Nanostructures Designed To Fall Apart

For a nanoparticle to successfully deliver a drug to a tumor, it needs to remain in the body long enough to find its target and release its payload. Particles with diameters smaller than about 10 nm usually don’t work: They get filtered quickly by the kidneys. Larger particles stick around, but they can overstay their welcome—especially when the tiny materials are made of potentially harmful metals. To address these retention issues, a team led by Warren C. W. Chan of the University of Toronto has designed “superstructures,” made of metal nanoparticles, DNA strands, and polymers, that are larger than 10 nm overall but that eventually break down into smaller bits that can be excreted (Nat. Nanotechnol. 2014, DOI: 10.1038/nnano.2013.309). At the center of these structures sits a single gold nanoparticle. This particle is surrounded by a number of other “satellite” gold particles coated with polyethylene glycol and linked to the core via short, degradable DNA sequences. Forty-eight hours after the researchers injected mice with a variety of these superstructures, the smallest satellites—3 nm in diameter—appeared in the rodents’ urine. Although his team didn’t test drug delivery, Chan says therapeutics could be anchored to the satellites, bound to the DNA linkers, or somehow squeezed between the core and satellites.