“We have found that oligonucleotides with a so-called secondary structure – that is, DNA aptamers with a quadruplex shape – can be heavily loaded onto the surface of gold nanostars (AuNS) using a citrate buffer at low pH,” says team leader Teri Odom. “Nanomaterials for biological applications – also called nanoconstructs – comprise a hard nanoparticle core and a soft shell of bimolecular ligands. Our nanoconstructs (*Apt-AuNS) are special since the gold nanoparticle has an anisotropic shape and the DNA aptamer we use is an anti-cancer drug in itself.”

Researchers have mainly studied spherical gold nanoparticles (AuNPs) for making nanoconstructs because they can be easily functionalized with ligands containing terminal groups such as thiols, phosphines, and amines. Loading is usually done by self-assembly of thiolated biomolecules (oligonucleotides, antibodies, and peptides). Nanoconstructs containing a high density of biomolecules are generally much better for treating tumours than the free form of the biomolecule – for example, Apt-AuNS are about 20% better in vitro at destroying a wide range of cancer cells compared to the free aptamer drug at over 10 times the concentration.

*Apt-AuNS are more deadly

The ligand shell is normally assembled onto AuNP cores using a salt-aging process. However, the problem with this technique is that it requires excessive amounts of oligonucleotides and as long as two days to complete. “Our approach using a recently published citrate-buffer method has three advantages over this conventional process,” Odom told nanotechweb.org. “The first is that the amount of DNA aptamer ligand employed is significantly less; second, assembly time takes only three hours; and third, we are able to load much more aptamer onto the nanoparticles – two-and-a-half times, in fact, than that possible with salt-aging.”

“Besides these key, practical advances, we found that cancer cells take up *Apt-AuNS containing higher amounts of aptamer drugs more rapidly,” she added. Once internalized and shuttled near the cell nucleus by a target protein (nucleolin), *Apt-AuNS can kill more cancer cells than AuNS with lower densities of aptamer, explains Odom. “Although we do not yet know the exact mechanisms behind this uptake, we believe that the greater amount of aptamer drug ends up binding to higher quantities of nucleolin, which then leads to cell death.”

The Northwestern team tested its *Apt-AuNS on pancreatic cancer and fibrosarcoma cells since these tumours represent two different subcategories of cancers that are difficult to treat with traditional therapies.

The researchers say that they will now focus on improving how the therapeutic oligonucleotides pack onto the AuNS by controlling the pH of the solution in which assembly takes place. “We will also look at adding two or more oligonucleotides onto the same nanoparticle,” said Odom. “We will optimise these nanoconstructs for in vivo animal studies and aim to obtain the highest loading possible, as well as improve circulation and accumulation times in tumours.” Adding polyethylene glycol to the assemblies might help us do this,” she added.

The present work is detailed in Nano Letters DOI: 10.1021/nl500844m.