Deep Ultraviolet Emitting Scintillators for Biomedical Applications: The Hard Way of Downsizing LuPO₄:Pr³⁺

Abstract

Pr3+‐doped LuPO4 emits UV radiation between 225 and 280 nm, where DNA shows strong absorption bands. Therefore, a systematic study of the luminescence of Pr3+ doped LuPO4 is performed: Different doping concentrations, particles sizes, and excitation schemes (vacuum UV at 160 nm and X‐rays 50 kV, 2 mA, tungsten target) are compared. The emission spectra in the UV range depends on the excitation energy and the particle size. Microcrystalline particles (6 µm) comprising 1% Pr3+ display the highest emission intensity at 234 nm upon vacuum UV as well as X‐ray excitation. Sub‐microscale particles (20–50 nm) of LuPO4:Pr3+ (1%) show the same UV emission under X‐ray excitation as the larger particles but do not emit under vacuum UV excitation. Colloidal nanoscale particles (5 nm) do not show emission in the UV‐C range. Based on the high‐density and strong X‐ray absorption of LuPO4, the implementation of Pr3+ doped LuPO4 particles of suitable size (20–50 nm) could improve the well‐established radiation therapy. Owing to the strong absorption and low penetration depth of UV‐C radiation in biological tissue, Pr3+‐doped LuPO4 particles located directly in cancerous tumors could allow for additional treatment with cell‐damaging UV‐C radiation.

Characterization of Micro‐ and Nanoscale LuPO4:Pr3+,Nd3+ with Strong UV‐C Emission to Reduce X‐Ray Doses in Radiation Therapy

“…These peaks correspond to the transitions from the lowest [Xe]4f 1 5d 1 crystal field component to the 3 H 4 , 3 H 5 , 3 H 6 , and 3 F 2 multiplets of the lowest [Xe]4f 2 ground state configuration of Pr 3+ . These results are consistent with previously published literature . The excitation spectrum monitored at 235 nm is composed of two broad bands peaking at around 146 and 188 nm.…”

“…The first set of results describes the effect of the Nd 3+ addition on the emission of LuPO 4 :Pr 3+ (1%) upon VUV excitation when systematically increasing the Nd 3+ concentration from 0% to 3% ( Figure ). The Pr 3+ concentration of 1% was found to yield the highest UV‐C emission intensity and therefore, was kept constant …”

“…The different optimal concentrations may be explained by the fact that X‐rays are ionizing radiation that interacts with the whole LuPO 4 particle, whereas VUV excitation at 160 nm (7.75 eV) mainly excites the surface due to its low penetration depth . Consequently, an inhomogenous distribution of Nd 3+ ions (donor clusters) in the particle and on the particle surface leads to quenching at lower Nd 3+ concentrations, as observed upon VUV excitation.…”

“…Previous studies have shown that the UV‐C emission of Pr 3+ ‐activated LuPO 4 is dependent on the particle size: for NPs of around 5 nm, the Pr 3+ emission was found to be quenched completely. For detectable Pr 3+ emission in the UV‐C range, 80–100 nm particles were necessary to produce emission . The majority of previous research has emphasized that the loss of the emission intensity is due to the nonradiative relaxation resulting from the interaction with surface defects .…”

“…In this paper, we propose Nd 3+ as a co‐dopant for LuPO 4 :Pr 3+ to increase UV‐C emission: based on the analysis of the energy level diagram of trivalent rare earth ions (Dieke's diagram), Nd 3+ ‐doped LuPO 4 shows a band at 190 nm caused by interconfigurational 5d‐4f transitions and therefore it possesses a big spectral overlap with the 4f‐5d transitions of Pr 3+ . Additionally, the use of Nd 3+ as a co‐activator could enhance the production of electron–hole (e–h) pairs due to thermalization processes upon X‐ray excitation …”