Fig. 3

a Fluorescence responses of BHN-Fe₃O₄@SiO₂ with various cations. Excitation wavelength was 415 nm. Spectra were recorded every 2 min after adding metal ions. b Competition of Fe³⁺-BHN-Fe₃O₄@SiO₂ towards cations. Fluorescent emission change of BHN-Fe₃O₄@SiO₂ (0.2 g/L) upon addition of metal ions (each metal ion is 5 × 10⁻⁵ M) in CH₃CN/H₂O 1:1 (HEPES buffer pH 7.36) at room temperature. c Time responses of BHN-Fe₃O₄@SiO₂ with Fe³⁺ and Cu²⁺. d UV-Vis titrations of BHN-Fe₃O₄@SiO₂ (0.2 g/L) with Fe³⁺. e Fluorescence titration of BHN-Fe₃O₄@SiO₂ (0.2 g/L) with Fe³⁺. Inset: the fluorescence intensities at 518 nm at various concentrations of Fe³⁺. f Job’s plot of BHN-Fe₃O₄@SiO₂ with Fe³⁺