Designing photon upconversion nanoparticles capable of intense emission in whole human blood.

The properties of upconverting nanoparticles (UCNPs) are crucial for their applications in biomedicine. For studies of organisms and biological materials, the penetration depth of excitation light is also essential as the depth from which the luminescence can be detected. Currently, many researchers are trying to obtain UCNPs with intense emission under excitation wavelengths from the biological transparency windows to increase the penetration depth. However, studies comparing the properties of various types of UCNPs in real conditions are rare. This article shows how deep the 808, 975, 1208, and 1532 nm laser radiation penetrates human blood. Moreover, we determined how thick a layer of blood still permits for observation of the luminescence signal. The measured luminescence properties indicated that the near-infrared light could pass through the blood even to a depth of 7.5 mm. The determined properties of core/shell NaErF/NaYF materials are the most advantageous, and their emission is detectable through 3.0 mm of blood layer using a 1532 nm laser. We prove that the NaErF/NaYF UCNPs can be perfect alternatives for the most studied NaYF:Yb,Er/NaYF. Additionally, the setup proposed in this article can potentially decrease reliance on animal testing in initial biomedicine research.