Optical imaging for biological applications requires more sensitive tools. Near-infrared persistent luminescence nanoparticles enable highly sensitive in vivo optical detection and complete avoidance of tissue autofluorescence. However, the actual generation of persistent luminescence nanoparticles necessitates ex vivo activation before systemic administration, which prevents long-term imaging in living animals. Here, we introduce a new generation of optical nanoprobes, based on chromium-doped zinc gallate, whose persistent luminescence can be activated in vivo through living tissues using highly penetrating low-energy red photons. Surface functionalization of this photonic probe can be adjusted to favour multiple biomedical applications such as tumour targeting. Notably, we show that cells can endocytose these nanoparticles in vitro and that, after intravenous injection, we can track labelled cells in vivo and follow their biodistribution by a simple whole animal optical detection, opening new perspectives for cell therapy research and for a variety of diagnosis applications.

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Information on the energy that is needed to transfer an electron from the valence band of an inorganic compound to a trivalent lanthanide impurity is presented. The energy is a measure of the location of the ground state of the divalent lanthanide relative to the top of the valence band. A variation with type of lanthanide is found that is the same irrespective of the type of compound (fluorides, chlorides, bromides, iodides, oxides, sulfides). The variation is anti-correlated with the known variation in fd transition energies in divalent lanthanides. Because of the anti-correlation, the energy difference between the first 4fn−15d state and the bottom of the conduction band is relatively invariant with type of lanthanide ion. The difference is largest for Eu2+, and decreases gradually towards the end of the lanthanide series by 0.5 eV for Y b2+. Based on the systematic variation in charge transfer energy and fd energy, a three-parameter model is presented to position the energy levels for each divalent lanthanide relative to valence and conduction band states. Using a similar model the levels of trivalent lanthanides are positioned.

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