Microstructure And Sensing Properties Of Monolayer Fluorescent Films At The Gas-liquid Interface

Fluorescent sensing film has been widely recognized as one of the most approved detection methods for trace analyte detection due to its plenty of advantages such as high sensitivity,rapid detection and real-time determination.Although,most of the recently developed fluorescent sensing films have good sensing performance,they suffer from the limitation of widespread use because of their several drawbacks such as the photodecomposition of fluorophores on the air-solid interface and the poor diffusion of the analytes within films.In order to solve these problems,our group has been focusing on developing new strategies for fabricating fluorescent sensing film with excellent performance.At the same time,we found that the substrate and the structure of the amphiphilic fluorescent molecule have significant impact on sensing performance.Apparently,it is of great importance to study the orientation of the sensing unit and the microstructure of monolayer film assembled on the air-liquid interface for optimizing the sensing performance and clarifying the sensing mechanism.To this end,we firstly investigated the orientation,distribution of the sensing unit and the regulative effect of self-assembly process on its photophysical properties.Based on this,the highly sensitive and rapidly reversible sensing of nerve agents was realized.Specifically,this thesis is mainly composed of the following two parts:In the first part,the synthesis,characterization and self-assembly behavior of an amphiphilic BODIPY derivative(OBN)are presented in details.The characterization of OBN was performed using FTIR,NMR,MS and element analysis.Then the selfassembly behavior of OBN in solution and at the air-liquid interface,besides the microstructure of monolayer film were studied by UV-vis absorption spectra,fluorescence spectra,LB technique and molecular dynamics simulation.The studies reveal that at lower concentration OBN exists as a single molecule.As the concentration increases,OBN begins to aggregate,as reflected by the redshift of the signals and the greatly reduced fluorescence intensity in the fluorescence spectra.Moreover,selfassembled OBN molecules are neatly arranged at the air-liquid interface with the hydrophilic tail obliquely inserting into the water and the hydrophobic head orientating to the air.In addition,the benzene rings connected to the amino groups in OBN have the same spatial orientation and the center-to-center distance is about 5.8 ?,indicating the presence of ?-? interaction to form J-aggregates.Since the sensing process mainly contains the interaction between the sensing unit and the analytes,the sensing performance of the fluorescent films primarily depends on the aggregation state,distribution and orientation of the sensing unit.The study on the microstructure of monolayer film at the air-liquid interface lays a foundation for fabricating fluorescent monolayer films,optimizing the sensing performance and clarifying the sensing mechanism.The second part deals with the self-assembly behavior and the photophysical property studies of OBN in ionic liquid([BMIM][BF4]).Furthermore,it demonstrates the fabrication and characterization of micropatterned fluorescent monolayer film OBN/[BMIM][BF4].The film was fabricated by the interfacial self-assembly of amphiphilic molecules and microcontact printing technology and characterized by metallographic microscope,fluorescence microscope and confocal laser scanning microscope.It was found that a well-proportioned fluorescent sensor array with a diameter of about 50 ?m was successfully formed on the gold substrate.The height of a microbead is about 31.5 ?m and OBN molecules are successfully assembled at the microbead surface.Importantly,it shows excellent photochemical stability and the fluorescence intensity remains unchanged within 1800 s under the irradiation.Whereas the fluorescence intensity of OBN solid films spin-coated on Au or glass substrate decayed about 24%and 31%respectively at the same conditions.Based on these findings,a highly sensitive and rapidly reversible sensing of diethyl chlorophosphate(DCP)was realized.Interestingly,a 4.04-fold enhancement in fluorescence intensity was observed upon exposing the fluorescent monolayer film in DCP vapor.The response time is just 2.43 s and the detection limit is as low as 111 ppt.More importantly,the sensing process is completely reversible because of the hydrogen bonding between OBN and DCP.The fluorescent monolayer film can be used repeatedly more than 50 times.What's more,some common solvents and other nerve agent mimics have barely influence on it.In a word,the as-prepared fluorescent monolayer film by the interfacial self-assembly of amphiphilic molecules not only shows good stability,but also offers excellent sensing performance.