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),　ABTS2-,　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.　©　2013　American　Chemical　Society.