Photo- and pH-dually responsive hydrogel containing spirooxazine groups

The development of a low-cost and multifunctional sensing and monitoring device would be a meaningful endeavor. In this research, a novel spirooxazine with a two double-bonded structure, designated as (1-butyl-3,3-dimethylspiro[indoline-2,3’-naphtho[2,1-b] [1, 4]oxazine]-5,8’-diyl)bis(methylene)bis(2-methylacrylate) (SO-DB), was successfully synthesized and incorporated as a functional cross-linker into a polyhydroxyethyl methacrylate hydrogel system. Notably, this hydrogel system based on a single responsive component exhibits photo-pH dual responsiveness, which avoids the addition of multiple components that may affect other properties of the hydrogel. Particularly, in comparison to traditional surface grafting and physical blending, crosslinking with SO-DB leads to the formation of a denser and stronger three-dimensional polymer network in the stimuli-responsive hydrogel. Under UV light irradiation, spirooxazine absorbs energy and undergoes cleavage of the C spiro -O bond, transforming the molecule from a colorless closed-ring state to an open-ringed photomerocyanine (PMC) state. Under acidic conditions, hydrophobic spirooxazine undergoes a reversible protonation open-ring reaction, forming a stable intermediate form, a protonated photomerocyanine (PMCH). Furthermore, the addition of SO-DB as a crosslinking agent to the hydrogel system increases its stability and fatigue resistance, and maintains good repeatability over 24 light cycles and 7 pH cycles without performance loss. Such approach to generate stimulus-responsive hydrogels could have great potential in both wearable biosensors and food packaging field.