Preparation And Optical Regulation Of In-situ Copolymerized Hydrogel From Polymerizable Quantum Dots

Short-wave blue light（below 455 nm）can cause retinal damage,but long-wave blue light plays an important role in non-visual functions such as biological rhythm regulation.It is of great significance to develop a blue light filter with a narrow absorption transition band(wavelength span of 0.05 T_max-0.8 T_max)to filter out harmful short-wave blue light while retaining the high transmittance of beneficial long-wave blue light.Quantum dots（QDs）mostly have strong absorption in the ultraviolet and short-wave blue regions,and their maximum absorption wavelength can be adjusted by the quantum size effect.In this paper,cadmium sulfide（CdS）and zinc sulfide（ZnS）quantum dots were synthesized,and the optical properties of quantum dots were adjusted by adjusting the size of the quantum dots and the coating amount of surface modifiers by using the quantum confinement effect of quantum dots;the polymerizable monomers are chemically grafted on the surface of the quantum dots,and the in-situ copolymerization realizes the anchoring of the quantum dots on the gel skeleton,reduces the release of the quantum dots,and overcomes the thermodynamic instability that the quantum dots are easy to agglomerate.Preparation of high refractive materials and blue blocking materials with narrow absorption transition bands.Firstly,CdS QDs capped with mercaptoethanol（ME）were synthesized by hydrothermal method,dispersed in N,N-dimethylacrylamide（DMA）monomer,and thermally cured to prepare CdS composite hydrogel（CdS-PDMA）.The effects of reaction temperature on the crystal structure of quantum dots were studied by X-ray diffraction（XRD）spectroscopy.The morphology and lattice structure of CdS quantum dots were observed by high-resolution transmission electron microscopy（HRTEM）.The absorption spectra of CdS quantum dots and their composite films were measured by UV-visible（UV）spectroscopy.The results show that at a lower reaction temperature（80℃）,hexagonal crystal CdS QDs with a particle size of about 2.5 nm were formed;a higher reaction temperature（160℃）helps to form cubic crystal CdS QDs with a particle size of about 4 nm.The synthesized hexagonal and cubic CdS QDs both exhibit discrete energy level structures,with band gaps of 3.59 e V and 3.1 e V,respectively.The CdS-PDMA composite hydrogels retain the optical properties of CdS QDs,and the absorption transition band of the 30 wt%cubic CdS QDs composite hydrogel membrane is only 20 nm（440-460 nm）,can completely block ultraviolet and short-wave blue light below 440 nm.Secondly,by extending the reaction time（10 min-540 min）,cubic crystal CdS QDs with different particle sizes（3.6-4.8 nm）were synthesized.The transmission spectrum shows that with the increase of the particle size,the 50%Tmax is red-shifted from 445.1nm to 447.8 nm;The polymerizable monomer isocyanoethyl methacrylate（IEM）was grafted onto the surface of cubic crystal CdS QDs through the chemical reaction between isocyanate and hydroxyl group on the surface of CdS QDs to synthesize polymerizable CdS QDs;The infrared and proton nuclear magnetic resonance spectra of polymerizable quantum dots were measured,and the results showed that polymerizable IEM was successfully grafted on the surface of cubic CdS QDs.Polymerizable quantum dots and methyl methacrylate（MMA）,initiators form the polymerization system,and the transparent CdS-PMMA composite hydrogel is prepared by copolymerization.The effect of the recombination process on the crystal structure of CdS QDs was studied by XRD spectroscopy,the compound content of CdS QDs was determined by the thermogravimetric analysis（TGA）method,the light transmittance and blue blocking effect of CdS-PMMA composite hydrogel were measured by UV spectroscopy.The results showed that the CdS-PMMA composite hydrogel exhibited the characteristic diffraction peaks of CdS QDs,and the size and lattice structure of CdS QDs did not change during the in-situ polymerization.The actual compounding amount of CdS QDs in the gel can be adjusted between 39.17 wt%and 65.69 wt%.As the size and compounding amount of the composite CdS QDs increase,the cut-off wavelength is red-shifted,and the cut-off wavelength is adjusted in the range of 436.9 nm-447.8 nm.This makes it possible to prepare a filter that accurately blocks harmful short-wave blue light and has high transmittance of beneficial long-wave blue light.Finally,three kinds of ZnS QDs with different amounts of mercaptoethanol（ME）capping were synthesized.The amount of ME capping on the surface of ZnS QDs was measured by thermogravimetric method.The morphology and lattice structure of three ZnS QDs were analyzed by high-resolution transmission electron microscopy and X-ray diffraction spectroscopy.The refractive index（RI）of ZnS QDs was tested by a refractometer.The results show that the ZnS₁,ZnS₂,and ZnS₃ have an ME capping of23.59%,27.7%,and 33.84%,and their sizes are 3.25 nm,3.25 nm,and 2.95 nm,respectively,the lattice fringes are all about 0.275 nm,ZnS₂ has the highest apparent refractive index.Disperse ME-capped ZnS QDs in monomers such as methacrylic acid-β-hydroxyethyl（HEMA）,DMA,MMA,etc.,add a little functional monomer glycidyl methacrylate（GMA）,solvents N,N-Dimethylformamide（DMF）,a transparent ZnS QDs composite hydrogel was synthesized through a"one-step method".The mechanism of the“one-step method”for preparing composite hydrogels was studied by ¹H NMR.The results showed that under the catalysis of DMF,the epoxy group on GMA was opened and grafted onto the surface of ZnS QDs capped with ME,and further copolymerized with other monomers to prepare ZnS composite hydrogel.The transparent ZnS-PDMA,ZnS-PDMA-HEMA and ZnS-PMMA composite hydrogels were prepared.The light transmittance,swelling degree,RI value,mechanical properties and photoluminescence of the composite hydrogel were tested.The results show that the three composite hydrogels are highly transparent;increasing the composite amount of ZnS,the swelling degree of the gel decreases,the tensile strength and the Young’s modulus increase;the refractive index also increases with the increase of the composite amount of quantum dots,when the composite amount of ZnS₂ is about80 wt%,the RI value of the ZnS-PDMA-HEMA hydrogel membrane reaches 1.52 at the swelling equilibrium,and the RI value of the dry gel reached1.764.Under the excitation of 365 nm ultraviolet light,the ZnS composite films exhibit photoluminescence.In summary,this paper prepared CdS QDs composite hydrogels with narrow absorption transition band and adjustable cut-off wavelength and ZnS QDs composite water with higher refractive index by in-situ copolymerization of quantum dots and polymerized monomers.Gel film,which can accurately block harmful high-energy short-wave blue light,provides new ideas through beneficial low-energy long-wave blue light and high refractive index composite hydrogel.