Fig. 1: Characterization of AEP-CaF₂ NCs.

a Molecular structure of an AEP ligand at pH 7. b Cryo-TEM AEP-CaF₂ NCs (scale bar 20 nm). c STEM-EDS elemental maps of an agglomerate of dried AEP-CaF₂ NCs (scale bar 10 nm). Shown are a combined map of the NCs elements, a high-angle annular dark-field (HAADF) image (white) and elemental maps of each element, i.e., F (pink), Ca (blue), and P (green). (d) Liquid-state, high-resolution ¹⁹F-NMR spectrum of water-dispersed AEP-CaF₂ NCs and its deconvoluted signals, attributed to ¹⁹F atoms at the surface of the NCs (-105 ppm, pink) and in the NC core (−109 ppm, light blue). e Schematic illustration of the CaF₂ NC with its surface (pink) and core atoms (light blue) and the equation (1) used to evaluate the size of NCs from the ¹⁹F-NMR spectrum (Eq. (1) and Supplementary Note 1). f Correlation between the sizes measured with NMR and TEM of various CaF₂ and SrF₂ NCs bearing different ligands (AEP, Citrate (Cit), Oleic Acid (OA) and Polyethylene Glycol (PEG)); (1) Cit-CaF₂ (Fig. 5b) (2) AEP-CaF₂ (3) AEP-SrF₂ (Fig. 3) (4) OA-SrF₂ (Supplementary Fig. 3b) (5) PEG-CaF₂ (Supplementary Fig. 3a) (6) OA-CaF₂ (Supplementary Fig. 3c).