Polydispersity and assembling phenomena of native and reactive dye-labelled nanocellulose

The assembling (self-association vs. aggregation or agglomeration) of highly polydispersed nanocelluloses (NCs) is a well-known but difficult to identify phenomena. In this research complementary analytical tools were applied for tracking and assessing this phenomena under different conditions (base vs. buffer) using both native and dye-labelled cellulose nanofibrils (CNFs) and cellulose nanocrystals. For this purpose, dichlorotriazine-type reactive blue 4 (RB4) dye was covalently (through alkyl-aryl ether bond) and regioselectively (to C6-OH groups) attached to the NC, being identified by ATR-FTIR and C-13 CP/MAS solid-state NMR spectroscopies, and quantified by UV-Vis spectroscopy. The introduced RB4's anthraquinone (i.e. hydrophobic) moieties evoked aggregation of those CNFs being quantified from the shifting and broadening of size-distribution profiles within differential light scattering analysis and their zeta-potential reduction. In addition, their high polydispersity profiles was assessed by qualitatively supported differential centrifugal sedimentation, nanoparticle tracking analysis, and transmission electron microscopy. A desorption of chemically non-bonded, yet highly substantive (i.e. partially or fully hydrolysed), dye from both types of NCs was also identified by capillary electrophoresis, which simultaneously excluded the non-labelled fractions, thus obtaining free dye-absent NC dispersions. Finally, labelling extent-triggered separation within the RB4-labelled CNFs was identified by applying the micellar capillary electrophoresis, thus confirming the conformational changes affecting NCs' hydrodynamic (size) profiles.