Crystalline clathrates can be templated, from nanoparticles using strands of DNA, according to researchers at the Northwestern University and the University of Michigan in the USA. Their approach to the synthesis of these materials could allow them to literally program their formation through choice of sequence and nanoparticle shape. [Lin, H et al. Science (2017) 355(6328):931-935; DOI: 10.1126/science.aal3919]
Clathrates are defined as inclusion compounds in which the guest molecule is held within a cage formed by the host molecule or by a lattice of host molecules. In other words, they are chemical substances comprising a crystal lattice that traps or contains molecules. The word derives from the Latin clatratus meaning with bars. Originally, the team referred to polymeric compounds that completely envelop their guest molecules, but modern usage takes into account host-guest complexes and inclusion compounds. So the definition encompasses calixarenes and cyclodextrins as well as inorganic materials such as zeolites. They have wide ranging potential applications as catalysts, gas storage materials, drug-delivery agents and much more, so finding novel clathrate structures or building designer compounds in this class is high on the materials science agenda.
There has been a focus in recent years on using the information-bearing properties of DNA for templating the synthesis of novel materials from nanoparticle building blocks with nanoscopic precision, an approach first introduced by Chad Mirkin, a coauthor on the paper, in 1996. The collaboration between the Northwestern University and Michigan University teams has followed this lead and may well have opened up a new artery in the field of programmable materials by using DNA to make complex clathrate compounds.
In a proof of principle demonstration Chad Mirkin and his colleagues worked with 250 nanometer gold crystals (oblate trigonal bipyramids, a shape crucial to the process too) held in suspension with synthetic DNA. The DNA strands attach to the gold particles and guide them into certain positions during a hybridization process. Depending on the length of the DNA sequences and the arrangement of the base pairs, different three-dimensional lattice structures form. Through DNA programming we can more or less determine the structure of the crystal lattice in a very precise manner.
The team's approach could be rather general with controlled production of colloidal clathrates opening up a wide range of possible applications. Such materials could be used to recognize proteins or viruses or be used for targeted manipulation of a crystal lattice to develop more complex material properties not seen in simpler colloidal crystals.
In their paper in Science, the team reveals how electron microscopy shows at least three different structures formed as large single-domain architectures or as multidomain materials. "Ordered assemblies, isostructural to clathrates, were identified with the help of molecular simulations and geometric analysis," the team reports. "These structures are the most sophisticated nanoparticle architectures ever made by any technique."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".