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/H2O2. 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.