Determining the molecular weight of polyelectrolytes remains a persistent challenge because conventional techniques such as size-exclusion chromatography (SEC) often require added salt, calibration against neutral polymers, and complex optimization to overcome chain-column interactions. Here we introduce a kinetics-based method that employs Localized Surface Plasmon Resonance (LSPR) to measure the real-time electrostatic complexation of poly(ethylenimine) (PEI) and poly1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido-1,2-ethanediyl, sodium salt] (PAZO). By extracting association and dissociation rate constants from concentration-dependent binding transients, we establish a calibration-free approach to determining the number-average molecular weight (Mn) of polyelectrolytes under dilute, salt-free conditions. Using the known Mn of PEI, the Mn of PAZO was calculated as 257,400 g mol-1, corresponding to a degree of polymerization of 642. This analysis assumes symmetry of the association rate constant (kon) across binding orientations, an assumption justified by the reciprocal nature of electrostatic interactions. The results highlight LSPR-based kinetics as a powerful, surface-sensitive alternative to traditional methods, offering a broadly adaptable strategy for characterizing charged polymers and other biomolecular systems. This study is presented as a proof-of-concept demonstration, emphasizing the novelty and feasibility of the LSPR-based kinetic framework rather than exhaustive validation of molecular weight determinations. By focusing on methodological innovation, the work highlights a generalizable strategy for polyelectrolyte characterization that can be extended and refined in future investigations.
Mooney et al. (Wed,) studied this question.