Photostimulable Fluorescent Nanoparticles for Biological Imaging
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Photostimulable Fluorescent Nanoparticles for Biological Imaging Andres Osvet1, Moritz Milde2, Sofia Dembski2, Sabine Rupp2, Carsten Gellermann2, Miroslaw Batentschuk1, Christoph J. Brabec1 and Albrecht Winnacker1 1 2
Chair Materials for Electronics and Energy Technology, University of Erlangen-Nuremberg, Martensstr. 7, 91058 Erlangen, Germany Fraunhofer Institute for Silicate Research, Neunerplatz 2, 97082 Wuerzburg, Germany
ABSTRACT Spherical monodisperse core/shell-type nanoparticles, comprising an amorphous SiO2 core coated with a luminescent phosphor layer were synthesized by the modified Pechini processes. The sol-gel method allows covering the 50 – 500 nm core particles with different inorganic phosphor layers of about 10 nm thickness, doped with rare-earth or transition metal ions which determine the luminescent properties. Particles comprising a Zn2SiO4 shell, doped with Mn2+ ions, are not only fluorescent under UV irradiation (260 nm), but store the activation energy by trapping electrons/holes at lattice defects. This energy is released as phosphorescence in the time scale of seconds and minutes, or as photostimulated luminescence under the excitation of red light (650 nm). Traps related to these processes are different, and their concentration is affected by the preparation conditions of the particles. INTRODUCTION Inorganic luminescent nanoparticles have many possible applications including phosphors for displays and conversion phosphors for white LEDs [1], transparent luminescent materials, or sensors [2]. Semiconductor quantum dots (QD) are considered as an alternative to the organic luminescent markers in biological and medical research and diagnostics [3], having broad absorption bands, less photobleaching and less reactions with nearby molecules. The problems related to QDs are the presence of toxic heavy metals, the luminescence intermittency, and the unwanted background luminescence created by the blue or UV excitation. Oxide- or fluoride-based nanoparticles may serve for marker applications as well; they have high chemical stability and brightness if doped with rare-earth (RE) or transition metal (TM) ions. Gated detection may be used to avoid the background emission, profiting from the long, micro- or millisecond decay time of the RE and TM ion emission. It is also possible to use infrared-excited upconversion luminescence like in NaYF4:Er3+,Yb3+ [4] with an additional advantage of the deep penetration of the infrared light and its smaller damaging potential to the cells. However, due to the low efficiency of the two-photon excitation, very high excitation intensity is needed. In this work we focus on Zn2SiO4:Mn2+ which belongs to phosphors in which the excitation energy may be stored by capturing the electrons/holes in traps. Afterglow over minutes, resulting from thermal releasing from the traps, opens further possibilities for marker applications as demonstrated in ref. [5]. Even more interesting is the possibility of triggering the luminescence of the previously charged particles at a des
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