Fig. 1

a Schematic representation of the synthesis of Ho-DOTA; b Chemical stability: detecting the leakage of Ho3 + in Ho-DOTA and HoCl3 in PBS by using the XO indicator. The solutions from left to right are pure XO solution (0.2%), pure 0.01 mM HoCl3 solution, pure 10 mM HO-DOTA solution, 0.01 mM HoCl3 solution containing the XO indicator, and 10 mM HO-DOTA solution containing the XO indicator; c FTIR spectra of DOTA and Ho-DOTA; d Cellular viabilities of MCF-10A cells and COS-1 cells after being treated with Ho-DOTA (25, 50, 100, 200, 400, 600 μg/mL) for 24 h