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50 C. Sealy

reatment is required to remove the gaps betweenanosheets.

‘‘The concept is very simple,’’ says Tsapatsis. ‘‘We makeanometer-thin, high-aspect ratio layers with molecular-ized pores running through the layer thickness and theneposit them (pack them) on a porous surface to form aontinuous molecular sieve filter.’’

Despite the simplicity of the approach, it has been impos-ible to demonstrate until now. The researchers say thathe films fabricated from exfoliated zeolite nanosheets haveolecular sieving properties, which they demonstrate with

he standard separation of xylene isomers. The approachould well be applicable to other microporous layered mate-ials, they add.

‘‘The potential impact in the area of selectiveembranes for separation and purification processes is enor-ous,’’ says Tsapatsis. ‘‘Unlike regular zeolite or otherolecular sieve crystals, these layers can pack well form-

ng compact thin films and have considerable flexibility toccommodate substrate roughness.’’

esearchers set the rules for designing superlattices

ordelia Sealy

cientists have long desired to be able to control the pre-ise arrangement of atoms and molecules into structures,ut the process can be hard to predict and dependent onany different factors. Now researchers from Northwesternniversity and Argonne National Laboratory have defined sixesign rules that can be used to prepare nanoparticle super-attices [R.J. Macfarlane et al., Science 334 (2011) 204].

In a step towards the ultimate goal of controlling theormation of materials, Chad A. Mirkin and his team haveevised a way of predictably creating superlattice struc-ures from Au nanoparticles connected by single-strandedNA (ss-DNA). The researchers define six design rules thatetermine all the different lattice structures into which theanoparticles can form, some that are found in nature andthers that are completely new.

‘‘Using these new design rules and nanoparticles asartificial atoms’, we have developed modes of controlledrystallization that are, in many respects, more powerfulhan the way nature and chemists make crystalline materialsrom atoms,’’ says Mirkin.

By controlling the size, shape, type and location of

Nicholas Kotov of the University of Michigan believes thatthe breakthrough is significant. ‘‘The new type of zeolitematerial has outstanding prospects in catalysis,’’ he toldNano Today. ‘‘The sheet morphology of the material opensthe door for the preparation of most interesting layeredcomposites with polymers.’’

The easy formation of large-scale membranes by fil-tration makes the approach particularly attractive whenusing low-cost, commercially available porous supports. Theresearchers are now working to scale up the production ofthe nanosheets from laboratory-scale milligram quantitiesto the volume needed for large area membranes and areexploring options for commercialization.

E-mail address: cordelia.sealy@googlemail.com

1748-0132/$ — see front matterdoi: 10.1016/j.nantod.2011.10.004

‘‘Once we have a certain type of lattice, the particlescan be moved closer together or farther apart by changingthe length of the interconnecting DNA, thereby providingnear-infinite tunability,’’ explains Mirkin.

The researchers start with just two solutions of nanopar-ticles coated with ss-DNA. Adding DNA strands that bindto these DNA-functionalized particles prompts the ‘stickyends’ to bind to each other, creating a superlattice of thenanoparticles. The simple process of mixing and heating cre-ates an ordered crystal lattice out of a disordered startingsolution.

The design rules could be applied to other types ofnanoparticle as well, say the researchers. Mirkin suggeststhat software could also be developed that would allowscientists to create almost any crystal structure from theright nanoparticle and DNA combination. The approach turnssuperlattice engineering on its head. Instead of the nanopar-ticle type determining the structure that is synthesized,scientists will be able to choose the building blocks that willgive them the structure they desire.

Hicham Fenniri of the University of Alberta in Canada

anoparticles and the distance between them (dependingn the length of the linking DNA molecules), the researchersreated 41 different colloidal structures, which adopt onef nine distinct crystal lattices (Fig. 1).

am

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grees that the design rules should the creation of newaterials that might not be possible through other methods.‘‘The strength of this work is the ability to pre-

ictably design specific arrangement of nanoparticles (with

Nanowires beat at the heart of tissue repair 551

Figure 1 The six design rules allow the prediction of the relative stability of a particular structure for a given set of designparameters, such as nanoparticle size or DNA length. These rules enable the construction of both nanoscale analogues of atomic

quivarsity

lattices, and lattices that have no naturally occurring mineral eCr3Si, AlB2, CsCl, NaCl and Cs6C60. (Credit: Northwestern Unive

predefined dimensions) in a three-dimensional space,’’ he

told Nano Today.

However, the approach does appear to be limited to afew thousand nanoparticles, which could limit its immediateapplication in areas like plasmonics, photonics or catalysis.

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lent. The lattices shown here are isostructural with (from left).)

Cordelia Sealy has many years’ experience asa scientific journalist and editor in areas span-ning nanotechnology, materials science andengineering, physics and chemistry. She hasserved as Editor of Materials Today and NanoToday, and more latterly as Managing Editor ofboth titles. She has also worked in academicpublishing as a books acquisitions editor andin business-to-business publishing as a jour-nalist on European Semiconductor. She has aFirst in Physical Sciences (BSc) from University

ollege London and a DPhil in materials science from the Universityf Oxford, and is a Member of the Institute of Physics. Cordelia isurrently a freelance science writer for her own company, Oxfordcience Writing, and News and Opinions Editor for Nano Today.

E-mail address: cordelia.sealy@googlemail.com

748-0132/$ — see front matteroi: 10.1016/j.nantod.2011.10.008