Functionalized Photonic Crystal Materials Based On Poly(Vinyl Alcohol)Hydrogels

Colloidal crystal arrays(CCA),so called photonic crystals(PC),based on nanoscale colloids can modulate electromagnetic waves due to their regular periodic structures,but the assemblies of colloids depend on the electrostatic force between the particles,and thus the array is meta-stable.A simplest but effective method to enhance the stability of CCA is to embed the colloids into hydrogels,so that not only the periodicity of the CCA can be maintained,but also broaden the applications of the CCA-based materials.Based on this,in this paper,poly(vinyl alcohol)(PVA)hydrogel was utilized to prepare a series of CCA materials.First,a two-dimensional(2D)CCA-PVA film for anti-counterfeiting coatings was prepared by utilizing the high transparency,hydrophylic PVA polymer.This thin film has the bright structural color of PC and the unique Debye diffraction ring due to its 2D structure,and thus can be applied as anti-counterfeiting layers.In addition,due to the high transparency of the film,the protected content can be clearly identified,while the oleophobicity of PVA prevents the protected content under the coating from being tampered with and reprinted.The good adhesion of PVA makes the coating layers easily transferred to different substrates.PVA hydrogels have good self-healing performance due to large amount of free hydroxyl groups in it.A PVA hydrogel-based PC with self-healing properties was prepared.After being damaged,the self-healing of PVA-PC can be realized by utilizing freeze-thaw treatment,and the material showed comparable mechanical properties as those before being destroyed.If the PVA hydrogel undergoes a volume change,the distance between the colloids will be changed,and the diffraction peak shift will occur according to the Bragg's law,showing a change in structural color.Therefore,the PC can be used as a stress sensor.Furthermore,in this research,the quantum dots printings are written at the fracture interfaces.The quantum dots are embedded after self-healing of PVA-PC,which showed a structural color under natural light while a fluorescent color could be observed under UV exposure.This work can be applied to anti-counterfeiting or color printing in the future.Chitosan(CS)can be widely used as metal ion adsorbents due to a large number of amino and hydroxyl groups in it.CS/PVA hydrogel PCs were prepared by the freeze-thaw method as a copper ion sensor.After removing the copper ions,an ion-imprinted photonic crystal(IIPC)sensor with a specific response to copper ions was obtained.The volume of the hydrogel swells due to the adsorption of the copper ion by the hydrogel,which leads to the change of the lattice spacing.In addition,the color of copper ions changes the average refractive index of the hydrogel,which causes the shift of the diffraction peak or even a color change,which enables visual detection of copper ions.The sensor has high selectivity and good repeatability.This method provides a new idea for efficient detection of copper ion concentration,and also shows the broad application prospect of PCs in the detection of trace substances.