Time-dependent complexation of cysteine-capped gold nanoparticles with octadecylamine langmuir monolayers at the air-water interface

In this paper, we present time-dependent studies on the complexation of cysteine-capped gold nanoparticles with octadecylamine (ODA) Langmuir monolayers. The cysteine molecules bound to the colloidal gold surface via thiolate linkages impart a net negative charge to the particles due to deprotonated carboxylic acid groups. Strong attractive electrostatic interaction between the negatively charged gold nanoparticles and positively charged ODA monolayer drives the complexation process. The extent of complexation of the gold nanoparticles and subsequent Langmuir-Blodgett (LB) film formation is a function of charge on the particles/monolayer. The charge on the nanoparticles/monolayer may be controlled by simple variation of the subphase pH. At pH 9, the carboxylic acid groups on the particles are highly ionized leading to strong electrostatic attraction with the protonated ODA monolayer, while at pH 12, the ODA monolayer is deprotonated leading to a reduction in the electrostatic interaction. The nanoparticle complexation with the ODA Langmuir monolayer has been followed in real time by a host of techniques such as surface pressure-area (π-A) isotherms, pressure-time (π-t) isotherms, Brewster angle microscopy, ellipsometry, and pendant drop analyses. LB films of the nanogold-ODA composites have been characterized by UV-vis spectroscopy, Fourier transform infrared spectroscopy, and contact angle measurements. These measurements clearly indicate uniform nanoparticle deposition at pH 9 (pH of maximum electrostatic interaction). The LB films of the gold nanoparticles were also tested for thermal stability.