Scientists have found that gold clusters assemble into double-helical supercrystals, using pairing principles similar to those found between DNA base pairs.
DNA has been used before to create helical hybrid DNA-nanoparticle structures, but this is the first time that nanoclusters have been known to organize themselves into a double helix. Without the help of biological molecules, helices are difficult to build because they require a delicate balance between repulsive and attractive forces – in DNA these are ionic interactions, stacking or hydrogen bonds.
The new nanoclusters, synthesized by a team of chemists in the US and China, each have 29 gold atoms and 19 adamantane thiolate ligands Au29(SAdm)19th. They form two enantiomeric strands that arrange themselves into a double or even quadruple helix. Each helix pitch contains 16 nanoclusters and measures 12.8 nm – much larger than the 3.4 nm pitch length of DNA.
The arrangement arises from four different substructures or motifs that form two pairs, similar to the base pairs in DNA. Each pair is held together by balanced attractive conformational adaptations and repulsive steric interactions. A similar gold compound – Au30th(SAdm)18th – also creates two enantiomers, but does not build helices, as it only has a single pair of matching motifs.
The double helix was found to have a 65 times longer photoexcited state lifetime than the non-helical Au30th(SAdm)18th. In addition, the extensive coherence of the helix enhances its photoluminescence.
The researchers suspect that the information these nanocluster assemblies reveal could help them design other atomically precise supercrystals and better understand processes in biological supramolecules, such as protein folding.