DNA‐Templated Silver Nanorings

Nanostructures of noble metals with well-defined shapes and sizes are increasingly attracting the attention of scientists in the fields of catalysis, electronics, photonics, information storage, optoelectronics, biological labeling, etc. The further development and practical applications of nanostructures are expected to increase rapidly because of their interesting optical, electronic, and magnetic properties. In this context, important knowledge of the direct preparation of metallic nanostructures of controlled size and shape has been developed over the past few years, and various morphologies, such as spherical nanoparticles, nanocubes, nanoprisms, nanoplates, or nanobelts, can now be prepared in a controlled way. However, since these techniques are based on the directed growth of particles in the reaction medium, they can only lead to shapes of a simple topology, such as spheroids, ellipsoids, or polyhedrons. In contrast, nanoparticles with a toroidal shape (nanoring) can not be produced by a direct growth technique. Hence, the only way to produce such a morphology is to use a toroidal template of nanometer-scale dimensions. Elaborate and successful methods to prepare silver or gold rings based on the use of a nanoparticle array or a mesoporous membrane as a primary template, were recently described by Xia and co-workers and Yan and Goedel, respectively. However, these techniques provide rings with a minimal size of 0.5 lm that can not be directly dispersed in an aqueous medium. On the other hand, due to the specific interaction between DNA and silver ions, DNA is an ideal template to build silver nanostructures. This principle has been used successfully to produce nanoparticle arrays on a DNA scaffold, or DNA-templated silver nanowires. However, the ability of DNA chains to form toroidal condensates as a result of the DNA-folding transition (DNA condensation) has not been hitherto noticed by materials scientists. The ability of DNA to condense into well-defined toroids provides a unique opportunity to use them as templates to create silver toroidal nanostructures (nanorings) of controlled shape and dimensions. In this communication, we describe a one-pot, three-step, simple preparation of well-defined silver nanorings (100 nm in diameter) dispersed in water, based on the use of dilute solutions of DNA condensates as nanostructured templates. DNA is a semiflexible, highly charged polyelectrolyte that assumes an elongated-coil conformation in water due to the electrostatic repulsion between the negatively charged monomers. DNA molecules usually fold into tightly packed toroidal condensates with an outer diameter of typically 70–90 nm in the presence of hydrophilic neutral polymers, or upon the addition of a small amount of condensing agent, such as cationic polyamines, multivalent metal cations, and cationic surfactants, to a dilute DNA solution. The role of the condensing agents is to induce an attraction between the DNA monomers (chain neutralization or crowding effect), and the toroidal morphology is adopted because of the native rigidity of the DNA double-stranded chain. Advanced Materials 0000, 00, 0–0 1