Assembly of Gold Nanoparticles on a Molecular Ultrathin Film: Tuning the Surface Plasmon Resonance

A number of methodologies for immobilizing metal nanoparticles in 2-dimensional aggregate structures on various substrates, some with concomitant tuning of the surface plasmon resonance (SPR), have been reported. Many of them involve special functionalization of the nanoparticles, multiple fabrication steps or lengthy procedures. The present study demonstrates that monolayer Langmuir-Blodgett (LB) film of a hemicyanine-based amphiphile with cationic headgroup is an easily fabricated platform for harnessing citrate-stabilized gold nanoparticles. It is shown that a single immersion step can be used to immobilize the nanoparticles uniformly on large area films and that systematic variation of the immersion time from, 10 min to 6 h leads to controlled assembly of the particles and tuning of the SPR band over similar to 100 nm. A model for the structural reorganization in the LB film that facilitates the assembly of nanoparticles is presented and the advantages of the current methodology over earlier protocols are pointed out. The versatility of LB films in terms of the molecular level control of fabrication it enables and the variety of film structures that can be realized, point to the wide scope for future explorations, expanding upon the present observations.