Surface Chemistry-Dependent Binding Interactions between Kraft Lignin and Polyelectrolyte-Encapsulated Gold Nanoparticles

This study investigated the relationship between gold nanoparticle (AuNP) surface chemistry and the binding affinity of unfractionated kraft lignin to polyelectrolyte-coated AuNPs. Specifically, fluorescence quenching titrations were employed to determine lignin's binding affinity (<i>K</i> _a) to different charged polymer surfaces displayed on 90 nm citrate-stabilized AuNPs, (poly-(allylamine hydrochloride)) (PAH), polyacrylate (PAA), and poly-(diallyldimethylammonium chloride) (PDADMAC). AuNP-lignin conjugates were characterized by UV-vis absorbance spectroscopy, ζ-potential analysis, and dynamic light scattering (DLS) measurements. The characterization data showed that the size, surface charge and aggregation state of the lignin-AuNP conjugates depended on the original surface chemistry of the AuNP, in conjunction with the lignin concentration. Fluorescence quenching titrations indicated that lignin's binding affinity for the polyelectrolyte-wrapped AuNPs was significantly higher for PDADMAC-AuNP, while the Cit-, PAH-, and PAA-AuNPs gave statistically indistinguishable affinity constants (<i>p</i> < 0.05). <i>K</i> _a values for PAA-AuNP, Cit-AuNP, PAH-AuNP, and PDADMAC-AuNP were determined to be 87 ± 8 nM^-1, 92 ± 11 nM^-1, 107 ± 13 nM^-1, and 240 ± 13 nM^-1, respectively. This fluorescence data indicates that electrostatic interactions are not the primary driving force in lignin adsorption to these AuNP surfaces. Instead, van der Waals forces (such as hydrophobic interactions and hydrogen bonding) are likely more important in mediating lignin-AuNP adsorption on this length scale. Understanding how the surface chemistry of polymers mounted on nanoscale surfaces impacts lignin adsorption will inform the process of eco-corona formation on polymer-coated nanoscale surfaces.