Preparation And Properties Of Responsive Fluorescent Composite Films Based On Nano Cellulose

Cellulose is a research hotspot in the field of polymer materials.The nanocrystalline celluloses(CNCs)obtained by acid hydrolysis have the characteristics of high aspect ratio,Young’s modulus,and easy chemical modification.Under natural conditions,evaporative self-assembly can form colored thin films with left-handed chiral nematic structure.This colored film is thought to be a natural polarizing filter with a special photonic band gap(PBG).Circularly polarized luminescence(CPL)is a phenomenon in which chiral luminophores emit left-and right-handed polarized light with different intensities.However,it is still challenging to obtain high-performance chiral light-emitting materials in batches and control their CPL.The surface of CNCs has negatively charged carboxyl groups,which can be assembled with achiral light-emitting substances under the action of electrostatic force to construct CPL materials of binary hybrid system.Based on the above characteristics of nanocellulose,two aspects of research are carried out in this thesis:1.Preparation of silver nanoclusters and cellulose nanocrystal fluorescent composite films and their application as multicolor inks:In this work,metallic silver nanoclusters(Ag NCs)were prepared by reducing silver nitrate using reduced glutathione(GSH).Polyvinylpyrrolidone(PVP)and Ag NCs were added together into the CNCs solution,mixed uniformly,evaporated and co-assembled to form a thin film.The thin films show bright structural color with the addition of appropriate amount of Ag NCs,and obvious reflection peaks can be seen in the UV-vis spectrum.Circular dichroism(CD)plots show that the composite films have a strong positive Cotton signal.Under the excitation of 365 nm ultraviolet light,the film emits fluorescence.The left-handed nematic film can reflect the left-handed light and transmit the right-handed light.The asymmetry factor of the composite film under a certain Ag NCs concentration reaches-0.12.Adjusting the pH from 5.04 to 11.11,the emission peak of the fluorescence spectrum was red-shifted from 525 nm to 775 nm.With the increase of humidity in the environment,the fluorescence of the composite film can be weakened or even quenched,and the fluorescence intensity is restored when the humidity is restored.Taking advantage of this property,anti-counterfeiting patterns were designed on the composite film using water,and the film was irradiated with ultraviolet light to reveal the pattern,and the quenched fluorescence recovered after the water evaporated,and could be used repeatedly.At the same time its suspension can also be written on paper as colored fluorescent ink,we designed different fluorescent patterns,such as chameleon,Chinese knot.2.Flexible nanocellulose/shellac acid optical composite film with pH response:In this work,polyvinyl alcohol(PVA)and shellac acid(LD)were added together to microcrystalline cellulose(MCC)as the raw material in the dispersion of CNCs prepared by evaporation and co-assembly to prepare circularly polarized luminescent films.The composite film has certain flexibility,enhanced thermal stability,and exhibits pH-responsive properties.As a mechanical property enhancer,PVA greatly improves the weak toughness of the cellulose film.When the absolute dry ratio of PVA and CNC in the composite film is 3:1,the film can even be bent,and the mechanical strain increases from 2.6%of the pure CNCs film to 10%.LD is a natural water-soluble dye that exhibits stimulation and responsiveness to the pH of aqueous solutions.After LD participated in the self-assembly,the chiral nematic phase structure of the composite film was still maintained,showing an obvious left-handed signal in the CD spectrum.The LD solid and aqueous solution hardly produce photoluminescence,the composite film emits orange fluorescence under the excitation of 365 nm ultraviolet light,and the fluorescence spectrum shows an obvious emission peak at 615 nm.The asymmetry factor of the composite films can reach about-0.25.The composite films showed different shades of purple in different concentrations of Na OH solutions.The Na OH solution with a concentration of 1 mmol/L can change the color of the film,and the composite film can be used as a pH detection test paper to detect pH.Fluorescence is also quenched by the presence of OH~-.Using the response to the alkaline solution,a purple cartoon head was drawn using 0.1 mol/L Na OH solution,showing the quenching ability of Na OH solution on fluorescence in the small jellyfish pattern.