Dipyramidal-Au@SiO2 nanostructures: New efficient electromagnetic nanoresonators for Raman spectroscopy analysis of surfaces

Plasmonic structures act as electromagnetic nanoresonators leading to a local enhancement of the intensity of the electric field of the incident radiation, which leads to an increase in the efficiency of Raman scattering for molecules in close proximity to such nanoresonators. Because many biological molecules can change their structure during interaction with metal surfaces, plasmonic nanostructures are sometimes covered with very thin layers of chemically inert oxides, which prevent direct interaction between the sample being analysed and the metal surface. The Raman analysis of surfaces using surface-protected plasmonic nanoparticles is called shellisolated nanoparticle-enhanced Raman spectroscopy (SHINERS). In this contribution, the first example of using dipyramidal-Au@SiO2 nanoparticles as nanoresonators for SHINERS is presented. Since such nanostructures contain many sharp edges and apexes (on which the highest field enhancement is usually generated), the Raman enhancement factor induced by the dipyramidal nanoparticles is significantly (about one order of magnitude) larger than the enhancement factor generated by standard semi-spherical nanostructures. Synthesized dipyramidal- Au@SiO2 nanoparticles were tested as nanoresonators in SHINERS measurements of some thiolate monolayers formed on a platinum surface and for detecting the pesticide thiram deposited on a tomato skin. For example, the limit of detection of thiram in the last system was estimated as about 0.9 ng cm−2. The method of synthesis and purification of dipyramidal-Au nanoparticles applied in this work makes it possible to obtain samples of SHINERS nanostructures having very high homogeneity, significantly better than the homogeneity of anisotropic gold nanostructures previously used for SHINERS measurements.