In their study, the researchers correlated nanoparticle arrangement on the surface of the film with local SERS activity over the same sampled area by combining atomic force microscopy (AFM) and scanning confocal Raman microscopy measurements (made using an alpha 300R system from Witec).

Design details

The key to the huge sensitivity in SERS is the way that light induces collective oscillations of free electrons known as surface plasmon resonances at particular points or "hotspots" located between nanoparticles (nanogaps) or at their edges or tips. To fabricate highly active SERS substrates, the researchers used anisotropic silver nanoparticles of triangular and hexagonal-like shape, which were synthesized and stabilized in chitosan solution. A very low concentration of analyte – here, adenine molecules at about 10–12 M – was mixed in solution with the chitosan-coated silver nanoparticles and a simple drop coating method was used to cast the film onto a solid substrate. Water evaporation caused isolated nanoparticles existing in solution to get closer and form small ensembles like dimers or trimers, which can protrude through the film surface. Detailed characterization of the film surface was obtained by both AFM and SEM. (see top image).

Assessing SERS efficiency

To explain the origin of the most intense hotspots enabling single-molecule detection, the team compared the morphology of the film surface with the spatial distribution of the SERS signal collected from adenine molecules. The picture on the right illustrates an example of correlated AFM-Raman images obtained by overlaying the spatial map of SERS signal on the AFM picture of the same sampled area. The high SERS enhancement areas coincide with the locations of nanogaps residing in nanoparticle assemblies such as dimers, trimers and some larger clusters.

A highlight of this research is that the chitosan biopolymer not only provides the support for the silver nanoparticles to adopt distinct and stable arrangements with junction and gap sites that can generate enormous SERS enhancements, but also serves as a biocompatible and permeable coating, which allows analyte molecules to diffuse and immobilize in close vicinity to the silver surface. The fabricated plasmonic substrate with such endowed biocompatibility can hold significant potential for biomedical sensing and imaging via SERS.

The researchers presented their work in the journal Nanotechnology.