Optimizing SERS performance through aggregation of gold nanorods in Langmuir-Blodgett films

Abstract

Nanoparticles with a high degree of anisotropy are characterized by large electromagnetic field enhancements at the single-particle level; however, their arrangement into aggregated structures does not lead to significantly better performance. Surprisingly, for nanorods a superior accumulation of hot spots is present at surface coverages above 60%, which ensures a higher efficiency for surface-enhanced Raman spectroscopy (SERS). In this study, we produced pegylated gold nanorods in Langmuir-Blodgett layers in order to optimize SERS performance for the ultrasensitive detection of molecules, by tuning plasmonic coupling and nanoparticle surface coverage. To understand the arrangement of the nanorods in Langmuir and Langmuir-Blodgett layers, we performed spectroscopic and microscopic analysis of the obtained films. Based on the compression isotherms of the nanorods, we proposed the best conditions for nanoplatform design. The Raman enhancement factor was around 2·105 with a deposition surface pressure equal to 12 mN·m−1. Due to the polymer’s presence at the surface of the nanorods and the dewetting process, specific dendritic-like aggregates were observed on the solid substrate. The results obtained indicate the significance of further studies on the arrangement of metallic nanoparticles in monolayers, taking into account the size and shape of the nanoparticles, as well as the type of stabilizing ligands at their surface.