DNA 'origami' glitters gold

Materials World magazine
7 May 2012

A synthetic nano-optical material produced using DNA ‘origami’ - the
folding of DNA on the nanoscale into two- or three-dimensional
structures - may facilitate a new class of versatile sensors and optical
lens systems for environmental or medical monitoring applications.

The material is created using a method of self-assembly that exploits the fact that DNA bases naturally seek out their counterparts. ‘All we did is write up a blueprint,’ says Professor Tim Liedl of the Nanosystems Initiative Munich, Germany, who led the team.

‘The blueprint involves two main components – one long strand of DNA called a scaffold that contains about 8,000 bases, and smaller staple strands that have between 30 and 50 bases. ‘As the bases that make up these staples bond to the bases on the scaffold, they fold the scaffold into the appropriate shape.’

The staples are further modified to enable other molecules to be attached – in this case 10nm diameter gold nanoparticles – and the modified staples are chosen to give a left- or right-turning helix.

Actual assembly entails mixing hundreds of billions of DNA staple strands with billions of scaffold strands. The mixture is then heated and allowed to cool slowly. As it does so, each DNA region finds its partner region, while the gold particles also find their target sites by themselves.

The material modulates light by taking advantage of plasmons – waves of changes in electron densities in metals, where electrons behave like a gas, and photons can excite the oscillation of electrons in metal nanoparticles. These oscillations can in turn be re-converted into photons, which together create a modified electromagnetic wave.

The research has shown that if metal particles are arranged in a helix, they selectively absorb light of a defined circular polarisation, and that the material can also rotate linearly polarised light. It also shows that the optical response can be tuned, because the intensity of the response rises strongly with the size of the particle, and that the optical resonance shifts from the red end of the spectrum to the shorter wavelengths of the blue end if the gold particles are coated with a layer of silver. These macroscopic effects (visible with the naked eye) are a first, Liedl says.

DNA origami also improves on methods such as top-down lithography. For example, lithography cannot create features smaller than 10nm, but DNA ‘origami’ allows the position of the gold particles to be defined with an accuracy of better than 2nm.

And while lithography is limited mostly to two dimensions, this technique makes it far easier to create 3D structures that can then float in liquid – and the liquid is completely isotropic, so the observed collective behaviour of the particles will be the same regardless of the direction from which it is viewed.

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