Preparation and Characterization of Aggregation-induced Dual Emission Solid Red Carbon Dots
Due to the unique luminous properties,Carbon Dots(CDs)has become one of the research hotspots in the field of new materials.The existence of aggregation-induced fluorescence quenching effect,however,severely limits the application of carbon dots in the field of solid-state luminescence.The development of long wavelength red CDs in solid state has always been a difficult problem.Most long-wavelength red CDs display quenching phenomenon and single fluorescence emission,which greatly limits their utilization in the field of anti-counterfeiting.Therefore,in this study,an environmentally friendly and easy to prepare solid red fluorescent CDs(FD-CDs)was designed and synthesized by doping nitrogen and sulfur elements.Dithiosalicylic acid and formamide were used as raw materials,and acetic acid was used as solvent.The dual emission peak and aggregation induced emission characteristics of FD-CDs make it suitable for fingerprint detection,ink printing and anti-counterfeiting applications.The structural analysis shows that FD-CDs is spherical and evenly distributed.It is composed of surface functional groups containing nitrogen,sulfur and other elements as well as carbon graphite nuclei.FD-CDs exhibit blue emission in the dispersed state and red emission in the aggregated or powder state.FD-CDs solution has only one emission peak at 367 nm,while FD-CDs powder has double emission peaks at 482 nm and 612 nm.The fluorescence quantum yield of FD-CDs in solid state is as high as 41.01%,while that in solution state is only 17.68%,showing a typical phenomenon of aggregation induced enhanced emission.The structure and optical characterization indicate that the double emission of FD-CDs has different luminescence centers.In low concentration or dispersed state,FD-CDs only has short wavelength emission,which may be caused by carbon nucleus emission.With the increase of concentration,FD-CDs agglomerates,π-π accumulation occurs in carbon nuclei with large conjugated structure,and short-wavelength fluorescence is quenched,therefore blue emission is inhibited.When FD-CDs aggregates,the symmetric heterocycles induced by S-S bonds limit the intramolecular rotation,and the main surface energy transition will be converted to fluorescence,resulting in red emission.By further calculating the ratio of radiative and non-radiative transitions,the non-radiative transition channels of FD-CDs in powder state are significantly reduced.It is inferred that aggregation can inhibit the movement of surface groups,prevent the dissipation of energy,inhibit the non-radiative transition process,and significantly improve the fluorescence emission efficiency of FD-CDs.According to the unique double emission and aggregation induced emission effect,a double switch ink based on FD-CDs was designed.Apply the prepared FD-CDs solution to the"butterfly"filter paper with a glue head dropper.After the water evaporates,the overall"butterfly"appears red.The filter paper emits strong red fluorescence under ultraviolet irradiation at 365 nm.Then the surface of the"butterfly"was coated with anhydrous ethanol,and the red fluorescence disappeared and showed blue fluorescence.After the anhydrous ethanol volatilized,the water was coated again,the blue fluorescence disappeared,and the red fluorescence appeared again.The use of anti-counterfeiting ink to print a pattern"two-dimensional code"on acid-free paper,in visible light,acid-free paper without any trace,and in 365 nm ultraviolet irradiation can see a red two-dimensional code pattern,with a smart phone scanning the two-dimensional code can be swept out of the official website of"Shaanxi University of Technology".The excellent long wavelength red emission can also be used as fingerprint recognition,and the finger containing FD-CDs can be printed on the tape with a fingerprint pattern.Under 365 nm ultraviolet irradiation,the bifurcation,termination,island,eye,nucleus,cross and scar information on the fingerprint were clearly presented.
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