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

Stannous fluoride (SnF(2)) is an effective fluoride source and antimicrobial agent that is widely used in commercial toothpaste formulations. The antimicrobial activity of SnF(2) 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 SnF(2) (0.454 wt% SnF(2), 0.34 wt% Sn) and the amorphous, semi-solid nature of the toothpaste. Here, we show that dynamic nuclear polarization (DNP) enables (119)Sn solid-state NMR experiments that can probe the Sn speciation within commercially available toothpaste. Solid-state NMR experiments on SnF(2) and SnF(4) show that (19)F isotropic chemical shift and (119)Sn chemical shift anisotropy (CSA) are highly sensitive to the Sn oxidation state. DNP-enhanced (119)Sn magic-angle turning (MAT) 2D NMR spectra of toothpastes resolve Sn(II) and Sn(IV) by their (119)Sn chemical shift tensor parameters. Fits of DNP-enhanced 1D (1)H → (119)Sn 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).