br Acknowledgments This work was supported by

Acknowledgments This work was supported by FAPESP, São Paulo, Brazil, (Grants # 2014/15982-6 and 2013/21075-9); CNPq and CAPES (Brasília, DF, Brazil). We thank Dr. Julian Sale (Medical Research Council Laboratory of Molecular Biology, Cambridge, UK) for critical reading of this review.
Introduction DNA micelles have benefited from the programmable design of nucleic ampk pathway ligands and size-controllable hydrophobic assembly of lipid molecules. As such, they have been widely developed as an efficient tool in biochemical research with numerous applications, such as intracellular imaging, targeted drug delivery, and immune response initiation. However, even after several years of development, accurate structural profiling of DNA micelles has not been achieved. In spite of the unparalleled properties of such exquisite structures, critical micelle concentration (CMC) remains a limiting factor that militates against expanded applications of DNA micelles,5, 6 a challenge shared by all kinds of micellar materials. Meanwhile, since hydrophobic forces exist widely in bilayer structures and biological environments, DNA micelles can be easily degraded upon incubation with cells. As a consequence, part of the DNA-lipid monomer may insert on the cell surface such that only a small number of micelles are able to traverse the bilayer by a fusion and shedding process to finally function inside the cell. This could be solved by stabilizing intra-micelle interactions by crosslinking each monomer of a DNA micelle. One approach is using Hoogsteen hydrogen bonding to crosslink lipid monomers by forming a G-quadruplex in the presence of potassium ions.4, 8 Compared with hydrogen bonding, a more stable crosslinking strategy is to use covalent bonds.9, 10, 11 However, such a solution, as demonstrated in most reported methods thus far, is thwarted by extended time investment, sophisticated synthesis of monomer required, or unsatisfactory stability improvement. In this paper, we describe a facile and universal method for in situ crosslinking of DNA micelles, using spherically directed reduction of metal ions. As shown in Scheme 1A, a single-stranded DNA (ssDNA) consisting of a ligand domain, a template domain, and a lipid domain was designed as a monomer to form the DNA micelle. Some reported specific DNA sequences were incorporated here as a template domain to selectively adsorb different kinds of metal ions (poly C for Ag,12, 13, 14 poly A for Au,15, 16 and poly T for Cu). In the presence of a reducing agent, metal ions enriched around the template domain will be reduced to zero-valent metal under very mild conditions at room temperature and finally crosslinked into a hollow (Cu) or solid (Ag and Au) metal core, resulting from the dense packing of DNA ligands in the micelle. The size of the metal core can be tuned by simply changing the length of the template domain, whereas the ligand remains on the surface of the metal core as a DNA corona because only the template domain will induce the enrichment of metal ions. The detailed structural profile of DNA micelle was further determined for the first time after a series of calculations and simulations, and the results matched very well with the subsequent experiments.