Natural elimination of upconversion YVO$_4$ : Yb, Er nanoparticles from a grape snail

We report on a method for detection of upconversion nanoparticles (UNPs) in biological preparations by using confocal microscopy technique. Approbation of the method was carried out by monitoring the natural excretion of an injected colloidal solution of YVO$_4$ : Yb, Er nanoparticles (0.2 ml, containing 15 mg ANP, dosage 600 mg/kg) with sizes from 10 nm to 700 nm into the snail. The experiments showed that the animal naturally eliminated the nanoparticles from the body within three days, while the excretion rate decreased over time. The threshold of sensitivity in the experiments was equal to one nanoparticle in 1 $\mu$l.