Closer to the polydopamine structure: new insights from a combined 13C/1H/2H solid-state NMR study on deuterated samples

Abstract

Despite the large number of reported applications for polydopamine (PDA), a fundamental challenge still exists in this field: no complete structural elucidation of PDA is available to date. Herein, considering that this is due to the limitations of standard analytical techniques when applied to complex and heterogeneous systems, a solid-state NMR approach is introduced, which brings important novelties with respect to both sample engineering and the employed experimental techniques. ²H solid-echo, ¹H fast-MAS and ¹H double-quantum filtered ss-NMR experiments on PDA samples prepared under three different deuteration schemes are reported for the first time in the present work. The obtained results together with the results from ¹³C CP-MAS/CPPI ss-NMR experiments at 25 kHz MAS provide key structural and dynamic information, which can eliminate some of the uncertain structural characteristics present in the literature on PDA. It is found that at least half of the phenyl ring positions remain protonated in PDA and the majority of phenyl/indole rings are rigid, which is most probably due to π–π stacking. Considering the numerous models proposed to date, these results offer strong experimental evidence in favor of one particular monomer coupling and oligomer aggregation scheme in PDA. It is also evidenced from the ²H and ¹H ss-NMR data that water molecules undergo slow diffusion inside the sample as well as in and out the sample. All these findings combined with recent theoretical predictions represent important advances in understanding the complex structure of PDA.