Hydrophobic core and hydrophilic shell: Now we know more about the surface structure of lignin nanoparticles

Lignin is the second most abundant biopolymer after cellulose and is a most intriguing natural material that can be utilized across a wide range of applications. With modern technological advancements, lignin nanoparticles, a material with potential applications in biomedical- as well as environmental fields, have been developed. Published literature in this field has revealed that by varying the source- and synthesis conditions, it is possible to obtain lignin nanomaterials in the form of nanotubes or as spherical particles with either a hollow or a core−shell substructure. Further, it has been shown that a thorough comprehension of the morphology of the surface of the nanomaterials is essential for their efficient modification. In contrast to other techniques used for studying their surface morphology, nuclear magnetic resonance (NMR) spectroscopy can provide direct insight into the structure of the molecules on the lignin nanoparticles (LNPs) surface. Indeed, NMR analysis of lignin has become a routine way of exploring the structure of lignin, but the liquid-state NMR data for lignin nanoparticles have not yet been reported since LNPs lose their primary structure in most organic solvents.

Recent literature has reported that the ¹H NMR and two-dimensional (2D) heteronuclear single quantum coherence (HSQC) methods are able to detect surface carbons and protons in nanocellulose, thereby showing that NMR is a powerful technique for the chemical characterization of aqueous suspensions of nanocellulose. Moreover, using liquid-state NMR, it is possible to detect the amount of water adsorbed on the silica. One can therefore hypothesize that it should be possible to identify signals from lignin NPs in aqueous solution and thereby acquire a better understanding of their surface structure. To test this hypothesis, KTH Royal Institute of Technology researchers: Dr. Ievgen V. Pylypchuk, Par A. Linden (PhD candidate), Prof.. Mikael E. Lindstrom, and Associate Professor Olena Sevastyanova, proposed to apply a modified sequence for liquid-state ¹H NMR using a combination of pre-saturation water suppression and excitation sculpting water suppression to analyze LNPs prepared from fractionated softwood and hardwood kraft lignin. Their work is currently published in the research journal, ACS Sustainable Chemistry & Engineering.

In their approach, a series of lignin nanoparticles were prepared from spruce- and eucalyptus kraft lignin fractions with narrow molecular weight distributions and functionalities. The research team then characterized particle size, charge, and surface morphology by transmission electron microscopy and dynamic light scattering.

The authors reported that according to liquid-state NMR, methoxy groups from syringyl and guaiacyl units of lignin were seen to be the main groups present on the surface of LNPs. In addition, their study also reported and confirmed the presence of aliphatic moieties, mainly from the side chains of lignin molecules. Moreover, by taking into consideration the chemical composition of the lignin fractions, the structure of lignin NPs as shown by NMR spectroscopy, and their size and surface charge, a pattern of lignin self-assembly into LNPs was suggested.

In summary, the study demonstrated the successful application of a modified ¹H NMR water suppression technique on suspensions of LNPs, that made it possible to gain a clear insight into the surface composition of the LNPs and gave a better understanding of the mechanism of LNP formation. By using this approach, it became possible to identify signals from the surface of the LNPs originating from methoxy groups of the corresponding syringyl- and guaiacyl units of spruce and eucalyptus lignins. It was clearly illustrated that LNPs consist of nonoxygenated aliphatic core and oxygen-rich surface. In a statement to Advances in Engineering, Associate Professor Olena Sevastyanova and Dr. Ievgen Pylypchuk explained that based on their results, one could therefore suggest that the unexpected sharpness of NMR signals for a polymer of such high molecular weight is due to a shell on the NPs consisting of a low molecular weight fraction of lignin molecules.