  The silicification of DNA origami structures increases their mechanical and
thermal stability and provides chemical protection. So far, it is unclear how
silicification affects the internal structure of the DNA origami and whether
the whole DNA framework is embedded or if silica just forms an outer shell. By
using in situ small-angle X-ray scattering (SAXS), we show that the
net-cationic silica precursor TMAPS induces substantial condensation of the DNA
origami, which is further enhanced by the addition of TEOS at early reaction
times to an almost 10 % size reduction. We identify the SAXS Porod invariant as
a reliable, model-free parameter for the evaluation of the amount of silica
formation at a given time. Contrast matching of the DNA double helix Lorentzian
peak reveals that silica growth also occurs on the inner surfaces of the
origami. The less polar silica forming within the origami structure, replacing
more than 40 % of the internal hydration water causes a hydrophobic effect:
origami condensation. In the maximally condensed state, thermal stabilization
of the origami up to 60 {\deg}C could be observed. If the reaction is driven
beyond this point, the overall size of the silicified origami increases again
due to more and more silica deposition on the DNA origami. DNA origami objects
with flat surfaces show a strong tendency towards aggregation during
silicification, presumably driven by the very same entropic forces causing
condensation. Our studies provide novel insights into the silicification
reaction and hints for the formulation of optimized reaction protocols.
