Preparation And Application Of Poly (Vinyl Alcohol) Gelated Crystalline Colloidal Array Photonic Crystal Sensing Materials

Photonic crystal (PC) is a dimensionally periodic dielectric structure that exhibits a photonic band-gap (PBG). Photonic crystal materials inhibit photons from propagating for a certain band of frequencies with the fabricated PBG comparable to the wavelength of light, which have been extensively applied as functional optical devices. Among the applications, there is intense interest in the fabrication of photonic crystal sensing materials.Highly charged, monodisperse colloidal spheres self-assbemble into non-close-packed crystalline colloidal array (CCA) due to the electrostatic interaction between the particles. The CCA is a generally three-dimensional (3D) face-centered cubic (FCC) complex fluid, which Bragg diffracts ultraviolet, visible, or infrared light based on the modulated interparticle lattice constant and/or refractive index contrast. Polymerized crystalline colloidal array (PCCA) optical sensor was developed by using CCA encapsulated in polymer hydrogels via in situ photopolymerization. Such optical sensors represent visually distinguishable structure color changes as the stop-band shifts rely on the reversible swelling and shrinking of the hydrogels responding to certain analytes. The CCA could be disordered when subjected to environmental influences such as mechanical shock, thermal forces, and the presence of ionic impurities, few monomers or macromers coexist with CCA to fabricate PCCAs. Herein, we developed physically cross-linked poly(vinyl alcohol) (PVA) hydrogel to form a gelated crystalline colloidal array (GCCA) photonic crystal material. PVA hydrogels, with their nontoxicity, biocompatibility and desirable mechanical properties, have been investigated for use in various applications, such as drug delivery, articular cartilage, contact lenses, and wound dressing. PVA hydrogels could be formed through either chemical or physical gelation of PVA polymer solutions. Chemically cross-linked PVA hydrogels utilize crosslinkers such as glutaraldehyde, while the most widely used method for preparing physically cross-linked PVA hydrogels is the well-known freezing-thawing process. Thus, we systematically studied the GCCA photonic crystal system based on PVA hydrogel/CCA composites.Emulsion polymerization was utilized to prepare modispersed polystyrene (PS) colloidal spheres. CCA solution was obtained via dialyzed～10% w/w suspensions of PS particles against ultrapure water. The iridescent CCA solution was obtained due to Bragg diffraction as the colloids were sufficiently cleaned and self-assembled. The CCA was then consisted with PVA hydrogel was formed by utilizing a physically controlled chilling-thawing method while the CCA was physically immobilized within PVA hydrogel matrix. After being chilled at 2℃for 24 h, the gel could be formed without disturbing the CCA. With the repetition of chilling-thawing cycle, the hydrogel network was reinforced. We further functionalized the PVA hydrogel with Chitosan (CS), and the pH sensing behavior of the PVA/CS GCCA was observed. It revealed that the sensitivity of the PVA/CS GCCA correlates with the CS concentration. Without restriction by UV radiation, physically cross-linked GCCA can be fabricated of any size and geometry with the simplicity of its synthetic process and independence of photoinitiator. Furthermore, the GCCA can efficiently diffract visible light, and demonstrate thermoreversible and rehydratable behaviour. Physically cross-linked GCCA network can be dissolved by increasing the temperature, and rebuilt by simply lowering the temperature while the sample maintains the ability of diffraction. The GCCA can also be reversibly dried and rehydrated, which promises a long-term storage in rigorous conditions.Shortly afterwards we developed solvent-assisted physically cross-linked GCCA photonic crystal materials. Since PVA solutions could form high-strength hydrogels at low temperature consisting of water and a water-miscible organic solvent such as ethanol (EtOH), dimethyl sulfoxide (DMSO), ethylene glycol (EG) and glycerin (GC). The addition of the organic solvent could promote structural stability, and help produce firmer hydrogels because of the penetration of PVA gel matrix. The GCCA films efficiently diffract the visible light, and the diffracted colour can be tuned by simply stretching the sample, which could be easily distinguished by the naked eye.We then functionalized this solvent-assisted GCCA to build a fast response, multi-sensitive photonic crystal sensor. The resulting GCCA sensor could rapid respond to various external stimuli such as solvent, pH, cation and mechanical force, and the shifting of stop-band could be observed easily by the naked eye. The results show that GCCA can be designed in a precise ways by integration of functional groups such that the swelling/shrinking process of the polymer network could be controlled. We also demonstrated three different sensing mechanisms for the future design of other photonic crystal optical sensors.