Construction Of Red Afterglow Aerogel With Citric Acid Carbon Dots And Its Performance

Carbon dots(CDs)have been widely noticed and studied since their discovery for their excellent optical properties(e.g.,bright fluorescence emission,fluorescence stability and good water solubility).Among them,CDs using biomass as carbon source also have lower toxicity,better biocompatibility and environmental friendliness,so the preparation of CDs from green biomass materials has been a hot research topic,but most solid CDs powders have no phosphorescent emission limiting their applications.Citric acid as a natural product widely found in nature has been used to prepare CDs,and citric acid is an organic acid with high content in many kinds of peel wastes,therefore,using citric acid as raw material not only can obtain CDs with good performance,but also can make full use of peel wastes and solve the environmental pressure.Compared with fluorescence,organic room-temperature phosphorescent(RTP)materials can eliminate the interference of fluorescent background and still observe afterglow emission after the external excitation light source is turned off,so they have great application value in the fields of anti-counterfeiting,information encryption and printing ink.Aerogel is a porous material with a three-dimensional network structure inside,which can be obtained by drying to replace the water in the hydrogel with air,and the rich pore structure can provide a good rigid environment for chromophores.Therefore,the strategy of embedding the fluorescent chromophores in a rigid polymer aerogel matrix can be used to effectively prevent phosphorescence quenching.In addition to the ability to suppress molecular vibrations and rotations,the protection provided by the rigid polymer aerogel matrix can also reduce the quenching effect of water molecules and oxygen,allowing the triplet excitons of RTP materials to maintain a long enough emission time.However,the emission of most aerogel afterglow materials is still blue or green,and very little has been reported for red afterglow aerogels.Blue fluorescent CDs with excellent luminescence performance are obtained by a simple one-step hydrothermal method using biomass citric acid as the carbon source and ethylenediamine as the nitrogen source,and the abundant functional groups on the surface of CDs can form hydrogen bonding interactions with the polymer matrix more easily,resulting in long-lived RTP materials.Polyacrylamide aerogels are common chemical polymers with multihydrogen bonding and porous structures.Aerogels are obtained by freeze-drying after crosslinking by free radical chemistry using monomers and cross-linking agents,etc.Under the limitation of rigid matrix of aerogel,long afterglow carbon dots composite polyacrylamide(CDs@PAM)aerogel with 500 nm phosphorescence emission center and 453.9 ms lifetime can be obtained by adding fluorescent CDs to it.The obtained phosphorescent aerogel has excellent p H stability and solvent stability,and the lifetime of the phosphorescent aerogel can be tuned by different additions of CDs and temperature/humidity sensitivity.Phosphorescent aerogels with red afterglow emission are of great application value but difficult to realize.The phosphorescent emission spectra of CDs@PAM aerogels have a large overlap with the UV-vis absorption spectra of rhodamine B(Rh B),therefore,on the basis of phosphorescent CDs@PAM,using CDs and Rh B as donor receptor energy transfer pairs,efficient and feasible phosphorescent energy transfer can occur in chemical polymer polyacrylamide aerogels to obtain carbon dots-rhodamine B composite polyacrylamide(CDsRh B@PAM)aerogels with long-lived(215 ms)red afterglow emission.The efficiency of tripletto-singlet F?rster resonance energy transfer(TS-FRET)reached 92.7%.The red afterglow aerogel also has significant p H,solvent stability and temperature/humidity sensitivity,and exhibits reversible lifetime stability in multiple "humid-dry" cycles.The combination of phosphorescent emission and the low thermal conductivity of aerogels allows the use of CDsRh B@PAM aerogels as heat preservation coatings and for leak detection.Meanwhile,fibers loaded with CDs-Rh B@PAM aerogel can be made into phosphorescent embroidered textiles.Cellulose is a biomass material abundant in nature,and cellulose aerogel is also a new generation of green aerogel that can replace traditional aerogel,which not only retains the properties of aerogel,but also shows environmental friendliness.Nanocellulose is rich in hydroxyl groups,which can form strong hydrogen bonding interaction to inhibit non-radiative leap.Using natural biomass nanocellulose(CNF)as the matrix,no additional cross-linking agent is needed compared with PAM,and only the green composite physical doping method of adding CDs can be used to obtain carbon dots composite nanocellulose(CDs@CNF)phosphorescent aerogels.The carbon dots-rhodamine B composite nanocellulose(CDs-Rh B@CNF)aerogel with red afterglow emission can also be obtained by constructing energy transfer system with Rh B receptor.This result verifies that phosphorescent energy transfer is a universal strategy to obtain red afterglow aerogels,which can be achieved with both chemical polymer aerogels and natural biomass aerogels as matrixes.Multicolor afterglow aerogels were also obtained by verifying the universality of dye receptors.The afterglow color tunability of the loaded afterglow aerogel phosphorescent paper was utilized for aesthetic phosphorescent craft folding paper and anti-counterfeiting applications.It enriches the strategy and application area of preparing CDsbased multicolor room temperature phosphorescent materials from biomass materials.