Cu2+-Induced “Off–On–Off” Switching Luminescence of Cellulose-Based Luminescent Hydrogels

Stimuli-responsive luminescent hydrogels based on natural cellulose are emerging soft materials with potential applications because they simultaneously show unique luminescence properties, biocompatibility, biodegradability, and stimuli-responsiveness, and they particularly serve as functional biomaterials for bioimaging, biosensing, therapeutics, and so on. Herein, we provide a new method to prepare transparent, bulky, and multi-color luminescent hydrogels based on cellulose, ionic liquids, and lanthanide complexes (LnCs). Cellulose-based luminescent hydrogels were constructed first via dissolution–regeneration processes by dissolving cellulose in 1-allyl-3-methylimidazolium chloride (AmimCl) and then immersed in an aqueous solution of LnCs to fill the uniform porous structure of cellulose. The color-tunable emission colors ranging from red to green were emerged through regulating the molar ratio of Eu3+/Tb3+ in LnCs. Additionally, the presence of Cu2+ results in the luminescence "turn-off" process of resultant hydrogels by the naked eye through the photo-induced electron-transfer (PET) mechanism, including color and intensity, resulting in the selective recognition of Cu2+ ions. Moreover, the strong coordination between glutathione (GSH) and Cu2+ could destroy the PET process and restore the luminescence to "turn-on". Therefore, cellulose-based hydrogels could be utilized as biocompatible and biodegradable "off–on–off" luminescence switches in aqueous medium.