A concept for a pure DNA hydrogel has been demonstrated in which triplex structures act as the core components and are assisted by single strands and artfully designed Y-shaped structures. Typically, the triplex structure which is based on protonated cytosine-guanine-cytosine (C-G·C+) is formed at pH 5.0 and disassembles at pH 7.0, whereas the thymine-adenine-thymine (T-A·T)-based triplex structure is formed at pH 7.0 and disassembles at pH 10.0. Therefore, such triplex DNA structure-based pure DNA hydrogels provide pH-controlled reversible self-assembly of DNA structures with a transition between gel and liquid states. More significantly, the introduction of both a fluorophore (FAM) and a quencher (BHQ1) to the hydrogels provides an innovative method for monitoring the self-assembly and disassembly processes through a fluorescence technology.