Transformations of photoluminescence spectra of upconversion nanophosphors by phantoms of biological tissues

Phantoms are often used to imitate biological tissues in laboratory conditions. Phantoms are usually made on the basis of natural and synthetic materials, as well as hydrogels and various bioactive compositions. Today to visualize biological tissues and study the processes occurring to them in in vitro and in vivo researches in real time, upconversion nanophosphors (UCNPs) are actively used. They have a whole set of unique photoluminescent properties and are promising components of modern tools for non-invasive optical diagnostics of the human and animals body. We have carried out the synthesis and complex characterization of $\beta$-NaYF$_4$:Yb$^{3+}$:Er$^{3+}$/NaYF$_4$ nanoparticles, which effectively convert radiation from the near-IR range into the visible region of the spectrum. The process has been developed to encapsulate them into the structure of aliphatic polyesters and to form bioresorbable polylactoglycolide scaffolds by anti-solvent 3D printing. We formed two types of tissue phantoms based on agarose, ultra-pasteurized cow’s milk and melanin. Characterization and analysis of their optical properties were carried out. We studied the transformations of the photoluminescence spectrum of the synthesized UCNPs during the passage of their radiation through these phantoms, and performed the visualization of the photoluminescent polyester matrices placed in them.