Interaction of Grafted Polymeric <i>N</i>-oxides with Charged Dyes

Grafted polymeric <i>N</i>-oxides have recently attracted interest for antifouling applications, drug delivery, wastewater purification, and electronic devices. Their function depends on the efficiency of the grafting process and the following postgrafting oxidation step. These two parameters govern the solvent-accessible charge density on the surface, an important parameter, which is notoriously hard to determine. In this study, a novel colorimetric quantitative assay for polymeric <i>N</i>-oxides was developed. It allows the determination of the surface charge density of grafted polymeric <i>N</i>-oxides. The method is based on the adsorption of acid fuchsin (AF) to grafted <i>N</i>-oxides through reversible electrostatic interactions between the positively charged nitrogen atoms of the <i>N</i>-oxide functionality and the sulfonate groups of the dye. The process depends thus on the pH-switchable properties of polymeric <i>N</i>-oxides. Adsorption was achieved at a pH value of 3, where <i>N</i>-oxides are almost fully protonated (typical p<i>K</i>_a 4-5). AF was desorbed from the surface at pH 7 and quantified via visible adsorption spectroscopy (UV-vis) at 556 nm to determine the amount of surface-grafted functional groups. Charge densities of diverse <i>N</i>-oxides grafted by free radical polymerization from polyethylene (PE) were determined to be in the range 1-3 × 10¹⁵ N⁺-O^-/cm². Notably, <i>N</i>-oxides can form covalent bonds with electron-deficient triarylmethane dyes like AF. This nucleophilic reactivity of <i>N</i>-oxides does not compromise the proposed assay, but it may be of relevance for dye adsorption and desorption in wastewater purification.