Organic X-ray excitement enables innovative imaging

Organic-based materials could transform X-ray imaging by improving fabrication methods and providing reliable high-resolution imaging results. Now researchers at the King Abdullah University of Science and Technology (KAUST) have developed a novel approach for designing and building such high-performance scintillator materials for detecting X-rays at low doses, with their work published in the journals Nature Photonics and Matter.

A scintillator is a material that, when hit by high-energy ionising radiation such as X-rays, absorbs the energy and re-emits some of it as low-energy visible light. Scintillators are widely used for X-ray imaging screens in multiple applications, from airport security scanners to medical radiography. However, most existing scintillators are made from ceramic or perovskite materials, which are often fabricated under harsh conditions and can be plagued with poor stability over time when exposed to light and air.

“Organic-based scintillators have inherent advantages, such as low toxicity, high mechanical flexibility, low cost and straightforward large-scale production,” said KAUST postdoc Jian-Xin Wang, who worked on the project under the supervision of Omar Mohammed. “However, balancing the X-ray absorption capability, exciton utilisation efficiency and photoluminescence quantum yield of organic scintillators has proven challenging.”

To date, organic scintillator materials have been hampered by the small range of X-ray frequencies that they can naturally absorb. However, Wang and colleagues realised that X-ray absorption should increase dramatically as the atomic number of the incorporated elements increases. In particular, the team hypothesised that the addition of heavy atoms to the scintillator material could resolve this issue. X-ray photons can interact efficiently with heavy atoms due to their photoelectric effect — the emission of electrons under excitation from radiation.

“We used a simple molecular engineering strategy to design novel organic scintillators,” Wang said. “We began by introducing chlorine, bromine or iodine to thermally activated delayed fluorescence (TADF) chromophores. We then observed how these heavy atoms altered the efficiency and resolution of the resulting X-ray images.”

TADF chromophores are useful because they exist in an excited quantum ‘triplet state’ in the form of excitons — bound states of electrons and electron holes that are created when a high-energy X-ray photon is absorbed, effectively lifting an electron out of its hole. The triplet state converts to a singlet state when the chromophores absorb thermal energy. They can then de-excite to the ground state and emit light in a process called delayed fluorescence.

“This means that, due to the minimised singlet-triplet energy gap, TADF chromophores can harness both the singlet and triplet excitons that are generated when they are exposed to X-ray radiation,” Wang said.

Mohammed added, “This dramatically improves the exciton utilisation efficiency of the scintillator, which in turn provides much higher X-ray spatial imaging resolution and ultralow detection sensitivity.”

The technique of fabricating screens using scintillators doped with heavy atoms has proved successful so far. One of the team’s scintillators, made using TADF-Br (bromine) chromophores, exceeded the resolution of most reported organic and organometallic scintillation screens.

“These fabricated screens provide a powerful design approach and promising new alternative materials for making X-ray imaging scintillators with outstanding sensitivity, low cost and high stability,” Mohammed said.

The team are currently manufacturing a portable X-ray sensor with their fabricated screen for high-resolution medical imaging, including dental examinations and health checks. Their design could also advance the development of tiny wearable X-ray devices.

Image caption: X-ray images of fish using the high-performance scintillator material that can detect X-rays at low doses. Image ©2022 Elsevier Inc. Jian-Xin Wang et al.