Synthesis Of Room Temperature Phosphorescent Carbon Dots And Their Application In Information Encryption And Fingerprint Imaging

As a new type of photoluminescent material,room temperature phosphorescent carbon dots have advantages of small particle size,wide source of synthetic raw materials,low toxicity or even non-toxicity,good biocompatibility and stable chemical properties.It shows great potential for applications in sensing,light-emitting devices,and information encryption and anti-counterfeiting.When constructing room temperature phosphorescent carbon dots,in order to optimize their phosphorescent properties,the following two aspects can be considered:(1)Suppress the non-radiative transition of triplet excited electrons.For example,dispersing CDs into a suitable matrix(polymer matrix such as polyvinyl alcohol,polyacrylamide,etc.,organic small molecule matrix such as melamine,urea,etc.)can provide a rigid protective environment for light-emitting group to suppress triplet excitons non-radiative transitions occur.(2)Promote the intersystem jumping of singlet excitons to generate more triplet excited electrons.For example,the introduction of heteroatoms(B,N,P,S,etc.)or carbonyl groups containing lone pairs of electrons can effectively improve the intersystem channeling.These two strategies have the advantages of simplicity,high efficiency,cheap and readily available raw materials in designing and synthesizing CDs with room temperature phosphorescence properties.Moreover,host material and CDs material can be selected and designed according to the needs,so it is expected to design and synthesize room temperature phosphorescent materials with long lifetime and wide application through these two strategies.In this thesis,according to the above-mentioned means of optimizing phosphorescence properties,carbon dots with room temperature phosphorescence properties were synthesized by high temperature calcination.and we apply these materials to the fields of information encryption and fingerprint imaging.The main contents of this research are:(1)Preparation,characterization and application of water-soluble room temperature phosphorescent CDs.Using benzoic acid derivatives as raw materials,they were ground in a mortar and then placed in a muffle furnace for 12 hours at 260 °C to prepare carbon dot-based room temperature phosphorescent materials(PTA-CDs).The synthesized PTA-CDs powder was dissolved in ethanol to prepare an ethanol solution,and then mixed with a 10% polyvinyl alcohol solution and stirred uniformly to obtain a PTA-CDs@PVA composite solution.The composite solution was uniformly coated on glass or raw paper substrate and dried in a blast drying oven to obtain a transparent PTA-CDs@PVA composite film with a phosphorescence lifetime of 148.8 ms.Based on the photoluminescence properties of the PTA-CDs@PVA composite,it can be applied in information encryption.The PTA-CDs@PVA composite solution was used as the ink to print the double crane pattern on the substrate by screen printing technology.After the ink was dried,the substrate was irradiated with 365 nm ultraviolet light,and the blue flying crane pattern could be clearly observed,and the light source was removed.After that,a green flying crane pattern was observed.(2)Preparation,characterization and application of boron-doped room temperature phosphorescent CDs.The benzoic acid derivatives and boric acid were ground and mixed according to a certain mass ratio,and then placed in a muffle furnace for 12 hours at 260 °C to obtain boron-doped room temperature phosphorescent carbon dots(B-CDs).The B-CDs material exhibits a room temperature phosphorescence lifetime of 1.06 s,and its color can be tuned by adjusting the wavelength of the excitation light.It is found that the doping of boron is crucial for the phosphorescence emission of CDs.Boric acid generates boron oxide after thermal decomposition,which provides a rigid protective environment for light-emitting groups,inhibits the movement of molecules,and makes triplet excitons emit phosphorescence in the form of radiation transitions.Based on its better photoluminescence properties and unique color tunable properties,we filled the B-CDs powder into the prepared molds,and according to the interesting luminescence-dependent behavior of B-CDs and their water-sensitive properties,we apply it to more complex information encryption.Bright blue light emission can be observed when the powder is irradiated with a 254 nm light source and when the light source at this wavelength is removed.Similarly,when the powder is irradiated with a 365 nm light source,the powder emits blue-white fluorescence,and when the light source is removed,it emits green phosphorescence.(3)Preparation,characterization and application of nitrogen-doped room temperature phosphorescent carbon dots.Using urea and p-phenylenediamine as carbon and nitrogen sources,N-CDs materials with fluorescence and room temperature phosphorescence properties were prepared by one-step pyrolysis.The material is a reddish powder under natural light.When irradiated with an ultraviolet light source with a wavelength of 365 nm,the powder emits blue fluorescence.When the light is turned off,it exhibits green room temperature phosphorescence,and the lifetime is about 782.39 ms.Due to many carbonyl groups,amino nitrogen and pyrrole nitrogen on the surface of N-CDs,hydrogen bonds are easily formed between N-CDs and effectively inhibit the vibration,rotation,collision of the luminescent group,and hinder the nonradiative transition of excited excitons.Thus,the room temperature phosphorescence performance of the material is effectively improved.Based on the fluorescence and room temperature phosphorescence dual emission behavior of N-CDs,we use them in fingerprint imaging.The ground N-CDs powder was evenly covered on the surface of the object with fingerprints,and then the excess powder was gently brushed off with a brush,leaving only the powder on the fingerprints of the fingers.When irradiated with a 365 nm UV light source,the shape,width and edge profile of the fingerprint ridges can be clearly observed.