Ho3+-Yb3+:YMoO4 core@shell nanoparticles for enhanced visible upconversion and security applications

Ho3+-Yb3+ sensitized YMoO4 core@shell upconversion (UC) nanoparticles (NPs) have been structurally and optically characterized. Bright Field Transmission Electron Microscopy (TEM) imaging and the corresponding Fast Fourier Transform (FFT) pattern has been done to evidence the formation of core@shell nanostructures. Optical properties viz. UV-Vis absorbance spectra, Dynamic Light Scattering (DLS), zeta potential measurement and upconversion (UC)/downconversion (DC) spectra of core and core@shell nanoparticles have been reported. The core@shell NPs exhibits both the colloidal stability and solubility in well suited biological liquids. Due to silica shell across the Ho3+/Yb3+ codoped YMoO4 NPs, the frequency UC emission intensity of all bands have been increased compared to that of core NPs. The colour emitted from the core@shell NPs lies in the green region having colour purity ∼95%. In biocompatible solvents under 980 nm excitation, the reduction in the overheating problem is due to small decrement in the UC emission intensity of core@shell NPs compared to that of the core NPs.