The development of nanoparticles for NIR imaging and diagnostics is an area of considerable interest. Among the different imaging modalities, optics emerged has an interesting technique since it is a non-invasive, cheap imaging technique allowing real time imaging. In-vitro, this technique is very useful, however in-vivo fluorescence imaging suffers from suboptimal signal-to-noise ratio, which is caused by the strong tissue autofluorescence under constant external excitation. To address this limitation, novel types of optical nanoprobes are actually being developed in the deep red/near infrared (NIR) range and among them, persistent luminescence nanoparticles (PLNPs), with long lasting near-infrared luminescence capability. These NPs allow optical imaging to be performed in an excitation-free and consequently autofluorescence-free manner. This chapter will first introduce the physical phenomenon associated to the long luminescence delay of such nanoprobes, from minutes to hours after ceasing the excitation, and will then highlight the tools used in physico-chemistry laboratories to characterize these nanoparticles with a focus on the ZnGa2O4 nanoparticles which are widely studied over the world. Then their biocompatibility will be mentioned and finally the evaluation in term of new advances for in-vivo bioimaging theranostics nanoprobes will be presented. We will conclude this chapter by envisioning perspectives for such nanomaterials.