Multimodal underwater adsorption of oxide nanoparticles on catechol-based polymer nanosheets

Abstract

Multimodal underwater adsorption behaviour of catechol units was demonstrated by examining the adsorption of different oxide nanoparticles on nanoscale-integrated polymer nanosheets. Catechol-based polymer nanosheets were fabricated using the Langmuir–Blodgett (LB) technique with random copolymers (p(DDA/DMA)s) of N-dodecylacrylamide (DDA) and dopamine methacrylamide (DMA). The p(DDA/DMA) nanosheets were immersed into water dispersions of SiO₂, Al₂O₃, and WO₃ nanoparticles (NPs) respectively. The results show that the adsorption properties can be altered by varying the NP type: SiO₂ NP adsorption was observed only below pH = 6, at which the o-quinone form in p(DDA/DMA) nanosheets transforms into the catechol form or vice versa. However, their transition point for Al₂O₃ NP adsorption was found at approximately pH 10, at which the surface potential of Al₂O₃ NPs changes the charge polarity, indicating that the electrostatic interaction is predominant. For WO₃ NPs, adsorption was observed when citric acid, which modifies the surface of WO₃ NPs by complex formation, was used as a pH-controlling agent, but no adsorption was found for hydrochloric acid used as a pH controlling agent. FT-IR measurements proved that miniscule amounts of water molecules were trapped in p(DDA/DMA) nanosheets and that they acquired hydrogen bonding network formations, which might assist nanoparticle adsorption underwater and make the catechol units adjustable. The results indicate that the nanoscale spatial arrangements of catechol units in films are crucially important for the application of multimodal adsorption of oxide nanoparticles on catechol-based polymer materials.