Multiresponsive Microgels with Phase-Separated Nanodomains and Self-Regulating Properties via Incorporation of Anthraquinone Moieties

Of the multitude of stimuli-responsive microgels, it is still a challenge to achieve multiple responsivenesses to one single stimulus, which can even revert to the corresponding original state autonomously after stimulus. In this work, we reported a series of anthraquinone functionalized microgels (PNI-xVAQ) with thermosensitivity and redox-actuated self-regulating color, size, and fluorescent properties, which were easily synthesized via surfactant-free emulsion copolymerization (SFEP) with N-isopropylacrylamide (NIPAm) as the monomer, 2-vinylanthraquinone (VAQ) as the comonomer, and N,N′-methylenebis­(acrylamide) (BIS) as the cross-linker in an aqueous solution at 70 °C. The hydrophobic interactions of comonomer VAQ also led to the formation of internal phase-separated hydrophobic nanodomains in the obtained PNI-xVAQ microgels. The self-regulating color, size, and fluorescence changes of the PNI-xVAQ microgels were reliant on the nonequilibrium redox process of anthraquinone moieties by the addition of sodium dithionite as the chemical fuel to activate the positive feedback that was the reduction of anthraquinone to transient anthraquinone radical anions, following the slow oxidation of anthraquinone radical anions by autonomous “breathing” oxygen in air as the delayed negative feedback. These autonomous self-regulating properties of the PNI-xVAQ microgel were recyclable to a certain extent by repeated feeding of sodium dithionite.