  A key objective in DNA-based material science is understanding and precisely
controlling the mechanical properties of DNA hydrogels. We perform
microrheology measurements using diffusing-wave spectroscopy (DWS) to
investigate the viscoelastic behavior of a hydrogel made of Y-shaped DNA
nano-stars over a wide range of frequencies and temperatures. Results show a
clear liquid-to-equilibrium-gel transition as the temperature cycles up and
down across the melting-temperature region for which the Y-DNA bind to each
other. These first measurements reveal the crossover of the elastic
G'({\omega}) and loss modulus G'({\omega}) when the DNA-hydrogel formed at low
temperatures is heated to a fluid phase of DNA nano-stars well above the melt
temperature Tm. We show that the crossover relates to the life-time of the
DNA-bond and also that percolation coincides with the systems' Tm. The approach
demonstrated here can be easily extended to more complicated DNA hydrogel
systems and provides guidance for the future design of such transient,
semi-flexible networks that can be adapted to the application of molecular
sensing and controlled release.
