Making higher-energy light to fight cancer

Researchers use nontoxic silicon nanocrystals
to convert low-energy photons into high-energy ones, bringing scientists closer to developing
photodynamic treatments for cancer. Materials scientists at the University of
California, Riverside and The University of Texas at Austin have demonstrated that it
is possible to achieve photon up-conversion, the emission of light with energy higher than
the one that excites the material, when using carefully designed structures containing silicon
nanocrystals and specialized organic molecules. The accomplishment, published in Nature Chemistry,
brings scientists one step closer to developing minimally invasive photodynamic treatments
for cancer. The advance could also hasten new technologies
for solar-energy conversion, quantum information, and near-infrared driven photocatalysis. High energy light, such as ultraviolet laser
light, can form free radicals able to attack cancer tissue. Ultraviolet light, however, doesn’t travel
far enough into tissues to generate therapeutic radicals close to the tumor site. On the other hand, near-infrared light penetrates
deeply but doesn’t have enough energy to generate the radicals. While photon up-conversion can overcome this
limitation, up-converted materials have either low efficiency or are based on toxic materials. Silicon is well-known for being nontoxic,
but until now, researchers have not been able to demonstrate that silicon nanocrystals can
up-convert photons, leaving this promising cancer treatment tantalizingly out of reach. The research group learned how to attach ligands,
which help bind molecules together, to the nanoparticle that are specifically designed
to transfer the energy from the nanocrystals to surrounding molecules. The team then shined laser light into the
solution. They found silicon nanocrystals with appropriate
surface ligands can rapidly transfer the energy to the triplet state of surrounding molecules. A process called triplet-triplet fusion then
converts the low-energy excitation to a high energy one, resulting in the emission of a
photon at shorter wavelength, or higher energy, than the one originally absorbed by the nanoparticle. This discovery could also lead to improved
photocatalysis, which uses light to drive chemical reactions. The environmentally sustainable silicon-centered
approach is also relevant for quantum information science and singlet fission-driven solar cells.