| The development of hydrogels gets more and more attention in the recent years.Inspired by the bioluminescence in nature,scientists design various polymer hydrogels with tunable fluorescence.Due to the smart responsiveness of hydrogel matrix and fluorophore,the fluorescent hydrogels are widely applied in the fields of illumination,chemical sensor,bioimaging.The design of functionalized fluorescent hydrogels by using different fluorophore and hydrogel matrix is always research highlights.To expend the application of fluorescent hydrogels,following challenges need to be addressed:（i）Preparation of fluorescent hydrogel with high strength and high Young’s modulus.The reported tough fluorescent hydrogels are usually double-network hydrogels with physical cross-linking,which have high fracture strength but low modulus.Such fluorescent hydrogels are difficult to maintain their shapes under external force,which limits the application in the application of display.Therefore,the combination of fluorophore and hydrogel matrix with high strength and high Young’s modulus is of practical significance to expand the application of fluorescent hydrogels.（ii）Fluorescence tunability of fluorescent hydrogels.In the practical application of fluorescent hydrogels,the tunability of local fluorescence is important in information display and hiding.Recently,the most researches of patterening fluorescent hydrogels are based on ion printing,which reduces the stability and resolution of the pattern.（iii）Fabrication of a new kind of white fluorescent hydrogels.White fluorescent hydrogels show great potential in the fields of lighting and information display.However,the fabrication of them generally requires the introduction of several chromophores and accurately adjustment of recipe,which are complex and time-consuming.It is of great significance to explore new preparation methods and develop new kinds of white fluorescent hydrogels.To solve the issues mentioned above,we did the following investigations:（i）We developed a fluorescent hydrogel by incorporating a donor-acceptor chromophore unit,4’-（N-vinyl benzyl-4-pyridinyl）-2,2’:6’,2’’-terpyridine perchlorate（VPTP）,into poly（1-vinylimidazole-co-methacrylic acid）network,which possesses high stiffness and toughness,as well as temperature-mediated shape-memory property,due to the dense inter-chain hydrogen bonds.Additionally,the hydrogel shows photo-mediated tunable fluorescence through unimer-to-dimer transformation of the chromophores.By photolithography and origami/kirigami techniques,fluorescent pattern is encoded in the hydrogel sheet which is deformed into specific three-dimensional（3D）configuration for information protection.The encrypted information in architected hydrogel becomes readable only after sequential shape recovery and UV light irradiation.Both fluorescent patterns and 3D configurations are reprogrammable,facilitating repeated information protection and display.（ii）We prepared a fluorescent hydrogel with ideal white light emission by micellar copolymerization of hydrophilic acrylamide and hydrophobic VPTP monomer,in which the unimer and dimer of the chromophore coexist and generate high-and low-energy emissions under excitation.The fluorescent behavior of the hydrogel can be well tuned by photo-or heat-treatment that induces unimer-to-dimer transformation of the chromophore and thus variation of the fluorescent color from blue to white and then to yellow.The fluorescence can also be reversibly switched off by forming terpyridine-Cu2+chelate complexes and recovered by using chelating agent to extract the Cu2+ions out of the gel matrix.These properties afford patterning the fluorescent hydrogel,which is transparent under daylight yet shows the pattern under UV light.Base on the difference between white light emission and background,a protected quick response code was patterned on the fluorescent hydrogel,which was hidden under daylight yet became readable under UV lamp.（iii）We prepared a class of tough supramolecular hydrogels facilely by polymerizing methacrylic acid precursor solution in the presence of hexadecyltrimethylammonium chloride（CTAC）micelles.After swelling the as-prepared hydrogel in water,strong polyelectrolyte/surfactant complexes（PESCs）are formed between the weakly charged polymer chains and oppositely charged surfactants,serving as the physical crosslinks of the gel matrix.The equilibrated hydrogel is transparent with water content of 50-85 wt%and possess excellent mechanical properties,with tensile breaking stress of 0.1-5 MPa,breaking strain of 600-1200%,and Young’s modulus of 1-70 MPa.The mechanical properties of hydrogel could be well tuned by p H,temperature,and ionic strength that influence ionization of polymer chains and the strength of PESCs.These dynamic behaviors render the hydrogel with self-recovery and shape memory properties.Furthermore,the nano-sized hydrophobic pockets within the hydrogel afford the capacity of loading functional hydrophobic molecules and promising applications as fluorescent materials and drug delivery systems.The fluorescent hydrogel can be easily obtained by previously addition of hydrophobic chromphore in the precursor solution.（iv）We fabricated a hydrogel with reprogrammable gradient structures through micellar polymerization of hydrophobic coumarin monomer and hydrophilic acrylic acid.The presence of CTAC micelles increases the local concentration of coumarin units and also improves the mechanical properties of the hydrogel by forming robust PESCs that serve as the physical crosslinks.High-efficiency photodimerization and photocleavage reactions of coumarins are realized under 365 and 254 nm light irradiation,respectively,affording reversibly tuning the network structure of the hydrogel.Through photolithography,different gradient structures are sequentially patterned in one hydrogel that direct the deformations into distinct configurations. |
