Development of an acid fuchsin dye-based method for the quantification of solvent accessible surface charges of polymerized N-oxides

Biofilm formation on surfaces utilized in medical and industrial settings can lead to a series of severe impacts. In the pursuit of designing materials with low fouling properties, the grafting of polymeric zwitterions has been a commonly employed and effective approach to prevent the attachment of biomolecules and microorganisms on surfaces. Recently, polymeric N-oxides have emerged as promising candidates for such applications, exhibiting low-fouling properties and additionally, antibacterial activity. To achieve effective antifouling and antibacterial properties, the surface charge density of grafted N-oxides plays a crucial role, as higher charge density is associated with increased antifouling and antimicrobial activity. Polymeric N-oxides were attached to polyethylene foils through a plasma-mediated grafting process. This was initiated by radical polymerization of a tertiary amine monomer which was oxidized to the corresponding N-oxide with H2O2 post-grafting. Successful grafting was confirmed by IR spectrometry and by the determination of the water contact angle on the grafted foils. While surface characterization was previously thoroughly conducted via XPS and ToF-SIMS, the evaluation of the charge density of grafted N-oxides was not investigated so far. To determine the zwitterionic charge density of polymerized N-oxides, as a central part of this work, a new dye-based assay was developed and validated. This assay is based on the ion exchange mechanism between the positively charged nitrogen atom of the N-oxide functionality and the anionic sulfonate group of the dye acid fuchsin. The effect of different parameters, such as pH of the dye solution and treatment time, were thoroughly investigated. The assay conditions determined in this investigation were effectively employed for the determination of the charge density of diverse grafted N-oxides exhibiting values in the range of 1-3 x 1015 N+/cm2. These values are close to previously measured charge densities of similar polycationic brushes. Moreover, the chemical reactivity between the acid fuchsin dye and the N-oxide was studied for the trapping of triarylmethane dyes in solution. The zwitterionic nature of grafted N-oxides can be exploited to achieve a reversible adsorption of the dye on the surface via nucleophilic addition of the negatively charged oxygen of N-oxide groups and electrophilic dyes. The potential of this reversible adsorption process presents a promising opportunity for the efficient removal of synthetic dyes from wastewater.