Construction Of Carbon Dots-based Fluorescent Hydrogel Films And Their Performance

Fluorescent hydrogels have attracted much attention because of their potential uses in sensors,drug delivery,and bioimaging based on their unique properties,such as photoluminescence,multi-responsiveness,and biocompatibility.At present,traditional fluorescent hydrogels have been created by directly embedding various fluorescent substances including organic dyes,quantum dots(QDs),metal nanoparticles(NPs),etc.,in hydrogel matrix by physical or chemical interaction.However,this method not only brings disadvantages such as high toxicity,photo-bleaching,and poor photoluminescence stability,but also causes carbon dots leakage in different harsh chemical environments,thereby weakened fluorescence properties and mechanical strength.Carbon dots(CDs),newly discovered fluorescent carbonaceous nanoparticles(<10 nm),show more advantages,not only have excellent water dispersibility,high biocompatibility,low cytotoxicity,stable photoluminescence and easy surface modification,but also the surface of carbon dots has abundant hydroxyl,carboxyl,and other surface active groups.Therefore,the introduction of carbon dots into the fluorescent hydrogel films can effectively improve the existing problems of the conventional fluorescent hydrogel.Thus,the main purpose of this paper is to prepare a fluorescent carbon dots with good fluorescence stability.The carbon dots is covalently modified by using the feature that the carbon dots is easily surface modified,and the modified carbon dots is used as a covalent crosslinking center.Then,the modified carbon dots is introduced into the water by covalent crosslinking.The gum base is to produce a new type of fluorescent hydrogel films with high mechanical properties.In addition,silica colloids and carbon dots were hydrothermally treated to prepare fluorescent composite colloids.The fluorescent composite colloids were modified and then introduced into the fluorescent hydrogel films.The purpose of this study was to explore the influence of the size of the covalent cross-linking point size on the mechanical properties of the fluorescent hydrogel films.The main results are listed as follows:(1).Carbon dots were synthesized using alginate,ethylenediamine as precursors through a facile,eco-friendly and inexpensive hydrothermal carbonization route(reaction at 200 ? for 3 hours).The spherical carbon nanoparticles were measured by transmission electron microscopy(TEM)and the particle size distribution was between 2-4 nm.At the same time,High-resolution transmission electron microscopy(HRTEM)measured carbon dots with lattice fringes,and the 0.21 nm lattice spacing was related to the in-plane lattice of(100)graphene.The UV-visible absorption spectrum of the carbon dots measured by the fluorescence spectrophotometer showed a broad peak in the range of 270-300 nm,the excitation wavelength increased from 360 nm to 460 nm,and the emission peak shifted from 460 nm to 530 nm.The test results show that as the excitation wavelength increases,the red shift of the emission spectrum from the normalized fluorescence spectrum observation is very obvious.In addition,Fourier transform infrared spectroscopy(FT-IR),X-ray powder diffraction(XRD)and X-ray photoelectron spectroscopy(XPS)tests showed that the surface of carbon dots prepared in this experiment had more hydroxyl,carboxyl and epoxy groups and other surface activities.The group supports the advantages of carbon dots with excellent water dispersibility and easy surface modification.(2).Construction of carbon dots-based fluorescent hydrogel films and their performance.The carbon dots were covalently modified by using the characteristics of easy-to-surface modification of carbon dots,and modified carbon dots were used as covalent crosslinking centers.A high-intensity fluorescent hydrogel films with good mechanical properties was prepared by ultraviolet light polymerization.The fluorescence properties of the carbon dot-based fluorescent hydrogel films measured by a fluorescence spectrophotometer are similar to those of the carbon spot aqueous solution and tend to be stable over a long period of time.In addition,the fluorescence intensity of the fluorescent hydrogel film can be changed by controlling the content of carbon dots.As the carbon content increases,the fluorescence intensity increases.More importantly,the mechanical properties of the fluorescent hydrogel can be adjusted by changing the water content and carbon content.The tensile stress of the fluorescent hydrogel with 1%carbon content can reach 12 MPa under the condition of 10%water content.The freeze-dried fluorescence hydrogel was characterized by scanning electron microscopy(SEM)and found to have a relatively ordered "ladder-like" structure inside.Finally,we infer that the reason for its good mechanical strength is the relatively ordered microstructure inside the fluorescent hydrogel.(3).Construction of fluorescent composite colloids-based fluorescent hydrogel films and their performance.First,fluorescent composite colloids are prepared by hydrothermal treatment of silica colloids and carbon dots precursors.Then,the fluorescence composite colloids were modified to prepare a fluorescent composite colloids-based fluorescence hydrogel films with high mechanical properties by ultraviolet light polymerization.The particle size of the prepared monodispersed silica colloids and fluorescent composite colloids(SiO2/CDs)were 93.21 nm and 96.4 nm,respectively,measured by nanometer particle size analyzer.The TEM test also confirmed the particle size range of silica colloids and fluorescent composite colloids.Through the mapping scanning test of the fluorescent composite colloid,the carbon content in the fluorescent composite colloid was measured to be 9.61%.Subsequently,the FT-IR test of the fluorescent composite colloid strongly demonstrates that the carbon dots can effectively compound with silica colloids.The fluorescence properties of fluorescent composite colloidal-based fluorescent hydrogels measured by a fluorescence spectrophotometer are similar to those of aqueous carbon spot solutions and tend to be stable over a long period of time.At the same time,the fluorescent properties of the fluorescent composite colloids before and after the modification were compared and it was found that the fluorescence properties of the two were almost the same.Similar to the first part of the work,the study of the mechanical properties of fluorescent hydrogels was carried out on the fluorescent composite colloid content and water content.It was found that the fluorescence hydrogel with 1%fluorescent composite colloid content has a tensile stress of up to 13.2 MPa at 10%water content,which is much higher than that of the abovementioned carbon-based fluorescence hydrogel and other conventional fluorescent hydrogels.Furthermore,the SEM test of the fluorescent composite colloid-based fluorescence hydrogel shows that it has a well-ordered"hierarchy" structure inside.Finally,we conclude that the unique "hierarchy" microstructure of fluorescent composite colloids-based fluorescent hydrogels is the main reason for enhancing the mechanical properties of hydrogels.