Chemists Create Gold Quantum Rods with Unusual Optical Properties

The three-atom wide molecules could have applications in high-resolution, noninvasive medical imaging

Carnegie Mellon University chemists have synthesized a series of gold quantum rods that strongly absorb and emit in the near-infrared, making them 100 times stronger than commercially available organic dyes. The work, published in the Proceedings of the National Academy of Sciences, highlights the quantum rods' promise for use in biomedical imaging applications and in boosting solar cell efficiency.

Developing molecules that emit light in the near-infrared, specifically wavelengths between 1,000-1,700 nanometers (nm), offers advantages in a variety of medical applications, including deep, non-invasive imaging and image-guided cancer surgery.

"We need near-infrared materials because these wavelengths are very important for high-resolution bioimaging," said Chemistry Professor Rongchao Jin.

Unlike visible light, which has wavelengths between 400-700 nm, near-infrared wavelengths can penetrate tissues more deeply — and dramatically improve the image resolution.

"These results are very promising. I'm quite excited for the quantum rods' bioimaging potential," said Jin, who is collaborating with faculty in Carnegie Mellon's Department of Biomedical Engineering to further test the quantum rods in imaging applications.

Jin's research group has long been interested in making atomically precise gold nanoclusters. These tiny particles, which can vary from 1 to 100 nm in size, have unique optical, electrical and magnetic properties. Of particular interest is their photoluminescence when the tiny particles are in the quantum-size regime. By manipulating the nanoclusters' size, structure and composition, they can be tuned to span the visible to the near-infrared regions of the electromagnetic spectrum.

"There are two great advantages to going to longer wavelengths/near-infrared: you can go much deeper and dramatically improve the resolution," he said.

While Jin's lab has been primarily focused on creating spherical gold nanoclusters, he said he found that the round shapes allowed for very low absorption at longer wavelengths (e.g., above 1,000 nm), making them unsuitable for high-resolution bioimaging. So he changed his approach.

"We decided to come up with a new design — non-spherical, or anisotropic, gold nanoclusters that would intensify near-infrared absorption at higher wavelengths," Jin said.

Their first success came in 2022 when former doctoral student Yingwei (Joanna) Li created a quantum rod made of precisely 42 gold atoms protected by a ligand. The rod's elongated shape gave it stronger absorption at 800 nm, which marks the beginning of the near-infrared region.

Doctoral student Lianshun (Evan) Luo, whose work is detailed in the PNAS paper, expanded upon that design. He has created a series of quantum rods that range in size from 42 to 114 gold atoms. Jin said the various sizes showed very strong absorption and emittance further into the near-infrared.

The 96-atom quantum rod is particularly impressive, Jin said. Luo showed that it emits at 1,650 nm, and he measured the absorption coefficient at 10⁶ —100 times stronger than any existing near-infrared dyes or nanomaterials that have been reported in the literature.

Jin's group is the first to develop these types of gold nano-structures. While there are organic dyes and other elongated structures — such as carbon nanotubes — that provide near-infrared absorption and luminescence, they are difficult to solubilize in water, do not have a strong light intensity (e.g., carbon nanotubes), and are not sufficiently stable (e.g., dyes).

"Our material is quite unique," Jin said. "Gold is very stable, and the quantum rods are very bright so you'd only need to use a tiny amount. So this is really an advantage."

Another promising field for the gold quantum rods is in boosting solar cells performance, Jin said. Roughly 50% of incident solar energy is wasted during the conversion of solar energy to electricity using a silicon-based cell. Although silicon cells can utilize wavelengths up to 1,100 nm, which is already very impressive according to Jin, a large portion of solar energy at the higher wavelengths (up to 2,500 nm) is not being utilized. The new quantum rods' ability to strongly absorb 1,000-2,000 nm wavelengths and potentially up-convert into the reaches of the silicon cells could make them ideal for use in solar cells to boost their efficiency.

Funding for this research was provided by the Charles E. Kaufman Foundation.