Hybrid graphene oxide (GO)/metal nanocomposites have been recently proposed as novel surface-enhanced Raman scattering (SERS) substrates. Despite an increasing interest in these systems, standardization in their fabrication process is still lacking but urgently required to support their use for real-life applications. In this work we investigate how the assembly of GO should be conducted to control adsorption geometry and optical properties at the interface with plasmonic nanostructures as monolayer assemblies of silver nanocubes, by tuning main experimental parameters including GO concentration and self-assembly time. We finally identified the experimental conditions for building up a close-fitting soft dressing of the plasmonic surface, which shows optimal characteristics for flexible and reliable SERS detection. KEYWORDS: plasmonic nanoparticles, sensing, self-assembly, Raman spectroscopy, quartz crystal microbalance
■ INTRODUCTIONSurface-enhanced Raman scattering (SERS) spectroscopy is as a robust and versatile analytical technique with the ability to offer detailed and label-free chemical information with molecular selectivity and ultrasensitivity. 1−5 The SERS effect relies on huge electromagnetic fields concentrated at the metal surface of a plasmonic nanostructure, which dramatically enhance the Raman signal of molecules placed in its close proximity. In this respect, efficient SERS enhancements are typically generated within less than 10 nm from the metal surface and are mostly confined within the so-called "hot spots", i.e. highly curved nanoregions or gaps and junctions between adjacent nanoparticles. The ability to gather molecules in these regions in a homogeneous and reproducible manner represents a current impediment that is still preventing further diffusion of SERS as a systematic analytical tool.A recent trend toward upgrading the performance of SERS technology relies on the combination of plasmonic nanostructures made of silver and gold with graphene. 6−9 Graphene is a single-atom-thick sheet of sp 2 -hybridized carbon atoms arranged in a honeycomb lattice. A number of unique chemical, electronic, optical, and structural properties make this material a preferred choice for greatly improving electronic and photonic devices. 10,11 With a focus on SERS, current efforts are taking advantage of a thin graphene coating in drawing and concentrating target molecules as well as in conferring the plasmonic surface with a passivating layer that discards possible disturbances and signal variability induced by metal−molecule interactions. 12 Main fabrication methods of graphene sheets include mechanical peel-off, epitaxial growth, and chemical vapor deposition (CVD), which, however, appear unsuitable for large-scale production of graphene, ultimately limiting its practical application. A convenient alternative is now represented by graphene oxide (GO), which is derived by oxidation of graphite in the form of graphene sheets with abundant oxygen-containing functionalities. These confer chemical st...