Selective adsorption of azo dyes on engineered polydopamine nanoparticles: Insights into structure-affinity relationships

In this study, polydopamine nanoparticles (PDA NPs) were synthesized in hydroalcoholic media containing ethanol (EtOH), 2-propanol (2-PrOH), or their mixtures to evaluate their adsorption capacity toward azo dyes. The NPs were characterized in terms of size, colloidal stability, chemical structure, and morphology as a function of solvent composition. Thirteen dyes with different chemical structures were tested, revealing higher PDA affinity (>75 mg/g) for aromatic and sulfonated dyes. Adsorption was mainly driven by π-π stacking, hydrogen bonding, and electrostatic interactions. Among the synthesized NPs, those obtained with a 25:75 (% (v/v)) EtOH:2-PrOH ratio (PDA(Mix) NPs) exhibited the most efficient adsorption. Isotherm analysis followed the Freundlich model, while kinetics fitted a pseudo-second-order model, suggesting adsorption governed by dye-NP interactions, supported by EDX and XRD analyses. Competitive adsorption experiments showed coexistence of compatible dyes without interference, whereas weaker binders were displaced by stronger adsorbates. At dye concentrations above 30 mg/L, PDA NPs displayed morphological alterations, including loss of sphericity and internal cavities, likely due to structural reorganization upon dye accumulation. Overall, this work highlights the influence of synthesis conditions on PDA NP performance, and would provide insights for designing PDA-based adsorbents for industrial wastewater treatment.