Fig. 4

Normalized upconversion luminescence spectra of core, core@shell particles in cyclohexane and UPG micelles in PBS under 980 nm excitation (a). The insets are full-color photographs of the cyclohexane solutions of the core (left), the core@shell particles (middle) and aqueous solution of UPG (right) taken in the dark under excitation of a 980 nm laser beam. b T₁-MRI maps & values, and c plots of R₁ versus Gd³⁺ concentrations for UPGs in aqueous solution. The corresponding relaxivity value of UPGs is estimated to be r₁ = 3.84/mM/s