Letters in 2012

siRNA delivery has so far been hampered by carriers that inefficiently encapsulate RNA, and by its degradation prior to cellular uptake. Now, self-assembled crystalline microsponges consisting solely of cleavable RNA strands — which are converted to siRNA only after cellular uptake — achieve, with three orders of magnitude lower concentration, the same degree of gene silencing as conventional siRNA nanocarriers.

The observation of a superconductive current flowing through a topological insulator is considered the first step towards the observation of the elusive Majorana fermions. This is now achieved in a superconductor/topological insulator/superconductor junction in which direct evidence of Josephson supercurrents is reported.

The development of reliable diagnostic tools to investigate the performance of a battery in situ is required at present. Techniques based on magnetic resonance imaging are now shown to be able to non-invasively visualize and characterize the changes occurring in Li-ion battery electrodes and electrolyte.

Raman spectroscopy has already proved to be a powerful tool for studying the properties of single graphene layers. It is now shown that this technique can also provide information on the interaction between graphene sheets in multilayered graphene structures. In particular, a Raman peak corresponding to the interlayer shear mode, and probably linked to the interlayer coupling, is unveiled.

The large-scale synthesis of single-walled carbon nanotubes (SWCNTs) with controlled chirality—which could find applications in fields such as electronics—remains a great challenge. It is now shown that the growth rates of SWCNTs are directly proportional to their chiral angles, suggesting a route towards selective synthesis based on kinetic control.

Lumped elements such as resistors, capacitors and inductors play a crucial role in electronic circuits. Now, inspired by metamaterials technology, the experimental realization of lumped circuit elements for optical frequencies provides a standardized platform for applications such as mixing and multiplexing of optical signals.

Interfaces between insulating oxides have revealed exotic electronic and magnetic properties. It is now shown that a complex magnetic structure can emerge in an oxide superlattice, and that specific interfaces can unexpectedly exhibit exchange bias. The observations reveal the induction of antiferromagnetism in a material that is usually paramagnetic.

It is demonstrated that graphene coatings do not alter the wetting behaviour of copper, gold or silicon surfaces. Such wetting transparency—shown to occur only for surfaces where surface–water interactions are dominated by van der Waals forces—and graphene’s ability to suppress copper oxidation result in a 30–40% increase in condensation heat transfer on copper. The findings have implications for graphene-based coatings with independently tunable electronic and wetting properties.

Among other exotic properties graphene exhibits the highest thermal conductivity observed so far. This is true at least for graphene composed of only ¹²C atoms. However, it is now shown experimentally that regions of ¹³C atoms can substantially reduce the thermal conductivity. Aside from their fundamental importance, these results suggest that thermal conductivity can be tailored by varying the relative amounts of carbon isotopes used.

The growth of microcrystals can be controlled by various agents such as ions, small charged molecules and polyelectrolytes. However, their use is specific to the crystallizing material. It is now shown that oppositely charged nanoparticles can act as ‘universal’ surfactants for controlling the growth and stability of microcrystals of inorganic salts and of charged organic molecules.

A quantum critical point occurs when different stable phases of matter are in equilibrium at absolute zero temperature. Describing quantum criticality with a theoretical framework that unifies different types of transitions is highly desirable to understand how phenomena such as superconductivity and magnetism interact in correlated electron systems. A study now provides an indication of an underlying universality of quantum criticality, and highlights the role of dimensionality in such a unified theory.

Dissolution processes affect the performance of oxides in applications ranging from power generation to catalysis. A study on polyoxometalate ions, which are thought to model oxide surfaces, now suggests that dissolution is controlled by the stability of transient oxygen-stuffed structures.