Fig. 3: Solid-state NMR of PDA formed in absence and presence of HNTs.

a Carbon-13 cross-polarization (CP) magic angle spinning NMR spectra for polydopamine formed in the absence of HNTs (PDA-A) after 24 h of polymerization time. Spectra are shown for short (80 μs, black) and long (2 ms; red) CP contact times. The former experiment shows only protonated carbons, where the latter also includes non-protonated carbons. b Analogous data for polydopamine deposited on the HNT surface (HNT-PDA), after 24 h. The inset highlights the lack of the peak at 105 ppm in the case of the HNT-PDA sample. These spectra were recorded at 18 kHz MAS on a 600 MHz NMR instrument.