Nontoxic Material Used to Upconvert Low-energy Photons

Researchers at the University of California, Riverside (UC Riverside) and the University of Texas at Austin demonstrated the use of nontoxic silicon nanocrystals to convert low-energy photons into high-energy ones. The use of nontoxic materials for photon upconversion could help advance the development of photodynamic treatments for cancer.

Ultraviolet (UV) light can form free radicals that can attack cancer tissue. However, it does not travel deep enough into tissues to generate therapeutic radicals close to the cancer site. Near-infrared (NIR) light, in contrast, can penetrate deep into tissues, but is not high-energy enough to generate free radicals. While photon upconversion can provide the advantages of both UV and NIR light, to date the materials used for converting incoherent long-wavelength light into the visible range have contained toxic elements, prohibiting their use in biological or environmentally sensitive applications. Silicon is known to be nontoxic, but has never before been used for upconversion.

A green lower-energy laser light goes through silicon quantum dots. The quantum dots reemit, or upconvert, the low-energy light into a higher-energy blue light. Courtesy of Lorenzo Mangolini and Ming Lee Tang/UCR.

A research team led by UC Riverside doctoral student Pan Xia addressed this challenge by chemically functionalizing nontoxic silicon nanocrystals with triplet-accepting anthracene ligands. “We functionalized silicon nanocrystals with anthracene,” Xia said. “Then we excited the silicon nanocrystals and found that the energy was efficiently transferred from the nanocrystal, through the anthracene molecules, to the diphenylanthracene in solution. It means we got higher-energy light.”

The group studied the surface chemistry of silicon nanocrystals and learned how to attach ligands, which help bind molecules together, to a nanoparticle designed to transfer energy from the nanocrystals to the surrounding molecules. When the researchers shined laser light on the nanocrystals, they found that the crystals with appropriate surface ligands could rapidly transfer energy to the triplet state of surrounding molecules. Through a process called triplet-triplet fusion, a low-energy state was converted to a high-energy state, resulting in the emission of shorter wavelength photons.

Silicon nanocrystals are formed by a silane gas in a plasma process. Courtesy of Lorenzo Mangolini/UCR.

“To turn the low-energy photons into high-energy photons, you need to use triplets, you need to use quantum-confined nanoparticles, and you need to hold the nanoparticles and the organic molecules very close together,” professor Ming Lee Tang said. “This is how you get the triplets to combine energy to get the high-energy photons.”

Professor Lorenzo Mangolini said that the UC Riverside group was the first to find a way to get the two parts of this structure — the organic molecules and the quantum-confined silicon nanocrystals — to work together.

Ultrafast lasers were used to investigate how the energy in this hybrid structure was transferred. Professor Sean Roberts from the University of Texas at Austin said that the process was fast and efficient. “The challenge has been getting pairs of excited electrons out of these organic materials and into silicon,” Roberts said. “It can’t be done just by depositing one on top of the other. It takes building a new type of chemical interface between the silicon and this material to allow them to electronically communicate.”

A silicon-to-molecule Dexter energy transfer drives photon upconversion. Courtesy of Sean Roberts/The University of Texas at Austin.

The team’s demonstration of spin-triplet exciton transfer from silicon to molecular triplet acceptors could enable new technologies for solar energy conversion, quantum information, and NIR-driven photocatalysis, in addition to opening the way for new, minimally invasive photodynamic cancer treatments.

The research was published in Nature Chemistry (www.doi.org/10.1038/s41557-019-0385-8).