Extension of poly(diphenylamine) monolayer at the air/liquid interface promoted by incorporation of gold nanoparticles

Abstract

Gold nanoparticles (AuNPs) were adsorbed from an aqueous solution onto a poly(diphenylamine) (PDPA) monolayer at the air/liquid interface for preparing composite PDPA/AuNPs molecular films. The AuNPs were surface-modified by mercaptosuccinic acid (MSA) to enhance the electrostatic interaction between PDPA and the AuNPs. The behaviors of PDPA and mixed PDPA/AuNPs monolayers at the air/liquid interface were investigated using surface pressure-area (π-A) and surface potential-area (ΔV-A) isotherms, as well as the direct observation of Brewster Angle Microscopy (BAM). The surface structures of the corresponding Langmuir-Blodgett (LB) films were characterized by an Atomic Force Microscopy (AFM). The results show that the PDPA molecules form a condensed layer on pure water subphase, with small compressibility. The limiting molecular areas of PDPA estimated from both π-A and ΔV-A isotherms are very small, indicating that PDPA molecules were highly aggregated on pure water. This inference was confirmed by the observations of BAM and AFM. On the contrary, the mixed PDPA/AuNPs monolayer demonstrates a more expanded characteristic with a much larger limiting area in comparison with that of PDPA. Furthermore, the PDPA/AuNPs monolayer reveals a high uniform morphology without significant aggregative phases under the observation of BAM and AFM. It is inferred that the PDPA-AuNPs interaction fixes the PDPA chains on the surface of AuNPs, decreases the mobility and interaction of polymer chains, and therefore, inhibits the folding and aggregation of PDPA molecules. Therefore, highly extended PDPA molecules were obtained in the composite PDPA/AuNPs monolayer.