Switchable catalytic acrylamide hydrogels cross-linked by hemin/G-quadruplexes

Copolymer chains consisting of acrylamide units and guanine (G)-containing oligonucleotide-tethered acrylamide units undergo, in the presence of K ⁺ ions, cross-linking by G-quadruplexes to yield a hydrogel. The hydrogel is dissociated upon addition of 18-crown-6 ether that traps the K ⁺ ions. Reversible formation and dissociation of the hydrogel is demonstrated by the cyclic addition of K⁺ ions and 18-crown-6 ether, respectively. Formation of the hydrogel in the presence of hemin results in a hemin/G-quadruplex-cross-linked catalytic hydrogel mimicking the function of horseradish peroxidase, reflected by the catalyzed oxidation of 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid), ABTS^2-, by H₂O₂ to ABTS^.- and by the catalyzed generation of chemiluminescence in the presence of luminol/H₂O ₂. Cyclic "ON" and "OFF" activation of the catalytic functions of the hydrogel are demonstrated upon the formation of the hydrogel in the presence of K⁺ ions and its dissociation by 18-crown-6 ether, respectively. The hydrogel is characterized by rheology measurements, circular dichroism, and probing its chemical and photophysical properties.