Design and Characterization of Gelatin-Coated Upconversion Nanoparticles: Insights into Structural, Relaxometric, Luminescent, and Cytotoxic Properties

Abstract

T he present study aimed to develop a theranostic nanoprobe for application. This nanoprobe is composed of upconversion nanoparticles (NPs) coated with gelatin. Initially, erbium-ytterbium co-doped gadolinium oxide (Gd₂O₃:Yb/Er) was synthesized using the homogeneous precipitation technique. The Gd₂O₃:Yb/Er particles were coated with gelatin (Gd₂O₃:Yb/Er@Gelatin) using the desolvation method. Four syntheses were conducted with different gelatin concentrations and the use of glutaraldehyde (GA) as a cross-linking agent. The characterization of the nanoprobe included structural, relaxometric, luminescent, and cytotoxicity analyses. The results indicate that cross-linking with GA reduces the size of the NPs, suggesting a greater compaction of the gelatin chains. It was observed that the gelatin coating increases the concentration of water molecules near the NPs through hydrogen bonding interactions and modulates their diffusion time near the paramagnetic center, influencing the decrease in proton relaxation time. On the other hand, cross-linking with GA restricts the mobility of water molecules, by all relaxivity values were found to be higher than those of commercial contrast agents. The luminescent data showed that although the spectral emission profile of upconversion between Yb³⁺ and Er³⁺ ions did not change compared to the oxide, the emission intensity ratio (I_R/I_G) decreased with coating, and the emission in the green region is generated by the absorption of three photons, while the emission in the red region is generated by the absorption of two photons. It was also observed that Gd₂O₃:Yb/Er and Gd₂O₃:Yb/Er@Gelatin NPs had no cytotoxic effect on healthy cells, with cell viability above 90%. The developed nanoprobe showed interesting luminescent and relaxometric properties, making it a promising tool for optical and magnetic bioimaging.