Schematic of the linking strand attached to a DNA-stabilized silver cluster hybridizing onto the complementary docking strand of a DNA nanotube.

Materialistically, society perceives the element silver second only to gold, however silver wins the favor of engineers, who take advantage of its unique conduction properties of heat and electrical current. A group of UC Santa Barbara researchers are applying silver’s behavior at the nanoscale for purposes of biomedical imaging, proving that even a small amount of silver is worth more than one may think.

Specifically, Dr. Beth Gwinn and her research team stabilize silver clusters with biopolymers such as DNA and RNA, and utilize the optical properties of these nanoparticles for biomedical applications. The group’s most recent research can be found in the April 2015 issue of ACS Nano.

As the size of metal clusters shrink down to just a few tens of atoms, these metal clusters can become fluorescent, allowing them to emit light when they are energetically excited. Silver is special in that its light emission is especially bright due to its electronic properties. Other metals such as gold and copper yield less of a vivid fluorescence, which is why Gwinn’s team primarily focuses their research on silver.

Using a nanoscale breadboard, a structural base for particle deposition in order to test photonics and electronics, the researchers arranged silver nanoclusters at precise locations to yield the desired fluorescent markers.

“Our DNA breadboard design is inspired by the field of DNA nanotechnology, which uses the natural base-pairing of DNA in order to engineer new nanomaterials,” Stacy Copp said, a graduate student in UCSB’s Department of Physics and the lead author of the study. “We have exploited DNA nanotechnology to place silver clusters at precise locations just seven nanometers apart on such a DNA breadboard.”

(from left): Co-authors, Steven Swasey, Beth Gwinn, and Stacy Copp

Achieving a uniform organization and distribution of nanoparticles for this type of array is a difficult task, but with a ligand like DNA to select for specific metal clusters, researchers can more precisely control the size and shape of the cluster assemblies at an atomic level.

“This is a remarkable degree of control that is promising for realizing new types of nanoscale photonics,” Gwinn said, principal investigator of the lab and a professor in the Department of Physics.

Designing photonic arrays that self-assemble require researchers to specifically program the positions of the clusters. In order to manipulate the silver clusters, the team engineered a ‘docking’ DNA strand that embedded itself within DNA nanotube, and a ‘linker’ strand that was fused to the cluster, forming the basis to the experiment’s design scaffold. These DNA-stabilized silver clusters could then be directed to hybridize, or bind, specifically to the complementary docking strand hanging from the DNA nanotube.

Silver’s fluorescent abilities encompass a large range of wavelengths within the electromagnetic spectrum, from visible light to infrared. Members of the Gwinn group were able to fine tune their silver clusters so that different colors are discernable on their breadboard design. By changing the sequences of DNA that they use, the researchers can manipulate the size and color of the silver cluster that they make.

“This sequence tenability of silver cluster color is unique to DNA-stabilized silver clusters and provides us with a whole range of cluster colors,” Copp said. “In the process of studying DNA-stabilized silver clusters, we are also learning more about how silver itself interacts with DNA.”

In addition to its effectiveness in sensing and logic devices, silver has long been used as an antibiotic agent, which suggests that the element’s usefulness in biomedical research and therapeutics will increase exponentially in the coming years.

Although silver has already proven to be an efficient candidate for metal cluster photonics on DNA nanostructures, researchers from the Gwinn lab hope to explore other alternatives.

“There are still many metals that we and others have not tried to make clusters with,” Copp said. “We are planning to investigate some of these in the future.”