Scientists from the Goa University (Goa, India) and Chemistry Paris-Tech (Paris, France) successfully demonstrated that Cr doped ZnGa2O4 was an excellent biomarker emitting in the red/near infrared region. Therefore, it has been shown that its activation could take place in-vivo. Those results, published in Physical Chemistry, Chemical Physics, pave the way for its use in optical imaging of vascularisation, tumours and grafted cells.
A biomarker, or biological marker, generally refers to a measured characteristic which may be used as an indicator of some biological state or condition. In medicine, a biomarker can be a traceable substance that is introduced into an organism as a way to examine organ function or other aspects of health. Researchers at Goa University, Goa, India and Chimie Paris-Tech, Paris, France were involved in developing novel biomarkers emitting red Long-Lasting Phosphorescence* (LLP) for in vivo small animal optical imaging. These biomarkers are first excited by ultraviolet light or X-rays for a couple of minutes outside the animal body, and then injected to the animal where they emit red/infrared light detectable for several hours after the injection.
Figure: Experimental Cr-K edge XANES spectrum (EXP) compared with spectra calculated with different environments for Cr; 2. Experimental Cr-K edge XANES spectrum along with computed LCF for (a) ZGO, (b) MGO, (c) ZAO ; 3. Variation of NIRPL intensity values taken at different times versus the % of Cr clusters present in ZGO, MGO and ZAO.
Cr doped ZnGa2O4 (Cr-ZGO) was reported to be an excellent biomarker emitting in the red/near infrared region (peak emission at ~695 nm) with UV/X-ray light excitation. Further studies also revealed that LLP can also be achieved through excitation by visible light. In this study, the electron–hole excitation occurs through the crystal field split 3d levels of Cr ions. This opened a new possibility of prolonged emission by re-excitation of the Cr-ZGO biomarker within the body of small animal using relatively harmless visible light. This in-vivo activation of ZGO-Cr makes it useful for optical imaging of vascularisation, tumours and grafted cells.
The visible light excitation of ZGO-Cr also indicates that some defects responsible for LLP are centred around Cr ion and hence it becomes necessary to understand the local structure of Cr in ZnGa2O4 matrix. Cr K edge XANES and EXAFS spectra were recorded at the SAMBA beamline of the synchrotron facility SOLEIL. These data collected allowed establishing the presence of cationic site inversion defects around the Cr3+ ion and their role in LLP in a low photon energy (visible light) charging process. The ab-initio calculations of Cr K XANES spectra showed the presence and the key role of Cr clusters. The analysis showed presence of a clear correlation between the amount of Cr clustering and the quenching of the visible light induced LLP intensity.
With the aim of improving the understanding of the LLP mechanism, it is now important to explore the consequences of this Cr-clustering by correlating this strong inhomogeneity of Cr3+ distribution with other studies like thermally stimulated luminescence studies, EPR spectroscopy and first principles calculations.
*LLP is caused by the presence of defect levels in the forbidden energy gap. Irradiation of material with ultraviolet light leads to creation of free electrons and holes which get trapped in the defect levels. These trapped electrons and holes are later released due to thermal excitation and recombine at a luminescent centre leading to emission