Manipulation of plyethylene glycol capped gold nanoparticle packing density and film morphology at the air/water interface

Abstract

Nanoparticle self-assembly (NP SA) at the air/water interface is a promising method for fabricating films with novel optical, electronic properties that are useful for state-of-the-art diagnostic and therapeutic applications. Interfacial NP films are often rigid and the NPs aggregate as a result of very strong NP-NP interactions. This prevents the film from correcting defects, causing poor performance. In this study polyethylene glycol (PEG) capped gold nanoparticle (AuNP) systems were investigated to potentially address this issue by tuning AuNPs’ main forces for self-assembly: electrostatic, hydrophobic, hydrogen bonding, and steric. A minimum concentration of 0.5 M NaCl was determined to be required for initial film formation at the interface from aqueous solution. Three types of PEG-capped AuNPs with various terminating PEG groups (methyl, amine and carboxylic acid) were synthesized and then tested on three different subphases pH’s (4, 6.5, and 8) to induce varying amounts of charge on the PEG terminating group. The effect of AuNP charge on NP film rigidity, morphology, and NP packing density were analyzed using compression isotherms along with atomic force microscopy (AFM) and transmission electron microscopy (TEM) imaging. The various PEG ligand terminating groups complicated AuNP purification and the 0.5 M NaCl subphase altered AFM and TEM sampling protocol. Results showed that, firstly, ligand functionality and charge impact both film rigidity, morphology and, secondly, that NP interfacial packing density varied with compression to create responsive AuNP films.