Lanthanide-Based Upconversion Nanophosphors for Bioimaging - The Chemistry of Molecular Imaging - page 317

13.5.2.2 Long-Term Toxicity of Ln-UCnPs To date, no obvious toxicity has been reported for Ln-UCNPs. The research
work evaluated the behaviour, weight, and histological assessment of tissues. Li and co-workers reported that after injection
of 15 mg/kg wt PAA-UCNPs (~11.5 nm), no changes in the common behaviour were observed for mice [48]. In other work,
mice injected with 25 nm to 60 nm NagdF 4 :Yb,Er,Tm nanoparticles could also survive for more than one month without
showing abnormal behaviour [92]. Body weight is another parameter to evaluate the living conditions of the mouse. Zhang
and co-workers reported that injection with a dose of 10 mg/kg wt of silica-coated NaYF 4 :Yb,Er UCNPs showed no obvious
effect on the body weight of a mouse [57]. Li and co-workers observed tiny weight differences between a mouse injected
with 15 mg/kg wt PAA-UCNPs (~11.5 nm) and the control group [48]. Neither death nor a significant body weight drop were
observed for PAA-UCNPs and PEg-UCNPs treated mice [103].
other analysis methods include the histological assessment of tissues and the routine analysis of blood. Li's group [48]
and Liu's group [103] reported related results for PAA-UCNPs and PEg-UCNPs, and at general dose levels, they observed
no abnormal data in these analysis.

13.6 Future dIrectIonS

As described in the previous sections, UCNPs can now be treated as candidates suitable for bioimaging. The fabrication of
the labels and the imaging techniques are founded both in principle and in practice. However, along with the detailed inves-
tigation of these nanomaterials, some disadvantages need to be overcome before moving toward actual applications in exper-
imental or clinical practice.
The most important disadvantage is the problem of the upconversion efficiency of UCNPs. Due to the large surface-to-
volume ratio of small-sized nanoparticles, surface-quenching centres limit the upconversion efficiency. Although core-shell
structures and some other techniques have been developed to overcome this problem, current materials still need significant
improvement in their absolute efficiencies.
Another important question is the evaluation of the biosecurity of UCNPs. Some efforts have been made to investigate
this point, but new techniques still need to be developed to monitor the metabolism, biodistribution, and possible toxicity of
UCNPs. Furthermore, the long-term bioeffects of UCNPs need to be considered before the widespread clinical application
of these materials.

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