KAUST researchers have developed a highly efficient organic X-ray imaging scintillator with significant potential in medical radiography and security screening applications. Image: 2021 KAUST; Ella Marushchenko.A nanocomposite that absorbs X-rays and then, with nearly perfect efficiency, re-emits the captured energy as light could help to improve high-resolution medical imaging and security screening. The material’s near-100% energy transfer could bring efficiency gains in devices ranging from light-emitting diodes (LEDs) to X-ray imaging scintillators to solar cells.
During a medical imaging procedure, X-rays passing through the body are absorbed by a scintillator material, which converts those X-rays into light for a digital-camera-type sensor to capture. “To date, high-performance scintillators consist mainly of either ceramic that needs harsh and costly preparation conditions, or perovskite materials that have poor air and light stability and high toxicity,” says Jian-Xin Wang, a postdoc in the lab of Omar Mohammed at the King Abdullah University of Science & Technology (KAUST) in Saudi Arabia, who led the work.
In contrast, organic scintillator materials have good processability and stability but low imaging resolution and detection sensitivity due to the low atomic weight — and so limited X-ray absorption — of their component atoms. Mohammed and his colleagues have now improved the X-ray capture of organic scintillators by combining them with a metal-organic framework (MOF) termed Zr-fcu-BADC-MOF, which incorporates high atomic weight zirconium within its highly ordered structure.
When the MOF layer of the nanocomposite was struck by X-rays, excitons – excited pairs of negatively charged electrons and positively charged holes – were generated. These excitons readily transferred from the MOF to the organic TADF (thermally activated delayed fluorescence) chromophore, aided by the ultrashort distance between them, where they emitted their energy as light.
Critically for the nanocomposite’s overall efficiency, the TADF chromophore emitted light regardless of the form of the exciton. 'Singlet' excitons resulted in direct light emission, while the TADF chromophore readily converted non-emissive 'triplet' excitons into the emissive singlet state.
“The direct harnessing of singlet and triplet excitons of the TADF chromophores contributed greatly to its remarkably enhanced radioluminescence intensity and X-ray sensitivity,” Wang says.
Due to its near-100% efficient transfer of energy from X-rays into light, the nanocomposite scintillator reached imaging resolution down to a few hundred micrometers, and a detection limit 22 times lower than typical X-ray medical imaging doses.
This concept was confirmed when the team employed a closely related strategy, showing that the TADF chromophore could also be combined with perovskite nanosheets to produce other nanocomposites with excellent X-ray imaging scintillator performance. Again, efficient energy transfer due to the ultrashort distance between layers, and the TADF chromophore’s direct use of both singlet and triplet excited states, were key. In this case, the detection limit of the material was enhanced even more, reaching 142 times lower than a typical X-ray medical imaging dose. The researchers report their results in papers in Matter and ACS Energy Letters.
“Our energy transfer strategy promotes organic X-ray imaging scintillators from an almost-dead research field into one of the most exciting applications for radiology and security screening. It also applies to other light-conversion applications including light-emitting diodes and solar cells,” Mohammed says. “We are planning to further improve the performance of our large-scale X-ray imaging scintillators before we take it to the market.”
This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.