Structural characterization of tin in toothpaste by dynamic nuclear polarization enhanced ¹¹⁹Sn solid-state NMR...

Stannous fluoride (SnF₂) is an effective fluoride source and antimicrobial agent that is widely used in commercial toothpaste formulations. The antimicrobial activity of SnF₂ is partly attributed to the presence of Sn(II) ions. However, it is challenging to directly determine the Sn speciation and oxidation state within commercially available toothpaste products due to the low weight loading of SnF₂ (0.454 wt% SnF₂, 0.34 wt% Sn) and the amorphous, semi-solid nature of the toothpaste. Here, we show that dynamic nuclear polarization (DNP) enables ¹¹⁹Sn solid-state NMR experiments that can probe the Sn speciation within commercially available toothpaste. Solid-state NMR experiments on SnF₂ and SnF₄ show that ¹⁹F isotropic chemical shift and ¹¹⁹Sn chemical shift anisotropy (CSA) are highly sensitive to the Sn oxidation state. DNP-enhanced ¹¹⁹Sn magic-angle turning (MAT) 2D NMR spectra of toothpastes resolve Sn(II) and Sn(IV) by their ¹¹⁹Sn chemical shift tensor parameters. Fits of DNP-enhanced 1D ¹H → ¹¹⁹Sn solid-state NMR spectra allow the populations of Sn(II) and Sn(IV) within the toothpastes to be estimated. This analysis reveals that three of the four commercially available toothpastes contained at least 80% Sn(II), whereas one of the toothpaste contained a significantly higher amount of Sn(IV).