"Our use of the effect is directed towards energy harvesting for solar energy applications," explained James De Yoreo, Professor at the Pacific Northwest National Laboratory. "When one looks at light harvesting complexes in biological systems, they often utilize a similar architecture."

The electrons in a metallic nanoparticle undergo collective oscillations known as a surface plasmon resonance, when exposed to certain frequencies of light. The nanoparticle then behaves as a plasmonic antenna, concentrating light within a few nanometers of the nanoparticle surface.

"This provides a means of creating an intense electromagnetic field near a light absorbing centre, thus allowing us to greatly increase the capture of light," De Yoreo told nanotechweb.org. Scaling up the number of fluorophores affected helps increase the amount of light captured even further.

Nanoscale assembly

The researchers, from Lawrence Berkeley National Laboratory, the Pacific Northwest National Laboratory, University of California Berkeley and Arizona State University, combined two self-assembly approaches to collect hundreds of fluorophores and position them next to the gold nanoparticle.

First, they used a virus 'capsid' to form a container for the fluorophores. A virus capsid, which would normally encapsulate a virus, is made of a protein that self-assembles with many copies of itself to form a shell. For this application, the researchers modified the inner surface with fluorophore attachment sites, capturing almost 180 fluorophores and giving a density of one fluorophore per 14 nm2. The capsid was further modified so that the outside was coated with DNA strands.

The second step used so-called DNA origami – a collection of several hundred synthetic DNA strands that self-assemble into shapes around 100 nm in size. The researchers formed a tile to act as a molecular breadboard, with two binding locations for attaching objects.

Finally the researchers modified gold nanoparticles with a set of DNA strands that recognized a location on the origami. The DNA sequences on the outside of the capsid were designed to bind to a different location.

Mixing the three components together – the fluorophore loaded capsid, the origami tile and the gold nanoparticle – resulted in the capsid and nanoparticle being held nanometres apart on the origami. By adjusting the origami design, the capsid-nanoparticle separation distance could be adjusted.

Characterizing the system

The researchers confirmed the system had formed as designed, using atomic force microscopy and electron microscopy. They then investigated how the capsid-nanoparticle separation affected the fluorescence characteristics by first studying a sample using confocal microscopy. The separation distance, was determined using atomic force microscopy. The combined measurements demonstrated increased fluorescence intensity for a number of separation distances.

In order to better understand the experimental results, the researchers modelled the interactions of the fluorophores with the nanoparticle, demonstrating that the system behaved as they expected. For larger gold nanoparticle sizes, the model showed the fluorophores would undergo significant increases in fluorescence.

"The model enabled us to explore changes to the nanoparticle size, choice of fluorophore, arrangement of fluorophores and even the capsid shape in order to optimize the performance," said De Yoreo.

Full details are reported in ACS Nano DOI:10.1021/nn5015819.