Figure 1.

a) Scheme of the formation of lanthanide–mannose (Ln–Man) complex through the coordination of mannose with lanthanide ions (Tb³⁺/Eu³⁺). b) Scheme of the formation of lanthanide–mannose–gelatin (Ln–Man–Geln) hydrogel by introducing Ln–Man into gelatin network, and the property of fluorescence “On/Off” upon Fe²⁺/EDTA. c) Illustration of the Ln–Man–Geln hydrogel as a 3D cell culture matrix for reversible fluorochromic “On/Off” switch upon Fe²⁺/EDTA.