Chromium Doped Persistent Phosphors for Medical Imaging

Persistent luminescent materials or glow-in-the-dark compounds have been known for centuries, but due to their limited performance in terms of total light output, they were never used for large scale applications. It is only since about 20 years ago, when SrAl₂O₄:Eu,Dy was introduced as a new persistent phosphor [1], that a leap forward in both brightness and decay time was achieved. This paper has initiated substantial research, which has led to an entire family of materials, showing an appreciable afterglow for more than 24 hours [2,3]. 

These developments have opened up a whole new series of applications, ranging from emergency illumination, decoration and toys to medical imaging. For the latter application, materials are needed with deep red to near infrared emission, which falls in the transparency window of biological tissues. This, together with the need for red emitters for indicating emergency situations, has led to an interest in long wavelength persistent luminescence.

Eu²⁺ is by far the most common dopant in persistent luminescent compounds. However, only very few chemically stable hosts are known, notably a number of nitrides, that allow to shift the Eu²⁺ emission to the deep red. Current research therefore focusses on Mn²⁺ and Cr³⁺ as valuable alternative dopants [4], as they allow to reach longer wavelengths than possible with Eu²⁺.

Fig. 1: Principle of in vivo imaging using persistent luminescent materials.

In this work, we will focus on Cr-doped ZnGa₂O₄ (ZGO), which is one of the most promising dopant-host combinations for near infrared emission [4]. ZGO has a normal spinel structure, where the Zn²⁺ and Ga³⁺ ions are located in octahedral and tetrahedral voids respectively. Due to their similarity with Ga³⁺ in charge and size, the Cr³⁺ ions occupy Ga-sites in the host. It has been shown that the trap states in ZGO:Cr, which are necessary to store the energy, liberated during the afterglow, are related to antisite defects: a small fraction of Ga-ions occupying Zn-sites or vice versa [5]. In addition, authors have tried to increase the number of traps by introducing different codopants and/or elements to form solid solutions with ZGO.

This presentation will discuss different methods to synthesize ZGO:Cr nanoparticles which are potentially useful for in vivo medical imaging. In addition, the effects of different types of dopants on the defects and thus on the persistent luminescence will be presented.

Finally, some alternative hosts for emission from Cr³⁺ - SrAl₂O₄ (ubiquitous in visible light persistent luminescence) and YAGG (yttrium aluminium gallium garnet) - are evaluated, and the toxicity of the materials, which is obviously very important for biocompatibility, is discussed.

[1] T. Matsuzawa, Y. Aoki, N. Takeuchi and Y.A. Murayama, J. Electrochem. Soc. 143, 2670–2673 (1996).

[2] K. Van den Eeckhout, P.F. Smet, D. Poelman,  Materials 3, 2536-2566 (2010)

[3] K. Van den Eeckhout, D. Poelman, P.F. Smet, Materials 6, 2789-2818 (2013)

[4] Y.X. Zhuang, Y. Katayama, J. Ueda and S. Tanabe, Optical Materials 36, 1907-1912 (2014)

[5] T. Maldiney, A. Bessière, J. Seguin, E. Teston, S.K. Sharma, B. Viana, A.J.J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, C. Richard, Nature Materials 13, 418-426 (2014)