Figure 4. Signature of apoferritin filling cobalt from cobalt salt CoNTA (CO₃) Na₂.

(A) UV-Vis spectrum of ApoF incubated with CoNTA (CO₃) Na₂ for 22 hours in acidic medium in comparison to ApoF alone and HoloF. Note the growth of the absorbance shoulder at 280 nm indicating the filling of the protein cage. (B) HRTEM of ApoF incubated with CoNTA showing crystalline structures with diffraction pattern (C) characteristic of the cobalt oxide (CoO) along [111] zone axis. (D) Distribution of crystal sizes (mean diameter 3.5 ± 0.6 nm) suggesting a partial filling of the protein cage, in line with UV-Vis spectrum. (E) STEM-HAADF images of ferritin nanocrystals and (F) EDX analysis of the red area, indicated characteristic lines of cobalt and sulfur.