Effects of intense hot-spot-specific local fields on fluorescein adsorbed at 3D porous gold architecture: evolution of SERS amplification and photobleaching under resonant illumination

Plasmonic nanostructures with a high density of hot spots, which are characterized by high local electromagnetic fields, are needed to increase the sensitivity of surface-enhanced Raman spectroscopy (SERS) and fluorescence (SEF). These nanostructures can be used in both identifying biological molecules and monitoring photochemical reactions that occur on a metal surface when electromagnetic radiation interacts with these molecules. Such analysis methods are widely used both in medicine and pharmacology.

In this work, using the method of pulsed laser deposition, three-dimensional (3D) porous wedge-shaped arrays of gold nanoparticles were obtained with structural parameters varying along the substrate, such as film thickness, porosity, nanoparticles size, and distance between them . The resulting arrays were close-packed multilayer structures with a varying density of hot spots along the substrate, in which the field intensity enhancement correlates with the nanostructure geometric parameters. Using the SERS of fluorescein molecules adsorbed on the surface of the obtained nanostructures, we studied the dynamics of fluorophore photobleaching under the influence of electromagnetic field. The obtained results provide the information about the highest electric field intensity region in hot spots on the surface of plasmonic nanostructures. We observed the similarity in the evolution of SERS signals and fluorescence in time, which indicated the similarity of the SERS amplification and photobleaching mechanism for both processes. An increase in the photobleaching rate of fluorescein for the structure with the highest enhancement of SERS and fluorescence has been observed.