excellent thermal stability,salt resistance,pH stability and photobleaching resistance,and the aqueous suspension of boron nitride nanodots emits blue fluorescence with the optimal emission wavelength of 405 nm under UV excitation.Besides,by utilizing the boron nitride nanodots as the fluorescent sensor,a rapid and wide-range fluorescence analysis method for Fe3+was established.In addition,the applications of boron nitride nanodots in cell imaging,fluorescent dyes and fluorescent anti-counterfeiting inks were explored.(2)Preparation of boron nitride nanodots-based materials with tunable lifetime room-temperature phosphorescence and investigation of the related luminescent mechanism.Here,a method of preparing boron nitride nanodots-based materials with adjustable lifetime has been developed by a facile strategy.The powder of BN nanodots material emits blue fluorescence under the UV lamp and green afterglow after the removal of UV lamp,and the lifetime can be adjusted between 116.3 ms and 1091.5 ms.The comparison and analysis of a series of samples show that heteroatom doping and amorphous carbon matrix are very important for the RTP emission of BN nanodots.Heteroatom doping can promote the intersystem crossing of excited electrons.The hydrogen bonds between amorphous carbon film matrix and BN nanodots can prevent the nonradiation transition of triplet excitons.The lifetime of room-temperature phosphorescence is mainly controlled by the oxidation of amorphous carbon film substrate and the change of doping concentration in BN nanodots.Based on the different RTP lifetime of the materials,a time-resolved multi-level information encryption system is constructed.(3)Synthesis of room-temperature phosphorescence boron nitride nanodots without matrix protection and its photophysical properties.BN nanodots doped with carbon and oxygen were synthesized by a rapid microwave method.The BN nanodots powder can emit stable blue fluorescence and green RTP without matrix protection,and has a long RTP lifetime of 1.57 s,and the afterglow duration is more than 8 s.The effective doping of carbon and oxygen elements in BN nanodots reduces the energy gap between single excited state and triple excited state,which promotes the intersystem crossing and the population of triple excitons.The formation of compact cores via crystallization and effective inter-/intra-dots hydrogen bonds further stabilizes the excited triplet states and reduces the nonradiation transition.Based on the double emission properties of fluorescence and RTP,we explored their applications in label anti-counterfeiting and information encryption.(4)Design of boron nitride nanodots-based materials with long lifetime room-temperature phosphorescence in both powder and suspension states and their photophysical properties.By combining hydrothermal and microwave-assisted heating,the composite structure materials with boron nitride nanodots embedded in the homologous boron oxide matrix were prepared.The lifetime of the sample is 1.7 s and the afterglow time is more than 10 s.The RTP lifetime in aqueous suspension is about 0.9 s and the afterglow time is more than 8 s.Detailed study shows that the boron oxide matrix not only provides a rigid environment for BN dots to disperse uniformly and reduce aggregation caused quenching,but also stabilizes the triplet excitons and inhibits the nonradiative transition through the interaction of covalent bond and hydrogen bonds.Moreover,because the rigid structure of boron oxide matrix is not easy to collapse,the RTP of the obtained samples in different solutions(water,oxidant,organic solvent,acid and alkaline solution)is realized.Based on the above properties,we explored the potential application of BN nanodots based room-temperature phosphorescent materials in the field of multi-level anti-counterfeiting and information encryption in wet environment.