The bis(trihydroxyphenyl)-appended ligand 1 was found to display dramatically enhanced fluorescence upon the addition of Cs⁺, but not with any other metal ions. The self-assembled nanofiber of 1 was changed into a spherical structure upon addition of Cs⁺ in aqueous solution. Furthermore, 1 as a chemoprobe displayed high-resolution fluorescence imaging for cesium in Arabidopsis.