Persistent luminescent nanocrystals enable high-resolution X-ray imaging

NUS researchers have developed a technology for high-resolution X-ray imaging of irregularly-shaped objects using lanthanide-doped nanomaterials with ultralong persistent luminescence. They have coined this technology as X-ray luminescence extension imaging (Xr-LEI). This innovation represents a significant step towards next-generation, low-cost X-ray imaging technology.

Current X-ray imaging technologies involving flat-panel detectors have been widely applied to medical diagnostics, security screening and industrial inspection. These X-ray sensing devices typically require the integration of a flat-panel substrate, equipped with a layer of scintillators for fast X-ray energy conversion and compact silicon sensors for image reconstruction. However, the compact sensor electronics associated with each pixel which is connected to its own application-specific integrated circuit limit the number of pixels per unit area and often cause heat buildup due to the power requirements of low-noise amplifier electronics. Existing X-ray detectors are also unsuitable for high-resolution imaging of curved three-dimensional (3D) objects. On the other hand, although existing persistent luminescent phosphors have shown promise for X-ray imaging, their low X-ray sensitivity largely limits their use in practical applications. Additionally, conventional phosphors require harsh growth conditions and have low solubility, imposing constraints on thin-film processing and flexible device fabrication.

A research team led by Prof Xiaogang LIU from the Department of Chemistry, NUS, has discovered that a series of solution-processable, lanthanide-doped nanoscintillators can be used to achieve ultralong-lived X-ray trapping and potentially enable flat-panel-free, high-resolution imaging of 3D electronics (see Figure). This research is in collaboration with Prof Bolong HUANG from The Hong Kong Polytechnic, and Prof Qiushui CHEN and Prof Huanghao YANG, both from Fuzhou University. Optical characterisations reveal that these nanocrystals can emit persistent radioluminescence for up to 30 days after X-ray irradiation. By incorporating these X-ray-responsive nanocrystals into highly stretchable silicone rubber, the researchers have demonstrated high-resolution X-ray imaging (less than 25 micrometers). The recorded images on the flexible substrate can be retained for more than 15 days. This imaging method can be used to detect defects in electronics, to authenticate valuable works of art, and to examine the exterior and interior of archaeological fossils at a microscopic scale.

Apart from imaging applications, the team has also investigated the mechanism underlying ultralong persistent luminescence in these lanthanide-doped nanocrystals. Theoretical studies suggest that the X-ray radiation triggers anion migration to the interstitials in nanocrystal lattices and generates traps for charge carriers. Under optical or thermal stimulation, these trapped holes migrate towards the lanthanide emitters, forming hole–lanthanide centres that radiatively recombine with released electrons. Such electron–hole recombination results in lasting persistent radioluminescence from the lanthanide ions. These displaced anions can diffuse back to their original lattice sites upon heat treatment.

Prof Liu said, “Over the past few years, many research groups including ours have been taking on challenges in X-ray imaging. This Xr-LEI technology may provide a much needed solution for imaging highly curved 3D objects and enable the development of point-of-care X-ray detectors and flexible X-ray mammography devices.”

Figure shows X-ray luminescence extension imaging (Xr-LEI) of the detailed circuitry of an iPhone 6 Plus smartphone. [Credit: OU Xiangyu]

Reference:

Ou X; Qin X; Huang B; Zan J; Wu Q; Hong Z; Xie L; Bian H; Yi Z; Chen X; Wu Y; Song X; Li J; Chen Q*; Yang H*; Liu X*, “High-resolution X-ray luminescence extension imaging”, NATURE DOI: 10.1038/s41586-021-03251-6. Published: 2021.