Evolution of Highly Biocompatible and Thermally Stable YVO₄:Er³⁺/Yb³⁺ Upconversion Mesoporous Hollow Nanospheriods as Drug Carriers for Therapeutic Applications

In recent times, upconversion nanomaterials with mesoporous hollow structures have gained significant interest as a prospective nano-platform for cancer imaging and therapeutic applications. In this study, we report a highly biocompatible YVO₄:1Er³⁺/10Yb³⁺ upconversion mesoporous hollow nanospheriods (YVO₄:Er³⁺/Yb³⁺ UC-MHNSPs) by a facile and rapid self-sacrificing template method. The Rietveld analysis confirmed their pure phase of tetragonal zircon structure. Nitrogen adsorption–desorption isotherms revealed the mesoporous nature of these UC-MHNSPs and the surface area is found to be ~87.46 m²/g. Under near-infrared excitation (980 nm), YVO₄:Er³⁺/Yb³⁺ UC-MHNSPs showed interesting color tunability from red to green emission. Initially (at 0.4 W), energy back transfer from Er³⁺ to Yb³⁺ ions leads to the strong red emission. Whereas at high pump powers (1 W), a fine green emission is observed due to the dominant three-photon excitation process and traditional energy transfer route from Er³⁺ to Yb³⁺ ions. The bright red light from the membrane of HeLa cells confirmed the effective cellular uptake of YVO₄:Er³⁺/Yb³⁺ UC-MHNSPs. The resonant decrease in cell viability on increasing the concentration of curcumin conjugated YVO₄:Er³⁺/Yb³⁺ UC-MHNSPs established their excellent antitumor activity. Therefore, the acquired results indicate that these YVO₄:Er³⁺/Yb³⁺ UC-MHNSPs are promising drug carriers for bioimaging and various therapeutic applications.