Mechanism of X-ray excited optical luminescence in NaGdF4-based nanoparticles

X-ray irradiating rare-earth nanoparticles could emit fluorescence (∼50 keV) and luminescence (∼eV) photons simultaneously. Utilizing this phenomenon, it is expected to achieve online multi-modal imaging and integrated radiation and photodynamic therapy for deep tumor. In this scenario, ALnF4: RE3+ (A = alkali metal; Ln = lanthanides acting as matrix; RE3+: rare-earth element used for luminescence center ion) nanoparticles are often selected due to their stable chemical and optical properties. In the application of imaging and therapy, the quantitative calculation of luminescence yield is meaningfully significant for nanoparticle design and to improve the imaging quality and therapeutic effect. However, there is a lack of theoretical calculation on the X-ray excited optical luminescence (XEOL) yield. In this paper, we propose a method to calculate the XEOL yield of ALnF4: RE3+ through theoretical derivation. Taking NaGdF4: Tb3+ as an example, the luminescence mechanism under X-ray irradiation is analyzed and the semi-quantitative formula of luminescence yield is obtained, which is based on the interaction theory of X-ray with materials and the energy transfer principle between different lanthanides. Then, the quantitative XEOL yield formula is gotten by fitting experimental results and the correctness of this formula is verified by the correlation coefficient between fitting results and experimental results. Our results could provide theoretical guidance for the design of NaGdF4-based nanoparticles to manipulate XEOL spectrum and increase XEOL yield. It also laid the foundation for the subsequent development of effective drugs or probes for X-ray driving integrated diagnosis and treatment of deep tumors in vivo.