Cation-Ligand Complexation Mediates the Temporal Evolution of Colloidal Fluoride Nanocrystals through Transient Aggregation

Colloidal inorganic nanofluorides have aroused great interest for various applications with their development greatly accelerated thanks to advanced synthetic approaches. Nevertheless, understanding their colloidal evolution and the factors that affect their dispersion could improve the ability to rationally design them. Here, using a multimodal in situ approach that combines DLS, NMR, and cryogenic-TEM, we elucidate the formation dynamics of nanofluorides in water through a transient aggregative phase. Specifically, we demonstrate that ligand-cation interactions mediate a transient aggregation of as-formed CaF2 nanocrystals (NCs) which governs the kinetics of the colloids’ evolution. These observations shed light on key stages through which CaF2 NCs are dispersed in water, highlighting fundamental aspects of nanofluorides formation mechanisms. Our findings emphasize the roles of ligands in NCs’ synthesis beyond their function as surfactants, including their ability to mediate colloidal evolution by complexing cationic precursors, and should be considered in the design of other types of NCs.


Sample preparation of X-ray crystallography:
AEP with Ca 2+ : A solution containing AEP (0.17M) and Ca(NO3)2 (0.04M) in 1 ml of ddH2O was neutralized with ammonium hydroxide to pH=7 was placed in a fume hood at 25 o C for 72h to evaporate the water solvent.
AEP with F -: A solution containing AEP (0.17M) and NaF (0.08M) in 1 ml of ddH2O was neutralized with ammonium hydroxide to pH=7 was placed in a fume hood at 25 o C for 72h to evaporate the water solvent.
Single crystal X-ray diffraction (XRD): Single crystal XRD were measured by sealed tube diffractometer Rigaku Synergy-S diffractometer dual source equipped with Dectris Pilatus3 R CdTe 300K detector and microfocus, with MoKa (l=0.71073 Å). Data collection was performed in low temperature under LN. Data were processed with CrysAlisPRO (Rigaku). Structures were solved using SHELXT 1 and refinement performed based on F2 with SHELXL 2 and OLEX2 3 with full matrix least-squares. All non-hydrogen atoms were refined aniostropically. Hydrogens were placed at calculated positions and refined using a riding model. See crystallographic details in the

Synthesis of AEP-CaF2 NCs
Synthesis Route 1: A solution containing AEP (0.17M) and Ca(NO3)2 (0.04M) in 1 ml of ddH2O was neutralized with ammonium hydroxide to pH=7. Next, NaF (0.08M) was injected and immediately vortexed for 3 sec. The reaction solution was either sampled at different time points for cryo-TEM measurements or was placed inside a 5 mm NMR tube for in-situ 31 P-NMR / 19 F-NMR experiments. (Scheme S1a) Synthesis Route 2: A solution containing AEP (0.17M) and NaF (0.08M) in 1 ml of ddH2O was neutralized with ammonium hydroxide to pH=7. Next, Ca(NO3)2 (0.04M) was injected and immediately vortexed for 3 sec. The reaction solution was either sampled for cryo-TEM measurements or was placed inside a 5 mm NMR tube for in-situ 31 P-NMR / 19 F-NMR experiments. (Scheme S1b).

Cryo-TEM
Cryo-TEM specimens were prepared by applying 6.5 μl sample (according to the two synthetic routes investigated following the injection of the second precursor) to a 200-mesh copper grid coated with holey carbon (Pacific Grid-Tech supplies). The grids were subjected to a 1 min glow discharge before sample application. Samples were blotted at 22°C and 95% relative humidity and then plunged into melting ethane using a Leica EM-GP Automatic Grid Plunger. Specimens were equilibrated at -178°C in the microscope prior to imaging. Cryo-TEM was performed on a Thermo-Fisher Scientific (TFS) Tecnai Typical recording times for EDS maps were 5 min. The EDS hyperspectral maps were processed using Velox (Thermo Fisher Scientific Microscopy Solutions, Hillsboro, USA).
All elemental maps and quantitative composition data were obtained from deconvoluted and background subtracted spectra.

High resolution (HR) TEM images
Solutions of AEP-CaF2 NCs were prepared according to both synthetic routes. After 1000 min at 25°C, the solution was centrifuged and washed with ethanol/ddH2O. By drop cast deposition on an ultra-thin carbon support foil the sample were dried and prepared for HR-TEM measurements. HRTEM images (

NMR Experiments
Sample for 31 P diffusion of AEP with Ca 2+ : A solution containing AEP (0.17M) and Ca(NO3)2 (0.04M) in 1 ml of ddH2O was neutralized with ammonium hydroxide to pH=7 was placed inside a 5 mm NMR tube.
Sample for 31 P diffusion of AEP with F -: A solution containing AEP (0.17M) and NaF (0.08M) in 1 ml of ddH2O was neutralized with ammonium hydroxide to pH=7. 31 P diffusion NMR measurements were conducted on a 9.4 T (162.06566 MHz) AVANCE Ⅲ NMR spectrometer (Bruker, Germany) equipped with a 50gauss/cm Z gradient system.  Table S1.   Table S1.          Table S1. Diffusion and relaxation (T1) 31 P-NMR. Experiments performed on solutions prior the reaction of AEP-CaF2 initiation, i.e., the pre-synthesis conditions.