Dear Researchers,
I am new to this imaging field, so I am not familiar about 1 question.
(1) Why people use anti-Stokes emission for imaging?
Generally anti-stokes emission is always weaker than Stokes emission (2 or 3 photon process),right?
Then why not to use Stokes emission for imaging?
e.g. mostly used Er case, excite at 800 nm and strong Stokes emission appears at 980 nm.
Fulfill all the criteria needed for imaging so far I know, like optical window of body tissue, Detector sensitivity (well available Si-detector), availability of excitation source (800 nm strong laser) and so on.
Can anybody suggest me, proper purpose of Anti-Stokes emission in imaging?
One of the main problems for down-shifting emission is low absorption efficiency of exciting radiation by the rare earth ions. Once you cannot use fundamental absorption of the host or any charge transfer bands, the direct absorption by the emitting ions is very inefficient. For up-converting materials, a sensitizer like Yb is often added because it has an intensive absorption band around 1050 nm.
Another issue is that with increasing the wavelength, the scattering inside the tissues decreases and, for a given intensity of exciting radiation, it will penetrate deeper in the studied specimen.
Anyway, you are thinking in a right direction, Stokes emission indeed can be used, you just need an ion with a prominent emission line even further into IR than the peak of absorption of a chosen sensitizer. If you search for NIR-to-NIR fluorescence imaging (or somehow similar it can be called), you will find some already published papers on this topic.
Thank you Roman for your nice information.
One more point is that , I checked 10 mol% Er doped Y2O3 host and compared to the 10 mol% Yb and 1 mol% Er doped Y2O3. The Er absorption at 970 nm wavelength is as strong as to 10 mol% Yb absorption at 1000 nm. However, Er emission band at 980 nm is much better for the former than the later.
Anyway I am learning and hope to get more information and discussion in near future.
Search for papers experimentally comparing the two ways of imaging and, if you don't find good publications, cary out the experiments with the compositions you've peaked in your lab - it will be a good paper.
One point maybe still has to be mentioned - oxides are not perfect hosts for upconversion. Although there is some logic behind a direct comparison of yttria-based down-shifting and up-conversion, it would be more relevant if you compare not the same host with different doping but "best in class" compositions for both types of emission. You will need something like sodium yttrium fluoride as a host for upconversion (to be put against an optimised oxide-based material exhibiting Stokes emission).
The best hosts for upconversion seem to be LiLuF4 with quantum yields of 5% for Er and 7.6% for Tm. See http://onlinelibrary.wiley.com/doi/10.1002/anie.201309503/pdf (Angew. Chem. Int. Ed. 2014, 53(3) 1252.
(Note: I have not yet seen confirmation of these yields)
In addition, Roman is perfectly right; exciting in the NIR leads to much less light scattering and absorption than if one has to excite in the UV-vis; Most of the downshifting probes need excitation in the range 350-450 nm, which is really not ideal, so that time-resolved detection has to be used. This is not the case with upconversion. Moreover, the new UC probes are bright enough to use low (=biologically safe) excitation power - much less than in confocal microscopy for instance. For a comparison of luminescent bioprobes, see Table 1 in J. Lumin. 170 (3), 866 (2016), http://www.sciencedirect.com/science/article/pii/S0022231315004135/pdfft?md5=a0a6ab7c486bf96818058a595eddd7ce&pid=1-s2.0-S0022231315004135-main.pdf .
Now, another point is that 970-980 nm excitation is not optimum because biological tissues still has noticeable absorption at that wavelength (0.6 cm-1). A way out is to use Nd as sensitizer which has sizeable (I mean considering f-f transitions!) at around 800 nm which seems to be an optimum wavelength. Ongoing research is targeting broad-band excitation in order to get brighter probes, but it is a difficult task.
Finally, regarding your experiment. You should have seen an increase in the absorptivity of the Yb-doped compound since Yb absorb at the same wavelength with a molar absorption coefficient 10 times higher.
Thanks for your nice comments.
Now I can differentiate downshifting and up-converter for imaging.
Dear prof Bünzli, Thanks for nice references.
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