Quantum dots are particles of semiconductor that can efficiently absorb and emit light at specially chosen wavelengths. DNA, another darling of nanotechnologists, is unique in that it only binds to a complementary sequence of molecule. Now, Shana Kelley, Edward Sargent and colleagues in Toronto have combined the two materials to make nanoantennas using self-organization techniques that involve pre-selected numbers and types of quantum dots and DNA with three different "domains".

Conventional antennas increase the amount of an electromagnetic wave (such as a radio frequency) that is absorbed and work by funnelling the electromagnetic energy to a circuit. Nanoantennas are similar except that they work at optical frequencies. The nanoantennas made by Kelley and Sargent's team funnel light to a single site within the DNA-quantum dot complexes.

"Like photosynthesis"
"This concept is already used in nature, in the constituents of leaves that make photosynthesis so efficient," explained Sargent. "Our complexes work in the same way, by also using wavelengths found in sunlight."

The DNA used was designed to have three domains: a quantum dot binding domain that ligands the nanoparticles; a spacer domain; and a DNA binding domain for recognizing complementary sequences in subsequent self-assembly of the complexes. The researchers first combined the DNA and a precursor containing cadmium, and then introduced a tellurium precursor.

"Crucially, we used the length of the quantum dot binding domain to program the valency of the quantum dots – that is, the number of DNA molecules per quantum dot, and hence the number of available binding sites per dot," Kelley told nanotechweb.org.

To then build higher-order complexes, the Toronto team used DNA binding domains that were complementary. "For example, to form a complex consisting of a central red nanoparticle clad by four yellow nanoparticles, we designed the red particle to have a valency of four; the yellow nanoparticles to have a valency of one; and for the DNA binding domains on red and yellow to be complementary," said Sargent.

Self-build
"The amazing thing is that our antennas build themselves – we coat different classes of nanoparticles with selected sequences of DNA, combine the different families in one beaker, and nature takes its course. The result is a beautiful new set of self-assembled materials with exciting properties." The complexes made might be used in light sensing and solar cell applications, he adds.

The scientists say that they now plan to identify and build a new class of solid-state electronic or optoelectronic devices that exploit this novel breed of materials. "The present work presents the synthesis of the material system and its application merits continued efforts," concluded Sargent.

The results were published in Nature Nanotechnology.