Photo- and Thermosensitive Polymer Membrane with a Tunable Microstructure Doped with Graphene Oxide Nanosheets and Poly(N-isopropylacrylamide) for the Application of Light-Cleaning.

Traditional polymer membranes exhibit a constant structure that makes adjustment of the filtration process difficult, such as flux changing and contaminant cleaning. Inspired by the automatically closing behavior of leaf stomata under strong light, we prepared a membrane with thermo- and photosensitivities, whose microstructure, as well as filtration properties, could be controlled by adjusting the light condition. The membrane was fabricated by the immersion phase inversion method with a casting solution of polyvinylidene fluoride-g-poly(N-isopropylacrylamide) (PVDF-g-PNIPAAm) and graphene oxide (GO) nanosheets. Additionally, the membrane could be heated to a high temperature in a short time under illumination, causing shrinkage of its PNIPAAm chains and expansion of its membrane pores. On the basis of the reversible photoinduced structural transformation, the membrane exhibited a high water gating ratio under the switching of light on/off. Moreover, we proposed a novel and simple method to clear the contaminant from the pores of the membrane via light, which we named "light-cleaning". Light-cleaning had a flux recovery rate of 99.2%, substantially higher than that of back-washing (62%). This work not only extends the controllability and functionality of the polymer membrane but also develops a new membrane cleaning system.

Tetraphenylethylene@Graphene Oxide with Switchable Fluorescence Triggered by Mixed Solvents for the Application of Repeated Information Encryption and Decryption.

Aggregation-induced emission (AIE) materials present unique solid-state fluorescence. However, there remains a challenge in the switching of fluorescence quenching/emitting of AIE materials, limiting the application in information encryption. Herein, we report a composite of tetraphenylethylene@graphene oxide (TPE@GO) with switchable microstructure and fluorescence. We choose GO as a fluorescence quencher to control the fluorescence of TPE by controlling the aggregation structure. First, TPE coating with an average thickness of about 31 nm was deposited at the GO layer surface, which is the critical thickness at which the fluorescence can be largely quenched because of the fluorescence resonance energy transfer. After spraying a mixed solvent (good and poor solvents of TPE) on TPE@GO, a blue fluorescence of TPE was emitted during the drying process. During the treatment of mixed solvents, the planar TPE coating was dissolved in THF first and then the TPE molecules aggregated into nanoparticles (an average diameter of 65 nm) in H2O during the volatilization of THF. We found that the fluorescence switching of the composite is closely related to the microstructural change of TPE between planar and granular structures, which can make the upper TPE molecules in and out of the effective quenching region of GO. This composite, along with the treatment method, was used as an invisible ink in repeated information encryption and decryption. Our work not only provides a simple strategy to switch the fluorescence of solid-state fluorescent materials but also demonstrates the potential for obtaining diverse material structures through compound solvent treatment.