Composite fast scintillators based on high-Z fluorescent metal–organic framework nanocrystals

Abstract

Scintillators, materials that produce light pulses upon interaction with ionizing radiation, are widely employed in radiation detectors. In advanced medical-imaging technologies, fast scintillators enabling a time resolution of tens of picoseconds are required to achieve high-resolution imaging at the millimetre length scale. Here we demonstrate that composite materials based on fluorescent metal-organic framework (MOF) nanocrystals can work as fast scintillators. We present a prototype scintillator fabricated by embedding MOF nanocrystals in a polymer. The MOF comprises zirconium oxo-hydroxy clusters, high-Z linking nodes interacting with the ionizing radiation, arranged in an orderly fashion at a nanometric distance from 9,10-diphenylanthracene ligand emitters. Their incorporation in the framework enables fast sensitization of the ligand fluorescence, thus avoiding issues typically arising from the intimate mixing of complementary elements. This proof-of-concept prototype device shows an ultrafast scintillation rise time of ~50 ps, thus supporting the development of new scintillators based on engineered fluorescent MOF nanocrystals.

Highly luminescent hetero-ligand MOF nanocrystals with engineered massive Stokes shift for photonic applications.

“…The nanocomposite shows a radioluminescence intensity five times greater than the reference composite made with the homo-ligand Zr-DPA (dotted line) which demonstrated a scintillation yield 𝜙 𝑠𝑐𝑖𝑛𝑡 , defined as the number of emitted photons for each MeV of deposited energy for ionizing radiation, of approximately 1000 ph MeV -1 . 39 The obtained data indicates therefore that the prototype 𝜙 𝑠𝑐𝑖𝑛𝑡 is assessed at around 5000 ph MeV -1 . This value demonstrates the success of the proposed strategy to enhance the scintillation performance of composite materials based on fluorescent MOF nanocrystals, which show a 𝜙 𝑠𝑐𝑖𝑛𝑡 comparable to that of commercial plastic an inorganic scintillators.…”

“…1b, the activation of DPT luminescence occurs by energy transfer during the random diffusion within the framework of an excited DPA singlet exciton, which is created upon light absorption or free-charge recombination in scintillation. 39 Before spontaneous recombination, the singlet moves from the original position by an average diffusion length 𝐿. [40][41][42] This implies that if a DPT moiety is placed at a distance shorter than 𝐿 from the position where the DPA exciton is created, the energy transfer can most likely occur before singlet recombination, thus without energy dispersion.…”

“…In the case of homo-ligand MOFs, the use of high loading levels induces a shortening of the photoluminescence lifetime and efficiency reduction, because of a partial aggregation of poorly dispersed crystals that can limit also the surface passivation effect of the host matrix. 39 Conversely, in hetero-ligand MOFs this effect appears absent, suggesting that DPT ligands are successfully incorporated as separated and protected units, whose emission ability is insensitive to the nanocrystal environment and aggregation-induced losses. 39,46 The absence of significant reabsorption is further highlighted by low temperature experiments.…”

Nearly 100% energy transfer at the interface of metal-organic frameworks for X-ray imaging scintillators

“…MOFs are robust chemical structures constructed from highly ordered organic linkers and heavy metal clusters with good stability, low toxicity, and an outstanding level of structural and compositional control. [30][31][32][33][34][35][36][37] The flexible selection of inorganic building blocks with different atomic numbers could ensure that their ionizing radiation absorption performance is sufficient. [38][39][40][41][42] On the other hand, the luminescent center should also harvest both singlet and triplet excitons, since they are generated with a 1:3 ratio from the recombination of electron-hole pairs after the X-ray excitation.…”

The Sensitization of Scintillation in Polymeric Composites Based on Fluorescent Nanocomplexes

“…The second contribution relies on an active sensitization process that can be observed if the heavy systems employed to increase the material density are luminescent. Indeed, under specific energetic resonance conditions, the organic emitters can be further activated by a second energy transfer from the heavy components (Figure 1a) [4][5][6]. While the passive sensitization can be seen, in first approximation, as a trivial consequence of the increment of the average scintillator density, the active sensitization requires a deeper analysis to be modelled.…”