Diffusion 19F-NMR of Nanofluorides: In Situ Quantification of Colloidal Diameters and Protein Corona Formation in Solution

Abstract

The NMR-detectability of elements of organic ligands that stabilize colloidal inorganic nanocrystals (NCs) allow the study of their diffusion characteristics in solutions. Nevertheless, these measurements are sensitive to dynamic ligand exchange and often lead to overestimation of diffusion coefficients of dispersed colloids. Here, we present an approach for the quantitative assessment of the diffusion properties of colloidal NCs based on the NMR signals of the elements of their inorganic cores. Benefiting from the robust ¹⁹F-NMR signals of the fluorides in the core of colloidal CaF₂ and SrF₂, we show the immunity of ¹⁹F-diffusion NMR to dynamic ligand exchange and, thus, the ability to quantify, with high accuracy, the colloidal diameters of different types of nanofluorides in situ. With the demonstrated ability to characterize the formation of protein corona at the surface of nanofluorides, we envision that this study can be extended to additional formulations and applications.

Keywords: 19F-NMR; colloids; diffusion; nanocrystals; protein corona.

Figure 1

Characterization of nanofluorides. Size distribution of colloidal nanofluorides as measured with DLS for (a) AEP-CaF₂, d_DLS = 4.6 ± 1.1 nm, (b) Cit-CaF₂, d_DLS = 4.9 ± 1.1 nm, (c) AEP-SrF₂d_DLS = 7.1 ± 1.7 nm and for (d) OA-CaF₂, d_DLS = 10.0 ± 1.8 nm. High-resolution, liquid-state ¹⁹F-NMR (376.7 MHz, 298 K) of AEP-CaF₂ (e), Cit-CaF₂ (f), AEP-SrF₂ (g), and OA-CaF₂ (h). (a–c) and (e–g) Colloids dispersed in D₂O. (d) and (h) Colloids dispersed in cyclohexane-d12.

Figure 2

Diffusion experiments of nanofluorides. (a) Schematic illustration of the diffusion NMR characteristics of the ligand (¹H NMR) and core (¹⁹F-NMR) content of the studied colloid. (b) Representative stack-plot of ¹⁹F-NMR signal (of AEP-CaF₂) decay as a function of the applied diffusion gradient. Logarithmic normalized ¹⁹F /¹H signal intensity as a function of the experimental b-values obtained from (c) AEP-CaF₂ in D₂O, (d) Cit-CaF₂ in D₂O, (e) AEP-SrF₂ in D₂O, and (f) OA-CaF₂ in cyclohexane-d12. Colored spheres are the ¹⁹F diffusion measurements of NCs, colored circles are ¹H diffusion measurements of ligands in NCs sample, and the dashed gray line is the line fitting of the diffusion NMR of free ligands. In the insets is a schematic of the equilibrium between the ligands and the NCs.

Figure 3

Diffusion ¹⁹F-NMR of nanofluorides. (a) Logarithmic normalized ¹⁹F-NMR signal intensity as a function of the experimental b-values (eq S1, see supporting notes); AEP-CaF₂ (pink), Cit-CaF₂ (blue), and AEP-SrF₂ (turquoise) dispersed in D₂O. (b) Correlating NCs diameter evaluated from diffusion ¹⁹F-NMR and DLS.

Figure 4

Diffusion ¹⁹F-NMR of protein adsorption on NCs in water (a) Schematic illustration of the protein adsorption on AEP-CaF₂ NCs surface. (b) Diffusion ¹⁹F-NMR of AEP-CaF₂ in D₂O before and after the addition of lysozyme proteins with an increased protein-to-nanofluoride ratio, as noted.