Development of CaS:Eu2+/Dy3+ Persistent and NIR Photo-Stimulated Nanophosphors

Persistent luminescent nanophosphors are nano-sized materials that have recently come into the limelight and have attracted attention as potential fluorescent imaging probes due to their attractive optical properties. The synthesis and characterization of red persistent luminescent CaS:Eu2+/Dy3+ nanophosphors were investigated. These nanophosphors are capable of energy storage due to presence of electron trap sites. This provides a new approach of avoiding exposure of biological tissue to high-energy irradiation and eliminating the generation autofluorescence since these nanophosphors are charged ex vivo with UV irradiation. Once injected in the body, in vivo images can be acquired by detection of the red persistent luminescence originating from the shallow traps present in the nanophosphor. After the persistent luminescence has subsided, energy stored in deeper traps can be released upon near infrared light (NIR) irradiation, resulting in a red photo-stimulated emission. With the goal to develop CaS:Eu2+/Dy3+ persistent and NIR photo-stimulated nanophosphors, synthesis and characterization of the optical properties of CaS nanoparticles and CaS:Eu2+ and CaS:Eu2+/Dy3+ nanophosphors were carried out. The nature of the electron trap sites and the trapping and de-trapping mechanisms were studied by wavelength resolved thermally stimulated luminescence. In addition, the generation of strong red light emission following NIR excitation of the CaS:Eu2+/Dy3+ nanophosphors is demonstrated. This basic understanding is primordial in the development of a new nano-sized photonic materials in the field of biomedical luminescent probes for applications in bioimaging. The integration of the CaS:Eu2+/Dy3+ nanophosphors as an optical bioimaging probe is limited by the hygroscopic character of the CaS nano host. Consequently, different surface modification strategies were studied to render the CaS:Eu2+/Dy3+ nanophosphors water dispersible and at the same time prevent hydrolization. Among the surface modification strategies studied, the grafting of a silica shell, tetraethyl orthosilicate (TEOS), was chosen to carry out preliminary in vivo fluorescence optical imaging experiments using TEOS-CaS:Eu2+/Dy3+ nanophosphors as the luminescent probes