In SERS probes, the Raman-signature molecules are typically attached to the outer surface of metallic nanoparticles. Such assemblies can have a tendency to aggregate, resulting in desorption of the Raman probes and leading to fluctuation and poor reproducibility of the Raman signal.

Recently Nick Wu’s group at West Virginia University, US, has developed sandwich-structured nanoparticles to upgrade the technique. Here, the Raman-signature molecules are sandwiched between a gold core and a thin silica shell.

Characteristic features

The silica shell offers long-term stability without degradation and flexible conjugation with a variety of biomolecules. This sandwich structure also wraps a high concentration of Raman-signature molecules, which prevents the Raman-signature molecules from leaking out, but also improves the solubility of the nanoparticles in organic solvents and in aqueous solutions even with a high ionic strength.

Three types of gold cores (nanospheres, nanorods and nanostars) were evaluated as SERS substrates. The gold nanostars show the highest electromagnetic enhancement for SERS due to a high density of “hot spots” generated between sharp tips on the surface.

In the study, the gold-(malachite green isothiocyanate)-SiO2 sandwich nanoparticle behaved as a highly sensitive SERS probe with good water solubility and stability, low-background fluorescence, and an absence of photo-bleaching for biological applications. This SERS probe can be used for biosensing and bio-imaging, say the researchers.

Full details can be found in the journal Nanotechnology.